Estimating the Probability of Elevated Nitrate Concentrations in Ground Water in Washington State

Scientific Investigations Report 2008–5025

U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2008–5025

Table 1. Summary of anthropogenic and natural explanatory variables used for model development.

Variable	Number of samples	Minimum value	Median value	Maximum value	Source of data
Well depth (feet)	12,601	1	145	2,200	Washington Dept. of Health data base
Average annual precipitation (inches)	12,601	5.7	43.23	171.9	Spatial Climate Analsyis Service–Oregon State University, 2006
Recharge	11,655	3	354	1,982	Wolock, 2003
2000 population density at the well location (people per squared kilometer)	12,560	0	675	39,263	GeoLytics, 2001
Urban land use within 500 meters of well (percent)	12,601	0	16.12	100	Vogelmann and others, 2001;  Price and others, 2003
Agricultural land use within 4 kilometers of well (percent)	12,601	0	7.52	98.87	Vogelmann and others, 2001;  Price and others, 2003
Undeveloped land use within 2 kilometers of well (percent)	12,601	0	68.5	100	Vogelmann and others, 2001;  Price and others, 2003
Nitrate application within 2 kilometers of well (kilograms per year)	12,601	0	6,295	153,047	This study
Available water capacity (inch per inch)	12,464	.03	.09	.21	Schwarz and Alexander, 1995
Clay content (in percent)	12,464	0	8.20	44.3	Schwarz and Alexander, 1995
Organic matter (percent by weight)	12,464	.1	1.30	10.9	Schwarz and Alexander, 1995
Permeability (inches per hour)	12,464	.25	3.81	17.14	Schwarz and Alexander, 1995
Hydrologic group [rated 1 (high infiltration) to 4 (very slow infiltration)]	12,464	1	2.5	4	Schwarz and Alexander, 1995
Soil drainage [rated 1 (excessive) to 7 (very poorly drained)]	12,464	1.3	3.1	6.9	Schwarz and Alexander, 1995