River-induced flow dynamics in long-screen wells and impact on aqueous samples
Vince R Vermeul1, James P McKinley, Darrell R Newcomer
1Pacific Northwest National Laboratory, P.O. Box 999, MS K6-96, Richland, WA 99352, USA. vince.vermeul@pnl.gov
Ground Water
|November 20, 2010
Summary
Continuous monitoring of wellbore flow reveals how river stage changes induce vertical flow, potentially biasing contaminant concentration measurements in long-screened wells. This highlights the need for careful interpretation of groundwater data.
Area of Science:
- Environmental Science
- Hydrogeology
- Geophysics
Background:
- Vertical wellbore flow in conventional monitoring wells with long screened intervals can lead to sample bias.
- Aquifer hydrodynamics, particularly near fluctuating river boundaries, can induce significant wellbore flows.
Purpose of the Study:
- To demonstrate the utility of continuous ambient wellbore flow monitoring.
- To present field experiment results on river-induced wellbore flows and their impact on contaminant concentration measurements.
Main Methods:
- Continuous (hourly) ambient wellbore flow monitoring using an electromagnetic borehole flowmeter.
- Field experiment with induced wellbore flows (up to 4 L/min) near a fluctuating river boundary.
- Concurrent monitoring of river stage, wellbore flow, and aqueous uranium concentrations in long-screen and multilevel wells.
Main Results:
- Observed vertical wellbore flows strongly correlated with river stage fluctuations, alternating direction.
- River-induced flow significantly impacted uranium concentration measurements in the long-screen well.
- Measured concentrations in the long-screen well nearly encompassed the full range of aquifer concentration variation with depth.
Conclusions:
- Continuous wellbore flow monitoring is valuable for understanding flow dynamics.
- River-induced vertical wellbore flow can cause substantial bias in contaminant concentration data from long-screened wells.
- Careful consideration of wellbore flow is crucial for accurate groundwater contamination assessment.
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