Study: Nitrate Slowly Moving into Oregon-area Aquifer

Shallow aquifers supplying drinking water to rural residents of southern Deschutes and northern Klamath counties near La Pine, Ore., 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 EPA's drinking water standard of 10 parts per million. However, 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 (DEQ) to evaluate alternatives for protecting 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 DEQ 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 groundwater 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."

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.

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