Water Quality in a Wet Meadow, Platte River Valley, Central Nebraska
By P.J. Emmons
The Platte River Valley in Nebraska, and in particular the reach from
Kearney to Grand Island, is an extremely important natural habitat area.
Over 300 migratory bird species, including several threatened and
endangered species, have been observed along the Platte River. In the
spring, nearly 500,000 sandhill cranes, along with millions of ducks and
geese, use this reach as a staging and feeding area during their northerly
migration. Wet meadows (grasslands which have waterlogged soils much of
the year) are a critical part of this migratory-bird habitat. However, the
area of wet meadows between Kearney and Grand Island has declined nearly 50
percent due to the activities of man. The condition of the remaining wet
meadows is of vital importance.
The U.S. Geological Survey's National Water-Quality Assessment (NAWQA)
Program is designed to describe the status and trends in the quality of the
Nation's surface water and groundwater resources and to provide a sound
understanding of the natural and human factors that affect the quality of
these resources. A study of the groundwater beneath a selected wet meadow
was undertaken as part of the Central Nebraska Basins NAWQA study unit.
Observation wells were installed in the wet meadow at various distances
along two transects downgradient from the edge of a corn field. One to
five wells completed at depths of about 15 to 100 feet were located at each
of the 5 sites. The wells were completed in the Platte River alluvium or
the underlying Ogallala Formation. The depth to the water table ranges
from 0 to 5 feet below land surface. The general direction of groundwater
flow is parallel to flow in the Platte River. Selected wells were sampled
in February, March, June, and December 1994 for major cations and anions,
nutrients, and organonitrogen herbicides.
Pesticides and fertilizers are used extensively in Nebraska to enhance the
production of row crops. Some of these pesticides and fertilizers have
migrated into the groundwater. Atrazine was detected in water from all of
the wells sampled in February and June and most of the wells sampled at
other times, but only in concentrations of 0.1 to 0.6 micrograms per liter.
Concentrations of the other pesticides analyzed, including alachlor,
cyanazine, and metolachlor, were at or below the detection limit of 0.05
micrograms per liter. The highest concentrations of nitrate were found in
water from the shallow wells (about 15 feet deep). The concentrations of
nitrate as nitrogen in water from these wells ranged from 5 to 13
milligrams per liter in June.
Concentrations of major cations and anions decreased and their ratios
varied with depth. The major cations were calcium and sodium, and the
major anions were sulfate and bicarbonate. Water from the shallowest wells
was a mixed calcium sodium sulfate type, whereas the deepest
alluvial-aquifer water was a calcium sulfate type. The water from the
Ogallala Formation was a calcium bicarbonate type. The variability of the
groundwater quality reflects seasonal changes in recharge to and
evaporation from the alluvial aquifer and rates of movement and mixing
within and between the aquifers.
Emmons, P.J., 1996, Water quality in a wet meadow, Platte River Valley,
Central Nebraska: U.S. Geological Survey Fact Sheet FS-097-96, 4 p.