Background: Air Sparging
Air sparging is an in-situ remediation technology where air is injected into the saturated zone of an aquifer. It is an alternative to conventional ex-situ pump and treat systems for remediation of contaminated ground water.
It is necessary to understand the pattern of air flow that will occur in the subsurface in order to determine the optimal placement, number, and operating parameters of sparge wells. It is generally acknowledged that soil properties, injection pre s s u re, and injection rate will influence the flow of air during air sparging .
Horizontal air sparging wells offer some important advantages for ground water remediation. The use of a horizontal well can provide more uniform airflow along the axis of the well (as compared to a number of aligned vertical wells) and considerably reduces the impact of well installation at the surface.
Three air sparging pilot tests on horizontal wells were conducted at a coastal test site in Southern California:
- a 50-foot long horizontal well
- a 300-foot long horizontal well
- a 400-foot long horizontal well.
Goals of this program were to evaluate:
- the feasibility of installing both dualended and single-ended horizontal wells in the unconsolidated sand
- the pattern of induced airflow in the saturated zone around horizontal sparge wells
- the short-term efficacy of the process for remediation of dissolved phase contaminants
- engineering design parameters necessary for future well construction.
This paper discusses the results of the pilot test on one of the three wells, the 400-foot long well. Specific objectives for this installation and test were:
- determine the feasibility of installing a 400-foot long well in dune sand using jet drilling techniques
- measure the effectiveness of various well designs/air injection scenarios for delivering air over the entire 290 foot length of screen section
The ground surface, relatively flat at the site, is approximately 14.5 ft above msl. The water table is approximately two feet bgs and has a slight seaward gradient of 0.011 ft/ft. The subsurface consists of fine to medium grained, poorly graded sand to a depth of 21 feet. Peat and silty clay are known to be present at depth from 12 to 17 feet bgs.
The design for the horizontal well was completed by Unocal’s Environmental Technology Group. The sparging air flow was estimated by modeling site conditions using TETRAD, a three-phase flow simulator. The well was designed for a flow rate of 1000 cfm to 1300 cfm at 25 psia.
The sparge well was perforated (one 0.140 inch diameter hole every 0.4 foot) to distribute air equally along the length of the well. The sleeve was hooked up directly to the air injection hose, which was flanged to the well casing.
Drilling, Installation, and Development
A three-inch pilot borehole was drilled using a jetting tool. After the pilot hole was completed by exiting to the surface, a nine-inch reamer was attached to the drill rod and pulled back to the entrance pit. Drill rod was trailed behind the reamer to safeguard against losing the hole. The well casing and screen were pulled into the wellbore behind the final reamer pass.