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It is also important to keep in mind that in order for a glacial sediment aquifer to be usable, it not <br />only has to have permeable deposits <br />(sand and gravel) of a large aerial <br />extent, it also must be thick (typically at <br />least 30 feet thick) and it must he <br />saturated or have a substantial saturated <br />thickness. A sand and gravel unit that is <br />only saturated to about 10 feet is not a <br />good candidate for a water supply. <br />The Metropolitan Council (2007b) <br />ranked the availability of groundwater <br />water supplies in the seven-county <br />metropolitan area using criteria such as <br />number and productivity of aquifers, <br />presence of known contamination, <br />population growth, well-interference <br />effects, and presence of nearby natural <br />-- <br />Potential AquNer Yield .:. ~rr .. ~ <br />- RenlrkleA <br /> <br />l 3 <br />~ woderar~. 4 <br />~ <br />~ ~F <br />~ <br />A ~Id <br />~ <br />HIgIr MOOerate <br />® HMS ~j I <br />t <br />i <br />J ,.~, <br />~.. <br /> <br />y <br />.,{ <br />~ry Nqh - _ <br />~,i3 yr. <br />~.~. <br />~ <br />- <br />.,3 <br />, ~~ <br />~ <br />r ~ <br />~~ ti <br />s~; <br /> <br />°t ~ I <br />.~ <br />yy w-act <br />a <br />i ~~..: <br />M~`, <br />~ <br />~ <br />~ - ~ x; <br />.v ' ~1e~' ~ <br />r <br />nY ~' <br />[. c ~~ i <br />A - ~ sj <br />~~ <br />~n~, <br />~t~ ~~~~ <br />,, <br />`-' <br /> <br />1---~--~~ ~-:-i <br />- -- - --- ~ ~'I~ -- - ---~ <br />Figure 8 Relative Aquifer Yield (from Metropolitan <br />Council, z007b) <br />for <br />resources that are sensitive to groundwater withdrawals. 'Their map (included here as Figure 8) shows <br />areas of relative aquifer yield. The areas of highest aquifer yield are ir>, eastern Hennepin, Ramsey, <br />and Dakota Counties, where the Prairie du Chien-Jordan Aquifer System is available. Northwestern <br />Hennepin County and western Anoka County area are shown as having only "moderate" to high <br />moderate" potential aquifer yield. <br />2.4 Regional Groundwater Contamination and Water Duality <br />Contamination of groundwater impacts water-supply availability in some parts of the metropolitan <br />area. Non-point source pollutants, such as nitrate, pesticides, and herbicides have impacted <br />groundwater quality in agricultural (or formerly agricultural) areas of Dakota, Scott, and Washington <br />Counties, although no wells have been taken out of service duo to these contaminants. Nitrate, for <br />example, can be removed from water using reverse osmosis (RO) treatment. Pesticide and herbicide <br />levels are generally relatively lovv and higher levels are typically isolated and impact domestic wells. <br />The MPCA has identified many leaky underground storage tank (LL1ST) sites in the region. <br />Typically, these sites are not causes of coYitaminatiun to community water supply wells. Other <br />potential sources of contamination (e.g., dump sites, etc.) are in various stages of investigation and <br />P:\Mp1s123 MN\71\2371105 water Supply Alternative Study\FinalDeli~erables~Alternatives__Report_final.doc 15 <br />