Environmental Health PerspectivesVol. 83, pp. 31-37, 1989

Chemical Substance Transport in Soils andIts Effect on Groundwater Qualityby Martin G. Khublarian*

The problems of chemical substance applications in different spheres of industry and agriculture andtheir effects on groundwater quality and human health are described. Sources of groundwater contami-nation from industrial and municipal wastes, agricultural pollutants, etc., are listed. The experience inthe application of chemical fertilizers and pesticides in the USSR is described. A brief estimation ofgroundwater salinity is given for various regions of the USSR where irrigation is practiced, as well as theexperience in environmental protection. Special attention is given to methods of simulating water seepageand chemical substance transport in soils. Boundary problems for free-surface seepage and dissolved solidstransport in porous media are stated, and methods of solution are described in the example of the hydro-dynamic theory of seepage and dispersion. Some results of calculations with this method are presented.The influence of groundwater quality on the morbidity of the population is given and the main diseasesand associated medical problems are listed.

IntroductionIncreased anthropogenic loading on the environment

has produced an appreciable disturbance of natural con-ditions. Considerable amounts of pollutants are releasedinto water and air, causing their contamination. Forsome chemicals, the rate of release is greater than therate of removal. As a result, some chemical substancesare accumulated, particularly in water and soil.

Pollution of groundwater must be understood to beclosely related to that of the whole environment. Mois-ture in the atmosphere is connected with moisture insurface and groundwaters via the hydrologic cycle.Therefore, it is impossible to prevent groundwater pol-lution if the atmosphere, surface waters, and soils aresubjected to continuous contamination.Many pollutants are able to penetrate into ground-

water aquifers. This situation is even more dangerousthan a deficit in the water supply, since unconfinedgroundwaters-unlike groundwaters in confined aqui-fers-are not as well protected from pollution. Thus,deterioration of groundwater quality is becoming a mat-ter of great concern.The main sources of groundwater contamination are

municipal, industrial, and agricultural wastes (bothsolid and liquid), gangue rocks, sludge and slimes, re-

fuse, pesticides, herbicides, effluents from livestock andpoultry farms, etc. Pollutants have different migrationcapacities, toxicities, and other properties. Thus, even

*USSR Academy of Sciences, Water Problems Institute, 13/3 Sa-dovo-Chernogriazskaya, 103064, Moscow, USSR.

low concentrations of highly toxic pesticides can signif-icantly influence cellular behavior, genetics, and me-tabolism. The ability of pesticides to enter and accu-mulate in tissues causes contamination of the whole foodchain.

Unfortunately, at the present time in the USSR,chemical methods are used to protect agricultural areasand forests from pests and undesirable plants. In 1986,more than 20 million hectares were treated with pes-ticides, whereas biological methods were applied onlyon small areas. In the future, the areas where biologicalmethods of protection will be used are to be expanded.These methods are ecologically pure, unlike chemicalmethods that cause environmental pollution, entailingthe annihilation of useful insects, birds, fish, mammals,and the poisoning of people.

Mineral fertilizers, pesticides, and municipal and live-stock farm effluents contain heavy metals. Many ofthese elements such as arsenic, mercury, cadmium,lead, and copper are toxic, even at low concentrations.The main pollutants in livestock farm effluents are ni-trogen, phosphorus, and potassium. The migration ca-pacities of potassium and phosphorus are low, so theseelements are retained in the upper soil layer; the maincontaminants of groundwater are nitrogen and bacteria.Field observations, carried out in Uzbekistan from 1980to 1985, showed that a loam layer 140 cm deep retained82.4% of ammonia nitrogen, 90.3% of phosphates, 30%of total nitrogen, and 90% of potassium contained in theeffluents (1).The annual amount of applied fertilizers varies from

