DNAPL removal from a heterogeneous alluvial aquifer by surfactant-enhanced aquifer remediation

A demonstration of surfactant-enhanced aquifer remediation (SEAR) was conducted at Hill Air Force Base (AFB), Utah. Two surfactant floods were undertaken in a test section of a heterogeneous alluvial aquifer with a hydraulic conductivity range of 2.8 to 8.6 ft/day (10(-5) to 10(-4) m/sec) and a pore volume of approximately 15,000 gallons (57 m(3)). The wellfield installed for the demonstration consisted of lines of three injection and three extraction wells, a central monitoring well, and a single hydraulic control well. No physical barriers to flow, such as sheet-pile walls, were employed surfactant flooding was controlled entirely by hydraulic manipulation of the flow field. The interwell distance between injectors and extractors was 20 feet (6 m);, the distance between individual injectors and extractors in line was 10 feet (3 m). The water table was 25 feet (7.6 m) below ground surface with a saturated zone approximately 19-feet thick (5.8 m). Residual dense nonaqueous phase liquids (DNAPL) remained in a zone of alluvium 42 to 46 feet (13 to 14 m) below ground surface following extraction of free-phase DNAPL. The injectors and extractors were screened in this DNAPL zone. Three partitioning interwell tracer tests (PITTs) and two surfactant floods were conducted over four months during 1996. The surfactant floods removed 341 out of 346 gallons of residual DNAPL (1290 of 1310 L), according to the PITTs. This represents a total recovery of about 98.5% of the DNAPL volume present in the zone of residual DNAPL as determined by comparing the initial and final PITTs. There was no reduction in hydraulic conductivity due to colloid mobilization during the surfactant floods; in fact, the hydraulic gradient across the test zone decreased as the floods progressed. Concentrations of dissolved total chlorinated hydrocarbons in the test section decreased from 1000 mg/L before the floods to less than 10 mg/L following the floods. This demonstration is evidence of the technical practicability of DNAPL removal from alluvium.