Agenda - Council - 10/27/2026 - Laserfiche WebLink

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PROPOSED WELL SITING AREAS The 2021 study identified three areas for future wells. Each area has different strengths. The table below summarizes them. SEH will evaluate all three areas in detail. We will also screen City -owned parcels outside these areas, as the RFP requires. AREA A LOCATION Southwest, along the Mississippi River and Highway 10 TUNNEL CITY (FT) 100 to 150 B Northwest, around Lake 0 to 80 Itasca North of Well 7 along Armstrong Boulevard WONEWOC (FT) 45 to 60 35 to 100 90 to 100 15 to 35 POTENTIAL ISSUES AND SOLUTIONS Addendum No. 3 sets the pricing basis at four test holes finished to 375 feet and four 6-inch test wells finished to 220 feet. Our drilling subcontractor quote uses the addendum basis and assumes that 220 feet is the outer casing depth, not the total depth. To meet Minnesota Rules 4725 with a 6-inch casing, the driller must first advance a larger borehole and set an outer casing through the unconsolidated material. The quote includes a 10-inch outer casing to 220 feet, a 6-inch inner casing to 250 feet seated in upper bedrock, and open hole drilling to 400 feet in the Tunnel City-Wonewoc aquifer. Proposed observation wells for this contract are anticipated to be constructed as follows: PER- AND POLYFLUOROALKYL SUBSTANCES (PFAS) PFAS has been detected surrounding the Anoka Regional MSW Land Disposal. MPCA has strict guidelines for low flow sampling from observation wells. While not included in this scope, the City should consider following MPCA guidance on low flow sampling for PFAS from any installed observation wells. WELL INTERFERENCE The RFP requires an interference evaluation for wells within 500 feet of each site. We will also model interference with the existing wellfield. The 2021 model shows this may matter at the 1,500- to 2,000-foot scale. MNDNR SAFE -YIELD THRESHOLDS The MnDNR limits drawdown in confined aquifers to 50% and 25% of available head above the aquifer. Site selection must model potential interference. KEY CONSIDERATIONS Thickest combined section. Closest to Well 7. Inside the 20 to 40 ft modeled 2040 drawdown zone. Some City - owned parcels. Thickest Wonewoc in the City. Farthest from the wellfield. Lowest interference. Longest watermain run to the water treatment plant (WTP). Several City -owned parcels. Along the existing watermain and within City land. Lowest connection cost; however, Wonewoc is only 15 to 35 feet thick here, so a well may rely more on the less productive Tunnel City. Assumed Test Well Construction Four 6-inch test wells in the Tunnel City-Wonewoc aquifer. Construction per Minn. Rules Chapter 4725. Not to scale. Final depths will be set from the first test hole log. Discharge to approved location oft .,, 10-inch outer casing so 0 to 220 ft, through unconsolidated material, grouted to surface 100 Pressure transducer 2 data loggers per test: pumping well and one observation well 150 Test pump 72-hour pumping test 6-inch inner casing 0 to 250 ft, seated in upper bedrock, annulus grouted Open hole 250 to 400 ft in the Tunnel City- Wonewoc aquifer 200 7 220 — 250 — 300 350 Grout CI 10-inch casing 40D ❑ 6-inch casing 0 Open hole • r Assumed construction for fee estimating per Addendum No. 1 (four 6-Inch test wells). Final depths. casing, and grout per Minn. Rules 4725 and the first test hole log. /1/ Static water level (15 to 30 ft in existing City wells) Unconsolidated Sand, gravel, and clay 200+ ft per City well logs (Well 8. Unique No. 743833) Tunnel City Group Upper bedrock, top about 220 Upper 50 ft fractured. enhanced permeability; lower (Lone Rock) low permeability, yields 10 to 20 gpr Wonewoc Sandstone Highly permeable sandstone Highest yield unit, up to 400 gpm (Anoka County Geologic Atlas) Contact depth varies by site SHORT ELLIOTT HENDRICKSON INC. WELL SITING ASSESSMENT FOR MUNICIPAL WELLS #9 AND #10