Nitrate contamination of groundwater In the Republic of Lithuania

Nitrate contamination of groundwater In the Republic of Lithuania

v ALGIRDAS KLlMAS& BERNARDAS PAUKSTYS

Klimas, A.and paukstys,B.1993:Nitrate contam inationof groundwaterin the Republic ofLithua- nia.Nor.geol.unoers.Bull.424.75-85.

Inmanycountries nitrate contamination ofgroundwaterhas become aserious ecologicalprobl em and Lithuaniais no exception.In fact,irrational.unbalanceddevelopmentof the econom y coupled with speci ficgeological andclimaticconditio ns haveled to nitratecontamina tionon sucha scalethat it has become the object of interestfor specialistsof neighbouringcountries.

Con tam ination is worst in the vicinity of fertilizer factories and intensely manured agricultural land.Excessivenitrogenconcentrations ,typicallyas nitrates.are commonin unconfinedgroundwa- ter which is vulnerabl e to direct contami nation from the land surface and the atmosphere.In deeper aquife rs,apparentl y relativelyisolatedfromsurfacepollution,ammoniumis prevalent.Nitro- gencontamination of groundwater shows aclear increasing tendency with time.

Nitrogen compounds are not necessarily stable. Theycan beaffected by vario us biochem ical reactions.In these reactions.interconversion between nitrates.nitritesand ammonium canoccu r, as well as denitr ification (i.e.lossto the atmosphereas nitrogen gas).Thus,groundwater can purify itself from suchcontaminatio nwith time.

Algirdas Klimas.ARTVA.Eisiskiu pi.26, Vilnius.Lithuania.Bernar das Paukstys,StateGeolog ical Service of Lithuania.Konarskio35.2600 Vilnius.Lithuania.

Introduction

Groundwater is the sole source of drinking water in Lithuania (summary map - Fig. 1).

This is due to several factors; groundwater resources are very large (Juodkazis 1989, Juodkazis & Klimas 1991), there is abundant availablerecharge (annual precipitation avera- ges from 540 to 930 mm), and geological conditions are favourable (the thickness of sedimentary rock cover varies from several hundred metres to two kilometres or more).

The upper aquifers are composed of glacial (e.g. Samogitianand Baltija Hills,MiddlePlain - Fig. 1) and glaciofluvial sands and clays (e.g.

Coastal lowland, Southeast Plain). Beneath theseonecan also find other fresh groundwa- teraquifers.Itis calculatedthat potentialfresh groundwater resources are around 3.2million m'/day in Lithuania (Juodkazis 1989).Extensi- ve surface water resources (rivers,lakes) also exist, but in many places these are polluted and not suitablefor water supply.

During the past 30 years hydrogeologists have also investigated groundwater quality, and it is clear that groundwater is threatened by extensivepollution(Kondratas &Mikalaus - kas 1973, Mikalauskas 1976, Klimas 1979, Klimas 1990).Especiallyworrying is the threat posed to groundwaterbynitrogencompounds (Zabulis 1988, Klimas 1990a, Klimas 1991), particularly by nitrates. Particular sources of

intensive nitrogen contamination are industry (e.g. Jonava factory for nitric fertilizers ) and agriculture (e.g.large pig-breed ing farms).

The studies of the current,and earlier,aut- hors (Kondratas & Mikalauskas 1973, Klimas

&Paukstys 1990,Klimas1991)show that nitro-

gen contaminationof groundwater is occurring over pract ically the whole territory of lithua- nia. This phenomenon is largely due to regio- nal pollution sources; i.e. atmospheric pollution and agricultural activity.The effect of the for- mer is not great;only 5-10% of groundwater contaminationmay be explainedby atmospher- ic deposition('acid rain').The remaining 90-95%

is the result of agricultural activity.The promi- nence of agriculturalpollution is due to seve- ral factors. Firstly, during the past 50 years, an unbalanced agriculture,occupying an area of some 3.6millionhectares,has been develo- ped. Increasedcrop-y ieldshave been achieved almost purelyby theuse of minera lfertilizers, applied by aircraft with little attention being given to the actual requirements of the crops (fertilizers were spread both in autumn/winter and in spring).Typical applications have been around 299 kg active fertilizer per hectare. It has also been common practice to spread manure on the fields in wintert ime.Duringthe snowmeltfloods,the manure-ladenrun-off be- came a source of contamination for surface- and groundwater alike.

76 AlgirdasKlimas&BernardasPauk~tys ^NGU·BULL.424.1993

N

"-

" ^\ ...

