## 2 Materials and Methods  
### Hydrogeologic Setting and Plume Characteristics of the Pilot Test  
The source zone and hotspot areas, where the tests described in this paper were conducted, is underlain by interbedded gravel, sand, clay, and silt, typical of Rio Grande Basin alluvial deposits in northern New Mexico. The water table aquifer, referred to as the shallow zone, contains the source zone and hotspot areas [4]. The water table in these areas is encountered at approximately 1.5 m (5 feet) below ground surface (bgs) and extends to approximately 9.1 m (30 feet) bgs before encountering a clay aquitard. This upper section is comprised primarily of gravelly and sandy material with intermittent lenses of clay and silt. The arithmetic mean of the shallow-zone hydraulic conductivity is approximately 18.2 m per day (60 feet per day), ranging from approximately 1.5 m/day (5 ft/day) to 30.5 m/day (100 ft/day), based on in-well slug testing and numerical modeling conducted as part of the remedial investigation and design. Historical water level measurements from wells screened within this unit indicate that the groundwater flows south to southeast (slightly towards the Rio Grande) at a representative horizontal hydraulic gradient of 0.002 m per meter (m/m). Assuming an effective porosity of 0.15 (based on typical values for soils of the types found at this site) a representative lateral groundwater flow velocity is estimated at 0.2 m/day (0.8 ft/day). Considering the length of the treatment zone along the groundwater flow path at 67.1 m (220 feet) long, the time for groundwater to travel from the source zone to the periphery of the hotspot area is estimated at 275 days.  
The maximum anticipated depth for treatment varies spatially across the hotspot area and is equal to the depth to the lower surface of the shallow groundwater plume. The vertical extent of the groundwater plume has been limited by extensive but discontinuous silt and clay layers, with contaminated groundwater penetrating deeper into the shallow zone as the plume migrated downgradient of the source zone, to the south and southeast. The maximum depth of the plume penetration beneath the source area, using a PCE concentration cutoff value of approximately 5 micrograms per liter (\(\upg/\L\)), is between 6.4 and 7.0 m (21 and 23 feet) bgs. At the south end of the hotspot area the maximum depth approaches 9.1 m (30 feet) bgs. The representative pore volume for the source zone and hotspot areas (or its volume), assuming the total porosity is 0.25, is estimated to be 4842.2 cubic meter (171,000 cubic feet) or 4.85 million liter (1.28 million gallons).  
PCE concentrations within the source zone/hotspot portion of the groundwater plume (excluding the source area), along the plume centerline and prior to remedial action, range from approximately 10,000 \(\upg/\L\) to approximately 3000 \(\upg/\L\), installed during phase II of remedial design. Evidence of natural reductive dehalogenation of PCE appears to occur within the hotspot area, as 830 \(\upg/\L\) TCE has been detected since March 2006. 1,2-cis DCE has also been detected in the hotspot area in groundwater and soil samples. The groundwater concentration ranges were developed from only a few sampling points. Thus, there is considerable uncertainty in the structure of the plume in this area of the Site  
The source area within the larger hotspot area, encompassing approximately 83.6 square meters (900 square feet) of the 3344.5-square-meter (36,000-square-foot) treatment area, is characterized by high concentrations of PCE in the groundwater (dissolved phase) and on the sediment (adsorbed phase). Groundwater PCE concentrations in the source zone groundwater have been detected above 60,000 \(\upg/\L\), with sediment PCE concentrationsranging up to 800,000 micrograms per kilogram (at SB-04 at a sampling depth of 3.7 m (12 feet) bgs).