Idaho State University study links nitrate contamination in lower Portneuf Valley watershed to septic tanks

Another one-third likely have nitrate contamination from septic sources, said Sarah Godsey, ISU assistant professor of geosciences who worked with geographic information systems master’s student Courtney Ohr, and collaborators in ISU’s Department of Biological Sciences and the Idaho Geological Survey. The study is part of the National Science Foundation’s statewide Managing Idaho’s Landscapes for Ecosystem Services project that is running through 2020 in Idaho.

“We knew from past studies that there were some pretty high levels of nitrates in private wells in the Lower Portneuf Valley Watershed, but there was not clear evidence of where it came from,” Godsey said. “Using new tools that we have, we figured out the nitrates are coming from septic and not agricultural sources.”

The study found 41 wells that were confirmed as septic impacted and 11 of these wells had nitrate concentrations greater than the Environmental Protection Agency’s maximum contaminant level (MCL).  Researchers found that 22 other wells were likely impacted by septic systems, but there was a possibility other sources impacted them. In the 35 remaining wells, there was insufficient information to define the nitrate sources, but septic sources could not be rejected as a possibility of these wells. Two wells identified fertilizer sources for nitrate contamination, indicating agricultural contamination. All these wells were originally tested in 2015.

Godsey noted that these tests were done on the well water before it received any treatment or filtering before being used. Test results were sent to all well owners, but the individual results of the study are being kept confidential. She also noted that the municipal water system was not part of this study and, based on the City’s regular testing, is a safe water supply.  Private wells were tested on the east and west benches, Portneuf Gap, Mink Creek and Johnny Creek areas surrounding the cities of Pocatello and Chubbuck.  Ohr found two main nitrate hotspots, one on the east side and the other on the west side of the valley.

The researchers determined the source of nitrate contamination by tracking nitrate isotopes that can distinguish between waste and fertilizer sources, testing for the presence of water softener salt and by tracking different pharmaceutical or personal care products (PPCP) that would originate from a septic system.  The PPCPs detected in the Lower Portneuf valley wells included DEET, codeine, ibuprofen, fluoxetine (a selective serotonin reuptake inhibitor), sucralose (a non-nutritive sweetener), carbamazepine (an antiepileptic and mood stabilizer drug), sulfadimethoxine (an antibiotic for domesticated animals), sulfamethoxazole (an antibiotic used by humans) and disphenhydramine (an antihistamine). The PPCPs were found in very low concentrations, and were key indicators that the nitrate contamination was coming from septic sources because PPCPs are not usually found in agricultural fields.  The artificial sweetener sucralose was the most commonly found PPCP. Although safe drinking water levels haven’t been established for these PPCPs, they were found in such low levels that they are unlikely to pose a health risk.

“The results of this study and others like it can help the community make wise decisions about our area’s groundwater, and plan for the future to have clean water at a reasonable price,” Godsey said. “Overall, we are lucky to have such clean water sources.”

The title of the study is “Sources and Public Perceptions of Contaminants in the Lower Portneuf River Valley: A Case Study for Nitrates and Personal Care Products and Pharmaceuticals.”