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Rivers near La Pine, Oregon, Are Vulnerable to Nitrate Contamination from Septic Systems
Released: 11/29/2007 2:46:38 PM

Contact Information:
U.S. Department of the Interior, U.S. Geological Survey
Office of Communication
119 National Center
Reston, VA 20192
Steve Hinkle 1-click interview
Phone: 503-251-3237

John Williams 1-click interview
Phone: 503-251-3220



The Deschutes and Little Deschutes Rivers in southern Deschutes and northern Klamath Counties, which receive part of their flow from ground water, are vulnerable to contamination by wastewater from conventional on-site wastewater treatment (septic) systems, according to the findings of a study by the U.S. Geological Survey (USGS). The USGS investigation was a part of efforts by Deschutes County and the Oregon Department of Environmental Quality to assess environmental impacts in a 250-square-mile area near La Pine, Oregon, where increasing residential development has led to increases in nitrate concentrations in ground water that drains to rivers.

Nitrate from septic systems percolates to the water table and is carried by ground water towards the Deschutes and Little Deschutes Rivers. Nitrate is a nutrient that can stimulate excessive growth of aquatic plants and algae, which in turn can cause large variations in dissolved oxygen concentration and pH that can be harmful to fish and other aquatic organisms. In deeper parts of the aquifer, nitrate from septic systems is removed by denitrification, a natural process in which bacteria convert nitrate to harmless nitrogen gas. According to Stephen Hinkle, the USGS study's chief, "Additional denitrification occurs in some near-river portions of the aquifer, providing additional protection for rivers. However, in some near-river zones, denitrification does not occur or occurs only to a small extent, leaving rivers susceptible to ground-water nitrate."

The USGS study characterized the nitrate-removal capacity of the aquifer near the rivers to assess their susceptibility to nitrate carried by ground water. The study also documented the types of river locations that tend to be at greatest risk for septic system nitrate. Finally, the study mapped the most vulnerable river segments by using a computer model to track the paths that nitrate will follow from existing and future homes as it flows toward the rivers. This information can be used by resource managers and planners to prioritize river segments where, for example, nitrate-removal processes could be enhanced by protecting or restoring riparian areas.

At present, most ground-water nitrate in the La Pine area is in upper part of the aquifer and far from the rivers. However, nitrate already in the aquifer will continue to migrate towards the rivers, and projected future increases in the amount of nitrate added to the aquifer will increase the nitrate concentration of ground water.

Information about the La Pine nitrate study, including links to publications and data, can be accessed at http://or.water.usgs.gov/projs_dir/or186/ .

The nitrate modeling report, "Ground Water Redox Zonation near La Pine, Oregon: Relation to River Position within the Aquifer-Riparian Zone Continuum," is published as U.S. Geological Survey Scientific Investigations Report 2007-5239. The report, which is available only online, can be viewed on the Web at http://pubs.usgs.gov/sir/2007/5239/ .


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