The geology of the site is characterized by 20 to 30 feet of alluvial silt and sand terrace sediments underlain by the Sentinel Gap Formation of the Columbia River Basalts. The top of the basalt consists of brecciated and fractured zones. The interior of the basalt is still massive.

• Dense Non-aqueous Phase Liquids (DNAPLs)
• Fractured Bedrock

Historical process operations and waste management practices resulted in subsurface contamination by coal tar creosote. The creosote has spread over an area of 7 acres. It is present in three water-bearing zones and one intra-flow zone at 230 feet below ground surface. The majority of the creosote is primarily located at the base of the alluvium and within the brecciated and fractured zones of the basalt flow top. Downward migration of creosote has been attenuated by the massive basalt flow interior.

• Creosote (coal tar) (0 µg/L)
• Creosote (wood) (0 µg/L)
• Polycyclic aromatic hydrocarbons (PAHs) (0 µg/L)

• Coring

Comments:
Since 1985, three drilling methods  cable tool, air rotary, and rotosonic  have been used to investigate and install wells into the fractured basalt.

Cable tool was initially used at the site between 1985 and 1992. The advantages of the cable tool method of drilling are that (1) it is reliable, (2) the occurrence of creosote and the basalt and overlying alluvium can be logged with a good understanding, and (3) the work can be conducted in Level D personal protective equipment (PPE). Telescoping techniques were employed to install wells in the basalt aquifers. The disadvantages of this method are (1) the time required to drill; thus, the cost per well is higher, and (2) the large amount of ground water mixed with cuttings that are produced and must be managed as FO41 hazardous waste.
The air rotary technique was used to install shallow unconfined wells at the site between 1992 and 2002. The primary advantage of the air rotary technique is that wells can be installed quickly. It was possible to drill and install one 40-foot well per day. The disadvantages of this technique are that: (1) it is necessary to conduct the work in Level C PPE, (2) large amounts of ground water mixed with cuttings are generated, and (3) the cuttings are challenging to log and it is therefore difficult to identify the horizons where the creosote is migrating.

Rotosonic drilling was employed during the summer of 2003 to install additional recovery wells in the unconfined zone. Advantages of this technique are that (1) the speed of drilling is similar to the air rotary, (2) it produces 7-inch diameter cores, which allows for accurate logging of the geologic material and the creosote migration pathways, (3) it produces small amounts of cuttings and ground water compared with cable tool and air rotary, and (4) work is conducted in Level D PPE. The overall cost for this drilling method is lower than for the other two methods. The disadvantage of this method is that it cannot penetrate the massive basalt and, therefore, cannot be used to install wells in the deeper confined zones of the site.

• Flushing (In Situ)
  • Water
• Pump and Treat

Site remediation objectives are to remove the potentially mobile creosote oil so that only residual saturation of creosote remains.

The recovery end-point is based on the decline curve, which is the incremental DNAPL recovery rate plotted versus the incremental volume of DNAPL recovered. The DNAPL extraction target recovery rate for this site is 90 percent for potentially recoverable creosote.

To date, more than 30,000 gallons of creosote oil have been recovered from the shallow-unconfined water-bearing zone.

Advantages of water flooding include (1) increasing the rate that DNAPL can be recovered from a given well as compared with dual-phase pumping, and (2) reducing the costs associated with aboveground treatment of recovered ground water as the ground water is re-circulated.

Managing DNAPL saturations, hydraulic gradients, and the thickness of DNAPL are critical to the successful hydraulic recovery of mobile DNAPL. If the DNAPL thickness is reduced in a well, the less viscous and more mobile water phase will truncate the oil flow and severely reduce the transmissivity with respect to DNAPL. Thus, managing the thickness of DNAPL in the well while continuously pumping the DNAPL will provide optimal DNAPL recovery rates.

References:
Blischke, Heidi; Jason Cole; Ken Shump. 2004. Water Flooding Recovery of Creosote in a Fractured Basalt. The Fourth International Conference on Remediation of Chlorinated and Recalcitrant Compounds.