RECOMMENDED MITIGATION ALTERNATIVE
Table 13 compares the effectiveness, implementability, and cost of the five mitigation alternatives. The mitigation alternatives all meet the mitigation objective of providing a drinking water supply with sulfate concentrations less than 250 mg/L, but do so using different combinations of source control, plume management, and drinking water supply mitigation actions. The selection of a recommended mitigation alternative considered guidance at ARS § 49-286 pertaining to mitigation of non-hazardous releases. ARS § 49-286.A identifies possible mitigation measures as:
1. Providing an alternative water supply.
2. Mixing or blending if economically practicable.
3. Economically and technically practicable treatment before ingesting water.
4. Such other mutually agreeable mitigation measures as are necessary to achieve the purposes of this section
ARS § 49-286.B states “The director’s selection of mitigation measures shall balance the short-term and long-term public benefits of mitigation with the cost of each alternative measure.
The director may only require the least costly alternative if more then one alternative may render water usable as a drinking water source.”
The contingent water supply mitigation provisions of the mitigation alternatives satisfy the provisions of ARS § 49-286.A. To characterize the short- and long term benefits of the mitigation alternatives, the effectiveness and implementability of the mitigation alternatives were evaluated with respect to the following factors as discussed in Section 4:
Practically and cost efficiently provide the owner/operator of an existing drinking water supply impacted by the sulfate plume from the STI with a drinking water supply with sulfate concentrations less than 250 mg/ltr:
· Control of plume migration
· Reduction of plume extent
· Minimization of mitigation pumping
· Duration and magnitude of source control pumping
· Net sulfate mass removal
· Technical implementability
· Administrative implementability
· Potential water level changes due to the mitigation alternatives
On consideration of the benefits and costs of the mitigation alternatives, and consistent with ARS § 49-286, Alternative 5 is recommended as the preferred alternative because it would provide superior effectiveness (Section 4.4.1), the least amount of long-term water level impact (Section 4.4.2.4), and greater cost effectiveness (Section 4.4.3.2) compared to the other alternatives considered.
Sierrita would implement Alternative 5 using an adaptive management approach consistent with Section III.D of the Mitigation Order. The adaptive management approach, which includes the use of contingent measures, would allow the mitigation to respond to scientific (i.e., groundwater monitoring results or engineering data), administrative (e.g., new laws or evolving water supply constraints, etc.), or business (e.g., changes in mine production.
