Aquifer washing by micellar solutions:: 3 Field test at the Thouin Sand Pit (L'Assomption, Québec, Canada)

A field test was performed to recover DNAPL (Dense Non Aqueous Phase Liquid) in a shallow aquifer at the Thouin Sand Pit near Montreal, to evaluate a new technique of aquifer restoration involving surfactant solutions. Laboratory tests have shown that washing solutions containing alcohols, surfactants, and solvents are very efficient in recovering DNAPL as a miscible phase. The Thouin field test was designed to: (1) study in situ recovery of DNAPL; (2) evaluate an injection–pumping strategy; (3) test the use of polymer solutions to control the mobility of a washing solution slug and to improve the vertical sweep efficiency throughout the sand unit. The test was performed in a shallow medium sand aquifer containing both contaminated saturated and unsaturated zones. The washing experiment was done on 17 m 3 of the saturated zone with an average DNAPL initial concentration of 55 000 mg kg −1 dry soil. Solutions were injected through a central well and pumped into four wells arranged in a five-point square pattern. In the zone swept by the washing solution, 86% of residual DNAPL was recovered using only 0.9 pore volume of a surfactant solution. These results confirm laboratory sand column experiments although the washing solution used in the field experiment had not yet been optimized to meet site-specific criteria. The use of a polymer solution before and after injection of the washing solution proved beneficial in insuring that the washing solution effectively swept all the sand layers in spite of soil heterogeneities. As the rinsing cycles were not completed because of weather problems (freezing conditions), small but significant amounts of the washing solution ingredients were still present in the aquifer at the end of the test and their fate is being monitored. For this field test, DNAPL recovery could have been better if an optimal washing solution had been injected, a larger volume of the selected solution had been circulated, or a different injection–pumping strategy had been chosen. ©1997 Elsevier Science B.V.