cation exchange processes and human activities in unconfined aquifers

7/30/2019 Cation exchange processes and human activities in unconfined aquifers

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Cation exchange processes

and human activities in unconfined

aquifersG. Panagopoulos N. Lambrakis P. Tsolis-Katagas D. Papoulis

Abstract During the 19992002 water years, ahydrogeological research project was carried out onthe unconfined aquifer of Trifilia in the Peloponnese.Seawater intrusion due to overpumping, andintensive use of fertilizers caused the groundwaterquality degradation that is a typical case for thecoastal aquifers in Greece. Isopiezometric maps

along with ion distribution balances, iondistribution maps and factor analysis indicate theexistence of three zones of groundwater quality. Inthe first zone of saline water, a cation exchangeprocess between the Ca2+ of sediments and the Na+

of groundwater contribute to the formation of thewater type Na+-Ca2+-Cl). In the second zone, whichis considered as a transition zone, dominate theCa2+-Na+-HCO3

)-Cl) water type. In the third zoneof Ca2+-HCO3

)-SO42) water type, relationships

among Ca2+, SO42), NO3

) and NH4 can be attributedto the dissociation of ammonium nitrate and sulfatefertilizers on one hand, and Ca2+ derivation from

cation exchange processes between water, rocks andclay minerals, such as smectite and illite, on theother.

Keywords Cation exchange Nitrogenous fertilizers Nitrate contamination Clay minerals Factoranalysis Seawater intrusion

Introduction

The study area lies in the Trifilia province of southwesternPeloponnesus and covers a total area of 120 km2 (Fig. 1).The mean annual air temperature is 16.3 C, with aminimum of 11.1 C in February and a maximum of25.7 C in August. The mean annual precipitation atGargalianoi meteorological station is 784 mm (Panagop-

oulos and Lambrakis 2001). The dominant land use in thearea is cultivation of olives (83%), vegetables (12%, mainlytomatoes and watermelons), and greenhouse crops (5%).Irrigation demands within the coastal regions are coveredby exploiting the unconfined aquifer of Pleistocene sedi-ments through a large number of shallow wells and deeperboreholes.A widespread agricultural practice in the area is the use ofinorganic fertilizers, which pollute the groundwater.According to Sampatakakis and others (1994), the meanannual amount of fertilizers applied in the Trifilia area is1,850 Kg/ha. Compositions of these fertilizers havechanged during the last decades. In 1960, 7080% of the

fertilizers were ammonium sulfate [(NH4)2SO4], 1015%was ammonium nitrate (NH4NO3), 1015% was mixednitrogen (N), phosphorous (P) and potassium (K), and theremaining 510% were trace elements. In 1970, thesevalues changed to 5060, 2530, 2030 and 510%respectively, while in 1980 the percentage contribution was2030, 3040, 3040 and 10%, respectively.Nitrate contamination of groundwater is of environmentalconcern. One of the main sources of contamination is thehigh input of nitrogen fertilizers in intensive agriculture(Roth and Fox 1990; Johnson and others 1991). Yadav(1997) showed that 15% of the annually applied nitrogenfertilizer, and even greater percentages of residual nitro-

gen, were leached into groundwater. This load contributesto chemical, physical and biological processes.When applying factor analysis to the hydrochemical dataof a coastal aquifer, Gimenez (1994) found a factor with ahigh correlation coefficient between Ca2+ and NO3

). Heexplained this relationship as a result of processes devel-oped after the application of nitrogenated fertilizers. Theseprocesses add Ca2+ to the water as a consequence of alikely ion exchange between NH4

+ and Ca2+. Comber andothers (1967) refer that NH4

+ derived from nitrogenousfertilizer dissociation, can replace an equivalent quantityof Ca2+ in the interlayers of clay minerals and then reachesthe aquifer. A number of similar observations for Ca2+,

Received: 25 April 2003 / Accepted: 25 March 2004Published online: 29 April 2004 Springer-Verlag 2004

G. Panagopoulos (&) N. LambrakisGeology Department, Section of Applied Geology and Geophysics,University of Patras, 26500 Rio, GreeceE-mail: [email protected].: +3-2610-996294Fax: +3-2610-997782

P. Tsolis-Katagas D. PapoulisGeology Department, Section of Earth Materials,University of Patras, 26500 Rio, Greece

542 Environmental Geology (2004) 46:542552 DOI 10.1007/s00254-004-1064-6

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SO42), NO3

) and NH4+ interrelations have been referred in

the literature (Ruiz and others 1990; Covelli and others1998; Stigter and others 1998; Jeong 2001). In the hydro-chemical study of a coastal karst aquifer located in Malia,

