Determining the hydraulic conductivity from innovative SPIN® injection measurements

The successful design and implementation of reagents injections for the in-situ remediation of contaminated soil and groundwater relies on accurate knowledge of the subsurface hydro(geo)logical properties, with the hydraulic conductivity being the key factor in the transport of contaminants as well as in the distribution of injected reagents. The direct push methods which are conventionally used for performing such injections yet come with several limitations. Soil compaction and smearing caused by pushing the injection rod with conical tip into the subsurface may considerably reduce the local hydraulic conductivity and consequently increase the injection pressure required to adequately distribute the reagents in the surrounding subsurface. Besides the more heavy machinery needed, with restricted (indoor) application on built-up sites, working with higher injection pressures increases the risk of fracturing and short-circuiting, possibly witnessed by blowout, reflux, or daylighting of products at or near the injection location. The company Injectis LLC developed a new injection technology in order to address these restraints. Using this SPIN^® technology, injections are made quasi-continuously over the entire depth profile covered, with real-time monitoring of the injection parameters in depth. The ratio of the injection flow to the injection pressure can be interpreted as a measure of the hydraulic conductivity. This study aims at the development of a standard procedure for processing these injection measurements into more reliable, absolute data of the hydraulic conductivity. This includes a correction of the measured pressure for the frictional pressure drop and gravity accounting for the operational configuration of the injection system. Furthermore, different reference methods are considered for absolute calibration of the derived hydraulic conductivity values. The procedure is validated using available datasets for different contaminated sites. The acquisition of accurate high-resolution data of the hydraulic conductivity contributes to the improved characterization of the local subsurface properties providing fundamental input to contaminant transport modelling, and thereby supports the more efficient design of remedial actions and the monitoring of their results.