Field and laboratory investigations of contaminant natural attenuation and intrinsic remediation in soils and the vadose zone

Several techniques were employed to study the problem of whether or not potential groundwater contaminants are attenuated and intrinsically remediated in the vadose zone, thus reducing the threat of contaminating underlying groundwater. A simple mass balance computer spreadsheet model was developed to assess the capacity of soils to assimilate applied biosolids at a land application site. A commonly-used laboratory test methodology used to assess the leachability and mobility of potential contaminants was evaluated and compared to a laboratory column leaching method. Parameters governing intrinsic remediation of gasoline hydrocarbons were measured at a field site. Lastly, mechanisms governing constituent fate and transport in field and laboratory settings were assessed using monitoring data and computer simulations.; A simple mass balance computer spreadsheet model was used to calculate and balance the input of a human biosolids waste stream to agricultural land, with losses due to crop biomass removal and volatilization. The balance represented the amount available to be sorbed on site soils and to percolate as groundwater leachate. Nitrogen was the factor limiting to annual biosolid application rate at the site. Biosolids application rates ranging from 2,000-3,000 dry kg/ha/yr are appropriate for average site yields of small grains. Metals are not rate-limiting to the annual site application of biosolids. Despite loading of the soils with potential contaminants, groundwater quality at the site has not been impacted, even by the mobile anion nitrate.; The synthetic precipitation leaching procedure (SPLP) is routinely used to determine the mobility of soil constituents based on the extract from a highly disturbed, acidified, slurried, and agitated sample. The SPLP methodology was compared to leachate from a site-scaled, unsaturated flow column leaching regime. Comparative analytic results indicate that batch testing methodologies are not appropriate for assessing the leachabiity of inorganic, nonbiodegradable constituents in soils not subject to flow heterogeneities and preferential flow pathways. Laboratory soil column leaching tests are more representative of field conditions. Laboratory scale computer simulation results were consistent with the findings of the laboratory column leaching study and provided supporting evidence that column leaching approaches can be successfully used to evaluate flow and transport at the field scale under one-dimensional flow and transport conditions not subject to flow heterogeneities and preferential flow pathways. An analytic expression is derived to express the liquid phase concentration based on the total soil concentration and the water content.; In a field-scale test of vadose zone intrinsic remediation at a gasoline petroleum hydrocarbon impacted site, benzene, toluene, ethylbenzene, xylenes, and total petroleum hydrocarbons as gasoline showed a decrease in concentration over a 17 month period. Based on computer simulations, and site measurement of total soil concentrations, soil vapor concentrations, temperature, neutron moderation logs, oxygen, and carbon dioxide, the decline in petroleum hydrocarbon levels over time is attributed to a combination of vapor migration away from the originally impacted soil volume and in-situ biodegradation. Temperature measurements indicate that vapor migration is likely driven by temperature-induced advective pressure gradients.