Ultra-fine coatings on sediment grains influence nitrate and sulfate storage in soil

Tiny sediment grains are covered with a very fine-grained, complex mixture of minerals in an open fabric that results in a large surface area in contact with water between the grains. Scientists at the U.S. Geological Survey (USGS) are studying this microscopic layer and finding that the mineral composition of these coatings on sediment grains in the unsaturated zone (i.e., between land surface and the water table) can have a substantial effect on the retention of nitrate and sulfate. Their findings are particularly important for evaluating the long-term effects of agriculture on water quality, as the storage of these common components of fertilizer in the unsaturated zone can affect the quality of shallow groundwater for many years after fertilizer application ceases.

Nitrate and sulfate are important plant nutrients. Farmers and soil scientists routinely monitor the nutrient content of the soil in farm fields so that fertilizer application can be managed to maintain profitability and to minimize runoff or infiltration. Loss of excess nutrients can elevate levels in streams and shallow groundwater. Understanding the mechanisms of nutrient storage in the unsaturated zone is critical to protecting groundwater resources.

Timothy Reilly, the lead author of the study, stated “The unsaturated zone is potentially a large reservoir for anions (negatively charged ions) like nitrate and sulfate. The increased residence time indicated by these findings suggests that models in areas with similar mineral characteristics, which neglect or minimize storage, will not accurately predict nutrient transport to the water table.”

In the study, unsaturated-zone sediments and the chemistry of shallow groundwater underlying a small (about 3 square miles) watershed in southern New Jersey were studied to identify mechanisms responsible for nutrient storage. Lower unsaturated zone sediments and shallow groundwater samples were collected at 11 locations, and concentrations of nitrate and sulfate were determined. Nutrient storage in the very fine-grained mineral coatings on the sediment grains was attributed to a complex combination of chemical and physical storage mechanisms.

Results from the study, supported by the US Geological Survey Toxic Substances Hydrology Program, were published in the February 2009 issue of the Vadose Zone Journal. The research was also presented in Houston, TX at the joint annual meeting of the Geological Society of America and Soil Science Society of America in October 2008. These findings are particularly important for evaluating the long-term effects of agricultural land use on groundwater quality. Continuing research includes characterizing stream sediments, determining relations between sediment mineralogy and the concentration of pesticides stored on the sediments, and laboratory experiments evaluating storage mechanisms.

The full article is available for no charge for 30 days following the date of this summary. View the abstract at http://vzj.scijournals.org/cgi/content/full/8/1/75.

Vadose Zone Journal, http://www.vadosezonejournal.org/ is a unique publication outlet for interdisciplinary research and assessment of the biosphere, with a focus on the vadose zone, the mostly unsaturated zone between the soil surface and the permanent groundwater table. VZJ is a peer-reviewed, international, online journal publishing reviews, original research, and special sections across a wide range of disciplines that involve the vadose zone, including those that address broad scientific and societal issues. VZJ is published by Soil Science Society of America, with Geological Society of America as a cooperator.

The Soil Science Society of America (SSSA) is a progressive, international scientific society that fosters the transfer of knowledge and practices to sustain global soils. Based in Madison, WI, and founded in 1936, SSSA is the professional home for 6,000+ members dedicated to advancing the field of soil science. It provides information about soils in relation to crop production, environmental quality, ecosystem sustainability, bioremediation, waste management, recycling, and wise land use.

SSSA supports its members by providing quality research-based publications, educational programs, certifications, and science policy initiatives via a Washington, DC, office. For more information, visit www.soils.org.

SSSA is the founding sponsor of an approximately 5,000-square foot exhibition, Dig It! The Secrets of Soil, which opened on July 19, 2008 at the Smithsonian’s Natural History Museum in Washington, DC.

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