PNNL

Abstract

Pacific Northwest National Laboratory is conducting a treatability test designed to demonstrate that in situ biostimulation can be applied to help meet cleanup goals in the Hanford Site 100-D Area. The in situ biostimulation technology is intended to provide supplemental treatment upgradient of the In Situ Redox Manipulation (ISRM) barrier by reducing the concentration of the primary oxidizing species in groundwater (i.e., nitrate and dissolved oxygen) and chromate, and thereby increasing the longevity of the ISRM barrier. This report summarizes the initial results from field testing of an in situ biological treatment zone implemented through injection of a soluble substrate. The field test is divided into operational phases that include substrate injection, process monitoring, and performance monitoring. The results summarized herein are for the substrate injection and process monitoring phase encompassing the first approximately three months of field testing. Performance monitoring is ongoing at the time this report was prepared and is planned to extend over approximately 18 months. As such, this report is an interim data summary report for the field test. The treatability testing has multiple objectives focused on evaluating the performance of biostimulation as a reducing barrier for nitrate, oxygen, and chromate. The following conclusions related to these objectives are supported by the data provided in this report. Substrate was successfully distributed to a radius of about 15 m (50 ft) from the injection well. Monitoring data indicate that microbial growth initiated rapidly, and this rapid growth would limit the ability to inject substrate to significantly larger zones from a single injection well. As would be expected, the uniformity of substrate distribution was impacted by subsurface heterogeneity. However, subsequent microbial activity and ability to reduce the targeted species was observed throughout the monitored zone during the process monitoring period, and low nitrate and oxygen concentrations were maintained. Chromate concentrations in the treatment zone began to increase about two months after substrate injection, up to about 30 percent of the background concentration upgradient of the test site. The performance monitoring phase will provide additional data to interpret the performance of the biostimulation process and information for scale-up as a reducing barrier.