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2007 Ground Water Summit |
Wednesday, May 2, 2007 : 2:20 p.m.
Use of Natural Attenuation Capacity to Define Intrinsic Susceptibility of Public-Supply Wells to Contamination
The occurrence of contaminants in public-supply wells is highly variable. To better understand this variability, the U.S. Geological Survey National Water Quality Assessment Program is studying the transport of anthropogenic and natural contaminants to public-supply wells. As part of this effort, numerical models have been used to help define the intrinsic susceptibility of public-supply wells to contamination by determining the effective natural attenuation capacity of the aquifers, as currently stressed, within the zones of contribution of the wells.
Natural attenuation capacity depends on hydrologic and geochemical variables and can be used to help define intrinsic susceptibility. Natural attenuation capacity is defined as a function of the ground-water velocity, a degradation-rate constant, and hydrodynamic dispersion. The fraction of an initial concentration of a contaminant (C/Co) that remains after travel through the aquifer along a given flow path is a function of the natural attenuation capacity and the distance traveled. C/Co was computed for each simulated flow path to a well. The values for individual flow paths were weighted by the volume of water they represent to determine an effective weighted-average C/Co for a given well.
Degradation-rate constants vary by contaminant and oxidation-reduction conditions in an aquifer. As a result, effective C/Co values for a given well are contaminant-specific. Values of C/Co were computed for 112 wells from 8 study areas across theUnited States based on a range of degradation-rate constants in both oxic and anoxic zones of the aquifers. Effective C/Co values were correlated with measured concentrations in the wells for selected contaminants, suggesting that the C/Co values can be a useful indicator of intrinsic susceptibility. Correlation of effective C/Co to observed concentrations also can be used to show the range of effective degradation rates that might apply to different contaminants.
Natural attenuation capacity depends on hydrologic and geochemical variables and can be used to help define intrinsic susceptibility. Natural attenuation capacity is defined as a function of the ground-water velocity, a degradation-rate constant, and hydrodynamic dispersion. The fraction of an initial concentration of a contaminant (C/Co) that remains after travel through the aquifer along a given flow path is a function of the natural attenuation capacity and the distance traveled. C/Co was computed for each simulated flow path to a well. The values for individual flow paths were weighted by the volume of water they represent to determine an effective weighted-average C/Co for a given well.
Degradation-rate constants vary by contaminant and oxidation-reduction conditions in an aquifer. As a result, effective C/Co values for a given well are contaminant-specific. Values of C/Co were computed for 112 wells from 8 study areas across the