Publication Summary

Mill Creek in Kent, Washington was chosen for a case study of an urban area with a high density of industrial development. Monitoring took place in the fall of 2005 for "first flush," worst-case conditions.

Hourly water quality monitoring of Mill Creek during three storm events showed exceedances of acute water quality criteria for zinc during two storm events and for copper during one event.

There are indications that industrial stormwater discharges with elevated levels of zinc and copper may be the principal cause of water quality exceedances in the creek during storm events.

Many industrial facilities statewide under the Industrial Stormwater General Permit (ISGP) have been found to discharge stormwater with elevated levels of zinc and copper. Mill Creek in Kent, Washington is in an industrialized urban area with a large number of facilities discharging stormwater under the ISGP.

Water quality monitoring of Mill Creek during three storm events showed exceedances of zinc and copper acute water quality criteria specified in the Washington State Water Quality Standards. The monitoring took place in the fall of 2005 for conditions approaching worst case, the "first flush" of runoff from a storm event following a period of no precipitation.

Exceedances of criteria were found for zinc during two storm events and for copper during one event. Concentrations of dissolved zinc and copper were found to be as high as 100 µg/L and 10.4 µg/L, respectively. Concentrations of total recoverable zinc and copper were as high as 105 µg/L and 14.1 µg/L.

There are indications that industrial stormwater discharges with elevated levels of zinc and copper may be the principal cause of water quality criteria exceedances in Mill Creek during storm events.

Link to EIM data for User Study ID SGOL0008