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HomeMy WebLinkAbout1.03 Appendix C - Drawings-Soil Test-Dewatering ReportsProject EngineersVicinity MapSheet IndexOwner/ApplicantScope of Work Drawings and engineering design for DRMS 112 Permit, Garfield County Land Use Change,and Garfield County Floodplain Development Permit Applications.118 West Sixth Street, Suite 200Glenwood Springs, CO 81601970.945.1004 www.sgm-inc.com Project SiteScott Contracting, Inc.Rifle Pit No. 11-COVER - Cover SheetC1 - Disturbance Summary PlanC2 - Floodplain Specific Site PlanC3 - Dewatering Pipeline PlanC4 - Dewatering Pipeline GeometryC5 - Mining Plan (Stage 1 of 2)C6 - Mining Plan (Stage 2 of 2)C7 - Erosion Control PlanC8 - Reclamation PlanC9 - Notes & Details (1)C10 - Notes & Details (2)C11 - Notes & Details (3)Scott Contracting IncMarch 20219200 East Mineral Ave, Suite #400Centennial, CO 80112Title:Cover SheetRevision#Dwg No.Scott Contracting, Inc. Rifle Pit No. 1Job No.Drawn by:Date:File:QC:2019-341.002RPF03.29.2021RPFRiflePit-Admin-17X11118 West Sixth Street, Suite 200 Glenwood Springs, CO 81601 970.945.1004 www.sgm-inc.com Date By:1-COVER 12Of :PE:ABFPermit Set (Not For Construction)Project Milestone:Jevon Poston, P.E. 57316Richard (Chip) Fisher, P.E. 57906N.T.S.Permit Set (Not For Construction) © 2020 Microsoft Corporation © 2020 Maxar ©CNES (2020) Distribution Airbus DS Title:DisturbanceSummary PlanRevision#Dwg No.Job No.Drawn by:Date:File:PE:QC:2019-341.002RPF03.29.2021RPFRiflePit-SitePlanABF118 West Sixth Street, Suite 200 Glenwood Springs, CO 81601 970.945.1004 www.sgm-inc.com Date By: Permit Set (Not For Construction)Project Milestone:C1 12Of :Graphic ScaleIn Feet: 1" = 500'02505001000Scott Contracting, Inc. Rifle Pit No. 1 © 2020 Microsoft Corporation © 2020 Maxar ©CNES (2020) Distribution Airbus DS Title:Floodplain SpecificSite PlanRevision#Dwg No.Job No.Drawn by:Date:File:PE:QC:2019-341.002RPF03.29.2021RPFRiflePit-FloodplainPlanABF118 West Sixth Street, Suite 200 Glenwood Springs, CO 81601 970.945.1004 www.sgm-inc.com Date By: Permit Set (Not For Construction)Project Milestone:C2 12Of :Graphic ScaleIn Feet: 1" = 1000'050010002000Scott Contracting, Inc. Rifle Pit No. 1 © 2020 Microsoft Corporation © 2020 Maxar ©CNES (2020) Distribution Airbus DS Title:DewateringPipeline PlanRevision#Dwg No.Job No.Drawn by:Date:File:PE:QC:2019-341.002RPF03.29.2021RPFRiflePit-SitePlanABF118 West Sixth Street, Suite 200 Glenwood Springs, CO 81601 970.945.1004 www.sgm-inc.com Date By: Permit Set (Not For Construction)Project Milestone:C3 12Of :Graphic ScaleIn Feet: 1" = 500'02505001000Scott Contracting, Inc. Rifle Pit No. 1 Title:DewateringPipelineGeometryRevision#Dwg No.Job No.Drawn by:Date:File:PE:QC:2019-341.002RPF03.29.2021RPFRiflePit-SitePlanABF118 West Sixth Street, Suite 200 Glenwood Springs, CO 81601 970.945.1004 www.sgm-inc.com Date By: Permit Set (Not For Construction)Project Milestone:C4 12Of :Graphic ScaleIn Feet: 1" = 500'0250Scott Contracting, Inc. Rifle Pit No. 1 5001000 Title:Mining Plan(Stage 1 of 2)Revision#Dwg No.Job No.Drawn by:Date:File:PE:QC:2019-341.002RPF03.29.2021RPFRiflePit-GrDrPlanABF118 West Sixth Street, Suite 200 Glenwood Springs, CO 81601 970.945.1004 www.sgm-inc.com Date By: Permit Set (Not For Construction)Project Milestone:C5 12Of :Graphic ScaleIn Feet: 1" = 200'0100Scott Contracting, Inc. Rifle Pit No. 1 200400 Title:Mining Plan(Stage 2 of 2)Revision#Dwg No.Job No.Drawn by:Date:File:PE:QC:2019-341.002RPF03.29.2021RPFRiflePit-GrDrPlanABF118 West Sixth Street, Suite 200 Glenwood Springs, CO 81601 970.945.1004 www.sgm-inc.com Date By: Permit Set (Not For Construction)Project Milestone:C6 12Of :Graphic ScaleIn Feet: 1" = 200'0100Scott Contracting, Inc. Rifle Pit No. 1 200400 Title:Erosion ControlPlanRevision#Dwg No.Job No.Drawn by:Date:File:PE:QC:2019-341.002RPF03.29.2021RPFRiflePit-GrDrPlanABF118 West Sixth Street, Suite 200 Glenwood Springs, CO 81601 970.945.1004 www.sgm-inc.com Date By: Permit Set (Not For Construction)Project Milestone:C7 12Of :Graphic ScaleIn Feet: 1" = 200'0100Scott Contracting, Inc. Rifle Pit No. 1 200400 Title:Reclamation PlanRevision#Dwg No.Job No.Drawn by:Date:File:PE:QC:2019-341.002RPF03.29.2021RPFRiflePit-GrDrPlanABF118 West Sixth Street, Suite 200 Glenwood Springs, CO 81601 970.945.1004 www.sgm-inc.com Date By: Permit Set (Not For Construction)Project Milestone:C8 12Of :Graphic ScaleIn