『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
friction 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.
Keywords: Agriculture; Trends; Atrazine; Metolachlor; Alachlor;
Fertilizer; Pesticides; Dissolved oxygen; Ground water』
1. Introduction
2. Site descriptions
2.1. Lizzie, North Carolina
2.2. Portage County, Wisconsin
2.3. Eastern San Joaquin Valley, California
2.4. Sumter County, Georgia
3. Methods
4. Results and discussion
4.1. Chemical transformations: implications for the fate
of nitrate and pesticides
4.1.1. Dissolved oxygen loss
4.1.2. Nitrate reduction and excess N2
fraction
4.1.3. Pesticide transformations and degradate fractions
4.1.3.1. Atrazine
4.1.3.2. Metolachlor and alachlor
4.2. Trends in nitrate and pesticide concentrations
4.2.1. Nitrate
4.2.2. Atrazine, metolachlor and alachlor
5. Conclusions
Acknowledgements
References