Impact of the Clean Water Act on the levels of toxic metals in urban estuaries:: The Hudson River estuary revisited

To establish the impact of the Clean Water Act on the water quality of urban estuaries, dissolved ( <0.45 mu m) trace metals and phosphate concentrations were determined in surface waters collected along the Hudson River estuary between 1995 and 1997 and compared with samples collected in the mid-1970s by Klinkhammer and Bender. The median concentrations along the estuary have apparently declined 36-56% for Cu, 55-89% for Cd, 53-85% for Ni, and 53-90% for Zn over a period of 23 years. These reductions appear to reflect improvements in controlling discharges from municipal and industrial wastewater treatment plants since the Clean Water Act was enacted in 1972. In contrast, levels of dissolved nutrients (PO4) have remained relatively constant during the same period of time, suggesting that wastewater treatment plant improvements in the New York/New Jersey Metropolitan area have not been as effective at reducing nutrient levels within the estuary. While more advanced wastewater treatment could potentially reduce the levels of Ag and PO4 along the estuary, these improvements would have a more limited effect on the levels of other trace metals. Rather than inputs from point sources, the release of Pb and Hg from watershed soils, and Ni and Cu from estuarine sediments, may represent the primary contemporary sources of these metals to the estuary. These potentially important nonpoint sources need to be addressed in metal contamination control strategies. This is critical to control the high levels of dissolved Hg observed under high river flow conditions. While levels of dissolved Ag, Cd, Cu, Ni, Ph, and Zn were at least three times below current New York Water Quality Standards, Hg levels in surface waters of the estuary still exceed those standards.