Toxic Substances Hydrology Program

Methodology to Evaluate the Effect of Sorption in the Unsaturated Zone on the Storage of Nitrate and Other Ions and Their Transport Across the Water Table, Southern New Jersey

By Timothy J. Reilly and Arthur L. Baehr

Scientific Investigations Report 2006-5074

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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

Background Concentrations

Effect of Sorption on Transport of Chemicals across the Water Table

Effect of Sorption on Unsaturated-Zone Storage

Summary and Conclusions

References Cited

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).

U.S. Department of the Interior, U.S. Geological Survey
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