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I. GROUND WATER OBSERVATION RECORDS - WHAT DO THEY SHOW? <br />• The purpose of observing and recording water levels in wells is to <br />determine the state of the ground water in storage; i.e. are water levels <br />rising or falling and what is the cause of these fluctuations. Fortunately <br />W.E. Code begin measuring some wells in the late 1920's and that <br />network was expanded by CSU to measure over 200 wells in the 1950-76 <br />period. Measurement of some of those wells has continued through 2006. <br />Analysis of those records shows the impact of periods of drought and <br />above normal precipitation as well as man made influences of pumping, <br />recharge from irrigation and artificial recharge projects. <br />What do the measurements show? For the alluvial aquifer of the South <br />Platte, mainstem the aquifer is significantly impacted by recharge from <br />irrigation as a result of canal seepage and deep percolation of irrigation <br />water applied to the fields. Pumping from wells intercepts water which in <br />its normal course is flowing to the River. Where depth to the water table is <br />less than 10 feet, there is significant loss of water due to evaporation and <br />transpiration by phreatophytes. <br />Hydrographs for five wells are attached as figures along with their <br />tabulated historic data. Measurements have historically been made in the <br />spring (March or April) prior to the irrigation season followed by a fall <br />measurement (October or November) after irrigation is complete. These <br />records are similar to the hydrographs from other wells, which show: <br />1) The water levels in the South Platte River alluvium wells has not <br />deciined over time <br />2) Even during the drought periods of 1940, 1953-57, 1963-64, and <br />2002 the aquifer is still full. <br />3) Fall measurements beneath the highest ditch are higher than <br />spring measurements. The river acts as a drain. <br />4) Comparison of spring to spring measurements shows the water <br />table returns to the same level. The impact of pumping and deep <br />percolation of irrigation water are all swept to the River over the <br />non-irrigation period (Oct.-April). <br />5) No apparent carry over impact of well pumping from one year to <br />the next. For the oldest records dating back to the 1930's and <br />1940's there is not apparent fowering of the water table due to <br />increased pumping in the 1950's or 1960's or even the pumping <br />of 2002. <br />6) Above the highest ditch, the fall readings are significantly lower <br />than the spring readings, which shows the impact of pumping <br />without the impact of deep percolation of irrigation water or canal <br />seepage. <br />2