Mercury in sediments of Ulhas estuary

Abstract

Hg levels in water, suspended particulate matter and sediment of the Ulhas estuary are under considerable environmental stress due to the indiscriminate release of effluents from a variety of industries including chlor-alkali plants. Concentration ranges of dissolved (0.04-0.61 mu g l u-1) and particulate (1.13-6.43 mu g g u-1) Hg reveal a definite enhancement of levels in the estuary. The Hg burden in sediment upstream of the weir that limits the tidal influence is low (0.08-0.19 mu g g u-1) with low Cdorg content (1.8-2.9%). The high Hg content of the sediment just below the weir varies seasonally (highest concentration recorded being 38.45 mu g g u-1) due to incremental accretion of sediment as the fresh water flow over the weir progressively decreases. The 30 km segment of the estuary sustains markedly high levels of Hg in the sediment with an exponential decrease in the seaward direction from the weir. Higher concentrations than the expected background prevail in all the estuarine cores up to the bottom, though the overall concentration decreases from about 20 mu g g u-1 in core 7 (inner estuary) to 1 mu g g u-1 in core 31 (outer estuary). The Hg in sediment is associated with Cdorg, while its correlation with Al, Fe and Mn is poor. The Hg profiles in cores from the Arabian Sea (stations 34, 35 and 37) have a distinct horizon of enhanced concentration in the 5-60 cm segment. Based on u210Pb dating of core 37, the sediment at the bottom of this core is inferred to have been deposited in the year 1949, roughly two year prior to the establishment of the first chlor-alkali plant and represents the background (0.06-0.10 mu g g u-1). The Hg profiles in the offshore cores indicate a marked increase in transfer of Hg to sediment subsequent to 1980, with a peak around 1990-1992. Based on the index of geoaccumulation it is considered that the estuarine segment between stations 4 and 23 is extremely polluted, while the sediment from the open coast is moderately polluted in the top 25-30 cm with respect to Hg. The enrichment factor of Hg in the sediment is 350-700 for core 4 and decreases to 0-7 for the open-shore cores.