ORIGINAL ARTICLE

Groundwater salinization in arid coastal wetlands: a study case
from Playa Fracasso, Patagonia, Argentina

Marı́a del Pilar Alvarez • Cristina Dapeña •

Pablo J. Bouza • Ileana Rı́os • Mario A. Hernández

Received: 14 April 2014 / Accepted: 9 December 2014

� Springer-Verlag Berlin Heidelberg 2014

Abstract The origin of the high salinity in the ground-

water of a coastal wetland in an arid climate was studied in

the Playa Fracasso marsh, located on the northwest coast of

the extra-Andean Patagonia. Research was carried out by

means of the design of a network of soil pits and short

piezometers in the marsh and the surrounding landforms.

Continuous fluctuations of the water table, in situ physical

and chemical properties, major ions (Ca2?, Mg2?, Na?,

K?, Cl-, SO4
2-, HCO3

-) and stable isotopes (18O and 2H)

in groundwater, as well as soil salinity, were measured. The

combined analysis of the hydrodynamics, the ion ratios

rCa2?/rCl- and rMg2?/rCa2? vs. rCl- and the isotopic

composition made it possible to recognize an area within

the high marsh in which the origin of groundwater is

mainly marine and another in which the contributions are

of mixed origin. By means of the analysis of rCl- vs. d18O,

a salinization process with no change in isotopic compo-

sition was identified. Its interpretation, together with those

of the soil salinity profiles and the records of the fluctua-

tions in electrical conductivity associated with extraordi-

nary tides, was used to define a conceptual model of

salinization which could be useful to understand other

coastal wetlands under similar arid climatic conditions. It

consists in a cyclical mechanism of evapotranspiration,

precipitation, dissolution and transport of salts during tides.

Keywords Geohydrology � Hydrochemistry � Salt marsh �
Patagonia � Arid climate

Introduction

‘Salt marsh’ is an ecological/morphological term used to

refer to low, swampy lands located in the maritime–ter-

restrial transition and periodically flooded by seawater

(Codignotto 1987). They develop under stable geomor-

phological conditions, such as estuaries or coastal areas

protected by spits or barriers, and they are usually covered

by halophytic vegetation, tolerant to partial immersion,

anoxia and edaphic hypersalinization. The importance of

these environments and their study is recognized world-

wide due to their enormous primary productivity and the

fact that they provide shelter and feeding areas to several

invertebrate and vertebrate species of great economic and

social relevance (Adam 1990). Besides, the study of the

origin of groundwater and its salinity acquires special rel-

evance when assessing the movement of nutrients and the

environmental conditions within a marsh, which is a fun-

damental aspect in its ecohydrological analysis (Mitsch and

Gosselink 2000).

The patterns and variations in groundwater flow in a

marsh are controlled by the tidal fluctuations, the local

water balance (evapotranspiration–precipitation), ground-

water discharge from the continent, the geometry and the

hydraulic properties of the sediments that constitute it

(Wilson and Morris 2012), as well as by its geomorphology

(Carol et al. 2013). Tides cause frequent, predictable

M. d. P. Alvarez (&) � M. A. Hernández

Cátedra de Hidrogeologı́a, Universidad Nacional de La Plata

(UNLP), CONICET, La Plata, Argentina

e-mail: alvarez.maria@conicet.gov.ar

C. Dapeña

Instituto de Geocronologı́a y Geologı́a Isotópica

(INGEIS, CONICET-UBA), Buenos Aires, Argentina

e-mail: dapenna@ingeis.uba.ar

P. J. Bouza � I. Rı́os

Centro Nacional Patagónico (CENPAT-CONICET),

Puerto Madryn, Argentina

e-mail: bouza@cenpat.edu.ar

123

Environ Earth Sci

DOI 10.1007/s12665-014-3957-3



hydroperiods affecting the lower parts, whereas in the more

elevated ones only extraordinary tides are responsible for

the inundation patterns, with the hydroperiods originated

by precipitations also being of great relevance (Brinson

1989).

