Nitrate contamination of groundwater In theRepublic of Lithuania

vALGIRDAS KLlMAS & BERNARDAS PAUKSTYS

Klimas, A. and paukstys, B. 1993: Nitrate contam ination of groun dwater in the Republic of Lithua­nia. Nor . geol . unoers . Bull. 424. 75-85.

In many countries nitrate contam ination of groundwater has become a serious ecological problemand Lithuania is no exception . In fact , irrational. unbalan ced development of the econom y coupledwith speci fic geolog ical and climatic conditions have led to nitra te contamination on such a scale thatit has become the object of interest for specialists of neighb ouring countr ies.

Contamination is worst in the vicinity of fert ilizer factor ies and intensely manured agriculturalland. Excess ive nitrogen concentrations , typ ically as nitrates . are common in unconfined groundwa­ter which is vulnerable to direct contami nation from the land surface and the atmosphere. Indeeper aquifers , apparentl y relatively iso lated from surface pollution, ammonium is prevalent. Nitro­gen contamination of grou ndwater shows a clear increasing tendency with time.

Nitrogen compounds are not necessarily stable. They can be affe cted by various biochem icalreactions. In these react ions . interconversion between nitrates. nitrites and ammonium can occur,as we ll as denitr ificat ion (i.e. loss to the atmosphere as nitrogen gas). Thus, groundwater canpurify itself from such contamination with time.

Algirdas Klimas. ARTVA. Eisiskiu pi. 26, Vilnius. Lithuania. Bernardas Paukstys, State GeologicalService of Lithuania. Konarskio 35. 2600 Vilnius. Lithuan ia.

IntroductionGroundwater is the sole source of drinkingwater in Lithuania (summary map - Fig. 1).This is due to several facto rs; groundwaterresources are very large (Juodkazis 1989,Juodkazis & Klimas 1991), there is abundantavailable recharge (annual precipitation avera­ges from 540 to 930 mm), and geologicalconditions are favourab le (the thickness ofsedimentary rock cover varies from severalhundred metres to two kilometres or more) .

The upper aquifers are composed of glacial(e.g. Samogitian and Baltija Hills, Middle Plain- Fig. 1) and glaciofluvial sands and clays (e.g.Coastal lowland, Southeast Plain). Beneaththese one can also find other fresh groundwa­ter aquifers. It is calculated that potential freshgroundwater resources are around 3.2 millionm'/day in Lithuania (Juodkaz is 1989). Extensi­ve surface water resources (rivers, lakes) alsoexist, but in many places these are pollutedand not suitable for water supply.

During the past 30 years hydrogeologistshave also investigated groundwater quality ,and it is clear that groundwater is threatenedby extens ive pollution (Kondratas & Mikalaus ­kas 1973, Mikalauskas 1976, Klimas 1979,Klimas 1990). Especially wor rying is the threatposed to groundwater by nitrogen compounds(Zabulis 1988, Klimas 1990a, Klimas 1991),particularly by nitrates. Particular sources of

intens ive nitrogen contamination are industry(e.g. Jonava factory for nitric fert ilizers) andagriculture (e.g. large pig-breed ing farms).

The stud ies of the current, and earlier, aut­hors (Kondratas & Mikalauskas 1973, Klimas& Paukstys 1990, Klimas 1991) show that nitro­gen contam ination of groundwater is occurringover pract ically the who le terr itory of lithua­nia. This phenomenon is largely due to regio ­nal pollution sources; i.e. atmospheric pollutionand agricultural activity. The effect of the for­mer is not great ; only 5-10 % of groundwatercontam ination may be explained by atmospher­ic deposition ('acid rain'). The remaining 90-95%is the result of agricultural activity. The prom i­nence of agricultural pollution is due to seve­ral factors. Firstly , during the past 50 years ,an unbalanced agriculture, occupying an areaof some 3.6 million hectares , has been develo­ped. Increased crop-y ields have been achievedalmost purely by the use of mineral fertilizers ,applied by aircraft with little attention beinggiven to the actual requirements of the crops(fertilizers were spread both in autumn/winterand in spring). Typical applications have beenaround 299 kg active fert ilizer per hectare. Ithas also been common practice to spreadmanure on the fields in wintert ime. During thesnowmelt floods , the manure-laden run-off be­came a source of contamination for surface­and groundwater alike.

76 Algirdas Klimas & Bernardas Pauk~tys NGU · BULL. 424.1993

N

"­

"\............

I(L.,

/-\Dysnaplain

r J L.JLithuanian­Bielorussian hills

30o 60km,,=,=~=~I

Fig. 1. Location map. show ing the main geom orph ological elements of Lithuania.

itrate cont amination of grou ndwater is asourc e of concern the wor ld over. e.g. in Great'Britain (Foster et al. 1985). Czechoslovakia(Vrba 1985). India (Handa 1983). Republic ofSouth Africa (Heaton et al. 1985). U.S.A. (Hall­berg 1987) and Poland (Kowalik 1987). Inmany countries. it is obse rved that the nitro­gen content of groundwate r is increasing withtime. The rate of increase has been determi­ned in Belgium (de Smedt & Loy 1983), Den­mark (Overcaard 1985) and Germany (Such1985). It is noted that unconf ined gro undwa­ter , having a direct connection with the atmos­phere. is typically being contaminated by nitra­tes (Krainov et al. 1989). Reduced nitrogenspecies (ammonium and nitrite) are only foundin unconfined groundw ater at the centre of inten­sively contam inated areas (Klimas & Kadunas1983. Zabulis 1988). The reduced spec iesmay. however. accumulate in deeper aquifersas a result of the reduct ion of nitrates (Botcher& Strebel 1985. Klimas 1990a. Klimas 1991).

