They want us to do what?
Granted, I was new to the field at the time and didn?t know that asking a contractor to dig a huge trench across a contaminated area was a big deal. The idea, of course, was to oxygenate the water. ?Even though I was a rookie, I could still see that there might be some problems related to digging a trench across the parking lot of an active gas station. I could just imagine a sleepy truck operator driving into it late at night. Fortunately, the client decided not to move forward, and instead we looked at doing oxygen injections into the vertical wells that existed on site.
(An object I don?t want in my trench)
It?s a shame. A trench would have been more efficient than injecting oxygen in vertical wells. It probably would have increased the amount of groundwater exposed to oxygen by an order of magnitude or more. It just wasn?t feasible at this site.
Active remediation through vertical wells depends on mass transports of air and/or chemicals through the interface offered by the well screens. The problem: the interface is not that big! Most monitoring wells have 5 or 10 feet of screen. Also, a heck of a lot of wells would be necessary in order to effectively remediate the site.
So, to recap, we have a large plume and an active facility that can?t be disturbed. What to do? Drilling horizontal wells is one option to consider.
Advantages of horizontal wells in comparison to trenching:
- Very little soil to containerize and dispose of.
- Greater vertical reach.
- Can be installed beneath surface structures.
? (Trenching at a remediation site in TN)
Advantages of horizontal remediation wells in comparison to vertical wells:
- Horizontal wells have more screen in contact with the contaminant plume. This can result in a greater zone of influence.
- Natural flow patterns tend to create plumes that are longer and/or wider than they are thick. Horizontal wells can be oriented to take advantage of plume geometry.
- A single horizontal well can have the remediation impact of 10 to30 vertical wells.
- Horizontal wells can be installed beneath surface structures.
(Comparison image of vertical versus horizontal wells)
Disadvantages of horizontal wells:
- The removal of cuttings is difficult and must be done with great care to avoid borehole collapse.
- Horizontal well casings must have greater tensile strength than vertical wells. Horizontal casings are subject to higher stress during installation due to the power of horizontal drill rigs.
- Installing a traditional sand pack is difficult.? Some engineers have tried pre-packed screens but have found that the weight and stiffness of the screens makes installation difficult.
- In areas where groundwater levels fluctuate, horizontal wells may be dry at times during the year.
The cost per foot for a horizontal well can be high ? around $90 a foot. However, if one horizontal well replaces anywhere from 9 to 30 vertical wells, the cost savings may be considerable.
Have you tried a horizontal well as a remediation strategy? What were your results?
Written By: Chris Hollinger