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- Title
Permeability and Groundwater Flow Dynamics in Deep‐Reaching Orogenic Faults Estimated From Regional‐Scale Hydraulic Simulations.
- Authors
Alt‐Epping, Peter; Diamond, Larryn W.; Wanner, Christoph
- Abstract
Numerical modeling is used to understand the regional scale flow dynamics of the fault‐hosted orogenic geothermal system at the Grimsel Mountain Pass in the Swiss Alps. The model is calibrated against observations from thermal springs discharging in a tunnel some 250 m underneath Grimsel Pass to derive estimates for the bulk permeability of the fault. Simulations confirm that without the fault as a hydraulic conductor the thermal springs would not exist. Regional topography alone drives meteoric water in a single pass through the fault plane where it penetrates to depths exceeding 10 km and acquires temperatures in excess of 250°C. Thermal constraints from the thermal springs at Grimsel Pass suggest bulk fault permeabilities in the range of 2e−15 m2–4.8e−15 m2. Reported residence times of >30,000 and 7 years for the deep geothermal and shallow groundwater components in the thermal spring water, respectively, suggest fault permeabilities of around 2.5e−15 m2. We show that the long residence time of the deep geothermal water is likely a consequence of low recharge rates during the last glaciation event in the Swiss Alps, which started some 30,000 years ago. Deep groundwater discharging at Grimsel Pass today thus infiltrated the Grimsel fault prior to the last glaciation event. The range of permeabilities estimated from observational constraints is fully consistent with a subcritical single‐pass flow system in the fault plane. Plain Language Summary: Observations from warm springs discharging through a fault at Grimsel Pass (2,164 m.a.s.l.) in the Swiss Alps are used to constrain a numerical model of the deep water circulation feeding the springs. The springs are known to have been active for at least 3.3 million years and to be due to ascent of meteoric water that penetrated to depths exceeding 10 km, where it acquired temperatures above 250°C. Simulations show that the circulation along the fault connects a meteoric recharge zone at high altitude to the west with a sub‐vertical permeable discharge zone at Grimsel Pass. A key unknown is the permeability of the fault. Temperature, discharge rate and chemical composition of the spring water depend on flow conditions at depth and can be used to estimate the fault's bulk permeability. Our study shows that the range of fault permeabilities can be narrowed down to roughly half an order of magnitude: 1e−15 m2–5e−15 m2. This permeability range is consistent with a currently stable, single‐pass flow pattern. Long water‐residence times inferred from the isotopic composition of the spring water suggest low recharge rates during the last glaciation and the dominance of a multi‐pass flow pattern during that time. Key Points: The topography in the Grimsel region drives meteoric water to depths exceeding 10 km, causing discharge of thermal water at Grimsel PassBased on thermal and chemical constraints from the spring water, the bulk fault permeability is in the range of 2e−15 m2–4.8e−15 m2Recent periods of glaciation changed the pattern of flow in the fault plane and are likely the reason for fluid residence times >30,000 years
- Subjects
ALPS; HOT springs; PERMEABILITY; WATER springs; GROUNDWATER flow; FLUID intelligence; WATER depth; GEOTHERMAL resources
- Publication
Geochemistry, Geophysics, Geosystems: G3, 2022, Vol 23, Issue 12, p1
- ISSN
1525-2027
- Publication type
Article
- DOI
10.1029/2022GC010512