• Drilling fluids – clay based and organic polymers
• Set back distance – remember for every 1′ of depth about 5′ of set back
• Soil sampling – overcoming obstacles and a couple of case studies
• The use of horizontal wells at small sites – dry cleaners and UST

So, how can we apply HDD technology for remediation solutions? Before we get into examples, let’s look at why we use horizontal wells. Remember remediation is a contact sport; we need to get our treatment – whatever it is – in contact with the contamination.

Let’s try a thought experiment. Visualize all of the contaminant plumes you have ever seen. Close your eyes and think about all the plumes that were 100′ in diameter and went 500′ straight down. How many sites have that type of plume geometry? My guess is very few.

Next, think about the sites where the contamination encounters the groundwater and moves laterally along a relatively “skinny” front. Most plumes tend to be elongated along the groundwater flow path. This is where the magic of horizontal remediation wells shines; the screen can be placed along the full length of the contamination. Instead of placing 25 or more sparge wells offsite, to attempt to intersect a 600′ plume, we can install just one horizontal well, with 600 feet of screen in contact with the entire contaminated zone.

With that short review of why we use horizontal wells for remediation, let’s examine the options.

Extraction Wells – Most of the first horizontal remediation wells were for pump and treat, i.e., installed for groundwater and/or free phase extraction. The long screen lengths allowed great capture of contaminated ground water. Many sites had multiple wells, in fact one location in Louisiana had 30 horizontal ground water extraction wells (all blind, all over 700′ in length). We all know that pump and treat is a slow, expensive process and has fallen out of favor with everyone, but horizontal ground water extraction wells are still being installed, often for hydraulic control rather than source area capture.

Injection wells – Other than air, what could you inject into the plume via a horizontal well? Injection wells are the most common type of installation we do these days, and the variety of things being injected seems to keep growing. Here’s a short list of the technologies, contaminants, and some of the locations for which we have installed horizontal injection wells over the past couple of years.

• Hot air/BTEX – New Mexico
• Oxygen/creosote – Florida
• Zero valent iron/chlorinated solvents – New Jersey
• Sodium lactate/solvents – Montana
• Brine/desalination activities – California
• Emulsified vegetable oil/solvents and/or munitions bi-products- California, Florida, Texas
• Potassium permanganate/solvents – New Jersey, California
• Hydrogen/aviation fuel – North Carolina
• Oxygen enriched water/heating oil – New Jersey

Recirculating wells – Take a bit of extracted groundwater, mix with a dash of nutrients and re-inject it, voila! a recirculating system, which combines greater control of groundwater flow through the site with continuous treatment (instead of sporadic doses), and no need to treat contaminated water for discharge to municipal sewage systems. We have seen continued growth in this segment of the industry, and horizontal installations are very “well” suited to the technology.

One project, completed at an active DOD facility in the Southeast, that used a pair of horizontal wells for bioremediation is an excellent example of re-circulating wells. After installation, the wells were connected to the recirculating system. One well extracted groundwater which was then mixed with the remediation substrate (sodium lactate). The treated water was sent to the second horizontal well for injection. Continued operation of the system created a hydraulic cell that constantly circulated amended water across the source area, providing a carbon source for in situ organisms that consume the contamination. The site, which had undergone several previous remediation attempts, was de-listed within two years!

Soil vapor extraction – SVE wells remove contaminant vapor from the vadose zone and horizontal SVE wells have been used at hundreds of sites across the US over the last several years. Think about expanding the technology to mitigate vapor intrusion issues in occupied building situations. In fact, a recent site in New Jersey had seven SVE wells installed at a depth of three feet below the slab to prevent vapor intrusion. A small, unobtrusive rig and just a few days were required to complete the project.

Air sparge with SVE
– At high flow rates, injected air (or oxygen or ozone) quickly oxidizes many contaminants, but also volatilizes organic solvents and mobilizes them from groundwater, to be removed from the soil column above with a soil vapor extraction system. In many cases, both sparging and vapor extraction wells must be installed, or the contaminants will simply reabsorb into the soil, to be washed into the water table again by precipitation. Horizontal sparge/SVE well systems have been installed at many sites across the United State and have been the most common use of directionally drilled wells over the last several years. Recent projects using sparge/SVE wells range from small sites (a dry cleaner in California and a UST site in Georgia) to installations of thousands of feet of well
screen at DOD air fields in Georgia, North Carolina, and Hawaii.

Biosparge – At more moderate flow rates, a steady flow of air will stimulate the growth of in-situ bacteria, which then feed on the contaminants and break them into either less harmful or shorter-lived daughter compounds. The advantage of biosparge systems is that no contaminants are brought to the surface, which means no secondary treatment is needed, no carbon canisters must be discarded or regenerated, and maintenance generally consists of checking wellhead pressures and making sure the compressor is plugged in and running.

A pipeline release site in the Southeast was recently selected for horizontal biosparge well installation. Two wells were installed, completed in blind boreholes, with four-inch schedule 80 PVC. The wells each contained over 400’ of screen, which required detailed open area design to insure that air was distributed evenly along the entire length of the screen.

You may be thinking, “Hey I’ve heard remediation is not the only use for horizontal wells!?” You’re right, and sometime in the future we can provide details on:

• Potable water supply – Nebraska
• Monitoring wells – California
• Subway tunnel dewatering – Washington DC
• Mine tailings dewatering – Maine
• Pre-excavation dewatering – New Mexico
• Slope stability – Washington

Does the list above include all of the ways horizontal wells have been used for remediation activities? Absolutely not. I’m sure that we’ve probably left something out and new treatment options have yet to be applied horizontally. If you’ve personally been involved with a unique HDD application – even if DTD didn’t install the wells! – please share your experience with us. We might even invite a guest author into the newsletter! In any case, we always need to remember that remediation is a contact sport and plumes are not vertical, so think about directional drilling for your next project.