GEOCHEMICAL ASSESSMENT OF MANAGED AQUIFER RECHARGE ON GROUNDWATER CHEMISTRY IN NORTHEASTERN ARKANSAS

Farms throughout eastern Arkansas face groundwater supply issues as the Mississippi River Valley Alluvial aquifer declines due to overdraft. Many of these critical groundwater areas have soils with high clay and silt content that limits infiltration and therefore prevents groundwater recharge. Passive managed aquifer recharge (MAR) is a promising option to address the issue. A pilot test is occurring in northeast Arkansas that includes water being pumped from a pre-existing on-farm reservoir into gravel infiltration galleries. This allows source water to bypass the surface clay layers and infiltrate through several dozen meters of unsaturated sands and gravels, which aids in natural filtration. However, water quality is a major concern, and care must be taken to avoid impairing the remaining groundwater supply either by mobilizing geogenic contaminants in the subsurface or introducing new ones from the source water. Water samples of the reservoir source water and three monitoring wells were taken over two three-month injection seasons in the spring of 2022 and 2023 and analyzed for physiochemical parameters and select nutrients and trace metals. Preliminary analyses of the 2022 injection season reported a significant increase in groundwater concentrations of iron (+39.2%, p = 0.014), chloride (+48.9%, p <0.001), and sulfate (+28.4%, p = 0.036) compared to background concentrations. These were not found in high concentrations in the source water and are likely being mobilized in the unsaturated zone. A significant decrease in bicarbonate (-29.7%, p = 0.008), potassium (-49.1%, p = 0.025), and sodium (-38.9%, 0.044) compared to background concentrations was also observed. Further modeling will be conducted to understand if decreases are explained by adsorption, precipitation, or gas exchange. No significant change was observed in arsenic. Initial geochemical modeling and mineralogical analyses of MRVA sediment indicate the presence and precipitation of Fe(III) and Mn (oxyhydr)oxides are key phases to enhance the removal of harmful oxyanions such as As. These results are encouraging and could point to the suitability of additional MAR projects in Arkansas. Additional results will be presented from the 2023 injection season, and whether they support the initial injection season trends or add new implications.