U.S NUCLEAR REGULATORY COMMISSION:
In-situ leach uranium mining affects groundwater quality when leaching solutions travel beyond well field boundaries
Recent NRC document describes horizontal and vertical excursions at ISL mines in Wyoming and Nebraska, some lasting for as long as 8 years
Posted September 13, 2009
Powertech officials will swear on a stack on bibles that the company can mine uranium in Northern Colorado's Laramie-Fox Hills Aquifer and never contaminate the groundwater used by hundreds of nearby residents, ranchers, and farmers. But the fact is that in-situ leach uranium mines have a history of excursions, when leaching solutions containing high levels of uranium, radium, and other heavy metals escape from the mining area. When excursions are detected, the mine operator must attempt to reverse the excursion by changing pumping rates in the well field.
By law, excursions must be immediately reported to the appropriate regulatory agency upon detection. Below is the full text of Section 2.11.4 of the NRC's Generic Environmental Impact Statement for In-Situ Leach Uranium Milling Facilities (NUREG-1910), which provides a brief overview of excursions reported to the federal agency.
JW
2.11.4 Excursions (Vol. 1, Page 2-46)
As discussed in Section 2.4, ISL operations may affect the groundwater quality near the well fields or in overlying or underlying aquifers if lixiviant travels from the production zone and beyond the well field boundaries. Monitoring wells are designed and placed to detect any lixiviant that moves out of the production zone. A monitoring well is placed on excursion status when two or more excursion indicators exceed their respective upper control limits (UCLs) (NRC, 2003a). Alternate excursion detection procedures (e.g., one excursion indicator exceeded in a monitoring well by a specified percentage) may also be used if approved by NRC. NRC licensees are required by license conditions to identify reporting, monitoring, and response measures to be taken to determine the extent and cause of the excursion, as well as measures to recover the excursion and remove the well from excursion status.
Historical information for several facilities indicates that excursions occur at ISL operations (NRC, 2006, 1998a,b, 1995; Crow Butte Resources, Inc., 2007; Cameco Resources, 2008; Arbogast, 2008). For example, from 1987 to 1998, 49 wells were placed on excursion status at the Irigary and Christensen Ranch uranium recovery facility in Campbell and Johnson Counties in the Wyoming East Uranium Milling Region (NRC, 1998a). Most of these excursions were recovered within a period of weeks to months, but six vertical excursions proved more difficult to return to baseline, with two wells remaining on excursion status for at least 8 years. These excursions were believed to be due to improperly abandoned wells from earlier exploratory programs prior to regulation by a UIC program. In 2007, three wells were on excursion status at the Christensen Ranch project, with only one, originally identified in 2004, remaining on excursion status at the end of 2007 (Arbogast, 2008a). None of the earlier excursions that affected monitoring wells identified in NRC (1998a) were on excursion status in 2007 (Arbogast, 2008b). An additional well at the Christensen Ranch project was placed on excursion status in 2008 (Arbogast, 2008b).
From 1988 through 1995, 22 monitoring wells (11 vertical and 11 horizontal) were placed on excursion status for the Highland Uranium Project located in Converse County in the Wyoming East Uranium Milling Region (NRC, 1995). Most of the excursions were recovered within less than 1 year, but four horizontal excursions lasted up to at least five years. In two of these wells, the excursions were due to a thinning of the confining layer that separated two production zones. Groundwater pumping during restoration of the underlying production zone resulted in a hydraulic gradient that brought excursion fluids down from the overlying aquifer. One of the other excursions was believed to be the result of fluids migrating from an upgradient abandoned uranium mine (NRC, 1995). No cause was identified for the other long-term excursion at the Highland Uranium Project. Only one horizontal excursion was reported between 2001 and 2005 at the Smith Ranch-Highland uranium recovery facility, and corrective action brought the well back below the UCLs within less than one month (NRC, 2006).
At the Crow Butte ISL facility located in Dawes County, Nebraska (Nebraska-South Dakota-Wyoming Uranium Milling Region), the operator reported five vertical excursions into the overlying aquifer from the start of commercial operations in 1989 through the license renewal in 1998 (NRC, 1998b). In two cases, these excursions resulted from well integrity problems (borehole cement contamination and a failed casing coupling). One excursion resulted from a leak in a plugged and abandoned injection well, and the remaining two were believed to result from natural fluctuations in the groundwater quality (NRC, 1998b). Between 1999 and 2006, 17 wells at the Crow Butte facility were placed on excursion status (7 vertical and 10 horizontal). Most of these wells were restored below the UCLs within 1 to 6 months, although one vertical well took almost four years to restore (Crow Butte Resources, Inc., 2007). In the second half of 2007, three horizontal monitoring wells were on excursion status (Cameco Resources, 2008). These excursions were first identified in April 2000, December 2003, and September 2006 (Crow Butte Resources, Inc., 2007). The licensee believes that these longer term excursions resulted from well field geometry and well field flare as a result of ongoing groundwater transfer and well field restoration activities.
Operational experience at these facilities indicates that lixiviant excursions can result from
- Thinning or discontinuous confinement
- Improperly abandoned wells that may provide vertical flow pathways- Casing failure or other well leaks
- Natural fluctuations in groundwater quality
- Improper balance of well field hydrologic gradients
Most horizontal excursions were recovered quickly (weeks to months) by repairing and reconditioning wells and adjusting pumping rates in the well field, consistent with the findings of Mackin, et al. (2001 a). Vertical excursions tended to be more difficult to recover than horizontal excursions, and in a few cases, a well remained on excursion status for as long as 8 years.