The aim of this paper is to demonstrate the

exte nt of nitrogen contaminat ion of groundwa­ter in Lithuania. its lateral and vertical dist ribu­tion. and the distribution of ox idised and redu­ced nitrogen species in the gro undwater.

Nitrogen contamination ­common featuresGroundwater contam ination by nitrogen maybe local (point- source ) and regional (diffusesource). Sources of local contamination maybe associated with:

(a) towns and industry: sewerage systems,domestic and industrial waste disposa l. storesof industrial and raw mater ials. anthropogenicdepos its and others.(b) agriculture: stores of fert ilizers and chemi­cals. silos . manure storag e. large farms . sett­lement s.

As regards regional contamination there aretwo poss ible sources: polluted atmospheric

NGU · BULL. 424. 1993

deposi tion and agr icultural activity. Burningof foss il fuels and some ferti lizer factoriescontam inate the atmosphere with nitrogencompounds. In agriculture, organ ic and mine­ral fert ilizers are the main source of nitrogencontam ination (Gustafson 1983, Such 1985,Gerhart 1986, Scheffer & Walther 1988, Zabu­lis 1988).

Nitrogen is a chemical element of variableoxidation state . In groundwater it may be foundin the form of dissolved gases (NH" N" N,O,NO, NO" N,O,) or ions der ived therefrom (nitra­tes (NO,'), nitr ites (NO,"). and ammonium(NH,+)). Many of the gaseous nitrogen com­pounds are relatively soluble in water withoutbeing particularly react ive (N" N,O, NO), alt­hough on oxidation, they may react readilywith water, e.g.:2NO + 0 , - 2NO,2NO, + H,O - 2W(aq) + NO,' + NO,'.Ammonia reacts with water to form a basicsolution, i.e.:NH, + H,O - NH/ + OH',but the greater part often remains in the soluti­on in the form of dissolved ammonia (Krainovet al. 1989). Two main factors , oxygen andorgan ic mater ial, control the state of nitrogenin groundwater. The most stable form of nitro­gen in oxidised environments is nitrate . In aclosed system , dissolved oxygen is rapidlyused up in the oxidation of organic mater ial,whereafter oxygen from other dissolved spec i­es (e.g. SO,= or NO,') is consumed. The reduc­tion of nitrates and nitrites to ammonium oc­curs after (Le. at a lower Eh) reduction of ironand manganese oxides , but before (Le. at ahigher Eh) the reduct ion of sulphate to sulphi­de (Krainov et al. 1982). Bacter ial activity isintimately involved in all the trans itions betwe­en the various nitrogen species, Le.NO,' ... NO,' ... NH/ .

During the course of such transformations,a certa in port ion of nitrogen is liberated asnitrogen gas (N,), Le., the tota l amount of nitro­gen in the system decreases due to the proces ­ses of biodegradation and biodestruction (Bott­cher & Strebel 1985, Wolff et al. 1985, Trudelet al. 1986, Ronen et al. 1987, Zabulis 1988).The geochemical environment, as defined bythe redox potential (Eh) and acidity (pH), deter­mines the distr ibution of the various nitrogenspecies in groundwater (Krainov & Schwetz1987). As the tranformation react ions are equi­librium reactions, all three species (NO,' , NO,',NH,+) are usually found , to a greate r or lesser

Nitrate contamination of groundwater 77

degree , in groundwater at different pH andEh values (Krainov et al. 1989, Klimas 1991).

The mechanism of the transport path ofnitrogen compounds down to groundwater israther compl icated (Gustafson 1983, Gerhart1986, Thiery & Seguin 1986, Scheffer & Wai­ter 1988b). The main biochemical transforma­tions take place in the soil. The amount ofnitrogen leached out of the soil depends onmany factors : climate , soil-type , quantity offert ilizers , their type and time of application,agricultural pract ice, etc. Biotransformationprocesses cont inue in the unsaturated zone,and some denitr ificat ion may occur , return ingpart of the nitrogen to the atmosphere. Thus ,the concentration of nitrogen compounds inthe unsaturated zone decreases with depth.Also, infiltration of sewage into the groundby means of septic tanks, infiltration plantsand liquid manure spread ing fields is common­ly pract ised as a means of treat ing sewageeffluent , and it has been shown that the majori­ty of pollutants, including nitrogen, can reachthe water table (Klimas 1988b). The infiltratedwater initially contam inates the surface layerof the unconfined groundwater. The groundwa­ter velocity, hor izonta l & vert ical dispers ioncoefficients and hydraulic conduct ivity of un­derlying strata determine the further lateraland vert ical spread ing of the contam ination(Klimas 1990c).

A phenomenon commonly observed in con­nection with local and regional groundwatercontam ination by nitrogen is that of vert icaland lateral geochem ical zonation (Zabulis1988, Krainov et al. 1989). Typical of vert icalzonation is a decrease in nitrates with depthand a correspond ing increase in ammonium.In instances of regional contamination of dee­per conf ined aquifers , the offend ing speciesis typically ammonium, derived from reduct ionof nitrate. In the case of local contaminationa contrary picture has been observed; in thecentre of intensive contamination, ammoniumis usually dominant in near-surface groundwa­ter , with nitrates becoming more common fur­ther from the centre . All these cases can bedocumented by concrete examples from lith­uania.

