Publication Summary

A dye study was conducted in late summer 1992 in a remnant channel of the Stillaguamish River. The purpose of the study was to assess the present impact of the city of Stanwood's intermittent wastewater discharge and the future impact that a continuous discharge might have. This discharge was the only freshwater inflow to the channel, which was actually a tidal bay.

A dye study was conducted in late summer 1992 in a remnant channel of the Stillaguamish River. The purpose of the study was to assess the present impact of the city of Stanwood's intermittent wastewater discharge and the future impact that a continuous discharge might have. This discharge was the only freshwater inflow to the channel, which was actually a tidal bay.

Dye injection began on an ebbing tide and continued for one tidal day for the dual purposes of calibrating a mixing zone model and determining far-field accumulation of effluent. The EPA 3PLUMES model was validated for the acute boundary and was calibrated for the chronic boundary by adjusting one coefficient. Dye measurement data from six days and the principle of superposition were used to calculate far-field accumulation.

There were a number of noteworthy findings: 1) the ultimate daily maximum concentration of dye within the mixing zone (assuming a continuous discharge) was 10% of the concentration present in effluent; 2) flushing time is about three days; 3) acute and chronic dilution factors under the present situation are 13:0 and 205.6, respectively; 4) the copper criterion is violated at the acute boundary and the ammonia criterion at the chronic boundary; 5) in a future scenario which assumes a continuous discharge of higher flow rate, dilution factors will be 16.6 and 41.6; 6) the copper criterion will be violated at the acute boundary and ammonia at the chronic; and 7) there is a reasonable potential that the chronic criterion for nickel is presently being exceeded--a condition which would continue in the future situation to be joined by lead.