Injecting fluid into the Earth can trigger earthquakes, but not every injection well is related to fracking. While the controversial process has brought a particular method of resource extraction into our day-to-day vocabulary, other types of injection wells can be just as guilty of triggering unintended consequences.

Hydraulic fracturing (fracking) well site in Colorado. Photography credit: Brennan Linsley/AP


I’m seeing a lot of stories covering the link between injection wells and induced earthquakes in Oklahoma, but most of them make the same error: this study had nothing to do with fracking. The injection wells in Oklahoma causing the swarms of small-magnitude earthquakes are used to dispose of wastewater from dewatering operations. These particular wells are still part of oil and gas production, but don’t fracture the surrounding rocks. Instead, extraction wells suck up water already in the formation. After yanking out the oil and gas, the leftover wastewater is injected back into the ground. The change in water pressure distribution induces the earthquakes, some quite far away from the actual wells.

This isn’t to say that fracking is safe and harmless. Fracking probably does induce earthquakes, maybe even some of the earthquakes in Oklahoma. On top of that, the fluids have a nasty habit of sneaking into the groundwater. But fracking is not dewatering, and the particular story making the rounds in the news circuit isn’t about fracking.


This sounds like a simple error; but it’s important to acknowledge that a whole variety of industrial waste disposal methods are triggering unintended consequences. Conversely, quite a few injection wells have been in use without incident for decades, so a blanket-ban on all subterranean injection would be overkill.

A Brief History of Injection Wells

Injection wells have been in use for a long time. While injection started as a salt-extraction technique a few centuries ago, the first application in the United States is within the past hundred years. In the 1930s, oil production companies figured out that stashing leftover brine down wells not only worked for disposing of waste, but it preserved local fluid levels and sometimes even enhanced oil extraction. By the 1950s, chemical companies figured out that they could use the same technique as a cheap disposal method, particularly for hazardous byproducts. At about the same time, states start regulating how brine can be disposed.


The next decade, the first reports of injection wells inducing earthquakes popped up in Colorado. The 1960s also marked the first times we started tracking contamination from injection in the water supply. By the 1970s, waste injected by a paper mill was traced leaking out of an abandoned oil well. This is about when Congress realized it had a problem, and authorized the EPA to protect underground drinking water from injection. The Safe Drinking Water Act was the start of the Underground Injection Control regulations establishing the first five classifications of injection wells. The regulations started stepping back in 1980, with Congress amending the act to allow oil and gas programs to self-regulate. But that’s alright, because they also passed the Hazardous and Solid Waste Amendments, making well operators promise that any hazardous waste will either be rendered non-hazardous or be trapped for 10,000 years.

Sodium lactate and whey powder are injected into the groundwater at this injection well in order to feed microbes as part of a groundwater remediation project. Photography credit: Department of Energy/INL


After that, things got complicated. Large-capacity cesspools got their own special branch of regulation within the Class V catch-all. Oil companies were told to shut down shallow motor vehicle waste disposal wells. Academics circled around to run their first conference on deep injection wells. Florida got a special set of rules just for their Class I municipal wells. Congress exempted fracking fluids from federal regulation. The EPA started looking at carbon dioxide sequestration. Now it’s a whole mess of regulation with a whole lot of exceptions.

What Exactly Are Injection Wells?

Injection wells are used to store fluids underground in porous rock formations. The construction of the well depends on the fluid and where it’s being injected. A well can be a bored, drilled, or driven shaft, a hole that’s deeper than it is wide, an enhanced sinkhole, or anything else used to distribute fluid below the subsurface. Fluids are typically injected into porous rocks like sandstones or limestones, although for shallow wells, the fluid can be injected into a soil layer. The injected fluids may be water, wastewater, brine (saltwater), or water mixed with various chemicals.


The Environmental Protection Agency divides injection wells into six categories. Five of the categories are sets of wells that serve similar functions, so are regulated to have similar construction and operating features with consistent technical requirements. The sixth group is the catch-all other category of wells that don’t fit any other groups. The categories are:

Class I, II, III, and IV wells. Image credit: EPA

Class I: Industrial & Municipal Waste Disposal Wells

Class I wells are used for the injection of hazardous waste, industrial non-hazardous liquids, municipal wastewater, or radioactive waste beneath the lowest underground source of drinking water.


The roughly 680* wells in the US are deep, typically injecting 1,700 to 10,000 feet below the surface beneath an impermeable cap rock, and clustered around the Gulf of Mexico. Class I wells have some of the most stringent regulations, with extra regulations for the subclassification of wells used to dispose of hazardous waste.

* Well counts are from a 2011 inventory; the real number is probably higher by now.

Class II: Oil and Gas Related Injection Wells

Class II wells are used for the injection of brines and other fluids associated with oil and gas production (including fracking), and storage of hydrocarbons as part of the strategic reserve. Of the wells associated with production, about 80% are extraction wells and 20% are waste disposal wells.


Wastewater from a fracking operation in Midland, Texas. Photography credit: Pat Sullivan/AP

The United States has at least 172,068 Class II wells, mostly in Texas, California, Oklahoma, and Kansas. States are allowed to place their own individual regulation on Class II wells, but the amendments to the 1980s laws mean that the industry gets to self-regulate. The most recent set of amendments exempt fracking fluids from federal regulation entirely.


