A Numerical and Statistical Analysis of the Fractured Rock Aquifer System in Ploemeur, France to Quantify Local and Regional Recharge
Law, Stacey E.
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Groundwater recharge is an essential metric for understanding and protecting groundwater resources. Quantifying this parameter remains extremely challenging due to the uncertainties associated with the extent to which the vadose zone affects groundwater movement and the highly heterogeneous nature of the aquifer systems being monitored. The difficulty surrounding recharge quantification is compounded when considering a fractured rock aquifer system, where classification and modeling is complicated by highly complex structural geology. However, the ability to distinguish the character and geometry of fractured rock aquifers is indispensable for quantifying recharge to evaluate sustainable yields, as well as for implementing protective measures to manage these systems. The primary intention of this study is to assess the hydrogeologic properties that have led the unique recharge signals within the fractured crystalline-rock aquifer system near Ploemeur, France. Infiltration and groundwater movement are characterized via time-series hydraulic head and precipitation data collected at daily, monthly, yearly, and at decadal intervals. In spite of the nearly one million cubic meters of groundwater extraction, measured drawdowns are marginal, suggesting that local and regional recharge plays a significant role in moderating water-level declines and raising questions as to the origins of the substantial inflow required to sustain this complex system. A roughly two-month lag has been observed between seasonal water level and monthly precipitation at Ploemeur, which has previously been attributed solely to slow vertical migration of water through the low-permeability micaschist layer to the fractured contact zone and interconnected fault. However, results from this study suggest that a significant portion of the observed lag can be attributed to vadose-zone processes, particularly the thickness of the vadose zone. This investigation also reveals a recharge signal that continues throughout the calendar year, departing from the traditional simplified concept that recharge quantity is essentially equivalent to the value of evapotranspiration subtracted from infiltration.
General Audience Abstract
Groundwater recharge is the amount of water added to underground water sources, called aquifers. This occurs as precipitation falls to the ground, moves downward through the unsaturated subsurface, and accumulates at the top of the saturated zone, deemed the water table. The saturated zone is so named because all pore spaces between sediment grains or crevices in rocks are fully filled with water. Understanding groundwater recharge is important to the protection of groundwater resources, but is hard to estimate due to the lack of knowledge about water movement in the unsaturated zone and the uncertainties related to the systems being studied. Aquifers forming within fractured rocks are even more challenging to investigate, because the complex geological structures are difficult to replicate with computer modeling. However, fractured rock aquifers are an important groundwater resource, and understanding them is the first step in estimating recharge within the system. Recharge estimates are used to calculate how much water can be safely removed from the aquifer for years to come, so that the resource can remain protected. The aim of this investigation is to assess the aquifer properties that lead to the unique recharge signal in a fractured crystalline-rock aquifer in Ploemeur, France, where nearly 1 million cubic meters of water have been removed each year since 1991 but water table levels have not fallen significantly. This behavior raises questions about the water returned to the system as recharge that is sustaining such a highly productive resource. This site also shows a roughly two-month lag between seasonal precipitation falling and the reflection of that precipitation recorded in the water level of the aquifer. It was previously thought that the lag occurred because water travelled slowly through the mica-schist layer, which has little pore space for water to move, and into the contact zone and interconnected fault. However, this study shows instead that a majority of the lag is associated with the unsaturated zone properties and processes, particularly thickness. This investigation also shows recharge entering the aquifer system throughout the calendar year, a departure from earlier studies conceptualizations.
- Masters Theses