MethodsIn all of the following hydrochemical investigations. stan­dard analytical methods have .been applied; pH and Ehhave been measured in the field. and other parameters inthe laboratory following preservation and storage of sarnp­les. Pilot studies of local groundwater contamination were

78 Algirdas Klimas & Bernardas Paukstys NGU · BULL 424. 1993

Table 1. Some indices of the unconfined groundw ater quality in the region of the Jonava nitric fert ilizer factor y. otethat conc entrations of nitrate and ammonium are referred to in mg NO,' or NH.+/I throughout the paper. rather than mgNl1.

Unconf ined groundwater Number of Numbe r of Water quality indicators (arithmetic mean)sampling point obse rvat ion samples pH Eh NO,- 0 ,- NH.+

(samplingdepth.m) wells (mY) mgn mg/l mg1

Background groundwater qual ity 3 30 7.2 130 0 0 0.2(6 - 12 m)

9.3 170 475 60 3000Below the factor y's 14 25ammon iac water reservoirs(6 - 8 m)

400 19 0.15 2.1Below the factor y's 5 15 7.5emerge ncy storage ponds(8 - 10 m)

46 0.03 1.2Region of polluted prec ipitat ion 7 40 7.4 400(10 - 12 m)

carried out at specially equ ipped observation areas (Jonavanitr ic fert ilizer factory . three pig-b reeding farms. the karstreg ion of North Lithuan ia, etc.)

For the studies of regional pollution of unconfined ground­water . some hund red dr illed wells . and approx imately onethousand dug wells were sampled. Intermediate conf inedand deep conf ined groundwater chemistry studies are ba­sed on the sampling resul ts from approx imately 3000 dr il­led wells .

Nitroge n compounds were analysed in the laboratoryusing standard methods such as Nesslerization and co lori­metry .

Local contaminationThe Jonava factory for nitric fert ilizers is anexample of especially intensive local industr ialcont amination of gro undw ater by nitrogencompounds (Klimas & Kadunas 1983, Klimaset al. 1988). The factory is built at the intersec­tion of the valleys of two large rivers: theNeris and Sventoji (Fig. 1). Here, in the upperpart of the geological section , alluvial sandand grave l depos its dom inate. The tota l thick­ness of these sediments is typ ically 15-20 m.The deposit s contain a saturated zone whichis typ ically 5-12 m thick . Among the sou rcesof groundwater con tamination one can name(Table 1):(a) emergency flows from ammoniac waterrese rvoirs and infiltration of contam inatedammoni ac-rich water from emergency ponds.(b) infiltration of contaminated precipitation .

In the region of the factory 's storage reservo­irs and ponds , contam inated infiltration waters

mix with confined grou ndwater which dischar­ges in the locality. The latter natura lly has asmall cont ent of dissolved solids, but the cont a­minated mixture discharging into the valleysof the above- mentioned rivers is best descr i­bed as a high ly minera lized water, containingespec ially high levels of ammonium and sodi­um (cations) and nitrate, chlor ide and bicar ­bonate (anions). In the vicin ity of the factory .precipitation is contaminated by ammonia, lea­ding to a nitrate concentrat ion in unconfinedgrou ndwater some tens times higher than thebackground level. The process of nitrif icationof ammonium begins in the atmosphere, conti­nues in soil, and is completed in the aerationzone (Klimas & Zabu lis 1984).

Large catt le-breed ing and pig-bre edingfarms are the legacy of collective agriculturein Lithuania. During the past 15 years . morethan 40 such farm-complexe s have been bu­ilt. The capacity of the pig-breeding complexesis around 12,000 - 54,000 pigs per year. Themanure accumulated here is remov ed usingwater, and the resultant liquid manure is spre­ad on fields. Due to poor technology, greatquantities of liquid manure accumul ate on thefields. In winter and spring, fields of a limitedarea are intensively manured (up to 700-1100kgN/ha) when soils are still saturated. In thisway , unconfined groundwater can be severelyconta minated (Zabulis 1988).

Table 2. Chemical composition of unconfined groundwater below the manured fields of some piq-breeding farms. TDS =total disso lved solids . Corg = organic carbon .

Location Number of Samp ling Chemical composition of groundwater. mg/l(Aquifer) observat ion depth Arithme tic mean [range of var iatio n)

well s (m) TDS Corg NO,' NO,- NH.+dissolved

Valkin inkai 21 10-13m 400 10 15 0.2 1(sand. thick unsa tura ted zone) [300 -1200) [9-26) [7-710) [0.1-0.3) [0.3-7)Rokai 14 1-3m 700 39 450 1 3(sand, thin unsaturated zone) [400-2200] [17-61] [ 10-900) [0.1-2J [0.4-12)Sirvin ta (sandy loam) 9 5-7m 1300 45 30 2 30

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

NGU - BULL. 424.1993 Nitrate contam ination of groun dwater 79

N

f

Fig. 2. Trends in pollution of karst groundwater in Lithua­nia. 1-areas in which karst water was already polluted bynitrogen compounds in 1978-1979; 2-areas in which indi­ces of nitrogen pollution increased by more than a factorof two during a 10 year period.

excess ive nitrate concentrat ions . Even furtherdown-gradient , denitrif ication and dilution leadto an eventual attenuation of nitrate concentra­tions (Zabulis 1988).

