PNNL

Abstract

Previously published field investigations and modeling studies have demonstrated the potential for sample bias associated with vertical wellbore flow in conventional monitoring wells constructed with long-screened intervals. This paper builds on the existing body of literature by 1) demonstrating the utility of continuous (i.e., hourly measurements for ~ one month) ambient wellbore flow monitoring and 2) presenting results from a field experiment where relatively large wellbore flows (up to 4 LPM) were induced by aquifer hydrodynamics associated with a fluctuating river boundary located approximately 250 m from the test well. The observed vertical wellbore flows were strongly correlated with fluctuations in river stage, alternating between upward and downward flow throughout the monitoring period in response to changes in river stage. Continuous monitoring of ambient wellbore flows using an Electromagnetic Borehole Flowmeter (EBF) allowed these effects to be evaluated in concert with continuously monitored river-stage elevations (hourly) and aqueous uranium concentrations (daily) in a long-screen well and an adjacent multi-level well cluster. This study demonstrates that when contaminant concentrations within the aquifer vary significantly over the depth interval interrogated, even for screen lengths as small as 7.6 m, river-induced vertical wellbore flow can result in variations in measured concentration that nearly encompass the full range of variation in aquifer contaminant concentration with depth. An approach to mitigate these effects based on increasing hydraulic resistance within the wellbore by installing an inflatable packer was evaluated and was demonstrated to reduce vertical wellbore flows.