Study Finds Land Disposal of HDD Mud a Viable Option
For horizontal directional drilling (HDD) contractors, an important concern at the jobsite is how to deal with the drilling mud that is sent down the bore hole and recirculated back to the surface.
While HDD contractors sometimes employ mud reclaimers to recycle mud during the boring process, more often the mud is used just once, before being gathered up with a vacuum truck and dumped, usually on the jobsite, on the contractor’s property or at a municipal landfill facility.
Expensive to dump
Contractors that go the landfill route will quickly find their expenses mounting, with tipping fees ranging from $25 to $50 a ton, or $0.50 a gallon. Considering that some drilling contractors might need to dump up to 20,000 gallons a week, the cost of HDD mud disposal is clearly a major concern. These costs are passed on to the municipality, pushing the burden beyond the contractor to the ratepayer. As the HDD industry continues to grow, more and more mud is being generated that needs to be dealt with. Putting all these factors together, the desire for the industry to develop an effective, cost-efficient means to dispose of HDD drilling mud has become a high priority.
It was with these concerns in mind that Ditch Witch, a large manufacturer of HDD drilling equipment, approached Oklahoma State University (OSU) with the idea of conducting a series of experiments with HDD mud. Ditch Witch and OSU wanted to find out whether the ingredients in HDD mud were harmful to the environment, and in what quantities the mud might be limiting to forage yield and seed germination.
Putting HDD mud under the microscope
Typical HDD mud consists of water and soil from the borehole, along with additives that are mixed in to facilitate the drilling process. Bentonite clay, for example, is often added to increase fluid viscosity during the drilling process, but the residual material can improve the ability of the soil to retain nutrients after it is land applied. Polymers such as polyacrylamide are used for the same purpose. Soda ash is added to control pH and surfactants are used to prevent heavy clays from balling up at the drill bit.
In the first experiment, Ditch Witch collected 56 samples of HDD mud taken from 28 states. The samples were sent to OSU, which conducted a series of tests on them, including their levels of pH, salts, plant nutrients and heavy metals. While seven samples did have concentrations of heavy metals beyond those found in normal soils, the levels were well below those mandated by the U.S. Environmental Protection Agency for the land application of exceptional quality biosolids under Part 503 ruling, and thus safe for land application.
The rest of the samples had heavy metal concentrations within normal levels found in typical soils.
“As far as we know, there is no chemical present beyond allowable concentrations that can limit the land application of spent HDD mud as a means of disposal,” states an abstract of the study, titled “Can Urban Horizontal Directional Drilling Mud be Land Applied?” “Based on the study, it is highly unlikely that excessive trace metals will be applied to soils through land application of the residuals.”
While the study found that ingredients in drilling mud in the vast majority of samples present no harmful effects on the environment, it is important to recognize that all HDD mud isn’t appropriate for use as a pure potting soil. For example, using pure mud on its own to grow vegetables may not be safe. However, the researchers found the mud can be added as a soil amendment, for example, to grow hay or for grazing cattle.
“Adding it to an agricultural field in a uniform manner, either for cattle grazing or hay or some other agricultural activity, or in a bare plot soil situation as might be found on a construction site, didn’t show any negative indication in terms of its effects on that soil or the plant material that was there,” said Kelvin Self, research and development project manager for Ditch Witch, who spoke to CUI about the results of the study.
“What was most pleasing about it, from the industry standpoint and my own personal standpoint, is that we really didn't find anything of great concern,” said Self, an industry veteran with a doctorate in agricultural engineering. “A hundred percent of the samples, according to OSU, could be land-disposed quite safely. So that was a very nice result.”
Test contaminated soils
The good news for the industry, however, comes with a caveat. While most HDD mud is basically benign, contractors must be aware of where they’re drilling before they consider land-disposing their mud. If the contractor is drilling in an area that was historically contaminated, say where mining or oilfield activity took place, the HDD mud should be tested prior to disposal.