( I L.,

-

r

L.J

Lithuanian - Bielo r us sian hills

o 30 ^60km

,,=,=~=~I

Fig.1.Location map.showing themaingeomorph ological elements of Lithuania.

itrate contamination of groundwater is a sourceof concerntheworldover.e.g.in Great

'Britain (Foster et al. 1985). Czechoslovakia

(Vrba 1985). India (Handa 1983). Republic of South Africa (Heatonet al.1985).U.S.A. (Hall- berg 1987) and Poland (Kowalik 1987). In many countries.it is observed that the nitro- gencont ent ofgroundwate risincreasingwith time.The rate of increase has been determi- ned in Belgium (de Smedt & Loy 1983),Den- mark (Overcaard 1985) and Germany (Such 1985). It is noted that unconfined groundwa - ter,having adirect connection withthe atmos - phere. istypically beingcontaminated bynitra- tes (Krainov et al. 1989). Reduced nitrogen species(ammoniumand nitrite)are only found in unconfinedgroundw ater atthe centreofinten- sively contaminated areas (Klimas & Kadunas 1983. Zabulis 1988). The reduced species may.how ever.accumulate in deeper aquifers as a result ofthe reduction of nitrates(Botch er

& Strebel 1985.Klimas 1990a. Klimas 1991).

The aim of thispaperis to demonstrate the

extentof nitrogen contaminat ionofgroundwa- terin Lithu ania. itslateraland verticaldistribu- tion.andthedistributionof oxidisedand redu- ced nitrogen species in the groundwater.

Nitrogen contamination - common features

Groundwater contamination by nitrogen may be local (point-source) and regiona l (diffuse source). Sources of local cont amination may be associated with :

(a) towns and industry: sewerage systems, domesticandindustrial wastedisposa l.stores of industrial and raw materials.anthropogenic deposits and others.

(b) agriculture:stores of fertilizers and chemi- cals.silos.manure storage.large farms.sett- lements.

As regardsregionalcontamination there are two possible sources: polluted atmospheric

NGU·BULL.424.1993

deposition and agricultural activity. Burning of fossil fuels and some fertilizer factories contaminate the atmosphere with nitrogen compounds. In agriculture,organicand mine- ral fertilizers are the main source of nitrogen contamination (Gustafson 1983, Such 1985, Gerhart 1986,Scheffer&Walther 1988,Zabu- lis 1988).

Nitrogen is a chemical element of variable oxidation state.In groundwater it may be found in the form of dissolved gases (NH" N" N,O, NO,NO" N,O,)orions derivedtherefrom (nitra- tes (NO,'), nitrites (NO,"). and ammonium (NH,+)). Many of the gaseous nitrogen com- pounds are relatively soluble in water without being particularly reactive (N" N,O, NO), alt- hough on oxidation , they may react readily with water,e.g.:

2NO + 0, - 2NO,

2NO, + H,O - 2W(aq)+ NO,' + NO,'.

Ammonia reacts with water to form a basic solution, i.e.:

NH, + H,O - NH/ + OH',

but the greater part often remains inthe soluti- onin the form of dissolved ammonia (Krainov et al. 1989). Two main factors, oxygen and organicmaterial,control the state of nitrogen in groundwater.The most stable form of nitro- gen in oxidised environments is nitrate . In a closed system, dissolved oxygen is rapidly used up in the oxidation of organic material, whereafter oxygen from other dissolvedspeci- es (e.g.SO,=or NO,')is consumed. The reduc- tion of nitrates and nitrites to ammonium oc- curs after (Le.at a lower Eh) reduction of iron and manganese oxides, but before (Le. at a higher Eh) the reductionof sulphate to sulphi- de (Krainov et al. 1982). Bacterial activity is intimatelyinvolved in allthe transitions betwe- en the various nitrogen species, Le.

NO,' ... NO,' ... NH/.

During the course of such transformations, a certain portion of nitroge n is liberated as nitrogen gas(N,), Le.,the totalamountof nitr o- gen inthe system decreases due to theproces- ses of biodegradationand biodestruction(Bott-

cher & Strebel 1985,Wolff et al. 1985,Trudel

et al.1986,Ronen et al.1987,Zabulis1988).

The geochemical environment, as defined by the redox potential(Eh)and acidity(pH),deter- mines the distr ibution of the various nitrogen species in groundwater (Krainov & Schwetz

1987 ).

Nitratecontamination of groundwater 77

degree, in groundwater at different pH and Eh values (Krainov et al.1989,Klimas1991).

