Technical Memorandum Sycamore Tailings and Barrel Waste Alternative Stormwater Assessment

Transcription

1 Tucson Office 3031 West Ina Road Tucson, AZ Tel Fax Technical Memorandum Sycamore Tailings and Barrel Waste Alternative Stormwater Assessment To: Kathy Arnold From: David R. Krizek, P.E. Company: Rosemont Copper Company Date: March 05, 2010 CC: Mike Zeller (Tetra Tech) Doc #: 061/ Introduction This Technical Memorandum presents a Stormwater Assessment for the Sycamore Tailings and Barrel Waste Alternative being considered in the US Forest Service Environmental Impact Statement (EIS) for the proposed Rosemont Copper Project (Project). This analysis quantifies the potential impact of the Sycamore Tailings and Barrel Waste Alternative on downstream stormwater flows and average-annual runoff. In order to determine the potential stormwater impact associated with the Sycamore Tailings and Barrel Waste Alternative, predictions were made for the 100-year regulatory flood-peak [in cubic feet per second (cfs)] and the average-annual runoff (in acre-feet) at a common point associated with the affected drainages. With the exception of Sycamore Tailings, the affected drainages on the east side of the Santa Rita Mountains converge at the United States Geological Survey (USGS) Gauging Station No before storm flows pass beneath State Route 83 (SR 83) in a double-barrel box culvert. Per information associated with the station, the contributing watershed area is calculated to be 14 square miles in size. Figure 1 shows the watershed areas contributing to this gauging station. Stormwater runoff from the contributing watershed area associated with Sycamore Tailings, which is part of the Sycamore Tailings and Barrel Waste Alternative, does not flow to the USGS Gauging Station, since it is located on the west side of the Santa Rita Mountains. 2.0 Pre-Mining/Baseline Hydrology East Side Figure 1 shows the pre-mining or baseline watershed conditions on the east side of the Santa Rita Mountains associated with the Sycamore Tailings and Barrel Waste Alternative. These contributing watershed areas drain to the USGS Gauging Station prior to storm flows passing

2 or? q I ctt Memorandum To: Cc: From: Beverly Everson Tom Furgason Kathy Arnold Doc #: 010/ Subject: Transmittal of Stormwater Assessment Memoranda Date: March 15, 2010 Rosemont Copper is pleased to present a series of Technical Memoranda on Stormwater Assessments prepared by Tetra Tech and dated March 15, These analysis include the following alternatives: 1. Mine Plan of Operations 2. Barrel and McCleary Alternative 3. Barrel Only Alternative 4. Scholefield and McCleary Alternative and 5. Sycamore and Barrel Alternative There is also a discussion of the Visual Analysis as it relates to Partial Backfill. These Technical Memoranda are dated March 5, 2010 and I am providing two hardcopies and one electronic file to the Forest Service and one hardcopy and one electronic file to SWCA.

