Understanding of the Hydrologic Connections Between Wide-channel and Adjacent Aquifers Using Numerical and Field Techniques

Streambed sedimentary structure plays a vital role in controlling the interactions between surface water and groundwater. In the study, three rivers -the Big Blue River, the Little Blue River, and the lower reach of the Platte River in Nebraska were chosen to characterize the shallow streambed for the two types of rivers (braided and meandering rivers) and investigated the variations of the streambed electrical and hydraulic conductivities with depth. In-situ and laboratory permeameter tests were conducted to determine streambed hydraulic conductivity up to 20 m below the channel surface in the three rivers. Additionally, the electrical conductivity logs were obtained using Geoprobe direct-push technique to characterize the hydrostratigraphy of streambed sediments. Although the tributaries of the Big Blue River have low-permeability sediments lining beneath the stream bottom which generate smaller Kv values, the Kv values in the top 1-m of the streambed sediments are usually greater than 5 m/d in the three rivers, indicating very permeable streambeds. Therefore, shallow streambeds are permeable over the gaining reaches of braided and meandering rivers despite their differences on the watershed size, channel width, topographic reliefs, etc. In addition, the Big and Little Blue Rivers have more fine-grained sediments deposited in the deep streambed than the Platte River. Furthermore, streambed Kv values in the three rivers exhibit a tendency to decrease with depth in the depth of 0 to 6 m below the channel surface. The constant head boundary is proposed to be an alternative solution in the simulation of stream-aquifer interactions. This approach is applied in a regional groundwater flow model to evaluate the impact of groundwater irrigation on the streamflow in the lower reach of the Platte River. Additionally, the model provides an accurate estimation of the streambed leakage of the Platte River using numerical and field techniques. Furthermore, the statistical distribution of Kv values of shallow streambed sediments along a 300-km segment of the Platte River is also examined. It was found that they are normally distributed; this finding differs from the widely accepted concept that hydraulic conductivity in aquifers is log-normal distribution. Adviser: Xun-Hong Che