The mechanism of the transport path of nitrogen compounds down to groundwater is rather complicated (Gustafson 1983, Gerhart 1986,Thiery & Seguin 1986,Scheffer & Wai- ter 1988b).The main biochemical transforma- tions take place in the soil. The amount of nitrogen leached out of the soil depends on many factors: climate, soil-type , quantity of fertilizers, their type and time of application, agricultural practice, etc. Biotransformation processes continue in the unsaturated zone, and some denitrificat ion may occur,returning part of the nitrogen to the atmosphere.Thus, the concentration of nitrogen compounds in the unsaturated zone decreases with depth.

Also , infiltration of sewage into the ground by means of septic tanks, infiltration plants andliquid manure spreadingfields is common- ly practised as a means of treating sewage effluent,andithas beenshown thatthemajori- tyof pollutants,including nitrogen,can reach the water table (Klimas1988b).Theinfiltrated water initially contaminates the surface layer of the unconfined groundwater.The groundwa- ter velocity, horizonta l & vertical dispers ion coefficients and hydraulic conductivityof un- derlying strata deter mine the furth er lateral and vertical spreading of the contamination (Klimas 1990c).

A phenomenon commonly observed in con- nection with local and regional groundwater contamination by nitrogen is that of vertical and lateral geochemical zonation (Zabulis 1988, Krainov et al. 1989).Typical of vertical zonation is a decrease in nitrates with depth and a corresponding increase in ammonium.

Ininstances ofregional contaminationof dee- per confined aquifers , the offending species is typically ammonium,derived from reduction of nitrate. In the case of local contamination a contrary picture has been observed; in the centre of intensive contamination,ammonium isusually dominantin near-surface groundwa- ter,withnitrates becomingmore common fur- ther from the centre. All these cases can be documented byconcrete examples from lith- uania.

Methods

Inallof the following hydrochemical investigations.stan- dard analytical methods have.been applied; pH and Eh have beenmeasuredinthefield.and otherparameters in thelaboratory followingpreservationandstorage ofsarnp- les.Pilot studiesoflocalgroundwater contaminationwere

78 AlgirdasKlimas&Bernardas Paukstys ^{NGU·BULL424.1993}

Table 1. Someindicesof the unconfined groundw ater qualityin the region of the Jonava nitric fertilizer factory. ote that concentratio ns of nitrateand ammoniumare referred toinmgNO,'or NH.+/Ithroughout the paper.ratherthanmgNl1.

Unconf inedgroundwater Numberof Number of Water quality indic ators(arithmeticmean)

samplingpoint observation samples pH Eh NO,- 0,- NH.+

(sampling depth.m) wells (mY) mgn mg/l mg1

Background groundwaterquality 3 30 7.2 130 0 0 0.2

(6 - 12m)

9.3 170 475 60 3000

Belowthefactory's 14 25

ammoniacwaterreservoirs (6-8 m)

400 19 0.15 2.1

Belowthe factory's 5 15 7.5

emerge ncystorageponds (8- 10 m)

46 0.03 1.2

Regionofpollutedprecipitation 7 40 7.4 400

(10-12 m)

carried outatspecially equippedobservationareas(Jonava nitr icfertilizer factory.three pig-breeding farm s.the karst regionof North Lithuania,etc.)

Forthe studiesofregion al pollutionofunconfined ground- water.some hundreddrilledwells.and approximately one thousand dug wells were sampled. Intermediate confined and deep confined groundwater chemistrystudiesare ba- sed onthe sampling resultsfrom approximately 3000dril- led wells.

Nitroge n compounds were analysed in the laboratory usingstandard methods such asNesslerizationand colori- metry.

Local contamination

The Jonava factory for nitric fertilizers is an exa mpleofespecially intensivelocalindustrial contamination of groundwater by nitrogen compounds (Klimas & Kadunas 1983, Klimas et al. 1988).The factoryis builtat theintersec- tion of the valleys of two large rivers: the Neris and Sventoji(Fig.1). Here,inthe upper part of the geological section, alluvial sand and graveldeposits dominate.The totalthick- ness of these sediments is typically 15-20 m.

The deposit s con tain a saturated zone which is typically 5-12 m thick . Among the sources of groundwater contamination one can name (Table 1):

(a) emergency flows from ammoniac water reservoirs and infiltr ation of contam inated ammoniac-richwaterfromemergencyponds.

