Home >Documents >ENVIRONMENTAL TRACING IN COASTAL AQUIFERS: OLD …aguas.igme.es/igme/publica/tiac-02/ÁREA...

ENVIRONMENTAL TRACING IN COASTAL AQUIFERS: OLD …aguas.igme.es/igme/publica/tiac-02/ÁREA...

Date post:21-Jun-2020
Category:
View:0 times
Download:0 times
Share this document with a friend
Transcript:
  • 79

    ABSTRACTE n v i ronmental tracing, based on the use of physical, chemical and isotope parameters, is avalid support in the re c o n s t ruction of aquifer conceptual models under natural and exploita -tion conditions. In the case of coastal aquifers, the phenomenological approach gets compli -cated due to the overlapping effects of peculiar natural conditions and human activities.H y d ro geochemistry and isotope geochemistry deal with essential questions concerning sea -water intrusion: the role of different natural and human sources of salinisation, the conse -quences on water quality, aquifer pro p e rties and pollutant transport, the evolution of the phe -nomenon in time and space, the time and conditions for aquifer recovering. The paper dealswith the geochemical and isotope methods applied in the last decade in the field of seawateri n t rusion and with future potential goals of environmental tracing as well.

    Key WordsEnvironmental tracer; hydrogeochemistry; isotopes; coastal aquifer; seawater intrusion;salinisation; SGWD; heavy metals; contaminants.

    RESUMENLos trazadores medioambientales, basados en el uso de parámetros físicos, químicos e iso -tópicos, son un apoyo válido en la reconstrucción de modelos conceptuales de acuíferos bajocondiciones tanto naturales como influenciadas por la explotación. En el caso de acuíferoscosteros, la aproximación fenomenológica es más complicada debido al solape de los efec -tos de las condiciones naturales peculiares de este tipo de acuíferos y las actividades huma -nas. La hidrogeoquímica y la geoquímica de isótopos tratan con cuestiones esenciales refe -ridas a la intrusión de agua de mar: el papel de las diferentes fuentes de salinización, tantonaturales como antrópicas; las consecuencias sobre la calidad del agua, las propiedades delos acuíferos y el transporte de contaminantes; la evolución del fenómeno espacial y tempo -ralmente; el tiempo y condiciones que requiere la recuperación del acuífero. Esta ponenciatrata sobre los métodos geoquímicos e isotópicos aplicados en la última década en el campode la intrusión de agua de mar y sobre los objetivos potenciales para el futuro de la técnicadel uso de los trazadores medioambientales.

    Palabras claveTrazador medioambiental, hidrogeoquímica, isótopos, acuífero costero, intrusión de agua demar, salinización, SGWD, metales pesados, contaminantes.

    ENVIRONMENTAL TRACING IN COASTAL AQUIFERS: OLD PROBLEMS AND NEW SOLUTIONS

    Maria Dolores FidelibusAss. Professor of Applied Hydrogeology, Department of Civil and Environmental Engineering,

    Bari Polytechnical University, Bari, Italy

    E-mail:[email protected]

  • INTRODUCTION

    The interpretation of time and space vari-ability of physical, chemical or isotopic parame-ters measurable in ground waters, can allowrecognising the processes presently acting orwhich were active in the past within (and outside)the aquifers. Hydrogeologists define the entiregroup of such parameters environmental tracers.Whether or not their characteristics vary or mod-ify in time and in space, they are always strictlylinked to the history of ground waters.

    The use of environmental tracers in thestudy of seawater intrusion is relatively recent,but a lot of work has already been done, especial-ly in the last decade. In fact, as usual in the devel-opment of sciences, the urgent needs force andspeed up the research of solutions: so, under thepressure of economic and social demand, in thelast years the problem of the use and protection offresh water resources in the over- p o p u l a t e dcoastal areas received more attention than in thepast. Notwithstanding the research progress, mainproblems concerning coastal aquifers and seawa-ter intrusion have not been completely solved.

    Therefore, questions concerning the maxi-mum amount of fresh water can we exploit in acoastal aquifer, why and where does seawaterintrusion occur, how does it evolve in time in rela-tion to natural and human impacts, remain openquestions, especially when dealing with the largevariety of hydrogeological environments.

    For replying to the above questions, the con-ceptual model of the coastal aquifer, based on aphenomenological approach, must be outlined inadvance, especially for the eventual subsequentdevelopment of mathematical and numerical mod-els coping with both flow and reactive transport.

    Classical hydrogeological studies generallylead to more than one possible model: the envi-ronmental tracing plays the important role ofselecting the more reliable one.

    In the last decade the situation of coastalaquifers has deeply deteriorated, especially in theMediterranean area: presently, hydrogeologistsinvolved in the studies of seawater intrusion have

    to deal with the serious and growing effects ofhuman activities, which superimpose on theeffects of climatic change. As a result of theabove factors, natural flow regimes of coastalaquifers have been sometimes deeply disturbed,leading, sometimes, to the mobilisation of freshand/or saline palaeo-waters, previously isolatedfrom active flow, and to pollution short-cuts.Hence, coastal aquifers revealed concealed fea-tures and the progress of researches led to newrelevant questions:• Is seawater the only source of salinisation?• Is seawater the only fluid involved in

    s a l i n i s a t i o n ?• What is the origin of saline fluids found in

    coastal aquifers? How old are they? • What are the effects of seawater intrusion on

    groundwater quality and aquifer properties?• Can we recuperate salinised groundwater? • How much time does restoration of original

    water quality require?• What is the behaviour of pollutants under

    seawater intrusion?• What are the chemical and isotopic features

    of groundwater discharging into the sea? Groundwater pollution, over- e x p l o i t a t i o n

    and climatic change make matters worse and envi-ronmental tracing has to play an important role indeciphering many crucial dilemmas. However,hydrogeologists are used to make the best of a badb a rgain: so, in the last decade, they brought in newmethods, borrowed both from the fundamentaldisciplinary fields of chemistry, physics and geol-o g y, and from other applied disciplines related toearth sciences. Moreover, as occurred for tritiumin the sixties, many pollutants have risen today tothe role of tracers (man-made tracers).

    In the last decade, many reviews concernedgroundwater salinisation and/or seawater intru-sion. Richter and Kreitler (1993) dealt with theproblem of groundwater salinisation in varioustypes of aquifers of U.S.A. and gave a detailedreview of geochemical and isotopic methods fordistinguishing the various sources. More recently,Jones et al. (1999) proposed a general overviewof geochemical investigations in coastal aquifers;

    80

    HIDROGEOQUÍMICA E ISÓTOPOS

  • moreover, in Tulipano and Panagopoulos (Eds.,2003) a review of application of environmentaltracers to coastal karst aquifers can be found. Theabove-mentioned books and papers represent areliable reference for the fundamental informa-tion they contain.

    The present paper does not pretend to tack-le all the matter concerning the geochemical andisotopic study of seawater intrusion. The follow-ing paragraphs will only deal with some of the oldand new problems, through the selection of themore outstanding aspects of the current researchand the outline of the new perspectives and possi-ble goals of environmental tracing in the con-cerned context. All the numerous recent papersconcerning environmental tracing in coastalaquifers are worthy of mention: here text limitswill allow commenting only a few.

    SOURCES OF SALINISATION INCOASTAL AQUIFERS

    The study of groundwater salinisation incoastal aquifers is an apparently simple task, beingin most cases present seawater the most obvioussaline end-member. Nevertheless, groundwatersalinisation can derive either from other natural saltsources different from present seawater or fromhuman impact. The question is not singular,because different mechanisms of groundwatersalinisation require different remediation measures.

    Custodio (1997) provided a guide for study-ing seawater intrusion and listed the salt sources,which, besides present seawater, can be involvedin the salinisation process in coastal aquifers.Emblanch et al., 2003, propose a recent reviewwith reference to karst coastal aquifers. Stuyfzandand Stuurman (1994) recognise almost 11 sourcesof salt menacing groundwater in the Netherlands:agriculture, direct seawater intrusion from NorthSea, evaporation, hyperfiltration, infiltration ofnon-marine polluted surface waters, leaching ofrock salts, local pollution, mixing, marine trans-gressions, sea spray and sedimentation (syngene-sis with sediments).

    The ideal tracer for the detection of the ori-gin of groundwater salinisation should possess afew basic characteristics like very low concentra-tions in the fresh water component and distinctiveconcentrations in the different salt end-members.Besides, these concentrations should be suffi-ciently large for being measured. In addition, thetracer cannot be applied without the completeknowledge of its spatial and temporal variationsand without the assessment of its conservativebehaviour in the different hydrogeological envi-ronments. The different salt sources have differ-ent geochemical and/or isotopic imprints, and amulti-tracing approach normally allows therecognition of their involvement.

    Actually, coastal aquifers contain, more fre-quently than expected and according with theirgeological history, saline fluids different frompresent seawater. In most cases, the saline fluidcorresponds to seawater that entered the aquifersduring previous transgressions and resided for aperiod long enough for the fluid-composition tobe modified through water-rock interactionprocesses. The geochemical diagenesis of intrud-ed seawater gives origin to saline fluids whosechemical features resemble those of saline watersfound in large sedimentary basins (Aquilina et al,in prep.). Sometimes old brines of non-marineorigin, having complex geochemical history, canbe met as well.

    These saline fluids are normally still andbecome manifest under over-exploitation; fre-quently they are recognised as end-members inthe brackish coastal spring waters. The first find-ing reveals the disruption of the natural equilibri-um of both fresh and saltwater flow systems. Thepresence of imprints of saline fluids differentfrom present seawater in coastal discharge watersindicates, instead, that such salt waters belong toa regional flow system. Salt water componentswhich leave the aquifer through coastal dischargehave to be replaced by present seawater: it meansthat present seawater enters the aquifer alongselected intrusion fronts and exit the aquifereither, rapidly, along the same fronts or, after along residence time, along different fronts.

