Spatial and temporal variations in trace metal concentrations in sediments, pore water of the Forth estuary, and their potential impact on water quality

The concentrations of trace metals: cadmium, copper, \nnickel, lead and mercury, in addition to aluminium, iron and \nmanganese were determined in the sediments and pore water \n(except aluminium) of the Forth estuary. Organic matter (as \nloss on ignition), total organic carbon (TOe), total organic \nnitrogen (TON), particle size and redox potential were also \ndetermined in the sediments. In addition, chloride, sulphate, \nconductivity, alkalinity, pH, dissolved organic \ncarbon, phosphate and nitrate were determined in the \nsediments pore water. Trace metals in the pore water were \ndetermined using a modified preconcentration technique and \nmeasurements were carried out by graphite furnace. Mercury \nwas measured using the p.s.a mercury specific fluorescence \ndetector. organic matter (TOC & TON) in the sediments were \nmeasured using CHNS/O analyzer. \nSamples were collected at seasonal intervals for a \nperiod of 18 months on board the survey vessel "Forth \nRanger". Six stations were selected to represent upper, \nmiddle and lower estuary. Sediment cores collected at each \nstation were subjected to centrifugation under nitrogen gas \natmosphere to separate the pore water after being sectioned \nat 2.5 cm intervals for a depth of 10 centimetres. \nConcentrations of trace metals in the Forth estuary \nvaried from station to station and from one season to \nanother. Mean concentrations for sediments were as follows \n(mg\\kg): lead, 56.9; cadmium, 0.22; copper, 38.9; nickel, \n33.7 and mercury, 1.86. For pore water, the mean concentraxxi \ntions (Jlg\\l) were as follows: lead, 3.14; cadmium, 1.86; \ncopper, 31.9; nickel, 24.7 and mercury, 0.075. \nMeasurements of sulphates, nitrates, phosphate, \nconductivity and dissolved organic carbon were used to \nexamine the processes controlling their distributions in the \npore water. The results indicated that the distributions of \nnutrients (phosphate and nitrate) in the upper 10 cm seemed \nto be controlled by macro-infaunal irrigation activities and \ntheir values varied according to the overlying water \nconditions. \nThe distributions of trace metals in the sediments were \nfound to be controlled by particle size and organic matter \nboth spatially and temporally. Vertical distribution of \ntrace metals in the pore water were found to be controlled \nby redox potential variations especially for iron and \nmanganese. Concentrations of trace metals (Pb, Hg, Ni, Cd \nand CU) in the pore water were found to be higher than that \nin the overlying water. Enrichment factors were limited to \n10-50 fold increase. Lead and mercury were 10 fold higher \nwhile 50 fold higher were found for nickel, copper and \ncadmium. \nBenthic flux calculations showed a significant flux of \ntrace metals both in the summer and winter. However, flux of \ncopper, nickel and lead tended to increase in the summer. \nSpatially, the lower estuary showed a higher flux of trace \nmetals than the upper and middle reaches of the estuary.