(b) infiltration of contaminated precipitation . Inthe regionof the factory's storage reservo- irs andponds,contaminated infiltratio n waters

mixwith confined groundwater which dischar- ges in the locality. The latter naturally has a smallcontentofdisso lved solids,butthe conta- minated mixt ure discharging into the valleys ofthe above-mentioned rivers is best descri- bed as a highly minera lized water,containing especially high levelsof ammoniumand sodi- um (cations) and nitrate, chloride and bicar- bonate (anions).In the vicinity of the factory. precipitationis contaminated by ammonia,lea- ding to a nitrate concentration in unconfined groundwater some tens times higher than the background level. The process of nitrification ofammo nium beginsin the atmos phere,conti- nues in soil,and is completed in the aeration zone (Klimas & Zabulis 1984).

Large cattle-breeding and pig-breeding farms are the legacy of collectiv e agriculture in Lithuania. During the past 15 years.more than 40 such farm-co mplexe s have been bu- ilt. The capacity ofthepig-br eedingcomplexes is around 12,000 - 54,000 pigs per year.The manure accumulated here is remov ed using water,andthe resultantliquid manure is spre- ad on fields. Due to poor technology, great quantities of liquid manure accumul ateon the fields.In winter and spring,fields of a limited area are intensively manured (up to 700-1100 kgN/ha) when soils are still saturated.In this way,unconfin ed groundwater can beseverely contaminate d (Zabulis 1988).

Table2.Chemicalcompositionof unconfinedgroundwaterbelow the manured fieldsof somepiq-breeding farm s.TDS= totaldisso lvedsolids.Corg=organic carbon.

Location Number of Sampling Chemical compo sitionof groundwater.mg/l (Aquifer) obse rvation depth Arithmeticmean [range ofvariation)

wells (m) TDS Corg NO,' NO,- NH.+

dissolved

Valkininkai 21 10- 13m 400 10 15 0.2 1

(sand.thick unsatura tedzone) [300-1200) [9-26) [7-710) [0.1-0.3) [0.3-7)

Rokai 14 1-3m 700 39 450 1 3

(sand,thinunsaturated zone) [400-2200] [17-61] [10-900) [0.1-2J [0.4-12)

Sirvinta(sandyloam) 9 5-7m 1300 45 30 2 30

[200-2000) [11-64) [5-300) [0-3) [2-35)

NGU-BULL. 424.1993 Nitrate contamination of groundwater 79

N

f

Fig.2.Trendsinpollutionofkarstgroun dwate rin Lithua- nia.1-areasinwhich karstwaterwas alreadypollutedby nitrogen compoundsin1978-1979;2-areasin whichindi- ces of nitrogenpollution increasedbymore thanafactor of two during a10 year period.

excessive nitrate concentrat ions.Evenfurther down-gradient ,denitrif icationand dilution lead to an eventual attenuationofnitrateconcentra- tions (Zabulis 1988).

The territory of North Lithuania possesses an outcrop area in excess of 1000 km' of gypsiferous Upper Devonian deposits (Pauk- stys 1991).The upper part of these strata is karstifi ed. Recharge of ground wate r typically occursviathe direct infiltration of precipitation on the outcrop area,and karst water dischar- ges into small local rivers. In these gypsife- rous rocks, a calcium sulphate groundwater, witha moderatedisso lvedsolidscontent (2000 - 2500 mg/l) is typicallyfound. UpperDevonian sands and sandsto nesunderlying these gypsi- ferous stratatypicallycontainfresh groundwa - ter. Groundwater within a large area (about 400 km') ofthe gypsiferous deposits is,how- ever,whollyoralmost wholly unprotected from surface contamination,and is thereforebeing intensively polluted by organic materials or

Table 3.Thechemicalcompositionofpollutedgroundwaterin thekarst region ofNorth Lithuania.

Aquifer Some indices of chemicalcomposition,mgll (except Eh,pH) (Sampling dept h,m)

TDS Ca++ Mg++ SO,= Cl- Eh(mV) pH NH,+ NO; NO;

