The primary purpose of this study is to evaluate the effectiveness of oyster shell retrofits in stormwater catch basins for dissolved metals treatment and to determine whether this non-proprietary technology should be entered into the Technology Assessment Protocol – Ecology (TAPE) program. A secondary benefit of the project will be collection of data on the relationship between solids accumulation and pollutant reduction in catch basins with and without oyster shell retrofits. This project builds on the current Catch Basin Cleaning Effectiveness Study (Regional Stormwater Monitoring Program; RSMP study) and may support future studies to determine if 60% sump fullness is an appropriate threshold for catch basin maintenance.

It is well known that dissolved metals in stormwater can be toxic to fish and other aquatic life, especially copper, even at relatively low concentrations. Some metals, like copper and zinc, are especially toxic when hardness is low. To account for this the Washington State Water Quality Standards (WQS) for coper and zinc are based on site specific hardness concentrations. It is well established that stormwater can be a major source of copper and zinc to local lakes and streams, and hardness in stormwater is often very low (< 25 mg CaCO3/L). This results in stormwater concentrations of copper and zinc that are frequently above the acute and chronic WQS across the Puget Sound region. These conditions can have detrimental impacts on resident aquatic organisms in streams and lakes with substantial stormwater inputs.

Stormwater managers from municipalities with NPDES Phase I Municipal Stormwater Permits are required to implement structural stormwater controls to reduce or prevent impacts to waters of the state. There are several challenges to accomplishing this in areas of existing development: 1) there is limited above-ground space for new treatment installations, 2) installation costs and time requirements for new treatments can be substantial, 3) maintenance requirements for new treatments can be extensive, and 4) many common filtration treatments, including bioretention, are not consistently effective at reducing dissolved copper concentrations to levels below those shown to cause toxicity. Identification of a retrofit option that reduces relatively low level dissolved metals, increases hardness, requires no above-ground construction, and is relatively inexpensive to install and maintain could expand the retrofit options for permittees looking to prevent WQS exceedances and reduce impacts to waters of Washington State.

Adding oyster shells to catch basins is a retrofit option that may satisfy these goals. Oyster and mussel shells are known to increase hardness due to the calcium content of their shells. A number of studies have demonstrated that copper and zinc in stormwater is readily bound to the shells over a variety of influent concentrations. Oyster shells in stormwater studies have been shown to consistently reduce dissolved copper concentrations, sometimes reaching effluent concentrations less than 5 µg/L. Several studies have cited effluent concentrations between 5 and 10 µg/L as the lowest dissolved copper levels that can be achieved through bioretention technology. The mechanism of metals removal by the oyster shells is likely due to adsorption and ion exchange. In addition to removal of metals, the planned retrofit design (originally used by the Port of Seattle, Appendix A) also likely increases the settling of particulates from stormwater. Reduction of particulate material in the effluent likely enhances removal of particulate-bound metals and other pollutants (e.g., bacteria and organic chemicals). Furthermore, this retrofit design requires minimal construction, with no impacts to above-ground areas. When this type of retrofit was used by the Port of Seattle, maintenance requirements were generally low; oyster shells were vactored from the catch basins and replaced yearly at relatively low cost.