SEA WATER INTRUSION IN COASTAL CARBONATE FORMATIONS IN , E. Custodio, Dr. Ind. Eng. M. Pascual, Geologist X. Bosch, Geologist A. Bayo, Geologist International Course on Groundwater

Hydrology, Universitat Polit~cnica de Catalunya and Eastern Pyrenees Authority. . Spain.

Abstract

Extensive carbonate formations are found along the Southern coast of Catalonia, in NE Spain. High population density and elevated water demand in small coastal basins with permanently dry creeks has put a great pressure to get water from limestones. Most wells exploit brackish water or became salt water polluted soon. Preliminary surveys show the existence of a very permeable carbonate formation near the coast that changes into a low permeability aquifer where distance to the coast line increases. In some instances cation exchange appears and shows the advancing nature of the salt water front. Two main areas are discussed, the Vandellos massif in which some deep observation wells have been drilled and the Southern part of the massif, where salinity problems are acute, and show a wide variation with areal situation. In these sites a thin fresh water layer floats on saline water, the mixing zone presenting a thickness varying from one site to another. Carbonate minerals saturation indexes are discussed.

Introduction

Recent interest in research into the hydrochemical processes at work in mixing zones where fresh water and salt water come into contact in coastal aquifers has given rise to a series of publications by authors

147 all over the world: Hanshaw and Back (19791; Back et al. (19791, Maga­ ritz et al., (1980l; Back and Hanshaw (19831; Herman and Back (!9841; Magaritz and Luzier (1985); Mercado (19851; Tulipano and Fidelibus (1986); Custodio et al. (19861 amongst others provide a sound theoreti · cal and practical basis for investigation. This article will give a summary of the findings on a regional level, corresponding to the first phase of a survey aimed at establishing the hydrogeological and geohydrochemical characteristics of the carbonate formations of Garraf and Vandell6s carbonate massifs, located on the coast of Catalonia. They have been the subject of the following publica­ tions amongst others: REPO Cl9701, Custodio et al. (19761, Custodio and

Galofr~ (19761, Custodio et al. (1975), Pascual et al. Cl986al, Pascual eta]. (1986 b), Custodio, et al. (19841, Soler (19841. Details will be given on the preliminary findings of a second phase of investigation, carried out in greater depth on a more local scale, in which a more detailed study is made of the hydrochemical changes occu­ rring in the mixing zone where salt water comes into contact with fresh water. A series of piezometres, wells and springs, some located in the mixing zone and others outside it, have been selected, and samples are periodi­ cally taken along the length of the water column both in the Vandell6s Massif and in the southern part of Garraf.

2 Hydrogeology of the Garraf and the Vandell6s Massifs

An important tourist infrastructure catering for summer and weekend visitors has developed along the coast line of the situated south of the city of Barcelona (fig. 11. Groundwater is the sole source of water supply to the area. Surface water collects in a reservoir on the river Foix (fig. 1) as does much of the domestic sewage from Vilafranca del Pened~s. Significant infiltration occurs through the reservoir floor. The water from the reservoir is used for local irriga­ tion. It is not fit for human consumption. A boom in urban and tourist development over the past 20 years has brought about the present situation characterised by high density of

148 ~ 0 .• I rn TrlaMiO m POioo1olo (t£J ~o~uras1ic- Cretaoeou• -l ~~~~~~~~I I I __j

Figure 1. Geographical and geological situation population in summer (over 500.000 inhabitants) and peak water demands in an area with limited resources. Serious deficiencies in water supply and water quality exist. The majority of wells exploit brackish water or are salt water polluted. In some formely freshwater wells up to 16 g/l of Cl· ion can be found. The Garraf Massif is a Jurassic (dolsparite and dolsmicrosparitel and Cretaceous (micritic limestone, marly limestone and marly layers) carbonate formation that overlies a Triassic series which outcrops on the Northern edge of the Llobregat valley (figure 1). It is affected by intense brittle tectonics with N.W.- S.E. structural directions. A layer of calcarenitic miocene covers the mesozoic materials and fills small secondary depressions inland and along the coast. A karstic morphology