M. G. KHUBLARIAN

10 to 100 kg/ha. These fertilizers contain deleterioussubstances: nitrogen compounds, sulfates, chlorides,and phosphates. Nitrogen is found in four forms: organicnitrogen, nitrites, ammonium, and nitrates, the latteraccumulating in soils. A steady and stable increase ofnitrate content in groundwater has been observed.Sometimes it exceeds 50 mg/L, which is the maximumpermissible concentration determined by the sanitarystandards for drinking water. High nitrate contentcauses a drop in blood hemoglobin level (methemoglo-binemia) that can be fatal in newborn infants.The danger of organic pollution (humic substances,

phenols, hydrocarbons, fatty acids, and lignin) shouldnot be underestimated. In 1985, the USSR agriculturalareas totaled 210.3 million hectares, and the applicationof mineral fertilizers (evaluated on the value of 100%nutrients) was 25,389,000 tons, including 10,951,000tons of nitrogen fertilizers, 6,837 tons of phosphates,776,000 tons of phosphorite flour, and 6,817,000 tons ofpotassium fertilizers.At present, 250 million hectares (16%) of the world's

arable lands are irrigated. In 1986, the area of irrigatedlands of the USSR was 20 million hectares. By the year2000, these areas will be expanded up to 30 to 32 millionhectares. Irrigated lands are located mainly in CentralAsia, Southern Ukraine, Northern Caucasus, Trans-caucasian, and Volga regions.The construction of large irrigation systems in dif-

ferent regions of the country caused appreciablechanges in groundwater level and quality. For example,in the Volga region, the depth of groundwater aquifersis 2 to 10 m, salinity is 0.2 to 15 g/L. Sulfates, chlorides,and sodium compounds are typical constituents ofgroundwater in the Volga region (2). On a land plot thathas been irrigated for 30 years, salinity of the poresolution increases uniformly from 0.5 to 1.5 g/L to 10to 15 g/L, as depth increases from 1 to 2 to 8 to 14 m.At a depth of 5 to 7 m the concentration of chloride ion,(C-) has achieved a value of 8 g/L. At a depth of 10 to14 m the chloride ion concentration is 9 g/L. Sodiumcations prevail in pore solutions: at a depth of 3 to 5 mthe average Na+ concentration is 1.6 g/L, and at a depthof 5 to 7 m it is 3.0 to 3.5 g/L. Concentrations of calcium,magnesium, and some other elements also increase withincreasing depth below the surface.The construction of the Kara-Kum Canal in Soviet

Central Asia (its length is 1200 km; annual runoff is 9ki3) had both positive and negative effects on the en-vironment. Manmade lakes and drainage systems wereconstructed and vast lands were irrigated. For the 23-year period of the canal's operation, the total inflow ofsurface waters into it was 240,933 billion m3. The totalinput of salts was 142,124 million tons. Iniltrating ir-rigational waters carried salts from the aeration zone,thus increasing their content in the groundwater.Zones of groundwater level rise (so-called seepage

mounds) appeared under irrigation canals and on irri-gated land plots. Thus, in the Ashkhabad region thegroundwater level rose by 10 to 15 m; in a 4 to 4.5 km-wide zone along the drainage collection area, the

groundwater level rose by more than 15 m. Infiltrationlakes appeared in the drainage areas for these canals.Groundwater salinity around them increased by 3 g/L,as compared to the background value, which was ex-plained by intensive evaporation.The Kara-Kum Canal waters irrigate up to 850,000

hectares of land whereas the total area of irrigated landsin Soviet Central Asia is 6.8 million hectares. Depth,chemical composition, salinity, and other parameters ofgroundwater are diverse because of the variety of lith-ologic, geomorphologic and climatic peculiarities, hu-man activities, and many other factors. As a rule, sur-face water salinity is proportional to the distancebetween the river and the source of irrigational water(irrigational canal). Similar changes are observed in thesalinity of groundwater (3).The salinity in the Amudaria River increased to a

value of 0.5 to 0.8 g/L and in the Syrdaria River, salinitylevels were as high as 1.5 to 1.8 g/L. Groundwater sa-linity was also subjected to vertical variability. The sa-linity of water from a sampling bore-hole in the Vakhshriver valley was as follows: 1.5 to 2.48 g/L; 7.5 to 1.80g/L; 15.0 to 1.90 g/L; 20.0 to 1.13 g/L; 47.0 to 1.69 g/L;149.0 to 1.06 g/L; 197.0 to 1.15 g/L (4).Experience in irrigational system construction and

operation is valuable and instructive. Special attentionis given to environmental protection problems. Thus,in order to ensure standard salinity in projected irri-gational systems in the Volpa basin, the impact of thedaily release of 2 million m of drainage waters (theiraverage salinity being 4 to 7 g/L) is planned and pro-jected for the Urals and Kazakhstam regions. These areregions that are far away from the Volga region, butare connected with that region.