The greater or lesser influence of the continental water

supply could be dominant in certain areas (Custodio 2010),

which determines whether it is a fresh marsh or a salt

marsh (Manzano et al. 2002; Durán 2003; Carol et al.

2009). However, in the most elevated areas, the water

patterns are also determined by high salinity, which may be

caused by the higher evaporation rate (Silvestri and Marani

2004). Likewise, in arid regions, high salinity characterizes

most of the groundwater, with many processes contributing

to salinization, among which the mixing with seawater,

rainfall concentration, evaporation, evapotranspiration,

dissolution of salts and seawater concentration in coastal

lagoons are the most common and can lead to groundwater

salinities in excess of modern seawater (Ridd and Stieglitz

2002; Fass et al. 2007; Warner et al. 2013).

In the marshes of the arid regions in Patagonia, mainly

ecological and edaphological research has been carried out

(Bortolus et al. 2009; Bala et al. 2008; Bouza et al. 2008;

Rı́os 2010; Rı́os et al. 2012); and, even if studies on general

hydrogeological aspects have been conducted recently

(Alvarez et al. 2012), the origin of the high salinity in its

groundwater is still the subject of research.

To define a conceptual model of salinization for coastal

wetlands in an arid climate, the Playa Fracasso marsh—

which has been declared a Ramsar and WHSRN (Western

Hemisphere Shorebird Reserve Network) site—was chosen

as case study (Fig. 1). By means of the hydrochemical,

isotopic and hydrodynamic variables, the influence of

marine and continental water and the salinization processes

at work were determined.

Study area

The study area, located on the coast of the San José Gulf

within the Área Natural Protegida Penı́nsula Valdés (Pen-

ı́nsula Valdés Protected Natural Area), corresponds to an

open coast and bay marsh (Bouza et al. 2008) with no

inflow from permanent surface watercourses. It is located

in the discharge area of the regional aquifer system

(Alvarez et al. 2010), a phenomenon that is evident in the

spring occurring on the cliffs limiting the area to the NE

(Fig. 1).

The mean annual precipitation is 230 mm, with no

definite trend throughout the year, and a mean annual

temperature of 13.4 �C, fluctuating between mean

extremes of 6.4 �C in July and 20.4 �C in January. The

potential evapotranspiration, estimated for reference

following Thornthwaite and Mather (1957), is 700 mm/

year. As regards the relative humidity, the mean annual

value is 68.4 % and the monthly mean varies between

62.4 % in January and 75.5 % in June. Such behaviour is

expected for the extra-Andean Patagonian region, where

the relative humidity values are low and their concentration

in winter is aided by the low temperatures as well as by the

lower frequency and intensity of the winds.

Under such conditions, the climate is classified as arid,

mesothermal according to Thornthwaite (Burgos and Vidal

1951). To carry out the climate characterization, the

records of the weather station of the Centro Nacional Pa-

tagónico (CENPAT; National Patagonian Centre) between

1982 and 2002 (Frumento and Contrera 2013) were used,

together with the precipitation data ranging from 1901 to

2006 of the Estancia La Adela (La Adela Farm); these are

located approximately 95 km to the SW and 20 km to the

SE of Playa Fracasso, respectively.

The material outcropping in cliffs and in the erosion

scarps surrounding the marsh is mainly represented by

tertiary sediments of marine origin of the Puerto Madryn

formation (sand, silt and clay with an important tuffaceous

component and intercalations of shell and gypsum levels),

Plio-Pleistocene gravel of the Rodados Patagónicos (Pata-

gonian Shingle Formation) and quaternary alluvial and

colluvial deposits, which are a remobilization of the above-

mentioned deposits (Haller et al. 2001). On the coast, there

are thin eolian layers (no more than 0.5 m), which mainly

overlie the alluvial deposits.

From a geomorphological point of view, Playa Fracasso

is a vast tidal plain which is overlain by a marsh (M) of

approximately 35 ha (0.35 km2), crossed by tidal channels

and associated with spits (E1 and E2), bajadas (B1 and

B2), sandy layers (SL) and pediments (Pd and PdF)

(Fig. 1).

