Contaminant Fate Model Research Articles

Modeling alternative groundwater remediation methods in contrasting hydrogeologic settings

A contaminant fate and transport model was used to simulate the effects of 11 remediation schemes on benzene plumes in three hydrogeologic settings. Pump and treat, bioremediation, and a combination of those technologies were considered. Dissolved oxygen concentrations of 8 and 50 mg/l were used at injection wells. The simulated aquifers include sand and gravel, silty sand, and clayey sand. Interior injection (50 mg/l O2) was most successful overall, consistently attaining the highest rank in each setting. Interior extraction/exterior injection (50 mg/l O2) ranked second overall. Whether used for injection or extraction, interior well configurations generally outperformed alternative patterns. Interior extraction, the most successful pump and treat scheme, performed effectively in the sand and gravel aquifer, but was among the lowest ranking schemes in the lower velocity settings. In contrast, interior injection (8 mg/1 O2) performed better in the low velocity aquifers. Consistently, downgradient extraction and no action were the least effective schemes.

Spatial and temporal variation of isoproturon residues and associated sorption/desorption parameters at the field scale

Spatial and temporal variation of isoproturon (IPU) residues (IPUr), and their sorption (Kd) and desorption (Kapp) coefficients were monitored at the field scale in the cultivated horizon of a clay soil following pre- and postemergent IPU applications to a winter barley crop. The half life in 25 subplots ranged from 31 to 483 days with a median value of 90 days. Kapp increased as IPUr declined, ranging from a median value of 9.9 L/kg, 27 days after the pre-emergent application (autumn 1991) to 77.8 L/kg four days prior to the post-emergent application (spring of 1992). Kd's were more consistent, with median values ranging from 3.7 to 6.4 L/kg over the same period. However, median Kd's of 7.1 and 8.7 L/kg in May and June 1992 respectively would suggest that increases may occur during the summer months when temperatures increase and the soil dries. Sorption nonideality (Kd/Kapp) was observed on each sampling occasion, ranging from approximately 0.5, 27 days after the pre-emergent herbicide application, to approximately 0.2 four days prior to the post-emergent application. All parameters were positively skewed. Variation of all parameters at the local scale (i.e. within 1 m 2) was less than that at the field scale, but greater than that associated with the experimental methods employed indicating that both intrinsic and extrinsic factors influenced the distribution of IPUr, Kd and Kapp. Variability of IPUr and Kd on individual sampling dates was consistent with coefficients of variation generally ranging from 26 to 44% and from 18 to 44% respectively. Variability of Kapp was more erratic, increasing to over 200% and 100% following two major rainstorm events in November 1991 and February 1992 repectively. Normalization of Kd's on a soil organic matter basis failed to reduce the variability observed. The implications of these results for contaminant transport and fate modelling are discussed

Constituent bioconcentration in rainbow trout exposed to a complex chemical mixture.

Classically, aquatic contaminant fate models predicting a chemical's bioconcentration factor (BCF) are based upon single-compound derived models, yet such BCF predictions may deviate from observed BCFs when physicochemical interactions or biological responses to complex chemical mixture exposures are not adequately considered in the predictive model. Rainbow trout were exposed to oil-shale retort waters. Such a study was designed to model the potential biological effects precluded by exposure to complex chemical mixtures such as solid waste leachates, agricultural runoff, and industrial process waste waters. Chromatographic analysis of aqueous and nonaqueous liquid-liquid reservoir components yielded differences in mixed extraction solvent HPLC profiles of whole fish exposed for 1 and 3 weeks to the highest dilution of the complex chemical mixture when compared to their corresponding control, yet subsequent whole fish extractions at 6, 9, 12, and 15 weeks into exposure demonstrated no qualitative differences between control and exposed fish. Liver extractions and deproteinized bile samples from exposed fish were qualitatively different than their corresponding controls. These findings support the projected NOEC of 0.0045% dilution, even though the differences in bioconcentration profiles suggest hazard assessment strategies may be useful in evaluating environmental fate processes associated with complex chemical mixtures. 12 references, 4 figures,more » 2 tables.« less