National Water-Quality Assessment (NAWQA) Program
U.S. Geological Survey
Water-Resources Investigations Report 02-4289
Nutrients in Shallow Ground Waters Beneath Relatively Undeveloped Areas
in the Conterminous United States
By Bernard T. Nolan and Kerie J. Hitt
Abstract
Nutrient concentrations in shallow (well depth of 30 meters or less)
ground waters of relatively undeveloped areas were evaluated to
determine background conditions relative to agricultural and urban
land uses. Lands comprising 67 percent or greater forest or range, 10
percent or less agricultural land, and 10 percent or less urban land
were used to represent relatively undeveloped areas. Data subsets
from the U.S. Geological Survey's National Water-Quality Assessment
Program (81 wells) and retrospective studies (320 wells) yielded 75th
percentile nitrate concentrations of 0.51 and 1.1 milligrams per
liter, respectively, in shallow ground water beneath relatively
undeveloped areas. The value of 1.1 milligrams per liter is a
reasonable upper bound estimate of relative background concentration
of nitrate in shallow ground waters in the United States and
incorporates effects of nominal nitrogen load to susceptible
aquifers. Relative background concentration of nitrate is variable and
depends in part on land use, rock type, and climate. Median nitrate
concentration was significantly greater in ground water beneath
rangeland (1.20 milligrams per liter) than beneath forest land (0.06
milligram per liter). Median nitrate concentration in ground water
beneath rangeland was 1.4-2.7 milligrams per liter in susceptible
aquifers, which consist of coarse-textured deposits or fractured
rock. Increased relative background concentration of nitrate in
rangeland areas likely results from evaporative concentration of
nominal nitrogen load associated with natural organic and inorganic
sources in hydrogeologically susceptible settings.
Table of Contents
Foreword
Abstract
Introduction
Previous Studies
Purpose and Scope
Study Methods
Nutrient Concentrations in Ground Water
Land Use in Relatively Undeveloped Areas
Relative Background Concentrations of Nutrients in Ground Water
Variability of Relative Background Concentration of Nitrate in Ground Water
Summary
References
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