HYDROCHEMICAL MODELING OF A STREAM DOMINATED BY ORGANIC-ACIDS AND ORGANICALLY BOUND ALUMINUM

An existing hydrogeochemical model, the Birkenes model (BIM), has been extended to include production of dissolved organic carbon (DOC) in t he soil/water system and its chemical interactions with H+ and Al3+. T he model has been calibrated and verified using precipitation and runo ff data from the Svartberget catchment in northern Sweden. The catchme nt is impacted by moderate amounts of sulphur deposition (6 kg S ha(-1 ) a(-1)); the stream is high in DOC (7-35 mg L(-1)) and experiences ep isodically low pH-values (similar to 4.0). The refined BIM is able to simulate the main variations in major chemical components of the strea m water (H+, Ca2+, Mg2+, Al-i, Al-0, Cl-, SO42- and DOC), as well as s tream discharge and O-18, while Na+ is not well reproduced. Although v ery simplified relationships are used for the chemical interactions be tween DOC and Hf and aluminium, the model shows that these interaction s are essential to stream acidity and aluminium-species content. Some of the model parameters are poorly determined by calibration with runo ff data only. Soil and soil water investigations may contribute to the evaluation and development of the model structure and the representat ion of chemical processes. Further improvements of the model should em phasize DOC-production/absorption, detailed studies on DOC chemical be haviour and hydrological structure.