    81

    ENVIRONMENTAL TRACING IN COASTAL AQUIFERS: OLD PROBLEMS AND NEW SOLUTIONS

  • Seldom there is the opportunity to easilyrecognise and sample the salt sources that havethe potential of being involved in the salinisationprocess. When the chemical and/or isotopic char-acteristics of a potential salt source are known, itsinvolvement in the mixing can be traced back, ifthe tracers that distinguish it from other sourcesbehave as conservative tracers. This is not an easytask. In fact, salinisation (mixing of two fluids orsolution of salts) normally leads to the activationof water-rock interaction processes, which effectsoverlap those of simple mixing or salt solution.Therefore, chemical composition of watersderived from the mixing at different proportionsof fresh and salt waters rarely matches the com-position defined the conservative mixing. Forsake of simplification, it is better to calculate theconservative mixing using fresh water and pres-ent seawater typical of the hydrogeological sys-tem. Thus, deviations from this mixing lineinclude information both on end-members differ-ent from present seawater and water-rock interac-tions overlapping the mixing.

    When direct information on the characteris-tics of the potential salt source is lacking, itsinvolvement can be only assumed, if the effects ofwater-rock reactions overlapping the mixing canbe distinguished. In the worst case, more than onesalt source is involved.

    Direct recognition of saline fluidsdifferent from present seawater

    The direct sampling of salt waters inlandthrough a net of observation wells drilled for thecontrol of seawater intrusion (Fidelibus and Tuli-pano, 1996) was performed for the study of thekarst coastal aquifer of Salento Peninsula (Puglia,Southern Italy). The chemical composition of thesaline fluids resulted modified with respect topresent seawater due to water-rock interactionwith carbonate rocks. Mainly dolomitisation isresponsible for the decrease of Mg/Ca ratiofrom values of 6 (recently intruded seawater) to2 (figure 1a), being such a decrease closely con-nected to the relative ageing of seawater, as indi-cated by 14C data (figure 1b). Minor constituentsas well (Li, B and Sr) result enriched with respectto present seawater according to increasing resi-dence times.

    Direct sampling allowed Ng and Jones(1995) distinguishing the various saline waterssampled in the dolostone aquifer of Gran CaymanIsland: Mg/Ca ratio of such waters ranges from6.75 (present seawater) to 2.33 (evolved seawa-ter). The Authors attribute the ratio decrease towater-rock interactions, which take place as soonas seawater enters the carbonate system.

    The occurrence of saline waters of long res-idence time in many coastal aquifers of British

    82

    HIDROGEOQUÍMICA E ISÓTOPOS

    Figure 1 - (a) Mg/Ca ratio for fresh and salt waters sampled along observation-wells; (b) Mg/Ca ratio as to 14C contents(percent of modern carbon) (Salento karst coastal aquifer - Southern Italy); (from Fidelibus and Tulipano, 1996).

  • Isles is highlighted in a paper of Darling et al,1997, who reviewed a number of isotopic data(1 8O, D and 1 4C) concerning ground watersbelonging to various basins ranging from Car-boniferous to Lias age.

    Extrapolation of saline fluidscharacteristics

    When only data concerning salinisedground waters are available, cross plots of majorand minor ions as to Cl concentration should givethe first indication about both the non-conserva-tive behaviour of constituents and the existence ofeventual saline end-members different from pres-ent seawater.

    Fidelibus et al. (1992) used the deviations ofion concentrations from conservative mixingbetween fresh water and present seawater (sur-plus and/or deficits, *ion) in the recognition of thesaline end-members responsible of salinisation ofthe ground waters flowing in the plio-quaternarydetrital aquifer of Castellon Plain (Spain). Thedeviation trends of Ca, Na, K, Mg and sulphatesas to chloride increase allow revealing that chem-ical composition of ground waters originate fromthe mixing, accompanied by ionic exchange, of

    three main components: fresh water, present sea-water and a water having a CaMgClSO4 facies.The over-exploitation, depending on the perme-ability conditions of the plio-quaternary aquiferalong the coast, determines lateral seawater intru-sion or the withdrawal of sulphate waters fromdepth. This last occurrence comes highlightedthrough the comparison of ∆SO4, ∆Sr and ∆Limaps: the excesses of the three parameters, whichbehave as conservative tracers of sulphate watercomponent, coincide.

    Fidelibus and Tulipano (1996) used thesame method in the recognition of water-rockinteraction processes and saline end-members inthe karstic carbonate aquifers of Puglia, SouthernItaly. The most significant information comesfrom lithium deviations (figure 2): lithiumbehaves as a conservative tracer of the differentsaline fluids involved in the mixing, being alwaysmore concentrated in the modified seawater (RI =up to 700 *µ/l) with respect to present seawater(160 *µg/l).

    The study leads to the identification, in thewaters of the coastal brackish springs, which dis-charge close to the contact between carbonate for-mation and the thick clay deposits filling the

    83

    ENVIRONMENTAL TRACING IN COASTAL AQUIFERS: OLD PROBLEMS AND NEW SOLUTIONS

    Figure 2 - Enrichments and depletion related respectively to major and minor ions calculated for coastal spring waters ofMurgia aquifer with respect to conservative freshwater-saltwater mixing (zero line). RI and B represent two saline end-members. Spring groups are ordered according to their progressive position along the coasts and, within each group ofsprings, according to TDS increase.

  • graben of "Fossa Bradanica", of a salt water com-ponent coming from the carbonate basementburied under the clay deposits and that spent therethousands of years. The volume of old salt waterdischarging into the sea needs to be replaced bynew volumes of seawater. The conclusion is thatwithin the Murgia and Salento aquifers a regionalcirculation of salt waters should exist: presentseawater enters from limited intrusion fronts andflows, with a very low migration velocity,towards other coastal fronts (extrusion fronts).

    As synthetised by Budd (1997) in studyingdolomitisation in carbonate islands, platforms, oratolls, the mechanisms able to cause a flow ofseawater in coastal aquifers relate to differencesin hydraulic head or differences between fluiddensities (figure 3). Some of these mechanismscan be presumed to operate in the Murgia andSalento regional aquifers. The conceptualisationof these mechanisms represents an important ele-ment when dealing with the recognition of saltend-members and reconstruction of salt-water cir-culation within the carbonate coastal aquifer.

    Barbecot et al, 2000, extrapolate the charac-teristics of the saline end-member responsible ofsalinisation of the Bathonian and Bajocian coastalcarbonate aquifer of the Caen area (NorthernFrance) studying the geochemical evolution ofsalinised ground waters. The Authors find locallybrackish waters depleted in sulphates with respectto conservative mixing. The trend of mixing tends

    to a saline end-member (probably related to theFlandrian transgression) which might have beensubject to redox processes according to peatorganic matter oxidation: this is evidenced byboth high Br/Cl ratio and isotopic composition ofsulphates, which show a trend toward an end-member enriched both in 18O and 34S comparedto present seawater.

    Multiple salt sourcesGroundwater salinisation due to multiple

    salt sources is a frequent likelihood in SouthernMediterranean area: there, the diffuse presence ofevaporite deposits may cause a serious salinisa-tion superimposed to that due to the more com-mon salt sources, e.g. present seawater and salinefluids.

    Numerous studies mainly devoted to thestudy of the geochemical evolution of saline flu-ids in sedimentary basins proved the potential ofmajor, minor elements and classical isotopes indistinguishing the different salt sources. T h ereport of Richter and Kreitler, (1993) represents auseful review of literature in the related field.Many examples of multiple salt source recogni-tion may be found in the studies that are men-tioned in the following paragraphs with referenceto a specific salt source or method: they involvecommonly a multi-tracing approach.

    Just to exemplify how multiple sources arewidespread in coastal aquifers belonging to

    84

    HIDROGEOQUÍMICA E ISÓTOPOS

    Figure 3 - Schematic illustrations of six circulation mechanisms that can deliver Mg to potential dolomitisation sites: (A)tidal pumping, (B) seepage influx, (C) differential sea-surface elevation, (D) brine reflux, (E) coastal mixing zone andoutlying zone of entrained seawater and (F) thermal convection (from Budd, 1997).

  • Southern Mediterranean area, we can mention thepapers of Fakir et al. (2002) and Sanchez Martoset al. (2002). The former paper deals with therecognition, within the carbonate coastal aquiferin the Sahel of Oualidia (Morocco), of the multi-ple sources and mechanisms of salinisation, byusing, as natural tracers, bromide, strontium,nitrates and sulphates. The Authors identify morethan one mechanism of salinisation: seawaterintrusion in the coastal part, contamination fromthe surface by chlorides and nitrates, washingaway of the gypsum marl formation and rise fromthe depth of sulphate waters present in the reser-voir of Jurassic evaporite. In the latter paper, con-cerning the complex hydrogeological system ofthe Lower Andarax River Basin (Almeria, Spain),the salinisation has been characterized by jointlystudying the content of the minor ions B, Br andLi and a series of ionic ratios like B/Li, SO4/Cl,Na/Cl and Cl/B. The combined analysis of B andLi enabled the identification of the diff e r e n tmechanisms of salinisation: flushing of salinewaters from sediments of marine origin, seawaterintrusion and evaporite solution result variablyresponsible of salinisation.

    Temperature of groundwater: aphysical tracer for visualisinggroundwater salinisation

    Temperature measurements along deepwells are usually carried out for evaluating hearthheat flow. For this purpose, both temperature gra-dient and thermal conductivity of rocks must beknown; moreover, it is assumed that flow isentirely conductive, the regimen is stationary andno heat is transported for convection from wateror other fluids.

    The presence of groundwater flow involvesa convective time-depending transfer of heat,which modifies the thermal field. The effects ofthe movement of the water on the conductive heatflow can be as small to be insignificant, as largeto completely dominate the field of the tempera-tures. Such effects represent a not minor compli-cation in the evaluation of conductive heat flow.However, they turn out meaningful for hydroge-

    ologists, because they represent an indication ofwater flow. An appreciable water flow reduces, infact, the amount of the heat flow at the groundsurface, thus causing in the subsurface a tempera-ture gradient different from that determined byconductive flow alone. Within an aquifer, ground-water temperature modifies continuously in spaceand time in relation with the characteristics of thehydrogeological environment. The in depth trendof isotherms, reconstructed through the interpola-tion of data related to thermal profiles carried outalong wells, supply useful information on pat-terns of groundwater circulation. Vertical, hori-zontal and 3D representations of groundwatertemperature are useful in recognising the rechargeareas, in distinguishing zones of active flow fromstagnant ones and in outlining the preferentialflow directions.

