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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 m3). 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 inter‐well 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.

Field Demonstration of the Surfactant/Foam Process for Aquifer Remediation G.J. Hirasaki, SPE, C.A. Miller, SPE, R. Szafranski, D. Tanzil, SPE, and J.B. Lawson, SPE, Rice University, H. Meinardus, M. Jin (SPE), J.T. Londergan, and R.E. Jackson, Duke Engineering and Services, and G.A. Pope (SPE) and W.H. Wade, University of Texas Abstract The first field demonstration of the surfactant/foam process for removal of DNAPL from a heterogeneous alluvial aquifer was conducted during the spring of 1997 at Hill Air Force Base in Utah. The surfactant solution was designed to mobilize and solubilize the contaminant, which was located in the lowest part of the aquifer. During the demonstration, air was injected to form an in situ "foam" in the zones of highest permeability, the purpose being to divert surfactant solution to zones of lower permeability and thereby improve the efficiency of the removal process, as compared to continuous surfactant injection without foam generation. The process was successful in reducing the average DNAPL saturation of the swept pore volume to 0.03%. Introduction and Summary The EPA defines a DNAPL (Dense NonAqueous Phase Liquid) site as "a site where DNAPL has been released and is now present in the subsurface as an immiscible phase", i.e., either free-phase and residual DNAPL or simply residual DNAPL alone. Residual DNAPL is that immiscible liquid trapped by capillary forces within the pore spaces of the sand and silt that comprise the aquifer system. In the saturated zone, it consists of discrete drops or ganglia and is immobile. In contrast, by definition, the free-phase DNAPL is present along continuous pathways through the aquifer and is mobile. The accumulation of DNAPL in an aquifer is a persistent source of contamination that cannot be remediated by the traditional method of "pump and treat". Even if it was possible to detect and produce all of the free-phase DNAPL, the aquifer will continue to be contaminated by the residual DNAPL. Thus, complete remediation will require removal of all of the DNAPL, including residual DNAPL and unswept DNAPL due to aquifer heterogeneities. Surfactant-enhanced aquifer remediation (SEAR) is a promising technology for removal of DNAPL because of its history of recovering residual oil that remains after waterflooding. A common problem with surfactant flooding, both for recovery of petroleum and in aquifer remediation, is the effect of heterogeneities on the performance of the process. The effect of heterogeneity is mitigated by application of mobility control. Mobility control in surfactant flooding has been accomplished by addition of polymer, generation of a viscous microemulsion, and in situ generation of foam by injection of gas. The first field demonstration of the surfactant/foam process for removal of DNAPL from heterogeneous alluvial aquifers was conducted during the spring of 1997 at Hill Air Force Base (AFB) in Utah. The surfactant solution was designed to mobilize and solubilize the contaminant, which was located in the lowest part of the saturated zone of an alluvial aquifer contained in a buried paleo-channel eroded into thick clay deposits. The clay provided a capillary barrier to contaminant migration. During the demonstration, air was injected to form an m situ 'foam' in the zones of highest hydraulic conductivity or permeability, the purpose being to divert surfactant solution to zones of lower conductivity and thereby improve the efficiency of the removal process as compared continuous surfactant injection without foam generation. The demonstration was conducted in a 6.1-meter (20-foot) line drive well pattern with three injection and three extraction wells spanning the width of the buried channel (approximately 3.7 meters, or 12 feet). Hydraulic conductivity ranged from 10–4 m/s (permeability: 10 darcy) to more than 10-–3 m/s (permeability: 100 darcy) with the contaminated zones near the bottom of the channel being in the lower portion of this range. The bottom of the buried channel was some 13.7 meters (45 feet) below the ground surface. The contaminant itself contained approximately 70% trichloroethene (TCE), and smaller amounts of other solvents and dissolved greases.

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A groundwater arsenic plume, derived from arsenite wastes disposed at a chemical plant in Tacoma, WA, extends to the shore of the Hylebos Waterway. The plume is characterized by high-pH, high-silica concentrations generated by past disposal of high-pH brines on site. Aquifer K d values for arsenic decrease at least 10-fold as the pH increases from 8.5 to 11. Near the shore, aquifer sands are cemented, predominantly by opal quartz. Cementation reduces porosity to about 19%; however, very little pore space is interconnected. Along the shore face, a massive amorphous precipitate, high in Si and Mg, is found. SOLMINEQ calculations show that mixing high-pH, high-silica groundwater with seawater causes initial supersaturation of brucite [Mg(OH) 2 ] and magnesium hydroxysilicates. The cementation has likely considerably reduced the cumulative discharge of arsenic to the waterway.