MANAGED AQUIFER RECHARGE OF THE MISSISSIPPI RIVER VALLEY ALLUVIAL AQUIFER USING ON-FARM INFILTRATION GALLERIES

Arkansas ranks third in the nation for irrigated land, with irrigation water being primarily sourced from the Mississippi River Valley alluvial (MRVA) aquifer. Groundwater level declines have created two primary cones of depression in the MRVA, which has led to the designation of critical groundwater areas. Managed aquifer recharge (MAR) refers to a suite of methods used to intentionally replenish groundwater, with a focus on increasing availability while sustaining quality. This pilot study evaluates the early stages of two infiltration galleries (IG) and their capacity to recharge the MRVA aquifer in the Cache River Critical Groundwater Area (CRCGA) of northeast Arkansas. IGs are gravel-filled trenches excavated to the sand layer of the upper MRVA aquifer, with subsurface plumbing connected to a nearby on-farm reservoir as a recharge source. IG construction began in November 2020 with completion in April 2021. Initial infiltration rates within the upper MRVA aquifer were measured using the Encased Falling Head Procedure and ranged from 15.4 to 76.5 cm hr^-1. Soil samples were also taken at various depths to describe soil texture and permeability. The water distribution system was completed in January 2022, which included submersible water pumps powered by deep cycle marine batteries connected to a solar power source. Initial IG operation occurred from February 10, 2022 - April 30, 2022, with cyclic injection periods. The system had a mean daily pumping rate of 212.6 m³ d^-1, and an estimated 1.93 megaliters (1.57 acre-foot) were injected during this initial period. Physically, there were no issues with the IGs and the MRVA aquifer accepting water. Infiltration rates of the flux of water leaving the bottom of the IGs were quantified and will be presented in the context of infiltration capacity. Recharge water movement was captured by electrical resistivity (ERT) measurements, as increasing saturation reduces resistivity. Resistivity in the unsaturated zone was reduced by an order of magnitude as reservoir water was injected. Continuous groundwater data were collected, and water table mounding was not differentiable from spring recharge recovery. These initial findings support IGs as a MAR strategy to address groundwater declines in the CRCGA of northeast Arkansas.