Phreatic,sandyloam(borehole)(10-15 m) 352 61 27 32 12 <100 6.8 120 <0.01 2

Phreatic,dolomite (dug well)(10-15m) 1063 154 64 101 135 330 6.9 <0.1 <0.01 180

Phreatic,dolomite {borehole) (20m) 392 30 31 8 5 150 7.6 30 <0.01 <0.5

Deep confined,dolomite (borehole)(40-60m) 365 22 31 4 5 120 7.8 45 <0.01 <0.5

The nitrogen in liquid manure is largely in the form of ammonium and organic nitrogen. If (a) the subsoil beneath the manured fields is sandy,(b)the unsaturated zone is thickand (c)themanure application is not very intensive (i.e. less than 300 kgN/ha in total), then the unconfinedgroundwater may escape excessi- ve contamination. Such contamination as ex- ists is typically in the form of nitrates (Table 2 - Valkininkai).Down-gradient fromthemanured field, denitr ification and dilution by clean in- filtration water result in a rapid attenuation of the contamination plume. If, however, the manured fields are underlain by moreclayey, fine grained subsoil,with a relatively shallow water-table,and if manuring isintensive, hea- vy contamination by ammonium and organic material is most typical. Down-gradient ,active oxidation of ammonium and organic nitrogen takes place in the groundwater, leading to

nitrogen compounds (Fig. 2). This is due to agricultural activity,particularly the application of mineral and organic fertilizers . Especially highly contaminated groundwater is found in the vicinityof cattle-breedingfarms with stores of manure,silos,sewage and manureaccumu- lators and fertilizer storage areas (Klimas &

Paukstys 1990) - e.g. the first two rows of Table 3.The areal distr ibutionof contaminated karst water areas shows a regular pattern (Fig. 2), reflecting very well the hydrogeologi- cal structure of the karst. The water is most contaminated in local areas of recharge (in interfluves and the upper reaches of small ri- vers) and least in the zones of discharge of karst water (in river valleys). This simplistic picture is, however,disto rted by the abstrac- tion of groundwater (Paukstys 1991).

Thekarstic groundwateris typicallycontami- nated by ammonium, and the thermodynamic

80 AlgirdasKlimas& Bernardas_Pauk~tys

Fig .3. Unconfined groundw ater pollution and soilbonitet in Lith uania. 1-highly polluted unconfined groundwater;

2- soilbonitetin therange 50·100(o f 1st-5thclass).

and biochem ical stabilty of that species is consistent with a low redox potential (below 200 mV) and a neutralpH,around 7 (Krainov et al. 1989).In the water from wells in areas wherethewaterhas goodaccess to the atmos- phere,Eh values aremuchlarger and nitrates prevail.

Reg ional pollution Unconfined gro undwater

In Lithuania,shallow unconfined groundwate r typicallyexistswithin Holocene(alluvial,aeoli-

NGU-BULL424.1993

an,marine sand)and Pleistoce ne (glacial and fluviolacialsand,loam,andsandyloam) depo- sits.The chemicalcompositionof such waters is very variable. It is, however, possible to

show that the level of contamination is two

to three times greater below more clayey, poorly-drained areasthanbelow sandier,well- drainedplains(Kondratas&Mikalauskas 1973, Klimas 1991). This difference is both directly andindirectlyconnected with thegeology,as itistypically the moreclayey areas whichare most fertile(soil bonitetof 1st-5th class:note - in Lithuania, 10classes of soil bonitet are in use) and thusthe locationsofmostintens- ive agriculture (Fig.3).

In Lithuania, the nitrate concentration in shallow groundwate r varies widely, from around zero to severalhundred mg/l,but one can state that beneath aboutone third of the country the nitrate inunconfined groundwater exceeds the permissible drinking water stan- dard(45 mg/l).Onlyinoneeighth of the terri- tory is the concentration less than 10 mgll (Fig.4a).Low nitrateconcentrationsarefound only in theCoasta llowland andSouth-Eastern Lithuanian plains,wherethe agriculturalactivi- tyisnot intense, andthegroundwater resour - ces are large(Juodkazis & Klimas 1991).The organic material content of groundwate r is similarly distributed: permanganate oxidation inthe regionscont aminated by nitratesgener-

Fig.4.Nitrate(a, b.c)andammonium(d.e.f) concentrationsinunconfined(a.d),intermediate confined (b.el.and deep con fined groundwat er (c,f)inLithuania.

NGU•BUL L.424.1993

ally exceeds 5 mg/I and in some places it reaches 10 mg/1.

The amount of ammonium in unconfined groundwater is comparatively small;itexceeds 0.2mg/lover only 10-15%of the territo ry (Fig.

4d).

Intermediate confined groundwa ter

This is the transitional type of groundwater between unconfined and deep confined groundwater. Itis typicallyfound inintermorai- nic deposits (Klimas 1991). In Lithuania, one or two such aquifers can typically befound in a vertical section throughthe Quaternary depo- sits, but in some places up to five or six can occur.They are usually hydraulically intercon- nected and may have good hydraulic contact with both underlying artesianwaterandoverly- ing unconfined groundwater.

