An Examination of Geophysical Tools Used in Guiding Assessment of Deep Groundwater – Alluvial versus Fluvial

Past industrial activities within the Rialto-Colton Basin in southern California have resulted in deep perchlorate groundwater impacts in alluvial basin-fill sediments. These sediments are alluvial fan deposits derived from the adjacent San Gabriel Mountains and host internal basin faults that offset groundwater potentiometric surfaces, indicative of a complex flow field which complicates assessment, which in turn necessitates support using geophysical tools. These deposits host groundwater at approximately 400 feet below ground surface, with the base of the flow system at approximately 800 feet. Assessment techniques involve installing well borings using mud rotary techniques, followed by geophysical runs (electric, resistance, gamma) down the borehole. The logs are correlated with the geologist log to identify discrete horizons with the presumed highest contaminant flow velocities, whereupon either a multi-port well or a multiple well cluster within the same borehole is installed to collect data that supports assessment and groundwater modeling. High gamma counts and resistivity generally equate to more permeable horizons due to the granitic component of the sands, and to the inflow of low total dissolved solids in groundwater. Similar assessment and geophysical techniques are used in contamination assessment of deep groundwater in the Long Island Magothy aquifer system, a mosaic of interbedded fluvial deposits. In contrast to the Rialto-Colton Basin example, high gamma counts in fluvial deposits of the Magothy aquifer system tend to correlate to finer-grained material and low permeability.