WATER QUALITY ASSESSMENT OF MANAGED AQUIFER RECHARGE BY INFILTRATION GALLERIES IN NORTHEASTERN ARKANSAS

Agriculture in eastern Arkansas faces groundwater supply issues as the Mississippi River Valley Alluvial aquifer declines from overdraft. The critical groundwater areas have soils with high clay and silt content that limit infiltration and prevent groundwater recharge. Passive managed aquifer recharge (MAR) is a promising option to address the issue. A pilot test in northeast Arkansas includes source water being pumped from a pre-existing on-farm reservoir into two buried infiltration galleries. This allows the source water to bypass the surface clay layers and infiltrate through the gallery gravel fill and then through 30m of unsaturated sands and gravels. Groundwater quality is a concern, and care must be taken to avoid the mobilization of geogenic contaminants and the introduction of new contaminants. Water samples of the source water, five subsurface lysimeters, and three monitoring wells were taken over three injection seasons from spring 2022 to spring 2024. These samples were analyzed for physiochemical parameters and select nutrients and trace metals. Groundwater concentration changes were characterized as a statistical increase or decrease in mean concentration between the pre-injection period, January 2020 – February 2022, and the post-injection period, March 2022 – March 2024. Inconsistent and various cations in each monitoring well increased compared to the pre-injection concentrations. Groundwater Mn and K concentrations decreased significantly by 48% in each monitoring well along with other constituent decreases in wells 2 and 3. Source water concentrations of K, total organic carbon, dissolved organic carbon, and pH were measured at higher mean concentrations with all other constituents having lower mean concentrations compared to the groundwater. No significant change was observed in As. Lysimeter mean concentrations were similar to source water concentrations, except some geogenic constituents and N cycling analytes were detected. Initial geochemical modeling points to hematite as the primary Fe phase and Mn²⁺ as the primary Mn phase. Mineralogical analyses of MRVA sediment indicate that Fe(III) and Mn (oxyhydr)oxides are key phases to enhance the removal of harmful oxyanions. These results are encouraging and could point to the suitability of additional MAR projects in Arkansas.