Treatment with chemical coagulants at different dosing levels changes ecotoxicity of stormwater from the Tahoe basin, California, USA

Abstract

In recent decades, the transport of stormwater-associated fine particles and phosphorus into Lake Tahoe has led to decreased water clarity and related ecological changes. Polyaluminum chloride coagulants (PACs) have shown great promise in removing these constituents from stormwater before it enters the lake. However, the potential risks of coagulant treatment to aquatic organisms are not well understood. This study investigated stormwater and coagulant toxicity under non-dosed, optimally-dosed, and over-dosed conditions using the US EPA 3-species test through growth of green algae (Selenastrum capricornutum), zooplankton (waterflea, Ceriodaphnia dubia) mortality and reproduction, and larval fish (fathead minnow, Pimephales promelas) mortality and biomass. Stormwater samples were collected during a 2005 spring snowmelt runoff event from three sites representing various forms of developed regions around Lake Tahoe. Samples were dosed with two different coagulants (a chitosan and a PAC) at levels optimized with a streaming current detector (SCD). Non-treated highway runoff was toxic to zooplankton and fish. Optimal coagulant dosing increased algal growth and reduced zooplankton toxicity. Overdosing at two and three times the optimal level of a PAC decreased zooplankton reproduction and increased fish mortality. PAC-related toxicity was correlated with increasing total unfiltered aluminum and decreasing alkalinity, pH, and DOC. Because of toxicity risks, we recommend keeping PAC coagulant dosing at or below optimal levels in practice.

Keywords:

• Coagulants
• ecotoxicity
• stormwater
• runoff
• overdosing

Acknowledgments

This project was funded by a grant from the USDA Forest Service through the City of South Lake Tahoe. We thank Tim Delaney, formerly with the UC Davis Tahoe Research Group, for his remarkable efforts in completing field and laboratory work. We also thank current and past employees of the UC Davis Toxicology Lab, including Alyssa Gartung, Stephanie Fong, Linda Deanovic, and Daniel Markiewicz, who advised on and implemented the toxicity analyses. We appreciate the input from John Reuter of the Tahoe Environment Research Center for his help in envisioning this study. Finally, thanks to Steve Peck and Russ Wigart, former employees of the City of South Lake Tahoe, who worked hard to facilitate the implementation of these studies.

Notes

∗ Com/Ind categorization includes commercial, industrial, communications, and utilities.

∗identifies statistically insignificant correlations (P > 0.05)

∗∗identifies statistically significant low correlations (< 0.700, P < 0.05); bold type identifies high correlations (0.700–0.749); bold italicized type identifies very high correlations (≥ 0.750).