Cienega Cr Watershed Hydrologic Evaluation of Proposed Hermosa Mine Water Treatment Plant (WTP2) Discharge Presentation to PARA November 12, 2020 Sonoita Cr Watershed Laurel Lacher, PhD, RG, Lacher Hydrologic Consulting, Tucson, AZ Bob Prucha, PhD, PE, Integrated Hydro Systems, LLC, Boulder, CO Study Commissioned by PARA • Purpose of Study: − To evaluate potential hydrologic impacts of South32’s proposed discharge of treated water from the Hermosa Project to Harshaw Creek as described in August 2020 AZPDES and APP applications. South32’s Proposed Action • South32 (parent company to Arizona Minerals, Inc.) proposes dewatering of Hermosa Project area ~ 5 miles south of the Town of Patagonia to facilitate underground mining. 1) Taylor Deposit (zinc, lead, silver) 2) Clark Deposit (zinc, manganese, silver) Initial dewatering rate up to 4500 gpm Treated water discharged to Harshaw Cr. Hermosa Project Modified from Clear Creek Assoc., Aug 2020, APP Significant Amend. Applic P-512235, fig. 1. Motivating Factors for PARA Existing Flood Potential on Harshaw Creek and in Town of Patagonia South32’s conclusion of no surface water impact in Sonoita Cr. History of Flooding in Patagonia 2017 Watershed Management Plan Perennial flows along Sonoita and lower parts of tributaries shown Streamflow at Patagonia-Sonoita Cr. Preserve (m3/s) Recent Harshaw Elevated Baseflows – amplify flood potential Creek Flooding Data courtesy of The Nature Conservancy Sonoita Cr Watershed Precipitation (mm) Sonoita Cr Watershed Precipitation (mm) 8/7/16-8/12/16 7/15/17-7/24/17 Harshaw Creek Flows Harshaw Creek Flows July 22, 2017 August 9, 2016 Key Points • Monsoonal storms leading to Discharge on Harshaw Cr relatively typical annual monsoon storm events • Preceding storms increase surface saturation and runoff South32 Findings (presented July 21-22, 2020) Literature Review Documents Reviewed Proposed New Discharge from WTP2 to Harshaw Creek Study Objectives 1. Evaluate effects of 4500 gpm mine dewatering discharge from WTP2 on: • Streamflow − Harshaw Cr − Sonoita Cr • Conveyance of Mine Discharge − Natural regime PLUS mine dewatering − Along Harshaw Cr − Through town of Patagonia • Groundwater − Groundwater-dependent ecosystems − Aquifer storage changes 2. Assess baseline hydrologic conditions and controlling factors 3. Develop a robust numerical model of Sonoita Cr watershed applicable to other problems Integrated Modeling MIKESHE by DHI https://www.mikepoweredbydhi.com/products/mike-she MIKESHE / MIKEHydro flexible process descriptions • Choice of spatial and temporal scales (depends on processes): • Groundwater (days to years) • Surface water (seconds to hours) • Unsaturated flow (seconds to years) • Process time scales independent and automatically controlled • Choice of processes to include in model • Choice of simple to complex solutions • Groundwater flow virtually identical to MODFLOW. ● Green and Ampt • FEMA‐approved fully hydrodynamic, surface water hydraulic model MIKESHE Model Development Approach Distributed NLDAS Meterological Data USGS Data (3 hourly) ADWR Data (Geology and -Air temp Wells/Water levels) -Precip -PET Soil Vegetation Model Landuse Development Snow Depths Preliminary Stream Q (Phase 1) Heads Updated parameters, Time- varying distributed Recharge & AET Integrated Model ET Soil Moisture Calibration Water Quality (2000 to 2020) Preliminary Predictive Local Scale Uncertainty Scenarios Regional Model (Phase 2) (Phase 1) Analysis (Phase 2) Model Scenarios Hermosa Mine Refine Calibration (More accurate/improved resolution) (account for WTP Discharge uncertain inputs) (4500 gpm) Conceptual Model Springs – can occur at Generalized Conceptual Basin Flow Model faults when upgradient GW forced to surface. Alluvial ‘Valley’ Fill (along Sonoita 10 m Creek) - High permeability gravel/sand matrix interbedded with clay/silt lenses controls ‘bank’ storage. Clays/silts limit vertical flows and surface infiltration Relatively thin veneerUnweathered of Bedrockhigh 790 m permeability colluvium (3 to 6 m) supports vegetation. Unweathered Bedrock Weathered Bedrock zone – Most productive wells fractured, higher permeability than screened in alluvium; underlying unweathered bedrock. some enhanced by Perches GW in overlying colluvium faulting – critical control on streamflow. Fault – impedes flow across Unweathered Bedrock –low it. Heads higher on permeability, isotropic unfractured. upgradient side. Many wells screened in bedrock Conceptualization of Unsaturated Zone Flow Variable soil types Strongly control ET dynamics, infiltration & recharge to aquifer Saturation increases above clay layers Influences infiltration and recharge to groundwater Unsaturated Zone Moisture Deficit draws water Groundwater Recharge UPWARD from water table