Common practice is that any waste disposal is supposed to go back into the same formations that oil or gas was originally extracted from. The idea is that then the brine loaded with toxic metals and radioactive materials won’t get into surface water, but in practice the formation may not be totally isolated from underground water sources. That’s where we get problems like, “Uh, why is my water now flammable?” and some of the earthquake swarms.

Class III: Mining Wells

Class III wells are used for the injection of fluids used for solution mining of minerals below the lowest underground source of drinking water.


The United States has somewhere over 22,131 Class III wells, used to dissolve and extract minerals like uranium, salt, copper, and sulfur. It’s a popular mining method: about 50% of all salt and 80% of all uranium extraction in the United States uses Class III injection wells for in-situ leaching. Each of the minerals requires a different fluid — uranium uses lixiviant, salt uses plain water, copper requires sulfuric acid, and sulfur requires superheated steam. Only the injection wells are regulated by Underground Injection Control; production wells used to bring the mining fluids to the surface are not part of the regulations. The biggest rule is that more fluid must be extracted than injected so that fluids don’t spill outside the subterranean mining area.

It’s relatively common for uranium mines to require injection into a potential underground source of drinking water, so the mines need to apply to the EPA for an aquifer exemption.

Class IV: Shallow Hazardous and Radioactive Injection Wells

Class IV wells are used for the injection of hazardous or radioactive waste into or above underground sources of drinking water. Before you freak out, this particular class of well is banned except in special circumstances as part of a federal or state groundwater remediation project.


If Class IV wells sound like a terrifyingly bad idea, you’ll be relieved to know the United States only had 33 of them in 2012. It’s less reassuring to know they were only banned in 1984. Although it sounds utterly demented that they’re allowable in any circumstances, it’s actually a standard part of “pump and treat” mitigation:

  1. Pump contaminated ground water to the surface.
  2. Treat the water to remove as much contamination as possible.
  3. Re-inject the treated water into the same formation, even if it’s still somewhat contaminated.
  4. Repeat until the contamination concentration is too low to treat.

If the water is still contaminated during step 3, the injection well is a Class IV. If the water is totally clean and free from hazardous materials, the injection well is a Class V aquifer remediation well.


Class V and VI wells. Image credit: EPA

Class V: Other Wells

Class V wells are the “other” category of wells that don’t fit in the earlier categories. They’re generally shallow wells used for injecting non-hazardous fluids for on-site disposal, but a few wells inject below the lowest underground source of drinking water.


With more than 20 sub-categories of wells, Class V is by far the most numerous well-type within the United States. At last count, the inventory was somewhere over 650,000, and even then the EPA acknowledged it probably didn’t have a complete count. This category includes all the simple shallow domestic disposal wells — storm drains, cesspools, and septic system leach fields. It also includes all the industrial wells that don’t fall into the above categories, like aquifer storage and recovery wells, injecting geothermal fluids extracted from subsurface hydrothermal systems, subsidence control, mine backfill, carwashes, food waste disposal, and experimental new technology.

Please don’t dump hazardous materials down the storm drains. Photography credit: Toby Talbot/AP


It gets a bit creepy when you realize that Class V wells are most common in areas that don’t have sewer systems, which usually directly coincides with areas that are dependent on groundwater. While the regulations theoretically protect groundwater from someone from slapping down a septic system where it will leach into the aquifer, communities are pretty much left to fend for themselves to enforce drinking water protection.

Class VI: Geologic Sequestration of Carbon Dioxide

Class VI wells are used for geologic sequestration of carbon dioxide. This is the latest classification of wells, a belated addition permitted by regulation in 2010. The idea is to speed up natural sequestration processes by directly pumping the gas into the subsurface as part of carbon capture and storage.


Only a handful of wells have been authorized so far — somewhere between 6 and 10 projects are expected to be functional by 2016. Along with the usual “protect groundwater” mandate, regulations on Class VI wells include a few unique features to deal with carbon dioxide, particularly its relative buoyancy and mobility, its corrosive nature when mixed with water, and how to cope with the anticipated huge injection volumes.

Injecting carbon dioxide into the Mount Simon Sandstone. Photography credit: Daniel Byers/Illinois State Geological Survey


Which Wells Trigger Earthquakes?

Injection wells are a complicated topic, with understandably a lot of rules to go with them. EPA regulations were developed to protect the drinking water supply, but maybe it’s also time to start looking at protecting developments from induced earthquakes. Lubricating the subsurface with fluids and changing porewater pressure in areas with faults is clearly having an effect.

Directly tying a particular earthquake to a particular injection is hard work. Blending the slipperiness of statistics with the black magic of geophysics does not make for a whole lot of certainty, but we can make some pretty solid guesses. Lubricating fault lines, be it from fracking, dewatering operations, or whatever else we’re cramming back into the ground, can impact subsurface dynamics, triggering earthquake swarms.


When the Class VI wells start ramping up operations, we can potentially see a whole new world of unintended consequences. The initial wells are operating after extended research, but it’s difficult to predict the impact of sequestration and geological engineering when transitioning from small-scale models to large-scale real-world testing.

It’s about time we started agitating for policy changes to find a better balance between the necessity of resource extraction and the protection of public safety. Don’t be blinding into thinking fracking is the only problem. When advocating for protective policies in your community, remember that earthquakes can come from more types of injection wells than those used in oil and gas production. Keep demanding better monitoring, measuring, and reporting on not just fracking operations, but also on other forms of deep injection wells.

Just, try not to accidentally ban your septic system in the process.