Identifying a mechanism for an infiltration threshold from the Sunflower River, MS to the underlying alluvial aquifer

Long-term groundwater level and stream stage measurements at a USGS coupled groundwater stream-gaging station located on the Sunflower River in Sunflower, MS show an apparent stage-threshold for infiltration to the underlying alluvial aquifer. This site is located near the center of a large regional groundwater cone of depression in the Mississippi River Valley alluvial aquifer. The USGS well (termed well 3 in this study) was thought to be completed in the regional shallow aquifer, though often recording anomalously high water levels relative to other wells in the region. The purpose of this research was to identify the responsible mechanism for the apparent stage-threshold for surface-groundwater communication. Two possible mechanisms were considered: (1) scour of infiltration-limiting fine-grained bottom sediments during high flow-rate events corresponding to higher stage, (2) and lateral infiltration at high stage into more permeable coarse grained sedimentary layers intersecting the stream channel at higher elevation.;A channel bed sediment survey was conducted over 100 km of the river at high stream stage. The stream bottom was composed of cohesive, fine-grained sediments, eliminating the first hypothesis as a viable mechanism. Entrainment of bottom sediments at the higher velocities did not expose coarser-grained, higher-conductivity bottom sediments. Cores were taken throughout the west bank of the river near the USGS well (well 3) along vertical and horizontal transects to measure variations in grain size. A more coarse grained layer (higher sand content) was identified at an elevation of 29.2-31.6 m (msl), consistent with the second hypothesis.;Two additional monitoring wells (wells 1 and 2) were installed 1 km upstream, 12 m deeper than well 3. Water levels from these wells were generally 7 m lower than in well 3, were more consistent with regional groundwater levels, and showed no response to short-term changes in stream stage. Well 3 appears to be screened within a perched aquifer which is in connection with the Sunflower River at high stream stage through coarse-grained layers intersecting the stream channel at the higher elevations. The two deeper monitoring wells are screened within the regional aquifer, with river-recharge limited to gradual drainage from the perched aquifer.;The results have important implications for groundwater assessment and management for the Delta region of Mississippi, especially concerning the role that streams play as potential sources or sinks for the Mississippi Valley Alluvial Aquifer.