FAULT ZONES PROPERTIES AND GROUNDWATER RESOURCES IN CRYSTALLINE ROCKS
The first field case corresponds to an outstandingly productive crystalline rock aquifer – Ploemeur site, Brittany, France – that is exploited at a rate of about 106 m3 per year since 1991 as the main water supply for a town of 18,000 inhabitants. The geology of the area involves two main structures: a highly fractured contact zone between the Ploemeur’s granite and the overlying micaschists, and a steeply dipping fault striking North 20°. The contact zone in itself consists of alternating deformed granitic sheets and enclaves of micaschists, pegmatite and aplite dykes, and locally mylonites and pegmatite-bearing breccias that are often associated with major borehole inflows. At the site scale – typically a few square kilometer – and at relatively shallow depth (100 to 200 m), the connectivity of the main flow paths and the hydraulic properties are relatively well constrained and quantified thanks to cross-borehole flowmeter tests and pumping tests.
The second field case consists in a sub-vertical fault zone in crystalline bedrock – Saint-Brice, Brittany, France. High measured well yields were obtained during drilling (100 m3/h), essentially related to a permeable fault encountered at 120 m deep. Although fault transmissivity was similar to the Ploemeur site, a two-months large scale pumping test (45 m3/h) showed that groundwater productivity was much lower and much more dependent on superficial weathered reservoirs. Numerical simulations of groundwater flows in steady-state conditions allowed us to show that such a difference in fault zone productivity could be simply explained by fault dip variations that control under some conditions the recharge area of the aquifer. This shows how sub-horizontal and shallowly dipping permeable fault zones may constitute relative highly-productive aquifers, particularly in area of low permeability such as crystalline rocks.