Therefore, predicting seepage phenomena, solutetransport in soils, and estimating their effects requiremodels that carefully describe natural processes.Among theoretical models of mass transport in soils arethe black box model, the gray box model, structuralmodels, and others. These models are described in greatdetail in the literature. Because of the specific interestof the author, the present paper deals mainly with struc-tural models. They are based on hydrodynamic princi-ples for describing transient seepage processes and pol-lutant transport in groundwater.

Isothermal groundwater and pollutant transport canbe described by the following system ofgoverning equa-tions as

V = -(grad P + pg grad Z)

at - div (pV) = 0

at div (CV) = div [DP grad C

(1)

(2)

(3)

p = p(C)(

32

(4)

CHEMICAL SUBSTANCE TRANSPORT AND WATER QUALITY

Here V is seepage velocity; K is the coefficient ofpermeability; p. is dynamic viscosity; m is porosity; P isfluid pressure; g is the acceleration of gravity; Z is thevertical coordinate; t is time; p is the density of thesolution; D is the second-rank tensor of dispersion; andC is pollutant concentration in groundwater.

Solutions of these equations are possible at assignedparameter values and boundary conditions for certainconditions. Assuming that the density and viscosity ofthe solutions do not depend on the concentration (whichis observed when the concentration varied within a nar-row range), the hydrodynamic problem can be consid-ered independently. Then the concentration can be de-termined from the obtained values of seepage velocity.

Hydrodynamic Theory of SeepageThe theoretical basis of the model is described else-

where (5,6). For nonuniform porous media according tothe linear form of Darcy's law, the two-dimensional see-page velocity profile with a free boundary is describedby an elliptic equation for head, H as

a HiM[KX(X,Y) + 7[Ky(X.Y) =

0

Seepage velocity components and the free surface Y(X,t) are determined as

Vx = -KX(X,Y) aH Vy =-Ky(X,Y) M,

I = -Ky(X,Y) aY + KX(X,Y) aX ax + E(X,Y,t)

where Kx and KY are the coefficients of permeabilit3the X and Y directions; E (X,Y,t) is the function tdescribes free-surface infiltration and evaporation. 1initial condition is

O(X,O) = F(X) 0 i X K L

(5)

and the selected zero plain (or zero line, in the casewhen the horizontal impermeable layer is the lowerboundary); and B is the distance between the zero plainand the level line of the surface water body. This valuegenerally depends on time.

If X = 0, either symmetry is observed, e.g., aH/aX= 0, or the values of head or flow are to be assigned.Boundary conditions for other cases are described else-where (7).The solution of this nonlinear boundary value problem

can be obtained with the present generation computers.Complex hydrogeological media, characterized by non-uniformity and the presence of stratified layers of dif-ferent permeability, can be investigated only with thehelp of hydrodynamic models. In cases when the domainunder consideration is uniform, the seepage layer isthin, its spatial variations are minimal, and flow dynam-ics are studied over vast areas. Only then can the hy-draulic theory of unconfined seepage be applied. In thiscase, seepage can be described by the quasi-linear Bous-sinesq equation. It should be noted that this equationresults from the vertical averaging of the continuityequation and the application of free-surface conditionsto Darcy's law (8).

Unsaturated-Saturated Seepage'this problem can be solved by two methods described

in the scientific literature. The first method consists of(6) using the equation of moisture transport in unsaturated

media and the Boussinesq equation for free-surface sat-urated seepage. The other method requires the devel-opment of a coupled mathematical model.We consider both of these methods, as follows:

ina Case 1. For a one-dimensional case, the Boussinesqy inL equation is given as:"hatrhe

(8)For the lower boundary, which, in general, has an ar-bitrary contour and recharge rate, the boundary con-dition is given as

IIIh =h

X LKhh

+ E(Xt)L)tax ax'

(11)

where h is head; K is the coefficient of permeability;and E is the term describing the source of flow.The initial and boundary conditions for the saturated

zone are

aH= Q(X,t)aN (9)

where N is the inner normal to the boundary domain.Vertical boundary conditions at X = 0 and X = L, canbe different depending on the concrete statement of theproblem. If X = L, then

aH O KY BnO0)XB

H = B

H = Y

Bn < Y < B(10)

B < Y K q(O,t)

where B,, is the distance between the water body bottom

The one-dimensional equation of transient moisturetransfer is

C(P) ap = -a[K(P) (a + }I] (13)

where C(P) is the coefficient describing the water-hold-ing capacity or storativity of the aquifer; K(P) is the

O<X <Lh(X,0) = f(X)

h(O,t) = ho(t)

-XhAX IX=L=

t > 0(12)

t > 0

33

M. G. KHUBLARIAN

coefficient of permeability for the unsaturated zone; andP is liquid pressure.

