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Evapotranspiration control on groundwater migration at a uranium mine in the high desert

Paper from the Proceedings of Mine Closure 2015 conference held in Vancouver, Canada, June 1-3, 2015. (downloadable PDF)

Authors: C. Ardito and J.M. Sigda, INTERA Incorporated, USA; R. Blickwedel General Electric Company, USA

Published by InfoMine Inc.
ISBN: 978-0-9917905-9-3
Copyright: 2015, InfoMine Inc.


Abstract

The St. Anthony mine is located within the Grants uranium mineral deposit, a west-to-northwest trending mineral belt approximately 150 km (95 mi) long and 2.5–32 km (1.5–20 mi) wide (McLemore, 2007). In general, these deposits are formed within aquifers as defined by the New Mexico Water Quality Control Commission (WQCC) regulations. As these deposits create groundwater conditions that naturally exceed WQCC standards for groundwater and there is little to no premining groundwater quality data, permitting and closure of uranium mines in New Mexico require establishment of alternative groundwater concentration limits.

The St. Anthony mine is a combined open-pit and underground uranium mine near the eastern margin of the San Juan Basin in central New Mexico. Ore was mined from the Jackpile sandstone member of the Jurassic Morrison Formation. The very fine-grained Jackpile sandstone has a low permeability, approximately 0.03 m/day (0.09 ft/day), which forced development of alternative water sources during mining operations.

Surface mining took place in two pits: the large pit, which penetrates most of the Jackpile sandstone and has a small pit lake; and the small pit, which does not fully penetrate into the Jackpile sandstone. A fate and transport analysis of uranium and several other constituents of concern was completed to evaluate the potential for postclosure impacts to human health and the environment.

In its current state, the large pit of the St. Anthony mine captures groundwater via a cone of depression that has developed in response to evaporation of pit water; however, reclamation alternatives that include backfilling the large pit would eliminate the evaporation and ultimately result in the loss of groundwater containment. A feasibility study using a multiple accounts analysis (MAA) (Robertson and Shaw, 2004) resulted in the selection of both pit backfill and pit backfill waiver options as the highest scoring alternatives. The pit backfill waiver alternative ranked high due to containment of mineralised groundwater. As pit closure is the preference of key stakeholders, and regional groundwater flow conditions would result in the loss of containment of mineralised groundwater, a detailed hydrologic analysis was necessary to ensure there was no complete groundwater pathway resulting in risk to public health and the environment.

The climate of the southeastern San Juan Basin is arid to semi-arid with variable precipitation that is consistently exceeded by evaporation and transpiration demands. Recharge into the subsurface is negligible across most of the site because topography and the low permeability of the surficial materials severely constrain recharge, leaving most precipitation to be lost to evapotranspiration. Water balance calculations on the large pit revealed that evaporation is the primary outflow driver and that groundwater inflow through the Jackpile sandstone is on the order of 26.5 l/min (7 gpm). Similar calculations revealed that the small pit is a potential intermittent source of recharge.

Our hydrogeologic analysis and groundwater flow model revealed the following hydrogeologic conditions: (1) the Jackpile sandstone is predominantly a confined system; (2) precipitation is far exceeded by transpiration during the growing season and evaporation throughout the year, with negligible opportunity for areal recharge of the Jackpile sandstone; (3) current groundwater flow through the Jackpile sandstone is controlled by evaporation within the main pit, by evapotranspiration and seepage in the adjacent Meyer Draw drainage,and by outcrop evaporation; and (4) tamarisk transpiration in the Meyer Draw drainage is estimated to occur at a maximum rate of approximately 53 l/min (14 gpm).

Predictive modelling of postclosure groundwater flow demonstrated that local groundwater discharge to Meyer Draw via tamarisk transpiration or seepage into alluvium in contact with the Jackpile sandstone is more than sufficient to capture groundwater migrating from the pits and surrounding site area. Sensitivity analyses revealed that major changes in boundary conditions or Jackpile hydraulic conductivity did not change the capture of groundwater by Meyer Draw by transpiration and seepage. Relatively low outflow rates are required to keep any solute plume on site because of the Jackpile sandstone’s low permeability.


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