The origin of the marsh is related to sea level fluctua-

tions and the littoral drift—both of which generate sand

bars that lie along the coastline—as well as to the accretion

of a spit following a west–east trend (Rı́os 2010). Topo-

graphically, it is a low area, with heights that do not exceed

8 m asl and a gentle slope, where drainage is generally

deficient. The marsh shows a vegetation zonation charac-

terized by Spartina alterniflora in the lower part, and

Sarcocornia perennis and Spartina densiflora in the upper

part. In the tidal-channel levees, which constitute relatively

high areas, patches of Limonium brasiliense can be

observed.

The soils associated with the marsh were classified by

Rı́os et al. (2012), according to the USDA Soil Taxonomy

(Soil Survey Staff 1999), as Sodic Endoaquents and Sodic

Psammaquents in the area dominated by S. alterniflora,

Typic Fluvaquents in the area with S. perennis and in the

patches with L. brasiliense.

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The marsh is crossed in its middle and lower sections by

active tidal channels (Fig. 1). Depending on the tidal range,

the water flowing into these channels affects the marsh to a

greater or lesser extent, flooding it almost entirely during

extraordinary high tides, in which the range exceeds 8 m

according to the records of the Argentine Navy Hydro-

graphic Service (Hidrografı́a Naval Argentina 2010–2012),

whereas during ordinary high tides seawater is either cir-

cumscribed to the channel or it overflows, only reaching

the lowest lying areas.

Materials and methods

A monitoring network consisting of 13 shallow manually

drilled boreholes (depths up to 2.3 m) was set up to

carry out this research. The spatial distribution of these

piezometers was based on geomorphological information

obtained in the field and by satellite image interpreta-

tion. They mainly correspond to three transects perpen-

dicular to the coastline, extending from the littoral

bajadas to the high marsh (Fig. 1). The boreholes were

levelled with an optical level (Kern GK1-AC) and,

adjacent to each one, a pit was dug to describe the soil

profile, including soil texture, and to obtain samples of

the soil horizons to measure their electrical conductivity

(EC).

About 40 water samples [i.e. groundwater (PF), spring

water and seawater] were collected in three sampling

fieldworks (i.e. November 2010, August 2011 and February

2012), and analyzed for their chemical and isotopic com-

positions. The pH, temperature and EC, as well as the

piezometric levels, were measured in the field, always

during the low tide period.

Fig. 1 Location map and geomorphological units. Geological desig-

nations are as follows: SL sandy layer, SB sandy beach, M marsh,

F alluvial fan, B1 and B2 coalescent alluvial-fan piedmont or bajada,

S1 and S2 spit, Pd pediment, Pl. M palaeomarsh, Tt tertiary sediment

erosion scarp, AB cross section [modified from Alvarez et al. (Alvarez

et al. 2012)]

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To analyze the influence of the tides, by monitoring the

water-table fluctuations and salinity variations, three

automatic water-level dataloggers (two Schlumberger

Cera-Divers and one Schlumberger CTD-Diver) were

installed in piezometers PF5, PF6 and PF11. All of them

were set to record data at 15-minute intervals. The salinity

monitoring datalogger (EC) was located at point PF11.

Chemical analyses of major elements were carried out

by the Chemical Service of the CENPAT laboratory. The

Ca2?, Mg2?, Na?, K?, Cl-, SO4
2-, HCO3

-and NO3
-

determinations were performed by the conventional meth-

ods in accordance with the American Public Health

Association (APHA, AWWA and WPCF 1997). Ion ratios

(r) were calculated with the Easy Quim (Vázquez Suñé

2002) and AquaChem (Schlumberger 2010) applications.

The water isotopic analyses were carried out by the

Instituto de Geocronologı́a y Geologı́a Isotópica (INGEIS,

Geochronology and Isotope Geology Institute) of the

Consejo Nacional de Investigaciones Cientı́ficas y Técni-

cas (CONICET, National Scientific and Technical

Research Council) and the Universidad de Buenos Aires

(UBA, University of Buenos Aires), Argentina. Samples

were treated following conventional techniques and mea-

sured using Los Gatos Research high-precision laser

spectroscopy, i.e. off-axis integrated cavity output spec-

troscopy (Lis et al. 2008). Results are expressed using the

conventional d-notation, defined in Eq. (1).

d ¼ 1000 Rs � Rpð Þ=Rp &½ �; ð1Þ

where d is the isotope deviation in %, s the sample, p the

international reference, and R the isotope ratios (2H/1H,
18O/16O). Values are relative to the Vienna Standard Mean

Ocean Water (VSMOW) (Gonfiantini 1978). Uncertainties

are ±0.3 % for d18O and ±1.0 % for d2H.

