Heavy metals in sediments of two Chesapeake Bay tributaries — Wicomico and Pocomoke Rivers

Abstract

Aquatic sediments can be both sinks and sources of heavy metals. The objective of this study was to measure the concentrations of heavy metals in sediments and sediment-porewaters of two Chesapeake Bay tributaries — Wicomico (WR) and Pocomoke (PR) Rivers so as to better understand the contributions of point (sewage treatment plant — STP) or non-point (agricultural runoff) sources of pollution. Sediment samples from three different depths (3″, 6″ and 9″) at four different sites (one mile before, adjacent to, one and two miles after the STP) on each of the two rivers were collected. This part of WR is residential and receives only STP effluent; whereas this part of PR gets effluents from both the STP and agricultural runoff including runoff from a poultry production farm that generates and stores large amounts of litter. The samples were analyzed for zinc (Zn), lead (Pb), copper (Cu), cadmium (Cd) and arsenic (As) using atomic absorption spectrophotometer (AAS). The concentrations of metals in sediment were normalized against organic carbon content and the correlation coefficient was found to be high in all cases. Concentrations of these metals were significantly higher in the second (near the STP outlet) and third (near the poultry farm) sampling sites in WR and PR, respectively. Metal concentrations decreased significantly (p ≤ 0.05) with depth in both the rivers. High concentrations of ammonia and nitrate in the WR indicate STP as the source of pollution, whereas a large amount of organic matter in PR sediment points toward agricultural runoff (from the poultry farm). As there was no baseline or earlier data on metal concentrations in these rivers the exact anthropogenic metal input could not be estimated.