Greece (Lambrakis and Papatheodorou 2001), a similarfactor exhibited high positive loadings on Ca2+, Mg2+,NH4

+, NO3) and SO4

2), suggesting a nitrate contributionof anthropogenic origin. Daskalaki (2002) studying thehydrochemical character and quality of groundwater inGreece, found a clear relationship between Ca2+ and NO3

)

in almost all the alluvial aquifers of Greece, and manysedimentary and metamorphic carbonate aquifers ofCentral and North Greece. Ca2+ was attributed to themineralogical composition of the aquifer, and NO3 toagricultural activities.The aim of this article is to examine the impact of inten-sive land use and fertilizer application in western Trifilia of

Peloponnesus, Greece, and contribute to the interpretationof the hydrochemical processes taking place in suchunconfined coastal aquifers.

Materials and methods

The impact of fertilizer application in western Trifilia wasinvestigated by studying the hydrogeology and hydro-chemistry of the unconfined coastal aquifer. The inventoryof the water points indicated that more than 300 shallowwells and deep boreholes exploit the aquifer. A series of 75

wells was used for piezometric measurements. Amongthese, 35 sample points were defined for chemical analyses(Fig. 2). Eight boreholes of the aquifer were tested forhydraulic parameter determination, and three core sam-

ples were collected to determine mineralogical composi-tion. Statistical processing of the groundwater chemicalanalyses data was made using R-mode factor analysis.

Sampling and hydrochemical measurementsSampling runs were undertaken biannually from May 1999to October 2001. The samples were collected at thewell-pump outflow in two polyethylene bottles. Allsamples were filtered on-site through 0.45 lm pore sizeMillipore filters. Unstable parameters such as pH, electricconductivity (EC), dissolved oxygen (DO) and redox po-tential (Eh), were determined on-site by using an ion/ECmeter with combined electrodes. The other analyses were

performed soon after collection in the Laboratory of Hy-drogeology, University of Patras. The first sample of 0.5 Lvolume was acidified with 2 ml of 65% HNO3 for cationanalysis. The second non-acidified aliquot (1 L) was usedto determine bicarbonates (HCO3

)) and chloride (Cl))using Hach titration kits. Sulfates (SO4

2)), nitrates(NO3

)) and ammonium (NH4+) were determined using a

spectrophotometer. Finally, the major cations Ca2+, Mg2+,Na+ and K+ were determined by atomic absorption spec-troscopy. The overall precision of the analyses is within5%. Processing of the data was carried out using asoftware package developed in the Laboratory of Hy-drogeology, University of Patras (Lambrakis 1991).

Environmental Geology (2004) 46:542552 543

Fig. 1Geological map of western Trifilia (Perrier 1980,Filiatra sheet)

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A statistical overview of the measured parameters is pre-sented in Table 1 (May 2001).

Mineralogical compositionThree core samples (S1, S2, S3) were collected from thePleistocene sediments of the study area (Fig. 1). Themineral compositions of the clay fraction (


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extract as many factors as the ruling theory demands.According to this criterion, three factors have been chosenfor this study, which together explain more than 76% ofthe total variance (Table 2). Apart from Eh, all the other

parameters have high communality (0.650.98), whichshows that the 3-Factor model describes them well. Thecorrelation coefficients between variables and the factors(loadings) are listed in Table 2.

Geological and hydrogeologicalsetting

The bedrock of the research area belongs to the TripolisSeries and is composed of a thick sequence of Eocene anddolomitic limestones overlain by Oligocene flysch (Perrier

1980). Pleistocene and Holocene deposits unconformablyoverlie all of these sediments (Fig. 1). The Pleistocenesediments host an extensive unconfined aquifer and arecomprised mainly of calcitic sandstone and marly lime-stone, with a thickness ranging from 30 to 100 m, whichwere deposited in a shallow marine environment.The piezometric map (Fig. 5) provides a good descriptionof the aquifer and shows that the general groundwater flowis SW towards the coast of the Ionian Sea. The aquiferpresents low hydraulic loads, close to zero or negativelevels, in its coastal sector. Towards the inland and to theeast, the water table has positive values that reach 93 m.The hydraulic gradients are smaller in the west than theeast, and in the wider area of Filiatra and Marathopolisvary between 1.2 and 2.5%. Eastwards, in the area ofChochlasti, gradients are greater and vary between 5 and7%. This is due to the smaller transmissivity in the easternpart of the aquifer compared to the western part, as proved