The isotopic composition of rainwater was compared

with data from the rain collection station at Puerto Madryn,

Chubut province, for the periods October 1981–April 1985

and December 1998–November 2006, and with the local

meteoric water line as defined by Dapeña (2008) with 43

records, i.e. d2H % = (7.37 ± 0.22)�d18O-(0.36 ± 1.70) %
for this station. The rain collector is part of the Red Nac-

ional de Colectores (National Collection Network) setup by

the INGEIS within the framework of the Global Network

for Isotopes in Precipitation (GNIP), created by the Inter-

national Atomic Energy Agency and the World Meteoro-

logical Organization (IAEA/WMO 2002; Dapeña and

Panarello 1999, 2008). These measurements were supple-

mented by meteorological information, such as the mean

surface air temperature and the amount of precipitation.

The record is incomplete due to lack of precipitation or

insufficient amounts of rain; besides, there were periods in

which no samples could be obtained. However, a pre-

liminary meteoric line and some weighted averages for the

years 1999, 2000, 2005 and 2006 were calculated. The

arithmetic mean is d18O = -7.2 % and d2H = -52 %,

d = 6 % with 43 records. The isotopic composition of the

precipitation shows great dispersion, with values ranging

from -12.9 to -0.9 %, and from -86 to -4 % for d18O

and d2H, respectively.

The data were represented in a conventional d2H vs.

d18O diagram with the local meteoric water line. Also a

chloride vs. d18O graph was plotted to evaluate the sali-

nization mechanisms.

Results

The salinization processes in the marsh were analyzed

considering the hydrodynamics and the salt content in the

soil profile, together with the hydrochemistry and the iso-

topic composition of water.

Hydrodynamics

As regards the surface inflow of continental origin—pen-

insular, in this case—it is exclusively ephemeral, in a SE–

NW direction and it corresponds to the discharge, during

storm events, of the littoral bajadas, as shown by the

alluvial landforms that surround and overlie part of the

marsh (Fig. 1).

Concerning the hydrodynamic behaviour, three sectors

were identified: (1) of preferential recharge, represented by

units B1, B2 and SL; (2) of circulation, directly associated

with the marsh; and (3) of discharge along the coastline.

The groundwater flow defined has a general SSE–NNW

direction, perpendicular to the coastline and to the limit

between the marsh and the bajadas (Fig. 2).

The fluctuations of the water table, recorded by means

of the different divers installed along the AB transect

(Fig. 1), are shown in Fig. 3. In the one closest to the coast

(PF5), a cyclical repetition of sets of maximum fluctuations

can be observed approximately every 28 days (150 cm

between the minimum and maximum values), as well as

some minor ones intercalated every 14 days. In the diver

installed in an intermediate location (PF11), the oscillation

follows a similar pattern, with maximum peaks coinciding

in time with the ones recurring every 28 days in PF5,

although the range never exceeds 130 cm. In the case of

PF6, only some peaks coinciding with those of PF5 and

PF11 are reproduced, and its range barely reaches 48 cm.

Even though there is no tide gauge locally available

which would make it possible to compare the water table

levels with those of the tides accurately, the predictions of

the Servicio de Hidrografı́a Naval Argentino for Playa

Fracasso (Hidrografı́a Naval Argentina, 2010–2012) indi-

cate that spring tides, whose recurrence is approximately

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every 14 days, have a range between low and high tide

oscillating between 7 and 9 m and that these coincide in

time with the maximum water table fluctuations observed

in the records of the divers installed in PF5, PF11 and PF6.

