June 23-27, 2008 z Naples, Florida, USA 317 Combined Groundwater Quality and Groundwater Model Approach as Main Tool of an Aquifer Management for Sustainable Water Supply in the Santo Domingo Valley, Baja California Sur, Mexico Jobst Wurl, Miguel Imaz Lamadrid, Juan Eduardo Martínez Meza, Cynthia Nayeli Martínez García and Genaro Martínez Gutiérrez Universidad Autónoma de Baja California Sur, CA Geohidrología y Geoinformatica ABSTRACT The extraction of groundwater and especially the over-exploitation of the Santo Domingo aquifer since 1957 has caused modifications in the natural flow system and induced lateral flow of seawater from the coastline. As a result the groundwater quality in the Santo Domingo Irrigation District is deteriorating. Seawater intrusion and irrigation-return combined with the mobilization of deeper groundwater have been identified as important sources of salinization. Figure 1. Location of the study area in Mexico, in the southern part of the Baja California Peninsula and detailed map of the Santo Domingo Irrigation District with the most important catchments and drainage systems INTRODUCTION Since its colonization in the 1950s the Santo Domingo Valley has become the most important agricultural district of Baja California Sur. As typical for semiarid regions the main source of freshwater is groundwater. Important factors in the water balance of the region are the very intensive rainfalls caused by storms and hurricanes. The runoff from the mountains infiltrates in the subsequent alluvial fans where it recharges the aquifers. The extraction of groundwater and especially the over-exploitation of the aquifer since 1957 has caused modifications in the natural