    In coastal aquifers, groundwater tempera-ture distribution can be of help in visualising theshape of the different water bodies (fresh, brack-ish and saline). The interpretation of isothermtrend relies on the temperature contrast that nor-mally exists among fresh groundwater, seawaterand other saline fluids. Evidently, diff e r e n tabsolute temperatures of water bodies and, thus,different in depth trends are expected at differentlatitude. Thus, it may be possible to follow theevolution of seawater intrusion or distinguish thezones where ground waters are salinised due tolateral intrusion from the zones where over-exploitation mobilises salt waters present atdepth. The principles of the temperature studymethod and some recent applications to a fewcoastal aquifers of Mediterranean area have beenrecently synthesised in Pulido-Bosh (Ed., 2003).

    As an example of isotherm trend expected incoastal aquifers at Mediterranean latitudes, figure 4shows a vertical temperature section of Salentokarst coastal aquifer. In this aquifer, temperaturesof fresh ground waters range between 14 and17°C, while seawater and salt waters inland aremarked by temperatures around 20°C. The sec-tion outlines the zones of recharge (low verticaltemperature gradients), the preferential flowpathways (a zone of high permeability is marked

    85

    ENVIRONMENTAL TRACING IN COASTAL AQUIFERS: OLD PROBLEMS AND NEW SOLUTIONS

  • Figure 4 - Vertical thermal section (°C) of Salento karst coastal aquifer (Puglia - Southern Italy). Groundwater TDS contentsat sea level are on m.s.l. line (from Tulipano & Fidelibus, 1989).

    by 14,7°C), the brackish water bodies (with tem-peratures higher than 17°C), the upconingprocesses (inflection of isotherms towards sur-face) and lateral seawater intrusion. The isothermtrend coincides with groundwater salt contentdistribution (Tulipano and Fidelibus, 1989). Infigure 5 a vertical section of the east part of Cam-po de Dalias (Almeria, Spain) shows respective-l y, the isotherm trend and the isoconductivitylines.

    The isotherm trend suggests an inflow ofcold waters at the foot of the Sierra de Gádor dueto a rapid infiltration of preferential surface

    flows across the fractures and the carbonatematerials.

    To the right, a rise of isotherms towards thesurface outlines the role of the exploitationthrough the deep boreholes tapping the Gádoraquifer in causing the rise of deep warm waters,may be along fractures. The conductivity values(figure 5b) in excess of 10,000 mS/cm-1of watersdrawn from the most superficial levels of theaquifer suggest seawater intrusion across thePliocene calcarenites, though one cannot rejectthe possibility that the flow comes from the depthacross the Gádor limestones (Molina, L., 1998).

    86

    HIDROGEOQUÍMICA E ISÓTOPOS

    Figure 5 - Vertical sections (EE’- WNW - ESE) of the extreme east part of Campo de Dalias (Almeria, Spain). a) Isothermtrend (°C) and b) conductivity (µS/cm) distribution (from Molina, 1988).

  • New tools in tracing the salt sourcesIn the last years, many chemical and isotope

    parameters, already used as tracers of salt sourcesin other fields of hearth sciences, have gainedinterest in the context of seawater intrusion stud-ies. At present, the B and Sr isotopes, which takeadvantage of a well defined background knowl-edge about their variation within geo-sphere,result the most interesting: their main characteris-tics and possible use will be briefly illustrated.Afterwards, the possible role of chlorine stableisotopes, Rare Earth Elements (REE) and organicbiomarkers, whose potential in groundwatersalinisation studies has not been adequatelyexplored up to now, will be discussed.

    Boron isotopesBoron is a good groundwater tracer thanks

    to its high solubility in aqueous solution, naturalabundance, and the lack of effects by evaporation,volatilisation, and oxidation-reduction reactions.Of the two boron stable isotopes, 11B and 10B, 11Bis partitioned preferentially into the B(OH)3,while 10B is preferentially incorporated into theB(OH)4

    -, which enters the solid phase. Boron iso-topic ratio 11B/10B shows large variations in nat-ural reservoirs. In particular, marine-derivedsources have high δ11B (e.g. seawater = 390/00,Dead sea 570/00), while rock-derived sources haverelatively low δ11B (e.g. Sea of Galilee = 240/00,salt lakes from Qaidam Basin, China = -10/00 to120/00, hydrothermal fluids = < 0

    0/00). Adsorptiononto clay minerals in the aquifer, enhanced byhigh salinity, can modify the δ11B of ground-water boron. The isotopic shift associated withboron retention is an 11B enrichment of about 20and thus ground water may have higher δ11B val-ues relative to the original source. Therefore,boron isotopes can be considered good tracers ofdissolved salts in groundwater (Vengosh et al.,1998).

    A few studies deal with the use of boron iso-topes in coastal aquifers. As an example, Vengoshet al., 2002, propose a very complete isotopicstudy of Saline Valley complex system (Califor-nia) made up of alluvial sand, gravel and clay

    deposits. They delineate the impact of salt sourcesin the different areas of the valley through theconcurrent use of δ11B, the stable isotopes of H, Oand Sr, the 14C, and Br/Cl and Na/Cl ratios.

    Another application of δ11B in coastalaquifer studies is carried out by Xiao et al, 2001:they use the boron isotopes in the study ofLaizhou Bay region (China) for distinguishingbetween brine or seawater influence on ground-water.

    The use of δ11B is promising, even if themethod has to be tested in a larger number of casestudies concerning coastal aquifers. Its use, more-over, is not limited to the recognition of saltsources, given that boron isotopes can be useful intracing the contamination due to domestic wastewater and other anthropogenic sources (e.g., fer-tilizers, pesticides, landfills). Boron derives, inthese cases, from Na-borates, having δ11B rangingfrom 0 to 10: in particular, fertilizers originatefrom Ca-borates with low δ11B as -13 (Vengosh etal., 1998).

    Strontium isotopes Strontium occurs with four naturally stable

    isotopes, having the following approximate abun-dances: 8 4Sr = 0.55 %, 8 6Sr = 9.75 %, 8 7Sr = 6.96 %and 88Sr = 82.74 %. Only the 87Sr can vary withrespect to the other isotopes due to the β decay of87Rb, but the variations are small due to both thelow Rb abundance and the very long 87Rb half-life (T1/2 = 4.88 * 10

    10 y). Strontium can enter thelattice of minerals as aragonite, calcite, fluorite,gypsum, anhydrite and barite: their strontium iso-tope composition, when formed in equilibriumwith seawater, directly records the composition ofthe coeval seawater. A close relationship existsbetween the strontium isotope composition ofseawater and the geological time: at present, thevariations of the seawater 87Sr/86Sr ratios throughthe Phanerozoic time have been defined. Presentseawater shows a worldwide strontium isotoperatio close to 0.709198 * 0.000020 (De Paolo andIngram, 1985).

    Sr isotopes show no detectable fractionationby any natural process that involves water-rock

    87

    ENVIRONMENTAL TRACING IN COASTAL AQUIFERS: OLD PROBLEMS AND NEW SOLUTIONS

  • interaction or mixing: hence, the isotopic compo-sition of natural ground water would inherit its Srisotopic composition from the aquifer rocks,according to their different lythological character-istics and age. The longer the residence time ofground waters is the closer the chemical equilib-rium with the different minerals in contact. Inground waters of same age, Sr enrichmentdepends on rock mineralogy, while 87Sr/86Sr ratiois determined by the different age of rocks. Sr iso-topic composition of saline water may be affectedalso by base-exchange reactions in which Ca, andhence also Sr is derived from adsorbed sites onclay minerals; moreover, the re-crystallization ordolomitisation of carbonate rocks would reducethe original 87Sr/86Sr ratio of the saline water.

    Thus, the co-variation of Sr and 87Sr/86Srcan provide a diagnostic tool for both the recog-nition of water-rock interactions and the resi-dence time of groundwater.Values of 87Sr/86Sr forground waters (Banner, 1989, Brass, 1976) rangefrom 0.7036 (waters draining young volcanicrocks) to 0.7384 (drainage from old (*1000 my)igneous and metamorphic rocks).

    The 87Sr/86Sr ratio methodology has beenapplied in a variety of hydrogeological environ-ments to the study of surface waters and groundwaters, including brines (Banner et al., 1989,Banner et al., 1994, Muller et al., 1991, Oetting etal., 1996). Main applications in the context of sea-water intrusion relate to the distinction of salineend-members of mixing.

    The 87Sr/86Sr ratio of salt waters sampled indeep observation-wells of the Salento carbonatecoastal aquifer (Puglia, Southern Italy), rangesfrom 0.70911 to 0.70825 (Calò et al, in prepara-tion). The age of formations likely to interact withsalt ground waters ranges from the Upper Jurassicto the Upper Cretaceous: related rocks have87Sr/86Sr ratios ranging from 0.70668 to 0.70780.The lowest measured value in salt waters is0.07825, which is far from the upper limit of therange characterising the rocks. This meanswhether that salt groundwater did not reach theequilibrium with rocks or whether that they rep-resent the result of the mixing of present seawater

    with a salt water deeply evolved which has notbeen up to now sampled.

    B a r b i e r i et al. (1999), had already hypothe-sised, for the same aquifer, the existence of saltwaters having different 8 7S r /8 6Sr ratios. They repre-sent the extrapolated end-points of the whole possi-ble mixing hyperbola (8 7S r /8 6Sr ratios as to Sr con-centrations), having a common starting point (freshgroundwater) and intercepting the measured valuesof brackish coastal springs belonging to the aquifer(figure 6). The 8 7S r /8 6Sr values related to salt watersreally found in the aquifer (Calò et al., in prep.) cor-respond to most of the extrapolated values, but do notcover all the range. One of the hyperbola leads, infact, to hypothesise the presence of a salt end-mem-ber deeply evolved, characterised by about 100 mg/lof Sr and a 8 7S r /8 6Sr ratios relatable to Upper Creta-ceous carbonate rocks: this end-member potentiallyshould exist, but has not met up today.

