Vertical shaft monitoring to assess mixing processes in karst aquifers.

Abstract

Karst springs often are the key to inferring the global functioning of their whole catchment area. The drainage of such springs represents the total amount of effective infiltration on the recharge area, regardless of the transit time of these flow components within the carbonate aquifer. However, springs rarely provide information about sub-catchments or compartments nor enable access to intermediate positions upstream of the groundwater outlet. Jarastepar massif is an alpine karst aquifer affected by a major tectonic contact of the Betic Cordillera (South Spain). The aquifer s drained by three permanents springs placed on metamorphic rocks (75% of the annual recharge), as well as by an overflow spring (25%), known as Pozancón, sited between the recharge area (Jurassic limestones) and the permanent springs. This outlet corresponds to a vertical cave 30 m deep, linked to horizontal conduits, where the accessible water table varies from 643 (low ater) to 664 m a.s.l. (overflowing threshold). Aerial drainage (up to 25 m3/s) is only visible during the most significant recharge events for several hours. The analysis of the discharge and water level series recorded at the outlet has allowed defining four hydrodynamic stages with different proportions of flows in mixing processes: 1) piston-flow (deep saturated zone) during the first hours after recharge, 2) outflowing (fast flow -~15 hours- from the ground) when water table exceeds the overflowing threshold, 3) declined piezometric level just after visible drainage stops but recharge water still arrives to the shaft, and 4) depletion as the lowest hydraulic head (dry season).