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~ 01 ~ . 1 w c: c':':'2 '0 '" 0 " "ICP I'''' Note: Precipitation from rain gauge 4 - ~ Qj > 01 .... ~ OliO C ~ ~ subsequent dry winter-spring cycle will only exacerbate the situation. Non-tributary ground water paints a different pic- ture. The Denver Basin is a large structural basin underly- ing nearly 7,000 square miles of eastern Colorado. Consist- ing of layers of sandstone interbedded with layers of impermeable shale and silt- stone, as shown in the cross- section below, the stratigraphy of the basin has been subdi- vided into the Dawson, Den- ver, Arapahoe, and Laramie- Fox Hills aquifers. The basin is estimated to hold as much as 270 million acre-feet of water in storage, which is a tremen- dous volume, compared to the surface water storage in the South Platte River watershed of 1.1 million acre-feet. Much of the ground water in the Denver Basin is considered to be non-tributary due to the separation of the sandstone layers from surface water by shale layers and large horizontal distances to aquifer out- crops. This non-tributary ground water is adminis- tered differently than tributary ground water. Instead, holders of non-tributary ground water rights have decrees that specify how much water can be used dur- ing a year. This amount depends on land area and aquifer thickness, among other factors. Non-tributary ground water usage will not be curtailed when surface water supplies run short. During a drought cycle, non- tnbutary ground water can continue to be pumped at the decreed rates when surface water and tributary ground water usage may be curtailed. This gives hold- ers of non-tributary water rights some immunity from ~ t '.'111'1' II !:! , 20 AMJJASOND FMAMJJASOND FMAMJJASOND 1998 1999 2000 2001 Water levels in an observation well in the Foothills west of Denver respond ropidly to seasonal precipitation events. ADAPTED fROM JEFFERSON COUNTY PHASE I REPORT SUMMARY WATER RESOURCES ASSESSMENT OF THE TURKEY CREEK WATERSHED, 1990-2000 The crystalline pre-Cambrian bedrock underlying the fo.othills just west of the Front Range Metropolitan Area IS an example of a fractured bedrock aquifer. Most homeowners in the foothills rely on individual wells tapping this vast aquifer. The hydrograph above, from an observation well near Conifer, shows the immediate connection between the ground water fill- mg the fractures in the bedrock and water at the sur- face. In spring, the water level in the well rises in response to snow melt and wet spring precipitation, then the water level falls through summer, fall, and winter. The next cycle begins with the next spring. Wells tapping this type of aquifer are very vulnerable to an extended drought period of a dry winter fol- lowed by a dry spring, as happened in 2002. In addi- tion, there is little chance of recovery until the follow- ing spring, given typical precipitation cycles. A w , - ~ '" ~ " E: ~ Denver o ~ Dowson Aquifer Elev in Feel 7000 Wetkins 6000 5000 4000 3000 E . Elev in Feet 7000 Arapahoe Aquifer Laramie Formation Fox Hills A uifer 6000 5000 · . Pl.rr~ Shot. 4000 Verlicalexaggeration x2 3000 Sandstone aquifers underlie the vast Denver Bosin and supply many of the growing suburban communities, porticu- larly In Douglas County, with groundwater. ADAPTED FROM USGS HA.730C. 10 Colorado Geological Survey ROCKTALK Vol. 5, No. .4 I I I