3 beneath SR 83. The baseline stormwater analysis associated with this location is detailed in the Technical Memorandum titled Mine Plan of Operations Stormwater Assessment (Tetra Tech, 2010) prepared as part of the alternatives analysis for the Project. The baseline assessment is assumed to be the same for all the alternatives, including the Mine Plan of Operations (MPO), for storm flows generated on the east side of the Santa Rita Mountains. As indicated above, an additional baseline assessment was made for the Sycamore Tailings portion of Sycamore Tailings and Barrel Waste Alternative, which is presented herein. This is the only alternative with facilities on the west side of the Santa Rita Mountains. Table 1.0 shows the pre-mining results for the 100-year regulatory flood peak and the averageannual runoff arriving at USGS Gauging Station No , based on the analysis performed for the MPO stormwater assessment. These values also apply to the baseline or premining conditions associated with the Sycamore Tailings and Barrel Waste Alternative on the east side of the Santa Rita Mountains. Table 1.0 East Sycamore Tailings and Barrel Waste Baseline Hydrology Results Baseline Conditions (DA = 14.0 square miles) Point of Concentration Peak Discharge Average-Annual Runoff USGS Gauging Station 8072 cfs 1407 acre-feet DA = Discharge Area 3.0 Post-Mining Watershed Conditions East Side Figure 2 shows the estimated eastern boundary of the post-mining contributing watershed area associated with the Sycamore Tailings and Barrel Waste Alternative. For this alternative, it was assumed that the following stormwater controls would be applied: Stormwater drainage channels would be placed at every 100 feet of vertical rise on the outer slopes of the Waste Rock Storage Area. Stormwater would flow off these benches to stilling pools/drop-structures located on the outer slopes. Drop-structures located on the northern half and a portion of the western half of the Waste Rock Storage Area would convey flows to the USGS Gauging Station location. Dropstructures would also be placed on the southern half of the Waste Rock Storage Area. Stormwater runoff generated from the southern face would be routed to containment areas located between the toe of the Waste Rock Storage Area and adjacent natural ridge areas. These areas would generally be sized to contain the Probable Maximum Flood (PMF) event. Due to the configuration of the Waste Rock Storage Area, contouring and the creation of wide benches to pond stormwater runoff may not be achievable under this alternative. Stormwater runoff generated from the top surface of the Waste Rock Storage Area would be routed to stormwater control basins located on the southern edge of the facility. Decant structures would then pass overflow to stilling pools/drop-structures 2

4 located on the south face. Stormwater control basins would not be located above the closed and encapsulated Heap Leach Facility. Construction of a portion of the AMEC Earth & Environment, Inc. (AMEC) diversion channel is assumed. This diversion routes stormwater runoff around the Plant Site area to McCleary Canyon Wash drainage, which eventually drains to the USGS Gauging Station. The Pit Diversion, located to the south of the Open Pit, is expected to discharge to an area located between the toe of the Waste Rock Storage Area and an adjacent natural ridge and will not drain to the USGS Gauging Station. There are no flow-through drains associated with the Waste Rock Storage Area under the final closure configuration. 4.0 Post-Mining Hydrology East Side Figure 2 depicts the estimated post-mining contributing watershed area (about square miles) draining to the USGS Gauging Station for the Sycamore Tailings and Barrel Waste Alternative. The contributing basin area shown on Figure 2 is only applicable to the 100-year regulatory flood-peak and to the average-annual runoff, based on the following assumption: The perimeter areas associated with the Waste Rock Storage Area are assumed to contain storm runoff from up to the PMP event. Storm runoff from the Pit Diversion is not anticipated to reach the USGS Gauging Station. Table 2.0 below shows the anticipated post-mining results for the 100-year regulatory flood peak and for the average-annual runoff arriving at USGS Gauging Station No for Sycamore Tailings and Barrel Waste Alternative. Attachment 1 provides the backup data for the post-mining stormwater assessment. Table 2.0 East Sycamore Tailings and Barrel Waste Post-Mining Hydrology Results Post-Mining Conditions (DA = square miles) Point of Concentration Peak Discharge Average-Annual Runoff USGS Gauging Station 7106 cfs 1110 acre-feet 5.0 Pre-and Post-Mining Conditions West Side The pre-mining/baseline and post-mining watershed conditions for the Sycamore Tailings located on the west side of the Santa Rita Mountains, and associated with the Sycamore Tailings and Barrel Waste Alternative, are shown on Figure 3. 3

