Trace elements in groundwater of La Aldea aquifer (Gran Canaria, Canary Islands)

Abstract

La Aldea area is dedicated to intensive agriculture for export. Irrigation water comes mainly from three dams located upstream, but groundwater is intensively used in drought periods. There are more than 370 large diameter wells (depths ranging from 4 to 47 m), cutting alluvial materials (20 m thick) in the valley axis, with Miocene basalts located under the alluvial material and in the surrounding mountains. Several hydrogeochemical groundwater families can be distinguished, depending on the exploited materials and/or the existence of some modifying processes. Cl-Mg and Cl-Na-Mg waters are related to basalts, while Cl-Na high saline waters are associated with the hydrothermal alteration of volcanic materials (Tabladas family). These waters mix with the alluvial groundwater, producing Cl-Na-Ca or Cl-Mg waters that can evolve to CI-SO4-Na and SO4-CI-Na, with high nitrate contents (reaching 500 mg.L- 1) when irrigation returns are present. The hydrochemical fingerprint of the trace elements studied reflects the natural influences of the volcanic materials of the region as well as its alteration due to hydrothermal processes. The hydrochemical fingerprint of the hydrothermal altered materials in the northern end of the La Aldea valley is distinguished by high concentrations of Sr and B and, in a lesser degree of Rb, Cs, Li, U, Mo and As. The anthropogenic influence on the trace element content is inexistent or too low to be detected.