Effect of flow rate on clogging processes in small diameter aquifer storage and recovery injection wells

Abstract

This study investigates injection well clogging at low injection rates (<0.38 m3/ min or 100 gpm) through laboratory column experiments and geochemical analyses. Clogging in recharge wells is a major concern for aquifer storage and recovery (ASR) systems. Many of the current methods used to predict injection well clogging assume a higher injection rate. These methods may not adequately identify the clogging processes occurring at low flow rates. An ongoing project at the Kansas Geological Survey (KGS) investigates a low-cost, slow flow alternative to traditional ASR recharge systems. The project utilizes gravity-induced recharge and small diameter wells installed with direct-push technology to recharge and store ground water. The KGS ASR recharge system will have significantly lower injection rates than traditional ASR wells due to increased frictional losses in small-diameter wells and the absence of injection pumps. To examine clogging processes at low flow rates, laboratory columns packed with sand and gravel cores, taken from the Pleistocene Belleville formation during direct-push well installation at the Lower Republican River field site, were used for clogging experiments. Changes in geochemistry and hydraulic conductivity were monitored in three column sets running at flow rates of 1 m/day, 2 m/day, and 3 m/day, for seventeen days. Each set contains one column each of native aquifer water and one column of treated surface water, respectively. Hydraulic conductivity for all columns in this experiment increased above initial levels by at least one order of magnitude. Columns with the highest pumping speed had the greatest increase in hydraulic conductivity, suggesting sufficient pressure existed to mobilize clay particles and remove them from the columns. Experimental results and geochemical analyses indicate clay dispersion is the primary factor influencing hydraulic conductivity changes at the Republic ASR site. Observations of biofilm in effluent tubing suggest bacterial clogging of medium and low flow treated surface water columns, but bacterial abundance was not quantified in this study. Further understanding of clogging factors at low flow rates will aid in the selection of the most beneficial redevelopment and pretreatment methods for slow recharge ASR systems.