2020-11-25_HYDROLOGY - M1983194 - Laserfiche WebLink

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DAUB & ASSOCIATES, INC. 1985'Y2 SOUTH BROADWAY hi GRAND JUNCTION, CO 81507-9649 (970)254-1224 FAX(970)242-8438 email: gjdaub@daubandassociates.com www.daubandassociates.com November 19, 2020 -..� Mr. Nathan Wiser ;=a US EPA Region VIII Mail Code: 8WD-SDU NOV 2 5 2020 1595 Wynkoop Street Denver, CO 80202-1129 Re: Natural Soda LLC, EPA UIC Permit C030358-00000: BG-9 Elevated Groundwater Chemistry Dear Mr. Wiser: The BG-9 groundwater monitoring well is exhibiting elevated chemistry values for conductivity, total dissolved solids, and alkalinity. The BG-9 is a B-Groove Aquifer (BG) and subsurface subsidence monitoring well, recompleted from the DS-5, which was initially drilled as a core hole and completed as a Dissolution Surface (DS)Aquifer and subsurface subsidence monitoring well. The DS-5 began showing elevated chemistry and temperatures, indicating possible communication with the proximal solution mining operations. The casing was intact, so the decision was made to recomplete the well to utilize it both as a BG Aquifer monitor and a subsurface subsidence monitoring well. This was accomplished by setting a bridge plug near the end of the casing, placing cement on top of the bridge plug, and then perforated the casing across the BG Aquifer interval. The chemistry departure in the BG-9 began in late 2019, but has accelerated since April of 2020. Conductivity values have subsequently elevated to the 15-16K micromhos range. An investigation was initiated to determine the cause. A video log was performed in the BG-9 on October 12, 2020. Initially, the video log showed very little fluid movement in the well until the cement plug was reached, where there was evidence of minor upflow. Subsequent scrutiny, however, revealed four slight casing partings just above casing couplings at depths of 1676.9, 1760.9, 1782.2, and 1804.2. While haloclines due to saline stratification were observed at various depths, no apparent inflow or outflow was observed from these partings. Due to the casing partings but lack of apparent flow, it was decided to remediate the well by plugging it with a second bridge plug and cementing it back to just below the BG Aquifer perforations in order to continue utilizing it as a BG monitoring well. Subsequent to the plug back, the well was airlifted to clean out the well and ensure good flow through the perforations. Field samples indicated conductivity in the range of 45-50K micro-Siemens (field measurements), appreciably above the 16K micromhos (lab analysis) seen prior to plugging back the well. The well was airlifted at intervals for three days to remove any high conductivity fluids. Conductivity fell to 30k micro-Siemens before the effort was halted. It is believed that DS Aquifer water was migrating up inside the casing and mixing with the BG up through the first cement plug, and possibly through the casing partings. During subsequent operations, the DS water was forced out of the perfs, creating a localized concentration of higher conductivity water. NS proposes that the localized high conductivity water will dissipate. The 13G-9 will be sampled at a weekly interval to monitor the water chemistry (field parameters via sampling pump or dump bailer for temperature, conductivity, and pH). If the elevated conductivity continues to lower, then the SPECIALIZING IN PROFESSIONAL GEOLOGICAL, ENVIRONMENTAL, HYDROLOGICAL, GEOTECHNICAL AND PERMITTING SERVICES