Determination of Groundwater Flow Velocity in the Capulin Canyon Drainage, Molycorp, Inc

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

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

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.

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. think. deliver. 19 20 21 22 23 24 25 26 27 28 29 30 31 www.cdm.com

020415 2010

~---JANUARY----- S M T WTH F S 1 2 CDM 3 4 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----- M T WTH F 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 ------MARCH---- 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 ---- APRIL ---- S M T WTH F 5 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---- 5 M T W TH 5 1 2 4 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 TH F 5 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 ----JULY---- 5 M T WTH F 5 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--- 5 M T WTH F 5 1234567 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-- 5 M T WTH F 5 1 2 3 4 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 ( ( -----OCTOBER----- 5 MT WTH F 5 1 2 4 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24/31 25 26 27 28 29 30 NOVEMBER 5 M T WTH F 5 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 --DECEMBER-- S M T WTH F 5 ~::>-I-/) 1 2 3 4 /d 6 7 8 9 10 11 12 13 14 15 16 17 18 listen. think. deliver. 19 20 21 22 23 24 25 26 27 28 29 30 31 www.cdm.com

020416 2010

-----JANUARY--~- 5 M T W ffl 5 1 2 CDIVI 4 6 7 8 9 "3 . consulting. engineering. construction. operations 10 11 12 13 14 15 16 17 18 19 20 21 22 23 2%1 25 26 27 28 29 30 ---FEBRUARY--. M T WTH F 5 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--c----- 5 M T WTH F 5 1 2 3 456 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 ----- 5 M T WTHF 5 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 2021 22 23 24 25 26 27 28 29 30 ----MAY----- 5 M T W TH 5 1 2 3 4 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

23/30 24/31 25 26 27 28 29 ----JUNE---- 5 M T W TH 5 1 2 3 4 5 678 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 ----JULY---- M T WTH F 5 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--- 5 M T WTH F 5 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 31 SEPTEMBER-- 5 M T WTH F 5 1 2 3 4 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 ---OCTOBER--- M T WTH 5 1 2 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24/31 25 26· 27 28 29 30 --NOVEMBER 5 M T W TH F 5 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-- 5 M T WTH F 5 1 2 3 4 6 8 9 10 11 12 13 14 15 16 17 18 listen. think. deliver. 19 20 21 22 23 24 25 26 27 28 29 30 31 www.cdm.com

020417 Hydraulic Conductivity Values For Capulin Canyon

Capulin Canyon Source (RI Report, 2009) K (ft/d) Falling head slug test with 1 gallon of water in MMW -23A 0.11 Falling head slug test with 2.5 gallons of water in MMW-23A 1 Falling head slug test with 3.8 gallons of water in MMW -23A 0.9 Falling head slug test with 1.3 gallons of water in MMW-23A 0.9 Falling head slug test with PVC slug in MMW -23A 0.9 Falling head slug test with PVC slug in MMW-23A 1.8 Falling head slug test with PVC slug in MMW -23A 0.9 Falling head slug test with PVC slug in MMW -23A 2.8 Falling head slug test with PVC slug in MMW -23A 0.9 Falling head slug test with PVC slug in MMW-23A 0.1 Back calculated from yield analysis (51 gpm) 47.2

Geomean 1.04

Capulin K values.xls 7/6/2010

020418 NOTES: - Results are from April 2004 unless noted otherwise below. - RSTW values are from January 2004 - RR-11B2, RR11-B3, RR-13A, RR-13B and RR-15 are values for GSI piezometers sampled in March 2004 and only tested for dissolved concentrations (average of three consecutive days of sampling posted). - Spring 13-P1, P-1, P-2, and D1GW values are from May 2004. - Concentrations in MMW-30A can vary over time. - MMW-30A value in April 2004 is uncharacteristically low. - Data for MMW-16 are from June 2003, otherwise the well was dry or had insufficient water in it to sample. - Data for MMW-2 are from conventional sampling performed in November 2005 - Dissolved metal concentrations are similar to total cOI1ct'mh·Rtion!c;L Thus, the distribution and plume geometry for dissolved

