Ambiental pollen-based metodology for hydrogeological studies in karstic zones: Joya de Luna-Guaxcamá, S.L.P.

Abstract

Extensive outcrops of folded limestone rocks forming mountainous systems with intense karst development are found in areas of central Mexico. An important part of the water supply in these areas is derived from aquifers controlled by the karst system. We analyzed the underground flow between the towns of Joya de Luna (in an elevated part of the mountain) and Guaxcamá, topographically lower, to establish if there is a connection between these zones, and corroborate if the water infiltrated in the upper part is captured in lowland springs. A new method is proposed to qualitatively assess the hydraulic connectivity between the recharge and discharge zones in a karst and fractured system using pollen grains as tracers. Deuterium and oxygen-18 isotopic data were used to compare and evaluate the suitability of this method; in addition, excess deuterium was used as a factor in the hydrogeological analysis. Through a survey of the vegetation present within a 5 km radius in both zones, the pollen families existing in the recharge zone Joya de Luna and the discharge zone in Guaxcama were documented, and a database with 41 pollen species was generated for morphological characterization. Subsequently, 135 groundwater samples with suspended pollen grains were collected from 5 water wells in the recharge zone and 10 water springs in the discharge zone. The species analysis showed that pollen grains of Zea mays, Avena sativa, Cicer arietinum y Quercus sp., characteristic only of the recharge zone, were also present in water samples collected in the discharge area. The pollen-based results were consistent with those obtained with isotopic data; they allowed identifying two routes with hydraulic connection, G1 and G2. For each of these routes an hydraulic connection was established between samples from the recharge zone and the discharge zone. Grains of Zea mays, Avena sativa, and Cicer arietinum were found in samples from both zones. Pollen of Quercus sp., characteristic of the mountainous are was detected three samples of the recharge zone and seven points of the zone of discharge.