A GEOCHEMICAL CLASSIFICATION OF GROUNDWATER OF SRI LANKA

Absma : This paper presents a detailed geochemical classification of groundwater of Sri Lanka. In view of the fact that nearly 85% of the population of Sri Lanka use well water and other non-piped water for their drinking water supply; information on the chemistry of the growdwater is essential. Depending on its chemistry, the groundwater of Sri Lanka has been classified into 4 main types, namely, Ca,Mg,Na/K and non-dominant cation types. Each of these major groups are sub-divided into 2 or 4 sub-groups. It was revealed that the chemistry of the groundwater is markedly dependent on the underlying geology as well as the climate. The dry zone in particular is abundant in the Na/K type of water whereas the wet zone had Ca and non-dominant cation types. It is also shown that the proposed classification has useful applications in delineating areas susceptible to waterborne health hazards.


Introduction
The need for clean water as one of the most essential commodities for mankind can never be over-emphasized.Groundwater monitoring is one of the most important aspects of groundwater resource management and prevention of groundwater pollution.Most environmental research activities have been centered on rivers, lakes and the atmosphere of developed.nations.Very few case studies.have been reported from developing countries even though clean water is often a scarce commodity in such countries.

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In Sri Lanka, a country of 15 million people, only 10 -15% of the people have access t o piped water, and the majority of the country's health problems are related to its aquatic environment.Most people use small, unprotected wells, and in rural settlements, reservoirs and stream and river channels are the main sources of drinking water.The proper disposal of human and other wastes through sewerage systems and latrines is also severely limited, less than a third of the population having satisfactory latrine facilities.The poor water supply and excreta disposal systems have resulted in 40% of the Sri Lankan population being affected by typhoid, amoebic and bacillary dysentery, infectious hepatitis, gastro-enteritis, colitis and worm infections.The need to carefully monitor the groundwater quality of Sri Lanka, is therefore of high priority and upto now this aspect has been neglected.
Environmental geochemistry essentially deals with the geographical distribution of elements and forms the basis for a variety of interdisciplinary studies involving human and animal health, quality of groundwater, agriculture and nutrition, soil fertility, pollution and mineral exploration.The study of the abundance and distribution of some trace elements and the resulting biological manifestations involves geochemists, public health workers, soil scientists, ecologists and nutnbonists.
The chemical quality of groundwater is related to the geology of the area concerned.For example, areas underlain by acid igneous rocks such as granite or arenaceous sedimentary rocks generally contain lower levels of essential trace elements -particularly the first row transition elementsthan areas underlain by ultrabasic and igneous rocks or shale.These however, may sometimes contain sufficient concentrations of potentially toxic elements. 1 5 is the aim of this paper to present a detailed chemical classification of the groundwater of Sri Lanka.I t is hoped that this chemical classification would help, not only the hydrogeochemist, but also town and country planners and those engaged in the implementation of rural water supply schemes.

Materials and Methods
Figure 1 illustrates the general geology and climate of Sri Lanka and Figure 2, the locations of the sampling points for groundwater.All water samples were collected in acid-washed' polyethylene bottles and kept cool and dark until tested.All samples were collected during the period July-December 1982.The appendix shows the details of locations.Three samples were taken from each location, for the determination of the following :

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The total dissolved solids (TDS), Cl-, F-, ~0 ~~and HC03 determinations were carried out using 1000 ml of well-mixed unacidified filtered samples (Sample -1).Following the methods of Brown et, al., TDS sol2-CI-and HC03 measurements were carried out by gravimetry an titrimetry respectively.The fluoride contents of the water were determined by the use of specific ion electrode.'A 2000 ml sample (Sample 3) of filtered water, acidified with 3 ml of reagent grade conc.HNO was used for the determination of metal ions, and total Si02.The V and SiO, were determined spectrophotometrically following the method of Brown et al.The metals Cu, Co, Cr and Zn were determined by atomic absorption spectrophotometry following preconcentration using ammonium pyrrolidine dithiocarbamate (APDC)/methyl isobutyl ketone (MIBK).N, K, Ca, Mg, Fe and Mn were determined by 7 atomic absorption spectrophotometry without pre-concentration.

SAMPF E LOCATION MAP ( S E E APPENDIX FOR DETAILS)
Replicate analyses were made for all samples and inter-laboratory comparisons made.A relative standard deviation of % 1 -5% was observed.

