The concentration of phosphate is being monitored in the PO4 barrier and the concentration of iron is being monitored in both the ZVI and AFO barriers. These chemical constituents are being monitored to evaluate possible impacts of the barrier materials to offsite ground-water quality.
Elevated concentrations of phosphate are observed within the PO4 barrier and in the pea gravel upgradient of the barrier. The migration of phosphate into the pea gravel is in the opposite direction of the preinstallation ground-water flow. Heavy rainfall since barrier installation has probably resulted in temporary reversals in ground-water flow directions. Since the PO4 barrier is closest to Fry Creek, this barrier is likely to be the most sensitive to flow reversals. The persistence of high phosphate concentrations in the pea gravel is probably the result of the limited amount of metal oxides contained on the pea gravel. Metal oxides, especially iron and manganese oxy-hydroxides usually limit phosphate concentrations to significantly less than 1 mg/L (Hem, 1989). The low phosphate concentrations below the PO4 barrier are probably due to the large amounts of naturally occurring iron oxy-hydroxides in the colluvial sediments.