Publication Summary

The results of the first year of a two-year survey of construction stormwater sites in western Washington are reported. Samples were collected from 11 construction sites in Pierce and Thurston counties as well as from receiving waters upstream and downstream of discharges from three of the sites. Of 42 active construction sites visited, 11 sites were discharging runoff.

Turbidity ranged from 2.4 to >1000 NTU. Over 90% of the sites discharges ranged from 14 to 240 NTU.

Field turbidity determinations with a ratio-type nephelometer were found to be accurate. Transparency was found to be a usable surrogate for turbidity in some cases.

This report presents first-year results of a two-year survey of construction stormwater sites in western Washington. The purpose of the study is to evaluate the quality of stormwater discharged from construction sites during the winter wet season when potential impacts are greatest.

Under the NPDES and State Waste Discharge General Permit program, the Washington State Department of Ecology has regulated discharges associated with construction activity since 1992. Data representative of typical construction stormwater discharges in western Washington have not been available. This survey was initiated to obtain representative data to characterize stormwater discharged from construction sites.

Forty-two construction sites were visited as potential sampling locations. Samples were collected and measurements made from the 11 sites that were actively discharging. These 11 sites were in Pierce and Thurston counties. Upstream and downstream receiving water samples also were collected from three of the 11 sites.

Turbidity ranged from 2.4 to >1000 NTU. Over 90% of the discharges ranged from 14 to 240 NTU. Comparisons are made between three methods for determining or estimating turbidity.

The low incidence of active construction sites discharging runoff (26%) during the first year of the study is attributed, in part, to the permeable soils of western Washington, as indicated by soil permeability maps. Lower than typical groundwater tables resulting from below average precipitation also may have been a factor.

Field turbidity determinations with a ratio-type nephelometer were found to be as precise and accurate as laboratory turbidity. Transparency was found to be a good surrogate for turbidity in western Washington for estimated turbidity values below 300 NTU. Total suspended solids is not a valid surrogate for turbidity determinations.

Link to EIM data for User Study ID SGOL006