2008-08-14_REVISION - M1980244 (3)

Home Browse Search

14 August 2008 Pg. 3 (14) In Grassy Valley, the hydraulic gradient in the diatreme at depth is expected to be essentially opposite to that of the shallow alluvium (Fig. 4.9). It is difficult to comprehend that all infiltration will be captured by the deeper hydraulic gradient before being transported down the Grassy Valley and off site by the shallow gradient especially if the alluvium is more transmissive than the diatreme. This possibility seems more likely in the event of higher precipitation or runoff events, and such events could lead to metals loading to the Grassy Valley alluvium and points downgradient. The proposed mitigation plan in the event of this occurrence appears satisfactory, but the monitoring plan seems a bit deficient in ground water monitoring to detect the occurrence upgradient of the compliance points. Please address this concern in terms of how the proposed monitoring will detect such an occurrence, or provide an augmented plan perhaps consisting of additional monitoring wells upgradient of points of compliance to serve as early warning monitoring sites. (15) In the event of leachate release to the environment, it is always useful to have some knowledge of the expected geochemical interactions between the leachate and the material underlying the pile, bedrock or unconsolidated material, to assess the attenuation properties of the material. This can also be done using leach tests, but might be better approximated using sequential batch tests, which is a documented ASTM method. (16) For post-closure, they should provide a worst-case scenario of the quantity and quality of release from the overburden storage areas. They predict a long-term average release of 6.8 gpm, but ranges would be useful. And again for water quality they stress the bulk acid-base property of the rock. (17) The prediction that the overall seepage water quality will be similar to the Carlton Tunnel water is not entirely gratifying, since the Carlton Tunnel has sulfate running over 1000+ mg/L, which is 4x the drinking water standard. Although the Carlton Tunnel was discharging 1000 mg/L sulfate long before Anglo arrived, a new facility predicted to discharge a constituent with 4x the drinking water standard might cause concern, even though sulfate is a secondary standard. (18) The statement that the water quality will be similar to Carlton Tunnel water is of concern on two levels: A. First, these are two significantly different environments - one being an underground flow system that has been flowing for decades and has established some level of equilibrium as evidenced by the relatively consistent water quality of the tunnel discharge, the other a pile of fresh, crushed rock that will be exposed to a new environment of weathering and oxidizing conditions. It is unlikely the two systems will weather and leach similarly. B. Second, the Carlton Tunnel has been there for 60+ years, and if an assumption is made that the ground water flow system feeding it has established some kind of stasis or equilibrium, then I wonder about the existence of preferential flow paths causing selective dissolution and depletion of available minerals along those flow paths. The OB that will be crushed and piled will be fresh material that has not undergone that level of selective dissolution and depletion.