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It is estimated that between I % and 2% of the water stored in the Denver Basin aquifers is under confined conditions. The remainder is stored in the aquifers under unconfined conditions and will release from storage when a pwnp draws down the water level at a well below the top of the aquifer. 1.3.3 Ground Water Quality - Ground water from the Denver Basin aquifers generally meets the drinking water regulations established by the US Environmental Protection Agency for public water supplies. Table 1.3 below shows the average ground water quality parameters from a sampling of water supplies from the South Metro water providers. The highest quality ground water sources are in the Denver and Arapahoe aquifers. Lower quality ground water is found in the Dawson, Laramie-Fox Hills and alluvial aquifers. Table 1.3 Denver Basin Aquifers Representative Water Quality Drinking Dissolved Constituent Dawson Denver Arapahoe Laramie- Water Fox Hills Standard (mg/I) Calcium, mg/I 30 II 31 4.2 125 Iron, ug/I 80 30 170 100 300 Magnesium, ug/I 2.7 0.4 3 0.9 125 Manganese, mg/l 20 10 30 10 Potassium, mg/I 3.5 1.0 4.1 2.8 Sodium, mg/I 12 57 140 270 Bicarbonate, mg/I 120 ISO 250 640 Carbonate, mg/I 0 0 0 0 Chloride, mg/l 3.7 3.8 57 43 250 Fluoride, mg/l 0.5 1.6 1.1 2.7 1.8 Nitrate as N, mg/I 0.11 0.05 0.04 0.03 10 Phosphate, mg/I 0.03 0 4.1 2.8 Silica, mg/I 40 13 9.6 14 Sulfate, mg/l 12 13 110 7.6 250 Hardness (CaC03), mg/l 86 29 90 14 Dissolved Solids 164 175 479 662 500 The dissolved iron concentration in the Denver Basin aquifers ranges from 20 to 200 micrograms per liter. These concentrations are highly variable between wells due to the chemical environment in the aquifer. Oxidation of the dissolved iron leads to a visible black to reddish- Page 1-8 - . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 41 41