Feet: 1" = 200'0100Scott Contracting, Inc. Rifle Pit No. 1 200400 Title:Notes & Details (1)Revision#Dwg No.Job No.Drawn by:Date:File:PE:QC:2019-341.002RPF03.29.2021RPFRiflePit-DetailsABF118 West Sixth Street, Suite 200 Glenwood Springs, CO 81601 970.945.1004 www.sgm-inc.com Date By: Permit Set (Not For Construction)Project Milestone:C9 12Of :Scott Contracting, Inc. Rifle Pit No. 1 Dwg No.Job No.Drawn by:Date:File:PE:QC:2019-341.002RPF03.29.2021RPFRiflePit-DetailsABF118 West Sixth Street, Suite 200 Glenwood Springs, CO 81601 970.945.1004 www.sgm-inc.com Date By: Permit Set (Not For Construction)Project Milestone:C10 12Of :Title:Notes & Details (2)Revision# Scott Contracting, Inc. Rifle Pit No. 1 ····FIXED COVER4" [10.16 CM] W X 1/2" [1.27 CM] TBASE MOUNTING FLANGEFIXED ENERGYABSORBING PLATE60" [152.4 CM] I.D.MANHOLE BARREL(MIN 1/2" [1.27 CM] T)Ø12"FLOWØ2'[30.48 CM]SCH 80[60.96CM ]SDR 266'-4"[193.04CM ]PLAN VIEW(DOMED TOP NOT SHOWN FOR CLARITY)FIBERGLASS DOMED TOPWITH CONFINED SPACEENTRY SIGN AND STAINLESSSTEEL GAS STRUT, HINGE,AND HASP1'[30.48CM ]TYPICALGRADE6'[182.88CM ]MANHOLE DEPTH5"[12.70CM ]5 17/64"[13.39CM ]FOAM PADCONCRETE PAD (BY OTHERS)6" [15.24 CM] T MINIMUMELEVATION CROSS-SECTION VIEWFLOWDwg No.Job No.Drawn by:Date:File:PE:QC:2019-341.002RPF03.29.2021RPFRiflePit-DetailsABF118 West Sixth Street, Suite 200 Glenwood Springs, CO 81601 970.945.1004 www.sgm-inc.com Date By: Permit Set (Not For Construction)Project Milestone:C11 12Of :Title:Notes & Details (3)Revision# Scott Contracting, Inc. Rifle Pit No. 1 .TU1Apr. 16. 20095 1:23PM-1,.,_"'•UNITED COMPANIES Rifle---No. 3931 P. 2 108 354A I NOTTosc4el . ' _____ _J PROPOSE'.D GRAVEL PIT PROPERn' BORING2 • / 7 ( • ;?? BORINGS BORING 4 e ?/ LOCATION OF EXPLORATORY BORINGS I Figure 1 JLApr.16 . 2009~ 1:23PMHP-Gt UNIT ED COMPANIES Ri fle'-> ... No. 393 1 P. 3 BORIN G 1 BOR ING 2 BOR ING3 BO RING4 BOR ING 5 0 0 '7 3 ' ~ 7' .3 ( -:.--Ft ~ 5 5 5 - "/ - -- 10 I 10 17 ' 2 1 ,1 15 15 $ $ u. u.. I I .c ~ a. Q) ID Cl 0 20 20 25 25 30 30 35 35 Note: Explanation at symbols Is shown on Figure 3. 108 354A LOGS OF EXPLORATORY BORINGS Figure 2 ·Apr. 16. 2009" 1:23PM'H'"'-UNITED COMPANIES Rifle'"'""' 10 '"'"'"""' No. 3931 LEGEND: CLAY AND SILT (CL-ML); sandy, about 6 inches ot topsoil, medium stiff, slighUy moist, brown. GRAVEL (GM-GP); with cobbles, sandy, slightly silly to silty, occasional sand layers, dense, moist towel , brown. CLAYSTONE/SJLTSTONE BEDROCK: hard, moist, brown and grey. Free water level in boring at time of drilling. NOTES; 1. Exploratory borings were drilled on June 26 and 30, 2008 with 4-inch diameter continuous flight power auger. 2. Locations of exploratory borings were provided by the client and are shown approximately on the site plan provided. 3. Elevations at exploratory borings were not measured and the logs of exploralory borings are drawn to deplh. 4. The eitploratory boring locations should be considered accurate only to the degree Implied by the method used. 5. Tha lines between materials shown on the exploratory boring logs represent the approximate boundaries between material types and transitions may be gradual. 6. Water level readings shown on the Jogs were made at the time and under the conditions Indicated. Fluctuations In water level may occur with time. 108 354A H~ol LEGEND AND NOTES Figure 3 Hahn Water Resources, llc ________________________________________________________________________ 1 6589 Elaine Road ∙Evergreen, Colorado 80439 ∙ office 720.242.8639 ∙ cell: 303.870.5757 hahnwaterresources@gmail.com ∙ www.hahnwaterresources.com August 2, 2019 Angie Fowler, PE SGM 118 W Sixth St, Suite 200 Glenwood Springs, CO 81601 Letter Report: Evaluation of the Impacts of Dewatering at Rifle Pit No. 1 BACKGROUND AND OBJECTIVES This letter report provides a brief summary of my findings in an investigation of the impacts of dewatering of a proposed sand and gravel mine known as the Rifle Pit No. 1. The proposed mine is located in Garfield County, Colorado within the alluvial channel of the Colorado River. The effects of dewatering are of interest in terms of how they might impact several areas on the property that have been identified as wetlands. METHODS OF INVESTIGATION Basic Data Site-specific data available for this analysis was extremely limited. That data included drilling logs and water level measurements for 5 exploratory borings (HP Geotech, 2008), a