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' dispersivity, Dy) is modeled at 0 33 tunes DX, dispersion in the vertical direction (vertical dispersivity, Dz) is modeled at 0 05 tunes Dx(Connor,et al, 1995) ' So tion Contaminants partition between the aqueous phase and the soil matrix Adsorption onto the soil surface significantly retards migration but does not permanently remove BTEX which may desorb later Carbon is the most effective t sorption material in soils, and although clay minerals and amorphous minerals such as iron hydroxides also have some influence, only sorption to carbon in sot]is included in most contaminant fate and transport computer models ' Sorption is controlled by the organic carbon content of soil (fOe), the chemical specific organic carbon partition coefficient(Koc), the soil bulk density(ps), and the water content of the soil as measured by the porosity(0s) Koc is a measure of the affinity of a given chemical to sorb from water onto solid orgaruc material(Table 1) Once the ' porosity, bulk density, Koc and fOe have been established, the retardation factor(R) for the site can be calculated as follows R=(1 +ks * Ps/�s) where ks =foe *Koc ' The retardation factor is used in transport models (discussed below) as a measure of the degree to which the rate of plume migration is reduced by sorption processes ' Hydrolysis etc Other chemical reactions such as hydrolysis may reduce contaminant mass without microbial mediation Hydrolysis occurs when an organic molecule reacts with water or a component ion of water Unlike biodegradation,hydrolysis ' is not catalyzed by microorganisms Hydrolysis has not been observed to reduce BTEX concentrations, but is significant for halogenated volatile organics(solvents, etc ) Monitoring Groundwater For Natural Attenuation Assessment and monitoring of natural attenuation should be performed to confirm that intrinsic bioremediation and other forms of natural attenuation are occurring in the subsurface and are sufficient to lutut plume migration by achieving an equilibrium between hydraulic transport (advection) and removal/degradation/reduction of mobile ' contaminants To confirm natural attenuation, it needs to be demonstrated that intrinsic factors are limiting rnigratton, and that they will continue to do so until the plume has degraded to acceptable levels Natural attenuation can be evaluated by monitoring specific Indicator parameters over a given period of time As ' further confirmation, simple fate and transport models can be applied to the site using the site-specific information obtained Several lines of evidence will generally need to be combined to provide a convincing case of natural attenuation First, it is necessary to establish that the plume is stable or being reduced in terms of size and ' concentrations, by review of historical data, possibly including statistical analysis At least one year of monitoring data utilizing an adequate distribution of wells should be sufficient For all chemical parameters, background concentrations need to be established by sampling one or more clean wells In addition to plume concentrations, Rifat et al,(1995),recommends, at a mimmum, monitoring the following parameters • Microbial enumeration[total heterotrophic bacteria(plate count), and total hydrocarbon using bacteria(ASTM method G-2)] ' Temperature(field measurement) pH(field measurement) • Dissolved Oxygen(field measurement or EPA Method 360 1) ' If DO is depleted relative to background concentrations, additional monitoring for anaerobic processes may be considered and should include the following ' Eh(field measurement) Sulfate(EPA method 300 or 375 4) Nitrate/nitnte(EPA method 300, 353 1 or 353 2) Dissolved iron(EPA method 200 7) ' Total iron(EPA Method 236 1 or 6010) ' CLEARWATER GROUP(NATURAL ATTENUATION) 2 revised October 3,2002