The Glenn and Susan Buckley Lecture in Environmental Geology

Nitrate Concentrations in Regional Aquifer Systems of the United States: A Case of Creeping Normalcy?

Dr. Peter B. McMahon
U.S. Geological Survey
Denver, Colorado

Monitoring for nitrate in groundwater is a common practice in the United States yet detecting trends in nitrate concentrations in regional aquifer systems remains a formidable task. A combination of factors including complex nitrate source histories, reaction and mixing during transport, and location of wells in flow systems contribute to the noise commonly observed in nitrate time-series data. Gradual trends may be difficult to separate from this noise, a phenomenon sometimes referred to as “creeping normalcy.” One consequence of gradual upward trends in contaminant concentrations is that the contamination problem may go undetected because of a lack of long-term monitoring data. Once contaminated, deep zones in aquifers in which production wells are screened may not be remediated quickly because of slow recharge rates, long water residence times in the aquifer, and slow rates of contaminant degradation.

This talk will compare nitrate concentrations in regional aquifer systems, present examples of gradual upward trends in nitrate concentrations, examine factors controlling those trends, and discuss types of data that might be used to predict trends on the scale of regional aquifers.