SPATIO-TEMPORAL VARIATION IN THE HYDROCHEMISTRY OF TAWA RIVER, CENTRAL INDIA

Abstract

Studies on the fluvial geochemistry of a river are important, not only to determine its quality, but also to identify the sources of elements to rivers and to constrain weathering patterns. Central India enjoys tropical climate and in this region, a river's fluvial geochemistry is governed to a large extent by the Monsoons as well as lithology and anthropogenic inputs. The importance of these factors is better understood, if a high spatial resolution study is performed on a watershed of moderate size. Tawa, one of the longest tributary of Narmada has been selected for this study. Spatial and temporal variations in the hydrochemistry of the Tawa river, were studied based on seasonal sampling along the river and its tributaries, carried out for three seasons: post monsoon (October 2010), Monsoon (August 2011) and pre-monsoon (January 2012). The studies revealed that Monsoon indeed behaves as an important agent affecting the seasonal values with monsoon season ionic concentrations being relatively lesser and pre-monsoon season values being higher, though the latter could be due to groundwater interactions, which showed higher concentrations. The rains in the watershed were also found to be relatively acidic in nature. The basin is relatively pristine with most parts being covered by dense forests. However, human influence in the form of mining as well as agricultural activity is also present for some locations. The effect of anthropogenic activities is very clearly discernible, with the affected samples showing very high concentrations. Two major dams are also constructed on the river course and the reservoir samples show some of the least concentration values. Following Gibbs classification of rivers, it is identified that the water chemistry of the Tawa River basin is controlled by rock weathering, and the (Ca+Mg)/(Na+K) value of 3.89 suggest that the river water composition is controlled to a large extent by the weathering of carbonate lithology. HCO3 , Ca2+ and Mgt+ are found to be the dominant ions. Following the method of Raymahashay (1986), it is determined that on an average, 41.5% of HCO3_ in the Tawa river water is contributed by silicate weathering and 58.5% comes from carbonate lithology. The values of the Na-normalized ratios of Ca, Mg and Hco3- and their respective plots on the mixing diagrams suggest that both the Carbonate and Silicate lithology contribute to the hydrochemistry in the basin. In this study, we have also used annual water discharge data for the period 1979-80 to 2004-05 as well as bimonthly water quality and discharge data collected during the period from June 1991 to August 1992. The data confirms the effect of monsoon on water discharge (rapid increase) and other parameters like pH (more acidic values), TDS and ionic concentrations (dilution) as well as the effect of anthropogenic influence in the basin.