Effects of long-term pumping on recharge processes in an alluvial-bedrock aquifer system
Date
2019
Authors
Cognac, Kristen, author
Ronayne, Michael, advisor
Sanford, William, committee member
Sale, Tom, committee member
Journal Title
Journal ISSN
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Abstract
The response to pumping in multi-aquifer systems involves complex processes which can significantly affect regional water budgets. Particularly where long-term pumping has occurred, drawdown might take decades to propagate regionally. Failure to incorporate changes caused by long-term pumping into regional hydrogeologic conceptual models can lead to mischaracterization of critical water budget components like recharge, inter-aquifer fluxes, and groundwater-surface water exchange. Accurate description of these budget components is necessary for managing water resources and making predictions about future water supplies. This study analyzes long-term changes in an area of the Denver Basin aquifer system with high historical groundwater withdrawals to characterize the effects of long term pumping on recharge, inter-aquifer fluxes, and groundwater-surface water exchange. An evaluation of historical water level data (1960s to 2010s) documents large hydraulic head declines (>50m in some areas) and a deepening bedrock water table relative to the stream and alluvial aquifer. Results indicate a muti-decade transition from upward to downward hydraulic gradients in the vicinity of major streams, a change that affects the water budget of bedrock aquifers. Implications for regional water budgets are evaluated using a 2D variably saturated finite-difference model which quantifies fluxes across stream, alluvium, and bedrock interfaces in a vertical sequence. Modeling results demonstrate that long-term head decline can produce complex saturation conditions beneath the alluvial aquifer including a transition period of partial desaturation and ultimately a perched saturated zone in the alluvium underlain by an unsaturated region in the bedrock aquifer. The results illustrate how inter-aquifer fluxes eventually stabilize, with no further changes caused by additional lowering of the bedrock water table. Saturation levels and fluxes across interfaces are strongly dependent on geologic heterogeneity, particularly with respect to hydraulic conductivity contrasts between and within aquifers and the location and connectivity of channelized sandstones. Modeling results demonstrate the importance of considering heterogeneity and saturation when managing aquifers that have undergone long term pumping. The results of this study provide insight into the mechanics of long-term water budget change, including controls on the transition to induced recharge and recharge rates. This has important implications for assessing the aquifer response to ongoing and future stresses.