STUDY OF STREAM-AQUIFER INTERACTIONS IN THE PLATTE RIVER WATERSHED USING METHODS OF AQUIFER HYDRAULICS, SEDIMENTOLOGY, GEOPHYSICS, AND GEOSTATISTICS

Over the last years, the methodology for estimating pumping-induced stream depletion (SD) was subject to a considerable amount of theoretical work because of increased concerns over long-term consequences of groundwater development and climate change. In order to investigate the applicability of these models, an extensive four-component study has been carried out at a test site at the perennial Prairie Creek, in the Platte River watershed, Nebraska.

The test site is instrumented with a pumping well at a distance of about 55 m from the stream and a monitoring network of 26 permanent piezometers screened at different depths. Major aquifer hydraulics study included a series of pumping tests under drought, low flow, and high flow conditions in the summers of 2000 and 2001. 2D and 3D modeling, sensitivity analyses, groundwater temperature data and slug tests were used to delineate large-scale hydrostratigraphic features that are characteristic for the meandering stream environment at the site.

Further investigation of the hydraulic conductivity structure used small-scale constant-head injection tests in the streambed to obtain K data set from 456 points, in a 45m x 20m x 1.2m section of the streambed. K values ranged from 74 m/d to 0.15 m/d. Geostatistical analyses and 3D visualization of the data indicate the presence of a distinct high K feature closely resembling channel deposits along bends as observed in sedimentologic research (e.g. Bridge, 1977). Ground penetrating radar surveys confirm this pattern and suggest locations of scour surfaces.

These data on large-scale and small-scale heterogeneity were incorporated into SD models. Results of this first experimental study in the Platte River watershed indicate a significant degree of uncertainty in the SD predictions due to hydraulic parameters uncertainties. Approaches that reduce the uncertainty in SD estimates are discussed. Results have important practical ramifications for applications of hydraulic models for assessment of stream depletion under various conditions.