Year Established: 2016 Start Date: 2016-03-01 End Date: 2017-02-28
Total Federal Funds: $24,697 Total Non-Federal Funds: $49,486
Principal Investigators: Gregg Davidson, Andrew OReilly
Abstract: Groundwater depletion in regional aquifers in many parts of the world is becoming an increasing concern. In the United States, the Mississippi River Valley Alluvial Aquifer (MRVAA) has been identified as one with some of the highest deficits. In the Delta region of northwestern Mississippi, agricultural irrigation has created a depression in the water table that has drawn water levels below area streams and lakes. Effective management of this groundwater resource requires a thorough knowledge of sources of recharge. Oxbow lakes are ubiquitous on the Mississippi River floodplain and hold water that is potentially available for recharge, but their interaction with the underlying aquifer is largely unknown and left unaccounted for in many groundwater models. It is generally assumed that the fine-grained deposits accumulating in the bottoms of oxbow lakes form plugs that effectively eliminate vertical exchange. Many oxbow lakes in this region support forested wetlands that either surround the lake, or in some cases, fully occupy the meander loop. Decaying root networks and fallen, buried tree limbs create preferential flow pathways through the fine-grained sediments, potentially allowing recharge to the underlying aquifer. Preliminary data from Sky Lake, just south of the area of greatest groundwater depletion, suggests that recharge from the lake may be significant. The proposed project will include installation of a series of monitoring wells inside and around the Sky Lake meander loop, each equipped with a data-logging pressure transducer to determine if a groundwater mound exists beneath the lake consistent with vertical recharge. Select wells will be outfitted with a vertical array of data-logging temperature probes as a secondary data set to identify seasonal patterns that support either vertical recharge or recharge from the Yazoo River that subsequently flows beneath the lake. Cuttings from the wells will be used to determine the local thickness of the regional surficial confining layer, and cores collected from the wetland will be used to determine the thickness of the oxbow sediments. Evidence of downward flow within the wetland sediment and the possible presence of preferential pathways will be investigated in a small plot (100 m2) inside the wetland. A suite of thermistors will be installed in pairs at depths of 30 and 60 cm. The thermistors will be connected to an existing multi-channel data logger mounted to the side of an elevated boardwalk to record temperature changes over time that can be used to calculate the flux of water at each site. Results from this work are expected to find immediate use in groundwater flow models developed and used by the USGS, and incorporated into policy decisions regarding impoundment and control of water in oxbow lakes.