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.
