Clean Water Services (CWS) is a water resources management district in Washington County, Oregon responsible for wastewater treatment and stormwater management serving over 600,000 residents and several major industries. CWS co-implements the stormwater program in partnership with Washington County and its 11 cities. To comply with the regulatory monitoring requirements for the stormwater system, CWS has sampled five MS4 outfalls representing five different land use types combinations for at least three storm events per year over the last 12 years. The samples are analyzed for twenty different water quality constituents to help characterize the runoff from the land use contributing to the outfall and their potential impact to the Tualatin River. However, analysis of the current drainage areas of these sites has shown a lack of representation for the dynamic state of the urban area. In addition, CWS’ recently reissued MS4 permit requires determining if stormwater impacts river water quality. The limited MS4 monitoring locations and poor correlations with routine ambient sampling locations have made it difficult to understand the contribution of different land uses to runoff water quality and its effects on the Tualatin River. As part of the effort to create and maintain resilient watersheds, CWS is working on developing a MS4 monitoring plan that strategically optimizes sampling locations and sampling frequency. This presentation will be focused on the development process, the difficulties and some of the techniques used in the generation of an optimal and adaptive stormwater monitoring program.