In aquifer sediments hydraulically connected to the sea,

the level fluctuations occurring in the water body are

transmitted landward as continuous variations in the

groundwater levels (Ferris 1951; Erskine 1991; Smith and

Hick 2001; Bye and Narayan 2009). The cyclical rises in

the water table recorded by divers PF5 and PF11 show the

influence of the tidal fluctuations, with maximum peaks

with a recurrence similar to the one of the spring tides. The

combined analysis of these records and the isophreatic map

(Fig. 2) indicates inundation events in the marsh up to its

upper end, in the limit with the bajadas (Fig. 1).

By way of summary, in Fig. 4 a schematic cross section

of the hydrodynamic behaviour of the marsh and the

relationship between marine and continental groundwater

inflow are shown. This transect coincides with wells PF2,

PF5, PF11 and PF6 (Fig. 1).

Soil salinity profile

Together with the groundwater sampling and the level

and EC records of the divers, the EC of the soil was

measured in soil pits coinciding with wells PF5, PF11

and PF6 (Fig. 5). The soil profiles located in the marsh

(PF5 and PF11) are composed of an A horizon with a

silty to silty loam texture overlying sandy C horizons.

Soil pit PF5 shows a salinity profile with EC values

ranging between 13 and 13.8 mS/cm on the surface, and

it decreases gradually with depth up to a value of

9.14 mS/cm. In profile PF11, a high salinity value (EC

44.10 mS/cm) can be observed, being localized in the

first horizon and decreasing abruptly below the upper

20 cm, reaching lower values than those recorded in

PF5. It is important to highlight that in both edaphic

profiles the sediments are of marine origin and that they

belong to similar textural classes; however, the signifi-

cant difference in soil salinity in the upper centimetres

indicates that it is not the parent material that is

responsible for such a variation, but the presence of a

salinization mechanism. In turn, in the profile located

over the bajada landform (PF6) of the type A–AC–C, a

sandy texture with intercalations of shell remains and

gravels predominates and the conductivity in the differ-

ent levels is much lower than in the marsh soils. Up to a

depth of 113 cm, the EC values remain below 0.2 mS/

cm and, as from that point up to a depth of 140 cm —

coinciding at the time of describing the profile with the

Fig. 2 Equipotential map for

November 2010.

Geomorphological unit

references are as shown in

Fig. 1. Modified from Alvarez

et al. (2012)

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123



water table (saturated zone) and with the presence of

buried beach sand—they increase until they reach values

of 2.2 mS/cm (Fig. 5).

Hydrochemistry

Saline content

The water salt content varies spatially between 4.6 and

88.2 g/L depending on its location (i.e. marsh, fans, baja-

das, sandy layer, spring or sea), with an increasing gradient

towards the centre of the area, and temporally depending

on the season and the tides.

The temporal EC fluctuations recorded at PF11 show

that the most significant variations occur with a certain

cyclicity and are associated with the highest water fluctu-

ation levels. The highest EC records are associated with the

instance in which the water level is above ground level, a

situation that occurs cyclically and is related to the spring

tides. Besides, as the highest temperatures are approached

(November–January), the records increase in value. In

August, the contents do not exceed 40 mS/cm, whereas in

December they are higher than the measurement limit of

the instrument (80 mS/cm). As regards the behaviour in

between spring tides, it can be observed that when the

water table decreases, so does the EC, remaining in the

winter months between 20 and 30 mS/cm and in the sum-

mer months above the marine value (Fig. 3).

It should be clarified that the periods in which the

records are null (EC = 0 mS/cm) correspond to measure-

ments in which the diver was out of the water due to a

further decrease in the water table. Such decreases occur

between November and January, when evapotranspiration

is highest and the general level of the whole marsh is

deeper.

