DRIVE POINT MULTILEVEL SYSTEMS FOR CHARACTERIZING ATTENUATION OF WASTEWATER DERIVED CONTAMINANTS IN A SMALL EFFLUENT DOMINATED STREAM

Groundwater surface-water interaction studies often use clusters of hand driven wells installed to different depths below the stream bed. This increases the materials required, the footprint and accumulation of debris on the monitoring infrastructure, and the disturbance to the natural system. Alternatively, commercially available engineered multilevel systems (MLSs) divide a single drilled borehole into multiple depth-discrete monitoring intervals. We field tested a drive point MLS that merges a hand driven well and an engineered MLS into a single system at two sites in a small effluent dominated stream incised into Holocene alluvium. Two- and four-port systems were driven to a maximum depth of 2 m below the stream bed to characterize vertical changes in head and concentrations of pharmaceuticals, pesticides, and per- and polyfluoroalkyl substances (PFAS). The MLS accommodates up to six ports and is composed of stainless steel that includes a tip, a plain 1.9 cm diameter threaded pipe, and ports with a 100-mesh screen and a tube running from the port to the surface. These drive-point MLSs were assembled onsite and driven using a slide hammer. After initial installation, however, many of the ports did not produce water. Removal revealed that during driving very fine-grained sand was forced through the screens and into the port stem blocking flow. For the next attempt, a 3.8 cm diameter galvanized pipe closed off at the bottom by a bolt was driven to the target depth. The MLSs were lowered inside the pipe, the pipe was removed, and the formation materials collapsed around the MLSs. This installation approach yielded substantially improved results with 10 of 12 zones producing water. Head profiles collected near the effluent outfall indicate convergence of stream water and groundwater in the shallow channel sediments. Head profiles collected 5 km downstream from the outfall show upward gradients throughout the aquifer indicating groundwater recharging the stream. Contaminant profiles generally show substantial decreases in concentration with depth into the stream bed and large changes in mixture compositions. Future adjustments that warrant consideration include a dedicated drive casing tool to assist with installation, modifications to the port to reduce the opportunities for clogging, and possibilities for including more than six ports in a single MLS.