Revision 1 11/10/08 NOTES 1. Mine Site and vicinity topography provided by Molycorp - Questa Mine (2005). DRA\ URS Center 1,500 o 8181 East Tufts Avenue Feet Denver, CO 80237-2637 DATE Scale- 1 :18000-- or- 1 in- = 1500 ft (303) 694-2770

020419 Molycorp Remedial Investigation Report Section Three Revision No, 2 July 3, 2009 Page 3-90 of3-175

SECTIINTHREE Phvsical Characteristics

from 2.3 to 8.9 feet/day. The geometric mean value was 6.2 feet/day. Assuming a porosity of 0.25 with the gradient of 0.1, the seepage velocity is estimated to be 2 feet/day within the Goathill Gulch debris fan. However, the seepage velocity increases as the colluvial water mixes with the higher permeability Red River alluvium. The estimated flow from the colluvium to alluvium at the base of the Goathill Gulch drainage is about 300 gpm, using the Darcy flow equation (1). This estimate is based on the assumption that the saturated cross-sectional area of colluvium across the drainage near the mouth is approximately 190,000 square feet (Figure 3.5-25), the hydraulic conductivity of the colluvium is 6.2 feet/day, and the hydraulic gradient perpendicular to the cross-sectional area from the colluvium to the alluvium is 0.05 feet/feet (based on the water level elevation of 7,725 feet at MMW-44A and interpolated water level elevation of 7,690 feet in the alluvium, 700 feet downgradient ofMMW-44A).

3.5.1.4.7 Capulin Canyon Colluvial sediments line the narrow drainage of Capulin Canyon from its headwaters at the base of Capulin Rock Pile to Red River, approximately 1.5-miles southwest. The'colluvial sediments are derived from erosion of the bedrock slopes of the drainage. 'A hydrothermal scar is located on the west-facing slope of the drainage' about 0.5-mile upstream of Red River. The scar covers 3 percent of the total Capulin Canyon drainage area. The colluvium downstream of the scar is expected to be comprised of some amount of scar material. The extent of colluvium in the upper portion of the Capulin Canyon drainage is limited as shown on Figure 3.5-1. Monitoring well MMW-23A, which is about 200-feet downstream of the lower catchment of the leachate collection system, penetrated 12-feet of colluvium before reaching bedrock (welded tuff). the upper several feet of sediment was fill material for the drilling pad so the actual colluvial thickness is around 6 feet. A side drainage joins Capulin Canyon farther downstream in where the USGS installed two colluvial monitoring wells, CC-1A and CC-2A. The thickness of colluvium at these locations was 31 feet and 20 feet, respectively. Some of the colluvial thickness at CC-1A is believed to be weathered bedrock so the actual colluvial thickness is around 20 feet. The colluvial thickness near the mouth of Capulin Canyon is illustrated in cross section on Figure 3.5-26. Monitoring well MMW-2 is positioned in the axis of the drainage and encountered 58-feet of colluvium before reaching bedrock (andesite). The cross section shows the bedrock outcrops on the east and west side of the drainage that constrain the colluvium to a narrow cross-sectional area. Saturated colluvium in Capulin Canyon is limited in the upper reaches and increases toward the mouth of the drainage. It is not known if saturated conditions extend the entire length of the drainage. In the upper reach of the drainage, the depth to water ranges from 3 feet to 10 feet. Shallow depths to water typically occur in the spring when the snow pack begins to melt. Water levels decrease through the summer and fall, but may increase quickly in response to large rainfall because the water table is so shallow. Over the year, water levels may fluctuate up to 8 feet. The saturated thickness of colluvium in the upper reach of the drainage ranges from 1() to 18 feet. The saturated thickness is expected to decrease higher in the drainage. The width of saturated colluvium across the drainage is expected to be only 30 feet when water levels are low to as much as 60 feet when water levels are high in the spring and summer. URS

