Estimating infiltration speed and depth in unconfined fractured aquifers: Preliminary results from a numeric simulation study on Yucca Mountain

Stephan Matthai and Mohammad Sedaghat and Thomas Jerome and Khoa Tran Luat and Qi Shao. ( 2020 )
in: 2020 RING Meeting, ASGA

Abstract

What fraction of the sporadic rainfall in arid regions makes its way to the bottom of the vadose zone, contributing to the sustainable yield of regional aquifers, is an important question in groundwater hydrology. While many studies have treated this as a steady-state time-averaged problem because rainfall events are so short and infrequent that they are hard to simulate individually, this approach is at odds with increasingly well-characterised vadose zone dynamics, especially in unconfined fractured aquifers. This numeric simulation study investigates speed and penetration depth of infiltration pulses triggered by historic rainstorm events, drawing on the comprehensive multi-disciplinary characterisation of Yucca Mountain, Nevada, USA. Sequentially coupled simulations include a fully transient extended shallow-water equation model of run-off, run-on and near-surface infiltration, accounting for the spatially variable soil cover in the Yucca Mountain catchment. Its dynamic outputs constrain the transient boundary condition of a 500-m tall 3D fracture and fault zone model of the exceptionally profound vadose zone in the area of the underground “Exploratory Research Facility” (ESF). This adaptively refined, hybrid finite element – finite volume model contains discrete representations of a hierarchy of faults and captures the hydrologic impact of pervasive small-scale fractures with fracture-matrix ensemble saturation functions derived from small-scale discrete fracture and matrix (DFM) modelling. Before the simulation of infiltration events, water saturation is calibrated with measurements from boreholes. Preliminary simulation results indicate that infiltration due to small rain or snow showers typical for winter or early spring, remains largely confined to weathered bed rock or soil layers. However, one rainstorm that occurred in 1984 (2-days, >0.1-m total precipitation) was sufficiently intense and prolonged to infiltrate the fractured and faulted sequence of volcanic ash-flow deposits down to a depth of several hundred meters. In spite of the uncertain exact geometry of critical flow paths and the degree of flow focussing caused by downward convergent fault strands, these predictions indicate that it is very likely that this infiltration pulse locally recharged the groundwater aquifer. This result differs from earlier dual continua models based on volume averaging of unsaturated zone flow properties. By contrast to these models, our DFM model further predicts that the infiltration front velocity in the larger deformation structures is limited by fluid supply rather than permeability.

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BibTeX Reference

@INPROCEEDINGS{MATTHAI_RM2020,
    author = { Matthai, Stephan and Sedaghat, Mohammad and Jerome, Thomas and Tran Luat, Khoa and Shao, Qi },
     title = { Estimating infiltration speed and depth in unconfined fractured aquifers: Preliminary results from a numeric simulation study on Yucca Mountain },
 booktitle = { 2020 RING Meeting },
      year = { 2020 },
 publisher = { ASGA },
  abstract = { What fraction of the sporadic rainfall in arid regions makes its way to the bottom of the vadose zone, contributing to the sustainable yield of regional aquifers, is an important question in groundwater hydrology. While many studies have treated this as a steady-state time-averaged problem because rainfall events are so short and infrequent that they are hard to simulate individually, this approach is at odds with increasingly well-characterised vadose zone dynamics, especially in unconfined fractured aquifers. This numeric simulation study investigates speed and penetration depth of infiltration pulses triggered by historic rainstorm events, drawing on the comprehensive multi-disciplinary characterisation of Yucca Mountain, Nevada, USA. Sequentially coupled simulations include a fully transient extended shallow-water equation model of run-off, run-on and near-surface infiltration, accounting for the spatially variable soil cover in the Yucca Mountain catchment. Its dynamic outputs constrain the transient boundary condition of a 500-m tall 3D fracture and fault zone model of the exceptionally profound vadose zone in the area of the underground “Exploratory Research Facility” (ESF). This adaptively refined, hybrid finite element – finite volume model contains discrete representations of a hierarchy of faults and captures the hydrologic impact of pervasive small-scale fractures with fracture-matrix ensemble saturation functions derived from small-scale discrete fracture and matrix (DFM) modelling. Before the simulation of infiltration events, water saturation is calibrated with measurements from boreholes. Preliminary simulation results indicate that infiltration due to small rain or snow showers typical for winter or early spring, remains largely confined to weathered bed rock or soil layers. However, one rainstorm that occurred in 1984 (2-days, >0.1-m total precipitation) was sufficiently intense and prolonged to infiltrate the fractured and faulted sequence of volcanic ash-flow deposits down to a depth of several hundred meters. In spite of the uncertain exact geometry of critical flow paths and the degree of flow focussing caused by downward convergent fault strands, these predictions indicate that it is very likely that this infiltration pulse locally recharged the groundwater aquifer. This result differs from earlier dual continua models based on volume averaging of unsaturated zone flow properties. By contrast to these models, our DFM model further predicts that the infiltration front velocity in the larger deformation structures is limited by fluid supply rather than permeability. }
}