The territory of North Lithuania possessesan outcrop area in excess of 1000 km' ofgypsiferous Upper Devonian depos its (Pauk­stys 1991). The upper part of these strata iskarstified. Recharge of groundwate r typicallyoccurs via the direct infiltration of precipitationon the outcrop area, and karst wate r dischar­ges into small local rivers. In these gyps ife­rous rocks, a calcium sulphate groundwater,with a moderate disso lved solids content (2000- 2500 mg/l) is typically found. Upper Devoniansands and sandstones underlying these gyps i­ferous strata typ ically conta in fresh groundwa ­ter. Groundwater with in a large area (about400 km') of the gyps iferous depos its is, how­ever, who lly or almost wholly unprotected fromsurface contam ination, and is therefore beingintensively polluted by organic materials or

Table 3. The chemical composi tion of polluted groundwater in the karst region of North Lithuania.

Aquifer Some indices of chemical composit ion, mgll (except Eh, pH)

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

Phreatic, sandy loam (boreho le) (10-15 m) 352 61 27 32 12 < 100 6.8 120 < 0.01 2

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

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

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

The nitrogen in liquid manure is largely inthe form of ammonium and organ ic nitrogen .If (a) the subso il beneath the manured fieldsis sandy, (b) the unsaturated zone is thick and(c) the manure application is not very intens ive(i.e. less than 300 kgN/ha in tota l), then theunconfined groundwater may escape excessi­ve contamination. Such contam ination as ex­ists is typically in the form of nitrates (Table 2- Valkininkai). Down-grad ient from the manuredfield, denitr ification and dilution by clean in­filtration water result in a rapid attenuationof the contamination plume. If, however, themanured fields are underla in by more clayey ,fine grained subso il, with a relatively shallowwater-table, and if manuring is intensive, hea­vy contamination by ammonium and organ icmaterial is most typical. Down-gradient , activeoxidation of ammonium and organ ic nitrogentakes place in the groundwater, leading to

nitrogen compounds (Fig. 2). This is due toagricultural activity, particu larly the applicationof mineral and organic fert ilizers . Especiallyhighly contam inated groundwater is found inthe vicinity of catt le-breed ing farms with storesof manure, silos , sewage and manure accumu­lators and fert ilizer storage areas (Klimas &Paukstys 1990) - e.g. the first two rows ofTable 3. The areal distr ibution of contaminatedkarst water areas shows a regular pattern(Fig. 2), reflecting very well the hydrogeologi­cal structu re of the kars t. The water is mostconta minated in local areas of recharge (ininterfluves and the upper reaches of small ri­vers) and least in the zones of discharge ofkarst water (in river valleys). This simplisticpicture is, however, disto rted by the abstrac­tion of groundwater (Paukstys 1991).

The karstic groundwater is typically contam i­nated by ammonium, and the thermodynamic

80 Algirdas Klimas & Bernardas Pauk~tys

Fig . 3. Unconfined groundw ater pollution and soil bonitetin Lithuania. 1-highly polluted unco nf ined groundwater;2- soil bonitet in the range 50· 100 (of 1st-5th class).

and biochemical stabilty of that species isconsistent with a low redox potential (below200 mV) and a neutral pH, around 7 (Krainovet al. 1989). In the water from wells in areaswhere the water has good access to the atmos­phere, Eh values are much larger and nitratesprevail.

Regional pollutionUnconfined groundwaterIn Lithuania, shallow unconf ined groundwate rtypica lly exists within Holocene (alluvial, aeoli-

NGU - BULL 424. 1993

an, marine sand) and Pleistocene (glacial andfluviolacial sand, loam, and sandy loam) depo­sits. The chemical composition of such watersis very variable. It is, however, possible toshow that the level of contamination is twoto three times greater below more clayey,poorly-drained areas than below sandier, well­drained plains (Kondratas & Mikalauskas 1973,Klimas 1991). This difference is both directlyand indirectly connected with the geology, asit is typically the more clayey areas which aremost fert ile (soil bonitet of 1st-5th class: note- in Lithuania, 10 classes of soil bonitet arein use) and thus the locations of most intens­ive agriculture (Fig.3).

In Lithuania, the nitrate concentration inshallow groundwate r varies widely, fromaround zero to several hundred mg/l, but onecan state that beneath about one third of thecountry the nitrate in unconfined groundwaterexceeds the permissible drinking water stan­dard (45 mg/l). Only in one eighth of the terri­tory is the concentration less than 10 mgll(Fig. 4a). Low nitrate concentrations are foundonly in the Coasta l lowland and South-EasternLithuanian plains, where the agricultural activi­ty is not intense, and the grou ndwater resour­ces are large (Juodkazis & Klimas 1991). Theorganic material conten t of groundwate r issimilarly distributed: permanganate oxidationin the regions contaminated by nitrates gener-

Fig. 4 . Nitrate (a, b. c) and ammonium (d. e. f) concentrations in unconfined (a. d), intermediate confined (b. el . and deepcon fined groundwater (c, f) in Lithu ania.

NGU • BULL. 424. 1993

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

The amount of ammonium in unconfinedgroundwater is comparatively small ; it exceeds0.2 mg/l over only 10-15% of the terr itory (Fig.4d).

Intermediate confined groundwaterThis is the transitional type of groundwaterbetween unconfined and deep confinedgroundwater. It is typ ically found in intermorai­nic deposits (Klimas 1991). In Lithuania, oneor two such aquifers can typically be found ina vertical section through the Quaternary depo­sits , but in some places up to five or six canoccur . They are usually hydraulically intercon­nected and may have good hydraulic contactwith both underlying artes ian water and overly ­ing unconf ined groundwater.

