Characteristics of Snowmelt Runoff from Highways in the Tahoe Basin and Treatment Investigations for Improving Runoff Quality

Abstract

To reduce the current trend of decreasing water clarity of Lake Tahoe, the California Regional Water Control Board's Lahontan Region imposed numeric discharge limits for five pollutants (total nitrogen, total phosphorus, total iron, turbidity, and oil and grease) for both surface water and infiltration discharges in the Tahoe basin. To assess its compliance with these discharge limits, the California Department of Transportation (Caltrans) monitored six representative sites for snowmelt and other storm water runoff from summer 2000 through spring 2003. In addition, Caltrans has investigated several existing double-barrel sand traps for their effectiveness in removing pollutants and complying with the Tahoe basin discharge limits. The results obtained to date indicate that ( a) concentrations of pollutants found in the snowmelt runoff are higher than the specified storm water discharge limits; ( b) double-barrel sand traps were capable of capturing sand and sediment, but the capture was not complete; ( c) particles that range from 100 to 2,000 μm represented the highest proportion, but no correlation was found between particles and chemical concentrations; and ( d) sand traps did provide modest improvement in runoff quality, but this improvement was insufficient to meet Tahoe basin discharge limits, and hence additional treatment was found to be needed to bring the effluent water quality into compliance.