A PRELIMINARY STUDY ON THE CHEMICAL QUALITY OF GROUND WATER IN THE ANDELLA OYA BASIN OF SRI LANKA

A hydrogeological investigation was carried out within the area of Andella Oya basin of the eastern part of Sri Lanka. As a part of this survey, water samples were collected from the dug wells before the north east monsoon rainfall and were chemically analysedfor major ions. The results of these analyses were plotted on PIPER'S trilinear diagram and the results reveal that about 113rd of the samples fall into bicarbonate type while 2/3rd fall into chloride type. Comparative studies emphasized that all cliloride type of water correlate with electrical conductivity (EC) values higher than 500 micromhoslcrn and the rest are below the critical value. The pH values of all samples were above 7 and sodium and calcium are the major cations in high TDS water. According to the Piper's trilinear diagram and the result of dug well survey, the samples can be divided into four geochemical groups. The groundwater from higher lands show high bicarbonate water (Group A) and when it moves into the lower areas, the quality changes into the chloride type. Water in shallow weathered overburden areas also show the bicarbonate type (Group B). There are a few wells in the higher lands with higher chloride content (Group C) which may be due to the higher clay content of the area or stagnation of water in structurally controlled basins. Wells near the lagoon show higher chloride content due to sea water intrusion. However, some wells about 1-2 kms away from the lagoon towards land also show higher chloride content (Group D) caused perhaps by the mixture of recharging water and salt water and therefore it can be assumed that there are salt water or salt which have been deposited in the subsurface areas perhaps due to the percolation of sea water during the Quaternary period marine transgression. This is very good evidence for locating the boundary of old beaches. The general quality of water is good.


Introduction
Groundwater prospecting techniques and investigations are directed mainly at locating groundwater resources and also at understanding the formations of groundwater systems with respect to the environment in which it occurs. The groundwater located from deeper areas can be used for any purpose during the drought. Hence the investigations for exploiting groundwater has important applications.
Andella Oya basin lies within the area of theGal Oya development project which was started in early 1950 ( Figure 1). "Senanayaka Samudraya" at Inginiyagala which is the main reservoir in this project supplies water through its left bank main channel for agriculture and domestic requirements of the settlers. A geohydrological investigation was carried out in this area to evaluate the groundwater and surface water resources of the area. Chemical analysis of water collected from dug wells was carried out as a part of this research. The chemical quality of water is a valuable tool in water investigation. ! F i~u r e 1: Antlella Oya Basin of Sri Lanka

Location and Extent of The Area
The study area lying in the Andella Oya basin comes under Ampara and Batticaloa districts in the eastern province of Sri Lanka (Figure 1). This area is about 272 sq. kms and lies between 81 35' and 81 47' east and 7 20' and 7 34' north. The area under investigation is bounded by the Uhana main branch channel in the south and by the Samrnanthurei-Batticaloa minor road which is close to the southern strip of lagoon in the east and by the left bank main channel in the west and north (Figure 3).
Andella Oya basin is situated in the Dry Zone of Sri Lanka (Figure 1). The rainfall isohytes show that the arithmatic mean in the hilly areas and lower areas are 2000 tnm and 1735 mm respectively.7 The temperature of the area varies between 26'C and 32°C.

Geomorphology .and Geology
The groundwater basin under study is situated in the lowest peneplain6 which is continuous over most of the north-eastern and coastal parts of Sri Lanka. The land surface is of the gently undulating type with more resistant rock types standing out as ridges, isolated hills and turtle back shaped outcrops. The general elevation varies from sea level in the east to about 48 m. in the west. The hilly area in the north-western side rises steeply from 48 meters to about 3 16 meters.
Geologically the land can be divided into the following zones; (a) beach sands, dune sands and salt marshes (b) alluvium such as flood plains and braided stream deposits (c) rocky area with Precambrian basement outcrops.2 Major rock types throughout the area are hornblende biotite gneiss, hornblende biotite rich augen gneiss, migmatitic gneiss, pink feldspar granitic gneiss and granite, calc gneiss, basic dykes and pegmatites ( Figure 2). All joints in the outcrops are closed and very tight. Well developed flood plains and other alluvial deposits occur on both sides of Andella Oya and other tributaries. Medium to coarse grained well graded sand deposits which is subangular to subrounded in texture occur in the homestead areas along the eastern boundary of the area. The dug well survey indicated that the weathered overburden varies from place to place significantly. It was also found that except at a few places, the entire area consists of shallow weathered overburden. The maximum range of the weathered overburden lies between 5m to 10m with a few locations showing the formation of lateritic soils.

Method of Survey Dug Well Survey
This survey was carried out in order to obtain preliminary information on annual water level fluctuations, consumption of water, recharge of water into the well, taste of water, formation of the well sites and weathering condition of the rocks, effect of the seepage condition from the channels, temperature and conductivity of the well water and possible effects of water pollution. The wells near the channels were not selected for this survey. Wells which were generally more than 100 meters away from the channels were selected randomly ( Figure  3).
During this survey, several water samples were collected before the northeast monsoon rainfall from the selected wells for the analysis of major ions. The following factors were considered in the selection of wells for chemical analysis.

3.
Deep weathered rocks and deep hard rocks. 4. Highly rechargable wells and less rechargable wells.

Streams and springs.
Samples were collected from the top of the water column using a bucket used by the people for collecting water and filled into 500 ml capacity pre-cleaned plastic bottles. All water samples were collected within a day and handed over for chemical analysis to the chemical laboratory of the Water Resources Board, the following day. The temperature was measured in Centrigrade using a thermometer.

Results and Discussion
Very tight joints in the hard rocks, less decomposed rocks and thin in-situ chemically weathered products (lateritic types) indicate that the general groundwater circulation is limited in this studied area. The chloride' content is very high in lower ground and in a'few locations of higher lands. Comparative studies showed that all chloride types of water have EC values higher than 500 micromhos/cm and that the rest are-below the critical value of 500 micromhos/cm. The results of these analysis (Table 1) plotted on PIPER'S trilinear diagram ( Figure 5), reveals that about 113 of the samples fall into the bicarbonate type while 213rd falls into chloride type. Most of the water from the western higher areas show high bicarbonate concentrations and when it moves into the lower areas, the quality changes into the chloride type.   According to these chemical groups, groups B and C show that the water quality has changed with the thickness of weathered rock overburden. Due to the uneveness of the terrain, all water may be stored in structurally controlled basins and the groundwater levels of each of these basins may not get connected together during the dry season. Most-of the salts in solution in thick weathered overburden areas and thick sandy clay areas will be dissolved as water percolates through the soil zones and the TDS content therefore depends upon local factors. This may be the reason for the poor quality water in Group C.
Brackish water (sodium chloride type) is found in the eastern boundary area and has a TDS range higher than 1000 ppm. One well is located near the lagoon where sea water may intrude into the well. The other wells are however located more than 5m above sea level and about 1-2 krn away from the lagoon towards the land side. Hence sea water may not intrude the shallow areas according to the GHYBEN-HERZBEY relation between fresh water and saline water.' These wells are located in the raised beach area. Hence the percolation and recharging water throughout the year may be mixed with the salt water which had been deposited in the subsurface areas due to the percolation of sea water during the Quaternary period marine transgression or the left over sea water in the subsurface areas.
However the general quality of groundwater is found to be suitable for drinking.