149 has developed along the fracture lines with numerous caves, sink-holes and surface karstic canvings. Karstic solution paths develop particular­ ly at higher levels, that only reach the water-table at specific points where underground channels exist below sea level, forming submarine springs (La Falconera, Aiguadol9). Along the coastline, extensive areas exist where piezometric levels are just above or below sea level due to the effects of pumping, even when this occurs several kilometres inland. The piezometric head in a large number of wells is higher than the regional piezometric level, due to the presence of marly layers and marly limestone layers of Aptian age found in the upper Cretaceous carbonate series. This gives rise to numerous perched aquifers partially charged from the Foix reservoir. Preliminary studies show the existence of a thin, saturated, highly permeable formation near the coast that gradually becomes less permeable as the distance from the coast increases. This is the layer most affected by salt water pollution. Bayo et al. (1986) and Pascual et al. (1986 b) discuss the hypothesis that groundwater circulation is based on a two flow system: rapid non-saturated drainage or possibly partially saturated during recharge, similar to karstic flo~~ "sensu stricto," and underlying "slow drainage" where the groundwater flow is much closer to a diffusive one. The Vandellos Massif is an important formation in the Catalan coastal ranges. Some parts of the Massif reach up to 700 m above sea level less than 5 km inland. The basal formation of the zone is composed of a series of secondary dolomitic limestone and marly limestone material overlying in angular discordance paleozoic bedrock (schist and granite) and clay-marls of the Keuper. These pile-up of materials, over 1000 km in thickness,are affected by folds and predominatly NE-SW fractures, running largely parallel to the coast. The secondary permeability and porosity is due to fracturing and prefe- rential karstification of certain fracture systems. Primary porosity of the rock seems to be practically negligible. The piezometric level is close to sea level. An extensive unsaturated area exists where a rapid system of percolation and preferential drainage takes place through a series of faults following two main

150 directions !Custodio et al., 1984). The most permeable zone in the saturated medium is found along the coastal fringe. The mixing zone in this highly permeable zone is relati­ vely thin (transit occurs within a few metres), subject to heteroge­ neous reactions and very stable under peak recharge conditions.

3 Theoretical principles governing chemical changes in the mixing zone

According to Hanshaw and Back (1979) the mixing zone is, from a dynamic point of view, the most active part of the carbonate geochemical system. It is here that physico-chemical changes occur when two types of water with very different ion contents come in contact with each other. Hanshaw and Back 11980b) state that where two types of water of differ­ ing salinity, both saturated with one or more carbonate minerals, come in contact with each other, the resulting solution is generally subsatu­ rated relative to calcite and dolomite and thus capable of producing rock dissolution. The fact that certain mixtures usually become subsatu­ rated in calcite and dolomite has been confirmed under laboratory conditions !Thrailkill, 1968; Runnells, 1968, 1969) and has been observed "in situ" (Back et al., 1979; Herman and Back, 1984). Carbonate mineral saturation may occur due to the non-linearity of the solubility of the mineral with respect to temperature, salinity, partial pressure of CDe and ionic strength (Back et al., 1981). If the dissolut­ ion of carbonate minerals occurs in the mixing zone, thereby producing a transfer of mass from the rock to the water and, moreover, groundwater flow exists, conditions are optimum for an increase in secondary permeability. Despite the fact that changes occurring in the mixing zone may veer either towards subsaturation or oversaturation, the overriding tendency is towards dissolution ITrailkill, 1968: Plummer et al. 1978). This is because, for kinetic reasons, much more calcite is dissolved in short periods of subsaturation than precipitates over longer periods of oversaturation (Back eta!., 1979). Thus, Hanshaw and Back 11980 b) and Back and Hanshaw 119B2l venture an