Initial and boundary conditions for Eq. (13) are

P(Z,O) = C(Z)

P(O,t) = 0

_P R3Z IZ=Zo = K(P)

O S Z zo

t > 0

t > 0

where R is the infiltration flow rate through the soilsurface (R > 0) or the evaporation rate (R < 0).

If the effect of vegetation is to be taken into account,a term describing water absorption by the plant rootsmust be included in Eq. (13). The algorithm for thesolution of this problem, the coupling of the two equa-tions and the analysis of the results are described in (9).It can be concluded from this investigation that themethods give satisfactory results for simulation startup.When the inflow to the water surface is small, the lateralcomponent in the unsaturated flow domain is negligibleand the Boussinesq approximation hypothesis of Du-puit-Forchheimer is valid. For these cases a coupledmodel will be an effective, approximate solution of theunsaturated-saturated seepage problem, since the so-lution of practical problems using the rigorous modelrequires extensive computer time. I

Case 2. A rigorous coupled mathematical model oftwo-dimensional unsaturated-saturated seepage is

W---[K(v)- la.y[K(1) -J aKaL (15)dcv at ax Ax a ay avay

whereW is moisture content, and T is moisture tension.On the boundary between the saturated and unsatu-rated zones the pressure is atmospheric, (T = 0), whichallows us to determine the position of the depressioncurve as

a) in the unsaturated zone:

dW0 K=K(w)dw

(16)

b) in the saturated zone:

OV>0dW-=0dtv

K = Ko = const. or K = K(X,Y)

T determines the free-surface position.

The following relation is assumed between K and T:

approaches to the problem of water and salt transfersimulation in soils are given in (10).Many investigations (11-13) have dealt with the prob-

lems of unsaturated-saturated seepage. Algorithmsfor Eq. (15) have been developed and numerous exam-ples of calculations have been given. In addition to thecomputational difficulties, the authors were faced withlack of knowledge of dW/dT and the relationship forK(t).The use of a rigorous model of unsaturated-saturated

seepage is advisable when the groundwater aquifer isclose to the root zone and is subjected to large-scalevertical variations, thus endangering the plants' growthand in some cases, causing their destruction. This modelis also important in studying the interaction of surfaceand groundwater, as well as chemical substance trans-port in the domain. In the case of surface waters, whenthe water body bottom is covered by a layer of silt thathas runoff from agricultural activities, it can representan additional source of contamination to underlyinggroundwaters. The hydrodynamics of these processesare described in Khublarian et al. (14,15).

Chemical Substance TransportThe above models allow us to study seepage processes

in the soil, predict their changes, and solve the problemof solute transport in nonuniform media, because theseepage flow is the main carrier of chemical pollutantsin the porous media. The equation of pollutant transporthas been given in Eq. (3). At present there are manyinvestigations dealing with theoretical substantiationand validation of the solute transport equation and crit-ical analysis of the model's sphere of application. Thesepapers also present suitable boundary conditions andsuggest analytical or numerical methods for their so-lution.

Migration of chemical contaminants from agriculturalfields takes place mainly through the aeration zone. Theequation of transport in this zone is gives as

.a(WC) - div (CV) = div (D grad C)a)t (19)

where W is moisture content, determined from theequation of moisture transfer. The statement of theproblem of water and salt transport, numerical methodsfor their solution and results of calculations are givenby Watson and Jones (16,17).Let us now state the boundary problem for the equa-

tion, describing water and salt transport in a porousmedium, for which the hydrodynamic problem of free-boundary seepage has been investigated (Fig. 1). Thetwo-dimensional transient equation of convective trans-port in saturated porous media is

3 v °K = Ko 1- AV

I > 0(18)

where K(, is the saturated permeability coefficient andA is a constant for the soil type. Analyses of modern

a)c a) ac a aCUIC = a [D(V) +J Y [D(V) a7J

-v ac Vy ca

(20)

34

CHEMICAL SUBSTANCE TRANSPORT AND WATER QUALITY

The coefficient of hydrodynamic dispersion D(V) de-pends on the seepage velocity and certain parametersof porous media (5) and is given as

D(V) = Dg + iVI a2 (21)

where D, is the coefficient of molecular diffusion; 1 isthe coefficient that is a function of the structure of theporous media, and ot is an exponent, usually 1 - a - 2(18).The initial condition for Eq. (20) is

C(XXY,O) = CO(X,Y) (22)The boundary conditions are

a) Flow by dispersion does not exist across the bound-ary zones, and the boundary is impermeable for waterand salts.

