METAL BEHAVIOR DURING SURFACE GROUNDWATER INTERACTION, SILVER-BOW CREEK, MONTANA

This work documents the geochemical character of the hyporheic zone in the bed sediment of a small creek underlain by acidic, metal-rich groundwater. Use of a unique in situ method of sampling the solid-phase chemistry (elemental composition of coatings on installed ceramic beads) combined with water chemistry data allowed us to build a more complete picture of the geochemistry during surface groundwater interaction. A mixing ratio of 96:4 surface water (pH congruent to 7.9-9.1, [Fe] congruent to 0.2 mg/L) to contaminated groundwater (pH congruent to 4.2 - 4.9, [Fe] congruent to 400 mg/L) can explain the composition of the similar to 1 m thick hyporheic zone. Accumulations of metals on the ceramic beads support nonconservative behavior predicted by mixing ratios and show partitioning between the solid and aqueous phases. The behavior of metal contaminants in this system is discussed.