Variations in hydrochemistry, trace metal concentration and transport during a rain storm event in a small catchment

Abstract General hydrochemistry and trace metal concentrations were monitored during a runoff event in a small catchment (0.9 km 2 ) with an old abandoned copper mine and covered sulphidic waste. Five hours of rising hydrograph was sampled manually every 10 minutes while an automatic sampler was used during the recession phase. The hydrograph started to rise about two hours after the commencement of rain, but the hydrochemistry 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 rapidly, while most other elements exhibited a peak in concentration. Different origins and varying hydrological response could explain these observations. Chemical redistribution might, however, be important for some elements, notably Al and Fe. After this peak the concentrations of most elements decreased, probably because of dilution with precipitation 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 remained at high levels, while concentrations of Mn and Cd were lowered possibly due to depletion of the mobile pool. Increased pH during the event is probably caused by liming of soils surrounding the creek, in combination with soil buffering processes. The increase of alkaline and alkaline earths (notably K) concentrations could be explained by transport of accumulated elements as the groundwater level rose.