Everett, WA - ERH under active storm water retention basin - Case Study


Courtesy of TRS Group, Inc.

First below water ERH remediation of TCE in active stormwater retention basin

Project Summary

TRS Group, Inc. (TRS) worked as a sub contractor to Landau Associates on this unique ERH remediation project. This project represents the first of its kind using ERH for site remediation underneath an industrial storm water retention basin full of water. The treatment area is located beneath the north end of the Powder Mill Gulch storm water retention basin (see Figure 1). TCE source zone remediation using ERH is being performed as an interim action under an Agreed Order with the Washington Department of Ecology.


This very unique ERH remediation was performed underwater, beneath an operating 4-acre stormwater detention basin. The basin has an operational water volume of approximately 1.4 million gallons that resulted in much of the electrode field being covered by 8-ft of water during most of the ERH system operation. Storm events would increase the volume of water in the basin and the depth of submergence of the electrode field to maximum depth of 30 ft. In order to keep the basin operational during fall and winter storm events, the ERH electrodes and wiring, the temperature monitoring system, the vapor recovery wells and piping, and the groundwater monitoring wells, were designed to operate while completely submerged. An additional unique feature was that many of the electrodes and monitoring wells had to be constructed on the sloping sidewall of the basin.

Site Characteristics & Design Parameters

The ERH remediation design includes the use of 50 electrodes placed to an average total depth of 71.5 feet below basin bottom (bbb). This average depth includes the sloped area at the perimeter of the basin (see Figure 1). Each electrode contains a co-located vapor recovery (VR) well. An additional electrode element has been added at 19 electrode locations to provide treatment to 75 feet bbb.

A total of 10 groundwater monitoring wells are located between the electrodes. Each well contains a co-located temperature monitoring point (TMP) with 10 temperature sensors set at one per five vertical feet of treatment interval. Three of these TMPs were installed to a depth of 72 feet bbb and contain 12 sensors each.

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