Agenda - Council Work Session - 10/20/2020 - Special

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5.1.5.2 At the start of this period, Well 5 water levels were recovering to static at approximately 23 feet below ground surface. Pumping commenced at 15:15 on May 3rd and continued for the next 18 hours. Initial drawdown or instantaneous drawdown is depicted by the step increase in drawdown from 1651 to 18:27. Water levers over this period fell 80 feet. Water levels over the following 18 hours decreased another 40 feet. The well ceased pumWng at 10:30 on May 3fd and recharge to the aquifer took 17 hours to return to static water revels. Current well pumping and rates of recharge appear good with the aquifer being able to recharge water to static in a relatively low period of time. Well Drawdown and Available Head Water levels from the source water aquifer, the Tunnel City-VVonewoc Aquifer, are much higher than the topographic elevation of the top of bedrock. This scenario is known as a confined aquifer, where the potentiometric surface, or water head, is higher than the topographic location of the aquifer. Presently, water quantity throughout the region is good and there are no regulations actively being enforced on pumping levels within a well. Well interference and long term groundwater trends through pumping/drawdown is conducted on a case by case basis by the Minnesota DNR if a problem arises. Typically it is not a good idea to drawdown water revels in a confined aquifer below the top of bedrock as it can introduce oxygen into the formation. Additionally, water levels should stay sufficiently above the specifications of the water pump design. For confined aquifers, the Minnesota DNR has established a two -tiered aquifer protection threshold system to ensure the long-term viability of the pumped aquifer and to prevent exceedance of the aquifer safe yield as defined by MN Rule 6115.0630 Definitions Subps,15 and 16. These thresholds allow for appropriation from the aquifer, but establishes minimum water level elevations to be maintained as a safeguard to protect the structural integrity of the aquifer itself. Threshold elevations are set in observation wells completed in the source aquifer and not pumped wells. • The first threshold is set at an elevation that is 50% of the pre -pumping available head above the top of the aquifer. If water levels drop to the 50% threshold, pumping will need to be evaluated and a possible reduction in rate and volume may be required. • The second is a water level elevation associated with 25% of the pre -pumping available head above the aquifer. At the 25% threshold, pumping would need to cease to prevent exceeding the safe yield for the artesian aquifer. • If more than one aquifer is impacted by pumping, then thresholds are set similarly in the other aquifers. The table below depicts the static water level and the approxirnate available head above top of aquifer in the City of Ramsey's current municipal wells taking account the Minnesota DNR 50% and 25% thresholds. FEASIBILITY STUDY -DRAFT RAMSY 154354 Page 18