Seasonal and short-term variations in the concentration and supply of dissolved zinc to polluted aquatic environments

Monitored levels of dissolved zinc in the River Ystwyth, ranging from 0·17 to 0·88 mg/l, are related to discharge variation. Bulking of samples, infrequent sampling, and sampling without regard to the recording of discharges are inadvisable in such situations. Concentrations are highest at low flows (a 'dilution' effect), and for brief periods during the initial stages of strom runoff (a 'flushing' effect, but in terms of bulk zinc output the range in discharge variation (0·4 to 269·0 m³/sec in 1973-1974) far outweighs the effect of varying concentration. For two successive years an annual yield of 72t of dissolved zinc has been estimated, with individual monthly yields ranging between 1·0 and 12·8 t, the higher during greater winter discharges. This may explain observed variations in metal concentration in coastal waters and in certain biota, in addition to changes arising from varying rates of metabolic activity.