Table 2Eigenvalues and factor loadings for the Varimax Rotated 3-FactorsModel

Variables Factor 1 Factor 2 Factor 3

pH )0.74D.O 0.79Eh 0.62E.C 0.94Ca2+ 0.88Mg2+ 0.93Na+ 0.97K+ 0.89

HCO3)

)0.65Cl) 0.96SO4

2) 0.77NO3

) 0.89NH4

+ 0.81Eigenvalues 5.1 3.0 1.8% total

variance39.4 23.4 13.8 Fig. 5

Piezometric map of the unconfined aquifer of Pleistocene deposits(May 2001)

Fig. 4X-ray powder diffraction pattern of core sample S2 (


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by pumping tests. These tests show that transmissivityvaries between 410)3 m2/s and 4.910)2 m2/s in thewestern part.The water balance parameters have been computed basedon the procedures described by Thornthwaite and Mather(1955). The potential evapotranspiration has been com-puted based on temperature data using Thornthwaitesformula (1948). The average annual volume of actualevapotranspiration is 23.5106 m3 and corresponds to

48.1% of the annual precipitation. The mean annualvolume of the areas surface runoff is 14.7106 m3 andcorresponds to 30.2% of the mean annual precipitation,while the remaining 10.6106 m3 (21.7% of the meanannual precipitation) is the recharge groundwater.

546 Environmental Geology (2004) 46:542552

Fig. 6Piper plot showing the chemical types of groundwater in theunconfined aquifer

Fig. 7Distribution map of the three different water types of the unconfinedaquifer

Table 3Statistical hydrochemical overview of the three different groups (May 2001). Concentration is given in mg/L, otherwise indicated

Parameter Group 1 Group 2 Group 3

Average Std_Dev Average Std_Dev Average Std_Dev

n=8 samples n=14 samples n=13 samples

E.C (lS/cm) 2,292 539 1,435 428 1,442 869PH 7.4 0.4 7.3 0.4 7.2 0.5Eh (mV) 162 80 167 48 156 69D.O 6.6 1.9 5.0 2.3 6.1 1.7Ca2+ 180.3 40.6 158.3 34.3 209.3 73.8Mg2+ 36.9 5.8 22.1 15.4 17.8 7.2Na+ 274.6 87.6 85.6 41.2 68.2 90.6K+ 11.0 4.3 4.4 2.7 3.3 3.2HCO3

) 252.1 50.6 309.7 52.1 308.6 51.0Cl) 555.3 148.9 145.5 87.1 112.0 177.7SO4

2) 148.7 45.1 140.9 87.0 210.4 138.3NO3

) 95.7 34.1 71.6 35.7 141.9 82.5NH4

+ 1.06 1.16 0.63 0.76 1.43 1.34

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Results and discussion

Groundwater chemistryBased on the Piper diagram (Fig. 6), three different watertypes can be discerned in the study area. These are thefollowing: Na+-Ca2+-Cl) water type (Group 1), Ca2+-Na+-HCO3

)-Cl) water type (Group 2) and Ca2+-HCO3)-SO4

2)

water type (Group 3). Their distribution is shown in Fig. 7.The Na+-Ca2+-Cl) water type occupies the coastal zone(zone 1) and is dominated by Na+. The water samples ofthis zone are characterized by higher electrical conduc-tivity and Na+, K+, Mg2+ and Cl) concentrations comparedwith samples from the two other types (Table 3). Thechloride distribution map (Fig. 8) shows three regions inthe coastal zone with high chloride concentrations (Filia-tra, Agia Kyriaki, Marathopolis), while the highest con-centration (844 mg/l) is found in the Marathopolis region.The increased salinity of the groundwater in this zonecould be attributed to seawater intrusion due to the lowhydraulic load of the aquifer here. This low hydraulic load

(Fig. 5) is caused by the overexploitation of the aquifer tomeet increased irrigation demands and to the lack ofrational water management.Seawater intrusion causes ion exchange phenomena be-tween brackish water and the aquifers matrix. Presumingthat Ca2+ is the dominant ion, as is the case for thesediments of the study area, the following equations candescribe the above-mentioned process (Appelo andPostma 1993):

Na 1=2Ca X2 ! Na X 1=2Ca X2 1

where X is the soil exchanger.By calculating a composition based on conservative mix-

ing between seawater and freshwater of the area (Appeloand Postma 1993), we obtained the concentration of thedifferent ions as deficits (negative numbers) and excesses(positive numbers, Table 4), expressed as Dion=Cm Cex,where Cm is the measured concentration and Cex is theconcentration expected by freshwater seawater conser-vative mixing. Table 4 shows that all groups have excess inCa2+ concentration, while Na+ is obviously in deficit in thefirst group and in excess in the other groups.