    In a hydrochemical investigation of ground-water circulating in the Quaternary sequence of gla-cial and fluvio-glacial deposits at Stautrup Wa t e r-works, east coast of Jutland (Denmark), strontium-isotopes have been even used for the recognition ofthe saline end-members (Jorgensen and Holm,2001). The 8 7S r /8 6Sr ratios of ground waters as to1/Sr show a well-defined mixing hyperbola, with87Sr/ 86Sr ratios ranging from 0.7088 (low-Cl sam-ples) to 0.70840 (high-Cl samples). The value of0.7092 (seawater) from the nearby Arhus Bayresults significantly different from that of the mix-ing hyperbola, thereby excluding the possibility ofany significant present seawater involvement.

    In the coastal aquifer made up by Quater-nary marine sands and gravels in the Keta Lagoonarea and the Volta River estuary, Keta Basin,Ghana (Jorgensen and Banoeng-Yakubo, 2001),the strontium-isotope distributions (well-definedmixing hyperbola for strontium isotopes with sea-water as one end member) as well as oxygen andhydrogen isotope compositions (samples collect-ed from shallow and deep groundwater plot alonga seawater mixing line or evaporation line)demonstrate that present seawater is the only end-member involved in salinisation of ground watersand surface waters.

    88

    HIDROGEOQUÍMICA E ISÓTOPOS

  • Chlorine isotopesCl has two stable isotopes (35Cl and 37Cl),

    which are highly mobile in the hydrosphere andnot easily fractionated in nature. Fractionation of37Cl/ 35Cl is expected when diffusion is the mainmechanism of solute transport, because thelighter isotope will be diffused more readily(Desaulniers et al., 1986). Deviation from zero0/00 of δ

    37Cl would constitute evidence of a diffu-sion-controlled hydraulic regime, where maxi-mum depletion is of -2.5 0/00.

    As an example, in the coastal plain ofSuriname, Groen et al. (2000), were able toattribute the salinisation of palaeo-groundwaterin permeable Tertiary formations to the down-ward solute transport from overlying Holocenemarine clays and the upward transport fromsaline Cretaceous sediments, thus showing thatthe approach of combining chloride and δ3 7C l

    analyses and salt transport modelling is suc-cessful in reconstructing hydrogeologicalevents in coastal areas.

    Significant isotope variation exists in natu-ral chlorides: thus, stable chlorine isotope data,used in conjunction with other geochemicalparameters, are useful in determining the origin ofsolute in formation waters having salinity differ-ent that of present seawater (Eastoe et al., 2001).In general salt deposits and saline hydrothermalsprings tend to be enriched in δ37Cl with respectto seawater (Kaufmann et al., 1984). Dependingon local lithology, δ37Cl might be a useful tool fordetermining the mixing between regional andshallow ground waters as well (Nimz, 1998).Therefore, the isotopic variations of δ37Cl innature seem to be sufficient to hypothesise its useas a hydrologic tracer in the field of salinisationstudies.

    89

    ENVIRONMENTAL TRACING IN COASTAL AQUIFERS: OLD PROBLEMS AND NEW SOLUTIONS

    Figure 6 - Cross plot of 87Sr/86Sr values as to Sr+2 concentrations for fresh ground waters, brackish waters of the coastalsprings, salt waters and present seawater (stars) sampled in the Salento coastal karst aquifer -Southern Italy (from Barbieriet al., 1999, modified).

  • Rare Earth ElementsThe rare earth elements (REEs) form a

    unique chemical set in which the gradual decreasein ionic radius across the series leads to systemat-ic changes in geochemical behaviour. REEs ingroundwater derive mainly by rocks throughwhich they flow. The low concentrations inwaters (ppb level or less) for long time preventedtheir use both in the assessment of water/rockinteraction processes and as hydrological tracers.Because of the refinement of analytical tech-niques (ICP-MS), the interest in the chemistry ofdissolved REEs is increasing: they could be usedin the study of water-rock interaction, origin ofgroundwater and groundwater mixing.

    A recent example is the study of Johannes-son et al. (1997), who, with the aim of testing theutility of REEs as geochemical groundwater trac-ers, analyse the carbonate aquifer system ofSouthern Nevada, which conceptual model waswell known. They compare mixing proportionscalculated trough REEs with the proportionsobtained through the use of more classical tracersas D, 18O, Sr isotopes and major constituents: thecomparison shows that REEs may prove especial-ly useful for determining groundwater sourceswhen aquifer materials vary substantially andprovided the possibility to model the removal ofREEs by adsorption processes.

    Another example comes from Aquilina et al.(2002), who, studying the origin of saline thermalfluids of Balaruc Le Bains Peninsula, SouthernFrance, use REEs and trace elements to recognisediagenetically evolved "fossil" seawater, whoseorigin is attributed to the Triassic or to the crys-talline basement. This origin is deeper than theJurassic carbonate formation, through which ther-mal fluids interact with karst waters.

    The use of REEs as geochemical tracers inhydrogeology, until now, seems limited: surelymore work has to be done on collection of enoughdata concerning absolute concentration andmobility in fresh water and salt-water environ-ments. Dia et al. (2000), investigate temporal andspace variation in ground waters of theKervidy/Coet-Dan catchment, southwest of

    Rennes in Central Britanny, France: they statethat, although being potentially affected by chem-ical processes (redox, adsorption, complexation)and therefore considered as non-conservativetracers, the different REE patterns display finger-prints typical of each hydrological domainthrough space and time. Furthermore, their sensi-tivity to redox variation, complexation or uptakeonto sorptive surfaces strongly suggest that theREEs can be useful tracers of groundwater- rockinteraction elsewhere.

    Organic biomarkersBesides the approach through inorg a n i c

    chemistry, a group of researchers (Sukhija et al,1996) tackles with sources of salinisation in thecoastal aquifer by using organic biomarkers.Palmitoleic (PAL) and oleic acids (OL), vaccenic(VAC) and hopanoic (HOP) acids, have been usedas tracers in distinguishing old saline fluids frompresent seawater. The former two acids indicate,in fact, paleo-marine conditions, while the latterare typical of surface marine environment. Thus,the organic biomarkers may be useful to differen-tiate between ancient and modern salinities, sup-plementing conventional geochemical and iso-topic techniques commonly used.

    GROUNDWATER AGE

    The key information about the age of salineground waters should accompany their identifica-tion. In addition, also the renewability of freshwater resources should be assessed.

    Dating can be performed through geochem-ical data, but it is not quantitative. Moreover, sta-ble isotopes D and 18O are widely used for identi-fying "paleo-groundwaters", thanks to the shift inthe stable isotope content of past precipitation orto deuterium excess. In this case, as well theinformation is only qualitative, indicating thatground waters are fossil and that resources arefinite and not renewable. The only availableabsolute dating technique relays in the decay (oraccumulation) of radionuclides, which allow dat-

    90

    HIDROGEOQUÍMICA E ISÓTOPOS

  • ing waters from a few days to hundreds or thou-sands of years.

    Groundwater dating in hydrology andhydrogeology is the subject of a few recent exten-sive review-books (Clark and Fritz, 1997,Kendall and McDonnell, 1998, Cook and Herzeg,1999). Therefore, we will not go into details ofthe measurements techniques, nor into the appli-cation of most commonly used isotopes, as 3H or14C, which use in the hydrogeology has been longdebated. We will only deal with a few more recentor developing tools for dating such as 36Cl, 81Kr,CFCs and TFA, which might be useful in seawa-ter intrusion studies.

    New tools for dating

    Chlorine - 36Thermonuclear 36Cl is expected to develop

    as an indicator of young water as the thermonu-clear 3H in ground water will decay to back-ground levels over the next 20 years. 36Cl has acosmogenic component (in atmosphere, bycomics–ray spallation of 36Ar and stable 35Cl) anda bomb-produced component (atmospheric explo-sions of nuclear weapons between 1952 and 1958,which generated considerable quantity of 36Cl inseawater).

    36Cl is used both, for dating waters less than50 years BP and ground waters belonging to sys-tems with long pathways or low transmissivity.The former possibility is due to the fact that in theatmosphere 3 6Cl resides about 1 week: thus,events, which happened in the 50’s, mark younggroundwater. The second type of dating is possi-ble thanks to the half-life of 301,000 years, whichmakes 36Cl suitable for dating in the range of60,000 to 1 My.

    The abundance of 3 6Cl is usually defined asthe atomic ratio of 3 6Cl to total chloride in the sam-ple. The ratio is always quite low in naturalwaters, ranging from 10- 1 5 to 10- 11. T h e r m o n u c l e a r3 6Cl ratios are marked by values higher than 10- 1 2,while values derived from the in situ production(from U and Th) are of about 50 x 10- 1 5. Precipita-tion values are in the 20-500 x 10- 1 5 r a n g e .

    The determination of the age through 36Clof ground waters isolated from present atmos-pheric contribution, requires that certain condi-tions must be met: the only sink for 36Cl should beradioactive decay, no stable chloride should beadded during flow and the 36Cl/Cl ratio at the timeof recharge should be equal to the present day val-ue (Andrews and Fontes, 1992). Nevertheless,during flow radioactive decay reduces the ratio,while sub-surface production increases it; more-over, mixing with ground waters with different36Cl and chloride concentrations or dissolution ofsalts adding dead chlorides may occur as well.Mixing can be handled, if the end-members canbe identified and characterised. As for the lasttopic, Cresswell et al. (1999), were able to esti-mate the age of the ancient ground waters of theAmadeus Basin (Australia) on the base of expect-ed level of 36Cl/Cl of a hypersaline (150 g/l) brineand using a model which incorporated a smallamount of mixing with in situ salts.

    As for dating young ground waters, thelarge amount produced during bomb tests led to asort of "bomb peak" that can be used to identify ayoung groundwater component up to water agesof 40 years. The dating is based on knowledge ofthe initial 36Cl/Cl ratio of precipitation input andon evaluation of 36Cl sub-surface production fromstable 35Cl in the rocks and water, especially inthe case in which chlorinity is not constant (Nimz,1998).

