ORIGINAL ARTICLE
Isotope signatures and hydrochemistry as tools in assessing
groundwater occurrence and dynamics in a coastal arid aquifer
C. Tamez-Mele´ndez1,2 • A. Herna´ndez-Antonio1 • P. C. Gaona-Zanella1 •
N. Ornelas-Soto1 • J. Mahlknecht1
Received: 13 July 2015 / Accepted: 7 April 2016 / Published online: 5 May 2016
 The Author(s) 2016. This article is published with open access at Springerlink.com
Abstract Groundwater from the La Paz arid coastal
aquifer in Baja California, Mexico, is essentially the only
source of drinking water for the local population and
tourists, as well as irrigation water for agricultural needs.
The intensive exploitation of the aquifer and water cycling
has resulted in groundwater abatement (up to 10 m) and
high salinity (up to *5800 mg l-1). A study using
hydrochemistry, isotopic (deuterium, oxygen-18 and car-
bon-14) and gaseous tracers (chlorofluorocarbons CFC-11,
CFC-12, CFC-113), as well as multivariate statistics, was
developed to elucidate groundwater composition, flow and
occurrence. Groundwater is of meteoric origin, and a large
proportion is subject to evaporation. The primary natural
recharge is generated in the El Novillo and Las Cruces
ranges, and groundwater subsequently flows in a SE–NW
direction toward the coast. The initial water type is the
result of discordant dissolution of silicate minerals and ion
exchange on soils. In the lower plain portion, the aquifer
system is recharged from irrigation return flow and sea-
water intrusion, which significantly affects groundwater
chemistry. Nitrate and chloride concentrations indicate that
groundwater is highly affected by an overuse of fertilizers
in agricultural activities, but there is little effect from urban
activities. Seawater intrusion has progressed rapidly during
the past decade, and the impact currently extends 13 km
inland. Radiocarbon residence time calculations suggest
that groundwater is modern, with the exception of Cha-
metla and El Centerario sites in the central and western
lowlands with ages of up to *5000 years. These waters
indicate an additional recharge source for the upconing of
fossil groundwater or regional flow.
Keywords Coastal aquifers  Hydrogeochemistry  Arid
regions  Isotope hydrology  Contamination
Introduction
Coastal areas are preferred places for living because of
multiple benefits. According to recent data provided by the
United Nations Environmental Program (UNEP), approxi-
mately 60 % of the world’s population lives within 60 km
of a coast. This proportion will increase to 75 % within the
next two decades. Human settlement, together with the
development of agricultural, industrial and tourist activi-
ties, has adversely impacted coastal water resources (Car-
doso da Silva et al. 2010). Moreover, in semiarid and arid
coastal regions, groundwater frequently is the sole source
of fresh water, and often the periods of highest water
demand coincide with the periods of lowest availability
(Post 2005; Unsal et al. 2014). This situation can result in
groundwater deficits and saltwater intrusion and thus
overexploitation and degradation of available freshwater
resources. Although salinization of coastal aquifers is
mostly due to seawater intrusion, there are other factors
that affect the quality of groundwater that are related to
geochemical processes and anthropogenic impacts, and
hence the importance to differentiate the sources of salin-
ization and behavior of the aquifer in order to reduce
& J. Mahlknecht
jurgen@itesm.mx
1 Centro del Agua para Ame´rica Latina y el Caribe,
Tecnolo´gico de Monterrey, Ave. Eugenio Garza Sada 2501,
64849 Monterrey, Mexico
2 Postgraduate Program, Faculty of Geoscience and
Geography, University of Go¨ttingen, Goldschmidtstr. 3,
37077 Go¨ttingen, Germany
123
Environ Earth Sci (2016) 75:830
DOI 10.1007/s12665-016-5617-2
vulnerability of the aquifer (Han et al. 2014; Kaplan and
Mun˜oz-Carpena 2014).
At least 15 Mexican coastal aquifers are affected by
saltwater intrusion; almost all of them are located in the
desert northwestern portion of the country (Baja California
and Sonora) (CONAGUA 2014). The La Paz region on the
Baja California peninsula is an example of a coastal area
that is well known for its tourism and relatively high
quality of life. La Paz city, the economic and political
center of this region, and the surrounding irrigation-based
agriculture rely almost entirely on underlying groundwater
resources. The population has increased by 2.4 % per year
in recent years (CONAPO 2014). The groundwater
extraction volumes for water supply have increased as a
consequence of demographic growth and development of
agriculture and tourism. Official studies show that the
annual extraction of groundwater (32 million m3) is out-
growing the long-term annual recharge (28 million m3),
thereby creating a water supply deficit (CONAGUA 2010).
The ever-increasing heavy withdrawals have caused
groundwater table depressions of up to 10 m that have led
to the reversal of the hydraulic gradient in the La Paz area.
The hydraulic gradient reversal has enhanced seawater
intrusion and has resulted in an increase of groundwater
salinity of up to 4800 mg l-1 (TDS), especially along the
coastal plain (CNA 1997, 1999; CONAGUA 2010). The
migration of the groundwater salinity front has progressed
from 6 km inland from the coastline in 2005 (Cruz-Falco´n
et al. 2013) to 13 km in 2013 (Gaona-Zanella 2014).
According to recent studies (ICF, Fundacio´n FEMSA and
GeoAdaptive LLC 2012), the major challenge for sus-
tainability of La Paz is water supply for the residents in
sufficient quality and quantity. Thus, a multi-stakeholder
program was established recently to address water scarcity
and avoid future conflicts.
Previous studies conducted in the La Paz area concen-
trated on characterization of the aquifer system (Cruz-
Falco´n 2007; PRONATURA Noroeste 2010; Cruz-Falco´n
et al. 2010), identification of seawater intrusion (CNA
2001; Escolero and Torres-Onofre 2007), hydrodynamic
simulations (Monzalvo 2010; CONAGUA and UNAM
2010; CIBNOR and UNAM 2011), estimation of water
availability (CNA 2002; CONAGUA 2009) and ground-
water recharge based on the hydrologic balance (Cruz-
Falco´n et al. 2013).
The aim of the present study was to perform a comple-
mentary investigation to study groundwater processes by
using a suite of hydrochemical, isotopic and gaseous tracer
technologies. Hydrochemical and isotope tracers provide
valuable tools for evaluation of groundwater. It is well rec-
ognized thatmajor andminor chemical elementsmay give an
insight into water–rock interactions as well as anthropogenic
influences affecting the groundwater system as demonstrated
in numerous studies (Hem 1989; Mahlknecht et al. 2004;
Ledesma-Ruiz et al. 2015; Cruz-Fuentes et al. 2014; Her-
na´ndez-Antonio et al. 2015; Russak et al. 2015), while water
isotopes may define groundwater recharge areas, mixing and
evaporation sources (Edmunds et al. 2003; Horst et al. 2007;
Jime´nez-Martı´nez and Custodio 2008; Mohammed et al.
2014; Gonc¸alve`s et al. 2015), and radiocarbon isotopes res-
idence times of old groundwater and its distribution (Fontes
and Garnier 1979; Clark and Fritz 1997; Edmunds and
Smedley 2000; Mahlknecht et al. 2006a). More recently, it
has been demonstrated that chlorofluorocarbons are useful in
evaluating residence times of modern groundwater, mixing
patterns and aquifer’s vulnerability to contamination
(Busenberg and Plummer 1992; Cook et al. 1995; Goody
et al. 2006; Mahlknecht et al. 2006b; Horst et al. 2008;
Darling et al. 2012).
This study combines the aforementioned tools with the
objective to develop a robust groundwater flow concept.
The understanding of the complex processes of La Paz
coastal aquifer represents a valuable and essential tool for
the adequate management of the water resources to ensure
sustainability and protection of the coastal aquifer system.
Study area
General settings
The La Paz aquifer (1275 km2) is located in the valley of La
Paz and is part of the state of Baja California Sur (Fig. 1)
(Cruz-Falco´n 2007). The climate is predominantly arid with
an average annual temperature and rainfall of 23 C and
265 mm, respectively. Rainfall occurs primarily during
summer (August and September) and is mostly associated
with tropical storms and cyclones (CONAGUA 2005; Cruz-
Falco´n et al. 2011). Themean potential evapotranspiration is
approximately 2015 mm (CONAGUA 2009). The total
number of production wells and annual groundwater
extraction in the La Paz aquifer is 155 and 32 million m3,
respectively. Sixty-eight percent (68 %) of the extracted
groundwater is used for the public urban water supply, 27 %
for agriculture, 3 % for livestock and 2 % for industry and
other services. The well depths of the production wells vary
between 3 and 200 m (Monzalvo 2010). Crops cover
approximately 26,000 ha of the La Paz area and include
agave, avocado, guava, cotton, garlic, pepper, quelite,
tomato and forage, as well as other crops (INEGI 2005).
Geological and hydrogeological settings
The geomorphological structure of La Paz consists of
intermountain plains (valleys), table mountains (mesas)
and mountain ranges (sierras) with elevations of up to
830 Page 2 of 17 Environ Earth Sci (2016) 75:830
123
Fig. 1 a Geologic-structural map of the La Paz–El Carrizal valley showing main features, sampled sites and groundwater table elevation (m
a.s.l.) for the year 2012; b vertical section as indicated in (a); adapted from CONAGUA (2001)
Environ Earth Sci (2016) 75:830 Page 3 of 17 830
123
900 m above sea level (m a.s.l.). The valley of La Paz-El
Carrizal (the valley of La Paz) represents a tectonic graben
with a N–S orientation that is delimited on the W and E by
the La Giganta and La Paz faults, respectively, and on the
north by the Sea of Cortez (Fig. 1a, b). Hypsometric data
and the presence of structurally controlled wadies (arroyos)
suggest the existence of a series of secondary faults inside
the graben (A´lvarez et al. 1997; Cruz-Falco´n 2007).
The valley of La Paz is the result of distention tec-
tonics that created the Gulf of California, probably during
the Pleistocene, and the subsequent accumulation of
marine and continental granular sediments in the lower
topography. The base of the lithologic column in the La
Paz area is a metamorphic complex that consists of
Mesozoic shales, phyllites, schists, sillimanite and gneiss
(Cruz-Falco´n 2007; CONAGUA 2009; Monzalvo 2010).
This unit is overlain by Cretaceous intrusive rocks (gab-
bro, granite and granodiorite) in the Cordillera Oriental
portion of the La Paz fault. The basin-fill material of the
valley of La Paz consists of Miocene sandstones, con-
glomerates and fractured volcanic rocks (e.g., Comondu´
Formation, San Isidro Formation) in the lower part of the
valley and alluvial fans (sands, silts, clays) in the upper
part of the valley (Fig. 1a, b).
The hydrostratigraphy of the valley includes an upper-
most unit that corresponds to non-consolidated, granular
sediments and volcanoclastic material with little com-
paction and medium-to-high permeability. The underlying
second unit matches the volcanic flows of the Comondu´
Formation, with high hydraulic conductivity as a result of
fracturing, but is only of local importance due to its dis-
continuity. These two units represent an unconfined aquifer
system approximately 300–400 m thick and having an
average hydraulic conductivity of 1.2–14.3 9 10-3 m s-1.
Finally, the deeper third unit corresponds to the Pre-Ter-
tiary intrusive rock formations that outcrop in the Cordil-
lera Oriental portion of the study area and have low
permeability. The basement of these units is located
approximately 900 m below land surface and consists of a
metamorphic complex (CNA 1997, 1999, 2002; CIGSA
2001). This study concentrates on the upper, unconfined
aquifer units.
