This thesis describes the hydrochemistry of three sub-catchments of the Uda Walawe basin, Sri Lanka. The basin is underlain by crystalline hard rocks and parts of the sub-catchments have been developed under an irrigation scheme. Groundwater is used for agricultural and drinking water purposes, but is vulnerable to poor water quality, particularly from high fluoride (F) concentrations and microbiological contamination. Dental fluorosis is endemic in the basin where F concentrations reach 9 mg/L. The Uda Walawe basin is marked by two major geological zones, the Highland Series and the Eastern Vijayan Complex. Highland Series rocks are subjected to a greater fracture network density and have higher yields and transmissivities compared to the Eastern Vijayan Complex. Groundwater is found in the alluvial soils, weathered regoliths and in the fractured layers of the crystalline rocks. Shallow wells are recharged by water leakage from irrigation canals and provide significant amounts of groundwater. Groundwater movement is southwards and discharges to the River Walawe and the southern coast of Sri Lanka. A detailed hydrochemical survey of dug wells and tube wells completed as part of this thesis showed that a Ca-Mg-HCO3 water type is present in shallow and deep groundwaters and a Na- Cl type in a few shallow wells in southern coastal districts. Prevalence of HC03" with high pCO2 indicates H2CO3 as the main acid for silicate weathering and release of major cations (Ca2+, Mg 2+ and Na) and F. Biotite and hornblende are the commonly occurring F bearing silicate minerals in the study area, as seen in other parts of the world. The occurrence of accessory minerals such as carbonates, apatites and fluorite are very limited and are not considered an important source releasing F to groundwater. Groundwater in the discharge areas of the study area is saturated with respect to these accessory minerals indicating controls of mixing of different waters and an increase in dissolved constituents, but not providing a control on F concentrations. Of secondary importance, F concentrations are higher in areas where there is limited recharge that may result in a longer residence time that promotes greater rock-water interactions. F concentrations show a positive correlation with weathered depths in shallow and deep wells. High evaporation in the downstream catchment areas concentrates fluoride ions in the shallow waters but, in the upstream areas, F concentrations are additionally controlled by dilution effects. Nitrate (up to 136 mg/L) and phosphate (up to 116 mg/L) are contributed from fertiliser or sewage sources but only occur in high concentrations in a few shallow well localities in those areas with low recharge and less dilution, and where favourable oxidising conditions exist. Nitrate is generally reduced in paddy fields and the risk of contamination in this situation is limited. Arsenic occurs in high concentrations (up to 0.36 mg/L) in a few shallow well localities, also in areas with low recharge. Sulphide oxidation is likely to favour arsenic release in the study area. Total coliforms and E. coli. levels are high in the surface waters and shallow groundwaters. The presence of E. coll. bacteria in the groundwaters suggests the predominance of point sources of contamination. In general, areas under irrigation in both geological regions show low and medium groundwater This thesis describes the hydrochemistry of three sub-catchments of the Uda Walawe basin, Sri Lanka. The basin is underlain by crystalline hard rocks and parts of the sub-catchments have been developed under an irrigation scheme. Groundwater is used for agricultural and drinking water purposes, but is vulnerable to poor water quality, particularly from high fluoride (F) concentrations and microbiological contamination. Dental fluorosis is endemic in the basin where F concentrations reach 9 mg/L. The Uda Walawe basin is marked by two major geological zones, the Highland Series and the Eastern Vijayan Complex. Highland Series rocks are subjected to a greater fracture network density and have higher yields and transmissivities compared to the Eastern Vijayan Complex. Groundwater is found in the alluvial soils, weathered regoliths and in the fractured layers of the crystalline rocks. Shallow wells are recharged by water leakage from irrigation canals and provide significant amounts of groundwater. Groundwater movement is southwards and discharges to the River Walawe and the southern coast of Sri Lanka. A detailed hydrochemical survey of dug wells and tube wells completed as part of this thesis showed that a Ca-Mg-HCO3 water type is present in shallow and deep groundwaters and a Na- Cl type in a few shallow wells in southern coastal districts. Prevalence of HC03" with high pCO2 indicates H2CO3 as the main acid for silicate weathering and release of major cations (Ca2+, Mg 2+ and Na) and F. Biotite and hornblende are the commonly occurring F bearing silicate minerals in the study area, as seen in other parts of the world.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:273467 |
| Date | January 2003 |
| Creators | Rajasooriyar, Lorraine Dushyanthi |
| Publisher | University of East Anglia |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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