Managed aquifer recharge using a borrow pit in connection with the Mississippi River Valley alluvial aquifer in northeastern Arkansas

Abstract

The Mississippi River Valley alluvial aquifer (MRVAA) is one of the overexploited aquifers in the United States. Agriculture in Arkansas relies significantly on the MRVAA for irrigation, due to its accessibility and high yield. Increased irrigation demand since the early 1900s with continued expansion and inequitable recharge contributions resulted in groundwater decline. Overdraft of the MRVAA in Arkansas has resulted in the designation of critical groundwater areas. Managed aquifer recharge (MAR) methods intentionally replenish stressed groundwater resources. A MAR case study was conducted to determine whether infiltration basins, as repurposed borrow pits, could be used to enhance groundwater decline in critical groundwater areas of northeast Arkansas. This rehabilitation would be a practical solution to alleviate groundwater decline as well as economically feasible as land would not need to be taken out of production. In 2015, the Arkansas Department of Transportation contracted sand excavation of fallow land owned by a collaborating producer. This borrow pit would serve as a test case to measure infiltration rates into the MRVAA using nearby surface water as the recharge source. Initial soil core analyses revealed soil properties within the confining clay layer of red-brown clay and silty clay soils (0 to 3.7 m deep) with sand below. Excavation completed to a depth of ~6 m exposed the uppermost-unsaturated section of the alluvial aquifer, consisting of well-sorted medium grain size sand. The borrow pit floor was ~27 m above the existing water table, and it was hypothesized that this exposed unsaturated aquifer section would provide a natural filter and an avenue for increased water storage underground. Sediment samples were collected from the pit floor and sidewall pre- and postexperiment to characterize particle size, textural class, and organic matter. Submersible pressure transducers were installed within the pit and in a nearby irrigation well to monitor water level changes. Meteorological data were collected on-site to measure the water budget components of precipitation and evaporation. Water level declines and infiltration were evident throughout the experiment. An initial infiltration rate of 192 mm d⁻¹ was measured in February of 2016 that decreased until March, with steady state rates of 4.43 to 136 mm d⁻¹ that varied until June. An overall integrated infiltration rate of 36.4 mm d⁻¹ was calculated from the water budget. Total subsurface storage increased by 9.3 ML from February to June of 2016, and a two-dimensional simulation predicted a maximum groundwater mounding of 2.6 m during the experiment. Additionally, 14 borrow pits that had not been repurposed were identified in the area using remote sensing. Results of this study demonstrate that a relatively inexpensive MAR strategy could be implemented using former borrow pits repurposed as infiltration basins to alleviate groundwater decline in a critical groundwater area of northeastern Arkansas.