National Water-Quality Assessment (NAWQA) Program
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By Anthony J. Tesoriero, David A. Saad, Karen R. Burow, Elizabeth A. Frick, Larry J. Puckett, Jack E. Barbash
[Journal of Contaminant Hydrology, vol. 94, 2007, p. 139-155]Abstract
Tracer-based ground-water ages, along with the concentrations of
pesticides, nitrogen species, and other redox-active constituents,
were used to evaluate the trends and transformations of agricultural
chemicals along flow paths in diverse hydrogeologic settings. A range
of conditions affecting the transformation of nitrate and pesticides
(e.g., thickness of unsaturated zone, redox conditions) was examined
at study sites in Georgia, North Carolina, Wisconsin, and
California. Deethylatrazine (DEA), a transformation product of
atrazine, was typically present at concentrations higher than those of
atrazine at study sites with thick unsaturated zones but not at sites
with thin unsaturated zones. Furthermore, the fraction of atrazine
plus DEA that was present as DEA did not increase as a function of
ground-water age. These findings suggest that atrazine degradation
occurs primarily in the unsaturated zone with little or no degradation
in the saturated zone. Similar observations were also made for
metolachlor and alachlor. The fraction of the initial nitrate
concentration found as excess N2 (N2 derived
from denitrification) increased with ground-water age only at the
North Carolina site, where oxic conditions were generally limited to
the top 5 m of saturated thickness. Historical trends in fluxes to
ground water were evaluated by relating the times of recharge of
ground-water samples, estimated using chlorofluorocarbon
concentrations, with concentrations of the parent compound at the time
of recharge, estimated by summing the molar concentrations of the
parent compound and its transformation products in the age-dated
sample. Using this approach, nitrate concentrations were estimated to
have increased markedly from 1960 to the present at all study
sites. Trends in concentrations of atrazine, metolachlor, alachlor,
and their degradates were related to the timing of introduction and
use of these compounds. Degradates, and to a lesser extent parent
compounds, were detected in ground water dating back to the time these
compounds were introduced.
Table of Contents
Introduction
Site descriptions
Lizzie, NC
Portage County, WI
Eastern San Joaquin Valley, CA
Sumter County, GA
Methods
Results and discussion
Chemical transformations: implications for the fate of nitrate and pesticides
Pesticide transformations and degradate fractions
Trends in nitrate and pesticide concentrations
Conclusions
Acknowledgements
References
U.S. Department of the Interior |
U.S. Geological Survey
URL: http://water.usgs.gov/nawqa/nutrients/pubs/jch_v94/index.html
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