EFFECT OF AGRICULTURAL PRACTICES ON CHEMICAL QUALITY OF WATER IN KALA OYA RIVER BASIN

Abstract

An assessment of nitrates and phosphates in groundwater and surface water in two heavily agricultural areas (Talawa and Giribawa) within the Kala Oya River Basin was carried out to study the temporal effect on the groundwater upon fertilizer applications. Behaviour of some major cations, pH and conductivity were also studied. The total number of sampling points is 296 and weekly sampling was carried out at 20 locations within the two selected areas. The rate of fertilizer application was monitored during the sampling within three months period. The results of the study indicate that the pH is more alkaline in agricultural and dug wells whereas it is more acidic in tube wells. The health risk levels of WHO limits for nitrate and phosphate are not exceeded in almost all the water samples studied. However, a strong relation between the fertilizer application and the nutrient increase in water has been observed. Higher nitrate values were recorded from deep groundwater whereas some lake water is enriched with phosphates. The results of surface water analysis show that stream and canal waters are rich in Ca and Mg. In contrast, lake water is characterized by many dissolved ions except Ca and Mg. The highest values of Fe and Mn are found in lake water. The dissolved cations in all groundwater bodies studied are comparatively high. Although Ca and Mg levels are low, Na is high in agricultural wells. The total hardness is very high in some dug wells whereas lower values of Fe and Mn are recorded from groundwater. It was clearly observed that the intense agricultural practices increase nitrate and phosphate concentrations as well as other dissolved ions in groundwater. The geology of basement rocks and climatic conditions may also enhance the dissolved ion content in water. However, prevailing reducing conditions, overburden soil formation and pH of water act as buffers in reducing the intensity of accumulation of nutrients in water bodies.