Intermediate confined water is commonly only minimally protected against anthropoge- nic pollution (Klimas 1990a). Therefore,where unconfinedgroundwater is contaminated,inter- mediateconfined groundwaterisgenerally also contaminated (Fig. 4a and b). It is clear that this occurs where contaminated unconfined groundwater recharges intermediate confined aquifers (Le. a downward hydraulic gradient from unconfined to intermediate confined aqui- fers), most frequently in highlands. On the contrary, in plains and river valleys, where unconfined groundwater is sometimes highly contaminated, intermediate confined ground- water usually remains clean because the latter is protected by confining strata and generally by an upward hydraulic gradient.

The contamination pattern for intermediate confined aquifers is shown in Figs. 4b and e. Comparison of the diagram s shows that increased nitrate contentsinintermediate con- fined water are found,not in Middle Lithuania where unconfined groundwater contamination is greatest, but in the margi nal parts of the republic,especiallyin regionsof hightopograp- hy (Le., in the recharge areas of intermediate confined aquifers). It is clear that the areas ofintermediate confined groundwater contami- nated by nitratesare muchless extensive than those of unconfined groundwater.Nitrate con- centrationsinintermediate confined groundwa- ter rarely exceed 10 mg/I whereas in uncon- fined groundwater the value is usually 3-4 times greater.

Comparison of Figs. 4d and 4e indicates that intermediate confined groundwater is

Nitrate contamination ofgroundwater 81

more areally extensively contaminated by ammoniumthan unconfinedgroundwater.The- seareasofammoniumcontamination in uncon- fined and intermediate confined groundwater donot coincide. Inintermediate confinedaqui- fers the high ammonium values are found in areas where grou ndwater is better isolated from unconfi nedgroundwater(Iimnoglacial pla- ins,moraine dominatedareas and the Samogi- tian highlands - see Fig. 1).

Deep confined (artesian) gro undwater

InLithuania,deep,confinedwater can be found in deposits of all geological ages,from Cam- brian sandsto nes to Paleogene and Neogene sands (Juodkazis 1989). Such deep confined groundwater is one of the main sources for large,centralised water supplies in Lithuania.

Only a small propor tion of deep confinedaqui- fersare,however, reliably isolated and protec- ted from surface pollution. In recharge areas (topographical highs)deepconfinedgroundwa- ter is recharged from intermediate confined andunconfined groundwater.Due tointensive exploitation inWest and Middle Lithuania,lar- ge areas of depressed groundwater head have beenformed.Thus,some areas of natu- ral discharge of artesian water have become areas of recharge,creatingconditions suitable for the dow nward migration of polluted water to deeper confined aquifers (Klimas 1979).

Investigations reveal that deep confined water is only contaminated by nitrates to a limited extent (Fig. 4).The few areas of deep confin edwaterwith elevated nitrate concentra- tionsdo notcoincide withareas of unconfined groundwater and intermediate confined water which are highly contaminated by nitrates . Klimas(1991) has noted generally higher con- cent rations of ammonium in deep confined water s thaninintermediateconfined oruncon- fined grou ndwater (Fig.4f). The areas in which ammonium concentrat ion exceeds 1 mg/Iac- count for almost20%ofthe territory ofLithua- nia. Ammonium accumulates comparatively well in the isolated deep confined aquifers of West Lithuania but is practicallyabsent in the much less isolated, East Lithuanian , deep confined aquifers. A similar pattern can also be recognised in the distribution of organic materialin deep confined water. It thus appe- ars thatthelow nitrate and elevated ammoni- um contents typically found in deep confined

82 AlgirdasKlimas&BernardasPaukstys GU .BULL.424.1993

groundwater are the result of the chemical reduction of nitrate to ammonium (Klimas et al. 1988,Krainov et al. 1988, Klimas 1991).

Discussion

Investigations of localand regiona lgroundwa- ter pollution bynitrogencompounds in lithu a- nia revealregularitiesin the patternof contami- nationand allow one topredict itsfutureevolu- tion.

