SOME OBSERVATIONS FROM USING A COLLOIDAL BORESCOPE IN BASALT AQUIFERS

A colloidal borescope is a down hole tool and laptop computer with software to capture and track particle movement. A direct measurement of ground water flow direction and velocity is possible if a flow zone is found and sufficient number of colloids are present.

Two sites will be discussed where ground water occurs in basalt/interbed aquifer systems. The first site is a perched aquifer; the downhole surveys were conducted in three open boreholes. The presence of an active irrigation ditch near the three wells and the spatial orientation of the three wells make it improbable that standard methods for estimating flow directions in the perched aquifer would work. Lateral ground water flow occurs near the contacts between basalt flows and away from the ditch.

The second site is a regional aquifer. The borescope was used in completed monitoring wells that have wire-wrapped continuous slot screens and sand packs. Previously ground water was assumed to flow toward the southwest. The borescope showed that flow directions were spatially variable and toward the south and southeast in the wells surveyed.

Two observations developed from the borescope surveys. First, one completed well did not appear to have an active flow zone. This suggests the completion interval was too short and may have been completed across a single basalt flow without intersecting a flow zone. A second observation is related to a TCE contamination problem that appears to be dominated by vapor phase partitioning to the aqueous phase at the interface with the aquifer. A monitoring well virtually under the TCE source area has consistently exhibited TCE concentrations near the detection limit; two more distal wells have exceeded the MCL. The flow zone in this well was identified near the bottom of the well and not near the “water table” where vapor phase partitioning to ground water can occur.