020420 Molycorp Remedial Investigation .Report Section Thee RevIsion No.2 July 3, 2009 Page 3-91 of3-175

SECTIINTHREE Physical Characteristics

Near the mouth of the drainage, the depth to water in the colluvium is 32 feet based on measurements from MMW-2. The water level fluctuates up to 2 feet over the year, which is less than the water levels in the upper reach of the drainage. The saturated thickness of colluvium is around 26 feet, as illustrated on Figure 3.5-26. The saturated thickness is only about 10 to 15 feet greater than in the upper reach of the drainage. Water within the colluvium flows to the southwest in line with the drainage and eventually reaches the Red River alluvial aquifer as underflow. The upper reach of the drainage has a gradient that is approximated by the slope of the land surface, or 0.16. In the lower reach of the drainage, the gradient was estimated between MMW-2 and the elevation of the river near the Capulin Canyon mouth. The water table near the mouth of the canyon is known to be at the level of the river because this is an area of stream flow gain from groundwater. The reSUlting gradient between the well and the river is 0.11. 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 values ranging from 0.1 to 2.8 feet/day, with a geometric mean of 0.7 feet/day. The seepage velocity of colluvial water in the upper drainage was estimated using Equation (2) with an assumed porosity of 0.25 and gradient of 0.16. The. resulting seepage velocity is less than 1 foot/day. Near the mouth of the canyon, the hydraulic conductivity of the colluvium has been calculated for MMW-2 during single-well pumping (0.1 feet/day) and recovery (0.06 feet/day) tests, or average of 0.08 feet/day. The pumping test was conducted at a rate of 1.5 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 foot/day, using the measured gradient of 0.11 and assumed porosity of 0.25. Because the geometry of the water-bearing colluvium near the mouth of the drainage is well defined as illustrated on the cross section (Figure 3.5-26) and hydraulic testing has been performed, the approximate flow rate through the colluvium was estimated using Darcy's Law (Equation 1). The saturated cross-sectional area of the colluvium is estimated to be 1,300 square feet. Using the average hydraulic conductivity from the pumping tests at MMW-2 and gradient between the well and the river ofO.l1, the flow rate in the colluvium is estimated to be less than 1 gpm. The yield analysis presented in Section 3.3.7.3 estimated considerably more recharge or groundwater flow available within Capulin Canyon. It is possible that some of the flow within the colluvium of Capulin Canyon drains into the underlying bedrock and exits the canyon in the bedrock aquifer. However, this amount is expected to be low because pump testing of the bedrock companion well MMW-3 by MMW-2 resulted in hydraulic conductivity values that were equal to or less than the colluvial values. Another possible reason for the low colluvial flow rate of less than 1 gpm is that MMW-2 was completed in a clayey gravel that resulted in the very low hydraulic conductivity. This clayey gravel may not be representative of the entire colluvium/debris fan near the canyon mouth that quite possibly c6uld be comprised of more permeable sediment resulting in a higher flow rate exiting the canyon.

URS R:IProjects\22236246_RemediaUnvesLRep\Task_01\10.0_Word_ProcI4th Draft to CMIISection 3IMASTER_Section 3.0_SW_06-30·09JINALdoc 6/3012009(4:30:36 PM) 3-91

020421

Technical Memorandum: Determination of Groundwater Flow Velocity in the Goathill Gulch, Molycorp, Inc. Site

A groundwater flow velocity was calculated for the lower reach of Goathill Gulch from the rim of the south edge of the subsidence zone to MMW-44A located in the lower debris fan of Goathill Gulch. The calculation was performed to determine the transit time for a water particle to travel from the subsidence zone (i.e., a surface depression in the lower portion of the drainage) to MMW- 44A. This distance is approximately 2,700 feet. The subsidence zone formed as a result of block-caving mining techniques sometime after Phase III underground mining began in 1983. Assuming no further downgradient migration of leachate from the Goathill waste rock pile by 1983, the transit time calculation was made to determine if residual contaminants could still be traveling down canyon in the flowpath to the lower portions of Goathill Gulch. Summary of Results of Transit Time Calculations The resulting seepage velocity ranges from 0.9 foot/day (ft/d) to 3.56 feet/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 2.1 years to 8.2 years. Using the geomean value of the hydraulic conductivity (K), the transit time would take 3 years. However, using the yield analysis, the velocity is 0.12 ft/d and the associated transit time would be 62 years for groundwater to travel between the subsidence zone and MMW-44A. If the K value from the yield analysis is included in the geomean (not included in the RI), the transit time would increase from 3 to 4.04 years. Given the limitations of both calculations and assumptions and the K values, the range of transit times (2 to 62 years) most likely represents a realistic permeability of the drainage. Assuming that capture of seepage from the Goathill waste rock pile did not occur until 1983 or later, there is the possibility 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 lower reach of the Goathill Gulch drainage has a gradient that is approximated by the slope of the land surface, or 0.1 (Final RI Report, July 2009). The seepage velocity through the western half of the debris fan in lower Goathill

