Characterizing and preserving available groundwater resources within crystalline rocks is pertinent to understanding and predicting resources for ecosystems worldwide. Crystalline-rock aquifers, with favorable structure and climate, can be pumped year-round to meet local domestic demand. The Ploemeur hydrogeologic site, near the southern coast of Brittany, France, is characterized by a structurally complex fractured mica-schist and granite confined aquifer system. A contact zone, which acts as the main localized flow path through the aquifer, separates the two crystalline units, and a sub-vertical permeable fault zone cross-cuts the crystalline bedrock and contact zone. Using field observations, recharge estimates, and a calibrated three-dimensional numerical multi-zone MODFLOW 6 model, we present preferential flow paths of recharge infiltrating the complex geology of the Ploemeur hydrogeological site during pumping conditions. Using MODPATH to track groundwater and recharge path lines, we determine that water extracted from the aquifer originates from higher elevation areas west of the pumping site. Particle tracking analyses indicate that precipitation simulated over the pumping zone takes a minimum of two years to reach the pumping wells and travels up to 100 m in distance. Analyses of the water budget of the aquifer system using Zonebudget show that storage contributes significantly to the productivity of the system. Based on these analyses, we determine that recharge mechanisms such as piston flow and preferential flow play important roles in the Ploemeur hydrogeologic site. Though the Ploemeur site is unique in its composition and geometry, the methods used to characterize and monitor the aquifer can be applied to fractured crystalline-rock aquifers globally. Fractured crystalline-rock aquifers make up 10% of the region's freshwater sources, thus understanding their flow mechanisms contributes greatly to the management of freshwater resources. / Master of Science / Groundwater aquifers are a common source of freshwater worldwide as groundwater makes up 30% of Earth's freshwater resources. Porous, sedimentary aquifers, made of materials such as sand or gravel, are well studied; however, the less understood aquifers found in crystalline bedrock are also found all over the world. Generally, igneous and metamorphic crystalline rocks are not porous and have low permeabilities, but fractures and faults in the crystalline rock can increase the ability for water to travel through the system. The Ploemeur hydrogeologic site, located on the southern coast of Brittany, France, is a productive fractured crystalline-rock groundwater aquifer producing freshwater year round. The productivity of this aquifer is attributed to the increased hydraulic conductivity associated with the intersection of two permeable features: a subvertical fault zone and a sub-horizontal contact zone. Despite the aquifer's output, recharge travels very slowly into the system due to the depth, heterogeneity, and clay content in an overlying layer of weathered rock fragments and soil. In this study, we create a three-dimensional numerical model using MODFLOW to simulate precipitation in different locations to see how it travels through the aquifer to the site of groundwater pumping. We see that the recharge prefers to travel topographically from regions of higher elevation to lower elevation. The recharge preferentially travels through the geologic features with higher permeabilities, including the fault zone, regolith, and contact zone, but it does still travel through the less permeable, crystalline bedrock units. Even in the features with the higher permeabilities, simulated recharge requires a minimum of 2 years to travel from the land surface to the pumping wells. The pumping wells extract significant water from storage, as seen in our water budget calculations of each geologic unit. We see two recharge mechanisms present in the hydrogeologic site: piston flow, where young water displaces older water from the storage, and preferential flow, where recharge prefers to travel through regions with higher hydraulic conductivity. Understanding the recharge mechanisms in crystalline aquifers is pertinent to our knowledge of freshwater resources as crystalline aquifers make up approximately 10% of all groundwater supplies.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/103905 |
| Date | 17 June 2021 |
| Creators | Humm, Cathleen Hana |
| Contributors | Geosciences, Burbey, Thomas J., Pollyea, Ryan M., Widdowson, Mark A. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf |
| Coverage | France |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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