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Modeling Fluxes and States of Moisture and Heat through a Coarse Fluvial Aquifer

The Boise Hydrogeophysical Research Site (BHRS) and associated alluvial aquifer has been an important site for hydrogeophysical investigation over the past 20 years but there has remained a lack of sufficient understanding of boundary conditions between the aquifer and atmosphere (i.e. vadose zone), and aquifer and adjacent river (i.e. river boundary). In this work we provide methods for quantifying boundary condition behavior of the BHRS aquifer by investigating full-scale groundwater flow and seasonal trends in recorded data (Chapter 1), the nature of unsaturated flow through the vadose zone (Chapters 2 and 3), and through quantification of flux across the river-aquifer boundary including effects of inundation (Chapter 4).

Vadose zone tension (ψ) and moisture (θ) response to artificial precipitation was used to estimate in situ material parameters of the van-Genuchte Mualem (VGM) relationships and predict full ψ and partial θ data in a 1D model and then full ψ and θ data in a 2D model that incorporates heterogeneity in material structure and parameters. Results from the 1D model (Chapter 2) show high uncertainty in optimized distributions of individual parameters but far less uncertainty in predicted VGM relationships (e.g. θ(ψ) and K(ψ)), which ultimately control unsaturated flow. Results from the 2D model (Chapter 3) show that the inclusion of heterogeneity greatly reduces parameter uncertainty and also shows that small differences in individual parameter values can significantly alter predicted flow rates. Result from both models conclude that unsaturated flow relationships developed for agricultural soils (e.g. the VGM model) and not typically applied to coarse conglomeratic material, can be used to predict flow and moisture distribution of such material.

Well head (hwell) and temperature (Twell) response to short-term and seasonal changes in river stage were used to quantify flux across the full inundated river-aquifer boundary of the BHRS along a 2D cross-section perpendicular to river flow. Hydraulic and thermal aquifer properties were calibrated by matching hwell and Twell responses to 1) a single rise and fall in river stage coincident with a 10°C cooling of average daily air temperatures and 2) the full-year river hydrograph and seasonal air and river temperatures. After calibration, the model was used predict flux rate and total flux along the entire inundated river boundary and to simulate subsurface temperature distribution and estimate the extent of GWSW mixing. The influence of inundation on well head and temperature response, and GWSW mixing was investigated and shown to be only slightly significant at the spatial and temporal scales addressed in the model; mostly to the high conductivity and coarse nature of the BHRS aquifer, and sparse distribution of measurements.