In La Paz the catchment recharge represents 15.9 %
(42 mm) of the rainfall in the basin, surface runoff repre-
sents 3.6 %, and the rest of the rainfall is evaporated.
Recharge occurs mostly in the Las Cruces and Novillo
ranges located in the E and SSE, respectively, and
groundwater flows in a NNW direction (Cruz-Falco´n et al.
2011) (Fig. 1a). Groundwater extraction in excess of
recharge has caused groundwater abatement. The water
table currently slopes from 150 m a.s.l. in the southern
portion of the study area, and from 0 m a.s.l. in the
northern portion close to the sea, to 10 m below sea level in
the central part of the study area, which is indicative of
groundwater recharge from seawater intrusion in the cen-
tral portion (Fig. 1a).
Methods and materials
Field and laboratory
Forty-six production wells and one spring were sampled for
evaluation of ion chemistry and isotopes in August 2013.
Physical parameters, including temperature, pH, electrical
conductivity (EC), total dissolved solids (TDS) and dis-
solved oxygen (DO), were measured in situ using
portable meters until measured levels did not vary, allow-
ing for temperature compensations and calibrations with
appropriate standards, such as calibration of the pH meter,
which was performed at every sampling site. Alkalinity
was determined in the field by titration with 0.02 N H2SO4.
The groundwater samples were filtered using 0.45-l
membrane filters.
Activation Laboratories LTD, Ontario, Canada, per-
formed the laboratory analyses of major ions and minor
ions. Water samples were analyzed for cations (K?, Na?,
Ca2?, Mg2?, Sr2?) using a Perkin Elmer ELAN 9000 ICP/
MS and, if upper limits were exceeded, by a Perkin Elmer
Optima 3000 ICP OES. Samples were spiked with internal
standards to correct for matrix differences over the entire
mass range. Additionally, a SLRS-4 and a NIST 1640
standard were run for every 32 samples for quality control.
Anions (Cl-, NO2
-, NO3
-, Br-, SO4
2- and PO4
3-) and
SiO2 were analyzed by ion chromatography (DIONEX
DX-120 Ion Chromatography System). Nitrite concentra-
tion was absent (lower detection limit = 0.01 mg l-1) in
all except one sample (0.3 mg l-1). This is because nitrite
is usually rapidly transformed to nitrate. Phosphate con-
centration was absent (lower detection limit: 0.02 mg l-1).
A charge balance error of 10 % was accepted for use in
further analyses and interpretations. Duplicates of samples
LP10, LP20, LP30, LP40 and LP 41 were re-sampled for
quality assurance and quality control.
Stable water isotope ratios in groundwater (d2H, d18O)
were analyzed at Environmental Isotope Laboratory of
Tecnolo´gico de Monterrey. A volume of 0.2 ml of water
was pipetted into an Exetainer vial and analyzed by a DLT-
100 Liquid–Water Isotope Analyzer (Los Gatos). The
results were determined and reported in the usual delta
notation in % with respect to the VSMOW standard.
Evaluations of carbon-13 (13C) ratios, radiocarbon (14C)
activities and chlorofluorocarbon (CFC) concentrations
were conducted for 26 representative samples. Carbon-13
determination was performed with a MM-903 spectrometer
on carbon dioxide previously released by acidifying sample
830 Page 4 of 17 Environ Earth Sci (2016) 75:830
123
water. The results are reported in d13CDIC with respect to
the VPDB (Vienna Pee Dee Belemnite) standard. The
analytical precision was 0.15 %. Radiocarbon activity was
measured by AMS at the NSF-Arizona Accelerator Facility
and is reported as percent modern carbon (pmC). The
analytical precision (1 - r) ranged from 0.2 percent
modern carbon (pMC) for pMC values near 30, to 0.5 pMC
near 100 pMC.
Chlorofluorocarbon (CFC-11, CFC-12 and CFC-113)
content was analyzed at Spurenstofflabor, Wachenheim,
Germany, by gas chromatography with an electron capture
detector after preconcentration using a purge-and-trap
technique (Oster et al. 1996). The concentrations are
reported in pmol l-1. The analytical precision (1 - r)
ranged from 0.05 pmol l-1 for pmol l-1 values near 0, to
3 pmol l-1 near 12 pmol l-1.
Interpretation
A bivariate method, the Pearson correlation analysis, was
applied for a preliminary evaluation of relation between
two hydrochemical parameters. High correlation coeffi-
cients approximating 1 or -1 in combination with low
significance levels (p\ 0.05) represented a significant
correlation. A hierarchical cluster analysis (HCA) grouped
samples according to their similarity. Parameters were
progressively merged into clusters, beginning with indi-
vidual elements of a data set. Each agglomeration step used
the distance between clusters from the previous agglom-
eration. A dendrogram helped to graphically identify the
genealogical relationship between clusters and elements.
The results were proved for geographic correspondence.
All statistical analyses were performed using the Minitab
17.0 program (Minitab 2013).
Deuterium excess (d) was calculated with the formula
d = d2H–8d18O. On the one hand, the values of d tend to
increase when there is a high kinetic isotope fractioning
during evaporation of sea water due high temperatures and
dryness of the environment; on the other hand, the effect of
evaporation before recharge will lead to a decrease of these
values; therefore, this parameter was used as an indicator
of vapor origins (Rindsberger et al. 1983; Jime´nez-Martı´-
nez and Custodio 2008).
Lumped parameter models (LPMs) have been applied to
support the interpretation of CFC tracers. The TracerLPM
(USGS 2014) was used to evaluate groundwater age dis-
tributions and residence times with the LPMs most com-
monly used in previous studies (IAEA 1996; Plummer
et al. 2001; Plummer 2003, 2005; Goody et al. 2006). Ages
were corrected for a recharge temperature of 23 C, a
recharge altitude of 208 m a.s.l. and a salinity in% (IAEA
2006). A recharge altitude was selected based on the study
of Cruz-Falco´n et al. (2011), which indicated that the
recharge area is mostly in the El Novillo and Las Cruces
ranges.
Radiocarbon data were used to evaluate older ([1 ka)
groundwater in La Paz. Unadjusted 14C ages were calcu-
lated from measured 14C activities of dissolved inorganic
carbon (DIC) using the Libby half-life (5568 years) and
assuming an initial 14C activity A0 of 100 modern carbon
(pmC), except for samples of recently recharged water,
whose initial activity was set at 105 pmC. Age adjustments
were made using the formula-based inorganic adjustment
models (Ingerson and Pearson 1964; Fontes and Garnier
1979—F&G). Alkalinity, pH, water temperature and car-
bon isotope (13C and 14C) data were used to make these age
calculations. The model approach corrects the age for
dissolution with ‘‘dead’’ carbon derived from the aquifer
matrix, assuming the following parameters to account for
the pristine arid sarcocauleschent scrub vegetation (Perea
et al. 2005; Michener and Lajtha 2007; Rodrı´guez-Estrella
et al. 1998): initial activity; isotope fractionation factors
after Vogel et al. (1970), Mook et al. (1974) and Deines
et al. (1974); d13C carbonates of 0 % VPDB and d13C soil
CO2 of -19 % VPDB. If a particular model resulted in an
unreasonable (negative) age, the adjusted age was desig-
nated as either modern (recharge after the mid-1950s), pre-
modern (recharge prior to the mid-1950s), or as a mixture
of modern and pre-modern water with respect to 14C based
on evaluating CFC tracers (Gardner and Heilweil 2014).
Results
Groundwater chemistry
The physical, chemical and isotopic data for the ground-
water samples are summarized in Table 1. Pearson method
indicates that positive correlations exist between several
hydrochemical parameters. The correlation between Mg2?,
Ca2?, Sr2?, Cl- and Br- ions is highly significant
(R[ 0.9; p\ 0.01). Meanwhile, Na? is significantly cor-
related with Cl- and Br- (R[ 0.9; p\ 0.01). TDS present
a highly significant correlation with alkali and earth alkali
(Na?, Mg2?, Ca2?, Sr2?), as well as Cl- and Br- ions.
None of the samples showed significant negative
correlation.
The classification of water samples was performed using
HCA with 17 variables (pH, temperature, EC, TDS, DO,
alkalinity, Na?, SiO2, K
?, Ca2?, Sr2?, Cl-, Br-, NO3
-,
SO4
2-, d2H, d18O and the distance of the sample location
from the coastline). Ward´s linkage rule was applied for
iteratively linking nearby clusters using a similarity matrix
and performing an ANOVA to evaluate the distance
between clusters (Ward 1963). The water samples merged
into three groups at the phenon line shown in the
Environ Earth Sci (2016) 75:830 Page 5 of 17 830
123
T
a
b
le
1
H
y
d
ro
ch
em
ic
al
d
at
a
fr
o
m
g
ro
u
n
d
w
at
er
sa
m
p
le
s
fr
o
m
th
e
L
a
P
az
aq
u
if
er
,
B
aj
a
C
al
if
o
rn
ia
L
it
h
o
lo
g
y
D
ep
th
(m
)
E
C
(l
S
cm
-
1
)
p
H
T
(
C
)
D
O
(m
g
l-
1
)
C
l-
(m
g
l-
1
)
H
C
O
3
-
(m
g
l-
1
)
S
O
4
2
-
(m
g
l-
1
)
N
O
3
-
(m
g
l-
1
)
B
r-
(m
g
l-
1
)
K
?
(m
g
l-
1
)
N
a?
(m
g
l-
1
)
C
a2
?
(m
g
l-
1
)
M
g
2
?
(m
g
l-
1
)
S
r2
?
