2020-09-09_REVISION - M1977348 - Laserfiche WebLink

Home Browse Search

Amy Eschberger Reference No. 20144265-REV1 Colorado Division of Reclamation,Mining and Safety September 4,2020 after each casing volume removed. The wells will be developed until turbidity is reduced and field parameters have stabilized, or after five casing volumes have been removed,whichever comes first. In the event the well goes dry during purging, subsequent purging may be needed after the well is allowed to recharge. Equipment used for well development will either be new materials (such as tubing, surge blocks, pumps, inertial foot valves or disposable bailers) or will be decontaminated using a low phosphate soap (such as Alconox) solution followed by a distilled water rinse. It is assumed purge water will be non-hazardous and can be discharged onto the ground surface. 3.0 MONITORING AND DATA EVALUATION 3.1 Monitoring After well installation, it is anticipated that at least six rounds of semi-annual groundwater monitoring (water level measurement and water quality sampling)will be performed. Due to the low hydraulic conductivities observed in existing monitoring wells and the responses observed post well install at MW-6 and MW-7, it is anticipated that the new wells may take considerable time (e.g., years)to stabilize and be reflective of in situ groundwater chemistry following the disturbance by drilling. As observed in well MW-6 and MW-7, the reported values the first few years of monitoring may not be reflective of true aquifer conditions. If increasing or decreasing trends are observed, additional monitoring may be necessary to confirm that the water quality is representative of formation water. This determination will be made through visual assessment of time series graphs. This monitoring will be conducted in accordance with the procedures outlined in the Sampling and Analysis Plan (Golder 2010). The new well monitoring events will be consistent with the monitoring events for the existing wells so that the results will be comparable. The constituents to be analyzed for this water quality sampling will be the same as currently being conducted on a semi-annual basis for the existing wells outlined in TR-9 (Golder 2018). 3.2 Evaluation of MW-8 Evaluation of parameters at MW-8 will be focused on parameters that have previously exceeded the Regulation 41- Colorado Basic Standards for Groundwater(5 CCR 1002, BSGW)at the existing site wells and total dissolved solids (TDS)greater than 10,000 milligrams per liter(mg/L). These parameters include barium, iron, manganese, boron, selenium, uranium,fluoride, chloride, sulfate, nitrate, nitrate+nitrite, gross alpha, and TDS. It is noted gross alpha results have been consistently reported at low or negative concentrations with large uncertainty(e.g., -50±26 picocuries per liter(pCi/L))and vary widely within each well and between wells. Therefore, the gross alpha results have been ineffective with regard to assessing groundwater quality and the large variability an uncertainty associated with the results will likely make any comparison to background well MW-8 inconclusive. To evaluate the results from MW-8, an approach has been developed to determine the comparability of the new samples with the existing water quality at MW-4, MW-6, and MW-7. Two methods will be used for determining the magnitude of difference between the samples: a visual/graphical comparison and a mathematical comparison. For the visual and graphical comparison, time series of the collected data will be updated and reviewed following each sampling event, and box and whisker plots will be updated. For the mathematical approach, MW-8 will be assessed using a relative percent difference(RPD)method,treating the samples as though they are laboratory duplicates,to determine if they are likely to have been drawn from the same population.This approach was selected because it will allow for the comparison of individual data points rather than a GOLDER 4