Occurrence and Generation of Nitrite in Ground and Surface Waters in an Agricultural Watershed

Background/Need: Approximately 70% of the population in Wisconsin relies on groundwater as a drinking water source, and 10% of the State’s private wells have nitrate (NO₃^–) concentrations that exceed the EPA’s maximum contaminant level of 10 mg/L. Nitrite (NO₂^–) may be formed as a bi-product of NO₃^– enrichment, and prior research revealed its presence in surface waters of agriculturally dominated areas of the State. This observation leads to a compelling need to determine if NO₂^– is also present in Wisconsin ground waters and to identify the sources and pathways of NO₂^– generation in surface waters. By examining NO₂^– formation and distribution, we addressed multiple UWS groundwater research priorities, including: (1) transport of pollutants in groundwater; (2) impact of agricultural practices on groundwater quality; and (3) interactions of groundwater and surface water including chemical transformations in the hyporheic zone and influence of groundwater discharge on water quality.
Objectives: The goal of this project was to address the question: What conditions lead to the accumulation of nitrite (NO₂^–) in surface water environments in an agricultural watershed? This overarching question was addressed via three specific questions:
Q1. Is NO₂^– present in groundwater in N-rich areas of southern Wisconsin?
Q2. When and where is NO₂^– present along groundwater flow paths?
Q3. What processes and conditions are responsible for surface water NO₂^– accumulation?