D I)C , 0 (23)

where N is the normal to the boundary.b) Salt concentration on the boundary between the

water body and the porous medium is equal to its con-centration in the water body:

c = Cr (24)

c) The following condition can be assigned for the zonewhere the solute flows out of the porous media into thesurface water body:

C = CB; D .3c = 0aN (25)

On the free surface,

D )N = EN(C - Cinf) (26)aN

where EN = E/ 1 + (d4b/dx)2 is the value of infiltrationper unit length of free surface; and Cinf is the concen-tration of dissolved solids in infiltrating water.When infiltration does not take place (E = 0) and aC/

aN = 0. This means that dissolved solids move togetherwith the free surface.The problem, stated by Eqs. (5), (7), and (20) was

solved numerically with the help of the computer. Theseequations were solved successively. Once the positionof the free surface is known the hydraulic head value isfound from Eq. (5). Changes in the concentration aredetermined from Eq. (20), taking into account seepagevelocity [Eq. (6)]. Then the new position of the freeboundary at the subsequent moment of time was de-termined from Eq. (19), etc.Approximate forms of Eqs. (5) and (20) were devel-

oped with finite difference techniques and solved by thealternating direction implicit (ADI) method; Eq. (5) onthe basis of a longitudinal transverse run scheme; andEq. (20) on the basis of totally implicit splitting scheme(19,20).The following standard dimensionless variables were

used in the solution of the problem:

H X Y 0Bs Bs BS Bs

LEsj

B Bn=

F=Bs Bs Bs

(27)

Km _x_ D(V= t B vx Kin, vy = Ki d(V) = BK

Here Bs is the distance between the impermeable layerand the earth's surface; the coefficients of infiltrationand seepage are related to the maximum Km value ofthe coefficient of permeability in the domain. Variousproblems representing certain physical processes wereinvestigated.

Figure 1 presents a case of a horizontal drainagecanal, parallel to the irrigational canal, with water levelsbeing different in both the canals. Free-surface seepageand pollutant transport occur through the soil layer,separating the two canals. The soil layer is representedby the two-dashed lines.The plots in case 1 of Figure 1 present the results of

calculating free-surface position, hydraulic head valuesat T = 0.5 (head iso-lines), and values of dissolved solidscontent (concentration isolines) at T = 0.5 and T = 2.5(T is time, dimensionless). The values for porosity andpermeability coefficient in the upper, middle, and lowerlayers are

mu= 0.5

mm = 0.25

mI = 0.25

KU 1.0

Km 0.1 (28)

K, = 1.0

The initial position of the free surface is O = 1, thecoefficient of hydrodynamic dispersion is D = 0.001 +IVI and the initial concentration of the pore solution isCO = 0. When T > 0, the solution concentration in theirrigational canal is Cr = 1.0.Case 2 of Figure 1 shows that liquid flow moves from

the source of contamination (the zone of high pressure,Ph - 0.4) to the open channel. The source of pollutionis in the permeable layer (Km = 1.0), located betweenthe two impermeable ones (K1 = Ku = 0.1); 4XO = 0.8;D = 0.001; CO = 0; and, Cr = 1.0.The above examples illustrate the significant role of

porous media characteristics on the transport dynamicsof water and solutes. That is why the assumption thatphysico-chemical parameters such as permeability hav-ing constant values at all positions are not always jus-tified. However, the value of these models is that a largenumber of hydrogeological settings can be examinedvery quickly. This makes it possible to determine ex-posure concentrations of toxic chemicals in drinkingwaters derived from groundwater for many combina-tions of system parameters. These models make it pos-sible to examine the impact on groundwater quality ofagricultural practices and waste management activitiesby taking into account the interaction of hydrogeological

