The Story in the Sediment: Tracing Stormwater Pollution Sources at Superfund Sites

Since 2001, the lowest five miles of Seattle’s Duwamish River (known as the Lower Duwamish Waterway or LDW) has been designated as a 412-acre Environmental Protection Agency (EPA) Superfund site. The LDW’s Superfund status results from decades of historical industrial activity. On top of the historical contamination, the LDW has ongoing issues with contaminated stormwater runoff. Rain hits the abundance of impervious surfaces – e.g., asphalt roads and lots, building roofs – in the industrial areas next to the river, collects contaminants from those surfaces, and carries it to the nearest storm drain and into the river.

Sediment sampling in Seattle's Lower Duwamish Superfund area helps identify contaminates and cleanup strategies to improve water quality. 

Among the many tasks in cleaning up Superfund sites is the ongoing detective work to sleuth what contaminants are there and where they came from. In the LDW cleanup, one of the key clues isn’t even in the river water itself, but in the sediment carried by stormwater. 

Aspect staff have sampled stormwater sediments across the LDW Superfund site – previously for the City of Seattle and King County and currently for the Port of Seattle at Harbor Island. Our efforts studying these solids in stormwater runoff provide key information about the recent history at a site and the extent of contamination.
 

The Benefits of Sediment Sleuthing: Unlike Water, It Accumulates

Unlike stormwater, which runs through and beyond an outfall to receiving waters, heavier sediments and other settleable solids (relatively heavy substances that sink in water) carried by runoff drop out and accumulate. This accumulation, which occurs in key locations such as stormwater catch basins, vaults, and low-gradient pipes, provides a rich environment for valuable leads on water quality contaminants that may eventually end up in streams, rivers, lakes, wetlands, and Puget Sound. Sediment monitoring often provides a more comprehensive historical picture of pollutants compared to instantaneous or short-term sampling of water alone.

At Superfund areas, and many other sites challenged by stormwater runoff, sediment monitoring benefits clients by: 

  • Providing a historical picture of pollution—through accumulated sediment analysis--associated with stormwater runoff and industrial discharge.
  • Identifying chronic types of pollution that may deteriorate water quality and habitat.
  • Tracing the sources of pollution to their origin for purposes of management, treatment, or elimination.
  • Determining sediment accumulation rates in sewers and catch basins to improve maintenance and operation needs and to anticipate and prevent flooding.
  • Complying with permits, records of decision, and other legal requirements for preventing environmental degradation or requiring cleanup of polluted sites.
  • Measuring the effect of land use activities and stormwater treatment best management practices (BMPs).
     

Tracking Hot Spots Across 600 Acres of Pavement and Buildings

Boeing Field (aka King County International Airport or KCIA) is one of the nation’s busiest primary non-hub airports and covers over 630 acres of mostly impervious surface. Managing stormwater runoff over this much area and with many industrial tenants is a challenge, especially because KCIA faces the challenge of being responsible for all discharge to the LDW from its property, even runoff or discharge in tenant-operated areas. 

 Sediment traps in a storm sewer manhole

Sediment traps in a storm sewer manhole

Aspect staff previously performed inexpensive but high-resolution sediment monitoring throughout KCIA’s storm drainage infrastructure that ranged from shallow old brick manholes to deep new stormwater treatment vaults. The results from the sediment monitoring provided a finer-grained picture of accumulated sediment quality than had ever been collected at KCIA. This allowed King County to identify hot spots of likely pollution sources coming from both individual tenants and from legacy airport infrastructure and helped prioritize an action plan to address these areas.

Using Sediment Data to Track Down Drainage Ditch Polluters

In a different area of the LDW, sediment data helped the City of Seattle identify the source of intermittent toxic metal pollutants from a far upland drainage area to the LDW, despite having outdated drainage maps. 

 Sediment collection in a storm sewer manhole

Sediment collection in a storm sewer manhole

Because the area was previously in unincorporated Seattle, sewer records were incomplete. With the assistance of an Ecology inspector who knew the area and businesses well, Aspect staff collected sediment samples from both the public and private drainage systems. The sediment samples helped both Ecology and the City efficiently trace the source of the metals pollution to a business that had a previously unknown illicit connection from its industrial waste drainage system to the ditches outside, which served as the public storm drainage.

Long-term Sediment Monitoring at Harbor Island to Support Environmental Compliance

Aspect is currently assisting the Port of Seattle with sediment monitoring at a 15-acre marine terminal on Harbor Island, a discrete Superfund site located downstream of the LDW Superfund area. The sediment monitoring supports the Port in demonstrating compliance with a Record of Decision (ROD) to rehabilitate the site. As a site that drains directly to Puget Sound, the objective of the cleanup (which included dredging and removing contaminated soil) is to reduce concentrations of hazardous substances in runoff to levels that will have no adverse effect on marine organisms. 

 Sediment traps mounted on the side of a storm sewer manhole

Sediment traps mounted on the side of a storm sewer manhole

 Sample bottles with accumulated sediment at the bottom

Sample bottles with accumulated sediment at the bottom

To evaluate this over the required 10-year monitoring period, Aspect is monitoring accumulated sediments in the new stormwater drainage system at the terminal for metals, tributyltin, polycyclic aromatic hydrocarbons (PAHs), and polychlorinated biphenyls (PCBs). The results from the sediment monitoring are compared to target concentrations in the Washington State Sediment Management Standards and show the Port’s commitment to compliance with the ROD and to ensuring that the site rehabilitation was successful. 

Sediment Sampling Provides Key Historical Context to Water Quality Evaluation

Measuring sediment quality is an excellent – and affordable – complement to measuring water quality. Aspect’s sediment sleuthing has helped clients in the LDW create a more holistic picture of both historical and ongoing stormwater pollution, as well as flooding potential. From this picture, they are better able to identify sources of contamination and create specific plans to address them—leading to a healthier LDW for all.