CONSTRUCTED VERTICAL-FLOW WETLANDS FOR TREATMENT OF CHLORINATED ETHENES

Chlorinated ethene contamination of groundwater is the most prevalent long term remediation challenge on most industrial sites across the United States. The typical pump and treat technologies have no definitive end point, with estimations of 50 years or more operation at millions of dollars per year O&M costs. Recent exploration of in situ treatment techniques has met with limited success and is generally very sensitive to site-specific conditions. The technology presented here offers the reliability of traditional plume capture pumping techniques and surface treatment with minimal capital cost (<$1M) and virtually no O&M costs beyond pumping and monitoring treatment efficiency. It offers universally applied specifications for most all sites in temperate climate zones and no unnatural chemical additions to the soil matrix with unexpected consequences. Replacing current pump and treat with this constructed wetland technology at any site is expected to achieve payback within one to two years.

The reported effort is studying a recently constructed field-scale upward flow wetland treating chlorinated ethenes in groundwater. It is confirming sequential biochemical reaction zones in wetland sediment layers and phytoremediation phenomena in the upper root zone as suggested in current literature (methanogenesis to aerobic metabolism) as well as characterizing the hydraulic flow regime. A 3-D piezometer grid has been installed throughout the constructed cell sediment layers for observation of piezometric head and collection of water samples. Results are providing the foundation for developing optimum design criteria for general application of the technology. Preliminary results reveal delivery of 50 ppb tetrachloroethylene (PCE) to the bottom layer of the wetland. Concentrations of chlorinated species emerging at the wetland surface are an order of magnitude lower and are non-detect in most cases. Preliminary results also indicate organic acids at lower-than-expected levels, given the apparent success of reductive dechlorination within the high organic sediment layers. The layered sequential treatment scheme suggests several opportunities for engineering optimization.