GOATHILL GULCH GW TRANSIT TIME CALC TEXT RV4 7-20-10.DOC

020422

Gulch was estimated. Slug testing of wells MMW-44A and MMW-48A was performed during the RI, and also in MMW-42A but this well was not used in the analysis because it intersected more permeable Red River alluvium and is not entirely representative of debris flow material. A total of nine tests were performed in MMW-44A and MMW-48A wells with hydraulic conductivity values ranging from 2.3 to 8.9 ft/d. The geometric mean value was 6.2 ft/d. Assuming a porosity of 0.25 with the gradient of 0.1, the seepage velocity is estimated to be 2.5 ft/d within the Goathill Gulch debris fan. However, the seepage velocity increases as the colluvial water mixes with the higher permeability Red River alluvium. 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 slower transit times. In addition to using the pump test data from MMW-44A and MMW-48A, the hydraulic conductivity and seepage velocity were also calculated using the flow (Q) derived from the RI yield analysis for Lower Goathill West. The calculation was based on the hydraulic gradient of 0.1 as measured from the subsidence zone to MMW-44A, a cross-sectional area of 90,000 square feet (ft2), and an effective porosity of 0.25. The cross-sectional area value was based on a measured width of 600 feet and a saturated thickness of approximately 150 feet at MMW-44A as described in the Final RI Report. The width of the colluvial channel was measured approximately 500 feet north of MMW-44A at a point where the drainage widens into the debris fan (from Figure 3.5-1, Final RI Report). The yield analysis value in the Final RI Report for the lower reach of Goathill Gulch is 14 gallons per minute (gpm). Chevron Mining, Inc (CMI) assumes that 0 percent of this yield drains to the mine workings below the gulch. Thus, 100 percent of this water is available to exit the drainage and discharge into the Red River. The K value was calculated to be 0.30 ft/d with a seepage velocity of 0.12 ft/d. Limitations and Uncertainty Based on K values obtained from slug testing in MMW-44A and MMW-48A, a hydraulic gradient of 0.1, and a cross-sectional area of approximately 90,000 ft2, flow through this representative cross section would be approximately 290 gallon per minute (gpm). This estimate is unrealistic and essentially represents the estimated total yield (293 gpm) from the entire mine site that could potentially discharge as groundwater into the Red River. This unrealistically high discharge value is most likely due to the slug test results. Slug tests are better suited for low permeable material and may not accurately measure K values in coarser grained material. These wells should have been pump tested using single well or multiple well tests.

GOATHILL GULCH GW TRANSIT TIME CALC TEXT RV4 7-20-10.DOC

020423

The K value and seepage velocity were also calculated from flow of 14 gpm derived from the yield analysis. This value appears to be reasonable as a percentage (5 percent) of the total discharge (293 gpm) from the mine site since it reflects a fairly small drainage area below the subsidence zone. While the K value is somewhat low, 0.30 ft/d (1.06x10-4 centimeter per second [cm/sec]), it was considered a reasonable lower bound to the range of K values obtained from the slug testing. The value corresponds to silty sand (Freeze and Cherry 1979) and may not be completely representative of debris fan material. Despite these limitations, the K value from the yield analysis was used to calculate a lower bound to the seepage velocity and higher bound to transit times. Under these conditions, it would take a molecule of water approximately 62 years to travel down the lower portion of Goathill Gulch to MMW-44A.

