Permeable Reactive Barriers (Proceedings of the International Symposium held at Belfast, Northern Ireland, March 2004). IAHS Publ. 298, 2005, 32-42.

1 Geomatrix Consultants, Inc., Oakland, California, USA

2 General Electric Company, King of Prussia, Pennsylvania, USA

Abstract The year 2004 marks the tenth year of operation for the first commercial permeable reactive barrier (PRB) composed of granular iron metal. Recent performance assessment techniques, including the use of tracer dilution testing, analysis of groundwater samples for dissolved gases (including hydrogen and methane) and inorganic constituents have shown that the system continues to function successfully after nearly 10 years since its installation. The PRB, which is intended to reduce the concentration of aliphatic volatile organic compounds in groundwater to state water quality standards, consists of a 1.3-m thick (flow-through distance) by 10-m long by 6-m deep zone of granular iron; the 1.3-m thick granular iron zone is sandwiched on its up- and down-hydraulic gradient (laterally) sides by 0.7-m thick mixed iron/gravel zones. Cement-bentonite and soil-cement-bentonite slurry walls extend up to 100 m upgradient from the PRB to contain and route groundwater through the treatment area. Assessment of the PRB has utilized both chemical and hydraulic methods intended to evaluate not only the current conditions of the PRB, but to provide information pertinent to projecting future long-term performance. The evaluation of the analytical results indicates that at 10 years, groundwater within the iron core of the PRB is: (a) limited in biological activity; (b) strongly reducing (–500 mV as equivalent Eh), high in pH (11 pH units) and low in calcium and magnesium compared to ambient groundwater outside the PRB; and (c) high in dissolved hydrogen with concentrations as high as 600 000 nanomolar (nM), which is close to the aqueous solubility of hydrogen under standard conditions. A geochemical transition zone (including moderate hydrogen concentration, moderately reducing conditions, elevated pH, and higher methane concentration relative to influent groundwater) continues to occur within the upgradient iron/gravel zone. The results of tracer dilution testing in wells installed in the PRB suggest that groundwater velocity within the PRB system ranged from approximately 0.07 to 0.2 m per day during the testing. These velocity values are generally consistent with expectations 10 years ago where a design velocity of approximately 0.16 m per day was anticipated. Based on the current performance results, the PRB continues to meet the general intent of the design developed 10 years ago.

Key words commercial installation; hydraulic and chemical performance; permeable reactive barrier