151 alternative hypthesis to the commonly held belief that all paleokarst are the result of phenomena produced by surface or subsurface meteoric: water. They suggest that high porosity and secondary permeability zones in present coastal carbonate aquifers are the result of a process of dissolution in the mixing zone real ted to oscillations in sea level during the Pleistocene. Apart from the above-mentioned process of dissolution and precipitation, other changes take place in the theoretical c:hemic:al composition of the water in the mixing zone as the result of the interaction between the water and minerals in the aquifer. Ion exchange reactions, even in carbonate aquifers, are very important. Attention has frequently been drawn to their inc:idenc:e and effects (Custodio, 1976, 1981, 1985, 1986; Custodio et al., 1986; Mercado, 1985). As salinity increases, some Na+ may be retained by the reactive part of the roc:k and exchanged for Ca•2 and Mg+ 2 present in the minerals. Changes that oc:c:ur in the theoretical mixture that may be considered characteristic: of the process of salinisation are: Ci) increase in the concentration of ca• 2 and Mg•a together with a drop in Na•, Cii) a drop

in the concentration of HC0 3 - compensated by a slight decrease of pH, (iii) temporaly drop in the ratio Na/Cl to below the values normaly associated with freshwater and seawater (0.86) and Civ) temporaly drop

in the ratio of sodium absortion mNa/(mCa + mMg)~; m = molarity. These facts have been confirmed in the Pliopleistoc:ene sand and limesto­ ne aquifer in the coastal plain of Israel CMerc:ado, 1986) and in the confined limestone gravel of the deep Bes6s Delta aquifer, (Custodio, 1982). A clear distinction c:an be made between mixing zones where salt water has intruded and zones where salt water has been trapped in formations and is being pushed bac:k. Other processes at work in the mixing zone are dolomitization, c:hertifi­ c:ation, deposition of manganese oxide CFrank, 1981~ Bac:k and Hanshaw, 1983; Magaritz, et al., 1980; Mercado, 1985). Tulipano and Fidelibus 1985 suggest that dolomitization may oc:c:ur when sea water intrudes beneath the mixing zone, basing their observations on the decrease in the ratio Mg/Ca associated with intrusion. The longer the seawater remains in the terrain, the lower the ratio.

152 Hydrochemical characteristics of the southern half of the Garraf Massif

Sea water intrusion that largely affects the Southern half of the Garraf Massif determines the chemical characteristics of recharge water, changing it from a calcium or magnesium bicarbonate type to a sodium chloride type (fig. 2 a and bl, except in those areas where infiltra­ tions from the Foix reservoir occur. Down-river from the reservoir, more or less following the same trajecto­ ry as the creek (permanently dry), the anion content of the water would appear to show the influence of modifying factors that reduce the theoretical amount of sodium that should be present in a theoretical mixing zone (fig. 2a). Taking the batch samples as a whole, the behaviour of the principal ions is variable in relation to the concentration of Cl-, although a first approximation shows the following tendencies (figs. 3, 4 and 51: (i) marked excess of Ca•• with a parallel deficit in Na• and Mg••, principa­ lly in those areas where salinization is most advanced, (iil sligth, negligible excess of so~-• in water with a concentration of less than 2000 mg/1 of Cl-, with a subsequent swing to a marked deficit in water most affected by salinization, (iiil slight excess in the concentration of HC0 3 - throughout the whole process of salinisation (ivl temporaly reduction in the ratio rNa/rCl, except in the water in the inland calcarenitic miocene strip where salinity is low (figs. 2 and 41. Simple preliminary treatment of the samples as a whole revealed the possible existence of modifying factors such as cation exchange between Na• in the water and Ca+ 2 in the rock where salinisation is most advan­ ced. The results obtained from the first detailed samples taken when regular monitoring of hydrochemical changes began are not as conclusive as when samples had been previously treated as a whole. Samples taken at three different levels in a borehole located close to Calafell in December 1985 confirm the same processes to be a work as were identified by simple preliminary treatment (figure 6). The second batch of samples taken in March 1986 do not reflect these processes as clearly (figure

71, particularly the cation exchange between Na• and ca·2 • Only a marked

153 lA-··--· ·-so-.-·----·--·--·--~~~~5 I I Miocene • I • :1 I I Ca L------'"------>·N•• k I

0 I I II 4 II ---·--·

FIGURE 2. Hydrochemical facies in the Garraf Massif. Zonation derived from trilinear diagrams.

