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4. Summary and Conclusions <br />The interaction between the South Platte River and backwater slough channels <br />with the underlying alluvial aquifer at the Tamarack State Wildlife Area in eastern <br />Colorado is investigated. The Tamarack site is being suggested as a possible managed <br />recharge project to supplement flows in the South Platte River during critical low -flow, <br />high -demand periods. Pumping wells located next to the backwater sloughs and South <br />Platte River extract water from the alluvial aquifer. The interaction of the surface water <br />resources with groundwater in the alluvial aquifer is investigated using field tests to <br />document the degree of hydraulic interaction between the slough and aquifer and the <br />South Platte River and aquifer. <br />Falling -head permeameter tests are performed to estimate the hydraulic <br />conductivity of the streambed and sloughbed. Permeameter tests are performed using a <br />PVC -pipe pushed partially into the streambed. Water is added to this pipe to induce a <br />hydraulic head on the sediments inside the pipe. The water level inside the pipe is <br />allowed to fall while the displacement is measured. Falling -head permeameter tests <br />estimate the hydraulic conductivity of the riverbed to be approximately 150 m -d-1 in the <br />upper 25 cm and approximately 70 m -d-1 in the upper 40 cm. The hydraulic conductivity <br />of the bed materials in the slough is much less due to the presence of organic materials <br />that clogged the pore spaces of the coarse sand grains. Falling -head permeameter tests <br />estimate the sloughbed conductivity to range between 0.1 to 1.0 m -d-1. <br />Also, a stream/aquifer analysis test (i.e., pumping test next to the slough and <br />stream) is performed at the Tamarack site. Recently proposed analytical solutions <br />suggest that streambed conductance or conductivity can be inversely estimated by <br />matching the observed drawdown response in one or more observation wells to analytical <br />predictions of drawdown. The objective of this stream/aquifer analysis test is not only to <br />investigate surface water /groundwater interaction scenarios at the Tamarack site, but also <br />investigate the use of complex analytical solutions for estimating streambed conductance. <br />The stream/aquifer analysis test makes use of a single pumping well located 94 in from <br />the slough channel and 380 in from the South Platte River. Drawdown is measured over a <br />24 -hour period in five observation wells: two observation wells are located within the 94 <br />in between the slough and pumping well and three observation wells are located between <br />the slough and South Platte River. Finite stream width is noted to be critical in <br />influencing the interaction. between the slough channel and the alluvial aquifer since the <br />distance between the slough and pumping well is relatively small. In order to analyze <br />this data, this research combines the FDD analytical solution and the STRMAQ package <br />to develop a solution capable of addressing finite width effects of the slough channel. <br />Using this improved STRMAQ package, predicted drawdown is individually matched to <br />the observed drawdown in the two observation wells between the pumping well and <br />slough channel using a trial and error approach. Estimates of critical aquifer and <br />streambed parameters are obtained, and these estimates match reported parameter values <br />from other aquifer tests performed at the site. <br />14 <br />