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i particularly exposure to atmospheric oxygen, hence the preferred use of a downhole meter It is necessary to strictly adhere to instructions provided with a given model of instrument DO meters function by permitting a small quantity of oxygen to diffuse across a porous membrane Consequently, it is necessary to keep water moving in the vicinity of the membrane to prevent a depletion of DO immediately adjacent to the membrane This can be achieved manually, by a gentle raising and lowering of the meter in the well The membrane is delicate and must be carefully maintained A negative correlation should occur between DO concentrations and hydrocarbon concentrations Background concentrations should exceed 1 to 2 mg/L for effective aerobic degradation DO in groundwater is derived from the atmosphere at the recharge area or the vadose zone Surface water saturated with oxygen by contact with atmospheric air will contain between approximately 7 5 mg1L at 5°C and 12 75 mg/L at 30°C, though these figures may vary somewhat depending on other chemical parameters DO concentrations in groundwater are generally less than those for surface water by an amount dependent on the quantity of oxidizable materials (e g sulfides) in contact with the groundwater, and the length of time the groundwater has been stored in the aquifer Background groundwater DO concentrations in shallow aquifers can be as high as 12 mg/L in warm conditions or as low as 1 mg/L in cool conditions (Hem, 1985) DO may be increased by local groundwater recharge (e g irrigation) Aerobic degradation typically occurs when Eh is approximately+800 mV(discussed below) Anaerobic Electron Acceptors Analysis of water samples for nitrate, dissolved iron, and/or sulfate can provide data indicative of intrinsic bioremediation The higher the background concentrations the better,unless they are so high as to create toxicity for the microbes or exceed water quality standards Depleted dissolved electron acceptor concentrations (except iron, see below)in areas of high hydrocarbon concentration are indicative of microbial degradation Nitrate Nitrate concentrations may be derived by analyzing nitrate plus nitrite as N (EPA Method 353 2) This laboratory method calculates total nitrate, since nitrite is metastable in groundwater and seldom present in sufficient quantities to affect the ionic balance (Wiedemeier et al, 1995) The bulk of nitrates in groundwater are derived from human contamination (e g agricultural runoff/septic systems) Background concentrations vary widely with human activity in the site vicinity, and would otherwise be commonly less than 1 mg/L Concentrations considered indicative of a significant biodegradation capacity might be those in excess of 20 mg/L Denitrification/nitrate reduction typically occurs when Eh is approximately+750 mV(but more than 0 mV) Iron Laboratory analysis of iron concentration may be accomplished by collecting an unfiltered groundwater sample to obtain the total iron content (precipitated and dissolved), or by passing the sample through a 0 45 micron filter immediately after collection to obtain the dissolved iron concentration Iron in groundwater is derived primarily from soil minerals Dissolved iron concentrations are very sensitive to changes in pH and Eh Free dissolved ferric iron can only exist stably under extremely acidic conditions (pH<2) (Hem, 1985) Ferric iron reduction to ferrous iron occurs at intermediate Eh values Under aerobic, moderately acidic or alkaline conditions, dissolved iron is typically present as a hydroxide, the ferric species is ferric orthohydroxide (Wiedemeier et al, 1995) Dissolved ferric iron is usually rapidly reprecipitated as a sulfide,oxide or hydroxide Since microbes utilize insoluble sedimentary ferric iron oxides as their energy source, producing more soluble ferrous iron, an increase in total dissolved iron is indicative of microbial hydrocarbon degradation The solubility of ferrous iron is significantly reduced by the presence of sulfides,the end-product of sulfate reduction (Barker et al, 1995) Analytical results of dissolved ferrous iron concentration will likely give an underestimate, since it is not based on the actual amount of ferric hydroxide (the electron acceptor) present in the aquifer, but the amount of reduced ferrous iron(the end-product)remaining in solution at the time of sampling Typical background concentrations of total dissolved iron in groundwater are below 10 mg/L Results in excess of 10 mg[L indicate iron-reducing conditions (Cookson, 1995) which may have resulted from anaerobic hydrocarbon degradation High dissolved iron concentrations may also indicate the presence of very fine particulates, low pH, or high organic content High organic content induces stability of soluble iron complexes(Hem, 1985) Measurement of the total iron content of a sample is useful as a background datum against which to compare changes in the dissolved concentration Sulfate Sulfate is derived primarily from soil minerals The occurrence of sulfate reduction may be inferred from the presence of black acid volatile sulfide deposits on materials in long-term contact with contaminated groundwater CLEARWATBR GROUP(NATURAL ATTENUATION) 4 revised October 3,2002