2021-07-21_PERMIT FILE - C1981019 (5)

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Point of Compliance Well Investigation AECOM 4 2. Field Data Collection 2.1 Temporary Well Installation and Development The point of compliance groundwater investigation began with the installation of 16 temporary monitoring wells in the alluvium downgradient of Colowyo Mine (Figure 1). Notice of Intent to construct the temporary wells was provided to the State Engineer’s Office on October 2, 2019. The temporary well borings were advanced by Cascade Environmental using direct-push technology. The borings were completed over a period of five days from October 8 through October 12, with the installation depths ranging from 11 to 32 feet below ground surface (bgs). The diameter of each borehole was 2.5 inches. As shown in Table 1, a total of seven borings were installed in the Good Spring Creek drainage, two were installed in the Taylor Creek drainage, and seven were installed in the Wilson Creek drainage. An AECOM geologist was present on site during the drilling program to coordinate with the client, monitor the investigation progress, and log the cores recovered by the direct-push rig. The lithology encountered while drilling generally consisted of silt, sand, silty sand, lean clay, and gravel (Table 1). Upon completion of each borehole, a temporary monitoring well was installed using 1-inch inner diameter polyvinylchloride (PVC) well casing with ten feet of pre-packed 0.010-slot PVC well screen. The construction details and static water level for each well are summarized in Table 1. The wells were generally constructed with one to two feet of casing stickup to provide access for sampling, and were fitted with a well cap at the top of the casing to seal the well. The drilling contractor also added a small amount of bentonite chips in the annular space near the top of each boring to prevent surface runoff from entering the open boreholes. Well Construction and Yield Estimate Reports for each temporary well are provided in Appendix A. The temporary monitoring wells were developed the following week from October 14 through October 16, 2019. Well development was conducted using a bailer or peristaltic pump to purge a minimum of three casing volumes from each well. During development, the water temperature, pH, oxidation-reduction potential (ORP), specific conductance, turbidity, dissolved oxygen content, and total volume purged were measured at regular intervals. The field parameter readings are recorded on the groundwater purge log forms contained in Appendix B. 2.2 Baseline Sampling AECOM personnel collected the first set of baseline groundwater samples from the new wells in October 2019 immediately after well development. The samples were collected by pouring or pumping water directly from the bailer or pump discharge line into laboratory-supplied sample containers. Sample water was slowly poured or pumped into each container until it was appropriately filled, taking care not to overfill the container or spill the laboratory preservative contained in pre-preserved sample bottles. Samples collected for dissolved metals were field- filtered using a disposable 0.45-micron filter. The sample containers were then labeled and placed on ice in a sample cooler. At the conclusion of sampling, the samples were shipped under chain-of-custody (COC) control to Pace Analytical located in Sheridan, Wyoming. After the initial sampling event in October 2019, all subsequent samples from the temporary wells were collected by a Colowyo contractor following similar purging and sampling procedures. Up to six additional baseline samples were collected per well, with monitoring events occurring in March, April, June, July, September, and November 2020. Exceptions include POC-9, which only contained enough water to sample on two occasions, and POC-4,