GOATHILL GULCH GW TRANSIT TIME CALC TEXT RV4 7-20-10.DOC

020424 ",VIV -----JANUARy----- S M T W TH F S 1 2 CONI o 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/" 25 26 27 28 29 30 --FEBRUARY--. S M T W TH S 1 2 3 456 (Qv!C 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 6:",lc l 28 ---MARCH--- S M T W TH F S 1 2 345 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 2324 25 26 27 28 29 30 31 ::: 2 ) 7 0 Q ----APRIL ---- SMTWTHFS 123 > 4 5 6 7 8 9 10 7b. V'r- L-j .. ? 11 12 13 14 15 16 17 18 19 20 21 22 23 24 cr 25 26 27 28 29 30 ----MAy--- SMTWTHFS 1 2 345 678 9 10 11 12 13 14 15 16 17 18 19 20 21 22 "/'0 24/" 25 26 27 28 29 ----JUNE--- SMTWTHFS 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---- 5 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 29 30 31 -----AUGUST--- 5 M T W TH 5 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 31 ---SEPTEMBER-- 5 M T W TH F 5 1 2 3 4 6 7 8 9 10 11 1\1'= (f'. ~ ~+tl )(-~). ( ) s. r'G {'+/d 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 O. 'Zs- ---OCTOBER--- SMTWTHFS (0. I ,)" 1 2 3 4 5 6 789 10 11 12 13 1415 16 17 18 19 20 21 22 23 24/31 25 26 27 28 29 30 2 NOVEMBER-- SMTWTHFS 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 DECEMBER-- 5 M T W TH F 5 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 listen. think. deliver. 19 20 21 22 23 24 2S 26 27 28 29 30 31 www.cdm.com

020425 '"'V IV ---JANUARY-- SMTWTHFS 1 2 COM 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 jQe>. 7 8 9 10 11 12 13 _?'> £+ 14 15 16 17 18 19 20 21 22 23 24 25 26 27 '3.0-, ~0 28

--- MARCH~- SMTWTHFS 1 ·2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 ,7 18 19 20 21 22 23 24 25 26 27 28 29 30 31 ----APRIL -- SMTWTHFS j t\~,~{ yft S , . 123 4 5 6 7 8 9 10 UJ~G=OA 1 ( / 11 12 13 14 15 16 17 +0 18 19 20 21 22 23 24 25 26 27 28 29 30 (2~FtO~4 ") ----MAY----c-- SMTWTH 5 O

---JUNE~- SMTWTHFS 1 2 3 4 5 I 't dP~VJ 6 7 8 9 10 11 12 13 14 15 16 17 18 19 A : O.{ ((t1: 20 21 22 23 24 25 26 27 28 29 30 ( A S'J'U v.-., .x S ----JULY---- A::: SMTWTH S 123 -{L, I ::. ISo ~+ r;: I:) ""- 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 (QI!Jv/~( I 25 26 27 28 29 30 31 ---AUGUST-- ') SMTWTHFS 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 31 --SEPTEMBER-- S M T W TH F S 1 2 3 4 5 6 7 8 9 10 11 D.SO ~+Id 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 ( Qr 3 () JS+/ j o. I ') ---OCTOBER--- S M T W ~ S 1 2 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 "I" 25 26 27 28 29 30 --NOVEMBER-- M T W TH F 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- MTWTHFS 1 234 6 7 8 9 10 11 12 13 14 15 16 17 18 listen .. think.. deliver" 19 20 21 22 23 24 25 26 27 28 29 30 31 www.cdm.com

020426 il.UIU -----JANUARy----- S M T W TH F S 1 2 CDM 3456789 consulting. engineering. construction. operations 10 11 12 13 14 15 16 17 18. 19 20" 21 22 23 24/" 25 26 27 28 29 30 ---FEBRUARY-"- S M T W TH S 1 2 3 4 5 6 ~ f- Q Y", 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 ----MAY---- S M T W TH S CV 1 234 567 8 9 10 ·11 12 13 14 15 16 17 18 19 20 21 22 23/ 24/31 25 30 26 27 28 29 0.2- f:;27c1 ----JUNE--- SMTWTHFS 01 I 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 ----JULY---- S M T W TH F S ' 123 !.J4 A 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 W TH 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- SMTWTHFS 1 234 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 ---OCTOBER-- SMTWTHFS 1 2 3456789 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24/" 25 26 27 28 29 30 NOVEMBER-- 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 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. think. deliver. 19 20 21 22 23 24 25 26 27 28 29 30 31 www.cdm.com