(m
g
l-
1
)
S
iO
2
(m
g
l-
1
)
T
D
S
(m
g
l-
1
)
W
at
er
fa
ci
es
d1
8
O
(%
)
d2
H
(%
)
d
(%
)
L
D
L
N
A
N
A
N
A
N
A
0
.0
3
N
A
0
.0
3
0
.0
1
0
.0
3
0
.0
3
5
0
.7
2
0
.0
4
0
.2
N
A
N
A
N
A
N
A
G
ro
u
p
1
L
P
7
Q
(a
l)
1
5
0
2
1
0
9
7
.1
3
1
.1
7
.7
5
2
1
1
9
0
7
7
.1
6
.9
1
.3
4
.9
8
0
.9
1
5
5
.0
6
0
.3
1
.7
8
3
0
.7
1
1
3
0
C
a–
C
l
-
1
0
.0
-
6
9
.0
1
1
.6
L
P
1
4
Q
(a
l)
6
0
3
4
8
0
7
.1
2
9
.6
5
.6
9
3
6
3
7
0
1
3
6
9
.6
3
.6
4
.9
2
2
8
.0
2
6
3
.0
9
0
.7
4
.0
5
3
9
.0
2
0
8
5
C
a–
C
l
-
8
.4
-
5
8
.0
9
.2
L
P
1
5
Q
(a
l)
2
0
0
2
8
6
8
7
.2
3
0
.6
6
.6
7
0
0
4
0
2
1
6
6
1
1
.8
1
.9
8
.3
3
2
7
.0
1
3
4
.0
5
0
.7
1
.4
5
3
6
.2
1
8
3
9
N
a–
C
l
-
9
.7
-
6
9
.0
8
.3
L
P
1
6
Q
(a
l)
3
0
2
5
5
2
7
.3
2
9
.9
7
.9
5
6
7
3
9
5
1
6
9
1
0
.4
1
.9
7
.8
2
9
9
.0
1
0
4
.0
4
0
.8
0
.8
7
3
5
.7
1
6
3
1
N
a–
C
l
-
8
.6
-
6
4
.0
5
.1
L
P
1
7
Q
(a
l)
5
0
3
3
2
0
7
.1
2
9
.5
6
.8
8
3
3
5
4
0
1
8
0
1
2
.9
2
.0
9
.8
6
5
8
.0
1
8
2
.0
6
3
.7
1
.7
8
3
8
.4
2
5
2
2
N
a–
C
l
-
9
.6
-
7
2
.0
4
.8
L
P
1
8
Q
(a
l)
6
4
5
1
0
0
7
.0
3
1
.0
6
.3
1
4
0
0
5
7
8
1
9
5
2
7
.8
3
.4
9
.1
3
9
1
.0
3
3
5
.0
1
8
6
.0
2
.9
4
3
7
.9
3
1
6
6
N
a–
C
l
-
8
.2
-
6
2
.6
2
.8
L
P
1
9
Q
(a
l)
6
4
5
5
7
0
6
.9
2
9
.6
6
.4
1
6
9
0
3
7
2
2
9
0
6
.1
5
.0
1
0
.2
3
5
4
.0
5
0
0
.0
1
6
2
.0
4
.4
9
4
3
.5
3
4
3
7
C
a–
C
l
-
8
.8
-
6
7
.0
3
.2
L
P
2
0
Q
(a
l)
6
0
3
8
4
0
7
.1
2
9
.3
7
.2
1
1
2
0
4
3
2
1
4
8
4
.0
2
.9
7
.7
2
3
3
.0
3
4
7
.0
1
2
0
.0
3
.0
2
4
2
.4
2
4
6
0
C
a–
C
l
-
9
.1
-
6
7
.0
5
.8
L
P
2
1
Q
(a
l)
1
3
0
2
1
2
4
7
.1
3
3
.4
7
.8
5
5
0
4
0
6
1
0
7
6
.4
1
.4
5
.6
1
1
7
.0
1
9
9
.0
6
8
.5
1
.7
8
3
3
.8
1
4
9
6
C
a–
C
l
-
9
.3
-
6
7
.0
6
.8
L
P
2
2
Q
(a
l)
1
5
7
5
2
0
7
.0
3
0
.7
7
.5
2
2
6
0
4
9
8
4
9
0
1
2
.4
6
.8
1
0
.2
1
0
8
0
.0
3
5
6
.0
1
3
1
.0
5
.1
1
4
5
.0
4
8
9
5
N
a–
C
l
-
7
.2
-
5
3
.8
3
.8
L
P
2
3
Q
(a
l)
2
6
2
6
0
3
7
.3
3
0
.0
8
.3
7
4
4
5
1
2
1
5
2
4
.8
1
.9
6
.0
3
4
3
.0
2
0
5
.0
6
0
.8
2
.4
8
4
3
.9
2
0
7
6
N
a–
C
l
-
7
.4
-
5
3
.4
5
.8
L
P
2
6
Q
(a
l)
1
8
3
0
8
0
7
.2
2
9
.1
8
.1
7
2
8
7
2
4
1
6
0
1
1
.2
2
.3
3
.9
3
9
5
.0
1
6
4
.0
6
1
.1
2
.0
1
4
2
.9
2
2
9
4
N
a–
C
l
-
8
.2
-
5
3
.7
1
1
.5
L
P
2
7
Q
(a
l)
6
9
2
9
4
4
7
.2
2
8
.2
4
.8
5
9
0
4
4
2
3
1
6
4
5
.8
N
D
3
.2
4
4
3
.0
1
4
7
.0
5
6
.8
2
.0
8
5
1
.1
2
0
9
7
N
a–
C
l
-
9
.5
-
6
2
.6
1
3
.3
L
P
2
8
Q
(a
l)
1
5
6
8
8
0
7
.0
2
9
.0
8
.6
2
0
3
0
6
3
4
4
4
1
1
7
.0
6
.5
8
.9
7
6
3
.0
4
2
1
.0
1
8
6
.0
6
.6
9
4
7
.9
4
5
6
2
N
a–
C
l
-
9
.1
-
5
8
.7
1
4
.0
L
P
2
9
Q
(a
l)
2
0
3
4
6
0
7
.1
2
9
.7
5
.6
9
3
4
4
7
6
1
3
7
9
.4
2
.0
5
.9
2
8
3
.0
2
6
1
.0
1
1
0
.0
4
.0
2
4
4
.9
2
2
6
7
C
a–
C
l
-
9
.2
-
5
8
.6
1
5
.3
L
P
3
0
Q
(a
l)
2
2
5
1
6
0
6
.8
2
7
.9
3
.9
1
5
9
0
5
6
8
1
4
3
9
.9
4
.0
7
.0
3
9
5
.0
4
2
2
.0
1
5
7
.0
6
.5
2
4
2
.8
3
3
4
5
C
a–
C
l
-
8
.8
-
6
2
.3
8
.4
L
P
3
1
Q
(a
l)
3
0
4
7
7
0
7
.1
2
7
.1
5
.2
1
1
4
0
1
2
9
0
2
2
8
4
8
.8
2
.3
3
.5
6
9
3
.0
1
8
9
.0
1
1
0
.0
3
.0
8
4
3
.5
3
7
5
1
N
a–
C
l
-
8
.5
-
6
4
.3
3
.5
L
P
3
2
Q
(a
l)
7
0
2
5
8
9
7
.2
2
9
.0
5
.8
7
3
1
3
2
5
5
0
.3
1
0
.6
3
.0
3
.6
1
3
4
.0
2
3
7
.0
5
5
.9
3
.1
1
3
3
.2
1
5
8
7
C
a–
C
l
-
8
.5
-
6
3
.1
5
.0
L
P
3
3
Q
(a
l)
7
0
1
7
7
7
7
.4
2
9
.3
8
.7
3
9
8
3
4
0
6
9
.4
6
.3
1
.2
3
.1
1
2
2
.0
1
3
1
.0
3
8
.7
2
.0
8
3
2
.6
1
1
4
4
C
a–
C
l
-
8
.6
-
6
1
.6
7
.4
L
P
3
5
Q
(a
l)
7
0
2
7
5
1
7
.2
3
1
.3
6
.1
7
9
3
2
7
6
6
4
.3
8
.2
2
.6
3
.0
1
0
2
.0
2
5
5
.0
7
4
.5
2
.5
4
2
9
.3
1
6
1
0
C
a–
C
l
-
9
.0
-
6
6
.5
5
.3
L
P
4
0
Q
(a
l)
2
8
8
9
2
0
7
.2
2
8
.7
7
.1
2
9
6
0
9
8
4
2
4
3
5
.6
7
.2
1
4
.0
5
8
3
.0
6
5
8
.0
3
4
4
.0
8
.1
3
3
6
.2
5
8
4
3
C
a–
C
l
-
8
.1
-
6
5
.9
-1
.0
L
P
4
1
Q
(a
l)
5
0
6
0
0
0
7
.0
2
9
.4
5
.7
1
8
4
0
6
5
6
1
7
6
N
D
5
.2
7
.8
5
6
2
.0
3
4
2
.0
1
5
9
.0
4
.6
0
3
9
.7
3
7
9
2
C
a–
C
l
-
8
.3
-
6
4
.5
2
.2
L
P
4
3
Q
(a
l)
6
0
4
2
5
0
7
.3
3
0
.3
7
.4
1
1
4
0
5
2
8
2
9
2
8
.8
2
.9
9
.6
3
2
5
.0
3
1
1
.0
1
0
5
.0
2
.7
2
3
7
.5
2
7
6
3
C
a–
C
l
-
9
.0
-
6
6
.6
5
.2
G
ro
u
p
2
L
P
1
Q
(a
l)
N
A
8
9
3
6
.8
2
8
.9
8
.3
1
5
9
1
6
6
3
7
.1
1
0
.1
0
.4
4
.1
6
7
.5
5
1
.0
1
6
.5
0
.8
3
1
9
.6
5
3
2
N
a–
C
l
-
1
0
.9
-
8
1
.5
6
.4
L
P
2
Q
(a
l)
N
A
7
0
2
7
.1
3
0
.1
8
.1
1
0
7
1
7
5
2
7
.9
8
.8
0
.2
3
.9
5
3
.2
4
0
.8
1
4
.0
0
.6
4
2
1
.9
4
5
3
N
a–
C
l
-
1
1
.1
-
8
1
.0
8
.1
L
P
3
Q
(a
l)
N
A
1
5
3
7
6
.9
2
9
.2
7
.8
3
8
5
1
8
5
3
4
.3
4
.8
1
.1
5
.5
9
9
.9
1
0
3
.0
3
3
.7
1
.4
6
2
1
.9
8
7
6
C
a–
C
l
-
1
1
.0
-
8
2
.0
6
.3
L
P
4
Q
(a
l)
7
6
1
5
6
6
7
.2
3
1
.5
8
.6
3
4
2
3
0
5
4
3
.9
3
.7
1
.0
4
.4
1
3
7
.0
8
2
.5
3
9
.5
0
.7
1
3
0
.4
9
9
0
N
a–
C
l
-
9
.4
-
6
9
.8
5
.2
L
P
8
Q
(a
l)
5
7
1
1
4
2
7
.8
2
9
.9
8
.2
2
0
6
2
1
0
3
2
.7
3
.6
0
.6
3
.7
9
6
.9
5
2
.3
2
3
.0
0
.4
6
2
7
.8
6
5
7
N
a–
C
l
-
1
0
.4
-
7
3
.1
1
0
.7
L
P
1
1
Q
(a
l)
8
0
9
9
8
7
.4
2
5
.0
7
.9
1
6
8
2
3
2
3
6
.8
8
.9
0
.4
4
.0
1
0
5
.0
4
1
.1
1
7
.0
0
.5
5
2
7
.5
6
4
1
N
a–
C
l
-
1
1
.6
-
8
1
.1
1
1
.8
L
P
1
2
Q
(c
g
)
8
0
1
1
5
0
7
.4
2
5
.0
6
.1
2
8
4
3
2
5
7
5
.1
5
.5
0
.8
5
.6
2
2
3
.0
3
8
.7
1
0
.5
0
.4
5
4
2
.0
1
0
1
1
N
a–
C
l
-
7
.7
-
5
2
.1
9
.7
L
P
1
3
Q
(a
l)
6
0
1
1
5
5
7
.6
3
1
.8
7
.1
1
8
8
2
5
0
5
3
.5
5
.0
0
.5
5
.5
1
4
1
.0
4
3
.6
1
0
.1
0
.4
2
3
9
.0
7
3
7
N
a–
C
l
-
8
.2
-
5
5
.0
1
0
.3
L
P
2
4
Q
(a
l)
3
5
1
2
9
5
7
.5
3
1
.0
4
.5
2
6
4
4
2
6
4
2
.3
3
.9
0
.9
2
.6
1
8
7
.0
6
6
.3
1
8
.4
0
.9
4
4
3
.2
1
0
5
6
N
a–
C
l
-
8
.0
-
5
5
.5
8
.7
L
P
2
5
Q
(a
l)
3
8
1
2
9
2
7
.4
3
2
.7
7
.7
2
6
6
4
0
6
3
7
.9
2
.1
0
.8
4
.5
1
4
4
.0
8
1
.6
2
3
.5
1
.1
8
3
3
.5
1
0
0
1
N
a–
C
l
-
7
.1
-
5
7
.7
-1
.2
L
P
3
4
Q
(a
l)
7
5
1
0
2
3
7
.6
2
9
.7
8
.4
1
8
3
2
5
0
4
2
.6
2
.0
0
.5
2
.6
8
0
.1
6
4
.1
2
1
.1
1
.0
9
2
6
.4
6
7
3
N
a–
C
l
-
8
.3
-
6
1
.1
5
.2
L
P
3
6
Q
(a
l)
3
5
1
2
0
7
7
.4
3
1
.0
7
.1
2
4
5
2
3
6
2
9
.1
3
.2
0
.6
2
.6
7
0
.6
9
1
.1
2
3
.2
0
.9
9
2
8
.0
7
3
0
C
a–
C
l
-
8
.6
-
6
3
.3
5
.4
L
P
3
7
Q
(a
l)
6
0
1
6
5
8
7
.3
2
9
.4
5
.9
3
8
0
2
6
0
6
5
.9
2
.4
1
.1
3
.2
1
3
8
.0
1
0
6
.0
2
6
.7
1
.5
5
3
1
.5
1
0
1
6
N
a–
C
l
-
8
.6
-
6
3
.1
5
.3
L
P
3
8
Q
(a
l)
2
4
7
9
9
7
.8
2
9
.5
7
.3
1
6
4
1
8
6
1
1
.5
1
.5
0
.5
2
.1
5
9
.8
4
4
.5
2
0
.7
0
.5
2
3
6
.2
5
2
7
N
a–
C
l
-
9
.6
-
6
8
.0
8
.4
830 Page 6 of 17 Environ Earth Sci (2016) 75:830
123
dendrogram (Fig. 2) with geographical correspondence.