35

M. G. KHUBLARIAN

CASE 2y

FIGURE 1. Case 1: (a) Head isolines at T = 0.5; (b) concentration isolines at T = 0.5; (c) T = 2.5 for a case when there is a horizontaldrainage, parallel to the main irrigational canal. Case 2: (a) Head isolines at T = 2.5; (b) concentration isolines at T = 2.5; (c) T = 10 fora case when water flows out of the sources of contamination in the direction of the open channel.

characteristics of the subsurface and the chemical prop-erties of the pollutant.Environmental pollution and the deterioration of san-

itary conditions of surface and groundwater decreasedrinking water quality, which becomes unsatisfactoryin some of the regions. According to Soviet Water Leg-islation, groundwater as a source of drinking water isgiven first priority. However, groundwater quality, asmentioned above, is deteriorating. This is caused byincreased anthropogenic loading, manifesting itself inthe inflow of polluted waters into groundwater aquifers.Other activities that cause adverse effects on ground-water quality are pumping of toxic effluents into thesubsurface (deep-well injection), the burial of toxicwastes in unsaturated zones, etc.The influence of the human factor on spreading of

noninfectious diseases among the population is deter-mined by this type of pollution. These diseases are nu-merous and include practically all types of ailments.They include chronic cardiovascular and nervous systemdiseases, diseases of the digestive and blood-forming

organs, disturbance in fetal development, genetic af-fections, allergies and cancer. Bacterial pollution,caused by violation of water disinfection technology,results in outbreaks of infectious gastric and intestinaldiseases such as diarrhea, paratyphoid diseases, viralhepatitis, typhoid fever, dysentery, salmonellosis, andother waterborne diseases.Most frequently, waterborne diseases occur in re-

gions of intensive agricultural development, particu-larly in Soviet Central Asia. Sanitary conditions of theregion cannot but affect human health, with the mainproblem being gastric and intestinal infectious diseases.For example, morbidity from typhoid fever in CentralAsia is higher than anywhere else in the USSR. Blooddiseases and disturbance in both fetal development andchildbirth possibly caused by herbicides or pesticidesmigrating into groundwater resources used for drinkingwater are widespread in this region.Thus, the present situation necessitates paying

greater attention to environmental protection. Between1986 and 1990 (the 12th five-year period) 15 billion ru-

CASE 1

36

CHEMICAL SUBSTANCE TRANSPORT AND WATER QUALITY 37

bles have been allocated by the State for environmentalprotection, whereas between 1976 and 1985 (the 10thand 11th five-year periods) only 22 billion rubles wereallocated for environmental protection projects.

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2. Faibishenko, B. A. Water and Salinity Regime of Soils in Irri-gation [in Russian]. Moscow, 1986.

3. Kozlovskiy, E. A. (Ed.) Hydrogeological Base of GroundwaterProtection [in Russian]. Moscow, 1984.

4. Kats, D. M. Influence of Irrigation on Groundwater [in Russian].Moscow, 1976.

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9. Pikul, H. F., Street, R. U., and Remson, I. A numerical modelbased on coupled one-dimensional Richards and Boussinesq equa-tions. Water Resour. Res. 10(2): 295-302 (1974).

10. Nielsen, D. R., van Genuchten, M. Th., and Biggar, J. W. Water

flow and solute transport processes in the unsaturated zone.Water Resour. Res. 22(9): 89-108 (1986).

11. Todsen, M. Numerical solution of two-dimensional saturated/un-saturated drainage models. J. Hydrol. 20(4): 311-326 (1973).

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13. Davidson, M. R. Numerical calculation of saturated-unsaturatedinfiltration in a cracked soil. Water Resour. Res. 21(5): 709-714(1985).

14. Khublarian, M. G., Putyrskiy, V. E., and Frolov, A. P. Modellingof interrelated water flows in the system "unsaturated-saturatedsoil river" [in Russian]. In: Proceedings of the International Sym-posium on Groundwater Monitoring and Management, March, 23-38, 1986, Dresden, GDR, 1986, pp. 1-13.

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20. Khublarian, M. G., and Yushmanov, I. 0. Groundwater qualityappraisal. In: Hydrogeology in the Service of Man. Memoires ofthe 18th Congress of the International Association of Hydro-geologists, Part 3. Cambridge, 1985, pp. 119-126.