A/Cations B/Anions. miJinv line without reactions

10 I I T . I I v·y WATot.. g r I 'I I o,s·t. ..-, ..;.;;.- ..-+,l-;i"'..:.._-:. -+. - .... :-lr-..: ....:..::.: .. -:;_-:·:_:;.;;_::,._·· =---· ...-l.. t---+·---1 ! 7 ~ (.-~- -.-·.:~'::,..-, :-+~ I .... . I ' ATU,...... - i I ....- i .-:('' i • WATI:III... I I !! lf~· i 0 1J 0,5 I i • 01J5 0,1 0,5 1 5 10 •o I I cr lglll z ! i ~~I I . I I ... :.::. 0,1 / I i 0,05 ·v· /j I . I I ~y..... I ·. ' I I i i ' ops 0,1 10

0,5 i i '0 I ops 0,1 Q,5 10 crlglll ·~ 0,1 I I I ! [ -O.S·~--~-+-----+--~----~~-~;~I .~ I ~ ..- :. ::; . . 1,.._,.:::. I •o · .:·:··.1 :. :~.. • ;.V · 0 , ..... ;·... ·. : .... 1: .··::J: /_. Q,l ~.=. .:: ...... +-- OP6 . . . .. I ·-:r• I ' I

0 Q,l 0,5 1 0,(15 OJ Q,51 5 10 Cr (VII) cr lvlll Figure 3. Ratio of the principal ions to Cl- ion content. First batch of 110 samples from the Garraf Massif, Spring 1984. Neither temperature nor pH content were monitored.

reduction of Na~ and Mg+a in the most saline water at the bottom of the

well were detected. An excess of ca~• in dissolution is apparent. This increases as salinity goes up and is accompanied by a parallel slight

2 reduction in Mg•w. The differences observed in 504 - and HC0 3 - are neither significant nor conclusive. pH drops as depth increases, while

the opposite occurs with temperature and dissolved increase C0 2 This leads to a reduction in the saturation indexes with respect to carbona­ te minerals as calculated using the programme WATEQF (Plummer eta!.

155 1,1

~ 1,11 .."" E 1,4

I,:S

1,2 I (.'$ 1,1 FRESH Mixi~ line withont r1 :JC!ions l,e 1 WATER I 0,11 ·. -SEA 0,11 ~------~---+------~----~~~~----~··~~·-·------4-L---4WATER

0,7 1-- ... a,e ..

0,5

0,4 0,1 1 10 c1- (o/tl Figure 4. Ratio rNa/rCl and concentration of Cl- (from initial samples, Spring 1984) .

• 8

+ 0 z -ll~------+------4------~~-++---

'CJ -·~t===:::::*:-;;;::;:~;::::;;:::=:;;:;;;*~~~;:::==~=:; ' Ul -4~------~------~------~~--~~~

~ 0+------~~:=~~~~~~~~~~~~~~~~~

0,1 10

Figure 5. Changes registered in the average ionic concentration of all analyses, taken at intervals of 500 mg/1 of Cl-, with respect to those of theoretical mixtures in the Garraf Massif.

156 so=4 3 Na 20 I ' I ' ~ I I I :§ I I I ' I 40 r: I I Q. \ I c• \ 50 '- ~~ - \ \ I ) ~~ ""' 60 l

!() 40 50 80 0 5 100 5 10 !() 40 50 60 0 5 0 10 0 10 I 0 meq/1 meq/1 meq/1 meq/1 meq/1 m1q/l

ml>lng rldool 1m Depletion 0 Excess -measured

Figure 6. Hydrochemical variations with depth in a piezometer in Calafell (Garraf Massif). Batch 6/12/85 (neither temperature nor pH content was registered; analyses were carried out 10 days later).1976).

