WATER RESOURCES RESEARCH, VOL. 35, NO. 6, PAGES 1877-1894, JUNE 1999 Geochemical and boron, strontium, and oxygen isotopic constraints on the origin of the salinity in groundwater from the Mediterranean coast of Israel Avner Vengosh,1 Arthur J. Spivack,2 Yohanan Artzi, 3 and Avner Ayalon4 Abstract. In order to identify the origin of the salinityand formation of saline plumesin the centralpart of the Mediterraneancoastal aquifer of Israel, we determinedthe elementaland boron, strontium,and oxygenisotopic compositions of fresh and brackish groundwater(C1 up to 1500 mg/L). We distinguishbetween two key anthropogenic sources:(1) sewageeffluents used for irrigationwith high Na/C1,SO4/C1, and B/C1ratios andlow Br/C1 ratios relative to seawaterratios, low ;5•B values (0-10%o) and high ;5•80 values(>-4%0); and (2) importedwater from the Sea of Galilee that is artificially rechargedto theaquifer with high Br/C1 (3 x 10-3) and;5•80 values (-1%o) anda low 87Sr/86Srratio of 0.70753.The brackish groundwater from the salineplumes have relatively low Na/C1ratios (0.5-0.8) and high Ca/Mg, Mg/C1, and Ca/(SO4 q- HCO3) (> l) ratios relativeto seawaterratios; marine SO4/C1 and Br/C1 ratios; ;5•B values of 24.8-49.9%0; 18 87 86 ;50 of -2.95%0 to -4.73%0; and Sr/ Sr ratios of 0.708275-0.708532.The composition of most of the investigatedgroundwater from the saline plumesdiffers from those of the 87 86 anthropogenicsources, imported water, fresh uncontaminatedgroundwater ( Sr/ Sr of 0.70866,;5•B of 20-30%o),and saline water from the adjacentEocene aquitard. Only in ß 18 areas of artificial rechargedoes local groundwaterhave high Br/C1 and ;50 values that are typicalto the Sea of Galilee. The linear relationshipsbetween chloride and most of theions, including B andSr, the relatively high ;5•B (>30%0) and low ;5•80 (<-4%0) values,and the chemicalsignature of the salineplumes (e.g., marine Br/C1 and SO4/C1 ratios),suggest that (1) mixingprocesses control the chemicalcomposition of the brackish water within the aquifer, and (2) the salinepostulated end-member has a chemical compositionthat resembles modified seawater with a marineand higher ;5•B values, and a 87Sr/86Srratio of <0.7083.We proposethat most of thesalinization phenomena and the formationof salineplumes in the inner parts of the coastalaquifer are derivedfrom upconingof underlyingnatural salinewater bodiesand enhancedby overexploitationand draw-downof the overlyingfresh groundwater. 1. Introduction rechargeof salinewater) or nonpointsources (e.g., agriculture return flows,irrigation with sewageeffluent) derived directly Salinizationis one the most widespreadprocesses that de- from anthropogeniccontamination. Salinization can alsooccur gradeswater quality and endangersfuture water exploitation. becauseof "natural processes"such as seawaterintrusion and In manyareas, particularly in arid and semiaridzones, ground- saline-waterflow from adjacentor underlyingaquifers [e.g., water salinizationlimits the supplyof potablefresh water. This Maslia and Prowell,1990]. problemis intensifiedin coastalaquifers where human activi- The Mediterraneancoastal aquifer (Figure 1) is one of the ties (e.g.,water exploitation,agriculture, reuse of wastewater) importantwater resourcesin Israel, supplying20% of the na- result in acceleratingwater-quality deterioration. Monitoring tional water consumption.The salinityof groundwaterin this and identifyingthe origin of the salinityare crucialfor water aquifer has increasedduring the last few decades,and saline managementand remediation.Yet the variety of salinization plumeshave formed in its central region. Severalexplanations sources,particularly in unconfinedaquifers, makes this task to the riseof salinityin the aquiferinclude (1) recyclingof salts difficult.Groundwater salinization can resultfrom either point from irrigationof local groundwaterand/or evapotranspiration sources(e.g., leakageof industrialand domesticwaste water, of meteoricwater; (2) contaminationby irrigationand/or leak- age of wastewater;(3) rechargeand irrigationwith imported waterfrom the Seaof Galilee;(4) flowof salinewater from the •HydrologicalService, Jerusalem, Israel. 2Centerfor MarineSciences, University of NorthCarolina at Wil- adjacentEocene aquifer in the east;and (5) upconingof saline mington. water from underlyingsources [Magaritz et al., 1990;Mercado, 3jacob Blaustein,Institute for Desert Research,The Water Re- 1985; Ronen et al., 1987; Rosenthalet al., 1992; Vengoshand sourceResearch Center, Ben Gurion University of the Negev, Sede Rosenthal,1994; Vengoshand Ben Zvi, 1994]. Boqer, Israel. The purposeof the presentstudy is to establishthe mecha- 4GeologicalSurvey, Jerusalem, Israel. nism(s) for groundwater salinization and to identify the Copyright1999 by the American GeophysicalUnion. sourcesof salinity by utilizing an array of geochemicaland Paper number 1999WR900024. isotopictracers (boron, strontium, and oxygen).Each of these 0043-1397/99/1999WR900024509.00 isotopictracers has previouslybeen used separatelyto delin- 1877 1878 VENGOSH ET AL.: GEOCHEMICAL AND ISOTOPIC CONSTRAINTS A •0• Tel•viv/ ,• /' / Jerusalem .