WATER AND SEDIMENT GEOCHEMICAL CHARACTERISTICS OF KALLADA RIVER, KERALA

Abstract

The western coastline of India is home to many megacites and has higher population density that the eastern coast of India. The combined discharge of west flowing rivers accounts for 12.5% of the entire Indian river discharge and has 3.5 times the average surface run off of the Himalayan and eastern rivers; it is therefore necessary to study the westflowing rivers of India in the light of increasing domestic and industrial demand. The Kallada River is one of the two major rivers that flow through the Kollam district of Kerala. The study of small mountainous rivers, flowing over homogenous lithology is important to pinpoint the processes that control the geochemistry of river water. Kerala enjoys a tropical climate and the river is mostly controlled by the monsoon; the state of Kerala is urbanised - and the river-estuarine system is also a source of livelihood for the population. Water-quality parameters analyzed include pH, conductivity, alkalinity, total dissolved salts (TDS), Na', K, Ca2 , Mg2 , HCO3 , SiO2, F, CI, Br, NO3, S042 and P043 . Na and HCO3 are the dominant ions in the river stretch, while Na and Cl- are the dominant ions in the estuarine samples. Spatial variation in the hydrochemistry of the Kallada River and the contribution of the tributaries to the main channel was also studied. The tributaries indicate chemical contributions ranging from 2.8 to 13.0% to the main channel. Major ions show chemically conservative behaviour during transit along the river course. The composition of the river water reflects lithological influence accompanied by the influence of precipitation, runoff and anthropogenic effects. Though the basin is dominated by silicate lithology, estimates following the Na-normalisation of Ca2 , Mg2 , and HCO3 suggest that both the carbonate and silicate lithology contribute to the hydrochemistry. On an average, carbonate weathering accounts for —67% of HCO3 in Kallada River, while —32% of HCO3- is contributed by silicate weathering. High correlation of Na and Cl- indicates the influence of rainwater contribution to the composition of the river. The sources of nutrients and mixing processes in the upper reaches of the Ashtamudi Estuary are also studied. Grain size fining is observed along the course of the river and sillimanite, kyanite and hornblende are the dominant heavy minerals observed in the bank sediment samples which are representative of the charnockite and khondalite rocks the river drains during its course. Rainfall is observed to show a direct influence on river discharge, hydrochemistry, sediment load of the Kallada River and primary productivity in the coastal waters