Heavy metal concentration trends in the Thames Estuary

Trends in the concentrations of Cd, Cu, Hg, Ni, Pb and Zn in the estuary of the River Thames between 1980 to 1997 were examined using linear regression methods to determine whether stated reductions in metal discharges from all sources to the estuary have had an effect on water quality. Concentrations of all metals, except Pb, showed exponential declines. Lower dilution capacities triggered by drought events post 1989 and continued use of leaded gasoline, reversed initial declines observed in Pb concentrations. Control measures have achieved significant reductions in metal concentrations in the estuary and met most stated environmental objectives for the estuary. Cd and Pb were the exceptions. Achieved improvements compare favourably with data documenting experiences in other European estuaries. By 2000, metal concentrations in the estuary will approximate adopted European guidelines for freshwater, but will remain above estimated background levels. Given the importance of metals as determinants of water quality, improvements are argued to be likely initiators of subtle trends in biological recovery.