A model for trace metals that considers instream transport, metal oxide precipitation-dissolution and pH-dependent sorption is presented. Linkage between a surface complexation submodel and the stream transport equations provides a framework for modeling sorption onto static and/or dynamic surfaces. A static surface (e.g. an iron- oxide coated streambed) is defined as a surface with a temporally constant solid concentration. Limited contact between solutes in the water column and the static surface is considered using a pseudo-kinetic approach. A dynamic surface (e.g. freshly precipitated metal oxides) has a temporally-variable solid concentration and is in equilibrium with the water column. Transport and deposition of solute mass sorbed to the dynamic surface is represented in the stream transport equations that include precipitate settling. The model is applied to a pH-modification experiment in an acid mine drainage stream. Dissolved copper concentrations were depressed for a three hour period in response to the experimentally-elevated pH. After passage of the pH front, copper was desorbed and dissolved concentrations returned to ambient levels. Copper sorption is modeled by considering sorption to aged hydrous ferric oxide (HFO) on the streambed (static surface) and freshly- precipitated HFO in the water column (dynamic surface). Comparison of parameter estimates with reported values suggests that naturally-formed iron oxides may be more effective in removing trace metals than synthetic oxides used in laboratory studies. The model's ability to simulate pH, metal oxide precipitation-dissolution and pH-dependent sorption provides a means of evaluating the complex interactions between trace-metal chemistry and hydrologic transport at the field scale.
AGU Index Terms: 1871 Surface water quality; 1045 Low-temperature geochemistry; 1806 Chemistry of fresh water
Keywords/Free Terms: Copper, St. Kevin Gulch, Water Quality Modeling.
Water Resour. Res. 1999WR900259
Vol. 35
, No. 12
, p. 3829