Intermediate confined water is commonlyonly minimally protected against anthropoge­nic pollution (Klimas 1990a). Therefore, whereunconfined groundwater is contaminated, inter­mediate confined groundwater is generally alsocontaminated (Fig. 4a and b). It is clear thatthis occurs where contaminated unconfinedgroundwater recharges intermed iate confinedaquifers (Le. a downward hydraulic gradientfrom unconfined to intermediate confined aqui­fers), most frequently in highlands. On thecontrary, in plains and river valleys, whereunconfined groundwater is sometimes highlycontaminated , intermediate confined ground­water usually remains clean because the latteris protected by conf ining strata and generallyby an upward hydraulic gradient.

The contamination pattern for intermediateconf ined aquifers is shown in Figs. 4b ande. Comparison of the diagrams shows thatincreased nitrate contents in intermed iate con­fined water are found , not in Middle Lithuaniawhere unconfined groundwater contaminationis greatest, but in the marginal parts of therepub lic, especially in regions of high topog rap­hy (Le., in the recharge areas of intermediateconfined aquifers) . It is clear that the areasof intermediate confined groun dwater contami­nated by nitrates are much less extensive thanthose of unconfined groundwater. Nitrate con­centrations in intermediate conf ined grou ndwa­ter rarely exceed 10 mg/I wherea s in uncon­fined groundwater the value is usually 3-4times greater.

Compar ison of Figs. 4d and 4e indicatesthat intermediate conf ined groundwater is

Nitrate contamination of groundwater 81

more areally extens ively contam inated byammonium than unconfined groundwater. The­se areas of ammonium contam ination in uncon­fined and intermediate confined groundwaterdo not coincide. In intermediate conf ined aqui­fers the high ammonium values are found inareas where groundwater is better isolatedfrom unconfined groundwater (Iimnoglacial pla­ins, moraine dominated areas and the Samogi­tian highlands - see Fig. 1).

Deep confined (artesian)groundwaterIn Lithuania, deep, confined water can be foundin deposits of all geological ages, from Cam­brian sandsto nes to Paleogene and Neogenesands (Juodkazis 1989). Such deep confinedgroundwater is one of the main sources forlarge, centralised water supplies in Lithuania.Only a small proportion of deep conf ined aqui­fers are, however, reliably isolated and protec­ted from surface pollution. In recharge areas(topographical highs) deep confined groundwa­ter is recharged from intermediate conf inedand unconfined groundwater. Due to intensiveexploitation in West and Middle Lithuan ia, lar­ge areas of depressed groundwater headhave been formed . Thus , some areas of natu­ral dischar ge of artesian water have becomeareas of recharge, creating conditions suitablefor the downward migration of polluted waterto deeper confined aquifers (Klimas 1979).

Investigations reveal that deep confinedwater is only contaminated by nitrates to alimited extent (Fig. 4). The few areas of deepconfin ed water with elevated nitrate concentra­tions do not coincide with areas of unconfinedgroundwater and intermediate conf ined waterwhich are highly contam inated by nitrates .Klimas (1 991) has noted generally higher con­centrations of ammonium in deep conf inedwaters than in intermediate confined or uncon­fined groundwater (Fig. 4f). The areas in whichammonium concentrat ion exceeds 1 mg/I ac­count for almost 20% of the territory of Lithua­nia. Ammonium accumulates comparativelywell in the isolated deep conf ined aquifers ofWest Lithuania but is practica lly absent in themuch less isolated, East Lithuanian , deepconfined aquifers. A similar pattern can alsobe recognised in the distribution of organ icmaterial in deep confined water. It thus appe­ars that the low nitrate and elevated ammoni­um contents typically found in deep confined

82 Algirdas Klimas & Bernardas Paukstys GU . BULL. 424. 1993

gro undwater are the result of the chemicalreduction of nitrate to ammonium (Klimas etal. 1988, Krainov et al. 1988, Klimas 1991).

DiscussionInvestigations of local and regiona l groundwa­ter po llution by nitrogen compounds in lithua­nia revea l regularities in the patt ern of contami­nation and allow one to predict its future evolu­tion.

Redox potent ial (Eh) and acidity (pH) valuesreflect the processes of migrat ion and trans­form ation of nitrogen compou nds in groundwa­ter. Unconf ined groundwater aquifers usuallyenjoy a good connection with the atmosp hereand therefo re exh ibit high dissolved oxygenlevels, commonly up to 7-10 mg/l, Eh typicallyexceeds +200 mV and pH is around 7. Insuch water one typically finds a limited rangeof nitrate conce ntrat ion (2-10 mg/l). In centresof intensive contamination, prac tically all theoxygen is con sumed by the oxidat ion of orga ­nic contaminants. The amou nts of unox idisedorg anic material and ammo nium show elevatedvalues in such situations (see Table 2), and acorrespond ing reduction in Eh and nitrates(see Table 3) due to biochemical reductionby ammonification bacteria. The values of pHfound at such contaminated localities varywidely (Fig. 5). These chemica l cond itions areespec ially not iceab le in contaminated, uncon ­fined groundwater aquifers occurring in clay­ey, poorly-drained areas .

