High mountain karst aquifer vulnerability to climate change and groundwater transit times

Water resources in high mountain karst aquifers are usually characterized by high precipitation than in the surrounding lowlands, with significant recharge and discharge that assure the sustainability of the downstream ecosystems. Consequently, these hydrogeological systems are highly vulnerable to the climate change. The mean transit time (MTT) is a key parameter to describe the behaviour of these hydrologic systems and also to assess their vulnerability.

In high mountain zones, precipitation can be as rainfall and as snowfall. The latter generates snow accumulation that in many cases partially or totally melts in spring and summer, producing small runoffs. In this framework, the karst aquifer recharge show mainly two different mechanisms: (1) diffuse recharge, in which runoff from rainfall and snowmelt enters the epikarst through the whole outcropping area and percolates through low permeability fissured blocks in the way down to the saturated zone, (2) locally concentrated recharge through highly conductive hydrologic features, including different solutional forms (e.g. sinkholes, dolines, etc.), which are generally well-connected to vertical fractures and the drainage network of the aquifer. These recharge mechanisms condition the aquifer response observed at the outlet of the systems at different temporal scales.

This study is conducted in the Paleocene-Eocene karst aquifer of the Ordesa and Monte Perdido National Park (PNOMP) Spain, particularly focussing on the Garcés karst system, whose discharge forms the emblematic Horsetail fall of the National Park. Different karstic forms appear throughout the study zone, including sinkholes, dry and ice caves, dolines and karren fields, thus generating a heterogeneous karstified hydrogeological system.

In this work, the difference on the hydrological response of the fine fissures and the main drainage aquifer network is investigated in terms of the corresponding MTT. To this end, both environmental (d¹⁸O and d¹⁸H in water) and fluorescent dye (uranine, eosine, amino G acid and naphtionate) tracers are used. The former characterize the MTT associated to the diffuse recharge process by means of the seasonal variation of the isotopic content in both precipitation (d_P) and the Garcés spring discharge (d_GW). The dye tracers are used to study the hydrogeological organization of the highly conductive drains and to estimate the corresponding MTTs.

The obtained MTT are 1.3 years and 9 days for the environmental and the fluorescent dye tracers, respectively. These values are not very long and point out the difficulties of the aquifer to bear large interannual recharge fluctuations. Additionally, the difference between the estimated MTT values underlines how the heterogeneities of the unsaturated zone may condition both the hydrogeological system response to recharge and the aquifer vulnerability.