Underground mining in the Colorado Plateau had virtually ceased by the 1980s but mining has had a resurgence in recent years as the future for nuclear power has become rosier. In Wyoming, uranium mining was done in large open-pit mines but there are currently no operating open-pit mines in the US. A newer form of mining known as in situ recovery (ISR) or in situ leaching (ISL) is much less hazardous and environmentally damaging and has become the dominant form of mining in the United States. ISR accomplishes both the mining and the milling procedures in one step at the site of the uranium mine but does not create mill tailings.

How does ISR work? To answer that, we need to understand the geology of the uranium deposits. In the United States, most deposits of uranium occur in sandstone deposited by ancient meandering streams. Weathering of mountains containing uranium-bearing granite deposited the uranium in the sands of the streams that were later covered and formed into sandstone. If the sandstone was trapped between impermeable layers of shale, the conditions were set for trapping uranium in an underground aquifer. Uranium is highly soluble in water in its oxidized form. When rainwater with dissolved oxygen flowed into the sandstone aquifer containing uranium, it dissolved the uranium. As the water containing the oxidized uranium slowly percolated through the aquifer, it would accumulate in the presence of naturally occurring reducing agents such as sulfides (pyrite [FeS2], for example) or organic material. The uranium would then precipitate out to form what is known as a roll front—a convex surface containing a high concentration of uranium (Figure 11.1) (21).

ISR essentially reverses the process by which the uranium was deposited. The uranium is in an aquifer under chemical-reducing conditions. By pumping oxy­genated water and sodium bicarbonate (baking soda) into the aquifer, the oxi­dized uranium will go back into solution and can be recovered. A typical mining

configuration consists of a recovery well surrounded by four injection wells. Water containing the lixiviant (oxygenated water and sodium bicarbonate) is pumped into the injection wells and is drawn up through the recovery well (21).

The water containing dissolved uranium is pumped into a central processing plant, where it passes through ion exchange columns that extract the uranium from the solution, and the water is reused in the injection wells. About 1% of the water is bled off and stored in a holding pond, so there is always more water being recov­ered than is being pumped into the injection wells. This creates a negative pressure in the uranium aquifer to prevent dissolved uranium from leaving the aquifer. The ion exchange columns are eluted with a solution that extracts the uranium into a yellow slurry that is dried to form yellowcake, exactly analogous to the process in a milling plant (22, 23). In some cases—depending on the size of the mining opera­tion—the ion exchange columns loaded with uranium are not processed at the mine but are shipped to a central milling facility where the yellowcake is produced.

Just east of Fort Collins in the grasslands lies an underground formation that contains uranium in a roll front that is amenable to ISR. A Canadian uranium mining company, Powertech, has proposed to build an ISR facility to recover the uranium, though as of this writing they are not actively pursuing the mining option (24). The mining proposal raised a firestorm of concern by residents in the vicinity of the proposed mine, centered in the small town of Nunn. Making use of a caricature of radiation—that it makes you glow—they developed an anti-uranium-mining website called NunnGlow (www. nunnglow. com) and sprinkled the area with yellow signs saying “no to uranium mining in Colorado" So here is the conundrum—people want to have clean energy but the NIMBY principle (Not In My BackYard) or, in this case NIMS (Not In My State), means they don’t want it anywhere near where they live. Neighbors are opposed to a proposed wind farm north of where I live and to in situ uranium mining east of here. The huge Niobrara oil and gas field in eastern Colorado is being fracked to get natural gas, a fossil fuel that emits less carbon dioxide (CO2) than coal when it burns, but people along the Front Range of Colorado are opposed to it, too. Since nuclear power produces 20% of the electricity in the United States and contributes very little CO2, it is a clean energy source, yet if you can’t mine uranium, you can’t have nuclear power.

But do the citizens in the area have a right to be concerned? To be honest, few people, including myself, would be thrilled to have an ISR plant next to them because of the initial drilling of wells. No one likes industrial development in their neighborhood, but I would far rather have an ISR facility near me than a natural gas field that has far greater impact on roads from truck traffic and on water supplies. Once the ISR wells are drilled, there is very little disturbance of the area, only wellhead covers and a central facility containing the ion-exchange columns (Figure 11.2). The uranium extraction and recovery phases of an ISR mining operation usually take two to five years (25).

The main concern is that the mining would contaminate an aquifer used for agricultural or human use. However, the aquifer containing the uranium cannot be used for agricultural or human use anyway because it is naturally contaminated

with uranium, radium, radon gas and other elements such as vanadium and lead. The aquifer containing the uranium lies between layers of impermeable shale, so water can’t migrate into a lower aquifer. Monitoring wells are drilled outside the mined area, so any excursions of uranium into the outlying aquifer can be detected and dealt with simply by pumping more water out of the collection well. This creates a negative pressure on the surrounding aquifer and draws the contaminated water back into the well site. Also, there are thousands of existing water wells in the West and Midwest with uranium concentrations that exceed the EPA standard of 30 micrograms per liter (pg/l) because of the natural pres­ence of uranium (26), yet these high levels have not caused apparent health prob­lems. There are very few scientific studies—and no reliable ones—on this subject, however.