020427 Hydraulic Conductivity Values in Goathill Gulch

Goathill Gulch Source (RI Report, 2009) K (ft/d) Falling head slug test with solid slug in MMW-44A 3.7 Falling head slug test with solid slug in MMW-44A 2.3 Rising head slug test with solid slug in MMW-44A 6.2 Rising head slug test with solid slug in MMW-44A 6.7 Falling head slug test with solid slug in MMW-48A 8.7 Falling head slug test with solid slug in MMW-48A 6.7 Rising head slug test with solid slug in MMW-48A 8.6 Rising head slug test with solid slug in MMW-48A 8.9 Rising head slug test with solid slug in MMW-48A 8.6 Back calculated from yield analysis (14 gpm) 0.3

Geomean 4.58

Goathill K values.xls 7/13/2010

020428 -- Geophysical Line Cross Section location Tailing Pipeline -- Moly Tunnel Pre-Mine Drainage Red River

- - Open Pit Capulin Canyon Pumpback - - Subsidence Area

Current or New Underground Mine Workings

Old Underground Workings Vegetation Borescope Flow Direction Rockpile AJluvium [--I-. Colluvium Hydrothermal Scar Area

N Revision 1 11/10/08

URS Center 1,500 I o NOTES: Mine Site and vicinity topography 8181 East Tufts Avenue -- Feet provided by Molycorp - Questa Denver, CO 80237-2637 Scale--- 1 :18000 or 1 in = 1500 ft Mine (2005). (303) 694-2770

020429 Molycorp Remedial Investigation Report Section Three Revision No, 2 July 3, 2009 Page 3-88 of3-175

SlenONTHREE Phvsical Characteristics

saturation up gradient at the M&E. A greater thickness of saturated colluvium occurs at MMW-48A, which is west ofMMW-8B in lower Slick Line Gulch (Figure 3.5-25). The flow of water in colluvium in the lower reach of Slick Line Gulch (i.e., eastern approximate one-half of the Goathill debris fan) is to the south-southwest in line with the former drainage (Figure 3.5-4). Using the average water levels at the M&E area in wells MMW-21 and MMW-22 and in downgradient well MMW-8B, the gradient was estimated to be 0.2. This is the same gradient as the slope of the drainage prior to mining. The seepage velocity of the colluvium was estimated using pumping and slug test results from MMW-48A. Five values of hydraulic conductivity were determined from the tests that ranged from 6.7 to 8.9 feet/day. The geometric mean was 8.3 feet/day. Using Equation (2) with an assumed porosity of 0.25 and a hydraulic gradient of 0.2, the seepage velocity in Slick Line Gulch was estimated to be 7 feet/day. Using the range of hydraulic conductivity values and assuming that the porosity and gradient are fixed, the seepage velocity ranges from 5 to 7 feet/day. The estimated flow from the colluvium to alluvium at the base of the Slick Line Gulch drainage is about 200 gpm, using the Darcy flow equation (1). This estimate is based on the assumption that the saturated cross-sectional area of colluvium across the drainage near the mouth is approximately 170,000 square feet (Figure 3.5-25), the hydraulic conductivity of the colluvium is 8.3 feet/day, and the hydraulic gradient perpendicular to the cross-sectional area from the colluvium to the alluvium is 0.03 feet/feet (based on the water level elevation of7,753 feet in colluvium at MMW-48A and interpolated water level elevation of 7,740 feet in the alluvium, 500 feet downgradient ofMMW-48A).

