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Supplementary Site Assessment Report ' Former Mobil Bulk Plant 04-343 April 26, 1996 -1 Nutrients >' The primary inorganic nutrients for biodegradation are nitrogen and phosphorous. Nitrogen and phosphorous are frequently limiting factors for microbial growth. Groundwater samples collected from AW-3 and AW-9 were analyzed for nitrate, nitrite, and phosphorous to determine general nutrient content. Optimum levels of nitrogen and phosphorous have been suggested to be 50 mg/kg and 25 mg/kg, respectively (EPA, 1995). The nitrate levels found in AW-3 and AW-9 were 28 to 40 mg/kg, respectively, slightly lower than the optimum level. No nitrite and no phosphate was detected in either well, however, nutrient levels found in AW-3 should be adequate for hydrocarbon-degrading bacteria (personal communication, Regenesis, 1996). 1pH The pH of groundwater has an effect on the presence and activity of microbial populations in ' groundwater. The optimal pH for bacterial growth is approximately 7, with an acceptable range of 6-8. The pH values found in AW-3 and AW-9 (7.1 and 7.4, respectively) are favorable. Metals and hydroxides High concentrations of dissolved metals can be toxic to or inhibit growth and reproduction of 1 bacteria responsible for biodegradation. In general, concentrations of heavy metals greater than 2,500 ppm render biodegradation ineffective. Background dissolved iron was measured in two ' wells (AW-4 and AW-7) in December of 1992 (Alton Geoscience, 1993). The concentrations of Fe 12 in AW-4 and AW-7 were 86 ppm and 20 ppm, respectively, indicating high background dissolved iron with respect to MCL, however, the total heavy metal concentrations are below the 1 toxic level. One potential side effect of oxygen addition into an aquifer with high levels of dissolved metals ' is that the metals can precipitate as hydroxides and potentially cause fouling of the well casings. Oxygen--releasing compound, or ORCTM, is a magnesium-oxide compound which releases oxygen slowly into the aquifer through a filter sock emplaced in the well. ORCTM manufacturer ' Regenesis proposes that iron precipitates would have a tendency to deposit on the ORCTM filter sock, thus providing a mechanism to capture and remove the hydroxides when the ORCTM socks are removed from the wells. ' 5.2.2 Bio-parameter Summar The low levels of DO (1 to 3 ppm) and low bacterial counts (67 CFU/mL) in the hydrocarbon impacted groundwater at this site suggest that hydrocarbon degradation is not predominantly 7 1