Design of Research Wellfield for Calibrating Geophysical Methods against Hydrologic Parameters

296 A wellfield (Boise Hydrogeophysical Research Site or BHRS) is being developed in Boise, Idaho, for hydrologic and geophysical research in a shallow, coarse (cobble-and-sand), alluvial aquifer. Our goal is to develop cost-effective methods for quantitatively characterizing the distribution of permeability in heterogeneous alluvial aquifers using hydrologic and geophysical techniques. Responses to surface geophysical techniques (e.g., seismic, radar, and transient electromagnetics) will be calibrated against a highly characterized volume (the wellfield) with determined three-dimensional distributions of geologic, hydrologic, and geophysical properties. Well coring and construction methods, and the well arrangement in the field, are designed to provide detailed control on lithology (facies distribution and sediment properties), and to support a variety of single-well tests (e.g., borehole geophysical logging, permeability logging, and vertical seismic and radar profiling), crosshole tests (e.g., seismic, radar, and resistivity tomography), and multiwell tests (e.g., pumping and tracer tests). The wellfield has a central well surrounded by two rings of six wells each, and five outer “boundary” wells. Wells are screened through the cobble-and-sand aquifer to a clay that underlies the BHRS at about 20 m depth. Permeability will be measured with the flowmeter and pumping method in each well, and with a series of tests where successive discrete intervals will be pumped in a given well while multiple zones will be monitored in surrounding wells. Wellfield dimensions (e.g., ~20 m diameter of central well cluster) and design specifics are based on experience at the nearby Capital Station remediation site in the same aquifer material, and are designed to provide detailed control on the variation of properties over a range of volume scales (<1 m to ~6000 m). A Monte Carlo search was used to refine the wellfield design to optimize well-pair distances and azimuths for determination of geostatistical parameters and anisotropy. DESIGN OF RESEARCH WELLFIELD FOR CALIBRATING GEOPHYSICAL METHODS AGAINST HYDROLOGIC PARAMETERS