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(Barker et al, 1995) Pyrite may be precipitated in the soil Sulfate concentrations in groundwater are naturally . higher than those for nitrate Sulfate concentrations of 100 mg/L might be considered moderate and several hundred mg/L is not uncommon Concentrations below 40 mg/L are indicative of methanogenic conditions(Cookson, 1995) Sulfate reduction typically occurs when Eh is approximately-200 mV Methanogenests Under methanogemc conditions (Eh of approximately -250 mV),carbon dioxide and methane are both produced by hydrocarbon oxidation The utility of measurement of these compounds is discussed below (metabolic end-products) Carbonate/Hardness/Total alkalinity One of these associated analyses is typically conducted at the laboratory on collected water samples Increased carbonate concentration will commonly occur where acidity dissolves carbonates from the soil Sufficient concentrations of carbonate will buffer the pH and prevent acid toxicity that may result from hydrocarbon degradation Total alkalinity (as carbonate) concentrations exceeding 100 mg/L may be considered conducive to effective buffering Dissolved carbon dioxide may be assessed in conjunction with total alkalinity analysis Metabolic end-products Metabolic end-products of hydrocarbon biodegradation include carbon dioxide, water, nitrogen,nitrites,ferrous iron, sulfites, sulfides, hydrogen sulfide, and methane Carbon dioxide, hydrogen sulfide and methane may be measured with a gas meter at the wellhead Reduced ferrous iron, sulfite and sulfide may be analyzed in water samples Sulfides may precipitate into the soil and be under-represented in groundwater samples Nitrite is metastable and therefore nitrite detection (generally <0 1 mg/L) is indicative of ongoing denitrification Ammonium ions in excess of 10 mg/L may also be indicative of anaerobic conditions Elevated concentrations of all metabolic end-products should correlate positively with elevated hydrocarbons Field measurement of dissolved carbon dioxide (DCD) is of secondary importance but may provide useful data Dissolved carbon dioxide is derived primarily from the atmosphere Elevated DCD spatially correlated with decreased DO concentration,may be indicative of aerobic microbial hydrocarbon degradation as DCD is a metabolic end-product Elevated DCD may also result from anaerobic degradation High background DCD is a desirable feature in terms of the capacity of the groundwater to buffer decreases in pH produced by microbial hydrocarbon degradation which may otherwise limit biological activity Carbon dioxide is more soluble than oxygen and average concentrations are around an order of magnitude higher Contanunant Fate and Transport Modeling Plume transport can be modeled using simple analytical equations Transport assuming no attenuation can be modeled and the results compared with field data to provide a preliminary indication of the extent of natural attenuation Transport models can be modified to include various natural attenuation factors based on actual site data Comparison of these modeling results to actual field results can be used to confirm natural attenuation To model plume transport,the following basic site characteristics need to be determined • Historical dissolved hydrocarbon distribution • Hydraulic conductivity • Soil density/porosity • Aquifer thickness • Groundwater gradient/depth fluctuations • Possible preferential migration pathways • Organic content of the soil,foc Laboratory analysis of soil samples may be necessary to establish foci which is useful for modeling sorption Hydraulic conductivity may be obtained as an estimate from the literature based on soil type (for homogeneous lithologies),or by performing an aquifer test(slug or pump) For plumes under steady-state conditions, contaminant transport models such as the Domenico Transport Equation can be modified to include the processes of dispersion and sorption to predict contaminant concentrations at a given distance from the source(Connor et al, 1995) In addition,biological and chemical degradation may be collectively modeled by a first-order decay function requiring assignment of a literature-based decay half-life value(in days) for CLEARWATER GROUP(NATURAL ATTENUATION) 5 revised October 3,2002