5 5.1 Methodology The methodology used to compute the post-mining 100-year regulatory flood peak and averageannual runoff for the Sycamore Tailings located on the west side of the Santa Rita Mountains was identical to the methodology described in detail in the Technical Memorandum titled Mine Plan of Operations Stormwater Assessment (Tetra Tech, 2010). 5.2 Sycamore Tailings Pre-Mining Results Table 3.0 shows the pre-mining results for the 100-year regulatory flood-peak and the averageannual runoff arriving at a Stormwater Convergence Point located on the west side of the Santa Rita Mountains. Attachment 2 provides the backup for the pre-mining stormwater assessment on the west side of the Santa Rita Mountains. Table 3.0 Sycamore Tailings Baseline Hydrology Results Baseline Conditions (DA = 7.6 square miles) Point of Concentration Peak Discharge Average-Annual Runoff USGS Gauging Station 5055 cfs 799 acre-feet 5.3 Sycamore Tailings Post-Mining Watershed Conditions For Sycamore Tailings, it was assumed that the following stormwater controls would be applied: Stormwater drainage channels would be placed at every 100 feet of vertical rise on the outer slopes of the Dry Stack Tailings Facility. Stormwater would flow off these benches to natural ground and drain to the Stormwater Convergence Point. Storms up the 500 year, 24-hour storm event would be retained on top of the Dry Stack Tailings Facility in large, depressed areas. Storm runoff in excess of this event would be routed to side channels cut into natural ground. There are no flow-through drains associated with Sycamore Tailings under the final closure configuration. 5.4 Sycamore Tailings Post-Mining Hydrology Figure 3 also depicts the estimated post-mining watershed area draining to the Stormwater Convergence Point associated with Sycamore Tailings. The contributing basin area shown on Figure 3 is only applicable to the 100-year regulatory flood-peak and to the average-annual runoff, based on the following assumptions: The top of the Dry Stack Tailings Facility is assumed to contain storm runoff from up to the 500-year, 24-hour storm event. Table 4.0 shows the anticipated post-mining results for the 100-year regulatory flood peak and for the average-annual runoff arriving at a Stormwater Convergence Point located on the west 4

6 side of the Santa Rita Mountains. Attachment 1 also provides the backup data for the postmining stormwater assessment for the west side of the Santa Rita Mountains. Table 4.0 Sycamore Tailings Post-Mining Hydrology Results Post-Mining Conditions (DA = 5.5 square miles) Point of Concentration Peak Discharge Average-Annual Runoff USGS Gauging Station 4361 cfs 609 acre-feet 6.0 Conclusions The results of the baseline and post-mining hydrology assessment for the Sycamore Tailings and Barrel Waste Alternative indicate that flood-peak generated by the 100-year regulatory event on the east side of the Santa Rita Mountains, and arriving at the USGS Gauging Station, would likely be reduced by 12.0%, when compared to pre-mining conditions. Correspondingly, the average-annual runoff would likely be reduced by 21.1%, when compared to pre-mining conditions. On the west side of the Santa Rita Mountains, the results of the baseline and post-mining hydrology assessment for the Sycamore Tailings and Barrel Waste Alternative indicate that flood-peak generated by the 100-year regulatory event on the west side of the Santa Rita Mountains, and arriving at the Stormwater Convergence Point, would likely be reduced by 13.7%, when compared to pre-mining conditions. Correspondingly, the average-annual runoff would likely be reduced by 23.8%, when compared to pre-mining conditions. 7.0 References Krizek, David R., P.E. Tetra Tech (2010). Mine Plan of Operations Stormwater Assessment. Technical Memorandum to Kathy Arnold, Rosemont Copper Company. Technical Memorandum dated March 05,

7 FIGURES

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11 ATTACHMENT 1

12 SYCAMORE TAILINGS AND BARREL WASTE ALTERNATIVE Post Mining Conditions Model Tetra Tech, Inc.

13 Sycamore Tailings and Barrel Waste Alternative Post-Mining Conditions Model Tetra Tech, Inc.

14 Sycamore Tailings and Barrel Waste Alternative Post-Mining Conditions Model Tetra Tech, Inc.

15 Sycamore Tailings and Barrel Waste Alternative Post-Mining Conditions Model Tetra Tech, Inc.

16 ATTACHMENT 2

17 SYCAMORE TAILINGS AND BARREL WASTE ALTERNATIVE Baseline Conditions Model Tetra Tech, Inc.

18 Sycamore Tailings and Barrel Waste Alternative Baseline Conditions Model Tetra Tech, Inc.

19 Sycamore Tailings and Barrel Waste Alternative Baseline Conditions Model Tetra Tech, Inc.

20 Sycamore Tailings and Barrel Waste Alternative Baseline Conditions Model Tetra Tech, Inc.