Application of surface electrical resistivity surveys and MODFLOW modeling in Smoky Hill River aquifer, Kansas

Decreasing water resources require the utmost care in managing surface and groundwater supplies. In many areas these two sources are inseparable, and must be evaluated jointly. The Smoky Hill River Wellfield (SHRW) aquifer in semi-arid west-central Kansas is an example. The aquifer, providing half of Hays' water supply, is a typical stream-dominated incised-valley-fill aquifer composed of Late Quaternary silty terrace units hosting younger heterogeneous sandy channel deposits. Resistivity surveys were used to characterize this heterogeneous aquifer's internal and external geometries and hydraulic conductivity simultaneously. These were combined with other data to develop a three-dimensional geometric and facies model of the SHRW, showing that aquifer transmissivity is most heavily influenced by the location of a bedrock channel and downstream increase of conductivity by a factor of 1.5. Integrating this heterogeneity in MODFLOW models indicates the worst historical aquifer performance (minimal streamflow during the severe drought of 1991-1992) could have been moderated by optimized pumping. Consideration of heterogeneity is crucial, since simulations of similar hypothetical cases (no streamflow for 180 days) show the homogenous model underestimates the safe yield by 20%. Models of the heterogeneous aquifer under the worst-case of no streamflow and full water right extraction (2.81e6 m3/year), show optimal pumping would deplete the aquifer (drawdown exceeds 3 m) in 1.2 years. Hays' 2005 extraction rate (45% of water right) would deplete the aquifer in 2.6 years. An upstream reservoir currently releases water only twice annually. These releases would have barely sustained pumping of Hays' full water right, but would not sustain the downstream flows (2.47e6 m3) required. Models demonstrate that drawndowns would be significantly reduced by distributing the same release at more frequent intervals. Three regularly-spaced releases would decrease the maximum drawdown by 20%. The best option to support Hays pumping and the downstream flow requirements is a monthly release totaling 4.82e6 m3/year, yielding maximum SHRW drawdown of 0.9 m. Average natural streamflow since 1980 exceeds this requirement, but droughts (2002-2006) would require drawdown of the reservoir. Should Environmental Flow regulations be imposed, the same models can be utilized to determine the optimum releases that balance groundwater production with streamflow.