Hidrogeología de la unidad de Santa Eufemia-Ereñozar (Vizcaya): el sistema kárstico de Olalde

Abstract

The Santa Eufemia-Ereñozar unit, placed between Markina, Lekeitio and Guernica in the northeastern part of Biscay, is mainlv composed of urgonian limestone and locally of wealdian and Jurassic limestone. The outeropping area of these materials is about 70 km² and the average recharge approximately 56 Hm³/year; the natural regulation is very poor and the prospects of artificial regulation are limited owing to the special features of the karst system. A good example about this is the San Miguel de Ereñozar sub-unit of which the Olalde spring is the main drain; the mean flow of this spring is 240 l/sec, with a very high variability (ranging from more than 11.000 l/sec to 15-20 1/sec); the Olalde spring may be considered as receiving water not only from the area of limestone outerops, but also from drained closed basins, partially occupied by impermeable materials, of which the surface runoff recharges the karstic system through impressive sinks (ponors); however, this intense karstification seem to be restricted to certain layers and structures. This model is reinforced by the results obtained from the analysis of the recession curve of the Olalde spring and from the autocorrelation (and cross-correlation) and spectral (and crossspeetral) analysis of the above mentioned flow and Precipitation data, presented elsewere.