1999 GSA Annual Meeting -- Denver, Colorado
1999 GSA Annual Meeting -- Denver, Colorado
Author(s): REBOULET,Edward C., ecr@cgiss.boisestate.edu; and BARRASH, Warren, Center for the Geophysical Investigation of the Shallow Subsurface, Boise State University, Boise, ID 83725
Keywords: Permeability, Grain-size, Alluvial-aquifer, Facies, Heterogeneity
The three-dimensional (3D) distribution of permeability within alluvial aquifers is a primary factor controlling groundwater flow and contaminant transport. The Boise Hydrogeophysical Research Site (BHRS) is a research wellfield in a shallow, coarse, alluvial aquifer that is being developed to support the overall goal of developing methods to map 3D permeability distributions by combining geophysical and hydrologic data. At the BHRS, 18 wells were cored with a split spoon through ~18 m of unconsolidated, cobble-and-sand deposits which are underlain by very thin basalt and a red clay. These wells are being used to thoroughly characterize the BHRS with a wide variety of single-well, cross-hole and multi-well geophysical and hydrologic methods. Coring of the wells at the BHRS yielded >83% recovery including sand matrix with whole and truncated cobbles collected largely in place. Initially the core is photographed and a length-per-unit-length transect is taken that provides a first order approximation of relative cobble volume fraction. The core is then divided into <0.3m sample intervals based on estimated cobble and matrix volume fractions, and lithologic characteristics. Samples are separated into cobble and matrix portions for grain size distribution (GSD) analysis by standard sieve and hydrometer methods, and hand analysis to reconstruct broken cobbles. Four main types of GSDs have been recognized from ~600 samples analyzed to date (~40% of the recovered core). These four 'Type GSDs' are: (1) well sorted sand, (2) bimodal cobble and sand, (3) cobble-dominated GSD with poorly sorted matrix, and (4) very poorly sorted cobble, gravel and sand with an almost linear decrease of weight percent with decreasing log grain size. These four 'Type GSDs' have been matched with GSDs of samples collected in similar coarse braided-stream deposits at a quarry and outcrop in the vicinity of the BHRS. Current directions of investigation include multivariate analysis of grain size and other parameters (e.g., porosity from neutron logs) for (1) better definition of the hydrostratigraphic facies and their 3D distribution (as permeability data becomes available), and (2) for correlation with geophysical parameter distributions.
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