Analysis of nitrate (NO3-N) concentration trends in 25 ground-water-quality management areas, Idaho, 1961-2001

In Idaho, drinking-water supplies are pumped
from relatively shallow ground-water zones where
water quality has great potential for degradation
by land- and water-use activities. One indicator of
water quality, and one of the most widespread contaminants
in Idaho ground water related to land
and water uses, is dissolved nitrate.
In December 2000, the U.S. Geological Survey,
in cooperation with the Idaho Department of
Environmental Quality, began a study to compile
and assess nitrate data for ground water in 25
ground-water-quality management areas in Idaho.
The primary objective of the study was to determine
whether statistically significant trends in
ground-water concentrations were discernible. Data
included ground-water analyses but not analyses
of water from springs, drains, or thermal water
sources. A total of 8,465 nitrate analyses were
compiled from 2,931 wells in the 25 priority areas;
analyses dates ranged from June 1961 to February
2001.
A time-period comparison was used to assess
general trends in nitrate concentrations within
individual priority areas. A time-series comparison
was used to assess trends in nitrate concentrations
from selected wells in each priority area.
In time-period assessments, general trends in
an area were evaluated by compiling nitrate data
for selected time intervals to determine whether
the population distributions were significantly different
between one period and another. Data within
each priority area were sorted into decades—
1970s, 1980s, and 1990s—for long-term trend
assessment. The 1990s data also were divided into
sets of selected years corresponding to Statewide
Ambient Ground-Water Quality Program sampling
cycles—1991 through 1994, 1995 through 1998,
and a partial cycle, 1999 through 2000—for shortterm
trend assessment. Data were analyzed by
summary statistics, boxplots, and the Mann-
Whitney statistical test.
Long-term increasing trends in nitrate concentrations
were evident for 6 of 25 priority areas, and
long-term decreasing trends were evident for 4 of
25 areas. Short-term increasing trends were evident
for 7 of 25 areas, and a short-term decreasing trend
was evident for only 1 area. No long-term nitrate
trends were evident for 7 of 25 areas, and no shortterm
trends were evident for 15 of 25 areas. Data
were insufficient for long-term trend assessment in
8 areas and for short-term assessment in 2 areas.
Time-series trend analyses were conducted on
data from wells with 7 or more nitrate analyses
and longest periods of record, at least 10 years
between oldest and most recent analyses. Because
long-term records were available for so few wells
and well construction data were not available for
several of these wells, time-series analyses were
not helpful to the nitrate trend assessment study.
Trend results may be strongly affected by well
construction, hydrogeologic environments, and
changes in density and areal distribution of wells
and analyses. The utility of nitrate trend assessments
in priority areas would be improved by more
consistent and specific well location descriptions
between agencies; well construction and major
water-yielding zone information to accompany the
water-quality data for each well; and addition of
historical nitrate data to data bases, particularly
analyses prior to about 1990. Investigations of the
possible effects of changing priority area boundaries
and time periods on both long-term and short-term trend assessments are needed. Addition of
these kinds of information would allow assessment
of trends associated with hydrology and
geology of each area and would provide a much
stronger basis for trend assessment than currently
possible.