Fig. 5 is comp iled from stud ies carried outin fields spr ayed with liquid pig-manu re(300-600 kgN/ha) covering an area of 150hectares. Unconfined groundwater is found ata depth of 0-5 m in sand, morainic sandy loamand loam deposits . It can be seen that thelevel of contam inat ion of unconfined groundwa­ter , composit ion of the contamination compo­nents and the values of Eh-pH of unconfinedgroundwater found within the comparativelysmall area vary greatly. A patte rn can, howe­ver, be identi fied. First ly, the nitrate concentra ­tion in sandy deposits is dist inctly lower thanin clayey one s (Fig. 5c). Furthermore, increa­ses in NO,'concentrations correspond to inc­reases in Eh and pH. In addition. the concen­trations of ammon ium and iron show a corres­pond ence, and are related to depressed Ehand pH values. The depe ndence of ammoniumand iron on lithology is not so dist inct as isthe case for nitrates.

A similar pattern can also be observed in

pH8

~Ti ll

,

, ,I: "11 1:1 : ' I

I ,

7

EJ, I SandI I

I

20

30

10

40

Fig. 5. Eh,pH diagrams for a) ammonium b) iron and c)nitrate in polluted , unconf ined Lithuanian groundwater .

NH~ .

mg/ I

NGU - BULL. 424. 1993 Nitrate contamination of groundwater 83

Fig. 6. Ammonium concentrations plotted against depth forthe gro undw ater of the Lithuan ian karst reg ion.

the case of regional unconfined groundwatercontamination. It has already been demonstra­ted that nitrate tends to be the dominant nitro­gen spec ies in unconf ined groundwater in Lit­huania. Only in limnoglacial clayey formationscan elevated concentrations of ammonium befound (Fig. 4d). One exception, however, isthe karst region of North Lithuania where fissu­red dolomites with interbeds and lenses ofgypsum outcrop (Klimas & Paukstys 1990,Paukstys 1991). The groundwater within theseis stro ngly polluted with organic material andcontains pract ically no dissolved oxygen. Re­dox potentials are typically lowered to 60-180mV, and iron concentrations can reach 8mgll (Paukstys 1991). Generation of methaneand hydrog en sulphide can also be observed.Ammonium dominates among nitrogen com­pounds. The variation of its concentration withdepth is presented in Fig. 6.

Fig. 6 demons trates that in cases of intensi­ve contam ination, an inversion of oxidat ion­reduction zonation can take place: a zone lac­king oxygen, with low Eh values, and enrichedin ammonium is formed above an oxidisedzone (areas 2 on Fig. 2).

On a regional scale, however, a norma lzonat ion (Fig. 4) is typically observed, i.e. anupper, oxidised, nitrate zone over a deeperreduced ammonium-dominated zone. The factthat the amounts of ammonium and organicmaterial in intermediate conf ined and deepconf ined aquifers are increasing with time indi­cates that the phenomenon is really connectedwith anthropogenic pollution. This increase isdescr ibed by the following relations (Klimas1991):

in intermediate conf ined waterNH: = 0.012 t + 0.411 (r = 0.39)

P = 0.131 t + 1.61 (r = 0.82)

in deep confined waterNH: = 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 ox idisa­ble organic material in watert = time from the beginning of the analysis(1958) period, in years .r = correlation coeff icient, significance level0.5 % .

From these equations it follows that in deepconfined water organic material has been pre­sent since before 1958 (P "* 0, when t = 0),while the process of the accumulat ion of am­monium has begun later (NH. ~ 0, when t ~0). Furthermore, the corre lation coeffic ient forammonium is half that for organic material.This is presumably due to the instability ofammonium; with access to oxygen it readilyoxidises to nitr ite and eventually to nitrate.

The exp loitation of conf ined groundwatercan great ly alter the geochemical environmentwithin aquifers. It can induce leakage ofgrou ndwater of a different chemical composi­tion from strata above and below the exploitedaquifer (Klimas 1979). For example:

(1) water leaking to the aquifer from aboveis commonly enriched in oxygen and may thuslead to a progress ive increase in the nitrateconcentration in the exploited aquifer.

(2) water leaking to the aquifer from abovemay, however, be strongly polluted with orga­nic material and nitrogen compounds, leadingto consumption of oxyge n in the exploitedaquifer and increasing concentration of ammo­nium.

(3) water leaking to the aquifer from below(from deeper hor izons) is usually oxygen ­poor, leading to decreasing values of Eh andaccumulation of ammonium in the exploitedaquifer. All these cases can be observed atgroundwater works in Lithuania (Klimas 1973,1991).

Biotransformation of nitrogen compounds ,accumulation of unoxidised organic material,and a decrease in Eh can greatly alter thegeochemical environment in an aquifer, suchthat favourable conditions for the mobilisationof tox ic microelements (including heavy me­tals) may be formed in these aquifers (Krainov& Schwetz 1987, Klimas 1988a).

••-:. f··· •.: ...... ,• ••

IP

••

o 'f..

I

l P-

60

80

84 Algirdas Klimas & Bernardas Paukstys

ConclusionsStudies of groundwater pollution by nitrogencompounds have a long history. The informa­tion presented in this paper has been collec­ted from groundwater monitor ing data andfrom special hydrochemical maps compiled forindividual aquifers . The analysis 0 1 this materi­al indicates a complicated and rather ominousview of the pollut ion of the Lithuanian ground­wate r environment by nitrogen compounds .On the whole , it coincides rather well with theobservat ions of such pollution in many othercountries. Despite the complexity of the pro­cesses occurring in the subsurface, certainpatte rns can be identified in the data:1) Groundwater is contaminated by nitrogen

compounds locally and on a regional scale.There are many sources of local pollution,but only two sources of regional pollution- atmosp heric pollution and agriculturalactivity.