The US EPA regulates underground injection wells that are used in ISR min­ing and will not issue a permit unless stringent requirements are met. If the mine operator is injecting into an underground source of drinking water (USDW), they must get an aquifer exemption. An exempted aquifer must not currently serve as a source of drinking water and will not serve as a source of drinking water in the future (27). Besides requiring an aquifer exemption, the EPA specifies the well construction materials and requires casing and cementing to prevent any flow of water between the uranium-containing aquifer and any USDW aquifer. Monitoring wells must be drilled in the aquifer and at aquifers above and below the ore zone. If an excursion of uranium in solution occurs in the region of the monitoring wells, the recovery wells in the vicinity can be pumped to create a lower pressure within the producing aquifer and draw water back into the well field zone and away from the excursion. Finally, the operators must properly plug the wells after mining is completed (28, 29).

The bleed-off water contains a low concentration of radium (226Ra) that goes into the holding pond and must be disposed of properly according to Nuclear Regulatory Commission (NRC) or state regulations, the same regulations that govern conventional mill tailings and the required removal of uranium and other radionuclides that are often found in public drinking water systems. In some cases, the water can be injected into deep wells in an aquifer that is unsuitable for human consumption. The other radionuclide of interest is radon (222Rn) that is dissolved in the leach water and is released into the atmosphere, where it dis­sipates harmlessly but may need to be vented from the processing building (21). Overall exposure to the general public must be less than 1 mSv/yr at the boundary of the ISR facility, a dose that has no health consequences and is less than a quarter of natural background radiation in Colorado (see Chapter 8).

The overall operation of ISR facilities is governed by the NRC. The majority of states, including Colorado, are Agreement States that have formal agreements with the NRC to regulate radioactive materials through state agencies. They must meet or exceed the NRC regulations. Colorado drafted such stringent conditions that it may be impossible to have any ISR facilities in the state, leading Powertech to sue the state and, in fact, to place the project on hold, at least for now (24, 30).

ISR mining has been done for over 30 years in Texas, but ISR facilities also exist in Wyoming and Nebraska and are licensed in New Mexico (31). The NRC studied ISR well fields in Wyoming and Nebraska that have been restored after recovery of uranium. While the levels of some water parameters such as alkalin­ity, magnesium, manganese, sodium, lead, and radium did not return to exact baseline levels in some of the well fields,

for the approved restorations, the impacts to groundwater in the exempted aquifer met all regulatory standards for the state or EPA UIC (underground injection control) program, met the quality designated for its class of use prior to ISR operations, have been shown to decrease in the future due to natural attenuation processes, and have been shown to meet drinking water standards at the perimeter of the exempted aquifer. Therefore, the impacts of the exempted aquifer for each of the approved restorations do not pose a threat to human health or the environment. (32)

The NRC also studied the frequency of excursions from the ore site and found that, while there was a small frequency of excursions, they were adequately con­trolled by the pumping and injection process. Finally, the study analyzed well integrity failures. These have occurred infrequently and have not posed any threat to the environment or to human health.

You don’t just have to take the word of the NRC for it, though. Scientific epi­demiological studies have been done to determine whether ISR mining and mill­ing operations cause cancer in populations surrounding them. The region in the United States with the longest experience with ISR is Karnes County, Texas, where 40 mines and 3 mills operated beginning in the 1960s. The principal radia­tion concerns from mining and milling come from uranium, radium, and radon. Radium accumulates in the hard bone surfaces and can cause bone cancer, but recall that the tissue weighting factor of bone is 0.01, so it is very insensitive to radiation-induced cancer (see Chapter 7). Uranium can be inhaled into the lungs and cause lung cancer, but it can also be taken up by body tissues, mostly bone, kidney, and liver. Radon is a gas and can cause lung cancer, as it did in some of the Navajo miners. A cancer mortality study covering 50 years of potential exposure to uranium, radium, and radon from mining and milling activities found that there was no increase in cancer mortality rates for lung, liver, kidney, or bone cancer (or any other cancers) in Karnes County compared to control counties (33). In fact, the cancer mortality rates both in Karnes County and the control counties was less than the overall cancer mortality rate in the United States. This is consistent with the previously mentioned studies in Montrose County, Colorado, which also showed that milling and mining did not cause elevated cancer mortal­ity among the population, with the exception that underground uranium miners who smoked had higher lung cancer rates (17, 18).

So, in spite of the concerns raised about ISR being a health risk or contaminat­ing aquifers, there is no evidence to support any adverse health effects on the environment or on people in areas surrounding ISR facilities. This is a much more efficient method to obtain uranium than conventional mining, it does not pro­duce mill tailings that have to be carefully managed, and it uses far less energy (and hence production of CO2) than conventional mining. The alternative is to depend on coal mining and fracking for natural gas, which can be far more dam­aging to the environment.