Paul E. Stackelberg

Evaluation of the atmosphere as a source of volatile organic compounds in shallow groundwater

The atmosphere as a source of volatile organic compounds (VOCs) in shallow groundwater was evaluated over an area in southern New Jersey. Chloroform, methyl tertbutyl ether (MTBE), 1,1,1-trichloroethane, tetrachloroethylene (PCE), and carbon disulfide (not a VOC) were detected frequently at low-level concentrations in a network of 78 shallow wells in the surficial Kirkwood-Cohansey aquifer system. The atmosphere was sampled for these compounds and only MTBE concentrations were high enough to potentially explain frequent detection in shallow groundwater. A mathematical model of reactive transport through the unsaturated zone is presented to explain how variations in unsaturated properties across the study area could explain differences in MTBE concentrations in shallow groundwater given the atmosphere as the source. Even when concentrations of VOCs in groundwater are low compared to regulatory concentration limits, it is critical to know the source. If the VOCs originate from a point source((), concentrations in groundwater could potentially increase over time to levels of concern as groundwater plumes evolve, whereas if the atmosphere is the source, then groundwater concentrations would be expected to remain at low-level concentrations not exceeding those in equilibrium with atmospheric concentrations. This is the first analysis of VOC occurrence in shallow groundwater involving colocated atmosphere data.

Regression models for estimating concentrations of atrazine plus deethylatrazine in shallow groundwater in agricultural areas of the United States

Tobit regression models were developed to predict the summed concentration of atrazine [6-chloro--ethyl--(1-methylethyl)-1,3,5-triazine-2,4-diamine] and its degradate deethylatrazine [6-chloro--(1-methylethyl)-1,3,5,-triazine-2,4-diamine] (DEA) in shallow groundwater underlying agricultural settings across the conterminous United States. The models were developed from atrazine and DEA concentrations in samples from 1298 wells and explanatory variables that represent the source of atrazine and various aspects of the transport and fate of atrazine and DEA in the subsurface. One advantage of these newly developed models over previous national regression models is that they predict concentrations (rather than detection frequency), which can be compared with water quality benchmarks. Model results indicate that variability in the concentration of atrazine residues (atrazine plus DEA) in groundwater underlying agricultural areas is more strongly controlled by the history of atrazine use in relation to the timing of recharge (groundwater age) than by processes that control the dispersion, adsorption, or degradation of these compounds in the saturated zone. Current (1990s) atrazine use was found to be a weak explanatory variable, perhaps because it does not represent the use of atrazine at the time of recharge of the sampled groundwater and because the likelihood that these compounds will reach the water table is affected by other factors operating within the unsaturated zone, such as soil characteristics, artificial drainage, and water movement. Results show that only about 5% of agricultural areas have greater than a 10% probability of exceeding the USEPA maximum contaminant level of 3.0 μg L. These models are not developed for regulatory purposes but rather can be used to (i) identify areas of potential concern, (ii) provide conservative estimates of the concentrations of atrazine residues in deeper potential drinking water supplies, and (iii) set priorities among areas for future groundwater monitoring.