Ionic composition

According to the Piper classification, in the sampled

groundwater associated with the marsh, the littoral ba-

jadas and the spring, the predominant ions are Na? and

Cl-. Sodium increases linearly with respect to chloride,

showing a high correlation index (Fig. 6a), with the

Fig. 3 Groundwater level and electrical conductivity fluctuations

registered by divers located at points PF5, PF11 and PF6

Fig. 4 Schematic cross section of the hydrodynamic behaviour of the saltmarsh

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samples corresponding to the bajadas and the spring

being the least saline and the ones of the fans, the most

saline. In an intermediate position between such

extremes are the sea samples, whereas the marsh samples

are distributed practically along the entire correlation line

with a ratio close to 0.84, which is within the range of

the marine values measured (0.83, 0.81 and 0.9). Among

these, the PF11 samples fluctuate between lower values

(Cl- 238 meq/L) and similar or higher values (Cl-

551.5 meq/L) than the ones of the sea (Cl- 545.5 meq/

L). The PF2 values are similar to the ones of the sea,

except in the sample obtained in February (Cl-

633.8 meq/L) (Fig. 6a).

In general, the increase in Na? and Cl- content indicates

seawater intrusion (Lee and Song 2007), even though it

may be related to evaporitic levels or fossil marine water

(Castillo Pérez and Morell Evangelista 1988). Besides, the

halite content in marine aerosols may cause rain events

with a water composition of the sodium chloride type. In

addition, the groundwater discharge of continental origin is

Cl–Na; therefore, the contrast between the marine and

continental contributions is not evident.

Fig. 5 Schematic location of the soil profiles in the AB cross section.

Capital letters are used to designate master horizons; arabic numerals

are used both as suffixes to indicate vertical subdivisions within a

horizon and as prefixes to indicate discontinuities; suffix g: strong

gleying in which iron has been preserved in a reduced state because of

saturation with stagnant water

Fig. 6 Scatter plot of selected parameters from groundwater, spring and seawater samples, expressed in meq/L, and their ratios: rNa vs.r Cl (a),

and rMg/rCa vs. rCl (b)

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The rMg2?/rCa2? ratio increases as the proportion of

seawater in a mixture is higher, as in seawater there is an

excess of Mg2? with respect to Ca2? (Custodio 1997). In

Fig. 6b, the samples are organized into four groups. One of

them with rMg2?/rCa2? ratios near 1, among which the

spring sample can be found, as well as all of the samples

associated with the bajadas and one of the marshes (PF11),

in which the continental groundwater flow would pre-

dominate. Another group is composed of seawater (rMg2?/

rCa2? *5) and other samples with similar values (practi-

cally all of the marsh samples, as well as two of the fan

samples) whose origin is related to the marine contribution.

The third group consists of marsh samples associated with

wells PF4 and PF7, whose ratio lies between the marine

and continental values (3.69–4.55), suggesting a mixed

origin. Finally, there is a group of three samples associated

with the alluvial fans in which the ratio is much higher than

5, and in turn have a high chloride content (PF8 and PF9).

In this last group, given its high salinity and its spatial

location, it is probable that other modifying processes, such

as base exchange or Ca2? precipitation, could be distorting

the interpretation of this ratio (Custodio 1997; Appelo and

Postma 2005).

Isotopes

d 2H vs. d 18O

The isotopic compositions of all the samples (marsh, ba-

jadas, sandy layer, fans, spring and sea) are represented in

the conventional d2H vs. d18O plot (Fig. 7).

The isotope content of the groundwater samples of the

bajadas (PF6, PF10, PF12 and PF13) and of the sandy

layers (PF1) has d18O values between -7.0 and -4.6 %
and d2H values between -61 and -43 %, which reflects

the values of local precipitation. However, PF12 and PF13

have a more enriched composition, indicating evaporation

processes (Fig. 7). The fans (PF8 and PF9) have d18O and

d2H contents between -4.5 and -2.5 % and between -43

and -33 %, respectively. Together, the bajadas and fans

are aligned along an evaporation line, i.e. d2H = 5.3d18O-

19.9 (Fig. 8). Nevertheless, the fans have very high-salt

values which cannot be justified by the evaporation alone,

and which are probably related to the action of several

processes taking place simultaneously, like transpiration,

base exchange and salt dissolution.

The spring samples have a d18O composition between

-5.4 and -2.3 % and a d2H composition between -56

and -38 %, indicating variations in the isotope content of

the regional groundwater flow discharge (Fig. 7).

