Variables that affect agricultural chemicals in groundwater in Nebraska

Agricultural chemicals from nonpoint
sources in groundwater are present in the major provinces
of the High Plains aquifer in Nebraska. Nitrate and
triazine-herbicide concentrations in groundwater were
assessed to establish preliminary relations between these
constituents and selected hydrogeologic, climatic, and
land-use variables. Also, macropore flow paths were
measured in an attempt to delineate their contribution
to non-point source pollution from the study areas.
Water from 82 wells in six study areas was analyzed
for nitrate; water from 57 of the 82 wells was analyzed
for triazine herbicides. Twenty-one independent variables
were identified that could potentially affect chemical
concentrations in groundwater. Data for 9 of 21
independent variables suspected of affecting concentrations
of nitrate and triazine herbicides in groundwater
were collected from the well sites. The nine variables
and their measured ranges were hydraulic gradient,
0.0006–0.0053; hydraulic conductivity, 1.5–45.4 m
(5–149 ft) per day; specific discharge, 0.004–0.091 m
(0.0128–0.2998 ft) per day; depth to water, 0.91–76 m
(3–250 ft); well depth, 12–168 m (40–550 ft); annual
precipitation, 30–100 cm (12.0–39.3 in.); soil permeability,
1.9–23 cm (0.76–9.0 in.); irrigation-well density,
0–8 irrigation wells per 2.59 km² (1 square mile); and
annual nitrogen fertilizer use, 0–118 kg (0–260 lb) of
nitrogen per acre. Macropore flow is listed in percent,
average per study area based on determinations from
dye studies. In this instance, macropore flow is used to
also entail preferential flow paths. Nitrate concentrations
ranged from 0.1 to 45 mgL⁻¹. Triazine-herbicide concentrations
were detected in samples from five of the six
study areas in concentrations ranging from 0.1 to
2.3 μL⁻¹. Analysis indicated that there were significant
differences in nitrate concentrations (averages-at 95 %
confidence level using Kendall Test) among the six
study areas; no significant differences in triazineherbicide
concentrations were found. Concentrations
of nitrate and triazine herbicide were determined (using
contingency-table analysis), to be significantly larger in
more intensively irrigated areas compared to less intensively
irrigated areas. Preliminary correlations with the
independent variables and nitrate concentrations indicated
significant relations at the 95%confidence level with
variables hydraulic conductivity, well depth, and irrigation
well density. Correlations with triazine-herbicide
concentrations indicated significant relations with hydraulic
conductivity, specific discharge, well depth, annual
precipitation, and irrigation well density, as well as
nitrate concentrations. Simple multiple-regression technique
indicated that well depth and density and fertilizer
use explained about 51 % of the variation in nitrate
concentrations. Specific discharge and well depth explained
about 60 % of the variation in triazine-herbicide
concentrations. Macropore flow paths and specific discharge
explained 84 % of the total variation in triazineherbicide
concentrations. The use of trade names in this
report is for identification purposes only and does not
constitute endorsement by the U.S. Geological Survey.