    Krypton - 81When groundwater age is beyond the dating

    range of 1 4C, besides 3 6Cl, which has complex sub-surface production mechanisms, measurements ofnoble gas isotope 8 1Kr can be used with the advan-tage that all possible complications have minorimportance. 8 1Kr is able to date in the range from1 05 and 106 yr (half-life = (2.29 ± 0.11) x 105 y r ).The atmospheric concentrations of 8 1Kr areknown and constant and the human and subsur-face production are small or negligible. The onlypresent problem is the large volume sample(16.000 l) which have to be degassed for obtain-ing a significant mass of Kr to be subsequently

    91

    ENVIRONMENTAL TRACING IN COASTAL AQUIFERS: OLD PROBLEMS AND NEW SOLUTIONS

  • subject to a complex analytical procedure for Krseparation and accelerator mass spectrometry(AMS) measurement. 81Kr is a promising tool fordating very old ground waters thanks to the lowuncertainty connected with its interpretation: agesobtained by this method for ground waters ofGreat Artesian Basin (Australia) appear lower,but more reliable than those obtained by 36Cl(Collon et al., 2000).

    Chlorofluorocarbons and trifluoraceticacid

    Chlorofluorocarbons (CFCs) are man-madehalogenated alkanes produced for a range ofindustrial and domestic purposes. Current atmos-pheric lifetimes of CFC-11 (CFCl 3), CFC-12(CF2Cl2), and CFC-113 (C2F3Cl3) are 45 * 7, 87 *17, 100 * 32 years (Volk et al., 1997). Groundwa-ter dating with CFC is possible because they havea global source function not subject to geograph-ic effects and an increase of concentrations inatmosphere almost well known; moreover, con-centrations in young groundwater are relativelyhigh to be easily measurable.

    The CFC production is nevertheless declin-ing and modelling suggests that CFC will reachthe maximum before the turn of the century, afterwhich there will be a decline (Elkins et al., 1993).The error in apparent CFC ages is less than 1 yearfor groundwater recharged since 1960 (Dunkle etal., 1993). One of the assumptions of groundwater dating with CFCs is that concentrations inthe soil gas immediately above the water table arein equilibrium with the atmosphere. However,this is not the case if the unsaturated zone is thick(Weeks et al., 1982). CFCs in groundwater do notseem affected by aerobic degradation (Lovley andWoodward, 1992), while they can all be degradedunder anaerobic conditions (Sylvestre et al.,1997). Another process likely to remove CFCsfrom groundwater is sorption.

    Michel et al., (1994) used CFC and Tritiumconcentrations to estimate the rate of seawaterintrusion in the coastal aquifer system of Califor-nia. The Upper system consists of alluvialdeposits 125 m thick; the Oxnard aquifer, at a

    depth of about 70 m below surface, is underlainby another fresh water aquifer and overlain byperched saline aquifers. The lower system con-sists of continental marine deposits about 325 mthick. In the aquifer system, multiple potentialsalt sources exist (leakage from overlying salineaquifers through corroded casing of abandonedwells, movement of naturally occurring salinewater within the upper aquifer system in responseto pumping and invasion of brine). Relying in aconstant concentration of Tritium in the top 150 mof Pacific Ocean and on estimates of CFC con-centrations over the past 50 years and consideringthat CFC undergo the same mixing processes astritium, the Authors demonstrate that the rate ofseawater intrusion in the fresh aquifer underlyingOxnard Plain determined by using chlorides is inerror. Anyway, the interpretation of the tracer datais uncertain because input functions are not pre-cisely known and CFC concentration can changedue to physical and chemical processes that occurduring flow. Therefore, Authors conclude that Tand CFC might be used only to support conclu-sions made on the base of other data.

    H o w e v e r, this does not discourage researcherswho continue exploring the possibility to use anthro-pogenic compounds as tracers for dating and/or dis-tinguishing surface and young recharge waters fromolder ones. Trifluoroacetic acid (TFA), for example,is produced in the atmosphere as the result of thebreakdown of the chlorofluorocarbon replacementsHCFC-123, 124 and 134. T FA partitions in thewater phases occurring throughout the environment:as man-made tracer, T FA could be used as CFC.T FAconcentrations of fog and rain range from 31 to3779 ng l- 1; surface waters, included present seawa-t e r, vary from 55 ng l-1 to 41000 ng l- 1 depending onthe type and location (Wujicik et al., 1998). T h estudy carried out by Nielsen et al. (2001), estab-lished, through the analysis of pre-industrial groundwaters (> 2000 y old), that T FA is not a naturallyoccurring trace component of the fresh water envi-ronment. Further work is surely needed to measureT FA background concentrations in contemporaryprecipitation and to evaluate its general behaviourin different hydrogeological environments.

    92

    HIDROGEOQUÍMICA E ISÓTOPOS

  • EFFECTS OF SEAWATERINTRUSION ON GROUNDWATERCHEMICALAND ISOTOPECHARACTERISTICS AND ONAQUIFER PROPERTIES

    Ion exchange and hydrochemicalfacies

    The ionic exchange between water and sed-iments activates when ionic concentrations varyalong a flow path, as in the case of the movementof the salt water/fresh water interface. Therefore,hydrochemical variations of groundwater qualityis likely to occur in coastal aquifers in presence ofexchangers: ion exchange can completely alterthe groundwater cationic concentrations througha process known as ion-chromatography.

    In coastal aquifers, the reaction more easilyrecognised is the exchange between Ca2+ and Na+.Under the influence of recharge waters, washingaquifers already subject to seawater intrusion(where clays retain a great proportion of adsorbedNa+) (Appelo and Geirnaert, 1983), Ca2+ is selec-tively held up with respect to Na+, considered thenatural ion with the greater facility to theexchange. Therefore, if a Na-clay is exposed to asolution in which Ca2+ is the dominant cation, the

    solution enriches in Na+ and loses Ca2+, up to theattainment of a new equilibrium (Lloyd andHeathcote, 1985; Tellam et al., 1986). The reac-tion, in agreement with the selective character ofadsorption, is known as direct exchange Ca/Na.

    The inverse exchange Na/Ca (Howard andLloyd, 1983) occurs, on the contrary, during sea-water intrusion, when salt waters reach zones ofthe aquifer previously occupied by fresh waters.The clay-water system reacts provoking therelease of Ca2+ (occupying great proportion ofexchange sites) and the parallel adsorption ofNa+. A high Na/Ca ratio in ground waters turnsout to be dominant in comparison with the greaterselectivity of most part of the clays towards theadsorption of Ca2+. The selectivity coefficientdecreases with increasing ionic strength: thus,when salt water enters in contact with clays richin Ca2+, the preferential adsorption of the Na+

    occurs. Beekman (1991) studied such processes

    through laboratory simulations based on the prin-ciple of the ion chromatography. He used chro-matographic columns filled up of aquifer materi-al in equilibrium with a fluid that occupies thesediment pores, that is fresh water in the simula-tion of the seawater intrusion and diluted seawa-

    93

    ENVIRONMENTAL TRACING IN COASTAL AQUIFERS: OLD PROBLEMS AND NEW SOLUTIONS

    Figure 7 - (a) Simulation of seawater intrusion in a fresh aquifer, according to a geochemical/mixing cell model (from Appelo& Willemsen, 1987, modified). Cell represents the distance from the intrusion front. (b) Simulation of recovery of a porousaquifer. Flushing of the exchange complex develops from the left side towards the right side. The distance in Km representsthe distance from the front of the fresh waters (from Beekman, 1991, modified).

  • ter in the simulation of refreshing. At the top ofthe columns he injected, respectively, diluted sea-water and fresh water and analysed periodicallythe effluent chemical composition at their exit.The experimental data obtained through the chro-matographic process, were modelled (Appelo andWillemsen, 1987, Appelo et al., 1990) by using amathematical model (mixing cells) that includes theensuing geochemical processes as well. Figure 7shows the results of the simulations: the sketchesrepresent a sort of photograph of a section of theaquifer at a definite moment after the start of theprocesses.

    Later on, others codes, which can considernumerous complex and concurrent geochemicalprocesses, have been developed to simulate themulti-component transport, but little attention hasbeen paid both to the transport during seawaterintrusion and/or refreshing and to the validationof the results. The only example of validation canbe found in Xu T. et al. (1999): the Authors devel-oped a general 2-D finite element multi-compo-nent reactive transport code, TRANQUI, capableto deal with complex thermo-hydro-geochemicalproblems for single-phase variably water saturat-ed porous media flow systems and to reliablysimulate real situations. The model takes intoaccount a wide range of hydrological, thermo-dynamic and chemical processes. The code isused to model the hydrochemical evolution of theLlobregat Delta aquitard (Northeastern Spain)over the last 3500 years, during which fresh-waterflow from a lower aquifer displaced the nativesaline aquitard waters. The best match betweenmodelled and measured data is obtained consider-ing, besides ion exchange and calcite dissolution-precipitation, the redox reactions as well.

    A practical consequence of multi-compo-nent transport is that different sequences ofHYdrochemical Facies (HYF) mark the processesof seawater intrusion and refreshing (figure 7).The HYF of water can be defined following theprinciples defined in Stuyfzand (1986). He com-bines four essential aspects in a logical code:chlorinity, alkalinity, most important cation andanion and a Base Exchange index (BEX). BEX is

    defined by calculating the meq-sum of Na, K andMg corrected from the contribution of sea salts. Asignificantly positive BEX can be then translatedin to freshening (displacement of saltier ground-water), while a significantly negative BEX indi-cates salinisation (displacement of freshergroundwater). BEX = 0 indicates adequate flush-ing with water of constant composition. Somecomplications affect BEX interpretation: never-theless, the HYF Analysis (Stuyfzand, 1993),consisting of five consecutive steps (acquisitionof hydrochemical data, definition of the hydro-chemical facies, identification of hydrosomes,construction and description of maps and cross-sections of HYFs and hydrosomes, interpreta-tion), constituted for the Author a powerful tool inthe detailed exploration of the complex water-sediment reaction processes responsible of chem-ical variations in space and time of ground watersin the coastal dune area of the Netherlands.