Fig. 8Chloride distribution map

Table 4Recalculated water analyses of Table 3, showing the extent of reactions (cation exchange). Values in mmol/l

Parameter Group 1 Group 2 Group 3

Average Std_Dev Average Std_Dev Average Std_Dev

Dion n=8 samples n=14 samples n=13 samples

Ca2+ 3.34 1.51 2.39 0.69 3.34 1.72Mg2+ )0.11 0.26 0.36 0.29 0.34 0.16Na+ )2.20 1.28 0.24 0.91 0.22 0.45K+ )0.18 0.08 0.01 0.06 0.02 0.04HCO3

) 0.92 0.80 2.18 0.66 2.00 0.78Cl)

SO42) 1.06 1.51 0.94 0.55 1.65 1.08

NO3) 1.70 0.65 1.32 0.76 1.98 1.44

NH4+ 0.08 0.06 0.04 0.04 0.07 0.08

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Similar observations can be seen in Fig. 9a and b where thestraight line is the mixing line between the areas sea andfresh waters. Samples from the first zone belonging to thefirst group show ion exchange phenomena as describedabove, due to seawater intrusion.The chemical composition of group 2 with water type(Ca2+-Na+-HCO3

)-Cl)), is characterized mainly by thedominance of Ca2+ and lower salt content compared to thefirst water type. Groundwater becomes enriched with ionssuch as Na+ and Cl) (Table 3) from dissolving mineralsthat move along a flow line towards the sea.The third group, water type Ca2+-HCO3

)-SO42), has sam-

ples with a chemical composition dominated by Ca2+ and

HCO3) ions that characterizes fresher waters. Thus, zone

three can be considered as the recharge zone, while thesecond zone can be considered as a transitional zone be-tween fresh and saline water. The SO4

2) ion participationin this groups water type can be attributed to the con-tamination of groundwater by fertilizers. Table 3 showsthat the maximum values of NO3

) concentrations also

belong to the samples of this group. The excess in Ca2+

concentrations can be attributed to ion exchange phe-nomena due not to seawater intrusion, because thehydraulic load is high and Na+ concentrations are also inexcess, but to NH4

+ ions due to extensive application offertilizers (Comber and others 1967; Ruiz and others 1990;Gimenez 1994; Covelli and others 1998; Jeong 2001).

R-mode Factor AnalysisFollowing the application of R-mode Factor Analysis, athree-factor model was produced that accounted for morethan 75% of the total variance (Table 2). Factor 1 accountsfor nearly 40% of the total variance and has high positive

(0.890.97) loadings for EC, Mg

2+

, Na

+

, K

+

and Cl

)

, andmoderate negative loading for HCO3) (Table 2). The spa-tial distribution of factor scores for factor 1 (Fig. 10a) hasa discernible pattern; the higher scores occur in the coastalzone (zone 1 of Fig. 7), moderate scores are present in thetransitional zone (zone 2 of Fig. 7), and lower scores occurin the aquifers recharge area (zone 3 of Fig. 7). Thisparticular pattern in conjunction with high positive load-ings of EC, Mg2+, Na+, K+ and Cl), reflects the intrusion ofseawater in the coastal zone of the unconfined aquifer.Factor 2 accounts for 23.4% of the datas total variance andexhibits high positive loadings (0.770.89) on Ca2+, SO4

2),NO3

) and NH4+ (Table 2). The spatial distribution of this

factor shows that the area with higher factor scores has anelliptic shape and is located in Chochlasti region(Fig. 10b). Together with the loading profile, this geo-graphical pattern suggests that the second factor is acontamination factor of anthropogenic origin (fertilizers).Factor 3 accounts for 13.8% of the total variance. Theinterrelationship between the variables associated with thisfactor, dissolved oxygen (DO), Eh and pH, is well known(Freeze and Cherry 1979).

Nitrate contaminationDue to extensive use of fertilizers in the area, thegroundwater contains an increased concentration of NO3

)

and NH4+ (Fig. 11a, b), especially in the third zone (zone 3

of Fig. 7) of the aquifer (Table 3), with maximum valuesseen in Chochlasti region (290.4 and 3.73 mg/l respec-tively). Ca2+ and SO4

2) display a similar distribution pat-tern to NO3

) and NH4+ in the region of Chochlasti with

values of 340.8 and 460 mg/l respectively (Fig. 12a, b;Table 3), suggesting close relationships between thesechemical species.These characteristic chemical distribution patterns of thegroundwater are similar to the results of the R-mode factoranalysis shown in Fig. 10b. Nitrate, ammonium, calciumand sulfate distribution maps (Figs. 11a, b; and 12a, brespectively) also show higher concentrations of these ionsin the same region. The cross-plots in Fig. 13 support the


Fig. 9a,bCross-plots of Na+ (a) and Ca2+ (b) concentrations vs. Cl) contents.Lines indicate freshwater-seawater conservative mixing

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existence of strong relationships among these parametersin zone 3 of the aquifer, as the correlation coefficient be-tween Ca2+ and total nitrogen (NO3

)+NH4+) is extremely

high (0.958), while the correlation coefficient between Ca2+

and SO42) is 0.757.

