DIEL CYCLES OF NUTRIENTS AND TRACE METALS IN A HYPEREUTROPHIC STREAM, SILVER BOW CREEK, MONTANA

Butte, Montana, is historically significant for its extensive copper mining operations, which have led to widespread metal and arsenic contamination. Additionally, nutrient inputs from point and non-point sources produce elevated concentrations of ammonium (NH₄⁺) and nitrate (NO₃^-), which has led to hypereutrophic conditions in Silver Bow Creek (SBC). Significant mid-summer diel (24-hr) cycles in temperature, pH and dissolved oxygen (DO) have been noted in SBC, which induce diel cycles in concentrations of metals and nitrogenous species.

During warm summer months, DO concentrations in upper SBC are high during the day due to photosynthetic O₂ production, but drop to near zero during nighttime hours due to biological respiration and microbial oxidation of ammonium sourced from the municipal wastewater treatment plant (WWTP). Stream pH shows large diel cycles above the WWTP, but is damped below the WWTP because acid generated during ammonium oxidation offsets the daytime increase in pH from photosynthetic uptake of CO₂. As a result of changes in DO, pH and redox conditions, concentrations of heavy metals and nutrients also show significant diel cycles, with some constituents varying more than 800% over a 24-hr period. Concentrations of Zn and Mn decrease during daytime hours when pH and temperature rises, whereas As follows the reverse trend. In contrast, Cu concentrations show no clear diel pattern. In general, ammonium concentrations in SBC decrease with distance below the WWTP (e.g., 4.5 to 0.1 mg/L as N), while nitrate concentrations increase (e.g., 1.0 to 3.2 mg/L as N). These changes are caused by two-step microbial oxidation of ammonium to nitrite, and nitrite to nitrate. During nighttime hours, however, very low DO concentrations combined with cooler temperatures decrease the rate of ammonium oxidation. This causes a strong diel signal in the levels of both ammonium and nitrate when observed at a fixed location along the stream. The diel variability of metal and nutrient concentrations have significant implications for how water quality monitoring plans are designed and implemented, especially in reaches with hypereutrophic conditions and large variability in water temperature, DO, and/or pH.