Redox potential (Eh) and acidity (pH)values reflect the processes of migration and trans- form ationof nitrogencompou ndsingroundwa- ter. Unconfined groundwater aquifers usually enjo ya good connection withthe atmosp here and therefo re exhibit high dissolved oxygen levels,commonly up to 7-10 mg/l,Eh typically exceeds +200 mV and pH is around 7. In such water one typically finds a limited range of nitrateconcentration(2-10mg/l).Incentres of intensive contamination, practically all the oxygenis consumed bythe oxidat ion of orga - nic contaminants.The amou nts of unoxidised organicmaterialandammoniumshowelevated values in such situations (see Table 2),and a corresponding reduction in Eh and nitrates (see Table 3) due to biochemical reduction by ammonification bacteria.The values of pH found at such contaminated localities vary widely (Fig.5).These chemicalconditions are especially noticeab le in contaminated,uncon- fined groundwater aquifers occurring in clay- ey, poorly-drained areas.

Fig.5 is compiled from studies carried out in fields sprayed with liquid pig-manu re (300-600 kgN/ha) covering an area of 150 hectares.Unconfined groundwateris found at adepth of 0-5 min sand,mor ainic sandy loam and loam deposits. It can be seen that the levelof contaminat ionofuncon fined gro undwa- ter,composition of thecontamination compo- nents and the values of Eh-pH of unconfined gro undwater found within the comparatively small area vary greatly.A pattern can,howe- ver,beidentified.Firstly,thenitrateconcentra - tion in sandy deposits is dist inctly lower than in clayey ones (Fig. 5c).Furthermore, increa- ses in NO,'concentrations correspond to inc- reases in Ehand pH. In addition .the concen- trationsof ammoniumandiron show a corres- pondence, and are related to depressed Eh and pH values.Thedependence ofammonium and iron on lithology is not so distinct as is the case for nitrates.

A similar patte rn can also be observed in 8 pH

~ Till

,

,

, I: "

11

1:1 : ' I

I ,

7

EJ ,

I

20 30

10 40

Fig. 5. Eh,pH diagrams for a) ammonium b) iron and c) nitrate in polluted,unconfined Lithuaniangroundwater. NH~.

mg/I

NGU-BULL.424.1993 Nitra te contamination of ground water 83

Fig.6.Ammonium concentratio ns plotted against dep th for the groundw ateroftheLithuanian karst region.

the case of regional unconfined groundwater contamination. It hasalready beendemonstra- ted that nitrate tends to bethe dominant nitro- genspecies in unconfined groundwaterin Lit- huania.Onlyin limnoglacial clayey formations can elevated concentrations of ammonium be found (Fig. 4d). One excep tion, however, is thekarstregion ofNorth Lithuaniawherefissu-

red dolomites with interbeds and lenses of

gypsu m outcrop (Klimas & Paukstys 1990, Paukstys 1991).Thegroundwaterwithinthese is strongly polluted with organic material and contains practically no dissolved oxygen.Re- dox potentials are typically loweredto 60-180 mV, and iron concentrations can reach 8 mgll (Paukstys 1991).Generation of methane and hydrog en sulphide can also be observed.

Ammonium dominates among nitr ogen com- pounds.The variationof itsconc entrationwith depth is presented in Fig. 6.

Fig.6demonstrates that in cases ofintensi- ve contamination, an inversion of oxidat ion- reduction zonationcantake place:a zone lac- kingoxygen,with lowEhvalues,and enriched in ammonium is for med above an oxidised zone(areas 2 on Fig.2).

On a regional scale, how ever, a normal zonation (Fig.4) is typically observed,i.e. an upper, oxidised, nitrate zone over a deeper reducedammonium-dominated zone.The fact that the amounts of ammonium and organic material in intermediate confined and deep confined aquifersareincreasingwithtimeindi- catesthatthephenom enonisreallyconnected with anthropogenic pollution. This increase is described by the followin g relations (Klimas 1991):

in intermediate confined water

NH: = 0.012 t + 0.411 (r = 0.39) P

=

=

NH: = 0.0405 t - 0.02 (r = 0.46) P= 0.142 t + 0.32 (r = 0.75) where P = permanganate oxidation mg 0,/1, character ising the amount of readily oxidisa- ble organic material in water

t = time from the beginning of the analysis (1958) period, in years.

r

=

From these equationsitfollowsthatindeep confinedwater organicmaterial has beenpre- sent since before 1958 (P

"*

while the process of the accumulationof am- monium has begun later (NH. ~ 0,whent ~

0).Furthermore,the correlationcoefficient for ammonium is half that for organic material.

This is presumably due to the instability of ammonium; with access to oxygen it readily oxidises to nitrite and eventually to nitrate.

The exploitation of confined groundwat er cangreat lyalter the geochemicalenvironment within aquifers. It can induce leakage of groundwater of a differentchemicalcomposi- tionfromstrataaboveand belowthe exploited aquifer (Klimas 1979). For example:

(1) water leaking to the aquifer from above iscom monly enrichedinoxygen and may thus lead to a progressive increase in the nitrate concentration in the exploited aquifer.