3.5.1.4.6 Goathill Gulch Colluvial sediments occur along the entire length of Goathill Gulch drainage. The gulch drains to the south-southwest toward Red River. The colluvium in the gulch is comprised of surficial deposits of sediment derived from non-mineralized rocks, pyrite-mineralized scar areas, and waste rock debris from the Goathill North Rock Pile at the head ofthe drainage. The sediment is typically, poorly sorted loose material. Hydrothermal scars make up approximately 20 percent of the total Goathill Gulch drainage area. Water and seepage originating in the upper reach ofthe drainage, together with diverted water from the Capulin Canyon seepage collection system, flows in the drainage until reaching the subsidence zone where it temporarily collects and infiltrates through the fractured rock overlying the underground workings. Some of this water flows through the colluvial sediments that line the drainage. The thickness of colluvial sediments upstream of the subsidence zone is unknown but is near zero at the Narrows, which is approximately 3,000-feet upstream ofthe subsidence zone. Field work as part of hydrogeologic characterization of the subsidence zone area (SRK 2004b) found two exploration boreholes approximately 1,000-feet upstream of the subsidence zone that showed the colluvial water table to be 14- to 16-feet below the level of the flowing surface water in the drainage. The hydrogeologic characterization estimated that the colluvium reaches a maximum thickness of approximately 65 feet at that location. Darcy's Law

DRS

020430 Molycorp Remedial Investigation Report Section Three Revision No.2 July 3, 2009 Page 3-89 of3-175

SEODINTHREE Phvsical Characteristics

calculations of the flow through the colluvium upstream of the subsidence zone was estimated to range from 7 to 20 gpm, with an average of 12 gpm (SRK 2004b). Downstream of the subsidence zone, Goathill Gulch drains onto the large debris fan. Borehole drilling and several geophysical lines traverse the debris fan and provide geologic information on the occurrence of colluvium, which in this area is comprised largely of debris flow material. The extent of colluvium!debris flow material in lower Goathill Gulch is shown on Figure 3.5-1. A northwest to southeast cross section (Figure 3.5-25) was constructed through the debris fan and profiles the relationship between debris flow material and underlying bedrock. Beginning at the eastern end of the cross section, monitoring wells MMW-8A18B penetrated 117-feet of colluvium. Some ofthe sediment at this location is derived from Slick Line Gulch. Farther to the west, monitoring well MMW-48A encountered approximately 150-feet of debris flow material followed by about 60-feet of ancestral Red River alluvium before reaching bedrock (andesite) at a depth of214 feet. The change from debris sediments to alluvium was clearly recognizable by rounded gravel to cobble-sized material in a dark-brown silty, sandy matrix, which is characteristic of surficial alluvial deposits along the river. Farther to the west at MMW-44A1B, the debris flow sediments thickness reaches a maximum at 265 feet after which weathered bedrock was encountered. The location ofMMW-44AIB is within a bedrock depression identified by geophysics lines. At the western end of the section, MMW-42A1B encountered approximately 126-feet of colluvium that contained a 20-foot sequence offerricrete and possibly some Red River alluvium. The water within the central portion of the Goathill Gulch debris fan is typically found at a depth of about 100 feet. Saturated colluvium ranges in thickness from 40 to 70 feet on the east and west sides of the fan to about 150 feet in the center of the fan that overlies the bedrock depression. The elevation of the river is projected onto the line of cross section (Figure 3.5-25) and shows that the water levels within the eastern half of the fan are near the elevation of the river, which is about 700-feet south of the line of section. This suggests a flat gradient toward the river. At the distal end of the debris fan, a change from a southerly direction to a westerly direction occurs as mixing begins with more permeable Red River alluvium (Figure 3.5-4) and the gradient decreases to less than 0.1 in the direction from the wells toward the river. This same situation occurs at the base of Sugar Shack South Rock Pile where the gradient is relatively flat from the cluster of wells at the base of the rock pile to the river (Figure 3.5-21). A notable observation regarding the Goathill Gulch debris fan is that water levels are similar to the level of the river from the center of the debris fan upstream along the river and to the east. This is illustrated on Figure 3.5-25 and a result of the debris fan "damming" the valley and forcing water to upwell into the river. Downstream of the center of the debris fan, water levels are below the level of the river, resulting in a separation between the two and most likely cessation of groundwater upwelling into the river. The seepage velocity through the western half of the debris fan in lower Goathill Gulch was estimated. Slug testing of wells MMW-44A and MMW-48A was performed during the RI, and also in MMW-42A but this well was not used in the analysis because it intersected more permeable Red River alluvium and is not entirely representative of debris flow material. A total of nine tests were performed in the other two wells with hydraulic conductivity values ranging