• Ashdod/ / ' B Figure 1. (a) A generalmap of the Mediterraneancoastal aquifer in Israel and (b) the studyarea with the salinespots. Wells locationsare marked by solid circles.Note the locationsof the salinespots and chloride concentrations(in milligramsper liter) of the most salinewells in that area. eate fluid origins and chemical modifications,for example, the aquifer is not confined.In most parts of the aquifer, the oxygenfor tracing the source and nature of the recharged unsaturatedzone is made of calcareoussandstone (kurkar), fluids[e.g., Gat, 1974, 1981],boron for identifyingthe impacts loam (hamra), and heavysoils. In its westernpart the unsat- of anthropogenicsources [e.g., Vengosh et al., 1994],and stron- urated zone consistsof sandstonethat is reflectedby the high tium for evaluatingwater-rock interactions [e.g., Banner et al., qualityof the local groundwater(<50 mg C1/L). In most areas 1989; Chaudhuri et al., 1987; Fritz et al., 1987; Johnson and the aquifer overliesthick and relativelyimpermeable units of DePaolo,1997a, b]. Yet integrationof isotopicdata for a va- the SaqiyeGroup. The SaqiyeGroup (Figure 2) is composed riety of elementsprovides a better diagnostictool for deter- of (1) Oligoceneand early Miocene marls of the Bet Guvrin mining the origin of the salinity. Formation;(2) middleMiocene marlsof the Ziqim Formation; (3) upperMiocene evaporite units (representing the Messinian 2. Hydrogeological Setting event);and (4) Pliocene-agedunits of alternatingthick marine The coastalaquifer lies along the Mediterranean coast of shalesand limestonesandstone layers of the Yafo Formation. Israel (Figure 1) and is composedof Pliocene-Pleistocenecal- Along the easternmargin the aquifer restsupon the Eocene careoussandstone (Figure 2) and layersof clays[Garish and (ShefelaGroup) aquitardin the southerncoast (Figure 2). Friedman, 1969]. Its thicknessvaries from 200 m in the west The annualvolume of groundwaterpumped from the aqui- along the coastlineto a few meters in the eastern margins, fer, startingfrom the 1950s,has been 400 x 106to 450 X 106 20-25 km east from the seashore.In the westernmargins the m3/yr,about one fifth of thetotal water consumption in Israel aquifer is dividedinto severalconfined aquifer systems(suba- [WaterCommissioner, 1995]. Overexploitation beyond the nat- quifers) with different piezometriclevels. In its central and uralreplenishment (---340 x 106m3/year) caused a continuous easternparts there are no continuouspartitioning layers and drop in piezometriclevels between the 1950sand late 1980s. VENGOSHET AL.: GEOCHEMICALAND ISOTOPICCONSTRAINTS 1879 W CoastalPlain aquifer dunes Mediterranean te•rrestrial marine rocks •lays J Calcareoussandstone •_ _• • (Kurkar• __=--•••?' [•-:•-- m••------••neclaysandshales I Age Group Formatiol• Hydrogeology ______----- YafoFormation Kurkar coastalaquifer • ' SaqiyeGroup QuarternaryIPliestocene 5 10 15 Yafo aquiclude Km from coast ß Mavqi'im (evaporites) Ziqim i 12;[Miøcene Bet-Guvrin aquitard øligøceneMaresha Adulam Figure 2. Schematichydrogeological cross section of the coastalaquifer and lithohydrologicalunits. Asa result, hydrologic depressions formedin the central areas, and are used for domesticuse and irrigation over the coastal changingthe natural east-to-westflow regime in the aquifer. plain.In addition,a largevolume (--•20 x 106m3/year during Since the early 1990s,pumping has been reduced and water the lastfive years) has been recharged into the the aquifer levelshave consequently increased. Nevertheless, groundwater sincethe mid 1960sthrough Lake Azriqa'm (an artificiallake salinization,which began during the 1950s,has continuedde- in an old quarry), northwestof Be'er Toviyya; Lake Merar, spitethe restorationof the water levelsin the last 10 years.In southof Giva't Brener;and rechargewells in the Giva't Brener the Be'er Toviyyaregion (Figure 1) the parallel increasein area. water level and salinity has continued since the late 1960s [Vengoshand Ben Zvi, 1994]. As a result, salineplumes and 3.2. Analytical Techniques spotshave developed in the innerparts of the aquiferand in its Elementalanalyses were carried out in the analyticai labo- easternmargins (Figure 1). The locationsof mostof the saline ratory .of the HydrologicalService in Jerusalem.Boron con- plumesare associatedwith the hydrologicdepressions in the centrationswere determinedwith a modified spectrophoto- central part of the aquifer.