Vertical profiles of streambed hydraulic conductivity determined using slug tests in central and western Nebraska

Many issues of water-resources management
rely on modeling of ground-water/surfacewater
interactions, and streambed hydraulic
conductivity is a key parameter controlling the
water fluxes across the stream/aquifer interface.
However, in central and western Nebraska, this
parameter is generally undefined. The U.S.
Geological Survey, in cooperation with the
Nebraska Platte River Cooperative Hydrology
Study Group, performed slug tests at 15 stream
sites in the Platte, Republican, and Little Blue
River watersheds to determine the hydraulic
conductivity of streambeds in central and western
Nebraska. Slug tests were completed at several
discrete depth intervals using pneumatic or
mechanical methods, and the water-level response
was monitored on site using a pressure transducer
and laptop computer. Responses were analyzed
using either the Bouwer and Rice or Springer and
Gelhar methods. Vertical profiles of hydraulic
conductivity with depth were developed and were
compared to available information on lithology.
The profiles and corresponding lithology
showed that different types of streambeds were
tested and suggested that some streambeds
display a large variability in hydraulic conductivity
with depth. In some cases, hydraulic
conductivity values associated with nonstreambed
materials could be identified from nearby
lithologic descriptions. Seven of 15 sites had
streambed values that ranged over more than
3 orders of magnitude, and that variability
increased significantly when the measurements
considered to be from nonstreambed materials
were included. Streambed profiles from the Platte
and South Platte River sites generally were more
homogeneous and of larger hydraulic conductivity
than the other sites. No restrictive layers
were detected at any of the streambed sites on the
main stems or the flood plains of the main stems
of their respective watersheds. Alternatively, the
profiles characterized by a restrictive streambed
layer at some depth below the streambed surface
were all from tributary sites out of the main-stem
flood plain. These profiles can be used to represent
the streambed hydraulic conductivity in
central and western Nebraska in various applications,
including modeling ground-water/surfacewater
interactions.