Streambed Heterogeneity and Its Effect on the Computation of Streamflow Depletion

One potentially significant stream-aquifer interaction is streamflow depletion caused by groundwater pumping. In the computation of streamflow depletion, analytical and numerical methods often consider a simple streambed concept where a thin low-hydraulic conductivity (K) layer of sediment covers the whole width of the channel bottom and extends infinitely upstream and downstream. This paper demonstrates that channel sediments can have a complex layering condition. It will first describe the use of direct-push technology to generate electrical conductivity (EC) logs and collect continuous sediment cores in four rivers in Nebraska -- the lower Platte River, the Big and Little Blue rivers, and the Elkhorn River. EC logs and sediment cores suggest that channel sediments in some reaches consist predominantly of sand and gravel as deep as 24 m below the channel surface. The channel sediments in other reaches can include low-K silt-clay layers. However, these silt-clay layers are either interbedded with sand and gravel or occur as a major stratigraphic unit within the channel sediments. The vertical hydraulic conductivity (Kv) values of channel sediments can vary by four to five orders of magnitude in the same vertical profile. A low-K layer rarely occurs at the channel surface, or infinitely extends upstream and downstream. In contrast, the Kv values for upper most channel sediments are usually greater than Kv values for deeper sediments. This paper also analyzes the effect of low-K layers within channel sediments on streamflow depletion induced by near-stream groundwater pumping. MODFLOW simulations suggest that the effectiveness of low-K layers on the calculation of streamflow depletion depends on their dimensions and positions within the channel sediments. A low-K layer present at the channel surface seems to be the most effective hydrologic feature in reducing the hydrologic connectivity between stream and aquifer.