North Dakota researchers look at a reclamation strategy to bring salt out of the soil and put land back into production
Aaron Daigh has been tasked with a mighty challenge: figure out how to clean up the messes left by fracking brine spills.
The North Dakota State University soil scientist says most of the average 42 spills a week are relatively small, covering only a few hundred square feet or a couple acres. However, they are still severe and do take land out of agricultural production, possibly for thousands of years.
The Bakken oil field in western North Dakota is typical of hydraulic fracturing everywhere. Clean water is pumped into a well to open the seams. When the water is pumped out again, it is highly contaminated with sodium chloride and other minerals.
“The brine is dangerous to the local ecosystem if it escapes because it’s so saline. The (electrical conductivity) is over 200. Sea water has an EC of 40 to 50,” Daigh said.
By comparison, prairie cropland is considered to be severely saline if the EC is 10 to 15.
“The biggest spills come from pipeline breaks that go undetected,” he said.“These pipelines carry brine pumped out of a fracking well heading to a storage lagoon somewhere. After the brine settles for a while, it’s pumped into a deep well left over from the days of deep oil drilling.
“So it’s highly saline. We’ve had some of these pipeline breaks let loose three million litres in one spill. The brine ends up following the lay of land and it can carry on for miles and miles. It might only be 40 or 50 feet wide, but it covers a large area.”
Daigh said the fracking process fractures geological formations that are millions of years old and loaded with highly concentrated sodium chloride. Those salts and other minerals are caught in the brine. When everything goes as planned, the brine is injected before it has a chance to contaminate the Earth’s surface.
“Is there a way to harvest those minerals? We’ve had a lot of questions like that,” he said.
“Is there enough lithium in there to extract it commercially? How about the magnesium and the many other minerals? How to use reclaimed salt?”
Daigh said his role is to rehabilitate contaminated land, but these other questions will eventually be answered. He said fighting prairie salinity is difficult, even if there are no brine spills.
“Here in western North Dakota, we’re in a dry climate,” he said.
“Every time we try to leech salts back down into the ground, we’re fighting against what nature wants to do. If there’s salt in the soil and we get rain, the water wants to bring them right back up to the surface.”
The answer might be ferric hexacyanoferrate, a chemical that pulls salt out of the soil and crystallizes it on the surface. Early trials were encouraging.
An NDSU news release said researchers applied ferric hexacyanoferrate to samples of brine-contaminated soil in the laboratory. Twenty-nine to 57 percent of the salt was pulled to the surface after seven days for easier removal. Salt crystals that formed on the soil surface were high in water content and easy to remove. This technique shows promise as a quick and simple way to clean up salt-contaminated soil.
“We started playing with these crystalization inhibitors and found that they shift salt crystallization to where it’s above the soil surface and detached from the soil,” he said.
“Once you vacuum up the crystals, you end up with bare clean ground. There’s no soil in the vacuum and no sodium salt left on the soil. Bare clean ground.
“But we know that one treatment won’t do it. There’s a lower boundary of how much sodium salt has to be in the soil for this operation to work. It won’t work if there’s only a small amount of salt. We can take out the first 50 percent really fast. The other half will require some other remediation action, perhaps other traditional recovery methods such as amendments like gypsum or calcium based leeching techniques.
“We see the best potential for this in first response situations. Once a spill happens, get to it right away with the notion of trying to capture the greatest majority of the salts. Get half of it off in the first week. Maybe tile drain the area quickly to catch the salts before they go down into the ground.”
Daigh said the goal is to make the soil clean enough so it can be re-vegetated and eventually put back into agricultural production.
NDSU says traditional methods that are used to remove salt from soil contaminated by brine spills either take too long or involve removing the soil. Excavating contaminated soil and moving it to designated areas only relocates the problem. Salt still remains in the soil.
“No one has tried to bring the salt up, out of the ground, and harvest it. That would be a permanent remediation strategy,” the university said.
“There was one major obstacle. As water evaporates from the soil, salts get deposited in the soil pores. The deposited salt can form a hard, cemented crust which is very difficult to remove. We needed to find a way to stop this salty crust from forming.
“When the re-searchers applied this chemical to salt-contaminated soils in the laboratory, it made salts crystallize out of the soils and ‘bloom’ out on the surface.”
Twenty-nine percent of the salt had bloomed on sandy loam within seven days of application, 46 percent of the salt had bloomed on loam and 57 percent of the salt had bloomed on silty clay soil.
Daigh said safety was a concern, but toxicity toward humans and the environment is relatively minimal. He said the chemical is ex-traordinarily stable.
When it does decompose, he added, the chemical yields hydrogen and free-cyanides, which can be toxic, but it breaks down quite slowly, taking decades or even centuries under most soil conditions.
This slow breakdown can allow microbes to metabolize the cyanides and render them relatively harmless. As well, ferric hexacyanoferrate is just one example of a chemical that can be used.
“Other chemicals with low-toxicity reaction products may produce similar results,” he said.
“We need to find out how much to apply and how often to apply it.”
Daigh said he is also researching the possibility of using this method to rehabilitate typical prairie salinity but doesn’t think it is likely.
“This works because sodium chloride is highly soluble in water, but calcium salts and magnesium salts have low solubility,” he said.
“The natural near-surface salinity we see on the northern great plains is calcium, magnesium, sulfate based. They don’t stay dissolved in water.
“For this process to work, it re-quires salts to remain suspended in water until they start growing up out of the ground. The more calcium in the soil, the more it tends to cement up. The crystals that start never get a chance to get going and bloom on the surface.”