High-frequency nitrate-concentration data

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High-frequency nitrate-concentration data can be used to inform the development of best management practices to reduce nitrogen loading to Chesapeake Bay. Although nitrogen loads entering Chesapeake Bay have decreased in recent decades, they exceed levels that are compatible with a healthy ecosystem as a result of urbanization, agriculture, and other human activities in the bay watershed, and further reductions are needed. The transport of excessive amounts of nitrate (the most commonly occurring form of nitrogen in most streams and rivers) and other nutrients through streamflow and groundwater discharge can cause eutrophication and other adverse physical, chemical, biological, and economic consequences to the bay’s sensitive aquatic ecosystem. Mitigating these effects requires information not only about total loads, but also about time-varying sources of nitrate to streams and in-stream retention of nitrate. This information is difficult to obtain with traditional field-based study methods.

Although nitrogen loads entering Chesapeake Bay have decreased in recent decades, they exceed levels that are compatible with a healthy ecosystem as a result of urbanization, agriculture, and other human activities in the bay watershed, and further reductions are needed. The transport of excessive amounts of nitrate (the most commonly occurring form of nitrogen in most streams and rivers) and other nutrients through streamflow and groundwater discharge can cause eutrophication and other adverse physical, chemical, biological, and economic consequences to the bay’s sensitive aquatic ecosystem. Mitigating these effects requires information not only about total loads, but also about time-varying sources of nitrate to streams and in-stream retention of nitrate. This information is difficult to obtain with traditional field-based study methods. Results of a new USGS study show that collection and interpretation of high-frequency nitrate-concentration data from streams in the bay watershed (fig. 1) can be used to quantify time-varying sources and in-stream retention of nitrate which, in turn, can inform the development of best management practices to reduce nitrate input to the bay.

 

Results of new USGS research

Results of new USGS research published in the journal “Water Resources Research” (Miller and others, 2016) show that:

  • High-frequency nitrate-concentration data (in this study, collected at 15-minute intervals with water-quality sensors) can be used to quantify the fraction of total nitrate loading to streams that originates from groundwater and the fraction that originates from surface runoff, as well as the percentages of the stream nitrate load that are retained in the in-stream environment.
  • Groundwater contributed more than half the annual nitrate load (58–73 percent) to all three study streams (the Potomac River, Smith Creek, and Difficult Run) in the Chesapeake Bay watershed (fig. 2).
  • The proportion of the annual nitrate load retained in the in-stream environment of the three study streams ranged from 11 percent (in the two small streams) to 23 percent (in the Potomac River).

 

Implications for ecosystem management include:

  • Fifty-eight to 73 percent of the annual nitrate load delivered to the study streams is derived from groundwater. Therefore, protection and mitigation actions aimed at groundwater sources may be particularly effective at reducing nitrate loads.
  • Recent studies have shown that groundwater containing decades-old nitrate is being discharged to streams in the Chesapeake Bay watershed and elsewhere (Sanford and Pope, 2013; Tesoriero and others, 2013). Coupling the approach for quantifying nitrogen loading to streams and in-stream retention used in the current study with methods that estimate nitrogen transport and removal along groundwater flow paths will provide a more complete understanding of watershed-scale nitrogen processing and transport.

 

Sources of information

The findings in this Science Summary are reported in the article below, which should be used as the reference for this information:

Miller, M.P., Tesoriero, A.J., Capel, P.D., Pellerin, B.A., Hyer, K.E., and Burns, D.A., 2016, Quantifying watershed-scale groundwater loading and in-stream fate of nitrate using high-frequency water quality data: Water Resources Research, v. 52, no. 1, p. 330-347, doi:10.1002/2015WR017753, accessed May 4, 2016, at http://onlinelibrary.wiley.com/doi/10.1002/2015WR017753/full.

 

For more information about this science summary:

Contact Matthew Miller at mamiller@usgs.gov.

 

Locations of USGS streamgages in the Potomac River, Smith Creek, and Difficult Run Basins in the Chesapeake Bay watershed.

Figure 1. Location of U.S. Geological Survey streamgages in the Potomac River, Smith Creek, and Difficult Run Basins in the Chesapeake Bay watershed.

 

Predicted groundwater-discharged (GWD) and runoff (RO) nitrate (NO3) loads (in kilograms of nitrate per day [kg N/d])

Figure 2. Predicted groundwater-discharged (GWD) and runoff (RO) nitrate (NO3) loads (in kilograms of nitrate per day [kg N/d]) at (a) the Potomac River, (b) Smith Creek, and (c) Difficult Run. Sixty-nine, 73, and 58 percent of the annual nitrate load to the Potomac River, Smith Creek, and Difficult Run, respectively, was estimated to be derived from groundwater. The GWD and RO nitrate loads are the estimated loads delivered to the stream prior to any in-stream biogeochemical processing of nitrate. (From Miller and others, 2016)

 

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