Plotting of data and map making
As the eye is well adapted to the recognition of patterns inspatial data, maps have the capacity to present geochemical information with great impact, and the spatial component becomes an integral part of the compilation and interpretational process.Comparison of hydrogeochemical data with topographic geologic or geochemical information is made easier when all are in map form on the same scale.' Regional geochemical maps are best suited for application in agriculture, ecology and human health investigations.Such an approach has been taken by the Applied Geochemistry Research Group (AGRG) of Imperial College, London and the British Geological Survey, and has proved to be of immense value in a large number of disciplines.Generally this type of map provides the tbroadest view of large portions or the totality of country.The sampling densities of these maps are below 1 km2, over an area in excess 10,000 sq km and are usually presented as moving-average smoothed maps.Local irregularities of the sampling results often obscure rather than clarify any attempt to extract diagonostic patterns in the production of contour maps at regional scale.These irregularities need to be harmonized, first with suitable smoothing out procedures.Much of the studies in this direction had been carried out by ~a v i s ~ and 0lea.l'

The chemical basis for the classification of groundwater -
The major constituents in aquatic systems include Ca, Mg, Na, K, HC03 , C032-, ~0 ~~and Cl-.The proportions of these eight geochemically s i p ificant constituents in natural solution provides the basis for naming the water type.To provide a basis for comparison of water types and to relate them to specific environments, a graphic method of illustrating data' and appropriate terminology must be adopted.
The Piper diagram1 is a multiple trilinear diagram for graphic representation of the major chemical constituents of water, and effectively portrays analytical data.Similar analytical techniques were developed by ill,^ Langelier and ~u d w i ~" and ~0 m a n i .l ~ The model used in this study is a modification by ern.^Piper diagrams are used in various ways in hydrogeochemistry.The simpIest application is merely to display data to represent distinctions among individual water samples.A fairly recent and promising modification of the Piper diagram involves the use of component cation and anion diagrams to classify water.The water type is generally named after the dominant cations and dominant anions -defined as constituting more than 50% of the cation or anion.This has been accomplished graphically by joining the mid-points of each side of each triangular field which divides each triangular diagram into 4 smaller triangles.Thus a water type is easily named, based on the positioning of the points in the cation and anion triangles.Unless there are non-dominant cations or anions, the water type is named after the cations (Ca, Mg, Na/K) followed by a hyphen and a similar term selected for anion possibilities (SO4, Cl, HC03/C03).When a water type plots in the Piper diagram in the non-dominant cation or nondominant anion fields, it indicates that on percentage epm basis, no ion is present in an amount greater than 50%.In such instances, non-dominant cation (NDC) or non-dominant anion (NDA) forms the descriptive name.

The chemistry of the groundwater of Sri Lanka
The groundwater of Sri Lanka can be classified into the following 4 main water types.The appendix shows all chemical data pertainingto this study. ,1. Calcium type 2. Magnesium type 3. Sodium/potassium type 4. Non-dominant cation type Figure 3 illustrates the distribution of these 4 major water types in Sri Lanka.Each type is further sub-divided into the C1, SO*, HC03 and NDA types.Table 1 shows the average for the elements and ionic species.

The calcium type
Figure 4 illustrates the Piper trilinear diagrams for the calcium water type.In Sri Lanka, this type of water is distributed mainly in the northern, central and in some parts of southern, eastern and north central regions.The C1 type predominates in the northern parts whereas the HC03 type is prevalent in the central regions.The effect of salinity and the presence of carbonate rocks in the areas could possibly be attributed to such a distribution.Table 2 shows the correlation matrix for the elements and ionic species analyzed for the calcium type of water in Sri Lanka.The total dissolved solids (TDS) show significant correlations with K, Ca, HC03 and C1.The transition elements however do not show significant correlations for this type of water.

The magnesium type
When compared to the other types of water, the magnesium type is distributed only in relatively smaller areas, the southern parts of the country around Embilipitiya having higher concentrations.In this type of water, only the C1 and SO4 sub-types could be found.The correlation matrix and the Piper trilinear diagram for the Mg water type are shown in Table 3 and Figure 5 respectively.

3 The sodium potassium type
This type forms a major group and is distributed widely in Sri Lanka, particularly around the central region.The north western and north central and the south eastern dry zones mainly contain this type of groundwater.From among the sub-types, the C1 type is predominantly found in these regions.
Excessive evaporation and probably influence of salinity may have contributed to the prevalence of this water type.Table 4 shows the correlation matrix for the Na/K water type and Figure 6 illustrates the Piper trilinear diagrams.

