Reactive solute transport in streams: A surface complexation approach for trace metal sorption

A model for trace metals that considers in-stream transport, metal oxide pr ecipitation-dissolution, and pH-dependent sorption is presented. Linkage be tween a surface complexation submodel and the stream transport equations pr ovides a framework for modeling sorption onto static and/or dynamic surface s. A static surface (e.g., an iron-oxide-coated streambed) is defined as a surface with a temporally constant solid concentration. Limited contact bet ween solutes in the water column and the static surface is considered using a pseudokinetic approach. A dynamic surface (e.g., freshly precipitated me tal. oxides) has a temporally variable solid concentration and is in equili brium with the water column. Transport and deposition of solute mass sorbed to the dynamic surface is represented in the stream transport equations th at 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 3 hour period in response to the experimentally eleva ted pH. After passage of the pH front, copper was desorbed, and dissolved c oncentrations returned to ambient levels. Copper sorption is modeled by con sidering sorption to aged hydrous ferric oxide (HFO) on the streambed (stat ic surface) and freshly precipitated HFO in the water column (dynamic surfa ce). Comparison of parameter estimates with reported values suggests that n aturally formed iron oxides may be more effective in removing trace metals than synthetic oxides used in laboratory studies. The model's ability to si mulate pH, metal oxide precipitation-dissolution, and pH-dependent sorption provides a means of evaluating the complex interactions between trace meta l chemistry and hydrologic transport at the field scale.