Alluvial aquifer response to regulated reservoir releases: Implications for groundwater-surface water interactions along regulated rivers in the Mississippi River Valley alluvial aquifer

Abstract:

Measuring groundwater level change near regulated rivers is needed for developing accurate conceptualization and numerical simulation of groundwater-surface interactions for alluvial aquifers. Direct, detailed observations along individual stream reaches can provide realistic bounds on parameterization and validation of generalized models. In the northwestern part of the Mississippi alluvial plain—a region of extensive agricultural activity supported by groundwater irrigation—the Black River flows across the underlying Mississippi River Valley alluvial aquifer (MRVAA) where prior work has presented a conflicting categorization of the river as either a gaining or losing stream. This study presents a comparative analysis of variations in river stage at the Black River near Poplar Bluff and groundwater level and temperature data from seven wells ranging from shallow (<7 m) to relatively deeper (>15 m) depths located both near (<100 m) and distal (>500 m) to the Black River. Analyses of these data reveal several points relevant to understanding the MRVAA and other alluvial-aquifer regulated-river settings: 1, local hydraulic gradient relations show the Black River as a gaining stream, although groundwater temperatures at higher river stage indicated hyporheic expansion of 20 m, but only at the furthest upstream site 2, groundwater level responses near the river were attenuated by 50 % along a 10 km reach validating prior numerical simulations predicting groundwater response to follow high-flow peak attenuation; and 3, groundwater-level variations are greatest in shallow wells near (<10 m) the river, but attenuated and lagged responses were still observed further (>300 m) from the river suggesting relatively high hydraulic diffusivities. These results demonstrated a strong connectivity between surface water and regional groundwater levels in the northwestern MRVAA. Moreover, this strong connectivity between groundwater and surface water is likely the case for other streams in MRVAA where groundwater levels are near the surface, but at-present, not validated by direct observations nor modeled. Our findings add evidence that longitudinal variations of surface water-groundwater exchange along regulated rivers can drive variable groundwater flow in alluvial aquifers both near and distal to rivers.