(2) water leaking to the aquifer from above may,however,be stro nglypolluted withorga- nic materialandnitr ogen com pound s,leading to consumption of oxygen in the exploited aquiferandincreasingconcentration ofammo- nium.

(3) water leaking to the aquifer from below (from deeper hor izons) is usually oxygen- poor,leading to decreasing values of Eh and accumulation of ammonium in the exploited aquifer. All these cases can be observed at groundwaterworksin Lithuania(Klimas1973, 1991).

Biotransfo rmation of nitro gen compounds, accumulation of unoxidised organic material, and a decrease in Eh can greatly alter the geochemical environment in an aquifer, such that favourable conditions for the mobilisation of toxic microelements (including heavy me- tals)maybeformedintheseaquifers(Krainov

& Schwetz 1987, Klimas 1988a).

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80

84 AlgirdasKlimas& Bern ardasPaukstys

Conclusions

Studies of groundwater pollution by nitrogen compounds have along history.The informa- tion prese nted in this paper has been collec- ted from gro undwater monitor ing data and from specialhydrochemicalmaps compiledfor individual aquifers.Theanalysis 01thismateri- alindicates a complicatedand ratherominous viewof the pollution of theLithuanianground- water environment by nitrog en compo unds. On the whole, it coincides rather wellwith the observations of such pollution in many other countries. Despite the complexity of the pro- cesses occurring in the subsurface, cert ain patterns can be identified in the data:

1) Groundwater is contaminated by nitrogen compo unds locally andonaregiona lscale.

There are many sources of local pollution, but only two sources of regional pollution - atmosp heric pollution and agricultural activity.

2) Nitro gen compounds take an activepart in biochemical reactions,and their spsciation and concentrations in groundwater can vary. Eh and pH conditions are decisivein determining whether nitrate or ammonium is the dominant species.

3) Patternsin nitrate and ammonium distribu- tioncanbedetected.Theseareexpressed as verticaland horizon talzonation of redox potential and nitrogen speciation.In upper aquifer horizons,and in recharge areas of deeper aquifers,nitratesdominate and Eh is typically greater than +200 mY. In dee- per aquifers,and especially in their transit and discharge areas, ammonium is domi- nant in groundwater,with Eh typically less than +200 mY.

4) In unconfined groundwater,withacomp ara- tively good connection to the atmosp here and a sufficie nt amount of dissolved oxy- gen,nitrate isusuallydominant.Onlyin the

GU-BULL.424.1993

centres of intensive pollu tion (e.g. manure spreadingfields)where microorganisms are not able to oxidise organic material and nitrogencompounds,does ammoniumdom- inate in unconfined groundw ater.

5)The amount of nitrogen compounds in un- confined groundwater may decrease com- parativelyquick lydue todenitrification,dilu- tion by 'clean' precipitation (infiltration re- charge).These processesproceedquicker in sandy depo sits and more slowlyinclay- ey deposits. Unconfined groun dw ater is therefor e most strongly polluted by nitrate in the clayeyplainsregionof Middlelithu a- nia.

6) In deeper aquifers where thereis anincrea- sein concentrations of unoxidised organic material due to progressive pollution, the groundwaterenvironment becomessteadily more reducing, Eh values fall and nitrate is transformed to nitrite and eventually ammonium.The concentration of ammoni- um increases with time.

7) The increasingly reducing environment in deeper aquifersisone of themainreasons

for the accumulation of some metals in

groundwater. Major changes in geoc hemi- calconditions canbecaused bygroundwa - ter exploitation. During such exploitat ion, not only regional groun dw ater flow pat- terns,but also hydroch.emicalzonation are disturbed. Exploitat ion can thus seriously influence the distribution , concentrations and speciation of nitro gen compoun ds in groundwater.

Acknowledgements

DavidBanks of NGU.Tro ndnerrn,encouraged us towrite truspaper and kindlyhelpedwithEnglish languagecorrec- non s for whichweare mostgrateful.We arealsograte ful toPro!.Dr.V. Juodka zis(Vilnius) for readingthe Lithuanian versionof the manusc ript.andtoDr.V.Zakutin(Mo scow) for giving permissio n to use someofthe resultsfromhis studies of groundwaterqualityIn Lithuania.

NGU-BULL. 424.1993

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Manuscript received August 1992; final revised typescript accepted February 1993.