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Oyster Shell Catch Basin Retrofit, Mercer Island

Oyster Shell Catch Basin Retrofit, Mercer Island

Using oyster shells to clean up stormwater: a new pilot study

King County is partnering with the City of Mercer Island, with funding from the Stormwater Action Monitoring (SAM) program, to determine the effectiveness of treating stormwater with oyster shells from 2019-2020.

Study update

King County researchers collected samples during four storms in 2019 to test if stormwater in Mercer Island Town Center was cleaner after passing through oyster shells. Unfortunately, the results showed that the oyster shells were not reducing suspended solids and heavy metals in the stormwater and the study was halted in March 2020.

Oyster shells have been found to provide successful stormwater treatment in smaller scale applications, like treating runoff from parking lots or roofs (see Port of Seattle link ). The stormwater system at the Mercer Island Town Center is much larger. It’s likely that in this setting the ratio of oyster shells to stormwater was too small or the oyster shells were in contact with the stormwater for too short a time to provide sufficient treatment. The results suggest that this type of oyster shell treatment is not likely to be successful in larger stormwater systems like the Town Center area. For more information, please see the study report (coming soon!).

The main objective of this project is to see how well oyster shells remove dissolved metals from stormwater and whether this could be a relatively simple solution for dissolved metals treatment.

Sampling equipment at a retrofitted catch basin in Mercer Island Town Center with interpretive signage
Sampling equipment at a retrofitted catch basin in Mercer Island Town Center with interpretive signage (photo credit: Houston Flores, King County)

The stormwater problem

When it rains, water runs over roadways, sidewalks, and buildings where it can pick up pollutants from cars, roofs, pet waste, yard care products, and more. This water is called stormwater and it is one of the main ways pollution enters our waterways in the Puget Sound region. Stormwater can have many different types of pollutants, including heavy metals like copper and zinc that get into stormwater through sources like cars and roofing materials. These metals can be harmful to fish and other aquatic animals, especially if the water is also low in minerals such as calcium, magnesium, and potassium (also called low water hardness). These buffering minerals can be picked up when water percolates through soil, but most urban stormwater enters our waterways through storm drains, where it isn’t filtered through soil like it would be in a forest. Without these buffering minerals, heavy metals in stormwater can be particularly toxic, even at relatively low levels.

Common challenges in stormwater management:

Stormwater is managed by cities and counties to reduce stormwater impacts on local waterways. One way this is done is through retrofitting existing stormwater infrastructure to add stormwater treatment. Identifying retrofit options to fit particular needs can be difficult, and non-proprietary options are limited for some common situations. For example:

  • Areas to be retrofitted are generally already built out and above-ground space may be limited;
  • Typical heavy metals concentrations in urban stormwater are often within a range that is difficult to reduce through common treatment methods, such as bioretention (like rain gardens); and
  • Resources are limited, and retrofit options with low installation and maintenance costs could increase the feasibility of more widespread stormwater treatment.

Could stormwater treatment with oyster shells fit the bill?

There is some evidence that filtering stormwater through crushed oyster shells can reduce heavy metals like copper and zinc in stormwater. Oyster shells are also natural sources of the buffering mineral, calcium, which can be released into the stormwater, further reducing any harmful impacts from heavy metals to aquatic organisms.

In the City of Mercer Island Town Center, two catch basins – commonly known as storm drains – have been retrofitted to include oyster shell treatment. These basins have been partially filled with coarsely crushed oyster shells and fitted with a baffle over the outlet pipe. The baffle slows down the stormwater to increase contact time with the oyster shells to help increase the potential for treatment. This design may also cause additional particulate material to settle out of the stormwater into the basins, thereby trapping pollutants that are commonly bound to particulates like bacteria and petroleum chemicals. Furthermore, this retrofit design requires minimal construction, no impacts to above-ground areas, and is expected to have minimal maintenance requirements.

Monitoring effectiveness

Stormwater will be collected before and after passing through the oyster shell treatment for a total of ten storms. These samples will be analyzed for a variety of pollutants, including heavy metals like copper and zinc and compared to samples collected just upstream at catch basins fitted with baffles, but no oyster shells.

This comparison will show how well the crushed oyster shells reduce concentrations of heavy metals and other pollutants. If this pilot project shows that oyster shells successfully reduce heavy metals in stormwater, a more robust analysis could be conducted to have the treatment approved by the Washington State Department of Ecology for wide-spread use through the (TAPE) program.

Mercer Island Town Center location of the two catch basins retrofitted with oyster shells and the two untreated catch basins used as controls
Mercer Island Town Center location of the two catch basins retrofitted with oyster shells (red/blue circles) and the two untreated catch basins used as controls (red circles).

Technical project objectives

  • Collect time-weighted composite stormwater samples at the inlet and outlet of four catch basins (with and without oyster shell retrofits) for 10 discrete storms over two wet seasons. Samples will be analyzed for: total and dissolved metals, total suspended solids (TSS), total organic carbon (TOC), dissolved organic carbon (DOC), total and dissolved phosphorus, total nitrogen, hardness, pH, and flow.
  • Calculate changes in concentrations between influent and effluent samples for each catch basin and each storm.
  • Evaluate effectiveness of oyster shell retrofits in terms of dissolved metals treatment and phosphorus treatment as defined by the TAPE performance goals using the data collected from this pilot effort.
  • Compare “treated” catch basin effluent dissolved metals concentrations to freshwater quality standards.
  • Evaluate whether “treated” catch basins reduce the analyzed stormwater pollutants more than “untreated” catch basins.
  • Measure solids accumulation in “untreated” catch basins over the study timeframe, and observe whether oyster shell retrofits interfere with solids retention.
  • Evaluate the relationship between solids accumulation and pollutant removal in “untreated” catch basins. This can be used to support future evaluation of the current 60% sump fullness threshold for catch basin maintenance.
  • Communicate results with interested parties, including municipal stormwater permittees.

Project documents

Quality Assurance Project Plan: Pilot Study for Non-proprietary Dissolved Metals Treatment: Oyster Shell Retrofits in Catch Basins

Team members

The project team consists of personnel from King County’s Water and Land Resources Division, partners from the City of Mercer Island, and a SAM coordinator from Washington State Department of Ecology. The Port of Seattle provided the initial retrofit design and the oyster shells for this pilot project. King County team members include scientists from the Science and Technical Services Section and King County Environmental Laboratory.

Interpretive signage for the project utility boxes was done by the King County Information Technology Design and Civic Engagement team in coordination with the City of Mercer Island.


City of Mercer Island – Public Works/Stormwater Utility

Washington State Department of Ecology

Port of Seattle – Maritime Stormwater Program

Project schedule

Sampling Project Plan: March 2019
Retrofit installation: March 2019
Field Investigation: April 2019 through September 2020
Final Report: End of 2021

For more information about the Oyster Shell Catch Basin Retrofit on Mercer Island, contact Carly Greyell, Water Quality Planner/Ecotoxicologist, Toxicology and Contaminant Assessment Unit, Science and Technical Support Section, Water and Land Resources Division, King County Department of Natural Resources and Parks.