The marsh groundwater (PF2, PF4, PF5, PF7 and PF11)

has d18O values between -5.2 and 0.4 % and d2H values

between -42 and 1 %. Even though most values are

located near the position of the local seawater (d18O

between -0.3 and 0.1 %; d2H between -3 and -1 %),

they show a chloride content which is twice as high as the

one in seawater. However, PF4 and PF11 are relatively

more depleted, and PF11 in particular has very high

salinities but with a high variability among sampling

periods, also showing mixtures between waters such as the

ones in PF6 (bajada) and in PF4 (marsh). These mixes are

variable and close to 26 and 47 % in November and

August, respectively.

Discussion

The hydrodynamic and hydrochemical analyses (rMg/rCa

ratio) show a mixed origin for the marsh groundwater, but

cannot explain the hypersaline composition of some

waters, which indicates the existence of other salinization

processes. Although the rNa/rCl ratio close to the marine

value suggests that the evaporation of seawater below the

halite saturation state could be one of the salinization

processes, the hypersaline waters do not have an isotopic

signature consistent with marine water evaporation. Nei-

ther does it appear to be derived from geological or

anthropogenic sources, considering that the lithology of the

soil profiles analyzed at the cross section is the same for

both marsh points and that it is an unpopulated area. On the

basis of these data, it can be interpreted that the isotopic

compositions and salinities of the samples do not respond

exclusively to the marine contribution, but rather to the

joint action of several processes, such as evapotranspira-

tion, mixing, marine aerosols and dissolution/precipitation

(Gat 1981; Harbison and Cox 2002; Vengosh 2003; Fass

et al. 2007).

The conservative elements Cl- and 18O were ana-

lyzed, as they have traditionally been used to study the

salinization processes (Gonfiantini and Araguas-Araguas

1988; Yurtsever 1994; de Montety et al. 2008; Han et al.

2011). In Fig. 8, which shows the scatter plot of rCl- vs.

d18O, it can be observed that the samples located in the

marsh (PF2, PF4, PF5 and PF7) have a relatively con-

stant isotopic composition among samplings, which, on

the whole, is similar to the one of the sea. Their salin-

ities, however, exceed the values of the marine samples

in all cases.

The behaviour of these points, in which there are no

changes in isotopic composition and whose Cl- content is

higher than the value of the sea, can be explained as a

cyclical mechanism of evapotranspiration, subsequent salt

precipitation, new seawater inflow and the consequent

partial dissolution of the precipitated salts (Fig. 8). In

addition, another process that contributes to the high

salinity and to the lack of changes in the isotopic

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composition could be plant transpiration. For example,

according to Fass et al. (2007), highly saline groundwater

occurs beneath mangrove vegetation without inducing

isotopic fractionation, and this is attributed to the exclusion

of salts during the transpiration of tidal seawater.

On the other hand, the isotopic composition, as well

as the ion content measured in well PF11 during the

three samplings, reflects a mixing process between the

groundwater of the marsh and of the bajadas (Figs. 7,

8). However, the continuous records of salinity fluctua-

tions (Fig. 3) and the high salinity in the upper centi-

metres of the soil profile (Fig. 5) suggest that the

cyclical mechanism described in the previous paragraph

also occurs.

Fig. 7 d2H vs. d18O plot for all groundwater samples, spring, San José Gulf Sea and the local meteoric water line

Fig. 8 rCl- vs. d18O plot for

the marsh and bajada

groundwater samples and the

San José Gulf Sea. The arrow

shows marsh samples with no

significant change in isotope

composition but significant

variations in chloride

concentration

Environ Earth Sci

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Proposed conceptual model

The chemical evolution of solutes in arid continental

environments has been explained by Fitzpatrick et al.

(2000) for Australia by means of a salinization cycle that

involves salt accumulation on the ground, evaporation,

total desiccation, precipitation–dissolution of minerals and

then the flushing of the accumulated salts into the unsatu-

rated zone.

