Intertidal (Beach) Sediment Chemistry

King County has sampled intertidal (beach) sediments since 1988. The program started with two stations: Carkeek Park and North Beach. Sites have been added and removed since 1994, but the same 15 sites have been sampled from 2010 to present. These include sites at beaches near stormwater and wastewater outfalls as well as sites further away from these sources. Sampling sites are located along the shoreline from Edwards Point (north) near Edmonds to Dumas Bay (south) in Federal Way.

Samples were typically collected every year prior to 2005 but since 2005 have been collected every 5 years. These samples are collected in August and were originally collected alongside shellfish tissue samples in order to compare the amount of contaminants that settle into the sediments to those that are filtered by shellfish. The shellfish tissue monitoring program was discontinued after 2010, but intertidal sediments continue to be sampled in August to control for potential seasonal differences in the long-term dataset.

We collect the top 5 centimeters of sediment from above the average low tide line using a hand core. Several cores are mixed and placed in sample containers. Then, the King County Environmental Lab analyzes the samples for physical and chemical parameters. This includes the 47 chemicals of concern listed in Washington State’s Marine Sediment Management Standards (SMS) (Chapter 173-204 WAC), total solids, total organic carbon, grain size, and additional metals and organic chemicals.