Abstract
The U.S. Geological Survey, in cooperation with the
Papio-Missouri River Natural Resources District (PMRNRD),
conducted this study to map the water-level altitude of 2009
within the Elkhorn River Valley, Missouri River Valley, and
Platte River Valley alluvial aquifers; to present the predevelopment
potentiometric-surface altitude within the Dakota aquifer;
and to describe the age and quality of groundwater in the
five principal aquifers of the PMRNRD in eastern Nebraska
using data collected from 1992 to 2009. In addition, implications
of alternatives to the current PMRNRD groundwater-quality
monitoring approach are discussed.
In the PMRNRD, groundwater altitude, relative to National
Geodetic Vertical Datum of 1929, ranged from about 1,080 feet
(ft) to 1,180 ft in the Elkhorn River Valley alluvial aquifer and
from about 960 ft to 1,080 ft in the Missouri River Valley and
Platte River Valley alluvial aquifers. In the PMRNRD, the
estimated altitude of the potentiometric surface of the Dakota
aquifer, predevelopment, ranged from about 1,100 ft to 1,200 ft.
To assess groundwater age and quality, groundwater
samples were collected from a total of 217 wells from 1992 to
2009 for analysis of various analytes. Groundwater samples
collected in the PMRNRD from 1992 to 2009 and interpreted
in this report were analyzed for age-dating analytes (chlorofluorocarbons),
dissolved gases, major ions, trace elements, nutrients,
stable isotope ratios, pesticides and pesticide degradates,
volatile organic compounds, explosives, and 222radon.
Apparent groundwater age was estimated from concentrations
of chlorofluorocarbons measured in samples collected in
2000. Apparent groundwater-recharge dates ranged from older
than 1940 in samples from wells screened in the Missouri
River Valley alluvial aquifer to the early 1980s in samples
from wells screened in the Dakota aquifer.
Concentrations of major ions in the most recent sample
per well collected from 1992 to 2009 indicate that the predominant
water type was calcium bicarbonate. Samples from
4 wells exceeded the U.S. Environmental Protection Agency
(USEPA) Secondary Drinking Water Regulation (SDWR) for
sulfate [250 milligrams per liter (mg/L)], and samples from
4 wells exceeded the USEPA Drinking Water Advisory Table
for sodium (30–60 mg/L).
Eighteen of the 21 trace elements analyzed in samples
from PMRNRD wells have USEPA drinking-water standards.
Sixteen of the trace elements with USEPA standards were
detected in the selected samples. In the samples selected for
trace-element analysis, the only trace-element concentration
that exceeded an enforceable USEPA drinking-water standard,
the Maximum Contaminant Level (MCL), was for arsenic;
arsenic concentration exceeded the USEPA MCL of 10 micrograms
per liter (μg/L) in 4 percent of the samples. Trace-element
concentrations that exceeded the USEPA SDWR or
Lifetime Health Advisory level were iron (46 percent of the
samples were greater than USEPA SDWR of 300 μg/L), manganese
(70 percent of the samples were greater than USEPA
SDWR of 50 μg/L), and strontium (4 percent of the samples
were greater than USEPA Lifetime Health Advisory level of
4,000 μg/L).
The concentration of nitrate plus nitrite as nitrogen
(nitrate-N) in the most recent nutrient samples collected from
the network wells and from one randomly selected well in
the well nests from 1992 to 2009 for most wells (80 percent)
ranged from less than 0.06 to 8.55 mg/L, with a median value
of 0.12 mg/L. Concentrations of nitrate-N in 13 (7 percent)
nutrient samples, 1992 to 2009, were greater than or equal to
the USEPA MCL and Nebraska Title-118 standard of 10 mg/L,
and concentrations of nitrate-N in 35 (18 percent) nutrient samples,
1992 to 2009, were greater than or equal to 5 mg/L, which
is the PMRNRD action level for possible management implementation
to reduce nitrate concentrations in groundwater.
Of the 61 pesticides or pesticide degradates analyzed
from 2007 to 2009, 21 were detected. Three of the 21 pesticides
detected (alachlor, atrazine, and metolachlor) have established
health-based criteria; all detections of these compounds
were at concentrations less than their USEPA standards. From
2007 to 2009, 1 or more pesticide compounds were detected
in 16 of the 82 network wells and in 18 of the 26 wells in well
nests. From 2007 to 2009, the individual pesticide compounds
that were detected most frequently were alachlor ethane
sulfonic acid, a degradate of alachlor; deethylcyanazine acid, a
degradate of cyanazine; and atrazine.
Analytes with concentrations that exceeded 30 percent
of the applicable Nebraska Title-118 standard were identified
so that the PMRNRD can plan to monitor groundwater
in the area and consider possible actions should the analyte
concentrations continue to rise. The analytical results from the
most recent samples collected in the network wells and all the
wells in well nests from 1992 to 2009 indicate that, in at least
1 sample, there was a concentration that exceeded 30 percent
of the Nebraska Title-118 standard for at least 1 of 3 major
ions (chloride, fluoride, and sulfate), 1 nutrient (nitrate-N),
1 pesticide (atrazine), or 3 trace elements (arsenic, iron, and
manganese). In addition, 30 percent of the USEPA MCL or
Nebraska Title-118 standard for gross alpha activity likely was
exceeded in samples from three wells screened in the Dakota
aquifer. Study findings indicate that some alternatives to the
current PMRNRD groundwater-sampling approach that could
be considered are to collect fewer samples for nutrient analysis
and to collect samples periodically for determining concentrations
of additional analytes, particularly the analytes with
concentrations that were at least 30 percent or more than the
Nebraska Title-118 standard.