Project Abstract

Demonstration of an Interceptor Trench Technology for the Bioremediation of a Pentachlorophenol-Contaminated Ground Water

Principal Investigators

Sandra Woods
Oregon State University
E-mail: Sandra.Woods@orst.edu

Goal

The goal of this project is to demonstrate the applicability of an interceptor trench technology for the bioremediation of groundwaters contaminated with pentachlorophenol (PCP).

Rationale

Pentachlorophenol (PCP) is a fully chlorinated aromatic compound susceptible to reductive dechlorination, a common anaerobic biodegradation mechanism. The degradation of PCP by anaerobic reductive dechlorination is rapid and results in the production of lesser chlorinated phenols. As the degree of chlorination decreases, reductive dechlorination rates also decrease. Under aerobic conditions, however, degradation of the lower chlorinated phenolic compounds is rapid and may result in complete mineralization. This project will demonstrate a sequential anaerobic/aerobic biological treatment technique for PCP-contaminated ground water.

Approach

To allow for the physical, hydrogeological, biological, and regulatory conditions present, a diverse project team was assembled. Teams focused on the following:

Design considerations for the physical development of the permeable barrier technology;
Interactions of subsurface flow and mixing;
Biological process design including evaluation of electron donors and carbon sources;
Reactor construction;
Sampling design;
Regulatory and legal constraints.

Status

The construction of a 23" cylindrical down-borehole reactor (basket) was completed. The reactor was installed in the 24" diameter borehole during mid-June, 1997. Work focused on testing and validation of the reactor’s sampling and nutrient supply systems. In the absence of an inoculated biomass, several conservative tracer studies were conducted to characterize hydraulic conditions within the well and reactor. The observed retention time was approximately 24 hours.

Chlorophenol ground water concentrations were analyzed on a weekly basis beginning mid February, 1997. Maximum pentachlorophenol concentrations in the groundwater were measured in February at a level of approximately 1 mg/l. The spring and summer months marked a slow decline in aqueous concentrations. Minimum PCP concentration values of 0.2 mg/l were observed in July. Late summer and fall marked the increase in aqueous PCP concentrations and current levels are approximately 2.5 mg/L. Due to the variability in PCP concentrations, measurement of local ground water elevations and rainfall history was instituted to help predict seasonal chlorophenol variations. Observation of ground water elevations in the aquifer system show strong influence by the facility’s pumping wells and regional rainfall.

The reactor system was removed and inoculated with cells during the first week of December, 1997. The reactor was modified to include two recirculating flow cells for the characterization of the environmental conditions within the anaerobic and aerobic zones. Redox and pH measurements are coupled to a real time data acquisition system. Chlorophenols and sulfate present in the system are monitored by GC/ECD or IC analyses with space and time.

Current work focuses on the collection of baseline data for the inoculated system with no injection of electron donors or acceptors. Nutrient injection will begin in January, 1998. PCP removal will be evaluated under varying reactor operating conditions (addition of imitation vanilla flavoring, oxygen, sulfate, reactor orientation within the borehole, etc.).

Mineralization of pentachlorophenol was verified in laboratory scale batch bioassays. Experiments were conducted in 250 mL serum bottles under sequential anaerobic/aerobic conditions using imitation vanilla flavoring as the electron donor/primary substrate. Progress curves were developed for replicate reactors using 14C-labelled PCP or unlabelled PCP. In reactors containing unlabelled PCP, transient production of 2,3,4,5-tetrachlorophenol and accumulation of 3,4,5-trichlorophenol was observed during the initial anaerobic phase. There was no significant removal of 14C from the aqueous phase in the replicates containing 14C-PCP under anaerobic conditions. Upon injection of O2, there was immediate removal of 3,4,5-trichlorphenol in the unlabelled replicates concomitant with a decrease in 14C in the aqueous phase and a stoichiometric increase of 14CO2.