Water treatment process evaluation of wildfire-affected sediment leachates

Abstract

The recent increase in wildfire activity has raised concerns among many water utilities that rely on forested watersheds susceptible to wildfire for drinking water supplies. An understanding of post-fire treatment challenges, and guidance on how water providers can prepare is needed. In this study, dark, charred, riverbank sediment deposits were collected from the High Park fire burn area in northern Colorado, USA and leached in the laboratory to simulate the impact of post-fire runoff on source water quality. A comparison of the sediment leachates to post-fire river samples collected during flow events indicated the turbidity, phosphorus, nitrogen and dissolved organic carbon concentrations were similar, although the leachates do not reflect extreme concentrations observed in the field. The treatability of the leachates was evaluated with conventional coagulation and additional unit processes (pre-oxidation, powdered activated carbon, biofiltration), and samples were analyzed for disinfection byproduct (DBP) formation. Material leached from the sediments was enriched in haloacetonitrile (HAN) and chloropicrin precursors compared to baseline organic material of source waters, whereas total trihalomethane and haloacetic acid precursor reactivity were not statistically different from baseline source waters. Sediment leachates were amenable to conventional coagulation (25 mg L⁻¹ aluminum sulfate), but may require additional treatment to meet DBP maximum contaminant levels. Enhanced coagulation (50 mg L⁻¹) or pre-ozonation followed by conventional treatment are recommended as the best options for reducing carbonaceous DBP formation. Pre-ozonation/coagulation/biofiltration resulted in the lowest HAN and chloropicrin formation of the sediment leachates.