The non-dominant cation type
As illustrated in Figure 3 the non-dominant cation type of water is distributed mainly at the periphery of the central highlands and some parts of the north central and southern regions.The HCO and non-dominant

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anion sub-types predominate in these regions.Table 5 s ows the correlation matrix for the non-dominant cation water type and Figure 7 illustrates the Piper trilinear diagrams.

Effect of geology and climate on the chemistry ofg.roundwater
A closer study of the distribution of the groundwater types in Sri Lanka reveals that the underlying geology and the climate affects the chemical quality of water to a great extent.The wet zone of Sri Lanka .(See Figure 1) consists for the most part of non-dominant cationtypes and calcium -HC03 and non-dominant anion types.In the dry zone however, the Na/K type predominates and in this type of water the C1 sub-type is found covering vast areas of the dry zone.Evaporation under the,strong drought conditions as prevailing in the dry zone of Sri Lanka results in the accumulation of sodium salts in the soiI layers and this factor is lygely responsible for the abundance of the Na/K type in the dry zone.Further, the northern parts of Sri Lanka are underlain by sedimentary limestones, as a result of which the calcium type of water predominates in these parts.Increasing salinity has been observed in areas closer t o the shore-lines and in the Jaffna Peninsula in particular, this is commonly seen.The predominating anion in this type of water in the dry zone is C1.
When one considers the topography, the central highlands have groundwater of the Ca -HC03 type and with decreasing elevation, merges into the non-dominant cation type.In the lowlands the Na/K type predominates.Thus a Ca + NDC + NajK type of sequence is apparent with decreasing elevations from the highlands to lowlands.This sequence could well be due to the different geochemical mobilities of the elements concerned.Further, there are numerous shallow and deep seated fractures and lineaments within the central regions of Sri Lanka and these are mainly responsible for the migration of groundwater within the hardrock terrains.A Geochemical classification of Groundwater of Sri Lunka

Applz*cation in health
The delineation of areas of different water chemistry has applications in studies pertaining to human health and epidemiology.The effect of the chemistry of the .groundwateron the health of the human population in Sri Lanka is of paramount importance due t o the fact that the vast majority of the people use groundwater directly for their drinking and cooking purposes.
It is apparent from Figure 3 that the effect of Na, K and C1 is more pronounced in the dry zone areas as against Ca and non-dominant cation types in the wet zone.The people living in the dry zone regions are thus subiected to a different water chemistry than those living in other parts of Sri Lanka.The effect of water chemistry on the health of the population in Sri Lanka has been highlighted by Dissanayake et.~1 .~ and ~i s s a n a ~a k e .~ From these studies it was revealed that there is a correlation between water hardness and the incidence of cardiovascular diseases.Areas underlain by groundwater with high water hardness appeared t o have a low incidence of cardiovascular diseases as exemplified by the Jaffna Peninsula.On the other hand, certain regions in the wet zone where water hardness was low, had a higher incidence of cardiovascular diseases.Prior information on the chemical quality of the water of different areas helps in the delineation of disease prone regions.Among the other diseases dependent on the water quality are dental diseases such as dental fluorosis and tooth decay.Earlier s t ~d i e s ~? ~ have shown certain areas in Sri Lanka, particularly in the north central and eastern parts to contain anomalous fluonde concentrations in the groundwater.These areas coincided with a high incidence of dental fluorosis, particularly among school children.
The groundwater of Sri Lanka has been classified chemically and a map showing the distribution of the different water types prepared.The groundwater has been classified into 4 major types, namely Ca, Mg, Na/K and nondominant cation types.The Ca-HC03 type is found predominantly in the wet zone of the central highlands and appear to be associated &th the nondominant cation types.In the dry zone, the Na/K type is abundant whereas in northern areas particularly in the Jaffna Peninsula, the Ca-Cl type is abundant.It is apparent that the distribution of the different groundwater types is markedly affected by the underlying geology and climatic factors.The map showing the distribution of the groundwater types has application in delineating areas susceptible to health hazards depending on the chemical composition of water.q q a ; g ; q : 8 3 ?q g g ; ;

Figure 2 .
Figure 2. Map showing the locations of sampling points.

Figure 1 .
Figure 1.Map showing the climate and geology of-Sri Lanka.

Figure 3 .
Figure 3. Map showing the distribution of the major groundwater types in Sri Lanka.

Table 1 :
Elemental averages for the different water types of Sri LankaA ~e o c h e m i c a l classi'cation of Groundwater of Sri Lanka