DETERMINING DISTRIBUTIONS OF HYDROLOGIC AND GEOPHYSICAL PARAMETERS IN A HETEROGENEOUS FLUVIAL AQUIFER, BOISE HYDROGEOPHYSICAL RESEARCH SITE

The Boise Hydrogeophysical Research Site (BHRS) is a wellfield designed to support hydrologic and geophysical research with the near-term goal of developing methods for mapping three-dimensional (3D) heterogeneous distributions of permeability in coarse-grained sedimentary aquifers by combining geophysical data with hydrologic data. The approach for meeting this goal is to thoroughly characterize a field-scale control volume/test cell in a natural heterogeneous fluvial aquifer where the 3D distributions of hydrologic, geologic, and geophysical parameters – and relationships between them – are being determined, and then to develop methods to jointly invert geophysical and hydrologic data for the permeability distribution.

The BHRS is located on a gravel bar adjacent to the Boise River; the aquifer at the BHRS is shallow and unconfined. Eighteen wells were cored through 18-21 m of unconsolidated, coarse (cobble and sand) fluvial deposits and completed into the underlying red clay. The wells and the wellfield were designed to permit a wide range of hydrologic and geophysical testing and to capture short-range geostatistical information. In addition to providing local well density for single-well logging and profiling, the wellfield provides numerous well-pair transects for crosshole tomography, and provides overlapping volumes for multiwell/multizone hydraulic and tracer tests. Results to date from numerous methods of investigation indicate that the coarse sediments may be subdivided into five hydrostratigraphic units in the central portion of the BHRS, and that porosity and permeability vary within and between these units. Porosity data derived from neutron logs are highly correlated with radar and seismic data. Geostatistical analysis of porosity data is consistent with a three-level hierarchical system and leads to a multiscale, multifacies, generalized form of the variogram function. Recent experiments have tested new technologies for determining the 3D distribution of permeability (hydraulic tomography), and for combining tracer testing with time-lapse crosshole radar attenuation tomography to image the evolution and passage of an electrically conductive plume, and then to use these data along with chemistry and hydrologic data to model the heterogeneous distribution of hydraulic parameters in the investigated volume.