Group 1 (23 samples) is located in the central part of the
study area (south of El Centenario and Chametla) where
agriculture is practiced and groundwater extractions from
supply wells are high. Group 2 includes 16 samples located
mostly in the urbanized area of La Paz. This group is
similar to group 3 considering the statistical approach
(Fig. 2); however, based on groundwater chemistry
(Figs. 4, 5) it lies between group 1 and group 3. All sam-
ples of groups 1 and 2 are located in the 20-km fringe of
the coastal area. Finally, group 3 consists of 8 samples that
are located at higher altitudes and further away from the
coastline. They are closer to the recharge areas (Las Cruces
and El Novillo ranges). The three groups will be used and
discussed in the subsequent hydrogeochemical evaluation.
The study area exhibits a wide range of salinities,
ranging from very fresh (479 lS cm-1) to brackish
(8920 lS cm-1; r = 2047 lS cm-1), having slightly
acidic to alkaline pH values between 6.8 and 8.3 (r = 0.3).
High EC values are primarily due to elevated Cl-, Na? and
Ca? concentrations that increase from the recharge area
(group 3) to the coastal area (group 1) between El Cente-
nario and La Paz (Fig. 3a). Exceptions are water samples
LP38 and LP39 from group 2, which are located in the
coastal area but have low Cl- concentrations (164 and
95.6 mg l-1, respectively) in comparison to the rest of the
group. The locations of the wells where these samples were
collected coincide with an area where artificial recharge
takes place, i.e. the wastewater treatment plant of the city
of La Paz exchanges treated wastewater with farmers from
Centenario and Chametla that, together with Paraiso del
Mar, receive 6 million cubic meters per year (CONAGUA
2010). The pH–EC relationship indicates an inverse cor-
relation that reflects the drainage of non-carbonate rocks
and sediments. The temperatures range from 25.0 to
33.4 C (r = 1.7) Elevated temperature values were
measured in samples LP05, LP06 and LP10 a subgroup of
group three wells, and LP25 of group 2, possibly indicating
a slight upconing of deeper, regional groundwater flows
from lower, intrusive rock formations. An effect of
groundwater warming due to direct influence of air tem-
perature in the wells may be excluded because all wells
were tube wells, i.e. not open.
The molar Na/Cl ratio of groundwater compared to
seawater (0.86) varies from 0.19 to 1.49 (median 0.67) and
indicates that some waters (19 %) are enriched in Na?,
which means that molar Na? concentration is in excess of
Cl-, while most are depleted. The important depletion of
Na? reflects large evaporation effects from water recycling
probably linked to agricultural practices. Moreover, the
enrichment of Na? indicates weathering of silicates.
Another less important mechanism of Na? enrichment or
depletion is cation exchange in soils. Sodium is possiblyT
a
b
le
1
co
n
ti
n
u
ed
L
it
h
o
lo
g
y
D
ep
th
(m
)
E
C
(l
S
cm
-
1
)
p
H
T
(
C
)
D
O
(m
g
l-
1
)
C
l-
(m
g
l-
1
)
H
C
O
3
-
(m
g
l-
1
)
S
O
4
2
-
(m
g
l-
1
)
N
O
3
-
(m
g
l-
1
)
B
r-
(m
g
l-
1
)
K
?
(m
g
l-
1
)
N
a?
(m
g
l-
1
)
C
a2
?
(m
g
l-
1
)
M
g
2
?
(m
g
l-
1
)
S
r2
?
(m
g
l-
1
)
S
iO
2
(m
g
l-
1
)
T
D
S
(m
g
l-
1
)
W
at
er
fa
ci
es
d1
8
O
(%
)
d2
H
(%
)
d
(%
)
L
D
L
N
A
N
A
N
A
N
A
0
.0
3
N
A
0
.0
3
0
.0
1
0
.0
3
0
.0
3
5
0
.7
2
0
.0
4
0
.2
N
A
N
A
N
A
N
A
L
P
3
9
Q
(a
l)
N
A
6
8
3
7
.6
2
9
.9
7
.0
9
5
.6
2
1
8
1
4
1
.8
0
.3
2
.3
5
8
.4
3
3
.3
1
5
.2
0
.4
6
3
5
.9
4
7
5
N
a-
H
C
O
3
-
8
.3
-
6
5
.8
0
.2
L
P
4
2
Q
(a
l)
5
0
1
1
8
2
7
.4
3
0
.2
7
.3
2
8
9
2
0
6
1
9
.6
2
.9
0
.9
3
.5
5
9
.2
7
9
.3
3
9
.7
1
.0
3
3
8
.8
7
4
0
C
a–
C
l
-
9
.2
-
6
8
.2
5
.7
G
ro
u
p
3
L
P
5
Q
(a
l)
2
0
1
6
4
2
7
.1
3
2
.5
7
.8
8
0
.8
2
1
5
1
4
.4
2
.9
0
.3
2
.3
4
0
.6
4
4
.7
1
5
.9
0
.4
0
2
6
.8
4
4
4
C
a–
H
C
O
3
-
1
0
.7
-
7
3
.5
1
2
.4
L
P
6
Q
(a
l)
2
0
1
7
0
1
7
.4
3
2
.5
7
.7
1
1
1
1
7
5
1
0
.8
3
.0
0
.4
1
.7
4
4
.6
4
6
.8
1
4
.7
0
.4
5
3
7
.3
4
4
6
C
a–
H
C
O
3
-
1
0
.0
-
6
8
.0
1
1
.8
L
P
9
Q
(a
l)
2
0
1
8
6
5
7
.4
3
0
.5
6
.7
1
1
8
2
8
0
4
7
.5
1
.4
0
.3
2
.7
8
1
.7
5
2
.7
2
0
.0
0
.6
7
2
7
.4
6
3
2
N
a–
H
C
O
3
-
9
.0
-
6
1
.4
1
0
.7
L
P
1
0
Q
(a
l)
1
5
6
5
6
1
7
.7
3
3
.0
7
.3
6
9
.4
2
5
4
9
.8
1
.5
0
.2
3
.8
6
7
.1
3
4
.0
1
1
.4
0
.3
7
2
8
.7
4
8
0
N
a–
H
C
O
3
-
9
.5
-
6
3
.0
1
3
.0
L
P
4
4
Q
(a
l)
8
0
6
3
0
7
.2
3
0
.2
7
.1
8
9
.7
1
9
8
1
4
.5
8
.1
0
.3
1
.5
5
0
.1
3
3
.5
1
5
.6
0
.3
5
2
5
.8
4
3
7
N
a–
H
C
O
3
-
9
.4
-
6
5
.5
9
.8
L
P
4
5
Q
(c
g
)
5
0
6
1
0
7
.2
2
8
.5
7
.8
6
1
.9
2
7
5
1
1
.6
2
.9
0
.2
2
.1
3
6
.7
3
9
.1
2
1
.6
0
.3
8
2
0
.9
4
7
2
C
a–
H
C
O
3
-
1
0
.1
-
7
1
.0
1
0
.4
L
P
4
6
Q
(a
l)
2
1
9
2
4
7
.1
2
8
.1
6
.3
1
0
7
3
9
8
1
4
.6
2
.4
0
.3
2
.3
8
0
.9
4
9
.1
2
7
.6
0
.5
7
2
6
.7
7
0
9
N
a–
H
C
O
3
-
9
.1
-
6
2
.9
9
.6
L
P
4
7
K
(g
r)
0
4
7
9
8
.3
2
7
.4
1
4
.7
5
4
.5
1
8
2
7
.9
1
.8
0
.1
4
.1
3
9
.3
2
7
.8
9
.3
0
.2
8
2
1
.1
3
4
8
N
a–
C
l
-
9
.0
-
6
4
.0
7
.6
A
ll
sa
m
p
le
s
ar
e
g
ro
u
n
d
w
at
er
,
ex
ce
p
t
L
P
4
7
=
sp
ri
n
g
w
at
er
E
C
el
ec
tr
ic
al
co
n
d
u
ct
iv
it
y
,
D
O
d
is
so
lv
ed
o
x
y
g
en
,
N
A
n
o
t
av
ai
la
b
le
,
N
D
n
o
t
d
et
ec
te
d
,
L
D
L
lo
w
er
d
et
ec
ti
o
n
li
m
it
,
Q
(a
l)
Q
u
at
er
n
ar
y
al
lu
v
iu
m
,
Q
(c
g
)
Q
u
at
er
n
ar
y
co
n
g
lo
m
er
at
e,
K
(g
r)
C
re
ta
ce
o
u
s
g
ra
n
it
e
Environ Earth Sci (2016) 75:830 Page 7 of 17 830
123
enriched by exchange of Ca2? for Na? ions at clay sur-
faces, which produces Na–HCO3 waters (e.g., LP9, LP10).
Finally, Na? is possibly depleted from solution by reverse
ion exchange reactions of Na? for Ca2? and/or Mg2? ions
at clay surfaces in discharge areas (e.g., LP19, LP20).
Nitrate concentrations (NO3–N) range from 1.4 to
48.8 mg l-1 (r = 9.7 mg l-1). Eleven out of 47 samples
(23 %) do not comply with the Mexican drinking water
standard for NO3
-–N (10 mg l-1). Samples with values
exceeding the standard are mostly located in two agricul-
tural zones south of El Centenario and Chametla, and south
of La Paz (part of group 1) (Fig. 3b). Sulfate concentra-
tions vary from 7.9 to 490 mg l-1 (r = 114 mg l-1) and
increase towards the coastal areas. Chloride concentrations
and EC values indicate a similar trend. Two samples do not
comply with the Mexican drinking water standard for
Fig. 2 Classification by
dendrogram using the Ward’s
method. The phenon line
indicates the decision criteria
Fig. 3 Distribution of a electrical conductivity and b nitrate in groundwater. Including geological features, urban areas (hatched areas) and well
sites (group symbols)
830 Page 8 of 17 Environ Earth Sci (2016) 75:830
123
SO4
2- (400 mg l-1). The values of alkalinity
(HCO3
- = 166–1290 mg l-1; r = 219 mg l-1) and major
cations tend to increase considerably towards the coastline.
