Transport and Fate of Nutrients

Science Center Objects

Eutrophication, or excess nutrients in streams, is typically one of the top reasons that a stream is listed as impaired on the 303(d) list as part of the Clean Water Act. How nutrients, primarily nitrogen and phosphorus, are transported to streams and groundwater greatly affects the best management plan to keep them on fields and out of streams and groundwater. Likewise, environmental managers and the public want to know if these best management plans are improving the water quality. There are several ongoing USGS WSC projects and multiple completed studies that were designed to help provide information on this issue.

 

Why Nutrients are Important?

While nutrients are essential to the development, health and diversity of plants and animals in surface waters, excessive inputs of nutrients into streams have potential human-health, economic, and ecological consequences. Excess amounts of nitrogen (N) and phosphorus (P) have been shown to cause eutrophication in aquatic ecosystems, which has been linked to fish kills, shifts in species composition, taste and odor in drinking-water, and blooms of harmful algae as well as hypoxia in freshwater systems and estuaries, such as the Gulf of Mexico and the Chesapeake Bay.

To protect human health, the U.S. Environmental Protection Agency (USEPA) set drinking-water criteria (maximum contaminant levels) at 10 mg/L nitrate as N and 1 mg/L nitrite as N. The nitrate criteria were set mainly to protect against methemoglobinemia or “blue baby syndrome.” Other studies have linked nitrate and nitrite levels to adverse reproductive and developmental outcomes in animals and humans. Additionally, aquatic-life criteria for the protection of aquatic organisms have been developed for ammonia as N which varies with pH, temperature, and life-stage. However, these criteria do not address the effects on the biological communities from increased nutrients in rivers and streams. Typically, nutrient concentrations must be extremely high to be toxic to biological communities; such concentrations are rarely found in the environment. Exceptions are concentrations of ammonia associated with accidental discharges from wastewater-treatment facilities, combined-sewer overflows, or concentrated-animal feeding operations.

The Clean Water Act (CWA), as amended in 1976, established a national goal of achieving water quality that provides for the protection and propagation of aquatic organisms, wildlife, and recreation in and on the water. In 1996, the USEPA’s National Water Quality Inventory identified excess amounts of nutrients as the second leading cause of impairment in rivers and streams and as the primary cause of impairments in lakes and reservoirs.

The USEPA has placed many streams in Indiana, as well as the United States, on the CWA Section 303(d) list of impaired water bodies because of excess nutrients. To address these impairments, the USEPA in 2000 developed nutrient criteria to protect streams for total nitrogen as N (TN), total phosphorus as P (TP), periphyton and seston chlorophyll a (CHLa), and turbidity. These criteria are based on USEPA Aggregate Nutrient Ecoregions and Level III Ecoregions, which are areas with similar geographic features such as topography, soils, geology, land use, and biogeography. USEPA reviewed selected data and utilized the frequency distribution approach to develop criteria for nitrate as N, total Kjeldahl nitrogen as N (TKN), TN, TP, CHLa (periphyton and seston), and turbidity based on the lower 25th percentile value for all data or the 75th percentile from reference sites for each causal variable. However, few of the available data from Indiana were included in the criteria development or as was the case for CHLa, little or no data existed for many of the ecoregions, including in Indiana. Therefore, the proposed nutrient, CHLa, and turbidity criteria may not accurately reflect existing local conditions.

Why do we do Edge of Field studies?

Nutrient transport pathways in Leary Weber Ditch Watershed
Nutrient transport pathways in Leary Weber Ditch Watershed, Hancock, IN. Numbers in parenthesis are approximate transport amounts for a typical rain-fed watershed (Schnepf and Cox, 2006). Arrow is dashed and queried where pathway is uncertain.

To understand how nutrients are transported into streams all of the major hydrologic compartments are intensively sampled to determine the primary pathways of nutrients into streams. These are done on small fields or watersheds because the inputs and other important variables can easily be determined. The first study done in Indiana by the USGS was done in Leary Weber Ditch, a small tributary in the Sugar Creek watershed in 2003-04 as part of the National Water-Quality Assessment Program. For this study, water samples were collected from the various hydrologic compartments and analyzed for nutrients and sediment for several storms during the growing seasons of 2003-2004. Water samples were also collected from each hydrologic compartment during stable streamflow conditions between storms.

Nutrients primarily enter the watershed through application of fertilizers to crops. Most of the nitrogen applied to crops leaves the watershed through plant uptake and harvest; most of the phosphorus applied to crops attaches to soil particles and remains in the soil.

Most of the nitrogen not taken up by plants moves through soils (as nitrate) to tile drains which flow to the ditch. Runoff is not the major pathway for the movement of nitrate in Leary Weber Ditch; tile drains decrease surface runoff losses of nitrate while increasing losses through tile-drain water. Also, because nitrogen is injected into the soil, the direct runoff of nitrate to streams is decreased and the potential for nitrate movement to tile drains is increased.

Most of the phosphorus that does not attach to soils or is not taken up by plants moves in runoff to the ditch. Phosphorus concentrations increase in tile drains in response to increased rainfall; however, concentrations are nearly 10 times lower in the tile drain than in runoff. Phosphorus concentrations are low in tile drains between storms, indicating that phosphorus moving in matrix flow attaches to soils before reaching tile drains. For the same reason, negligible amounts of phosphorus move into ground water.

 

Super Gage-School Branch at CR750N at Brownsburg, IN
Edge of Field monitoring site in Indiana.

     

     

     

     

     

     

     

     

     

     

     

    Additional Resources

    Major Findings

    Occurrence and distribution of agricultural chemicals in Leary Weber Ditch Basin, Hancock County, Indiana, 2002-04

    • Nutrients that readily dissolve in water, such as nitrate, are transported into streams primarily through the tiles
    • Nutrients that bind to soil are transported primarily through overland flow
    • Because of the clay layer in the glacial till and the widespread use of tile drains, nutrients do not make it to the groundwater

    Preferential flow estimates to an agricultural tile drain with implications for glyphosate transport

    • Cracks in the clay and holes from roots and earthworms, often called preferential flowpaths, were an important pathway for glyphosate to tile drains and into the stream

    Water and agricultural-chemical transport in a Midwestern, tile-drained, watershed: Implications for conservation practices

    • Nitrogen and phosphorus have different pathways into streams
    • Based upon whether the nutrient is primarily transported through tiles or overland flow impacts the effectiveness of agricultural management practices used to reduce runoff into streams

    Acronyms

    • ACT - Agricultural Chemicals Team
    • WHMI - White, Great and Little Miami River Basins
    • NAWQA - National Water-Quality Assessment