Representation of karst units in the Aquifer Reference System of France - BDLISA: challenges of small-scale mapping

Abstract

International audienceThe Aquifer Reference System of France (BDLISA1) proposes a delineation of its hydrogeologicalunits, including aquifer reservoirs and impervious units, according to local,regional and national scale. Each unit is characterized by a class of permeability, porosity andtype of geological formation. The ranking number increases with increasing depth and thisenables identification of the vertical position at each point with respect to the other units.Thus, an aquifer in a sedimentary basin can be covered by several dozen other units. Theclasses enable identification of all carbonate rocks in France and their vertical position: fromsurface outcrops down to deep carbonates (Figure 1). Carbonate rocks cover more than 60%(350,000 km²) of Metropolitan France, of which 55% (193,000 km²) in outcrop.Each carbonate rock is a potential karst aquifer (World Karst Aquifer Mapping Project).Nevertheless, the absence or differing degrees of karstification demand a classification ofthese aquifers according to karst indicators. Specialists strive to find approaches that are bothadapted and homogeneous on a national scale.Initially, BDLISA was built on a regional scale, witheach region employing its own approach depending ondata availability: mapping of all surface disordersrelated to karst (swallets, sinkholes, dolines etc.),tracer tests in karst aquifers and identification ofgroundwater catchment areas, borehole analyses (flowout by fractures and karst)2, karst springs, wells withvery high specific discharge, stream sink, etc.In a later stage, and in order to harmonize the nationalscale of BDLISA, two approaches were proposed depending on the surface karstification rate:1) The whole unit is classed as a karst aquifer: four classes distinguish whether thegeological formation can be characterized by a network of underground karst systems or,moreover, a second or third network, all being part of the hydrodynamic system.2) Karst only constitutes part(s) of the unit: a supplementary layer enables the delineation ofareas inside the aquifer unit where the presence of karst is well known.The national cavity database3 provides a non-exhaustive listing of natural karst cavity as wellas karst springs and wells with very high specific discharge (BSS). These GIS layers wereoverlain on the extension of carbonate rocks. This information enabled a better understandingof many karst areas. For approach 1, the complete unit classed as karst aquifer, the mappingreveals 2/3 of the karst at outcrop and 1/3 in the subsurface (25%, corresponding to 545 unitsat local scale). For approach 2, 108 karst areas have been delineated and linked to their 60aquifer units.A new 1:1M scale hydrogeological map of France will be published in 2015, based on theinformation of the BDLISA units. It is designed for hydrogeologists and is a communicationand educational tool: the representation of karst aquifers had to be modified in order to allowvisibility at this scale by a differentiation of karst aquifer outcrops and deeper karst units, thedisplay of karst springs (World Karst Aquifer Mapping Project) and karst aquifer labels