| Abstract: | A useful tool for identifying the temporal and spatial ambient wellbore flow relationships near a dynamic river boundary is to monitor ambient vertical wellbore flow with an electromagnetic borehole flowmeter. This is important because the presence of the wellbore can result in significant mixing or exchange of groundwater vertically across the aquifer. Mixing or exchanging groundwater within the well-screen section can have significant impacts on the distribution of contaminants within the aquifer and adverse effects on the representativeness of groundwater samples collected from the monitoring well. Ambient monitoring data, collected from long screened wells at Hanford’s 300-Area Integrated Field Research Challenge site, located approximately 260 m from the Columbia River, demonstrate that vertical wellbore flow exhibits both a positive and inverse temporal relationship with periodic river-stage fluctuations that can change over short distances between wells. The spatial distribution of these vertical flows across the well field indicates two general regions of ambient wellbore flow behavior. The western region of the site is characterized by vertical flows that are positively related to river-stage fluctuations. In contrast, the eastern region of the site exhibits vertical flows that are inversely related to river-stage fluctuations. The cause of this opposite relationship is not completely understood; however, the positive relationships appear to be associated with high-energy Hanford formation flood deposits. These flood deposits have a well-defined northwest-southeast trend and are believed to coincide with a local paleochannel. The inverse relationships are attributed to an erosional, subsurface high in the Hanford/Ringold Formation contact between the site and the Columbia River. Under these complex hydrogeologic and hydrodynamic conditions, the behavior of ambient vertical wellbore flow in monitoring wells near a dynamic river boundary can have important implications for collecting groundwater-quality samples, for contributing to contaminant distribution within an aquifer system, and for implementing effective remediation strategies. |
