Two Mining-Related Examples

Midnite Uranium Mine, Wellpinit, Washington

The purpose of this project was to develop a ground water flow model (GWFM) of the bedrock underlying the Midnite Mine that could be utilized by an EIS contractor and other interested parties. The Midnite Mine technical working group (TWG) included representatives of the Spokane Tribe of Indians (SIT; landowners), U.S. Bureau of Mines (BOM; technical support), U.S. Bureau of Land Management (BLM; primary trustee), U.S. Bureau of Indian Affairs (BIA; secondary trustee), and the U.S. Geological Survey (USGS; technical support).

The TWG envisioned that the model would be used to simulate outcomes of proposed reclamation scenarios put-forth during the EIS process. Constraints put on this effort by the TWG include: 1. the model must be widely used and widely accepted; 2. the model must be able to be updated as new information is gathered. 3. the model must be able to be modified to reflect proposed reclamation scenarios with minimal pre-processing effort; 4. the model must be able to be run on a platform that is commonly used; and 5. the output of the model must be able to be conveyed to the decision makers with minimal post-processing effort. AESE determined that MODFLOW/MODPATH could meet the aforementioned modeling requirements and that a shell model developed using AutoCad could be used to minimize pre-and-post processing efforts.

The geologic shell model (GSM) consists of 14 filterable layers that can be viewed/modified using AutoCad Release 13. The term "shell" as used herein is a three dimensional feature bounded by irregular surfaces. A shell differs from a solid in that no data are present within or linked to the interior of each shell.

The (GWFM) discretizes the study area using 25,536 blocks that are 200 feet long by 200 feet wide by 100 feet high. This rather fine discretization interval serves several purposes: (1) it enables the model to accommodate proposed alternatives such as drains at the very localized scale without having to reconstruct/redescretize the model domain; (2) it allows for analysis of localized phenomena such as flow in the vicinity of the pits; and (3) it results in a more realistic model capable of utilizing structural and geomorphic information. The results of two simulations applied to the GSM are given. This wire-frame also has been animated using 3D Studio and is being used to educate participants involved in the EIS process.

Grouting Fractures in a Small Mine Near Coeur d'Alene, Idaho

Heavy-metals leaching from a small, caved, hard-rock, gold mine were polluting a nearby stream. AESE was charged to reduce metal loads using technology that would not require long-term management. Our experience with acid mine drainage (AMD) problems in similar mines suggested that most of the poor quality water was probably being produced near the remaining oxidizing low-grade ore-body. In order to test this hypothesis AESE team members stabilized and re-opened the mine using conventional mining techniques. Once this was accomplished, we began mapping geology, flowrates, and aqueous-phase geochemistry issuing from different portions of the mine. The results of this effort indicated that our hypothesis was correct. A single fault/fracture system was responsible for only 10% of the total mine flow; however, the system was responsible for 90% of the metal load.

AESE determined that the problem could be solved using a kaolin-based grout. This grout was selected for the following reasons: (1) its physical properties are only slightly affected by pH; (2) it is relatively plastic and heals itself in response to further ground-shift; (3) its low viscosity and long gel-times allow for relatively deep emplacement in low-permeable rocks; (4) unlike chemical grouts, kaolin is not hazardous to the environment; and (5) the grout is cheaper than Portland cement.

The team diminished flow and subsequently heavy-metals issuing from the structure by core-drilling five NX holes into and beyond the fault/fractures in a radial fan pattern. Pneumatic packers and a grout pump were used to emplace the grout.

Return to AESE