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Nov 09,2007
USGS nitrate study shows water supply threatened from residential septic systems
by Lapine Weekly News Sources

Shallow aquifers supplying drinking water to rural residents of southern Deschutes and northern Klamath Counties near La Pine, Oregon, are vulnerable to contamination by wastewater from on-site, residential wastewater disposal (septic) systems, according to the findings of a study by the U.S. Geological Survey (USGS).

At present, relatively few wells in the La Pine area have nitrate concentrations greater than the U.S. Environmental Protection Agency (EPA) drinking water standard of 10 parts per million. Nonetheless, the USGS study predicts that unless nitrogen loading from septic systems can be reduced, future development will lead to water quality problems in the area.

The USGS investigation focused on a 250 square-mile area that included rural residential areas between Sunriver and La Pine and was a part of efforts by Deschutes County and the Oregon Department of Environmental Quality (ODEQ) to evaluate alternatives for protecting the shallow aquifers that supply drinking water for most residents of the area.

According to David Morgan, the USGS study’s chief, “Our investigation agrees with previous assessments by ODEQ and others, which found that nitrate contamination is a threat to the quality of water in the primary aquifer underlying the La Pine area.”

Nitrate is formed when nitrogen in household wastewater is exposed to oxygen in soils around and beneath on-site septic systems. The presence of elevated nitrate levels in drinking water is significant because of its potential health effects. The most commonly known effect, methemoglobinemia, or “blue-baby syndrome,” occurs in infants and children, where the oxygen-carrying capacity of the blood is diminished.

USGS geochemist Stephen Hinkle, a member of the study team, noted “Groundwater moves very slowly in this area because of the low rainfall and recharge. We age-dated ground water in the area and found that most people are drinking water that recharged at least 30 to 50 years ago—before most of the area was developed. The most contaminated water is currently near the water table, but is moving downward.”

Some of the nitrogen from residential septic systems is already finding its way into domestic wells, however, as evidenced by the fact that 10 percent of more than 1,500 well-water analyses performed from 1989 through 2004 had nitrate concentrations greater than four parts per million, well above background levels for the area. ODEQ had similar results when it sampled 192 wells in the area in 2000.

Although water in most wells is within EPA drinking water standards, Morgan noted “Our models predict that, at planned residential development densities, nitrate levels in many parts of the aquifer will eventually exceed the standards unless nitrogen loading from residential septic systems can be reduced.”

In addition to constructing a model to predict the evolution of nitrate contamination in the shallow aquifer beneath the La Pine area, the USGS study developed a simplified tool that can be used by land and water managers to determine the maximum capacity of the aquifer to receive nitrate while still meeting water quality goals. Other aspects of the study (1) examined geochemical controls on the transport of nitrate from septic systems to the Deschutes and Little Deschutes rivers, (2) used indicators of groundwater age, nitrogen isotope and geochemical analyses, and characterization of groundwater flow patterns to verify residential septic systems as the primary source of nitrate in the aquifer, and (3) documented the occurrence and transport of personal care products, household chemicals, pharmaceutical products, and viruses in on-site septic system effluent.
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