Nitrate in GW, Lower Umatilla Basin, OR

Science Center Objects

Stretching from Pendleton, Oreg., to the Columbia River, the Lower Umatilla Basin covers 550 square miles. Concentrations of nitrate in the basin's ground water frequently exceed national drinking-water standards. The basin's complex ground-water system is exposed to five human-related sources of nitrate: septic tanks, feedlots, explosives, fertilizer, and land applications of food waste.

To help resource managers in the the Lower Umatilla Basin Ground Water Management Area (GWMA), the USGS is developing tools to identify sources of nitrate in specific wells in the GWMA. The USGS is evaluating the effectiveness of tools that use isotopes and tracers to "fingerprint" the different sources of nitrate.

9722-AUF - Developing and Utilizing Isotopic and Tracer Tools to Evaluate the Source and Movement of Nitrate in Contaminated Ground Water in the Lower Umatilla Basin, Oregon - Completed FY2006

Problem - There are a variety of nitrate sources present in the Umatilla basin including septic tanks, CAFOs, explosives, fertilizer, and food processing waste (Grondin and others, 1995). Therefore, development of isotopic tools to distinguish which of the five sources of nitrate have contributed to contaminating particular areas is needed in order to assist agencies in effectively focusing their resources on reducing nitrate loading to the ground-water system. With additional information about how long the water has been traveling, it can be determined whether the nitrate is from legacy or current management practices.

Objectives - The objective of this study is to develop isotopic and tracer tools to identify sources of nitrate in ground water in the Lower Umatilla Basin Ground Water Management Area (GWMA). This study will attempt to assess 1) the nitrogen and oxygen stable isotopic signature in ground water from each type of nitrate source if not adequately defined in the literature and 2) the age of the water carrying the nitrate to determine whether the nitrate is from current or historical land management practices. This work will examine the ability of isotopic tools to fingerprint selected nitrate sources in the GWMA and expose the limitations of the method.

Relevance and Benefits - The USGS Strategic Plan (https://www.usgs.gov/stratplan/stratplan_rev/) has a Mission Goal of improving the understanding of our Environment and Natural Resources. In particular, the USGS works to provide customers with an integrated understanding of natural systems at various scales, with more and better predictive tools and decision support systems. The USGS Washington Water Science Center Science Plan (http://wwwdwatcm.wr.usgs.gov/uo/internal/sac/scienceplan.2000.wa.pdf) identifies a need to better predict land-use effects on surface-and ground-water quality. This project will develop a tool to allow EPA and Oregon Department of Environmental Quality to better determine the sources of nitrate in the ground-water of the Lower Umatilla Basin. With this information, these agencies can focus on the appropriate management practices to reduce nitrate levels in the basin. This work is expected to have transfer applications to other basins.

Approach - The following site selection and sampling work plan is anticipated:

  • Initial site selection based on the interpretation of the following regional information: Lower Umatilla Basin groundwater management area report (Grondin and others, 1995), recent (2002) unpublished water table maps and landuse history (written and oral comm., Phil Richerson, ODEQ); and site visits and discussion with landowners.
  • Develop QAPP
  • Candidate sites will be subjected to a screening geochemical sampling in fall 2003 to include field parameters (by USGS) and chemical constituents (by USGS except where noted).
  • Evaluate sites for intensive geochemical studies during the winter of2003-2004 based on the accessibility of existing sampling infrastructure and permission to use the geoprobe in the downgradient direction of the site over the length scale appropriate for nitrogen reactions deduced during screening geochemical field tests of candidate sites; the biogeochemistry of nitrogen along the flowpath deduced during the screening field tests; knowledge of nitrate waste streams, and uniformity of applied nitrate waste; the historical use of each site; and the hydrologic setting, including lithology and flowpath.
  • Select additional physical parameters and chemical constituents to be determined for intensive geochemical sampling based on analytical costs and characteristics of selected sites.
  • Amend Project QAPP in early 2004 for intensive geochemical studies to include δ 15N and δ 18O on nitrate and tracer samples.
  • Prepare plan for intensive geochemical field sampling in winter of 2003- 2004.
  • Perform intensive geochemical sampling in spring of 2004.
  • Analyze results of intensive geochemical sampling in spring and summer of 2004.
  • Form conclusions, prepare journal manuscript or report for peer review