Agricultural activities across southeast Minnesota’s karstic terrain have resulted in extensive areas where greater than ten percent of wells are above the drinking water standard of 10 mg/l, leading to several policy and management strategies to mitigate the problem. Many of these strategies are relatively routine and have been in practice for many years, such as setbacks of manure storage and application from karst features. More recently, Minnesota’s Groundwater Protection Rule (GPR) was established, with the purpose of minimizing nitrogen fertilizer sources of nitrate pollution. The GPR is implemented under the direction of the Minnesota Department of Agriculture. Part 1 of the GPR prohibits application of nitrogen fertilizer in the fall or on frozen soils in areas of vulnerable groundwater (with some exclusions), or in the protected area around a public well with high nitrate. These “vulnerable” and “protected” areas are delimited by hydrogeologic characterization that relies on detailed maps of geologic materials, a comprehensive karst features database, and extensive water sampling.  Part 2 of the GPR includes additional mitigation efforts for protected areas around public wells with nitrate concentrations of 5.4 mg/l or greater.  The efforts progress from voluntary to regulatory based largely on whether nitrate concentrations remain near (8 mg/l) or above the drinking water standard or are projected to exceed that level in the next 10 years. The extent to which voluntary efforts have been adopted is also considered.

Measuring the effectiveness of the GPR Part 2 mitigation efforts in lowering or stabilizing nitrate concentrations in public supply wells is rendered difficult due to variability in groundwater “lag time”, which is the elapsed time along a flow path from the land surface to a well. Recent research in southeast Minnesota reveals that the most commonly utilized aquifers have decadal-scale lag times.  The GPR rules require consideration of lag time, with efforts currently underway to meet the challenge of integrating this factor into the regulatory procedures. Another difficulty is that contributions to wells often include contaminated groundwater recharged from outside of the protected area around a well, where GPR mitigation rules do not apply. Minnesota’s GPR exemplifies how current hydrogeologic understanding can be integrated into water policy and management but also demonstrates implementation that utilizes this information can be challenging.