Treatment cells were lined with a high-density polyethylene liner, which was then covered with a gravel leachate collection layer, in turn covered with a geotextile fabric. Impacted soil was mixed with yard waste and cow manure, and the soil mix was placed in the cells. Koa Haole trees were planted in the cells, 53 cm (21 inches) apart. Drip irrigation lines were installed to feed the trees with collected contaminated groundwater and recirculated leachate sump water.
The pilot-scale test ran for two years, proving the idea successful. The trees were found to be effective in remediating the contaminated soil and groundwater. Soil contaminant concentrations were reduced to be better than residential standards, and leachate water better than drinking water standards.
Based on the success of the pilot test, WSP and the property owner implemented a full-scale project in operation starting in 2008. It includes 35 groundwater extraction wells, equipped with pneumatic pumps to continuously pump water from the impacted parts of the aquifer. The impacted water is stored in a 34,000 liter (9,000 gallon) tank, and distributed to the phytoremediation treatment cells, containing the Koa Haole trees.
As of September 2017, the phytoremediation system has successfully treated over 18 million liters (4.7 million gallons) of groundwater to concentrations that were better than drinking water standards.
This project is particularly important because EDB and DBCP were in widespread use for many decades to fight nematode infestations on Oahu and many other locations, until the negative effects of these compounds resulted in an order to discontinue use. The fact that soil and water impacted by these compounds can be treated efficiently and cost-effectively using phytoremediation will help to provide a sustainable remediation approach for other similar projects.
WSP and Del Monte Fresh Produce received the American Council of Engineering Companies (ACEC) of Washington’s Best in State Gold Award for Uniqueness and/or Innovative Application of New or Existing Techniques for its work on this project.
* This work was performed by Golder professionals who joined WSP in an acquisition completed in 2021.