Technical Memorandum: Determination of Groundwater Flow Velocity in the Capulin Canyon Drainage, Molycorp, Inc. Site A groundwater flow velocity was calculated for the upper reach of Capulin Canyon from near MMW-23A to MMW-2 located in the lower portion of the canyon. The calculation was performed to determine the transit time for a water particle to travel from the lower catchment pond (pumpback), located approximately 3,600 feet from the toe of the Capulin rock pile to MMW-2. This distance is approximately 5,775 feet. The catchment pond was installed in 1992 and later upgraded in 2006 to prevent overflowing of the ponds during storm events. Assuming by 2006, no further downgradient migration of leachate from the Capulin rock pile, the transit time calculation was made to determine if residual contaminants are still traveling down canyon in the flowpath to the lower portions of Capulin Canyon. Results Summary of Transit Time Calculations The resulting seepage velocity ranges from 0.06 foot per day (ft/d) to 1.79 feet per day (ft/d) based on a Darcy’s Law calculation. The transit time for a water molecule to travel this distance would range from approximately 8.8 years to 247 years. Using the geomean hydraulic conductivity (K) value (0.7 ft/d), the transit time would take 35 years. However, using the yield analysis, the velocity is 30.2 ft/d and the transit time would take 0.5 year for groundwater to travel between the pumpback pond and MMW-2. If the K value from the yield analysis is included in the geomean (not included in the RI), it would reduce the transit time from 35 to 23 years. Given the limitations of both calculations and assumptions, the transit time (23 years), based on the recalculated geomean K value (1.04 ft/d), most likely represents a realistic permeability of the drainage. Assuming that capture of seepage from the Capulin rock pile did not occur until 1992 or later, it is entirely possible that contaminant migration could be occurring in the lower portions of the canyon for years to come. Supporting Information Hydraulic Parameter and Calculation Assumptions The upper reach of the Capulin Canyon drainage has a gradient that is approximated by the slope of the land surface, or 0.16 (Final RI Report, July 2009). The hydraulic properties of the Capulin Canyon colluvium have been measured in the upper reach at MMW-23A and near the mouth at MMW-2. At MMW-23A, 10 slug tests have been performed resulting in hydraulic conductivity (K) values ranging from 0.1 to 2.8 ft/d, with a geometric mean of 0.7 ft/d (Final RI Report, July 2009). Near the mouth of the canyon, the hydraulic conductivity of the colluvium has been calculated for MMW-2 during single-well pumping (0.1 ft/d) and recovery (0.06 ft/d) tests, or an average of 0.08 1 CAPULIN GW TRANSIT TIME CALCULATION RV6 7-06-10.DOC 020412 ft/d. The pumping test was conducted at a rate of 1.5 gallons per minute (gpm) for nearly 4 hours and the drawdown was 29 feet at the end of the test, which was near the bottom of the well. The low hydraulic conductivity of the colluvium is supported by the fact that the well had almost pumped dry at the low pumping rate. The resulting seepage velocity is much less than 1 ft/d, using the measured gradient of 0.11 and assumed porosity of 0.25. The calculation assumes that neither chemical reaction nor retardation of contaminants due partitioning of metals to solid phases is occurring. Partitioning of metals such as aluminum could result in much slower transit times. Instead of using the pump test data from MMW-23A, the hydraulic conductivity and seepage velocity were also calculated using the flow (Q) value (51 gpm) derived from the RI yield analysis. The back calculation was based on the hydraulic gradient (0.16), a cross-sectional area of 1,300 square feet (ft2), and an effective porosity of 0.25. The cross-sectional area value is also from the RI Report (July 2009). The yield analysis value in the RI Report for the entire reach of Capulin Canyon is 102 gpm. Although the Capulin rock pile covers approximately 30 percent of the Capulin drainage, Chevron Mining Inc (CMI) assumed in the RI Report that 50 percent of the yield of the total drainage derives from the rock pile because it is located at the head of canyon at the highest elevations. On this basis, 50 percent or 51 gpm potentially may be exiting the canyon into the Red River. The K value was calculated to be 47.2 ft/d with a seepage velocity of 30 ft/d. Limitations and Uncertainty Based on the K values, hydraulic gradients, and effective porosity values stated above, a discharge calculation was performed and included in the RI Report for Capulin Canyon. The results of this calculation indicate that less than 1 gpm is discharging from the mouth of Capulin Canyon into the Red River. This value is believed to be unreasonably low and considerably lower than groundwater discharge (51 gpm) from the canyon estimated from the yield analysis in the RI Report. The discrepancy may be attributable to several factors, one of which includes an inaccurate representation of the hydraulic characteristics at MMW-2 due to a poor choice of well placement. As acknowledged in the RI Report, the well is drilled in clayey gravel and may not have been located along the central axis of the of the colluvial drainage in coarser and more permeable material. As such, the pump test values are most likely not indicative of the actual hydraulic conductivity (K) and flow (Q) in the drainage. However, while flow from the yield analysis may appear to be reasonable, the hydraulic conductivity value and seepage velocity that were back calculated from Q (51 gpm) are believed to be unrealistically high. Although the discharge value (51 gpm) from the lower portion of Capulin Canyon below the rock pile appears to be a reasonable percentage (17 percent) of the yield estimate (293 gpm) of available groundwater leaving the mine site, the K value of 47.2 ft/d (1.66 x 10-2 centimeters per second [cm/sec]) is unrealistically high for the type of geological environment where it is located. The value corresponds to a clean sand (Freeze and Cherry 1979) and appears to be incongruous 2 CAPULIN GW TRANSIT TIME CALCULATION RV6 7-06-10.DOC 020413 given that the canyon is a result of colluvial processes with a predominately poorly sorted mixture of fine- and coarse-grained, clastic materials. One possible explanation for this inconsistency is that some of the flow (51 gpm) within Capulin Canyon drains into the underlying bedrock thus, reducing the outflow from the canyon. A lower Q would translate proportionally into a lower K and velocity values. Another less plausible explanation may be that the cross-sectional area value is too low. Despite these limitations, the K value from the yield analysis was used to calculate an upper bound to the seepage velocity and lower bound to transit times. Under these conditions, it would take a molecule of water approximately 0.5 years to travel down Capulin Canyon to MMW-2. 3 CAPULIN GW TRANSIT TIME CALCULATION RV6 7-06-10.DOC 020414 2010 -----JANUARy~--- S M T W TH F S 1 2 CDM 3 4 5 6 7 8 9 consulting. engineering. construction· operations 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24/31 25 26 27 28 29 30 -----FEBRUARy----- S M T W TH F S 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 ---'---MARCH---- S M T WTH F S 1 2 3 456 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 ---:--- APRIL ---- S M T WTH F S 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 ----MAy---- S M T WTH F S 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 2%0 24/31 25 26 27 28 29 -------JUNE------- S M T W ~ S 1 2 345 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 ----JULy---- S M T WTH F S 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 -----AUGUST---­ S M T WTH F S 1 2 34567 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 SEPTEMBER-- S MT W TH F S 1 2 3 4 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 1/ 26 27 28 29 30 -----OCTOBER----- S M T W TH S ( 1 2 3 4 5 6 789 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24/31 25 26 27 28 29 30 NOVEMBER-- S M T W TH S 1 2 3 4 5 6 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 DECEMBER-- S M T W TH F S 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 listen.