In the case of the marsh in Playa Fracasso, it is a coastal

environment in an arid climate, with tidal influence and a

limited brackish continental inflow. Taking into consider-

ation that the high salinity of the upper centimetres of the

soil profile corresponds to an evaporation profile and sug-

gests the presence of precipitated salts, together with the

coincidence of the highest groundwater salinization peaks

with the highest water tables recorded in well PF11 and the

hydrodynamic, hydrochemical and isotopic characteristics

described, a simplified salinization model is proposed, as

shown in Fig. 9.

In this figure, the cycle begins with the increase in

groundwater level induced by normal tides (1), followed by

the capillary rise and evaporation of saline water and

evapotranspiration in the unsaturated zone (2 and 3), which

results in the precipitation of salts in the upper portion of

the soil (4). Then, at the high spring tides, the salt water

that floods the marsh dissolves the salts accumulated on the

surface and in the upper centimetres of the soil (5), and

transports them at low tide towards the saturated zone,

causing the hypersalinization of groundwater (6).

Conclusions

The combined analysis of the hydrodynamics, hydro-

chemistry and isotopic composition indicates that there is a

section within the high marsh in which the source of

groundwater is mainly marine, and another in which the

contributions are of mixed origin. The groundwater in the

marsh, the surrounding landforms, and the spring is sodium

chloride type water and it has very high salinity, however,

the analysis of the soil profiles indicates that the parent

material is not responsible for the high salinity of the

system.

Regarding the marine influence over the marsh, the

water table fluctuation records measured by the divers

show a recurrence every 28 days, indicating the scope—

even in the distal section of the marsh—of the extraordi-

nary tides with the widest ranges. But the rMg2?/rCa2?

ratios vs. rCl- made it possible to differentiate an area with

continental contribution, represented by the bajadas, the

sandy layer and the spring, and another with marine con-

tribution, constituted by the marsh. However, within this

area, point PF11 shows a mixed origin. The analysis of

rNa? vs. rCl- was not useful to differentiate marine and

continental contributions but it was helpful to understand

the salinization processes.

On the basis of the isotopic composition (18O and 2H), it

was possible to identify clearly the continental and mete-

oric origin of the groundwater associated with the land-

forms of the bajadas, fans and the spring, as well as the

evapotranspiration processes. When Cl- was considered, a

Fig. 9 1 Saline groundwater

rise by normal tide fluctuations;

2 capillary evaporation of rising

groundwater; 3

evapotranspiration; 4 soil

salinization/salt precipitation; 5

dissolution and 6 transport of

soluble salts into the unsaturated

zone. MW marine water, MGW

marine groundwater, CGW

continental groundwater, NSZ

non-saturated zone, SZ saturated

zone

Environ Earth Sci

123



cyclical mechanism was identified for the marsh samples,

involving evapotranspiration, precipitation and dissolution

with no changes in the isotopic composition, whereas in the

high part of the marsh the mixture with continental water

was observed. The origin of the groundwater in the bajadas

can be associated with local rainwater infiltration and

regional discharge.

On the basis of the cyclicity of the salinity fluctuations

and the association with extraordinary tides, the soil

salinity profiles and the salinization with no changes in the

isotopic composition of groundwater, a conceptual model

of salinization for the marsh with a cyclical mechanism of

evapotranspiration, precipitation and salt dissolution by

seawater leaching was defined, which could be useful to

understand other coastal wetlands under similar arid cli-

matic conditions.

Acknowledgments The authors would like to thank Agr. Eng.

Claudia Saı́n for her assistance in the laboratory and Tech. Julio César

Rua (Bocha) for his help in the field. This work has been funded by

the ANPCyT (PICT2008 No 2127) and the CIUNPAT-UNPSJB-

PROPEVA (PI No 842).

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http://dx.doi.org/10.1007/s10533-010-9570-y

	Groundwater salinization in arid coastal wetlands: a study case from Playa Fracasso, Patagonia, Argentina
	Abstract
	Introduction
	Study area
	Materials and methods
	Results
	Hydrodynamics
	Soil salinity profile
	Hydrochemistry
	Saline content
	Ionic composition

	Isotopes
	 delta 2H vs. delta 18O


	Discussion
	Proposed conceptual model

	Conclusions
	Acknowledgments
	References