Some wells downhill from agricultural fields treated with bio-based fertilizers exhibited nitrate levels above EPA standards
Some shallow groundwater wells next to or downhill from Orange County, N.C., agricultural fields treated with bio-based fertilizers have nitrate levels above U.S. Environmental Protection Agency (EPA) standards set for public water supplies, according to a new U.S. Geological Survey (USGS) report titled “Effect of Land-Applied Biosolids on Surface-Water Nutrient Yields and Groundwater Quality in Orange County, North Carolina.”
Researchers took water samples from one watershed where biosolids had been used, and one where they had not, to determine what effect biosolids have on water quality. While they found noteworthy differences in the concentrations of nitrate, many of the levels of contaminants were comparable.
“Overall the study showed that there was not much difference in water quality conditions between the two watersheds,” said USGS hydrologist Chad Wagner. “The most significant difference was found in the shallow groundwater from monitoring wells adjacent to fields where biosolids had been applied where we saw nitrate concentrations greater than the maximum drinking water criteria.”
Many county residents get their drinking water from private wells. While EPA does not regulate or set standards for private water supplies, the agency provides information on potential risks of drinking water containing nitrates above the standards set for public drinking water. The agency also recommends all private well owners have their water tested periodically to ensure it is safe to drink.
Bio-based fertilizers have been applied to some Orange County fields for as long as 30 years. The fertilizers are made of biosolids—nutrient-rich organic materials that result from the treatment of domestic sewage in a treatment facility. After the biosolids have been treated and processed, they can be used as fertilizer.
USGS conducted the agricultural watershed study in the Collins Creek and Cane Creek Reservoir watersheds in Orange County from March 2011 to May 2013. USGS scientists compared environmental conditions associated with agricultural fields that have had biosolid applications for approximately the past 30 years, and an agricultural field that has not had any biosolid application. Samples of soil, biosolids, shallow groundwater and surface water were collected from both watersheds for chemical analysis over a range of hydrologic and environmental conditions during the study.
During the course of the study, none of the biosolid fertilizer samples that were tested exceeded the standards set by EPA for this type of fertilizer. Results also indicated that treatment processes and storage techniques used by Orange Water and Sewer Authority are effective in eliminating E. coli and fecal coliform bacteria from the biosolids.
Contaminants of emerging concern—substances such as pharmaceuticals, hormones and antibiotics that are becoming more prevalent in water resources—also were analyzed as part of the study. Although they were frequently detected in the biosolid and soil samples, these compounds were rarely detected in groundwater and surface water samples during the study.
“Given the frequent detection of emerging contaminants in the biosolid and soil samples, we did not expect to find such an infrequent number of detections of these compounds in the groundwater and surface water samples,” Wagner said.
Using municipal wastewater biosolids as fertilizer is the most common method of biosolids management used in North Carolina and the U.S. The high nutrient concentrations in biosolids are beneficial to soil and plants for agricultural purposes. Land application can take advantage of these beneficial qualities, whereas disposal in landfills or incineration poses no beneficial use of the waste.