ANALYSIS OF TIME DELAY BETWEEN PRECIPITATION EVENTS AND WATER-LEVEL RESPONSE IN A CRYSTALLINE AQUIFER SYSTEM

The aquifer system near Ploemeur in Brittany, France is characterized by crystalline rock containing a transmissive sub-vertical fault zone intersecting a sub-horizontal contact zone between an underlying granite and an overlying mica schist. Municipal pumping from within the fault zone supplies approximately one million cubic meters of water annually for the town of Ploemeur. The goal of this research is to quantify the local and regional recharge processes that produce such impressive quantities of water from crystalline rocks. A significant time delay occurs between precipitation events and the water-level response within the aquifer. Two potential sources of this delay can be the effect of seasonal variations in evapotranspiration influencing the downward flux of meteoric water through the variable thick vadose zone and the vertical migration of recharge water through the mica schist into the contact zone. To better understand the role of the structure of the system on recharge mechanisms, we intend to build a Modflow 6 model to investigate the role of a high hydraulic fault and contact zone contained within a low hydraulic conductivity host rock environment. We will use seasonal recharge estimates based on vadose-zone modeling work and seasonal water-level observations as parameter estimates to inversely estimate the hydraulic characteristics of the fault and contact zones. These modeling results should provide greater understanding as to the local and regional recharge processes that can both explain the significant time delay of water-level responses and the high productivity of this system that has experienced limited drawdown.