The chloride-to-bromide mass ratio (Cl/Br) in seawater
is typically 285. Cl/Br ratios in coastal groundwaters are
typically close to 290 (Davis et al. 1998; Anders et al.
2014). Since Br is slightly more soluble than Cl, the Cl/Br
ratio in evaporates (halites, gypsum) decreases due to
evaporative effects. In consequence, the dissolution of
these rocks may increase the ratio to 4000 (Davis et al.
1998; Anders et al. 2014). Wastewater loaded with NaCl
may increase the ratio up to 655. Finally, the ratio may be
decreased to 130 by the use of Br-based pesticides or
leaching farm-animal or septic waste (Alcala´ and Custodio
2008). The Cl/Br for groundwater samples ranges from 245
to 629 (mean = 364) (Fig. 4). Most samples close to sea
have ratios higher than seawater; they are likely the result
of evaporation of seawater. Some samples close to sea that
have ratios lower than seawater may be indicative of ani-
mal farming. In general the samples are in the expected
range of arid coastal aquifers.
A piper diagram elucidates the chemical patterns
(Fig. 5). The general dominance of cations is
Na[Ca = Mg[K, while the dominance of anions was
Cl[HCO3[[SO4. The central diamond in the diagram
shows a clear distinction between the three selected groups.
Group 1 (Ca–Cl and Na–Cl) near the coastline is charac-
terized by seawater intrusion, and a trend towards elevated
SO4, Cl and NO3 values indicates contamination by agri-
cultural practices. Group 3 (Ca–HCO3 and Na–HCO3) is
derived from rainwater and spreads along the HCO3 facies
type as it infiltrates the ground surface and undergoes
weathering processes. Finally, group 2 represents a tran-
sition between both groups 1 and 3.
Stable water isotopes and deuterium excess
The d18O and d2H values vary from -11.6 to -7.1 %
(r = 1.0 %) and from -82.0 to -52.1 % (r = 7.2 %),
respectively (Table 1). Plotted in a conventional diagram,
the general trend is d2H = 6.28 (d18O)–8.47; r2 = 0.78
(Fig. 6). It indicates that the samples are plotted mostly
along and below the Regional Meteoric Water Line
(RMWL) according to the Chihuahua IAEA GNIP station:
d2H = 7.1d18O ? 2.8 (Wassenaar et al. 2009). These d18O
and d2H data indicate that groundwater in the coastal
aquifer is of meteoric origin and a large proportion is
subject to evaporation effects which is consistent with
deuterium excess. Waters located at higher altitudes (group
3) plot along the RMWL, indicating direct infiltration of
local precipitation with little evaporation. Deuterium
excess for this group varies from 7.6 to 12.9 with an
average of 10.6 % (r = 1.73 %) showing an increment of
d values as temperature increases due to kinetic isotope
fractioning related to high temperatures and dryness in the
environment (Jime´nez-Martı´nez and Custodio 2008).
Group 2 shows a high variability of d18O and d2H values,
which is similar to the values of the other two groups. A
notable exception is a cluster of four samples representing
the urban area of La Paz (LP1, LP2, LP3 and LP11). These
samples show the most depleted d18O and d2H values of
the study area, indicating groundwater recharge in higher
altitudes (Figs. 1a, 6). Finally, group 1 shows the least
depleted d18O and d2H values among all groups. The
samples of group 1 and 2 have a similar distribution and
deuterium excess values: from -0.1 to 15.2, and from -1.1
Fig. 4 Cl/Br mass ratios versus Cl- for the three groundwater groups
and seawater
Fig. 5 Trilinear diagram. Circles represent the first group of samples.
Triangles represent the second group. Crosses represent the third
group
Environ Earth Sci (2016) 75:830 Page 9 of 17 830
123
to 11.8 %, respectively. These samples are plotted on and
below the RMWL (El Centerario and Chametla) which is
consistent with the extensive evaporation before and during
recharge in the sample area due to dry conditions and the
heavy extraction and cycling of groundwater to supply the
area with irrigation water. Waters plotted below the
RMWL are the most mineralized. The area in which they
are located is characterized by low infiltration rates and
long residence times over land compared to other areas
(Cruz-Falcon et al. 2011), which is congruent with their
isotopic signature. Waters from the locality of El Cente-
nario (LP22, LP23 and LP27) are derived from recharge of
the western conglomerates (LP12, LP13). These samples
show the most mineralogically enriched values in the study
area, indicating lower altitude and/or less humid climate
conditions during recharge (Fig. 6). The d18O and d2H
values and water chemistry are affected in contaminated or
saline groundwater. Brackish groundwater with enriched
18O values and high Cl- or NO3
- concentrations is
indicative of evaporation effects in agricultural lands, as
observed in samples LP18, LP27, LP28 and LP31.
Groundwater from the urban area of La Paz (LP1, LP2,
LP3 and LP11) exhibits relatively low Cl- and NO3
-
values, suggesting little influence from domestic sewage.
Groundwater age dating
Trace gases CCl3F (CFC-11), CCl2F2 (CFC-12), C2Cl3F3
(CFC-113) and SF6 (sulphur hexafluoride) are increasingly
used as tracers of young (e.g., up to decades old)
groundwater. The principle of age dating with CFCs is
based on comparing their concentrations in groundwater
with the known worldwide atmospheric concentrations of
these anthropogenic gases over the past 60 years to esti-
mate the year of recharge at each site (Darling et al. 2012).
However, several complications may arise. Local sources
of trace gases from cities and industries, sorption or even
the sampling process may bias the levels of CFCs (Plum-
mer et al. 2001). Microbial degradation may also occur
under reducing conditions (Cook et al. 1995; Plummer
et al. 1998).
Chlorofluorocarbon mixing ratios in the study area
ranged from 45.7 to 2479 (CFC-11), 93 to 1587 (r = 413)
(CFC-12) and 10 to 318 (CFC-113) pptv (Table 2). The
highest CFC-12 values were indicated for samples located
in irrigation areas (LP28, LP42), although elevated values
are also indicated in samples located towards the El
Novillo range (LP4, LP8 and LP18) and in El Centenario
(LP22) (Fig. 7a). CFC-11 values indicate a notable devia-
tion from CFC-12 values and also from CFC-113 values.
CFC-12 and CFC-113 values are more consistent. Dis-
agreement between tracers is expected for groundwater
samples because the amount of increase in CFC concen-
trations due to contamination is considered to be different
for each CFC gas (Horst et al. 2008; Kusano et al. 2014).
The tracer pairs of CFC-12 and CFC-113 in Fig. 8a
generally plot within the regions bounded by the piston
flow and exponential mixing models. In Fig. 8b, the sam-
ples generally plot below the exponential mixing model
curve. This lower CFC-11/CFC-12 ratio may be due to
Fig. 6 a Stable water isotope diagram of groundwater samples and regional meteoric water line (RMWL) according to the Chihuahua IAEA
GNIP station; b oxygen-18 vs. deuterium excess
830 Page 10 of 17 Environ Earth Sci (2016) 75:830
123
T
a
b
le
2
C
F
C
co
n
ce
n
tr
at
io
n
s
an
d
eq
u
iv
al
en
t
m
ix
in
g
ra
ti
o
s;
ap
p
li
ed
fl
o
w
m
o
d
el
an
d
m
ea
n
re
si
d
en
ce
ti
m
e
u
si
n
g
C
F
C
-1
2
C
o
n
ce
n
tr
at
io
n
(p
m
o
l
l-
1
)
M
ix
in
g
ra
ti
o
(p
p
tv
)
C
F
C
-1
2
R
T
C
F
C
-1
2
M
R
T
P
ro
p
o
se
d
fl
o
w
m
o
d
el
a1
4
C
(p
m
C
)
d1
3
C
(%
D
IC
)
U
n
co
rr
ec
te
d
ag
e
(y
ea
rs
)
C
o
rr
ec
te
d
ag
e
(y
ea
rs
)
C
F
C
-1
1
C
F
C
-1
2
C
F
C
-1
1
3
C
F
C
-1
1
C
F
C
-1
2
C
F
C
-1
1
3
P
F
M
(y
ea
rs
)
E
M
M
(y
ea
rs
)
G
ro
u
p
1
L
P
7
0
.5
0
.3
0
.0
3
4
6
9
3
1
0
2
8
2
0
6
E
M
M
7
9
.9
-
1
0
.5
1
8
5
5
M
ix
tu
re
L
P
1
4
1
.3
0
.8
0
.0
9
1
2
0
2
5
5
2
9
–
5
1
E
M
M
5
0
.7
-
1
1
.9
5
6
1
5
4
1
0
0
L
P
1
6
2
2
.0
4
.6
0
.1
8
2
0
1
7
1
5
1
9
5
7
–
–
C
o
n
t
7
8
.5
-
1
0
.8
2
0
0
1
M
o
d
er
n
L
P
1
7
2
7
.0
4
.8
1
.0
0
2
4
7
9
1
5
8
7
3
1
8
–
–
C
o
n
t
8
6
.8
-
1
1
1
1
7
0
M
o
d
er
n
L
P
1
8
1
5
.0
1
.9
0
.0
9
1
3
8
9
6
3
3
2
9
–
–
C
o
n
t
6
5
.1
-
1
0
3
5
4
8
M
o
d
er
n
L
P
2
2
2
.7
1
.3
0
.1
7
2
5
1
4
3
6
5
5
2
6
1
8
P
F
M
7
2
.4
-
1
0
.9
2
6
7
0
M
o
d
er
n
L
P
2
3
1
.5
1
.2
0
.1
7
1
3
8
3
9
7
5
4
2
8
2
3
E
M
M
4
1
.7
-
9
.7
7
2
3
1
2
1
0
0
L
P
2
8
2
.2
1
.6
0
.2
5
2
0
5
5
3
5
8
0
–
–
C
o
n
t
6
7
.9
-
9
.1
3
2
0
0
M
o
d
er
n
L
P
3
0
1
.5
0
.7
0
.1
3
1
3
9
2
4
6
4
2
3
7
5
5
E
M
M
7
7
.9
-
9
.9
2
0
6
5
M
o
d
er
n
L
P
3
2
0
.7
0
.6
0
.0
6
6
4
1
8
8
1
9
4
0
8
2
E
M
M
5
6
-
1
0
.5
4
7
9
3
M
o
d
er
n
L
P
3
5
0
.6
0
.3
0
.0
4
5
5
1
0
9
1
3
4
5
1
7
6
E
M
M
7
7
.8
-
1
0
.7
2
0
7
5
M
ix
tu
re
L
P
4
1
2
.2
1
.7
0
.1
4
2
0
4
5
6
8
4
5
–
–
C
o
n
t
8
4
.7
-
9
.8
1
3
7
3
M
o
d
er
n
G
ro
u
p
2
L
P
1
2
4
.0
1
2
.0
0
.2
8
2
1
9
6
3
9
5
4
8
9
–
–
C
o
n
t
9
9
.6
-
1
2
.2
3
3
M
o
d
er
n
L
P
3
2
6
0
.0
1
4
.0
0
.2
1
2
3
,8
1
7
4
6
1
8
6
7
2
7
–
C
o
n
t
9
6
.4
-
1
2
.2
3
0
3
M
o
d
er
n
L
P
4
1
.4
1
.3
0
.1
8
1
2
8
4
3
0
5
7
4
3
1
9
P
F
M
6
0
-
1
0
.2
4
2
2
3
M
o
d
er
n
L
P
8
1
.8
1
.2
0
.1
7
1
6
6
3
9
7
5
4
4
2
2
3
E
M
M
7
6
.5
-
9
.7
2
2
1
4
M
o
d
er
n
L
P
1
1
9
.3
3
.4
0
.2
2
8
4
9
1
1
1
8
7
0
3
5
–
C
o
n
t
9
6
.9
-
9
.5
2
6
0
M
o
d
er
n
L
P
1
3
1
.4
0
.9
0
.1
2
1
2
8
2
9
0
3
8
3
7
4
3
E
M
M
2
9
.5
-
1
0
.1
1
0
,0
9
2
4
7
0
0
L
P
2
5
1
.3
1
.0
0
.1
4
1
1
9
3
3
0
4
4
3
2
3
5
E
M
M
3
2
.4
-
1
0
.9
9
3
1
7
4
1
0
0
L
P
3
7
0
.7
0
.5
0
.0
5
6
4
1
5
2
1
6
4
2
1
0
1
E
M
M
6
6
.3
-
1
0
.4
3
3
9
7
M
ix
tu
re
L
P
3
8
0
.8
0
.5
0
.0
7
7
3
1
7
7
2
2
4
1
8
8
E
M
M
6
0
.2
-
1
1
.5
4
1
9
5
3
7
0
0
L
P
4
2
0
.5
0
.4
0
.0
5
4
6
1
1
8
1
6
4
4
1
5
9
E
M
M
6
7
.6
-
1
1
.3
3
2
3
7
9
0
0
G
ro
u
p
3
L
P
5
0
.7
0
.4
0
.0
6
6
4
1
4
6
1
9
4
2
1
1
3
E
M
M
9
1
.6
-
1
0
.7
7
2
5
M
o
d
er
n
L
P
6
0
.8
0
.5
0
.0
6
7
4
1
5
6
1
9
4
6
9
9
E
M
M
8
0
.1
-
1
0
.9
1
8
3
4
M
o
d
er
n
L
P
9
0
.7
0
.5
0
.0
6
6
4
1
4
9
1
9
–
1
0
7
E
M
M
7
4
.9
-
1
0
.3
2
3
8
9
M
o
d
er
n
L
P
4
4
2
.1
1
.0
0
.1
5
1
9
6
3
3
5
4
8
3
2
3
6
E
M
M
1
0
0
.4
-
1
2
.3
-
3
3
M
o
d
er
n
R
ad
io
ca
rb
o
n
ac
ti
v
it
y
,
u
n
co
rr
ec
te
d
an
d
co
rr
ec
te
d
ag
es
R
T
re
si
d
en
ce
ti
m
e,
M
R
T
m
ea
n
re
si
d
en
ce
ti
m
e,
C
o
n
t
co
n
ta
m
in
at
ed
sa
m
p
le
,
P
F
M
p
is
to
n
fl
o
w
m
o
d
el
,
E
M
M
ex
p
o
n
en
ti
al
m
ix
in
g
m
o
d
el
Environ Earth Sci (2016) 75:830 Page 11 of 17 830
123
microbial degradation of CFC-11, as also observed in
previous studies (e.g., Lindsey et al. 2003; Horst et al.
