The complex estuarine formation of six rivers (Cochin backwaters system on west coast of India) - Sources and distribution of trace metals and nutrients

Abstract

The largest backwater system on the western coast of India is the Cochin Backwaters to which 16 major industries discharge nearly 0.104 M m super(3) d super(-1) of wastes containing organic wastes at 260 t d super(-1). The backwater receives freshwater from 6 rivers - 5 in south, 1 in the north and 33% of river discharge is from Periyar. The river discharges of 19,000 M m super(3) y super(-1) also carries a substantial portion of the fertilizer load (20000 t y super(-1). Trace metals in estuarine sediment had shown a post-monsoon enrichment of Zn, Pb, Cu and in the northern and southern limbs of the estuary. The dissolved iron had an opposite tend to that of it in the sediments. Significant correlation of iron with other metals (except Mn) indicates that elemental accumulation in sediments may be controlled by precipitation of iron on to organic matrix. The significant correlation between metals (except Mn) shows a common source of metals. Natural processes control the distribution of most metals, while Zn is influenced more by anthropogenic input Cochin bar mouth and harbor region was not enriched in metals to greater levels. The northern part, an enrichment of metals, especially Zn is evident. Absence of build up in harbor and bar mouth may be due to periodic dredging and removal of recent sediment deposit. Strong negative correlation between salinity and nutrients within the estuary indicate that nutrient levels are controlled by upstream discharge. The estuary acts as sink for silicate during neap tide. It is estimated that the backwater is receiving 42.4 10 super(3) mol d super(-1) inorganic PO sub(4) and 37.6 10 super(3) d super(-1) of organic nitrogen through Periyar. The export to coastal waters is only 28.2 10 super(3) mol d super(-1) inorganic PO sub(4) and 24 10 super(3) mol d super(-1) of inorganic NO sub(3). Thus estuary seems to act as a sink for nutrients. Associated with the diminishing of flushing rate, a nutrient build up is taking place for the backwater system.