Paper No. 12
Presentation Time: 11:25 AM


BOUR, Olivier, ROQUES, Clément, LERAY, Sarah and DE DREUZY, Jean-Raynald, Geosciences Rennes, Université Rennes 1, Campus de Beaulieu, Rennes, 35042, France,

Faults are heterogeneities that may greatly influence groundwater flow, especially in crystalline rocks. Here, we present two field cases and some numerical simulations to investigate the relationships between groundwater resources in crystalline rocks and the hydraulic and geometrical properties of fault zones. In particular, we show how fault dip may control partly groundwater productivity of such heterogeneous aquifers.

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.