2008). It has been demonstrated that CFC-12 and CFC-113
are less degraded in waters (e.g., Plummer and Busenberg
2000). However, in our study no individual sample indi-
cated dissolved oxygen concentrations of \3 mg l-1,
which indicates that aerobic conditions predominate. One
explanation of this discrepancy could be that only part of
the flow paths arriving at a sampled well underwent
anaerobic conditions, or oxygenation occurred in the well
tube. Moreover, the different CFC tracers have distinct
temporal patterns of their input functions. The air-mixing
ratios of CFC-11 and CFC-113 indicate a turnover in the
1990s, which leads to ambiguity in age dating with modern
ages that span the 1990s. Thus, in this study the estimation
of residence times was performed using only CFC-12
tracers.
The estimation of groundwater residence times (RTs)
requires the knowledge of the aquifer geometry. The most
commonly used models are piston flow and exponential
mixing. Although hybrid models are also feasible, such as
exponential piston flow models, they were not considered
due to a lack of detailed knowledge of aquifer conditions at
individual wells. The investigated area represents an
aquifer that is primarily unconfined with well depths of up
to 200 m and long screen lengths. This geometry indicates
that the exponential mixing model may generally match the
groundwater conditions rather than the piston flow model.
The tracer plot in Fig. 9a confirms this assumption. The
individual samples do not plot on the piston flow curve,
with the exception of a few dispersed samples (LP4 and
LP22).
The RTs calculated with the TracerLPM model range
from 26 to 46 years and 18 to 206 years for PFM and
EMM, respectively (Table 2). Samples with CFC concen-
trations greater than levels of possible atmospheric equi-
librium for the northern hemisphere were considered to
have been contaminated by excess air and were not used
for RT calculations.
Carbon-13 values range from -12.3 to -9.5 % VDBP,
and 14C activities vary between 29.5 and 100.5 pmC
(Table 2). Uncorrected RTs vary from recent to 10 ka.
Radiocarbon data indicate a trend from elevated levels in
the eastern mountainous flanks to lower levels in the
western extremes (Fig. 7b). The highest values on the order
of 100 pmC mostly represent groundwater recharged in the
flanks of the El Novillo range/La Paz fault in the eastern
portion of the study area (e.g., LP1, LP3 and LP44). These
waters consistently indicate a depleted d13C signature and
low EC, Cl and DIC values (Fig. 9a, b). Two trends can be
observed in Fig. 9b: group 2 and 3 waters reduce 14C
activities with increasing DIC/salinity, and group 1 waters
raise 14C values with increasing DIC/salinity. This rise in
14C values may be explained by the increased admixture of
seawater and evaporation in group 1 that increases DIC/
salinity.
Fig. 7 Distribution of a CFC-12 mixing ratio and b radiocarbon activity in groundwater. Including geological features, urban areas (hatched
areas) and well sites (circles)
830 Page 12 of 17 Environ Earth Sci (2016) 75:830
123
The corrected values in Table 2 suggest that ground-
water is modern with the exception of several sites (LP13,
LP14, LP23, LP25, LP38 and LP42). These waters (0.9
and 4.7 ka) are located in the central and western low-
lands (El Centenario/Chametla). There is no general
positive correlation between groundwater age and tem-
perature; however, the two oldest waters (LP13 and LP25
with 4.0 and 4.7 ka, respectively), located in the west-
ernmost portion, coincide with two of highest tempera-
tures in the study area (31.8 and 32.7 C, respectively)
which could indicate local upconing of older groundwater
from the lower aquifer unit.
Discussion
Suitability of environmental tracers
The geochemical and isotopic investigations show consis-
tency with respect to the hydraulic-head configuration.
Generally groundwater moves in SSE–NNW direction
from recharge to irrigation areas. Due to heavy abstractions
for irrigation, the water table is modified along the coast
line, i.e. water table abatement causes seawater intrusion
and flow inversion along the coast line. Chloride concen-
tration and TDS increase dramatically near the coast line
due to recycling and seawater intrusion rather than as an
indication of long RTs.
CFC RTs from the study area calculated using the
exponential mixing model tend to become older along the
flow path in a SE–NW direction from upper areas to the
coast. Similarly to the hydraulic-head configuration, this
trend is modified in irrigated areas in the center of the study
area where the shortest RTs are observed. The rejuvenation
is most likely generated from the irrigation return flow that
is continuously recharging the alluvial aquifer. Ground-
water from urban and industrial areas sometimes indicates
elevated CFC values added from anthropogenic sources
(e.g., city samples LP1 and LP11, which appear
contaminated).
Fig. 9 a Radiocarbon activity
versus 13C ratios, b radiocarbon
activity versus DIC
concentration
Fig. 8 Tracer plots with selected models curves: a CFC-113 vs.
CFC-12 mixing ratios, b tracer plot CFC-11 vs. CFC-12 mixing
ratios. PFM piston flow model, EMM exponential mixing model
Environ Earth Sci (2016) 75:830 Page 13 of 17 830
123
Carbon-14 RTs differ from CFC RTs because the con-
centrations of anthropogenic atmospheric CFC gases in
groundwater provide information on groundwater resi-
dence times and mixing processes for waters up to 50 years
in age (Goody et al. 2006). In addition, radiocarbon is a
tracer that estimates groundwater RTs in the aquifer,
integrating all flowpaths and considering degradation of
their concentrations. Additionally, the impact of irrigation
return flow modifies CFC concentrations substantially,
resulting in relatively young RTs that contradict 14C ages
in irrigated areas.
Contribution to flow conceptualization
Stable water isotopes suggest that the primary groundwater
recharge of the valley of La Paz is originating naturally at
higher elevations of the Las Cruces and El Novillo ranges,
respectively. Other recharge sources evidenced by
stable isotopes and CFCs are irrigation return flow below
irrigated lands and seawater intrusion due to an inversion
of flow direction along the coastline as a consequence of
heavy, uncontrolled extractions. These results corroborate
the outcome of a previous study on groundwater recharge
based on hydraulic data (Cruz-Falco´n et al. 2011). In
addition, CFC levels in combination with concentrations of
some chemical elements suggest recharge from anthro-
pogenic sources in urbanized areas, while radiocarbon data
indicate an upconing of a regional groundwater flow.
Piezometric levels indicate that groundwater moves
from natural recharge areas in SSE–NNW direction and
discharges in the Sea of Cortez. However, from the
decrease of radiocarbon activity observed in this study it
can be inferred that groundwater more likely moves in a
direction between E–W and SE–NW. In addition, the
groundwater table configuration is heavily modified from
intensive groundwater extraction. An ever-increasing
groundwater level abatement of up to 10 m is observed
below irrigated agricultural areas where groundwater
extraction is not controlled. The abatement is corroborated
by the observation of radiocarbon and CFC modification in
groundwater along the flow gradient.
Spring water reflects cation/anion concentrations typical
for water derived from igneous/metamorphic rocks. Total
dissolved solids and EC values along groundwater flow
sections attain up to 5352 mg l-1 and 8920 lS cm-1,
respectively. Chloride and sodium exert the major control
on salinity. Sodium is primarily released by weathering of
Na-feldspars, and Ca2? is primarily released from plagio-
clase, hornblendes and pyroxene. The resulting ground-
water types are Ca–HCO3 to Na–HCO3 as a result of the
discordant dissolution of primary silicate minerals and
possibly an exchange of Ca2? for Na? in the soil. In
addition, seawater with Na? and Cl- as dominant ions is in
contact with sediments from the coastal area and its
intrusion induces an exchange of cations on the soil par-
ticles where Na? is taken up and Ca2? is released.
Anthropogenic impact
The result from excessive pumping and upconing of sea-
water is a Ca–Cl groundwater type and local increase in
temperature in the aquifer’s center. Groundwater used in
agriculture shows increasing Cl- and Na? concentrations
as a result of evapotranspiration of irrigation water. The
subsequent water type is Na–Cl because Na? is exchanged
for Ca2? on the soil exchangers.
Elements of anthropogenic origin such as NO3
-2 have a
notable distribution in the study area. Elevated concentra-
tions of NO3
-2 that exceed the Mexican drinking water
standard are located specifically below agricultural lands.
