Edison groundwater assessment

We're assessing groundwater at the Edison on-site sewage system drainfield near Samish Bay.

Our hydrogeologists installed eight shallow groundwater monitoring wells to evaluate the groundwater flow system and impacts of the Edison large onsite sewage system. The effort is directed toward compliance with the state groundwater quality standards for nitrate and bacteria.

Evaluating groundwater flow system

The small community of Edison is located near the mouth of Samish Bay, south of Edison Slough in unincorporated Skagit County. Before construction of the Edison large onsite sewage system (LOSS), domestic waste in the community received minimal treatment. Onsite sewage systems for many homes discharged minimally treated wastewater to street drains that flowed directly into Edison Slough.

The Washington Department of Health restricted shellfish harvesting in Samish Bay (with 2,300 acres of commercial shellfish beds) in recent years due to fecal coliform exceedances. Wastewater from the community of Edison had been considered a possible, though relatively modest, source of fecal coliform bacteria to Samish Bay. Shellfish closures occurred several times in 2014.

The present system, consisting of a recirculating gravity filter with UV disinfection, two buried infiltration trenches, and a smaller conventional drainfield area, serves approximately 72 connections, including seven food sites and one school. The LOSS is located on the playfield of the local elementary/middle school and is surrounded by agricultural land, including manured fields. The purpose of the study is to evaluate the groundwater flow system and impacts of the LOSS to meet the requirements of the groundwater standards and the facility state waste discharge permit.

We installed eight shallow groundwater monitoring wells; there were three deeper wells already at the site. A piezometer was installed next to the bordering slough that discharges to Samish Bay. Our scientists will sample the wells and piezometers every other month for 18 months. Well samples were analyzed for standard inorganics (nitrogen, chloride, bromide, organic carbon, conductivity, pH, dissolved oxygen), bacteria (fecal and total), and one time for anions and cations.

You can find more information in our Environmental Information Management database, study ID: BCAR006.