2) Nitrogen compounds take an active part inbiochemical react ions, and their spsc iationand concentrations in grou ndwater canvary. Eh and pH conditions are decisive indetermining whether nitrate or ammon iumis the dominant species.

3) Patterns in nitrate and ammonium distr ibu­tion can be detected. These are expressedas vert ical and horizontal zonation of redoxpotential and nitrogen speciation. In upperaquifer hor izons , and in recharge areas ofdeeper aquifers , nitrates dominate and Ehis typ ically greater than +200 mY. In dee­per aquifers, and espec ially in their trans itand discharge areas, ammonium is dom i­nant in gro undwater, with Eh typically lessthan +200 mY.

4) In unconf ined groundwater, with a compara­tively good connection to the atmosphereand a suff icient amount of disso lved oxy ­gen, nitrate is usually dominant. Only in the

GU - BULL. 424. 1993

centr es of intensive pollution (e.g. manurespreading fields) where microorganisms arenot able to oxidise organic material andnitrogen compounds, does ammonium dom­inate in unconfined groun dwater.

5) The amount of nitrogen compounds in un­confi ned groundwater may decrease com­paratively quickly due to denitrification, dilu­tion by 'clean' precipitat ion (infiltration re­charg e). Thes e processes proceed quickerin sandy deposits and more slowly in clay­ey deposits. Unconfined groundwater istherefore most strongly polluted by nitratein the clayey plains region of Middle lithua­nia.

6) In deeper aquifers wher e there is an increa­se in concentrat ions of unoxidised organicmaterial due to progressive pollution, thegroundwater environment becomes steadilymore reduc ing, Eh values fall and nitrateis transformed to nitr ite and eventuallyammon ium. The concentration of ammoni­um increases with time.

7) The increasingly reducing environment indeeper aquifers is one of the main reasonsfor the accumu lation of some metals ingroundwater. Major changes in geochemi­cal condit ions can be caused by groundwa­ter exploitation. During such exploitat ion,not only regional groundwater flow pat­terns , but also hydroch.emical zonation aredisturbed. Exploitat ion can thus seriouslyinfluence the distr ibut ion, concentrationsand speciation of nitrogen com pounds ingroun dwater.

AcknowledgementsDavid Banks of NGU. Trondnerrn, encouraged us to writetrus paper and kind ly helped with Eng lish language corr ec­non s for which we are mo st grat eful. We are also grate fulto Pro!. Dr. V. Juodka zis (Vilnius) for read ing the Lithuanianversion of the manusc ript. and to Dr. V. Zakutin (Moscow)for giving permission to use some of the results from hisstudies of gro undwater qua lity In Lithuania.

NGU - BULL. 424.1 993

Referencesaoncner, J. & Stre bel, O. 1985: Redoxpotential und Eh/pH

- Diagramme von Stoffumsetzu ng in reduzierendenGrundwasser (Beispiel Fuhrbsrqer Feld) [Redox potenti­al and Eh/pH diagrams of chemical transformatio ns inreducing groundwater - example from the Fuhrb ergerarea]. Geot. Jenrb. 3. 2, 3-34 [in German].

De Smedt, P. & Loy, N. 1985 . Les nitra tes dans I'eau sou­terrai ne en Belgique (Nitrates in grou ndwate r in Belg i­um). IAHS Publication t54 . 178-187 [in French] .

Foster. S.. Geake. A.K.& Lawrence. A. 1985: Diffu se ground­water pollution: lessons of the British experience. IAHSPublication t54, 168-172.

Gerna rt, J. 1986: Groundwater recha rge and its effects onnitra te concen tration beneath a manu red field site inPennsylvania. Ground water. 24. 4, 483-489 .

Gusta fson, A. 1983: Leaching of nitrate from arable landinto groun dwa ter in Sweden. Environm. geol. 5. 2, 65-71.

Hallberg , G. 1987: The impacts of agr icultural chem icalson groundwater quality. Geojournal 15. 3, 283-295.

Handa, B. 1983: Effect of fertilizer use on groundwa terqua lity in India. IAHS Publicat ion 14212, 1105-1119.

Heaton , T.. Talrna, A. & v oqel, J. 1983: Or igin and historyof nitrate in confined groundwater in the western Kalaha­ri. Jou rn. of hydrol. 62. No 1-4. 243-262.

Juodkazis, V.1. 1989: Regional hydroge ology of Baltic coun­tries. Mokslas, Vilnius , 220pp . [ in Russ ian].

Juodkaz is, V. & Klimas , A. 1991: The chang e of the ground ­wa ter quality in the Balt ic artesian basin. Scient ificpape rs I. 39 pp [in English) .

Klimas. A.J. 1979: Groundwate r qua lity predict ion underlong- ter m development condit ions. In Memoires ofIAHS Congress in Vilnius Methods for evaluation ofgroundwater resources. Vol. 15, pub l. Nauka , Mo scow,pp. 279-282 [in Russian] .

Klimas . A.J. 1988a: Trace elements in fresh groun dwaterof Lithuania. f eoxHMHn [Geochem istry] 3. 367-375 [inRussian) .

