Assessing irrigation-influenced groundwater flow and transport pathways along a reach of the Arkansas River in Colorado
Date
2016
Authors
Criswell, David Todd, author
Gates, Timothy K., advisor
Bailey, Ryan T., advisor
Covino, Timothy P., committee member
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Abstract
Recent studies have concluded that stream reaches are not simply gaining or losing to groundwater but are best described as a mosaic of exchanges that contrast between flowpaths of varying lengths and directions which inherently influence solute residence times. These residence times directly affect chemical speciation of solutes such as salts, nitrate, selenium, and uranium and have the opportunity to undergo microbial dissimilatory reduction in the shallow riparian zone and the deeper sub-surface. To improve water quality and the overall health of these natural systems requires engineering intervention supported by reliable data and calibrated models. A three-dimensional numerical flow model (MODFLOW-UZF2) is used to simulate unsaturated and saturated groundwater flow, with linkage to a streamflow routing model (SFR2), for a 5-km reach of the Arkansas River near Rocky Ford, Colorado. The reach-scale model provides increased discretization of previous regional-scale models developed for the Arkansas River Basin, using 50 x 50 m grid cells and dividing the Quaternary alluvium that represents the unconfined aquifer into 10 layers. This discretization facilitates an enhanced view of groundwater pathways near the river which is essential for future solute transport evaluation and for consideration of alternative best management practices. Model calibration is performed on hydraulic conductivity (K) in the upper three layers, K in the lower seven layers, and specific yield (Sy) of the entire aquifer by applying an Ensemble Kalman Filter (EnKF) using observed groundwater hydraulic head and stream stage data. The EnKF method accounts for uncertainty derived from field measurements and spatial heterogeneity in parameters calibrated using a Monte-Carlo based process to produce 200 realizations in comparison to error-prone measurements of hydraulic groundwater hydraulic head and stream stage as calibration targets. The calibrated transient model produced Nash-Sutcliffe Efficiency (NSE) values of 0.86 and 0.99, respectively, for the calibration and evaluation periods for calibration targets using the ensemble mean of realizations. Realizations of calibrated parameters produced by the EnKF exhibit the equally-likely spatial distributions of aquifer flow and storage characteristics possible in the area, while MODPATH simulations display the associated groundwater flowpaths possible under such conditions. The mean residence time of a stream-destined fluid particle within the riparian zone was estimated as 1.8 years. Simulated flowpaths to the stream were highly variable given different geologic conditions produced by EnKF, with flowpaths to some stream reaches differing in direction as much as approximately 90° and transit times differing sometimes by decades. The simulated average annual groundwater return flow to the stream was 70 m3 d-1. Simulated average annual return flow was highly variable along the study reach and ranged from -250 to a little over 250 m3 d-1 with a CV of 1.4. The mean percentage of shallow (within the top three model layers) groundwater return flow to flow in aquifer layers beneath the stream was 27% with a CV of 0.58. Simulated groundwater flowpaths were superimposed upon a map of shallow shale units residing in the study region, demonstrating how groundwater flowpaths may interact with regional seleniferous shale layers. Results hold major implications for biogeochemical processes occurring in the sub-surface of the riparian area and the hyporheic zone that have an important influence on solute concentrations. Results may be used to aid decision makers in the implementation of best management practices and to further understand contaminant sources and fate.
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Subject
flow paths
MODFLOW
riparian
groundwater
ensemble Kalman filter
reach-scale