IN SITU REMOVAL OF PERCHLORATE FROM PERCHED GROUNDWATER BY INDUCING ENHANCED ANAEROBIC CONDITIONS

摘要

The usage of perchlorate during explosive device manufacturing resulted in elevated concentrations of perchlorate in perched groundwater (35 to 50 feet [10.7 to 15.2 meters {m}] below ground surface [bgs]) at this confidential industrial site in central California. An area of the site, representative of maximum measured perchlorate concentrations (>200 milligrams per liter [mg/L]), was selected to implement an in-situ reactive zone (IRZ) field demonstration to evaluate the feasibility of an enhanced biodegradation approach. By design, the IRZ induces anaerobic groundwater conditions due to microbial metabolism of a dilute organic carbon substrate delivered directly into the permeable strata. Corn syrup was selected as the organic carbon substrate to stimulate nitrate-reducing microbes indigenous to the perched aquifer. The IRZ system variables include the following tested conditions: (1) effect of dosing adjustments (e.g., volume and concentration of substrate material and frequency of application), and (2) evaluation of indicator parameters and target constituent concentrations for optimum decay rates. Measurable reducing conditions were established within thirty days following the first dosing. Perchlorate concentrations decreased in groundwater samples from 200 mg/L in the baseline sample to 0.022 mg/L within 18 feet of the organic carbon feed source. An average 94.3 percent reduction of perchlorate detected in three monitoring points resulted over a twelve-month active remediation and observation period. According to the results, excessive reducing conditions appear to inhibit the rate of perchlorate degradation. Thus the full-scale design was modified to reduce the carbon substrate dose and frequency to achieve the optimal reducing conditions for favorable perchlorate reduction using the operative population of nitrate-reducing microbes. Monitoring and maintenance of the IRZ is necessary to institute consistent anaerobic conditions for full-scale application. This is particularly relevant in a perched groundwater regime, which is likely to have low flux rates and more variability in flow directions when compared with more extensive aquifers. The field demonstration was used to evaluate the feasibility of a full-scale, cost-effective remedy for other source areas with perchlorate-impacted groundwater.