Dense non-aqueous phase liquids (DNAPL) greatly complicate groundwater remediation because the heavy DNAPL will sink and follow topographic lows within an aquifer system, and because DNAPL is difficult to extract using conventional pumping methods. These problems are now being observed at the Motco Superfund Site near Houston, Texas, where DNAPL is present in a shallow surficial aquifer. The aquifer remediation program includes these elements:
Detailed stratigraphic interpretation of the aquifer to find “sinks' of DNAPL in the complex sand-silt aquifer system.
A pilot recovery test to determine the effectiveness of enhanced oil recoverytechnologies (EOR) for mobilizing DNAPL.
A conceptual remedial design for extracting mobile DNAPL and for managing residual DNAPL and DNAPL dissolution products
DNAPL accumulations were observed in wells screened in low spots in the shallow transmissive zone. The 60 boring and well logs at the site were supplemented with an additional seventy-three cone penetrometer logs to generate a continuous and laterally extensive stratigraphic record of the shallow aquifer system. The data were used to develop detailed topographic maps of the base of the transmissive zone to find DNAPL accumulation and to design a pilot test for recovery of DNAPL and affected groundwater. Results of this investigation showed DNAPL to be moving through fractures and other secondary porosity features of the silt stratum, in general accordance with the base topography of the unit.
The pilot test compared three recovery technologies: pumping, water flooding, and vacuumenhanced recovery. For the vacuum-enhanced pumping scheme, the downhole pump was augmented by a wellbore vacuum to increase the available drawdown and maximum yield of the recovery well. For the water flooding scheme, a freshwater injection well was operated at a distance of 100 ft from the pumping well to increase the hydraulic gradient. A three-week testing program demonstrated that some DNAPL could be removed by pumping alone, but that waterflooding and vacuum enhanced recovery greatly increased recovery rates.
A conceptual remedial design was developed for managing the mobile DNAPL fraction and the dissolved organic constituents in the aquifer. Waterflooding and well-bore vacuums can be used to induce a high artificial gradient and mobilize some fraction of the DNAPL. After the artificial gradient is removed, the residual DNAPL would be immobilized under background gradient conditions. Design of the DNAPL management system involves two key considerations: evaluation of fracture and capillary effects to estimate residual saturation in the unit before and after gradient application, and analysis of DNAPL dissolution rates for predicting the concentration of the soluble organic constituents in the aquifer over time.