Table 2.

Well variables determined to be important covariates for log nitrate concentrations in Iowa private wells. More details on the sources for the variables are available in Wheeler et al. (2015).

Variable(s)	Definition
Well Depth	Depth of the nitrate sampling well (0.305 m or ft).
Nitrogen Fertilizer use, 2000 and 2002	Annual 2000 and 2002 county-level nitrogen fertilizer (kg of N/year) apportioned to 2000 and 2002 agricultural lands within a 1 km well buffer.
Agricultural Land 1978 and 2002	Agricultural land in 1978 and 2002 within a 1 km radius well buffer, expressed as a percentage.
Slope	Average percent slope within a 1 km buffer.
Minimum Temperature	Mean annual minimum temperature at well point for the time period 1981–2010 (degrees Celsius times 100).
Karst Category	Karst categorical value at well point. Indicates areas within one mile of known sinkholes as well as areas that have carbonate bedrock within 50 feet of the land surface. Categories included no karst, less than a 304.8 m (1000 ft.) sink, and greater than a 304.8 m (1000 ft.) sink.
Groundwater Region Category	Groundwater vulnerability region at well point broken into categories that characterized wells by bedrock, shale, and drainage composition.
USGS Well Logs	Categorical variable created from grouping the number of wells reported to the USGS, including location, ownership, and construction details, within a 4×4 mile square around the well point. Grouping is based on general known accuracy of variables created from contributing well log data (e.g., higher well log counts indicate increased accuracy).
Fine-Grain Deposit Thickness	Total thickness of fine-grain soil deposits at the well (0.305 m or ft). Values were calculated from above the top of the measurement well screen (assumed to be the well depth minus three feet) using an interpolated grid created from well log data within a 6.44×6.44-km (4×4-mile) square around the well point.
Well Logs at 9.66 km	Number of USGS well logs within a 9.66×9.66-km (6×6-mile) square around the well point.
Well Logs at 6.44 km	Number of USGS well logs within a 6.44×6.44-km (4×4-mile) square around the well point.
Horizontal Hydraulic Conductivity	Average horizontal hydraulic conductivity of all glacial deposits within a 6.44×6.44-km (4×4-mile) square around the well point (0.304 m or ft/day).
Saturated Hydraulic Conductivity	Average soil saturated hydraulic conductivity within a 1 km radius well buffer (micrometers/sec)