Publication Summary

A two-year survey was conducted on stormwater quality discharged from western Washington construction sites during the winter, wet season. The study sites were located in Thurston, Pierce, King, and Snohomish counties.

From a total of 183 construction sites visited, 44 sites were actively discharging stormwater at the time of the visit. Samples were collected and analyzed for turbidity, transparency, and total suspended solids. Upstream and downstream receiving water samples also were collected from six of the 44 sites.

Stormwater discharges from construction sites have been regulated since 1992 by the Washington State Department of Ecology, under the NPDES and the State Waste Discharge General Permit programs. This survey in western Washington was initiated to obtain representative data to characterize stormwater discharged from construction sites.

Data were collected during the two winter, wet seasons of 2003-04 and 2004-05. Data were on general site characteristics and water quality parameters, including turbidity, transparency, and total suspended solids (TSS).

Of 183 eligible construction sites visited in four counties, 44 sites were discharging runoff and were sampled. The low incidence of sites discharging stormwater offsite (24%) during field sampling visits is attributed, in part, to lower than normal rainfall, permeable soils, and the use of water quality best management practices. It can also be attributed to the variable and intermittent nature of stormwater discharges, which makes it difficult to time sampling visits to coincide with stormwater discharge events.

During this snapshot study, only six of 44 sites (14%) discharged stormwater directly to receiving waters during field sampling visits. Two of the six caused an increase in measured turbidity downstream of the discharge point.

Stormwater from construction sites showed a wide range of water quality. Approximately 80% of sites had turbidity from 2.3 to 200 NTU, transparency tube depths from 10 to 60 cm, and TSS from 1 to 46 mg/L. None of these parameters correlated to site characteristic data (stage of construction, type of construction, size of site, disturbed acreage, or site slope).

Transparency was found to be a good surrogate for turbidity values below 250 NTU, which corresponds to 5.5 cm in transparency tube depth. TSS correlated poorly with turbidity.

Link to EIM data for User Study ID SGOL006