Horizontal wells can remediate a large area with a single well where dozens or more vertical wells might be necessary to do the same job. This can be a huge advantage where a contaminant plume may be difficult to reach with traditional vertical wells or direct push techniques due to the relative locations of roads, runways, buildings or other structures; or where property ownership issues constrain access. Horizontal wells can also have a cost advantage when the total cost of remediation is considered, including the number of vertical wells needed, the connective piping necessary to deliver the remediant to all of vertical wells, and total equipment and O&M costs.
A question that often arises is, “do horizontal air sparge wells actually work?” This question is often raised by stakeholders, including regulatory agencies and responsible parties, when horizontal air sparge wells are proposed for remediation.
How do we know that a horizontal sparge well actually works? What is a successful horizontal sparge well installation? I define a successful horizontal sparge well with two parameters:
- It delivers the desired design flow rate of air to the subsurface, AND
- The delivered air exits along the entire length of well screen
The first parameter is fairly easy to measure with readily available flow sensors. The second question is more difficult to answer. If air only exits the first fifty feet of a 300-foot long horizontal air sparge well the entire plume targeted by the horizontal well will not be remediated. This problem is addressed theoretically using calculations of air flow and pressure along the well pipe, but actual field verification is difficult. Even if we are delivering the design flow to the well, how can we be sure the air is exiting along the entire well screen, and not just from the first, say, 50 feet? We’ve come up with two methods for field verification of air flow out of a horizontal air sparge well:
- Checking for air flow or pressure; or an increase in dissolved oxygen levels in nearby monitoring wells, or
- Putting a camera inside the horizontal well to see if air displaces the water along the entire length of the screen section. If there is air along the entire length of screen AND air is flowing to the well, we can assume that air is exiting the well over the entire length of the screen section. This method was suggested by a horizontal well driller we have worked with – Jim Doesburg of Directed Technologies. Jim provided the camera and other equipment we used to get the video of the inside of a horizontal air sparge well I’ll show you in a few minutes.
In actuality, we combine both of these methods in the field for verification of the design.
Now the question of “do horizontal air sparge wells actually work?” arose at a petroleum release site we were working on in Salinas, just north of here. Horizontal air sparge wells appeared to be the best solution, but we wanted to verify that they worked as designed. So after they were installed we put a video camera inside and looked at what was happening at different locations along the screened section of each well.
This is the site – a construction company storage yard. Three USTs were removed from this area in the driveway in 1998.
When we were asked to look at the site in 2010, the dissolved petroleum hydrocarbon plume was approximately 450-feet long and 300-feet wide, and extended under a road onto adjacent properties. Some cleanup had been completed in the source zone, but high concentrations of hydrocarbons remained in groundwater. Benzene and MTBE were the primary contaminants of concern, but I’ll focus on the benzene plume for simplicity. Concentrations of benzene were in the 10,000 ppb range in the source zone and up to 40,000 ppb in the downgradient, off-site “hot” well. The cleanup goal for benzene in California is 1,000 to 3,000 ppb. The hydrocarbon plume was in unconfined first groundwater in sandy material. The water table surface was at approximately 30-feet below the ground surface. The base of the first water bearing zone was at approximately 50 feet below ground. These were all favorable conditions for air sparging and a pilot test (in a vertical sparge well) indicated air sparging was technically feasible at the site.
We developed a remediation plan that called for installing three blind horizontal air sparge wells, each with approximately 300 feet of well screen oriented approximately perpendicular to the predominant groundwater flow direction, which is to the north-northeast. Only the screened section of the wells is shown on this figure for clarity. By blind I mean each well only had one surface entry – at the drilling point. Blind wells were chosen because the responsible party didn’t want any work done on off-site properties. Entry / exit horizontal wells are easier to install and it’s easier to verify their performance.
The compressor is turned on at about 35 seconds. Water is flowing past the camera towards the distal end of the well.
At about one minute and twenty seconds, less than a minute after the compressor was turned on we can see the air “bubble” has reached the camera. I believe we are looking at the air bubble through water which still coats the camera lens.
At about 2 minutes 10 seconds we can see water flowing past the camera and more details of the inside of the well. The “ring” down the well is light reflecting off of the thickened threaded section of the 10-foot long section of well casing. We can also clearly see the closest slots.
At 3 minutes we skip ahead to 4 minutes in the video.
At 4:15 we skip ahead to 6 minutes in the video.
At 6:10 we skip ahead to 9 minutes in the video.
At 9:10 we skip ahead to 10:15 in the video.
At about 10:25 the compressor is turned off and we see water immediately starts to enter the well.
At 10:55 a wave of water comes down the well and the camera is again submerged.
Results/Lessons Learned. Video images show that air can be delivered along almost the entire 300-foot long screen section of an air sparge horizontal remediation well. Groundwater analytical data, probably the best measure of remedial effectiveness, indicates the horizontal air sparge wells are reducing the concentrations of petroleum hydrocarbons in groundwater at this site.