“They have to have some knowledge of the area they’re drilling through. Because if they suspect at all that there could be high concentrations of metals back down in those soils, then they’re going to have the same concentrations in their mud. In which case, they need to test it,” said Chad Penn, associate professor at OSU, who led the study along with graduate student Josh Daniel.
How much mud?
Assuming that isn’t the case, and the area being drilled through is uncontaminated, the next question becomes: if the HDD mud is spread over land, how much of it is appropriate for the mud to become a good host for plants, and when does it become limiting?
Ditch Witch and OSU sought to find out by conducting two experiments. In the first experiment, normal HDD mud gathered from a local contractor was sprayed as a slurry on a typical field of Bermudagrass hay. The second experiment spread the HDD mud over plots of bare ground, sprinkled with Bermudagrass seed. This experiment was meant to simulate a construction site where HDD mud would likely be applied in a realworld situation.
After four months, plots that received the HDD mud on the hayfield, at rates equivalent to 10-, 20-, 30, 40- and 50- tons of dry solid per acre, produced the same amount of grass as control plots that did not receive any HDD mud. Plots that received the HDD mud could handle up to 50 tons per acre without any negative impact on grass production.
For the bare ground plots, the same rates of HDD mud were applied. After 60 days, researchers found plots that had received 10 tons of solids per acre produced significantly more grass than a control plot and the other rates. Plots that received 20, 30 and 40 tons per acre produced the same amount of grass as the control, while plots receiving the heaviest rate of 50 tons per acre produced slightly less.
The results indicated that when HDD mud is applied on existing grass cover, grass growth was not significantly affected by the amount of mud applied up to 50 tons per acre. On bare ground, however, applying too much HDD mud – 50 tons per acre or more – will impede plant growth.
If all of this sounds academic, Self has come up with a formula that contractors can use to determine how much HDD mud they should apply to a field – assuming that land application is permitted in their jurisdiction, and they have determined that the area in which they have drilled is not known to be contaminated.
The first step is to pick the tons per acre at which to apply the material. Next, measure the mud density using a mud balance. Then, plug the mud density into an equation, to get the number of gallons per acre of mud residue needed to get the desired application rate.
For example, to get 40 tons of solid material per acre, with a mud density of 12.3 pounds per gallon, a contractor would need to apply 12,500 gallons of mud residue. From there, in order to apply the material uniformly across an acre of land, at a width of 10 feet per row, the contractor would apply 600 gallons of mud per row.
Minimal regulation likely required
CUI asked Self, from Ditch Witch, and Penn, from OSU, what the likelihood is of land application becoming standard practice in the industry, and achieving widespread acceptance among the general public. Both acknowledged that there is a role for regulators to play in determining whether the mud needs to be tested before being land-applied, and not leaving it up completely to a contractors’ discretion.
Self, however, said there is definitely a risk of over-regulation.
“Of the 56 samples we took, all 56 of them would be safe to apply. That doesn’t mean that if you took a 57th one, that you wouldn’t find something. It just means that there’s pretty strong evidence that we’re sort of rubbing our hands together for no real reason,” he said.
“I would say the challenge is determining how to regulate it without putting these guys out of business,” said Penn, noting the cost to the contractor of testing each soil batch.
Oklahoma State will be publishing the results of its experiments on HDD mud into peer-reviewed scientific journals. Self said those papers should be coming out towards the end of this year and, when they do, could form the basis of a body of research that helps determine to what extent the disposal of HDD mud will be regulated. Self indicated that interested readers are invited to email [email protected] referencing the “HDD Mud Disposal Research Info” to receive the latest information available.
For now, he said the evidence suggests that minimal regulation is required. “The bottom line here is what we found is that the mud basically reflects what’s in the soil that’s already there. What you’re drilling through is just normal soils near the surface and the mud reflects what’s in those soils.”
Further information on this topic, along with recommendations, is presented by OSU cooperative extension at: http://pods. dasnr.okstate.edu/docushare/dsweb/Get/ Document-9597/PSS-2916web.pdf
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