Klimas, A.J. 1988b: Spatial-tempo rary heter ogeneity ofgro und water recharg e, BOA uue pe,ccy pc fj

[Waterresour ces] 3, 66-72 [in RUSSian]. -Klimas . A.J. 1990a: Symptoms of intermo rainic artes ian

wat er contamination in Lithuania . Geologija (LithuanianHigh schools scientific papers) 11,29-38 [in Lithu anian ].

Klimas. A.J. 1990b: Ground wa ter monito ring. MUS4 gamta[Our natu re] 3. 10-11 [in Lithuanian] .

Klirnas, A.J. 1990c: Hydrodispersion parameters for theground water pollution fore cast . P031l ellK8 Hox paa e HeAp [Earth depth exp loration and protection]4, 25-28 [in Russian] .

Klimas, A.J. 1991: Groundwater quality formation underintensive development and contamination, Coae-rcxasfeolI or Hn[Soviet geology] 12, 62-70 [in Russian] .

Klimas, A.J. and Kadunas. K.S. 1983: Precipitation influen­ce on groundwater quality in urbanized terr itories . Xrep. hydro meteoro l. Coni., Palanga., p. 153 [in Russ ian] .

Klimas, A.J. & Zab utis. R.M. 1984: Experimenta l investigati­on of contaminants migration in the aeration zone,Poa uen xe H ox p BH8 HCA P [Earth dep th explorationand pro tection] 8, 36-40 [in Russian].

Klimas . A.J., Zabulis, R.M. & Kadunas, K.S. 1988: Hydrogeo-

Nitrate contamination of groundwater 85

logic al condition s of gro undw ater con tam ination by nitro­gen com punds . Feonor-ast [Geology - Lithuanian Highschools scientific papers ] 9, 53-64 [in Russian].

Klimas, A.J, & Pauk~tys . B.P. 1990: Litomonito rinq orqaniza­tion in karst region of North Uth uanla.H nx euepnenf eolIor HR[Engineering geology] 4. 75-80 [in Russian].

Kondratas, A.R, & Mikalauskas, V.V. 1973: An example ofregiona l gro undwater contamination. Nedra, Moscow,280 pp. [ in Russian).

Kowalik , P. 1987: Zarueczyszczenia or'ganiczne wad pod ­ziemnych [Groundwater contamination by organic subs­tances]. Gospodarka vodna. 47. 2. 31-3 3 [in Poli sh] .

Krainov, S.R, & Schwetz, V.M. 1987.: Geocnemistry of drin­kable qrounaweter. Nedra, Mos cow. 237pp . [in Russi­an],

Krainov , S.R., Zaku tin. V,P. & Solomin, G.A. 1989: Nitrogencompounds in drinkable groundwater . VIEMS . Moscow,66pp. [in Russian).

Madison, R.& Burnelt , J. 1984: Overview of the occu rrenceof nitrate in groundwate r of the United States . USGSWater Supply Pap, 2275, 93-105. '

Mikalauskas, V,V. 1976: Groundwater pro tection in the Lit ­huanian SSR. Moks las, Vilnius. 112 ·pp . [in Russian].

Overcaa rc , K, 1985: Trends in nitrate pollution of groundwa­ter in Denmark. Water Supply 3. 2, 195-203.

Paukstys, B. 1991: Anthropogenic activi ties influence ongypsum kars t evolution in North Lithuania. Cand. sci­ent. thesis. VSEGINGEO. Moscow. 25pp. [in Russian).

Ronen, D., Magaritz. M. & Almon, E. 1987: Antropogenicanoxi fica tion ("eut rophication") of the water table regionof a deep phreatic aquifer. Wat. Res. Res. 23. 8, 1554­1560.

Sche tter, B. & Waiter , W. 1988: St ickstoff umsetzu ngen imboden und fo lgen fOr die nitrata uswaschund [T rans­format ion of nitrog en compounds in soils and implicati­ons for nitrate leaching]. CWE: Wasser lA bwasser 129.7, 451-456 [in German].

Such, W. 1985: Nitra tbelas tund des grou ndwa ssers auslandwirtschatttlcner nutzung, erkenntnisse und untersu­chungsmethoden in feld [N itrate load on groundwat ersfrom agr icultur e: recog nition and investigation in thefield]. BBR: Brunnenbau. Bau vvssserwerken. Ronnel­tungsbau 36. 10. 379-382 . 384-385 [in German].

Thiery. D. & Seguin, J. 1986: Un modele hydrogeologiquegloba l de transfert de nitrate s dans un basin versant.Applicat ion aux sour ces de Provins et a la nappe dela Beance a Pithiviern (A global hydrogeological modelfor the migrat ion of nitrat es in a bas in. Application tothe spr ings of Provins and the water table of Beancea Pithiviern). Hydrogeologie I , 87-100 [in French],

Vrba , J. 1985: Impact of domestic and industrial water andagr icultural activities on groun dwater quali ty , IAHS Publi­cation 1985, 1581/ .91-1 17.

Wolf!, C.. Ebeling . J,& Nove ke, R. 1985: Abiot ische undbiotiscne stoffumsetzp rocesse in Pleistoz anen Was ser­kor pern (Abiotic and biotic chem ical reactions in Pleisto­cene aquifers). Zeitschrift Dtsch. geol. Ges. 136. 2,563-574 [in German] .

Zab ulis. R.M. 1988 : Groundwater pollut ion pro tection in aregion of larger pig -breeding farms in Lithuania. lIT ­NIGRI. Vilnius, 71pp. [in Russian].

Manuscript received August 1992; final revised typescript accep ted February 1993.