The relatively higher concentrations of nitrates andammonium in zone 3 could be attributed to the intenseapplication of nitrogen fertilizers to the soil, mostlyNH4NO3 and (NH4)2SO4. Thus, the applied fertilizers movethrough the unsaturated zone and infiltrate into theaquifer. Nitrate and sulfate salts are very soluble in water(over 10 g/l at 20 C; Hatziioannou 1985) and thus disso-

ciate according to the following reactions:NH4NO3 H2O ! NH4OH H

0 NO3 2

NH4 2SO4 2H2O ! 2NH4OH 2H SO24 3

causing an increase of nitrate and sulfate concentrations,which is facilitated by the oxidizing environment of theaquifer. NH4OH is a weak base and dissociates accordingto the following reaction (Hatziioannou 1985):

NH4OH $ NH4 OH

4

that causes an increase of ammonium concentration in theaquifer.

The relative high concentration of Ca2+ in zone 3 and itsrelationship with NH4

+ could be attributed to ionexchange reactions between water, rock and the clayminerals smectite and illite.Due to the dominance of calcite in the Pleistocene sedi-ments of the study area, it is reasonable to assume thatmost of the interlayer sites of smectite were occupied byCa2+. In such an environment, Ca2+ could replace the ca-tions which were originally present in the interlayer ofsmectite. The high NH4

+ concentration in the water sam-ples of zone 3, could promote the saturation of Ca-smectitewith NH4

+ because the replacing power of NH4+ is greater

than that of Ca2+

(Grim 1968), according to the ion ex-change reaction:

Ca - clay 2NH4 $ 2NH4 - clay Ca2aq: 5

Thus, the interlayer Ca2+ cations of smectite areexchanged out and liberated in the groundwater.This exchange of cations could explain the enrichmentof water samples of zone 3 in Ca2+ and the observedpositive correlation between Ca2+ and NH4

+. Illite mayalso contribute to the enrichment of Ca2+ in the aquifer,though to a lesser extent, due to its lower cation exchangecapacity.


Fig. 10a,bSpatial distribution of factor 1 (a) and factor2 (b) in the study area

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Conclusions

The hydrogeological and hydrochemical study of thewestern Trifilia region reveals marked seawater intrusionin the unconfined aquifer of Pleistocene sediments in thecoastal zone, due to the reduction of the water levelbelow sea level caused by intensive pumping. As a result,Na+, Ca2+ and Cl) ions dominate the groundwatercomposition in this zone. Two further zones, the secondand third, were defined in the region. These two zoneslack direct sea influences on groundwater chemistry. Ionbalances showed that an excess of Ca2+ concentrations inthe water samples of the first zone can more or less beequilibrated with Na+ and K+ concentrations that are indeficit due to ion exchange phenomena caused by theseawater intrusion. Ca2+ concentrations are still in excessin water samples of the other two zones and present ahigh correlation with nitrogen. The intensive land useand the nitrogen-based fertilizers applied in the studyarea have resulted in high nitrate, sulfate and ammoniumconcentrations (290.4, 460 and 3.73 mg/l respectively) inthe groundwater of the third zone. It is suggested thatthe hydrochemical composition of the groundwater isinfluenced by cation exchange processes, which arefacilitated by the presence of the clay minerals smectite

and illite in the Pleistocene sediments of the unconfinedaquifer.The high amount of NH4

+ in the water of zone 3,coupled with its high replacing power, promotes thesaturation of smectite with NH4

+. Cations, e.g. Ca2+,which were in the interlayers of smectite and, to a lesserdegree, illite, and that were subsequently substituted byNH4

+, were liberated into the groundwater. This cationexchange process leads to the high correlation betweenCa2+ and NH4

+ in samples contaminated by nitrates andsulfates.

Acknowledgements The current research was conducted on theframes of K. Karatheodori project, financed by the ResearchCommittee of Patras University. The first author would like tothank the Greek State Scholarships Foundation for theeconomical support of his research.

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Original article

cation exchange processes and human activities in unconfined aquifers

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