DRS R;\Projecls\22236246_RemediaUnvesCRep\Task_01\10,O_WOf(CProc\4lh Draft to CMI\SectiOJ'l 3\MASTE~Section 3.0_ SW_06-30-09JINAl.doc 613012009(4:30:36 PM) - 3 -89

020431 Molycorp Remedial Investigation Report Section Three Revision No.2 July 3, 2009 Page 3-90 of3-175

SECTlaNTHREE Phvsical Characteristics

from 2.3 to 8.9 feet/day. The geometric mean value was 6.2 feet/day. Assuming a porosity of 0.25 with the gradient of 0.1, the seepage velocity is estimated to be 2 feet/day within the Goathill Gulch debris fan. However, the seepage velocity increases as the colluvial water mixes with the higher permeability Red River alluvium. The estimated flow from the colluvium to alluvium at the base of the Goathill Gulch drainage is about 300 gpm, using the Darcy flow equation (1). This estimate is based on the assumption that the saturated cross-sectional area of colluvium across the drainage near the mouth is X approximately 190,000 square feet (Figure 3.5-25), the hydraulic conductivity of the colluvium is 6.2 feet/day, and the hydraulic gradient perpendicular to the cross-sectional area from the colluvium to the alluvium is 0.05 feet/feet (based on the water level elevation of7,725 feet at MMW-44A and interpolated water level elevation of 7,690 feet in the alluvium, 700 feet downgradient of MMW-44A).

3.5.1.4.7 Capulin Canyon Colluvial sediments line the narrow drainage of Capulin Canyon from its headwaters at the base of Capulin Rock Pile to Red River, approximately 1.5-miles southwest. The colluvial sediments are derived from erosion of the bedrock slopes of the drainage. A hydrothermal scar is located on the west-facing slope of the drainage about 0.5-mile upstream of Red River. The scar covers 3 percent of the total Capulin Canyon drainage area. The colluvium downstream of the scar is expected to be comprised of some amount of scar matetial. The extent of colluvium in the upper portion of the Capulin Canyon drainage is limited as shown on Figure 3.5-1. Monitoring well MMW-23A, which is about 200-feet downstream of the lower catchment of the leachate collection system, penetrated 12-feet of colluvium before reaching bedrock (welded tuft). The upper several feet of sediment was fill material for the drilling pad so the actual colluvial thickness is around 6 feet. A side drainage joins Capulin Canyon farther downstream in where the USGS installed two colluvial monitoring wells, CC-IA and CC-2A. The thickness of colluvium at these locations was 31 feet and 20 feet, respectively. Some of the colluvial thickness at CC-IA is believed to be weathered bedrock so the actual colluvial thickness is around 20 feet. The colluvial thickness near the mouth of Capulin Canyon is illustrated in cross section on Figure 3.5-26. Monitoring well MMW-2 is positioned in the axis of the drainage and encountered 58-feet of colluvium before reaching bedrock (andesite). The cross section shows the bedrock outcrops on the east and west side of the drainage that constrain the colluvium to a narrow cross-sectional area. Saturated colluvium in Capulin Canyon is limited in the upper reaches and increases toward the mouth of the drainage. It is not known if saturated conditions extend the entire length of the drainage. In the upper reach of the drainage, the depth to water ranges from 3 feet to 10 feet. Shallow depths to water typically occur in the spring when the snow pack begins to melt. Water levels decrease through the summer and fall, but may increase quickly in response to large rainfall because the water table is so shallow. Over the year, water levels may fluctuate up to 8 feet. The saturated thickness of colluvium in the upper reach of the drainage ranges from 10 to 18 feet. The saturated thickness is expected to decrease higher in the drainage. The width of saturated colluvium across the drainage is expected to be only 30 feet when water levels are low to as much as 60 feet when water levels are high in the spring and summer. DRS

020432