In contrast, the impact of domestic sewage from urbanized
areas on groundwater is relatively low.
Conclusions
A study using groundwater chemistry, environmental iso-
topes and gas tracers was developed to elucidate the
dynamics and occurrence of groundwater of a coastal
Holocene aquifer. Groundwater is mainly of meteoric ori-
gin. Recharge occurs mostly in Las Cruces and Novillo
ranges in the E and SSE portion of the study area,
respectively, and flows in NNW direction. While flowing to
the discharge area, groundwater acquires salinity from
incongruent silicate weathering and evaporation due to the
dry climate. Pre-development flow conditions suggest that
groundwater discharged in the bay of La Paz Ensenada.
The agricultural development in the central portion of
the study area gradually modified the piezometric water-
table configuration and chemical conditions. The heavy
extractions from irrigation zones in El Centenario, Cha-
metla and south of La Paz provoked an ever growing water
balance deficit and consequently water table abatements
reaching so far an elevation of up to *10 m below sea
level in the central part of the study area. On the other
hand, this water table decline provokes groundwater
recharge from seawater intrusion along the coastline,
impacting the water chemistry 13 km land inwards, i.e.
reaching water quality levels in the order of brackish water.
Residence time calculations using chlorofluorocarbons and
radiocarbon indicate that groundwater is in general modern
to submodern.
Other groundwater sources have been identified. Several
sites in the western portion demonstrate higher tempera-
tures and fossil water of up to *4.7 ka which suggests
upconing of deeper, regional groundwater flows from
830 Page 14 of 17 Environ Earth Sci (2016) 75:830
123
lower, intrusive rock formations. This indicates that the
upper aquifer is locally connected with the lower aquifer
units. Likewise, managed aquifer recharge has been
detected. Farmers from El Centenario, Chametla and
Paraiso del Mar use treated wastewater from La Paz city
for irrigation purpose, as part of an agreement with La Paz
city to exchange water for wastewater.
The anthropogenic impact of groundwater is reflected by
the distribution of nitrate and sulfate concentrations, which
can be attributed to the general overutilization of fertilizers
in agricultural activities. The elevated and increasing levels
of chloride and salinity in areas of intensive pumping
support the hypothesis that induced mixing and water
quality degradation have occurred over relatively short
time scales (e.g., decades).
Groundwater degradation is currently a limiting factor
for local economic growth, and there is potential for a
conflict between agricultural and urban water use. The
study contributes to the development of an adequate con-
ceptual and numerical model for simulating the hydrody-
namics of the aquifer of the La Paz coastal plain to help
water authorities with a decision tool for a sustainable
water use.
Acknowledgments This study was co-financed by Fundacio´n
FEMSA and the Water Science and Technology chair of Tecnologico
de Monterrey. Fundacio´n FEMSA had no role in study design, data
collection and analysis, decision to publish, or preparation of the
manuscript. We would like to thank R. Ledesma for his assistance in
the field work and mapping. Finally, we would like to acknowledge
the reviewers for their valuable comments on the initial version of the
manuscript.
Open Access This article is distributed under the terms of the Creative
Commons Attribution 4.0 International License (http://creative
commons.org/licenses/by/4.0/), which permits unrestricted use, distri-
bution, and reproduction in anymedium, provided you give appropriate
credit to the original author(s) and the source, provide a link to the
Creative Commons license, and indicate if changes were made.
References
Alcala´ FJ, Custodio E (2008) Using the Cl/Br ratio as a tracer to
identify the origin of salinity in aquifers in Spain and Portugal.
J Hydrol 359:189–207
A´lvarez AA, Rojas SH, Prieto MJ (1997) Geologı´a de la bahı´a de la
Paz y areas adyacentes. In: Ramı´rez RM, Urba´n RJ (eds) La
Bahı´a de La Paz: Investigacio´n y Conservacio´n. UABCS-
CICIMAR-SCRIPPS, Mexico, pp 13–29
Anders R, Mendez GO, Futa K, Danskin WR (2014) A geochemical
approach to determine sources and movement of saline ground-
water in a coastal aquifer. Ground Water 52(5):756–768
Busenberg E, Plummer LN (1992) Use of chlorouorocarbons (CCl3F
and CCl2F2) as hydrologic tracers and age dating tools: the
alluvium and terrace system of central Oklaholma. Water Resour
Res 28:2257–2283
Cardoso Da Silva J, Bocanegra E, Custodio E, Manzano M,
Montenegro S (2010) State of knowledge and management of
Iberoamerican coastal aquifers with different geo-hydrological
settings. Episodes 33(2):0705–3797
CIBNOR (Centro de Investigaciones Biolo´gicas del Noroeste),
UNAM (Universidad Nacional Auto´noma de Me´xico) (2011)
Modelacio´n de escenarios para el estudio de manejo integrado de
las aguas subterraneas en el acuı´fero de La Paz, B.C.S. La Paz,
BCS. UNAM
CIGSA (Consultores en Ingenierı´a Geofı´sica SA de CV) (2001)
Estudio de caracterizacio´n y modelacio´n de la intrusio´n marina
en el acuı´fero de La Paz BCS. Contrato CNA, GAS-013-PR01.
pp 284
Clark D, Fritz P (1997) Environmental isotopes in hydrogeology.
CRC Press LLC, Florida
CNA (Comisio´n Nacional del Agua) (1997) Censo de captaciones de
aguas subterra´neas y coleccio´n de datos geohidrolo´gicos en la
zona La Paz-EI Carrizal, BCS. Informe final, Contrato GAS-
026-PR097 para ADI Construcciones S.A. de C.V. Subdireccio´n
General Te´cnica, Gerencia de Aguas Subterra´neas pp 173
CNA (Comisio´n Nacional del Agua) (1999) Documento de Respaldo
para la Publicacio´n de la Disponibilidad. Acuı´fero BCS.-24 La
Paz. Gerencia Regional de la Penı´nsula de Baja California,
Mexicali, B.C. Comisio´n Nacional del Agua pp 17
CNA (Comisio´n Nacional del Agua) (2001) Estudio de Caracteri-
zacio´n y modelacio´n de la intrusio´n marina en el acuı´fero de la
Paz B.C.S. Me´xico DF, pp 285
CNA (Comisio´n Nacional del Agua) (2002) Determinacio´n de la
disponibilidad de agua en el acuı´fero de La Paz, Estado de Baja
California Sur. Direccio´n General Te´cnica. Gerencia de Aguas
Subterra´neas. Subgerencia de Evaluacio´n y Modelacio´n Hidro-
geolo´gica pp 18
CONAGUA (Comisio´n Nacional del Agua) (2005) Estudio para
Actualizar la Disponibilidad Media Anual de las Aguas
Nacionales Superficiales en las 85 (ochenta y cinco) Subregiones
Hidrolo´gicas de las 7 (siete) Regiones Hidrolo´gicas 1,2,3,4,5,6 y
7 de la Penı´nsula de Baja California, Mediante la Aplicacio´n de
la NOM-011-CNA-2000. Comisio´n Nacional del Agua p 75
CONAGUA (Comisio´n Nacional del Agua) (2009) Actualizacio´n de
la disponibilidad media anual de agua subterra´nea; Acuı´fero
(0324) La Paz, Estado de Baja California. Diario Oficial de la
Federacio´n. CONAGUA Subdireccio´n General Te´cnica: Geren-
cia de Aguas Subterra´neas, Subgerencia de Evaluacio´n y
Ordenamiento de Acuı´feros
CONAGUA (Comisio´n Nacional del Agua) (2010) Situacion actual y
posibles escenarios de intrusion salina en el acuifero la paz, baja
california sur y su aprovechamiento como fuente de desalacion
para abastecimiento de agua potable. Informe Final, Mexico City
CONAGUA (Comisio´n Nacional del Agua) (2014) Estadı´sticas del
Agua en Me´xico. Secretarı´a de Medio Ambiente y Recursos
Naturales, Mexico City. http://www.conagua.gob.mx/CONA
GUA07/Publicaciones/Publicaciones/EAM2014.pdf. Accessed
03 July 2015
CONAGUA (Comisio´n Nacional del Agua), UNAM (Universidad
Auto´noma de Me´xico) (2010) Informe Final: Situacio´n actual y
posibles escenarios de intrusio´n salina en el acuı´fero La Paz,
Baja California Sur y su aprovechamiento como fuente de
desalacio´n para abastecimiento de agua potable. p 433
CONAPO (Consejo Nacional de Poblacio´n) (2014) Datos de proyec-
ciones: Estimaciones y proyecciones de la poblacio´n por entidad
federativa. http://www.conapo.gob.mx/es/CONAPO/Proyec
ciones_Datos. Accessed 28 January 2015
Cook PG, Solomon DK, Plummer LN, Busenberg E, Schiff SL (1995)
Chlorofluorocarbons as tracers of groundwater transport pro-
cesses in a shallow, silty sand aquifer. Water Resour Res
31:425–434
Environ Earth Sci (2016) 75:830 Page 15 of 17 830
123
Cruz-Falco´n A (2007) Caracterizacio´n y diagno´stico del acuı´fero de
la paz, BCS. mediante estudios geofisı´cos y geohidrolo´gicos.
PhD thesis, Instituto Polı´te´cnico Nacional
Cruz-Falco´n A, Va´zquez-Gonza´lez R, Ramı´rez-Herna´ndez J, Salinas-
Gonza´lez F, Nava-Sa´nchez EH, Troyo-Die´guez E (2010) Depth
estimation to crystalline basement in the valley of La Paz, Baja
California Sur, Mexico. Geofı´sica Int 49(4):213–224
Cruz-Falco´n A, Va´zquez-Gonza´lez R, Ramı´rez-Herna´ndez J, Nava-
Sa´nchez EH, Troyo-Die´guez E, Rosas-Rivera J, Vega-Mayagoi-
tia JE (2011) Rainfall and recharge in the basin of La Paz, BCS,
Mexico. Universidad y Ciencia 27(3):251–263 (in Spanish)
Cruz-Falco´n A, Ramı´rez-Herna´ndez J, Va´zquez-Gonza´lez R, Nava-
Sa´nchez EH, Troyo-Die´guez E, Fraga-Palomino HC (2013)
Estimacio´n de la recarga y balance hidrolo´gico del acuı´fero de la
Paz, BCS, Mexico. Universidad y Ciencias 29(1):87–100
Cruz-Fuentes T, del Carmen Cabrera M, Heredia J, Custodio E (2014)
Groundwater salinity and hydrochemical processes in the
volcano-sedimentary aquifer of La Aldea, Gran Canaria, Canary,
Islands, Spain. Sci Total Environ 484:154–166
Darling WG, Gooddy DC, MacDonald AM, Morris BL (2012) The
practicalities of using CFCs and SF6 for groundwater dating and
tracing. Appl Geochem 27:168–1697
Davis SN, Whittemore DO, Fabryka-Martin J (1998) Uses of
chloride/bromide ratios in studies of potable water. Ground
Water 36(2):338–350
Deines P, Langmuir D, Harmon RS (1974) Stable carbon isotope ratios
and the existence of a gas phase in the evolution of carbonate
groundwaters. Geochim Cosmochim Acta 38:1147–1164
Edmunds WM, Smedley PL (2000) Residence time indicators in
groundwater: the East Midlands Triassic sandstone aquifer. Appl
Geochem 15:737–752
Edmunds WM, Guendouz AH, Mamou A, Moulla A, Shand P, Zouari
K (2003) Groundwater evolution in the Continental Intercalaire
aquifer of souther Algeria and Tunisia: trace elemen and isotopic
indicators. Appl Geochem 18:805–822
Escolero O, Torres-Onofre S (2007) Ana´lisis de la Intrusio´n de Agua
de Mar en el acuı´fero de La Paz. Boletı´n Geolo´gico y Minero.