Response to stream stage (capillary draw). Macropores rapid recharge Shallow, low Perched Flow – above shallow Surface Water Runoff permeability bedrock bedrock Mechanisms Shallow bedrock • Causes rapid surface saturation, increasing runoff • True in many areas Low permeability soils with deeper GW table & bedrock Shallow groundwater table • Runoff occurs as ‘Hortonian Flow’ • Limits aquifer storage (i.e., rainfall rate exceeds infiltration capacity). capacity • Unlikely in most storms • “Rejects” infiltrating water, low permeability increasing runoff bedrock Evidence for Runoff Mechanism Surface water discharge appears strongly controlled by shallow GW table (locally-perched or connected to regional aquifer) for several reasons: • Reported range of soil permeability doesn’t produce peaky discharge (i.e., Hortonian runoff) • Aquifer storage effects apparent in surface discharge at Preserve gage • Relatively shallow GW beneath town/Sonoita Cr • Perennial flow along lower reaches of most tributaries and downstream of Town where shallow bedrock forces GW to surface. (eg, TNC preserve) Shallow groundwater Shallow bedrock low permeability bedrock Streamflow at Precipitation at Patagonia-Sonoita Cr Preserve* Delayed stream flow response * Data courtesy of The Nature Conservancy REGIONAL Model Setup/Assumptions Model Domain, Discretization, and Hydraulic Network • 500 m grid cells Mt Wrightson • Entire Sonoita Cr watershed Cienega Cr Watershed • Sonoita Creek and main tributaries • Patagonia Lake Patagonia Town of Lake Patagonia Proposed WTP2 Discharge Location Sonoita Cr Watershed Climate Data NLDAS Zones (11.4 km x 13.8 km spacing) • NASA’s National Land Data Acquisition System (NLDAS) data • Hourly Rainfall, Potential Evapotranspiration (PET), and Air Temperature (2000 through 2020) Can supplement with • Benefits of NLDAS dataset: local weather station data • spatially distributed (~11.4 x 13.8 km grid) • no data gaps • hourly consistency between rain, PET and air temperature. These data ‘drive’ the hydrologic and snowmelt response. Precipitation (mm/day) • Elevation lapse rates adjust rain and air temp account for orographic effects. SNOWMELT PARAMETERS • Melting Temperature – 0º C (uniform) Hourly Air Temp (C) Potential ET • Degree Day Coeff – 4 mm/C/day (mm/day) • Maximum Wet Snow Fraction – 0.1 • Thermal melting (from rain heat on snow) included – Melt Coeff – 0.131/C B HermosaMine ran c h 17 MIKEHYDRO_HermosaMine_CrossSections Bran MIKEHYDRO_HermosaMine_Branches Unsaturated ch 5 MSHEmodel_Sonoita B r an ch Faults_MSHEcombinedREVISED 19 Zone Places (Outline) Bran Cnty Bnds (detailed, named) ch 2 Bran ch 18 SSURGO (0 to 100 cm) • USDA - SSURGO Soil Ksat values x1e-6 m/s 0 - 4.2 Survey data 4.3 - 9 B ranch 20 9.1 - 16.3 • Similar to AZGS 16.4 - 26.9 27 - 66.1 UV83 B Branch ra 24 66.2 - 189.1 Surficial Geology and 4 n c 1 B h h c ra 2 189.2 - 425 n n 2 a r c h supported by well logs B 6 UV82 3 • Saturated hydraulic h 2 Santa Cruz Branc Branch ch 12 2 an 1 conductivity values Br 13 ch 6 n 1 H a h a shown for top 100 cm. r rs c ha B n w ra C Branch B r 4 eek • These data converted into AlumGulch B ra n c h 8 MIKESHE soil types. 9 h c n S a on r o B itaCr eek • Assumed uniform to B ran ch higher elevation of 10 11 h c an either groundwater table Br or bedrock surface. UV289 §¨¦19 ± 02461 Miles Clays present in Harshaw Creek MUD CRACKS and Valley Alluvium Middle Harshaw Cr. (Nassereddin, 1967) 55‐536795 !< ") Hermosa Project Borehole Logs 8 h nc a ") Br Alu mG !< u ") lc!<h BH-14 #* ") Harshaw Creek BH-01 #* BH-12 #* #* ") TP-02 Newfield_Mine_boreholes_pits ") ") #* #* ME TP-25 BH-11 BH-10#* ") -520.7 - -480.1 #* TP-07 #* #* #* ") -480.0 - -100.0 #* BH-09 -99.9 - -50.0 TP-53 #* BH-16 ") #* #* ") -49.9 - -10.0 TP-06 ") ") BH-15 ") -9.9 - -5.0 #* ") -4.9 - -3.0 TP-08 #* -2.9 - 3.0 #* ") 3.1 - 5.0 ") 5.1 - 10.0 ") ") 10.1 - 50.0 ") 50.1 - 100.0 ") 100.1 - 390.4 MIKEHYDRO_HermosaMine_CrossSections MIKEHYDRO_HermosaMine_Branches ") GWmodelboundary Streams_all 00.1 0.2 0.4 0.6 Cnty Bnds (detailed, named) Miles Most logs show soils overlying bedrock have significant clays & silts – even within Harshaw Creek. Looking down Sonoita Creek toward 3D Lithologic Model – Deposits Overlying Bedrock Patagonia Patagonia Borehole Lithology from ADWR’s Well55 Registry Driller’s Logs Patagonia Lake Lithologic Color Legend Increasing Permeability Bedrock Patagonia Sonoita Cr Clays Silts Sands Gravels WTP2 Saturated Zone 4 model layers 10 m Layers 1 & 2 (alluvium & weathered bedrock) Unweathered Bedrock 790 m Layer 3 – Weathered Bedrock Unweathered Bedrock Layer 4 – Competent Bedrock Depths to bedrock increase rapidly into Unconsolidated Material Thickness - Isopach Cienega Creek watershed to north B ranc h 17 Thickness data shows