USING STABLE AND RADIOGENIC ISOTOPES TO RESOLVE MINE GROUNDWATER ISSUES

Physical hydrogeology data are often inadequate to fully evaluate the impacts of mining operations on non-mine water. In many cases solute and isotopic chemistry provide the insight to resolve difficult and often contentious issues. We present three examples where stable and radiogenic isotopes were used to 1) abrogate a notice of violation (NOV), 2) end the threat of water rights infringement lawsuits, and 3) evaluate leakage of contaminates from ponds.

Case 1 - alleged underground leakage. Colorado issued West Elk mine an NOV alleging in-mine groundwater leaked through a barrier wall into adjacent abandoned workings and that this water discharged to the surface. δ¹³C, δ²H and δ¹⁸O were used in conjunction with geochemical modeling to demonstrate that the water from the mine does not discharge to the surface and the NOV was rescinded.

Case 2 - alleged infringements of surface water rights by in-mine groundwater interception. Most Utah underground coal mines intercept groundwater and this interception has been attributed to declines in spring discharges. Using stable and radiogenic isotopes in conjunction with physical hydrogeology data in the 240 km long Wasatch Range and Book Cliffs, we developed a conceptual model of groundwater flow that describes active and inactive systems. Active zone groundwater flow paths are continuous, responsive to annual recharge and climatic variability, and have groundwater resident times “ages” that become progressively older from recharge to discharge area. This water discharges from thousands of springs and contains appreciable ³H and anthropogenic ¹⁴C.

Inactive zone groundwater has extremely limited or no connection with annual recharge. Groundwater may occur as discrete bodies, is encountered 300-700 m bgs, and has δ²H and δ¹⁸O compositions distinguishable from near surface groundwater. In general, deep waters have no ³H and mean ¹⁴C ages of 500 to 20,000 years (45.9 to 4.9 pmc). Based on this investigation the pending infringement actions ceased.

Case 3 – pond leakage. Evaporation of holding pond water often results in an evaporative δ²H and δ¹⁸O signature and leakage of this water will partially impart this signature onto downgradient groundwater. Using pond and up-gradient groundwater as end members the extent of leakage has been evaluated at several locations.