    Giménez et al., 1995, define a modified andsimplified code for the definition of HYFs. TheBEX, in this case, indicates the direction of theexchange on the base of the deviation of Na+ con-centration from the value defined by conservativeFW-SW mixing. The new base exchange index ispositive in case of direct exchange and negativein case of inverse exchange: it avoids many of thecomplications affecting the interpretation ofStuyfzand’s BEX. Moreover, the procedure takesinto account that the freshwaters flowing inMediterranean coastal aquifers have a non negli-gible TDS as in Northern Europe: thus, higherconcentrations of major and minor ions have to beconsidered for the fresh water component.

    According to the simplified classification,an essential sequence of NaCl(-) withNa/Cl0.85), from the recharge areatowards the coast. However, the sequences deter-mined in lab experiments are only partially recog-nised in real situations and HYFs belonging to

    94

    HIDROGEOQUÍMICA E ISÓTOPOS

  • both processes normally coexist. In natural condi-tions, in fact, seawater intrusion and refreshingalternate, without having enough time to interestthe whole aquifer: moreover, the inversion of theflow does not bring back the water chemical qual-ity to the original conditions, since the exchangeprocesses are not linear. Other factors, likeaquifer heterogeneity, and, therefore, the variabil-ity of the permeability, influence in differentiatingthe real cases from those simulated.

    Most of the researchers use the HFs withoutthe exchange notation. Condesso de Melo et al.(1999), for example, find the typical NaHCO3facies in the Aveiro Multilayer Cretaceousaquifer, NW Portugal, made up mainly of silici-clastic sediments: while the dominant siliciclasticsediments are responsible only for a very littlegeochemical evolution, the poor presence of clayminerals dominates in modifying the chemistry ofground waters. Edet and Okereke (2001) studysaltwater intrusion in southeastern Nigeria: HYFsbelonging to both seawater intrusion and refresh-ing (CaCl, CaHCO3, NaHCO3, NaCl) coexist inthe study area. The CaCl type sometimes is hid-den under a CaNO3 facies, due to the release ofhigh amount of nitrates coming from urbanwastewater.

    The Pico aquifer (being Pico the youngestisland of the Azores archipelago composed ofbasaltic volcanic deposits less than 300,000 yearsold), consists of very permeable recent lava flowsand groundwater is mainly of a sodium-chloridetype (Vi rgilio Cruz and Oliveira Silva, 2001): itscomposition is explained by a mixing processbetween fresh water and present seawater to whicha ion-exchange process overlaps. Petalas and Dia-mantis (1999) study the origin and distribution ofsaline ground waters in the aquifer system locatedin the coastal area of Rhodope, NE Greece, whichincludes two aquifers within coarse-grained allu-vial sediments. The main process controlling thechemistry of ground waters subject to salinisationis calcium/sodium exchange between water andsediments under seawater intrusion.

    Another example comes from Imerzoukeneet al. (1994). They study the Mitidja plain in the

    North Algeria, which holds two aquifers. Themost important consists of fine-grained (clay) andcoarse grained (gravel and sand) sediments and itis alluvial, coastal and unconfined: the spatialsuccession of HYFs shows a good general agree-ment with the general pattern of groundwaterflow and allows to identify the areas where activeseawater intrusion occurs ().

    Within the unconfined coastal aquifer ofMar del Plata (Argentina), composed of silt andfine sand, ground waters, of CaHCO3 type in therecharge zone, become of NaHCO3 type towardsthe discharge area (Martinez and Bocanegra,2002) due to flushing by fresh water of salinisedsediments previously subject to seawater intru-sion. Hafi (1998), analysing ground waters in theaquifer in the coastal area east of Tripoli, outlinesthe presence of the ion exchange. The concentra-tions of the Na, K, Ca, Mg, sulphate and bicar-bonate deviate considerably from the conserva-tive mixing. Typical CaCl facies appears underseawater intrusion. Sulphate and bicarbonateenrichments relate to the oxidation of sulphidesand dissolution of calcite minerals present in theaquifer materials.

    Sometimes Ca enrichment accompanies Naenrichment as well, giving a muddling picture ofthe situation. In the study of the coastal Campi-dano Plain (Sardinia, Italy), composed of Tertiaryand quaternary sediments (Barbieri et al., 1994),this parallel enrichment is ultimately referred tothe simultaneous occurrence of gypsum and car-bonate dissolution (both causing calcium enrich-ment) and refreshing (leading to Na enrichmentand Ca depletion).

    Stuyfzand (1993) gives many examples ofhydrochemical facies mapping along verticalcross-sections of the coastal dune area of westernNetherlands. Figure 8 shows the areal distributionof HFs up to the depth of 200 m under an areawhere inordinate pumping caused a severe salini-sation, which needed artificial recharge of thedunes with Rhine water. The map shows theextent of both, fresh natural and artificial rechargewaters, whose influence extends up to 120 m ofdepth.

    95

    ENVIRONMENTAL TRACING IN COASTAL AQUIFERS: OLD PROBLEMS AND NEW SOLUTIONS

  • The BEX notation, when HYF’s maps areavailable for different seasons, allows an insightin the dynamics of seawater intrusion and refresh-ing. The clear effects of the succession of seawa-ter intrusion and refreshing have been shown byGiménez et al. (1995), in the study of the Orope-sa Plain (Eastern Spain), formed by Plio-Quater-nary sediments, mainly represented by carbonateconglomerate in clayey matrix. In this case, eitherCl or piezometric maps give a muddling pictureof the real spatial and temporal extent of process-es that can be visualised only through the analysisof HYFs and their mapping in the two differentperiods (figure 9).

    Ion exchange develops not only in porousaquifers, but also in karst coastal aquifers, whenthey contain clays in the matrix and /or as filler inthe fractures and fissures. Even a very low per-centage of clay is able to modify groundwaterquality under the effect of seawater intrusionand/or recharge: the effect of ion exchange super-imposes to that of the water-carbonate rock inter-action processes typical of karst coastal aquifers

    (see later). Pascual and Custodio (1993) give oneinteresting example. They studied the coastalaquifer present in the southern portion of the Gar-raf carbonate massif on the Mediterranean Seacoast: the formation consists of a thick sequenceof limestone and dolostone of Cretaceous, coveredin some areas by Miocene calcarenites, with someinterlayer of marls. The comparison between themeasured major ion concentrations and thosederived by the conservative mixing of localr e c h a rge fresh water and present Mediterraneans e a w a t e r, shows that, in the range 5 to 20 % ofsalinity, an excess of bicarbonates and calciumcorresponds to a deficit of sodium and magne-sium. At high salinity, Na+ and Mg2+ deficits andCa2+ excesses (even when calcite precipitation ispossible) indicate the action of the ion exchange,involving all major cations, which superimpose tocarbonate mineral dissolution and precipitation.Mass-balance calculation indicates that inverseion exchange Na/Ca plays a dominant role.Dolomite formation is also recognised, while cal-cite precipitates when the release of calcium by

    96

    HIDROGEOQUÍMICA E ISÓTOPOS

    Figure 8 - Schematised cross-section over the coastal dunes south of Zandvoort aan Zee across the Leiduin Catchment area(Western Netherlands) with areal distribution of hydrochemical facies (from Stuyfzand, 1993).

  • ion exchange is high, and dissolves when theexchange process is decreasing. At high depthscalcite is mostly dissolving while dolomite isforming: the high salinity produces chemicaleffects that prevail on the increase of Ca2+ due toion exchange, determining under saturation ofwater with respect to calcite. There, preferentialkarstification is going on.

    Many researchers point out the existence ofsulphate depletion with respect to conservativemixing normally accompanying the ionexchange: this is commonly explained by bacter-ial sulphate reduction coupled to organic matteroxidation. Nyvang et al. (2001) and Chrinstensenet al. (2001), studying the coastal aquifer ofSkansehage, Denmark, located in marine sandand gravel with thin lens of peat, outline that thedominant redox processes are sulphate reductionnear the intrusion front (together with cationexchange) and methanogenesis in the area withlittle sulphate (fresh part), thus giving groundwa-

    ter variably enriched in sulphide, methane andbicarbonate. Sometimes the sulphate depletionobserved in brackish waters is attributable to thetracing effect of the saline old end-memberinvolved in the mixing, which is depleted in sul-phate (Fidelibus and Tulipano, 1996) rather thanto a process accompanying the mixing. Gomis-Yagues et al. (2000) suggest that the sulphatedepletion can be caused not only by sulphatereduction, but also by precipitation of gypsumcoupled with ion exchange during early stages ofthe advance of seawater.

    Karst aquifer diagenesis due tobrackish and salt waters: poro s i t yreistribution and permeability changes

    The diagenesis of karst coastal aquifers hasbeen the subject of numerous studies during lastdecade, although most of the related papersbelong to the field of sedimentology and petrog-raphy. The hydrogeological interest resides in the

    97

    ENVIRONMENTAL TRACING IN COASTAL AQUIFERS: OLD PROBLEMS AND NEW SOLUTIONS

    Figure 9 - Hydrochemical facies maps and Na/Cl value distributions related respectively to summer (a, b) and winter (c, d).(Gimenez et al., 1995).

  • fact that chemical diagenesis can explain somepeculiar hydraulic features concerning the func-tioning of such important aquifers and poses inter-esting questions about their continuous evolution.

    The great potential of chemical diagenesisin the development of secondary porosity incoastal carbonate aquifers was outlined in theseventies by Hanshaw and Back (1979) andWigley and Plummer (1976). Arenewing of kars-tification is observed within the transition zone,i.e. the zone where fresh waters and salt waterscoexist: the non-linearity of mineral solubilitywith respect to variation of ion strength, partialpressure of carbon dioxide and temperature, caus-es the brackish waters to be under-saturated withrespect to most carbonate minerals in a largerange of salinity. Besides solution and precipita-tion of carbonate minerals, dolomitisation canoccur as well. Dolomitisation takes place also inthe zones of the aquifers occupied by salt waters.This process is likely to occur if three main con-ditions result satisfied: the existence of a sourceof reactants (Mg and CO3), of a suitable flowmechanism apt to transport reactants and products(Ca and CO3) to and from the site of the dolomi-tisation and favourable kinetic and thermodynam-ic conditions. The already mentioned review ofBudd (1997) deals with dolomitisation and themechanisms able to cause a flow of seawater incarbonate aquifers.