site elevation survey, preliminary mapping of on-site wetlands, mapping of various ditches and laterals surrounding and traversing the property, a decree for water rights on an adjacent parcel that included a discussion of aquifer properties (SGM, 2019). and aerial imagery (Google Earth and NAIP imagery). Model Selection It was determined that a numerical model would be best suited for making the types of predictions required. Whereas analytical solutions can, in some instances, provide more precise solutions, they have difficulty representing the multiple boundary conditions that need to be addressed in this setting. It was further determined that given the very limited data available for the site, the model would need to be relatively simple in its representation of the hydrogeologic system. MODFLOW-2000 (Harbaugh, et. al., 2000) was selected as the modeling platform. MODFLOW is a three dimensional, finite-difference groundwater model developed by the U. S. Geological Survey. The model was constructed with the following attributes: Model Layering The model was constructed as a single layer model representing an aquifer consisting of coarse and highly permeable alluvial sands and gravels. The sands and gravels overlie claystone/siltstone bedrock. The bedrock is treated as an impermeable boundary. Considering the limited information on the underlying bedrock (depth, elevation), the aquifer was represented as being horizontal, with a flat bottom. Hahn Water Resources, llc ________________________________________________________________________ 2 6589 Elaine Road ∙Evergreen, Colorado 80439 ∙ office 720.242.8639 ∙ cell: 303.870.5757 hahnwaterresources@gmail.com ∙ www.hahnwaterresources.com Model Domain The model domain is square, measuring 7,000 feet x 7,000 feet. Model cells measure 100 feet x feet. The model grid is aligned in a north-south direction. Aquifer Properties The principal aquifer properties of interest in this investigation are:  Saturated thickness (b) – the thickness of the saturated porous medium measured as the difference between the static water level and the base of the aquifer, measured in feet. The model was based on an average saturated thickness of 18 feet.  Hydraulic conductivity (K) – a measure of the capacity of a porous medium to transmit a volume of water through a unit cross-sectional area, typically expressed in units of feet per day. The aquifer was assigned a uniform hydraulic conductivity of 1,000 feet per day.  Storage coefficient or specific yield (Sy) – the volume of water released from a unit volume of saturated aquifer per unit decline in head, expressed as a fraction. The aquifer was assigned a uniform value of 0.2 for specific yield.  Transmissivity (T) – a measure of the capacity of a porous medium to transmit a volume of water through the entire saturated thickness, expressed in units of feet2 per day. Transmissivity is obtained by multiplying the hydraulic conductivity by the saturated thickness (T = K x b). Transmissivity was therefore set equal to 18,000 feet2 per day. Boundary Conditions The northern edge of the model coincides with the Colorado River. The river is modeled using the “river package” of MODFLOW. The southern edge of the model terminates at the first apparent river terrace above the floodplain. This feature is mapped as a no-flow boundary (no water can enter or leave the model through this boundary). The eastern and western boundaries of the model are modeled using the “general-head boundary package” of MODFLOW. This type of boundary condition allows for movement into and out of the model, based on the difference between groundwater levels that are predicted to occur within the model domain and groundwater levels that are specified for locations outside of the model’s domain. Recharge It is understood that any surface water runoff, tail water from ditches and laterals, irrigation return flows, and other sources of surface water entering the site will be re-routed in advance of the mining. The Last Chance Ditch lateral (mapped as entering the site along the eastern boundary) will likely be routed into the dewatering trench. In light of these plans, there are no sources of surface water that were modeled as entering the groundwater system on site. Rainfall recharge was set at approximately 10 percent of total rainfall, and represents the net recharge to the groundwater