2021-10-27_HYDROLOGY - M2002120 - Laserfiche WebLink

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Bernhardt Pit SWSP Page 2 of 5 October 27, 2021 mining had not commenced within the liner boundary. Mining is complete and the pond is being reclaimed, therefore no depletions from aggregate production or exposed groundwater will occur at Pond 4. The liner of Pond 5 was also provisionally approved on August 14, 2019, because mining had not commenced within the liner boundary. Mining is anticipated to begin in 2022. The liner approval for Pond 4 and 5 is only applicable during the mining operations at the site and does not classify the structures as reservoirs capable of water storage. An alluvial well has been constructed to provide water for concrete batching. This well was constructed under permit no. 60729-F (WDID 0405678) and is located outside the boundaries of the existing slurry wall. The SWSP request does not seek to pump water from this well during the plan period. Well no. 60729-F has not been used for concrete batching since 2008 and all lagged depletions from previous pumping from the well have already impacted the stream. A flow meter has been installed on the well, as is required by condition of approval #9 on well permit no. 60729-F, and the meter is currently in compliance with the South Platte Well Measurement Rules. Depletions Mining operation at the site occurs within the approved liner of the slurry walls therefore no water lost with the mined product is accounted for in the SWSR Dewatering at the site commenced in October 2003 at a rate of 750 gallons per minute. In 2004 dewatering decreased to 350 gallons per minute. All water pumped in 2003 and 2004 for dewatering purposes was pumped from trenches into an on-site pond for recharge back to the Big Thompson River. In September 2004, pumping of the dewatering water into the pond ceased. All water pumped from the trenches since September 2004 has been pumped from within the slurry wall and used for the purposes described in this plan. Based on the analysis previously performed by the Applicant there are no lagged depletions from past dewatering operations that will accrue to the river during this plan period. The plan proposes to expose 4.3 acres of groundwater at the Bernhardt Pit within the settling ponds and dewatering trenches. Evaporative depletions were calculated using a gross annual evaporation of 44 inches (based on the NOAA Technical Report NWS 33) with a credit of 9.29 inches for effective precipitation (based on an average annual precipitation of 13.27 inches for the Longmont weather station). Evaporative losses for the period of this SWSP are 12.39 acre-feet (2.88 acre-feet/acre), as shown on attached Table 1. Consumptive use of groundwater from the Bernhardt Pit consists of 2.16 acre-feet for on-site dust control and 12.39 acre-feet from evaporation. The total consumptive use at this site during this plan period is 14.55 acre-feet (see attached Table 1). Depletions from mining operations at the Bernhardt site will impact the Big Thompson River upstream of the Evanstown Ditch (WDID 0400517). The depletions from evaporation and operational losses were lagged to the stream using a Glover analysis assuming a distance (X) from the centroid of the exposed ground water surface to the stream of 715 feet, a distance (W) to the impermeable boundary of 2,976 feet, a transmissivity (T) of 100,000 gpd/ft, and a specific yield (S) of 0.2. The total lagged depletions that are to be replaced in 2022 are 14.56 acre-feet (see attached Tables 2 and 3).