VARIATIONS IN HYDROCHEMISTRY, TRACE-METAL CONCENTRATION AND TRANSPORTDURING A RAIN STORM EVENT IN A SMALL CATCHMENT

General hydrochemistry and trace metal concentrations were monitored d uring a runoff event in a small catchment (0.9 km(2)) with an old aban doned copper mine and covered sulphidic waste. Five hours of rising hy drograph was sampled manually every 10 minutes while an automatic samp ler was used during the recession phase. The hydrograph started to ris e about two hours after the commencement of rain, but the hydrochemist ry showed only small changes for yet another hour and a half. At this time the concentration of total organic C (TOC), Cu and Zn decreased r apidly, while most other elements exhibited a peak in concentration. D ifferent origins and varying hydrological response could explain these observations. Chemical redistribution might, however, be important fo r some elements, notably Al and Fe. After this peak the concentrations of most elements decreased, probably because of dilution with precipi tation water whereafter a secondary concentration maximum was observed . This behaviour was most evident for K but also for Na, Mg and Ca. A noteworthy exception is pH that increased during the event and remaine d at high levels, while concentrations of Mn and Cd were lowered possi bly due to depletion of the mobile pool. Increased pH during the event is probably caused by liming of soils surrounding the creek, in combi nation with soil buffering processes. The increase of alkaline and alk aline earths (notably K) concentrations could be explained by transpor t of accumulated elements as the groundwater level rose.