A NEW DIRECT-PUSH-BASED APPROACH FOR THE CHEMICAL INVESTIGATION OF STREAM-AQUIFER INTERACTIONS

Stream-aquifer interactions are often indicated by contrasts in groundwater chemistry. For example, water quality in aquifers adjacent to the Arkansas River in Kansas is impacted by the intrusion of saline river water. In order to evaluate stream-aquifer relationships, we have developed a new direct-push-based approach for high-resolution, spatial characterization of chemical variability in unconsolidated aquifers. Conventional direct-push sampling methods were modified to allow for accurate field measurement of sensitive parameters (DO, pH, and ORP) and other constituents of interest(specific conductance, Cl, SO4, NO3). The approach incorporates a direct-push sampling tool configured with a stainless-steel screen that is attached to polyethylene tubing and is isolated from the push rods during sampling. Chemical profiles are obtained as the tool is advanced to depths of interest and sampled using low-flow sampling methods. Measurement of field parameters is conducted using a flow-through cell and a multi-parameter sonde. The approach was tested in an 11-m thick alluvial aquifer where large vertical contrasts in groundwater chemistry are known to occur. Comparison of direct-push samples collected on a 0.61-m interval and those simultaneously collected from multi-level samplers finished at comparable depths showed excellent agreement for all parameters, and demonstrated the method's ability to resolve steep chemical gradients. This direct-push-based approach allows for fine-scale determination of the lateral and vertical extent of chemical indicators of stream-aquifer interactions without the need for preexisting monitoring wells. The approach is being used in conjunction with direct-push electrical conductivity logging to rapidly delineate the zone of river-water intrusion at a field site along the Arkansas River.