system. The Last Chance Ditch (running approximately east-west through the southern portion of the model was assumed to leak water, with leakage ultimately recharging the groundwater system. Leakage from this ditch was modeled as a constant value of approximately 20,000 cubic feet per day over the portion of the canal lying within the model domain. Hahn Water Resources, llc ________________________________________________________________________ 3 6589 Elaine Road ∙Evergreen, Colorado 80439 ∙ office 720.242.8639 ∙ cell: 303.870.5757 hahnwaterresources@gmail.com ∙ www.hahnwaterresources.com Dewatering System Dewatering of the Rifle Pit No. 1 will be accomplished with a perimeter trench running along the northern and western edges of the pit. The trench will be excavated approximately 4 feet into the underlying bedrock. Water accumulating in the trench will be pumped out of the trench and conveyed offset, ultimately discharging to the Colorado River. The dewatering system was represented using the “drain” package. The drain representation requires that an elevation be specified for the drain invert. The model then compares the elevation of the adjacent groundwater to the invert elevation. If the groundwater elevation exceeds the invert elevation, the model causes groundwater to move to the drain (where it is subsequently removed from the groundwater system). The dewatering system is modeled as being fully installed and fully operational at the start of the simulation. Model Stress Periods The model was set up as a transient, 3-year simulation, consisting of 99 stress periods of equal length of 10 days, and a final stress period of 105 days. RESULTS The model predicts a rapid response of groundwater levels to the dewatering. The following figure (Figure 1) shows the predicted groundwater level decline as a function of time (hydrograph) at a location near the western edge of the pit. The water level hydrograph indicates that most of the groundwater level response will occur within the first 60 days of the initiation of dewatering. The hydrograph also indicates that groundwater level response will reach a near- equilibrium condition within about 150 days of the start of dewatering. Dewatering will most likely be developed in stages, over time, as the pit is advanced. Accordingly, water level response times could differ significantly from those predicted in response to a fully developed dewatering system that is fully operational from the start of mining. Figure 1. Predicted Groundwater Level Decline vs. Time in Response to Dewatering Hahn Water Resources, llc ________________________________________________________________________ 4 6589 Elaine Road ∙Evergreen, Colorado 80439 ∙ office 720.242.8639 ∙ cell: 303.870.5757 hahnwaterresources@gmail.com ∙ www.hahnwaterresources.com Figure 2 (below) shows the model-predicted groundwater level decline (drawdown) in plan view. The semi-concentric circles (with white labels) indicate lines of equal water level declin e in feet, predicted at 100 days following the start of dewatering. The model predicts that the water level lowering will be laterally extensive. This behavior is consistent with a highly permeable system as exists at the Rifle No. 1 pit. Figure 2. Contours of Groundwater Level Lowering (Drawdown) Following 90 Days of Dewatering To the extent that the on-site wetlands are dependent on the underlying groundwater system, it is likely that these wetlands will disappear in a relatively short period of time (months to years) following the onset of dewatering. To the extent that such wetlands depend on an influx of surface or near-surface water (tail water, irrigation returns, etc.), the current plans to intercept these inflows will also likely result in the disappearance of the wetlands in a relatively short period of time. Respectfully submitted, William F. Hahn, P.G. Hahn Water Resources, llc ________________________________________________________________________ 5 6589 Elaine Road ∙Evergreen, Colorado 80439 ∙ office 720.242.8639 ∙ cell: 303.870.5757 hahnwaterresources@gmail.com ∙ www.hahnwaterresources.com REFERENCES District Court, Water Division 5, 2007. Judgment and Decree in Case No. 06CW121 Concerning the Application for Water Rights of Gypsum Ranch Co, LLC and Silt Sand & Gravel, LLC. Harbaugh, Arlen W., Edward R. Banta, Mary C. Hill, and Michael G. McDonald, 