National Water-Quality Assessment (NAWQA) Program
Go to:
By Bernard T. Nolan and Jeffrey D. Stoner
[Environmental Science and Technology, vol. 34, no. 7, 2000, p. 1156-1165]Abstract
Results of a national water quality assessment indicate that nitrate is detected in 71
percent of ground-water samples, more than 13 times as often as ammonia, nitrite,
organic nitrogen, and orthophosphate, based on a common detection threshold of 0.2
mg/L. Shallow ground water (typically 5 m deep or less) beneath agricultural land has
the highest median nitrate concentration (3.4 mg/L), followed by shallow ground water
beneath urban land (1.6 mg/L) and deeper ground water in major aquifers (0.48 mg/L).
Nitrate exceeds the maximum contaminant level, 10 mg/L as nitrogen, in more than 15
percent of ground-water samples from four of 33 major aquifers commonly used as a
source of drinking water. Nitrate concentration in ground water is variable and depends
on interactions among several factors, including nitrogen loading, soil type, aquifer
permeability, recharge rate, and climate. For a given nitrogen loading, factors that
generally increase nitrate concentration in ground water include well-drained soils,
fractured bedrock, and irrigation. Factors that mitigate nitrate contamination of ground
water include poorly drained soils, greater depth to ground water, artificial drainage
systems, intervening layers of unfractured bedrock, a low rate of ground-water recharge,
and anaerobic conditions in aquifers.
Table of Contents
Introduction
Methods
Results and discussion
Occurrence of nutrients in groundwater
Nitrate concentration in groundwater
National patterns of nitrate concentration in groundwater
Relation of nitrate concentration in shallow groundwater to explanatory variables
Nitrogen Loading
Land use
Soil drainage characteristics
Irrigation
Biogeochemical transformations
High nitrate concentration in major aquifers
Acknowledgments
Literature cited