AGUAYO FERNANDEZ Y OTROSABSTRACT: Hydrochemist and Hydrodynamics of the Gatzume-Zestoa hydrogeological unit(Gipuzkoa).The main hydrochemical and hydrodynamical features of the aquifers re<strong>la</strong>ted to the Urgonian limestonesthat <strong>de</strong>termine the Gatzume-Zestoa Hydrogeological Unit (Gipuzkoa) are summarized in thispaper. In accordance with the subterranean flows, the Unit is divi<strong>de</strong>d in two sectors: one drained bythe Hamabiturri spring and other drained by the Granadarreka and Utzeta springs. These three sourcesare the only in the area (:::: 90 km 2 ) that show a volume greater than 5 Ils. in low water. The temporaryhydrochemical control in these three points of discharge makes evi<strong>de</strong>nt the great chemical variabilityof Hamabiturri, re<strong>la</strong>ted with the concentrated overload of the superficial waters at the Aizarna<strong>de</strong>pression. This hydraulic connection has been also <strong>de</strong>monstrated by tracers. The other two great sprin!jsshow a simi<strong>la</strong>r chemical homogeneity. The draining coefficients (a) (between 1,7 and 5,3 x 10- 2 days- )calcu<strong>la</strong>ted in short low waters point out a fast <strong>de</strong>crease in the springs volumes in absence of precipitations.The annual average resources for ali the Unit are estimated in 17-18 Hm 3 •Key Words: Karst, Hydrochemist, Hydrodynamics, Water tracing, Gipuzkoa, Basque Country.LABURPENA: Gatzume-Zestoa (Gipuzkoa) unitate bidrogeologikoaren bidrokimika e<strong>la</strong> bidrodinamika.Idaz<strong>la</strong>n honetan Gatzume-Zestoa Unitate Hidrogeologikoa gorputzen duten kararri urgondarreiloturiko akuiferoen jokabi<strong>de</strong> hidrogeologiko zein ezaugarri hidrokimikoei buruzko zenbait datu <strong>la</strong>burtzendira. Lurpeko fluxuen anto<strong>la</strong>menduari begira zait bitan dago Unitatea banaturik. Zestoa azpiunitatekodrenaia Hamabiturri sorburutik gertatzen da nagusiro eta Gatzume azpiunitatekoa, or<strong>de</strong>a, Granadaerrekaeta Utzeta sorburuetatik, hirurok Unitateko <strong>de</strong>skarga-leku garrantzitsuenak izanik. Hauetan buruturiko<strong>de</strong>nborazko kontrol hidrokimikoak Hamabiturrin <strong>de</strong>n aldakortasun kimiko handia erakutsidu, Aizarnako sakongune karstikoetako lurgaineko uren iragazketa bilduak sorburu horretan duen eraginnabarmenaren seinale. Lotura hidrauliko hau markagaien bitartez ere izan da egiaztatua <strong>de</strong>nboraldi<strong>de</strong>sberdinetan. Beste sorburu bietan antzeko homogenotasun hidrokimikoa ageri da, eurek drenaiaturikoakuiferoen jokabi<strong>de</strong> erregu<strong>la</strong>rragoa azalduz. Agortze- koefizienteak, 1, 7 eta 5,3 x 10· 2 egun·'bitartean dira, agorte ~araian emariek duten gutxitze arina erakutsiz. Unitatean urtero berrizten direnbaliabi<strong>de</strong>ak 17-18 Hm -tan zenbakitu dira.1.- GEOWGIA GENERAL.La <strong>Unidad</strong> Hidrogeol6gica Gatzume-Zestoa ( = 23 km 2 ) constituye un acuiferokârstico insta<strong>la</strong>do en <strong>la</strong>s calizas Urgonianas (Cretacico lnferior) situadas entreel monte Gatzume y <strong>la</strong> pob<strong>la</strong>ci6n <strong>de</strong> Zestoa (figura 1).1.1.- Encuadre geologico regional.La <strong>Unidad</strong> se localiza en <strong>la</strong> gran estructura <strong>de</strong>nominada Anticlinorio Norte<strong>de</strong> Bizkaia, una <strong>de</strong> <strong>la</strong>s megaestructuras, junto con el Anticlinorio <strong>de</strong> Bilbao y elSinclinorio <strong>de</strong> Bizkaia, que conforman el Arco Vasco. El Anticlinorio Norte es unaextensa y compleja estructura que se resuelve en numerosas Unida<strong>de</strong>s Tect6nicas(UT) menores. La <strong>Unidad</strong> hidrogeol6gica Gatzume-Zestoa se situa <strong>de</strong>ntro <strong>de</strong> <strong>la</strong>UT Indamendi-Zestoa <strong>de</strong>finida por CADEM (Grupo EVE), 1985.Lurral<strong>de</strong>, 11 : 1988 40
AGUAYO FERNANDEZ Y OTROS.~~.... _)c0-"' cuNcuE:::1N-c(!)"0c"0~ ï::~ ::>..ii:Lurral<strong>de</strong>, 11 : 1988 41
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