2021-12-08_GENERAL DOCUMENTS - C1981019

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DocuSign Envelope ID:6BC32598-B737-464E-898A-81 F1 F7131339133 these lineations were observed within the mine permit area.These are nearly vertical or vertical and strike approximately north 70'west, north 45'west,and north 30'east. In surface exposures, sandstone exhibits joints more readily than highly oxidized and weathered coal beds; however, both have undergone similar stresses and have developed similar readjustment features that are apparent in the subsurface. The structure of sub-basin in the area and the Cretaceous to recent stratigraphy is shown on the geologic cross-section in Exhibit A3-5 (from CGS, 1984). Mine exploration test borings have been conducted within the permit area and have been utilized for the following purposes: Location of Subsurface Water — Since the holes were generally dry, the primary circulation medium for most exploration bore holes drilled within the mine plan area was either compressed air or drilling mud when fractures were encountered. Several drill holes throughout the area encountered minor amounts of ground water; however, all of the data obtained to date by Colowyo and the USGS have indicated that groundwater occurrences within and above the mined coal seams in the area are not continuous but rather are a series of perched systems. Information on groundwater occurrence is set forth under Section 2.04.7. Drill Hole Logs — Geophysical logs used estimate the injection horizons and to determine geological and hydrogeological parameters for the AOR determination are attached. These include SP and resistivity logs from the Gossard #1 well and SP, resistivity, gamma, and density logs from McBride Gov Little Cr#1 well. The locations of these wells are shown on Exhibit A3-1. Regional Seismicity — Limited chance of damaging seismic activity is expected in the area. According to the USGS 2018 National Seismic Hazard Model, northwest Colorado might expect a 4%to 19%chance of minor earthquake damage in 100 years. A small 3.5 magnitude earthquake occurred near the town of Rangely, Colorado(approximately 50 miles wells of the Colowyo Mine) in 2017. No serious damage from this event were reported. Attachment B2. Proposed Formation Testing Program Characterization of well stratigraphy and formation porosity will be primarily conducted via geophysical logs, which are discussed further in Attachment C. A comingled interval step-rate test (SRT) may be performed per EPA Region 8 guidance to determine fracture pressure which will be used to calculate maximum allowable surface injection pressure(MASIP). This will be done by hanging bottom-hole gauges inside the tubing work string and injecting down the tubing-casing annulus. The proposed SRT rates are listed in Table 82-1. The anticipated injection rate is on the order of 160 gpm but will be variable depending on the seasonal seep flow rate. The proposed rates for the SRT are based on an assumed maximum injection rate of 300 gpm. 8