A study of the groundwater travel time distribution at a rural watershed in Iowa: A systems theory approach to groundwater flow analysis

摘要

This dissertation is a study of groundwater-surface water interaction in terms of the travel time distribution framework applied to a tile-drained, agricultural landscape at the watershed scale. Specifically, we examined the two dimensional, steady state groundwater flow characterizing a shallow, unconfined aquifer at a representative watershed. A groundwater flow model of the aquifer was constructed using MODFLOW (Harbaugh et al., 2000). Hypothetical particles were then tracked through the simulated groundwater flow field using MODPATH (Pollock, 1994) to determine travel times associated with advective solute transport. The resultant distribution of travel times was represented by an exponential decay function with a mean travel time of 20.51 years. We further examined the impact of various control variables on groundwater travel times. First, the influence of the model selection on the travel time distribution results was examined by comparing results obtained from three models - analytic, GIS-based, and MODFLOW. Distributions obtained from all three models were represented by exponential decays, with the mean travel time varying between 16.22 and 20.51 years. The agreement between the MODFLOW and GIS models was probed by analyzing their flowpath length and velocity distributions. The spatial distributions of travel times obtained from the two models were analyzed, and conclusions of the impact of model selection on travel times were drawn. We also examined the impact of depth of flow on groundwater travel times. The analysis, conducted using the sink strength threshold parameter as a surrogate to depth showed that travel times, and structure of the effective sink network, are significantly impacted by depth. We examined the impact of variable tile drainage density and incision depth on travel times and baseflow. The marginal impact of tile drainage density on travel times and baseflow volumes was observed to diminish with increasing density, while the impact of tile incision depth on the two variables was observed to be linear. Overall, tile drainage density was observed to have a stronger impact on travel times than baseflow, while tile incision depth impacted baseflow more than travel times. The impact of aquifer recharge on travel times was observed to vary based on landscape position.