118:637–648
Fontes JC, Garnier JM (1979) Determination of the initial 14C activity
of total dissolved carbon: a review of the existing models and a
new approach. Water Resour Res 15(2):399–413
Gaona-Zanella P (2014) Ana´lisis de Calidad de las Aguas
Subterra´neas en La Paz, Baja California Sur, como contribucio´n
a la Sostenibilidad Hı´drica de la Regio´n. MSc thesis, Tec-
nolo´gico de Monterrey, Monterrey, Me´xico
Gardner PM, Heilweil VM (2014) A multitracer approach to
understanding regional groundwater flow in the Snake Valley
area of the eastern Great Basin, USA. Appl Geochem. doi:10.
1016/j.apgeochem.2014.02.010
Gonc¸alve`s J, Vallet-Coulomb C, Petersen J, Hamelin B, Deschamps P
(2015) Declining water budget in a deep regional aquifer
assessed by geostatistical simulations of stable isotopes: case
study of the Saharan ‘‘Continental Intercalaire’’. J Hydrol
531:821–829
Goody DC, Darling WG, Abesser C, Lapworth DJ (2006) Using
chlorofluorocarbons (CFCs) and sulphur hexafluoride (SF6) to
characterise groundwater movement and residence time in a
lowland Chalk catchment. J Hydrol 330:44–52
Han DM, Song XF, Currell MJ, Yang JL, Xiao GQ (2014) Chemical
and isotopic constraints on evolution of groundwater salinization
in the coastal plain aquifer of Laizhou Bay, China. J Hydrol
508:12–27
Hem JD (1989) Study and interpretation of the chemical character-
istics of natural water. US Geological Survey, Water Supply
Paper 2254
Herna´ndez-Antonio A, Mahlknecht J, Tamez-Mele´ndez C, Ramos-
Leal J, Ramı´rez-Orozco A, Parra R, Ornelas-Soto N, Eastoe CJ
(2015) Groundwater flow processes and mixing in active
volcanic systems: the case of Guadalajara (Mexico). Hydrol
Earth Syst Sci 19:3937–3950
Horst A, Mahlknecht J, Merkel B (2007) A stable isotope (strontium,
carbon, deuterium, oxygen-18) study for detecting mixing and
origin in an overexploited aquifer in Mexico. Isot Environ Health
Stud 43(4):323–338
Horst A, Mahlknecht J, Merkel BJ, Aravena R, Ramon-Arroyo YR
(2008) Evaluation of the recharge processes and impacts of
irrigation on groundwater using CFCs and radiogenic isotopes in
the Silao-Romita basin, Mexico. Hydrogeol J 16:1601–1614
IAEA (International Atomic Energy Agency) (1996) Manual on
Mathematical Models in Isotope Hydrogeology. IAEA-TEC-
DOC-910, Vienna, Austria, pp 207
IAEA (International Atomic Energy Agency) (2006) Use of Chlo-
rofluorocarbons in Hydrology: A Guidebook. IAEA Library
Cataloguing in Publication Data, Vienna, Austria, p 277
ICF (International Community Foundation), Fundacio´n FEMSA,
GeoAdaptive LLC (2012) La Paz, Ciudad Sostenible: Reporte
Sistema de Indicadores para la Plataforma de Ciudades Emer-
gentes y Sostenibles. Banco Interamericano de Desarrollo,
Cambridge
INEGI (Instituto Nacional de Estadı´stica Geografı´a e Informa´tica)
(2005) Superficie total por distrito de desarrollo rural y
municipio segu´n uso de suelo y vegetacio´n. http://www.
oeidrus-bcs.gob.mx/. Accessed 1 April 2015
Ingerson E, Pearson FJ (1964) Estimation of age and rate of motion of
groundwater by the 14C-method. Recent Researches in the Fields
of Hydrosphere. Atmosphere and Nuclear Geochemistry Mar-
uzen, Tokyo, pp 263–283
Jime´nez-Martı´nez J, Custodio E (2008) El exceso de deuterio en la
lluvia y en la recarga a los acuı´feros en el a´rea circum-
mediterra´nea y en la costa mediterra´nea espan˜ola. Boletı´n
Geolo´gico y Minero 119(1):21–32
Kaown D, Koh DC, Lee KK (2009) Effects of groundwater residence
time and recharge rate on nitrate contamination deduced from
d18O, dD, 3H/3He and CFCs in a small agricultural area in
Chuncheon, Korea. J Hydrol 366(1–4):101–111
Kaplan D, Mun˜oz-Carpena R (2014) Groundwater salinity in a
floodplain forest impacted by saltwater intrusion. J Contam
Hydrol 169:19–36
Kusano Y, Tokunaga T, Asai K, Takahashi HA, Morikawa N,
Yasuhara M (2014) Occurrence of old groundwater in a volcanic
island on a continental shelf: an example from Nakano-shima
Island. Oki-Dozen, Japan
Ledesma-Ruiz R, Paste´n-Zapata E, Parra R, Harter T, Mahlknecht J
(2015) Investigation of the geochemical evolution of ground-
water under agricultural land: a case study in northeastern
Mexico. J Hydrol 521:410–423
Lindsey BD, Phillips SW, Donnelly CA, Speiran GK, Plummer LN,
Boehlke JK, Focazio MJ, Burton WC, Busenberg E (2003)
Residence times and nitrate transport in ground water discharg-
ing to streams in the chesapeake bay watershed, water-resources
investigations Rep. 03-4035, US Geological Survey, New
Cumberland, PA
Mahlknecht J, Steinich B, Navarro de Leo´n I (2004) Groundwater
chemistry and mass transfers in the Independence aquifer,
central Mexico, by using multivariate statistics and mass-balance
models. Environ Geol 45(6):781–795
Mahlknecht J, Garfias-Solis J, Aravena R, Tesch R (2006a)
Geochemical and isotopic investigations on groundwater resi-
dence time and flow in the Independence Basin, Mexico.
J Hydrol 324:283–300
830 Page 16 of 17 Environ Earth Sci (2016) 75:830
123
Mahlknecht J, Medina-Mejı´a MG, Ga´rfias-Solis J, Cano-Aguilera I
(2006b) Intrinsic aquifer vulnerability assessment: validation by
environmental tracers in San Miguel de Allende, Mexico.
Environ Geol 51(3):477–491
Michener R, Lajtha K (2007) Stable isotopes in ecology and
environmental science. Blackwel, London
Minitab (2013) Getting started with minitab 17. United States:
Minitab Inc. Retrieved from http://www.minitab.com
Mohammed N, Celle-Jeanton Huneau F, Le Coustumer P, Lavastre V,
Bertrand G, Charrier G, Clauzet ML (2014) Isotopic and
geochemical identification of main groundwater supply sources
to an alluvial aquifer, the Allier River valley (France). J Hydrol
508:181–196
Monzalvo M (2010) Simulacio´n hidrodina´mica del acuı´fero de La Paz
y su aprovechamiento como fuente de desalacio´n. PhD thesis,
Universidad Nacional Auto´noma de Me´xico, Mexico City
Mook WG, Bommerson JC, Staverman WH (1974) Carbon isotope
fractionation between dissolved bicarbonate and gaseous carbon
dioxide. Earth Planet Sci Lett. doi:10.1016/0012-
821X(74)90078-8
Noroeste PRONATURA (2010) Diagno´stico de la Cuenca de la Paz:
Reporte Final del Convenio Niparaja. Programa Agua y Hume-
dales, La Paz, BCS, Mexico
Oster H, Sonntag C, Mu¨nnich KO (1996) Groundwater age dating
with chlorofluorocarbons. Water Resour Res 37:2989–3001
Perea MC, Ezcurra E, Leo´n de la Luz JL (2005) Functional
morphology of a sarcocaulescent desert scrub in the bay of La
Paz, Baja California Sur, Mexico. J Arid Environ 62(3):413–426
Plummer LN (2003) Environmental tracers and how they are used to
understand the aquifer. In: Bartolino JR, Cole JC (eds) Geolog-
ical Survey Circular 1222
Plummer LN (2005) Dating of young groundwater. In: Aggarwal PK,
Gat JR, Froehlich K (eds) Isotopes in the water cycle. Springer,
Netherlands, pp 193–218
Plummer LN, Busenberg E (2000) Chlorofluorocarbons: Tools for
dating and tracing young groundwater. In: Cook P, Herczeg A
(eds) Environmental Tracers in Subsurface Hydrology. Kluwer
Academic Publishers, Boston, pp 441–478
Plummer LN, Busenberg E, Drenkard S, Schlosser P, Ekwurzel B,
Weppernig R, McConnell JB, Michel RL (1998) Flow of river
water into a karstic limestone aquifer, 2. Dating the young
fraction in groundwater mixtures in the Upper Floridan aquifer
near Valdosta, Georgia. Appl Geochem 13(8):1017–1043
Plummer LN, Busenberg E, Bo¨hlke JK, Nelms DL, Michel RL,
Schlosser P (2001) Groundwater residence times in Shenan-doah
National Park, Blue Ridge Mountains, Virginia, USA: a multi-
tracer approach. Chem Geol 179(1–4):93–111
Post VEA (2005) Fresh and saline groundwater interaction in coastal
aquifers: is our technology ready for the problems ahead? Hydrol
J. doi:10.1007/s10040-004-0417-2pp120-123
Rindsberger M, Margaritz M, Carmi I, Gilad D (1983) The relation
between air mass trajectories and the waterisotopic composition
in the Mediterranean Sea area. Geophysical Rev. Lett. 10:43–46
Rodrı´guez-Estrella R, Dona´zar A, Hiraldo F (1998) Raptors as
indicators of environmental change in the scrub habitat of Baja
California Sur, Mexico. Conserv Biol 12:921–925
Russak A, Yechieli Y, Herut B, Lazar B, Sivan O (2015) The effect of
salinization and freshening events in coastal aquifers on nutrient
characteristics as deduced from field data. J Hydrol
529:1293–1301
Solomon DK, Genereux DP, Plummer NL, Busenberg E (2010)
Testing mixing models of old and young groundwater in a
tropical lowland rain forest with environmental tracers. Water
Resour Res. doi:10.1029/2009WR008341
Unsal B, Yagbasan O, Yazicigil H (2014) Assessing the impacts of
climate change on sustainable management of coastal aquifers.
Environ Earth Sci 72:2183–2193
USGS (United States Geology Survey) (2014) The reston groundwa-
ter dating laboratory. http://water.usgs.gov/lab/software/air_
curve/index.html. Accessed 2nd February 2014
Vogel JC, Grootes PM, Mook WG (1970) Isotope fractionation
between gaseous and dissolved carbon dioxide. Z. Phys. doi:10.
1007/BF01394688
Ward J (1963) Hierarchical grouping to optimize an objective
function. J Am Stat Assoc 17:24:12 (pp 236–244)
Wassenaar LI, Van Wilgenburg SL, Larson K, Hobson K (2009) A
groundwater isoscape (dD, d18O) for Mexico. J Geochem Explor
102:123–136
Environ Earth Sci (2016) 75:830 Page 17 of 17 830
123