Delaware Agricultural Shallow Groundwater Monitoring Network

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Studies in the Delmarva Peninsula have demonstrated that groundwater in shallow unconfined aquifers near agricultural lands is susceptible to contamination from chemicals applied at the land surface.

Observed concentrations of nitrate in shallow groundwater commonly exceed the U.S. Environmental Protection Agency Maximum Contaminant Level of 10 mg/L on the peninsula. Groundwater discharge on the peninsula is a major source of nitrate in streams and coastal waters, contributing to the nutrient loads to Chesapeake Bay, Delaware Bay, and the Delaware Inland Bays.

Results of nitrate analysis from this study are intended to provide a baseline data set which could be used to observe trends in nitrate concentrations.

Duration: October 2014 - Ongoing

Partner: Delaware Department of Agriculture - Visit Partner's Website

Project Lead: Brandon Fleming - View Profile - bjflemin@usgs.gov

Background: Studies in the Delmarva Peninsula have demonstrated that groundwater in shallow unconfined aquifers near agricultural lands is susceptible to contamination from chemicals applied at the land surface (Bachman, 1984; Denver, 1986; Denver, 1989; Andres. A.S. 1991; Shedlock and others, 1999; Blaier and Baxter, 2000; Denver and others, 2004). 

Observed concentrations of nitrate in shallow groundwater commonly exceed the U.S. Environmental Protection Agency Maximum Contaminant Level (MCL) of 10 mg/L on the peninsula. Groundwater discharge on the peninsula is a major source of nitrate in streams and coastal waters, contributing to the nutrient loads to Chesapeake Bay, Delaware Bay, and the Delaware Inland Bays (Andres, 1992, Ullman and others, 2007, Ator and Denver, 2012). 

Past and recent studies (Shedlock, 1993; Sanford and Pope, 2013) indicate that changes in nutrient management practices on the land surface may take decades to improve water quality in groundwater discharge to Delmarva streams because of groundwater residence times.

Problem: Delaware has a large agricultural economy based primarily on poultry production and production of corn, soybeans, and small grains. The Coastal Plain of Delaware is underlain by an extensive surficial aquifer that is a major source of water for public and domestic water supplies, agricultural irrigation, and base flow to streams.

The agricultural community in Delaware has been working with various State and Federal government agencies to apply a number of conservation practices intended to reduce the amount of nitrate reaching the water table beneath agricultural land in support of Delaware’s nutrient reduction goals.

To understand if these changes in agricultural practices are reducing nitrate concentrations in shallow groundwater, and eventually in drinking water and Delaware streams, a long-term groundwater monitoring program is being implemented. This network of wells is located in areas with young, oxic, shallow groundwater, overlain by agricultural land, where change will be seen most clearly in a relatively short timeframe.

Groundwater Network in Delaware

(Public domain.)

Approach: A groundwater monitoring network, consisting of existing shallow wells from the Delaware Department of Agriculture’s Pesticide network (Blair and Baxter, 2001) and USGS networks, has been established based on geochemical and land-use characteristics including:

  • The presence of oxic aquifer conditions
  • Geochemical indicators of agricultural land use based on major ion and nutrient concentrations (Ator, 2008; Debrewer and others, 2008)
  • Location of wells with respect to agricultural lands
  • Depth of wells, and
  • Available age dates for groundwater.

 

Samples have been collected for analysis of nutrients, major ions, and groundwater age and the results will be used to characterize groundwater quality and compared to existing groundwater quality data.

To evaluate changes in nitrate concentrations between 1980 and 2014, statistical matched pair tests (Helsel and Hirsch, 2002) will be applied to new nitrate analyses from wells sampled during the study and historical analyses (where available).

Much of the documented variability in nitrate concentrations on the Delmarva Peninsula included results from networks with wells in both oxic and anoxic aquifer conditions (Debrewer and others, 2007). To maximize the discriminatory power of the statistical tests, the network will have a large sample size (up to 50 wells) and only include wells in oxic aquifer conditions.

Results of nitrate analysis from this study are intended to provide a baseline data set which, if supplemented by sampling results collected under similar hydrologic conditions in future studies could be used to observe trends in nitrate concentrations.