Neverthless, throughout the water column the water is saturated with respect calcite and dolomite and slightly subsaturated with respect aragonite. Herman et al. (1986) have found a similar situation on the island of Mallorca (Spain). They compared the saturation index of

carbonate minerals and the partial pressure of C0 2 observed with those that the mixing 2ones should theoretically have and have come to the conclusion that even in a state of saturation, water that contains more taht 50% seawater has a lower saturation index and higher partial

pressures of C0 2 than their corresponding theoretical values. For the time being they atribute the relative drops in saturation indexes to the

apparent increase in C0 2 in the water. Problems in flow dynamics may also produce variations in the saturation index, probably giving rise to a complex process of dissolution­ precipitation. Wet periods, when recharge is greatest, would be most favourable to dissolution because transmission of mass is greater. Dry periods with little recharge would be most favourable to precipitacion

157 inside the mixing zone. Verification of these hypotheses are the object of present studies based on systematic sampling in wet and dry periods, on the use of environmen­ tal isotopes to determine areas of recharge, water transit times, etc.

Electrical a so= c •• Conductivity a 4 II a ao ' ',.... ~ ~- '\ .... \ ~ !10 -.- - I I Cl- I E I I ~ I I 40 r: " I No:_ I I ... I ___.l'l:, I 0• ..... 50 ' ' \ I I l I ' I .. I 50 l 2 4 IS 10 12 30 40 110 eo 0 10 0 II 10 0 5 10 10 • meq/1 meq/1 meq/1 meq/1

SiOa Hardness

't- t- ' I I 40 .c. I I l : I~ 't- 50 - -- 't(IZC....; I II I I I i I 1 I I I t t I I 80 ~~~1~0~111~20~~7~---.~~~~10~~15~20~~_.--~4-~~ ' 10 II 12 Ill 1!00 1100 1000 1200 1400 16 17 meq/l C02 mo/1 Si 0 Alkalinity 2 mo/1 C05 Ca TAC 20 \ \ \ \ '\ "Y / I rldool mi•ioo (I I I I I -~I II• 40 c -&i .l!tr~-~ I ....."' Ol "

Figure 7. Hydrochemical variations with depth in a piezometer in Calafell (garraf Massif). Batch 10/3/86.

158 5 Hydrochemical characterisctics of the Vandell6s Massif

Just as in the Garraf Massif, groundwater in the Vandell6s Massif is characteristically of calcium magnesium bicarbonate type. The mixing of this water with sea water depends on the hydrodynamic conditions preva­ lent, and may be the result of an advective process in rapid circulation zones, or a diffusive-dispersive process in slow circulation zones. In several observation wells along the coast that penetrate the mixing zone, and in some springs in the upper reaches of the Massif, detailed samples are being taken to corroborate the general characteristics established in a survey that has not yet been published by Custodio, Galofre and Bay6 (19B5l, with reference to the location of the nuclear power site in Vandell6s. A preliminary study of the first samples is summarised in Fig. B. Figure Be shows the saturation indexes relative to dolomite, calcite and aragonite using the programme WATEQF

159 (a)

21------~----+------+----+------~----+-----~

-II -e -II -5 C -4 ~0%o o,oa 0,1 II 10 mQ/1 Cl o Sprin<;~ from the int.-ior of the -if, x Piezomelllr • Dolomite • Colcite A AraQonite (b) • Piezometer A, * Piezometer 8, • Piezomelllr D, & PumpinQ well( d ) .-----.--.------.---.------.--.---~ 4001------+-----+-----1 0 0 ~ . 500 ~---~~ --- - - __ __.____ -;------.-

g ~,.__-•X A ,., Jl '-~ • • ~ 200 ~------~--+-.--+-.----t---t-0=-·-.----~,

100+------1---* ------+~"-____!,·+======-==v • • • 0,011 o,t 0,11 10 mQ/1 Cl A, B, C, D' Piezomet.-a SprinQs from the interior of the mauif "0, o SprinQa from the in'-

We would like to thank the Confederacion Hidrogr~fica del Pirineo Orien­ tal, the Asociacion Nuclear de Vandellos and the Geological Service of Catalonia for their collaboration and without whose help this study would not have been possible. We would also like to thank the Joint US-Spain Committee for Science and Technology (Project CAA83/007) for sponsoring the project. S. Balague,ICHPOl, was responsible for the greater part of the chemical analyses. Isotopic analyses have been done the Cedex, Madrid, under an agreement with the Public Works Ministry of Spain.

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164