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Modeling Groundwater Flow and Transit Time Distributions in the Nebraska Sand Hills
Abstract
The estimation of the groundwater transit time distributions (TTDs) is critical for water resources management and assessment of the non-point source pollution to streams and rivers. This PhD study integrates field measurements and groundwater modeling & particle tracking to estimate the TTDs in the Upper Middle Loup River (UMLR) Watershed which overlies the northern part of the High Plains Aquifer in the Nebraska Sand Hills. In the first part of this study, we investigated the effects of spatial patterns of aquifer heterogeneity, groundwater recharge and systematic variation in riverbed hydraulic conductivity on the mean transit times (MTTs) and TTDs. Results showed that the transit times at upstream sites of the model domain were dominated by younger groundwater which were more sensitive to a change in the riverbed hydraulic conductivity. However, recharge had a greater impact on the MTTs and TTDs at the downstream sites, where much older groundwater was observed. In the second part of the study, we focused on improving the local groundwater discharge predictions in the stream network by developing a regional groundwater flow model which included a calibration process using different sets of head and baseflow targets in the model domain. Results showed that using local baseflow targets significantly improved the accuracy of the groundwater fluxes between the stream and the aquifer. In the final part of the study, we investigated the discrepancies between the transit times obtained from the calibrated groundwater flow model and advanced age-dating tracer of 3H/3He, 14C and δ13C taken from five different sampling sites in the UMLR Watershed. Results indicated that the groundwater TTDs had the characteristics of gamma and exponential distributions for both TTDs obtained from groundwater modeling and age-dating tracer analysis, where the overall transit times ranged between 0-386 years and 1-10620 years for the upstream and the downstream sampling sites, respectively. The results of this research provide important insights on the hydrological responses of the northern High Plains Aquifer in the Nebraska Sand Hills.
Subject Area
Hydrologic sciences
Recommended Citation
Zeyrek, Caner, "Modeling Groundwater Flow and Transit Time Distributions in the Nebraska Sand Hills" (2022). ETD collection for University of Nebraska-Lincoln. AAI29162479.
https://digitalcommons.unl.edu/dissertations/AAI29162479