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Anaerobic Indicators Ferrous iron concentrations have been monitored at 4 wells on site on a quarterly basis since March of this year Fe+2 concentrations have been relatively stable in the range of 0 1 to 2 mg/L, as would be expected for moderately aerobic conditions However,MW-8 was reported to have a spike of 22 mg/L Fe+2 in the most recent round of quarterly analysts It is not clear why there would be a sudden increase in ferrous iron or anaerobic biodegradation activity at tits off-site monitoring well TPH concentrations remain low at 220 ug/L One possible explanation would be the presence of other organic material (surface drainage of wastewater, leaks from nearby sewer lines, septic system contamination, etc)that would account for an increased anaerobic activity with the concomitant reduction of ferric iron to ferrous iron by iron reducing bacteria However, in order for iron reducing bacteria to be active the groundwater would have to be anaerobic(very low DO <0 i ppm and very low ORP, <100 mv) Populations of Bacteria Aerobic bacteria populations appear to be declining in all 4 monitoring wells tested quarterly since March of this year Populations of aerobic hydrocarbon degraders have dropped from 6-30,000 efu/ml to less than 500 cfu/ml in 3 ofthe 4 wells MW-5B is the only well that appears to retain relatively high densities of HC degraders at 3,000 cfu/ml The general decline in aerobic HC degraders over the past 6 months could be related to the depletion of N and P nutrients and other site conditions not evident in the data. Presumably as site conditions improve, or if nutrient levels are restored,the populations of aerobic HC degraders would rebound particularly near the source area given the concentrations of dissolved petroleum hydrocarbons found at MW-1 (5,400 ug/L TPHg) The populations of aerobic heterotrophic bacteria generally follow the decline of the aerobic HC degraders, with decreases in populations of over two orders of magnitude since March in MW-1 and MW-4 The heterotroph populations actually increased (20,000 efu/ml) in the most recent analysis of MW-5B, down-gradient from the former UST and with concentrations of TPHg as high as 12,000 ug/L The general decline in populations at MW-1, MW-4 and MW-8 may be due to depletion of nutrients and other site conditions The stability and slight increase in populations of aerobic bacteria to the southeast of the former USTs are consistent with increased biodegradation activity as more petroleum hydrocarbons migrate dawn-gradient In summary, intrinsic bioremediation appears to be most active in the down-gradient portion of the hydrocarbon plume to the southeast of the former UST tanks in the vicinity of MW-5B The site may be consuming available nutrient levels faster than the natural recycling rate The introduction of dilute concentrations of ammoma and phosphate solutions may help to rectify the nutrient depletion in the most active biodegradation zones Increasing dissolved oxygen concentrations in MW-1 and MW-5 would further enhance the aerobic biodegradation of residual petroleum hydrocarbons in these zones