    Even more important for the hydrogeologi-cal aspects, is the fact that these processes do notlimit their effects to the present. They were effi-cient along the geological time, after the emersionof the carbonate formations, due to eustatic varia-tions, which caused the vertical and horizontaldisplacement within the aquifers of the positionof transition zone and salt waters. The variation ofsea level during Quaternary covered approxi-mately 200 m, with a maximum of about 80 mover the present sea level during Calabrian and aminimum of -120 m in correspondence of the lastglaciation, approximately 18,000 years ago (Fair-bridge, 1972).

    Therefore, in general, sub-horizontal karsti-fication, following pathways created by fractur-

    ing and fissuring, took place according to anystand of sea level; always following the base lev-el variation, in the zones of more or less activecirculation of salt waters, dolomitisation con-tributed to enhance the overall permeabilitydegree as well.

    The results of water-rock interaction due tomixing can be clearly recognised by the existenceof karst dissolution features along the coasts, asshown, for ex. by Whitaker and Smart (1997),who describe the effects of the massive dissolu-tion of calcite and aragonite in the west coast ofthe South Andros Island (Bahamas). Dissolutiongives origin, working on a system of sub-verticalfractures, to coves that vertically extend a lotunder the sea surface (blue holes). The mixing ofwaters having contrasting salinity and partial car-bon dioxide pressure, accompanied from bacteri-al oxidation of the organic matter, locally gener-ates a potential for a significant diagenesis withsecondary development of porosity.

    The effects of the dissolution have beenrecognised directly on rock core samples fromFlorida coastal karst aquifer by Wicks et al.(1995), while Higgins (1980) outlines the pres-ence of cavities from dissolution near somecoastal springs in Greece. As confirm of themacroscopic observations, the brackish waters ofthe coastal springs of the coastal carbonateaquifer of Murgia and Salento (Southern Italy)turn out under-saturated with respect to calcite(Fidelibus and Tulipano, 1990) in the salinityrange 5 – 22 g/l; the under-saturation is accompa-nied from a parallel increase of the concentrationsof bicarbonate and calcium.

    Chemical diagenesis that developed duringstands of the sea at elevations lower than presentm.s.l. is at the origin of the submarine springs,which presently gush out from carbonate aquifersfar from the present coasts even at high depthswith respect to present sea level. When these sub-marine springs belong to regional flow systems,they carry to the sea fresh water components thatoriginate from recharge areas very far from thecoast and follow preferential flow path-ways thatpush down as to great depths with respect to sea

    98

    HIDROGEOQUÍMICA E ISÓTOPOS

  • level before discharging into the sea. The meanelevation of recharge of the fresh water compo-nent of a brackish water spring can be deducedfrom D and 18O dilution curves as to chloride con-centrations, if chloride concentration of freshgroundwater and both stable isotope content andchloride concentration of the salt end-member areknown (Alaimo et al., 1989).

    While many researches confirm the exis-tence and the efficiency of the chemical diagene-sis at local scale, there are not direct proves of thesame efficiency at regional scale. The reconstruc-tion of the distribution at regional scale of thespecific capacity Qs (where Qs is the capacityestimated for the first meter of depression duringpumping tests) for the carbonate coastal aquifersof Murgia and Salento (Southern Italy) offers apossible indirect prove. Specific capacity suppliesan approximate appraisal of the permeability ofsaid aquifers (figure 10, Tulipano and Fidelibus,1995): high values characterise the areas closer tothe coast of both aquifers, while very high valuesinterest the entire Salento. As a matter of fact,also the groundwater salinisation follows at pres-ent the same course: anyway, the simple overlap-ping of the high values of Qs to the high TDS val-ues is not enough for giving evidence of theaction of chemical diagenesis. To understand thelevel of its different efficiency in the two aquifersit is necessary to take into account numerous dis-

    tinctive factors, such as lithology, formationthickness, topographical and morphologic charac-teristics, tectonic history; all these factors, cou-pled with eustatic changes, suggest that the twoaquifers has a different exposure to the aforesaidphenomena.

    The possibility to estimate the position andthe entity of the porosity increase in carbonatecoastal aquifers can have a great importance inthe understanding of the evolution of suchaquifers. Sanford and Konikow (1989a, 1989b)attempted to estimate the possible effects of theporosity and secondary permeability variations ongroundwater flow (quantitative evaluation of thedissolution of calcite in the mixing zone undertypical hydrodynamic and geochemical condi-tions) using a coupled model of flow and trans-port. The porosity develops asymmetrically to theinside of the transition zone, following mostly thestreamlines: this is attributed to the effects of themovement of the fluid that overlap positivelygeochemical effects. Moreover, the developmentof the porosity is strongly influenced by therenewal rate of the fluid in the transition zone.The same authors establish also that, in heteroge-neous aquifers, the dissolution emphasises theinitial heterogeneity instead of lessening it. Animportant conclusion of the simulations is that themechanism might produce a meaningful increasein porosity and permeability in a relatively short

    99

    ENVIRONMENTAL TRACING IN COASTAL AQUIFERS: OLD PROBLEMS AND NEW SOLUTIONS

    Figue 10 – Specific discharge (l / s x m) and T.D.S. content (g/l) maps related to Murgia and Salento karst coastal aquifers(Southern Italy).

  • time, of the order of 10.000 years. Sure, thesedata do not place the phenomenon at the humantime scale; however, they supply quantitativeinformation on "the destructive" ability of thekarstic phenomenon in a transition zone.

    A hypothesis based on the results of theabove studies is that porosity redistribution andconcurrent permeability increase in its turn mayinfluence the flow field and therefore, again, thediagenetic process (Liu and Chen, 1996): throughthis feedback, an advance of the salinisationprocess might, in theory, be produced. Thus, incarbonate aquifers, porosity is destined toincrease in time with increasingly important con-sequences on the evolution of seawater intrusion.

    Influence of ionic exchange on naturaland artificial reclaim of salinisedaquifer

    In coastal aquifers subject to salinisation, ionexchange may represent the limiting factor in ther e c o v e r y, in a reasonable time, of the originalwater quality. The time required for refreshing adefined volume of salinised aquifer, in fact, isl a rger than the time required for salinising it. Wi t hreference to column experiments, the speed of exitof every cation from the column depends on the (I-R) / (I+) ratio, where (I-R) is the amount of thecation adsorbed in the solid fraction and (I+) rep-resents its concentration in solution. For example,in the case of seawater intrusion, the concentrationof Ca2 + adsorbed in the sediments is lower than theconcentration of displacing Na+ in solution: thus,the ratio results low. Consequently, fast variationsin the column effluent occur or, translating it inreal case, fast salinisation of the aquifer occurs. Tothe contrary, during refreshing, the amount of Na+

    ions adsorbed on sediments is higher than the con-centration of Ca2 + in fresh waters, and, therefore,the ratio turns out very high. This means that, inlab experiments, the variations of column eff l u e n tare slow and in the real case, the refreshing of theaquifer is slow too. Therefore, the seawater intru-sion process is normally very fast, while the com-plete recovery of the aquifer can last hundreds orthousands of years.

    Lambrakis and Kallergis (2001), using thegeochemical simulation codes PHREEQE andPHREEQM (Appelo and Postma, 1994), studiedthe multi-component ion exchange process andfreshening time under natural and artificial recharg econditions for three coastal aquifers of Greece: theQuaternary basin of Glafkos in Peloponnesus, theNeogene formations in Gouves (Crete) and the car-bonate aquifer of Malia (Crete). The refreshing timeof the carbonate aquifer of Malia, in the hypotheti-cal cases of pumping cessation and natural recharg econditions, results of only 15 years (figure 11), dueto both the low cation exchange capacity and thehigh recharge rate. Their combined effects result ina fast replacement of the water reserve. Evidently,refreshing times for different types of karst coastalaquifers, with variable presence of clay (variableCEC) and different natural recharge rates, varyaccording to the absolute and relative importance ofthe two limiting factors.

    For the two porous aquifers, still under thehypothesis of cessation of pumping, refreshingtime is very long due to mainly to high CEC. Com-plete restoration should be attained after more than5000 ys, while partial restoration, reaching a waterquality close to that of recharge waters, could beobtained in 600-800 ys (figure 12).

    100

    HIDROGEOQUÍMICA E ISÓTOPOS

    Figure 11 - Simulation of the refreshing process undernatural recharge conditions of the karst aquifer of Malia,Greece (Lambrakis and Kallergis, 2001).

  • For both the porous aquifers, a highrecharge rate (artificial recharge), the restorationof original water quality is reached in a shortertime compared with time required by naturalrecharge (Lambrakis et al., 2001).

    During natural or artificial recharge incoastal aquifers, local and irreversible decrease inthe hydraulic conductivity (HC) can occur. If anaquifer, in fact, contains clays, cation exchangeprocesses, changes of the electrical double layeraround clay particles (connected to flocculation-deflocculation) and quantity of water adsorbed onclay interlayers (swelling) may strictly influencethe characteristics of flow.

    The processes that can be relevant for the HCdecrease during refreshing are the cation exchangeNa/Ca on clay-particle surfaces and the decrease ofionic strength of groundwater causing in turn theswelling of the clay particles and the expansion ofthe double layer. Swelling depends on the relative(not absolute) changes in concentration and causesthe decrease of the pore section of the medium,thus reducing the HC. In the expansion of the dou-ble layer, more water is entrapped in the doublelayer and clay starts deflocculating at a certainthreshold of concentration; then clay particles sep-

    arate and act as gel-droplets, which clog smallpores reducing HC. Swelling and deflocculationare correlated phenomena and both reduce the per-meability of the medium because both cause theformation of gel-droplets.

    The effects of ion exchange on HC needlong time to develop. Swelling and deflocculationoccur quickly. Goldenberg et al. (1983) were thefirst to execute some laboratory experiments onHC changes using real waters (seawater and freshground waters) and real sediment samples. Theirattention focused on the HC decrease in Israel’scoastal sediments, usually sandy, with low claymineral content (< 5 %). They observed a fastdecrease in soil HC when seawater was replacedby fresh water. The decrease was exponentiallyincreasing with clay content in the sediments; acontinuous decrease was observed as well withincreasing percentage of fresh water in the mixedsolutions.

