INVESTIGATING HETEROGENEITY OF HYDRAULIC CONDUCTIVITY AND ITS INFLUENCE ON GROUNDWATER DYNAMICS WITHIN A SATURATED RIPARIAN BUFFER IN CENTRAL ILLINOIS

The use of Saturated Riparian Buffers (SRBs) in agricultural settings as a means of reducing contamination of surface and groundwater resources has gained popularity due to their trapping ability and solute removal (nutrient recycling) capacity. In light of SRB’s nutrient-trapping and removal ability, numerous studies have been conducted to characterize the effectiveness of SRBs. However, most studies on SRBs have paid minimal attention to how heterogeneity of hydraulic conductivity (K) contributes to the effectiveness of SRBs. Heterogeneity of K has been proven to be, if not the most significant, one of the contributing factors to the effectiveness of SRBs since heterogeneity in K controls water fluxes and solute trapping efficiency. This study investigates the heterogeneity in horizontal K and estimates the vertical groundwater specific discharge (q_v) between the upper weathered and its underlying unweathered glacial units of an SRB, adjacent to a tile-drained agricultural farm field in central Illinois. The glacial deposits at the study area are of the Tiskilwa Formation and are distinguished into an upper weathered clay, which becomes coarser and poorly sorted with depth, and an underlying unweathered diamicton. The diamicton comprises a clay dominated matrix with coarser sediment resulting in poorly sorted sediments. For 23 wells, geometric mean K values were calculated after reducing multiple slug test results using the Hvorslev (1951) method. From the preliminary analysis, mean K values from individual wells ranged from 1.92 x 10^-4 m/s to 7.63 x 10^-6 m/s within the weathered diamicton; whereas, the unweathered diamicton had K values as low as 4.79 x 10^-9 m/s. q_v was computed as the product of the average vertical hydraulic conductivity (K_V) and vertical hydraulic gradients (i_z) between depths of 1.5 and 4.6m as these depths represent locations in the weathered and unweathered diamicton, respectively. K_V values used were inferred from the horizontal K reduced from the slug test but as an order of magnitude lower. Typical i_z values ranged from 0.03 to 0.1 from one-time measurements. Groundwater flow direction was downwards except in a few areas where groundwater flowed up. The q_v ranged from 1.25 x 10^-5 m/s to 4.30 x 10^-7 m/s where areas with high K had higher q_v. In conclusion, the study area exhibits heterogeneity, depicted by the variation of K over orders of magnitude.