2000. MODFLOW-2000, the U.S. Geological Survey Modular Ground-water Model. USGS Open-File Report 00-92. HP Geotech, 2008. Location and Logs of Exploratory Borings drilled June 26 and 30, 2008. Job No. 108 354A SGM, 2019. Miscellaneous data, including site elevation survey, preliminary mapping of wetlands, boundary information, gradation analysis for multiple pit samples. Hahn Water Resources, llc ________________________________________________________________________ 1 6589 Elaine Road ∙Evergreen, Colorado 80439 ∙ office 720.242.8639 ∙ cell: 303.870.5757 hahnwaterresources@gmail.com ∙ www.hahnwaterresources.com July 8, 2020 Angie Fowler, PE SGM 118 W Sixth St, Suite 200 Glenwood Springs, CO 81601 Supplement to Hahn Water Resources Letter Report of August 2, 2019 on an Evaluation of the Impacts of Dewatering at Rifle Pit No. 1 This report presents supplemental findings to those in my August 2, 2019 letter report to you on the impacts of dewatering at the Rifle Pit No. 1. The Rifle Pit No. 1 is a proposed sand and gravel mine located in Garfield County, Colorado within the alluvial channel of the Colorado River. SGM filed a permit application to the Colorado Division of Reclamation, Mining and Safety (DRMS) entitled “Construction Materials Regular (112) Operation Reclamation Permit Application” (Application) in January 2020. SGM received comments to the Application from DRMS (through Scott Contracting, Inc.) and from Garfield County. Those comments included questions related to the proposed dewatering activities. This report is intended to address questions and requests for additional information contained in the DRMS and Garfield County responses to the Application. Dewatering Concepts The initial goal of my investigation was to evaluate the expected changes in on-site and off-site groundwater levels that would accompany the dewatering activities. Changes in groundwater levels were of particular interest in terms of their impacts to on-site wetlands. A numerical groundwater flow model (MODFLOW) was used to estimate water level impacts from pit dewatering with an emphasis on water level change that would result from dewatering rather than on a specific dewatering system and its performance. Site-specific data available for this analysis was extremely limited. That data included drilling logs and water level measurements for 5 exploratory borings (HP Geotech, 2008), a site elevation survey, preliminary mapping of on-site wetlands, mapping of various ditches and laterals surrounding and traversing the property, a decree for water rights on an adjacent parcel that included a discussion of aquifer properties, and aerial imagery. The dewatering was modeled as if it were accomplished with a fully penetrating perimeter drain. The model predicted a pit inflow rate of about 7 cubic feet per second (cfs) or about 3,000 gallons per minute (gpm) under steady-state conditions. In this case, steady state conditions represent the point in time following an extended period of dewatering when inflow rates have leveled off. Given the absence of site-specific information on aquifer properties, there is significant uncertainty in these predicted inflow rates. Hahn Water Resources, llc ________________________________________________________________________ 2 6589 Elaine Road ∙Evergreen, Colorado 80439 ∙ office 720.242.8639 ∙ cell: 303.870.5757 hahnwaterresources@gmail.com ∙ www.hahnwaterresources.com As the mining plans have been refined, the dewatering concept was modified such that the dewatering will be accomplished with a series of wells. Whereas the dewatering concept has not yet been finalized, for purposes of my investigation the concept now includes a series of wells surrounding the pit. These wells would be augmented with perimeter drains as needed to capture seepage that may enter the pit in areas at seepage faces, or other areas where the seepage cannot be intercepted by wells. The perimeter drains would extend a short distance below the floor of the pit. Locations of the drains cannot be predicted in advance – these would likely be installed on an as-needed basis as the mining advances and would be removed when no longer needed for dewatering. In some instances, water collecting on the floor of the mine may be removed with portable pumps that are repositioned as needed to remove seepage water. Discharge from the perimeter drains and portable