    The decrease of HC has been detected inmany cases under artificial recharge of aquifersaffected by salinisation. Konokow et al. (2001),discuss the results of a field experiment in Nor-folk, Virginia, which show that clay dispersionoccurred in the unconsolidated sedimentary

    101

    ENVIRONMENTAL TRACING IN COASTAL AQUIFERS: OLD PROBLEMS AND NEW SOLUTIONS

    Figure 12 - Simulation of the process of refreshing of a porous aquifer previously salinised. (a) Natural recharge rate,150mm/y; (b) Artificial recharge rate, 730 mm/y (Lambrakis et al., 2001).

  • aquifer due to cation exchange as freshwater dis-placed brackish formation water. Migration ofinterstitial clay particles clogged pores, reducedpermeability and decreased recovery efficiency,but a calcium preflush was found to reduce claydispersion and lead to a higher recovery efficien-cy. Authors state that the reduction in permeabili-ty by clay dispersion may be expressed as a linearfunction of chloride content.

    QUALITY OF GROUND WATERSIN THE DISCHARGE ZONE

    Radium quartet and 2 2 2Rn: fingerprintsof Submarine Ground Wa t e r D i s c h a r g e

    The attention of oceanographers has alwaysbeen devoted to the evaluation of terrestrial ele-ment fluxes to the oceans, considered mainly dueto the river input. Nevertheless, the terrestrial fluxshould be regarded according to the hydrogeolog-ical viewpoint as well, given that groundwaterdischarge contributes to the global discharge intooceans. As suggested by Moore (1999), the twoviewpoints can be reconciled introducing the newterm of subterranean estuary, i.e. "a coastalaquifer where groundwater derived from landdrainage measurably dilutes seawater that hasinvaded the aquifer through a free connection tothe sea".

    With the aim of evaluating the groundwaterflux, Moore (1999) recently dealt with the pres-ence of radium isotopes (223Ra, 224Rn, 228Rn and226Rn) and 222Rn (coming from the decay of 226Ra)in waters discharging from coastal aquifers,because these isotopes are considered potentialtracers of coastal and Submarine Ground WaterDischarge (SGWD) (Cable et al., 1996).

    In general, after separation from its urani-um-bearing rock, radium resides in the dissolvedphase: its mobility is limited in fresh water envi-ronments by adsorption on to solids on a timescale of the order of minutes. Salinity of groundwaters and presence of other dissolved ions thatmay affect the radium adsorption distributionc o e fficient control radium mobility. 2 2 6Ra, in

    fact, is preferentially adsorbed on clay sedimentswhen these sediments are immersed in waters oflow ionic strength and preferentially desorbedwhen clays are exposed to waters of high ionicstrength. In practice, the 2 2 6Ra mobility is con-trolled in a coastal aquifer by the ion exchangedirection, which depends in turn on the dynamicsof seawater intrusion. Moore (1997), studyingthe fluxes of barium and radium at the mouth ofGanges-Brahmaputra River, states in fact that,when the river discharge is low, the fluxes ofradium and barium are controlled by SGWD,being barium and radium desorbed from particlesof the aquifer due to seawater intrusion. He takesinto account the dynamics of intrusion, noticingthat this flux to the sea changes in quality whenfresh waters (low ionic strength) flush sediments.H o w e v e r, oceanographers work with a limitedknowledge of the behaviour of the tracers theyconsider in the aquifers. Thus, hydrogeologistsare called to elucidate and implement this knowl-edge in order to fill the gap and reconcile the twopoints of view.

    An example of this effort is the study ofFidelibus et al. (2002), which attempts to explainthe high contents of 222Rn and 226Ra of brackishwaters discharging from the Salento carbonatecoastal aquifer (Southern Italy). Lab experimentsdemonstrate that, in the fresh part of the karstaquifer, 226Ra escaped from uranium bearing car-bonate rocks is blocked in "terra rossa", which isthe final product of limestone dissolution. Conse-quently, 222Rn activity in fresh ground watersresults mainly due to the decay of 226Ra blockedin rock/soils. In brackish water, instead, 222Rnactivity is due to both the blocked radium and thedesorbed one: it means that more 222Rn can beproduced directly in the liquid phase alonggroundwater paths. Figure 13 shows the 226Ra dis-tribution in the study area and, for comparison,the TDS distribution. The match of all informa-tion collected about the selected area, leads to theconclusion that, to the build-up of 222Rn contentsin brackish waters of the carbonate aquifer, con-cur many factors, whose relative significance isnot easy to establish.

    102

    HIDROGEOQUÍMICA E ISÓTOPOS

  • The high contents should reflect: (a) thememory of the fresh water component (in thefresh water environment 222Rn comes from thedecay of 226Ra of variable activity blocked in ter-ra rossa); (b) the effect of the decay of desorbedRa (its activity in the liquid phase depends on theamount released from sediments under ionicexchange during mixing, which in turn dependson the 226Ra activity of the terra rossa deposits);(c) the specific surface available for contact withmixed waters (which should have relation withthe overall permeability) that should enhance226Ra release by ionic exchange; (d) the effect ofthe salt-water component that could cause dilu-tion (present seawater with zero Rn concentra-tion) or concentration (226Ra and 222Rn from oldsalt-water component).

    The information gained by the study shouldbe of help for oceanographers involved in therecognition and evaluation of submarine andcoastal discharge: in karst coastal aquifers,groundwater discharge can transport very differ-ent amount of radon not easily related to the saltcontent of waters.

    The proved desorption of Ra quartet and Bafrom sediments at freshwater-seawater interfacein coastal aquifers suggests that ionic exchangedoes not limit its influence to major constituentsmobility, but may embrace several other ions andisotopes. Therefore, ionic exchange and sorptionprocesses should be considered with great atten-tion in the hydrogeology of coastal aquifers, giv-

    en that such processes may have an important rolein the transport of pollutants within groundwaterand to the marine environment.

    Conservative pollutants do not interact withthe rock matrix and chemical and microbial reac-tions are not able to transform or disintegratethem; after entering groundwater, they flow aswater does. On the contrary, non-conservative(reactive) pollutants undergo processes that gov-ern the balance between their accumulation andsolubilisation in soils, sediments, surface watersand groundwater. Many of the most dangerouscontaminants belong to the last category: theirbehaviour at seawater-freshwater interface ispractically unknown.

    Detoxification at saltwater-freshwaterinterface

    Recent environmental debate focus theattention on the potential impacts from the slowaccumulation in soils and sediments over the longterm of toxic materials and the risk of their mobil-isation linked to the change of environmentalconditions.

    One of the most alarming issue concernsheavy metals. Soil, freshwaters and estuarine sed-iments can be considered as the ultimate long-term sinks for heavy metals: the question is ifthese sinks could revert into sources followingchanges in the factors that can control their chem-ical form (Stigliani, 1994). The Author points outthat "the ability of soils and sediments to serve as

    103

    ENVIRONMENTAL TRACING IN COASTAL AQUIFERS: OLD PROBLEMS AND NEW SOLUTIONS

    Figure 13 - 222Rn activity distribution (with location of sampled wells and springs) and T.D.S. contour lines (Salento aquifer– Ionian side – Southern Italy), (from Fidelibus et al., 2002).

  • large depositories for storing toxic chemicals canlead to a false sense of security when it isassumed that the stored toxic materials willremain forever locked away".

    Clays, especially montmorillonite, organicmatter, oxides and hydroxides (mainly Fe-, Mn-and Al-mixed oxides) supply the substratum forthe sorption of heavy metals. Organic-rich layers,as an example, exhibit at times anomalous enrich-ments of trace elements, heavy metals and rareearth elements (REE).

    The capability of soils of adsorbing andaccumulating heavy metals depends not only onintrinsic chemical properties, but also on theirchemical speciation (distributions of metalsbetween dissolved and un-dissolved forms).Chemical speciation of metals varies with thepH/Eh changes, with microbial transformation(i.e., methylation, ethylation, etc..) by sediment-dwelling microbes and salinity. Microbial methy-lation is a key feature, for example, of the Hgcycle in both polluted and non-polluted environ-ments because the organic-Hg compounds arevolatile. Other heavy metals exhibiting methyla-tion are As, Pb, Sn, Se, Te, Pd, Pt, Au and Tl.

    The effect of redox potential on mobility iscomplex. The Eh variation causes metal valencychanges: Fe, Mn and As are more soluble in thereduced state. Fe and Mn in the oxidised statebehave, instead, as affective absorbers of otherheavy metals: when redox potential decreasesthese oxides dissolve and adsorbed species arereleased. However, sulphates reduce to sulphideswhen Eh decreases and this leads to precipitationof heavy metal sulphides. Thus, both accumula-tion and mobilisation become possible when Ehdecreases. An Eh increase reverts the processes.

    Furthermore, increasing salinity affects thesolubility of a number of important heavy metalsby altering the ion-exchange equilibrium, increas-ing soluble complexation and decreasing chemi-cal thermodynamic activities in solution; more-over, salinity increase may cause the decrease ofmicrobial activity (Hesterberg et al., 1992). Thus,the alternation of seawater intrusion and refresh-ing, changing the salinity in the saturated zone,

    may have relevant effect on the mobility of heavymetals. Akpan et al., 2002, studying the heavymetal concentrations trends in the Calabar River,Nigeria, confirm that relatively high metal con-centration are obtained in pore waters during hightide in the estuary. They attribute these levels tothe change of redox conditions of the sedimentsand the subsequent displacement of sedimentpore water rich in metals by seawater intrusionduring high tide. Van Geen et al. (1991) suggestthat one possible mechanism suitable to explainthe presence in the gulf of Cadiz of shelf watersricher in metals than the off-shore waters is thedesorption of the sorbed metals during estuarinetransport, although they outline that it remains tobe completely proved.

    Grassi and Netti, 2000, observed in theground waters drawn by some wells tapping bothclastic and carbonate aquifers of southern Tu s c a n y(Italy), mercury concentrations above the admissi-ble limit for drinkable water (1 mg/l). Groundwaters salinity varies between 0.7 and 34 g/l dueto seawater intrusion and an increasing Cl contentis consistently associated with increasing Hg con-centrations in the sampled ground waters,although to diff

Click here to load reader

Reader Image
Embed Size (px)
Recommended