Environmental Health - Toxic Substances Hydrology Program

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Coatings on Sediment Grains Affect Nitrate Storage and Transport

Photomicrograph of a polished thin section showing a cross-sectional view of a quartz sand grain
Photomicrograph of a polished thin section showing a cross-sectional view of a quartz sand grain (labeled Q) from a soil sample. The white arrows are pointing to fine grain coatings that can absorb and store nitrate. The thin section was made with blue-green epoxy, which can be seen in the pore spaces between grains.

The fine-grained and complex mixture of minerals coating quartz sand grains in sediments between land surface and the water table (the unsaturated zone) can absorb and store large amounts of nitrate. This is what a team of U.S. Geological Survey (USGS) scientists found while studying the transport and fate of nitrate in the subsurface. Nitrate is a common chemical form of the nutrient nitrogen, which can promote eutrophication and is a concern when it's present at excessive levels in water supplies used for drinking water. Nitrate is introduced to the environment through numerous pathways including fertilizer and human and animal wastes.

The scientists discovered that these coatings can have a substantial effect on the retention of nitrate in the unsaturated zone, where they potentially could be released later with a change in the chemistry of recharging water. Their findings are particularly important for evaluating the long-term effects of row-crop and animal agriculture on water quality, as the storage of nitrate in the unsaturated zone can affect the quality of shallow groundwater for many years after fertilizer application ceases. This study was funded by the USGS's Toxic Substances Hydrology Program.


Reilly, T.J., Fishman, N.S., and Baehr, A.L., 2009, Effect of grain-coating mineralogy on nitrate and sulfate storage in the unsaturated zone: Vadose Zone Journal, v. 8, no. 1, p. 75-85, doi:10.2136/vzj2008.0053.

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