EVALUATION OF A RECHARGE PIT/BASIN WITHIN THE MISSISSIPPI RIVER VALLEY ALLUVIAL AQUIFER IN NORTHEAST ARKANSAS

The Mississippi River Valley alluvial aquifer is an important irrigation water resource in northeast Arkansas. Groundwater usage rates from the aquifer increased over 930% from 1960 to 2010, mostly attributed to irrigation, with withdrawals exceeding recharge, mainly from precipitation. The alluvial aquifer is overlain in many locations by a confining layer of varying thickness, which represents one of the limitations in effectively recharging the aquifer with surface water. The Grand Prairie region (Lonoke, Prairie, and Arkansas counties) and the Cache region (west of Crowley’s Ridge sections of Craighead, Poinsett, and Cross Counties) are locations with the main cones of depression, coinciding with the largest rice producing counties in the state. A Poinsett County producer formed a contract with Arkansas Highway Transportation Department (AHTD) for sand excavation of unfarmed land. Prior to excavation, soil-core analyses by AHTD revealed soil types within the confining layer consisted of red-brown clay / silty clay soil (0 - 3.7 m) with sand below. Once excavation was complete to a depth of about 6 m, the uppermost-unsaturated section of the alluvial aquifer consisting of well-sorted medium grain size sand was exposed, above the water-table depth of 33 m. The excavation pit provided an opportunity to measure infiltration rates of the uppermost section of the alluvial aquifer. Surface water from a diversion canal was pumped approximately 1 km through plastic polypipe at a rate of 2.9 m³ min^-1 into the excavation pit for 24 hours. The excavation pit was instrumented with submersible pressure transducers and staff gages to monitor water levels within the pit. In addition, continuous monitoring of groundwater levels, atmospheric temperature, wind velocity, evaporation rate, and precipitation measurements were completed. Initial infiltration rates across 6 February 2016 to 7 March 2016 went from 251 mm d^-1 to 18 mm d^-1 with a 60% decrease in water level. Significant precipitation events increased the water level by 50% across 8-14 March 2016, resulting in a higher infiltration rate of 57 mm d^-1. Further information will be presented about experiment setup, infiltration rates, basin soil characteristics, groundwater storage inputs, and potential managed groundwater recharge efforts using an infiltration basin.