Toxic Substances Hydrology Program
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A new field-based approach for determining sorption in the unsaturated zone and its effect on the storage of ions and their transport in recharge to ground water has been demonstrated for a small agricultural watershed in the Coastal Plain of southern New Jersey. Moisture-content and chemical-concentration data obtained from unsaturated-zone-core and shallow-ground-water samples were used to estimate the mass flux of chemical constituents across the water table, as well as sorption coefficients (Kd). The selectivity order of the Kd values for cations is consistent with the expected selectivity order: for example, Na+ > Mg++ > Ca++ for sands. Although calculated sorption coefficients, as expected, were greater for cations than for anions, sorption had a substantial effect on the transport of anions through the unsaturated zone; in particular, average Kd values for NO3- were 0.22 liters per milligram for sands and 0.62 liters per milligram for finer grained sediments. The unsaturated zone in the study area is a large reservoir for nitrogen. Models that do not account for sorption are likely to result in unrealistic predictions of contaminant transport rate and provide overly optimistic expectations for natural cleansing in this watershed and those in other similar hydrogeologic settings.
Purpose and Scope
Description of Study Area
Methods of Sample Collection and Analysis
Effect of Sorption on Transport and Storage
Calculation of Mass Fluxes and Sorption Coefficients
Mass Flux of Chemicals Across the Water Table
Unsaturated-Zone Sorption Coefficients
Effect of Sorption on Transport of Chemicals across the Water Table
Effect of Sorption on Unsaturated-Zone Storage
Summary and Conclusions
Appendix 1. Results of analysis of unsaturated-zone sediment and ground water, southern New Jersey, 2003-04
Appendix 2. Summary of recharge estimates for selected sites, Upper Deerfield Township, New Jersey
Appendix 3. Summary of sorption coefficients for selected ions in sediments, Upper Deerfield Township, New Jersey
Download: PDF of SIR2006-5074 (1.3Mb).
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