Water quality implications of post-wildfire erosion control with polymeric additives

Post-wildfire erosion to downstream surface waters can deteriorate water quality to levels that can create challenges for aquatic life and drinking water treatment. Polymeric additives, xanthan gum (XG) and polyacrylamide (PAM), have been demonstrated to be effective for controlling erosion in the presence of hydrophilic ash. However, with repeated rainfall applications, some of the applied XG and PAM may mobilize with the runoff and enter surface waters, which may pose water quality concerns. In this study, indoor rainfall simulation experiments were performed on plots containing wildfire-burned soil overlaid by hydrophilic ash collected after the 2021 Green Ridge Wildfire near Walla Walla, WA. The plots were treated with three concentrations (11, 33, and 60 kg ha^-1) of XG or PAM and subjected to three wet-dry cycles. Runoff water samples were collected at 5 min intervals during each wetting event. The pH, electrical conductivity, dissolved organic carbon (DOC), total dissolved nitrogen (TDN), and settled water turbidity (SWT) were measured for runoff water samples. The presence of XG in runoff from XG-treated plots increased SWT by up to 247% and DOC to as high as 16.6 mg_C L^-1. PAM treatment also increased DOC (up to 24.5 mg_C L^-1) and TDN (up to 5.8 mg_N L^-1) in runoff. DOC and TDN concentrations in runoff from treated plots increased with an increase in treatment concentrations and were generally greatest in the first wetting event. The results suggest that benefits of using polymeric additives for erosion reduction should be evaluated together with an assessment of dilution of downstream water bodies to alleviate the negative impacts of the additives on downstream water quality.