pumps would be combined with discharge from the wells prior to conveyance to the river. Although the Application identified a single well located near the western edge of the pit to accomplish the dewatering, the actual dewatering system would likely consist of multiple wells combined with a perimeter drain. It is important to note that the dewatering system described herein represents a concept only and should not be relied upon for design/construction of the dewatering system. The dewatering concept was developed to allow a “planning-level” investigation of probable water level impacts and dewatering flow rates. I recommend engaging a dewatering contractor who would make further investigation of subsurface conditions, design the dewatering system, and refine estimates of the timing and rates of inflow to be expected during dewatering activities. Supplemental Findings Performance of the conceptual dewatering system was evaluated in terms of expected discharge rates during dewatering and the effects on groundwater levels during the mining period and post- mining. This investigation made use of the numerical model developed during my initial investigation, modified to simulate wells rather than a fully penetrating perimeter drain. I also made use of an analytical model as an independent check on the solution provided by the numerical model. My analysis assumed that dewatering would be accomplished with 8 wells approximately evenly spaced around the perimeter of the mine. I adjusted pumping rates to achieve near-complete dewatering of the sediments to be mined. I did not attempt to balance the number of wells and the discharge rates of those wells relative to the efficiency of the dewatering scheme. The current investigation suggests that dewatering could be accomplished with wells operating at a combined discharge rate of about 7 cfs or about 3,000 gpm. These results agree with findings of my earlier investigation. For this analysis, it was assumed that dewatering would be continuous over an estimated 5-year life of the mine. Figure 1 shows the predicted water levels at a location near the western edge of the pit. As seen in this figure, groundwater levels drop relatively quickly, approaching steady-state water levels in about 6 months. Water levels are predicted to stabilize in about 12 months. This figure also shows that groundwater levels recover Hahn Water Resources, llc ________________________________________________________________________ 3 6589 Elaine Road ∙Evergreen, Colorado 80439 ∙ office 720.242.8639 ∙ cell: 303.870.5757 hahnwaterresources@gmail.com ∙ www.hahnwaterresources.com quickly following the cessation of dewatering, returning to pre-mining levels within about 6 months. Figure 1. Predicted Groundwater Level Decline vs. Time in Response to Dewatering Figure 2 (below) shows the model-predicted groundwater level decline (drawdown) in plan view as predicted for a dewatering system consisting of wells. The semi-concentric circles (with white labels) indicate lines of equal water level decline in feet, predicted at the end of 5 years of continuous dewatering. The model predicts that the water level lowering will be laterally extensive. This behavior is consistent with a highly permeable system as exists at the Rifle No. 1 pit. Figure 2. Contours of Groundwater Level Lowering (Drawdown) Following 5 Years of Dewatering Hahn Water Resources, llc ________________________________________________________________________ 4 6589 Elaine Road ∙Evergreen, Colorado 80439 ∙ office 720.242.8639 ∙ cell: 303.870.5757 hahnwaterresources@gmail.com ∙ www.hahnwaterresources.com Seasonal variations in the performance of the dewatering system are expected to be relatively minor. Groundwater levels in the alluvial aquifer, the target of the dewatering, are expected to track water levels in the Colorado River. River stage is estimated to vary over the course of a year by about 3 feet, with the exception of the month of June. Snowmelt runoff typically peaks in June, when river stage may peak at 5 feet or more above average. Changes in groundwater levels may require slightly more pumping to achieve the equivalent water level drawdown, or they may limit the depth to which mining can advance while the groundwater levels are elevated. Peaks in river stage typically last less than one month. Respectfully submitted, William F. Hahn, P.G.