Numerical modeling and hydrochemical analysis of the current and future state of seawater intrusion in the Todos Santos aquifer, Mexico
Date
2019
Authors
Fichera, Marissa M., author
Sanford, William E., advisor
Ronayne, Michael J., committee member
Bailey, Ryan T., committee member
Journal Title
Journal ISSN
Volume Title
Abstract
The Todos Santos aquifer, Baja California Sur, Mexico, provides the sole source of freshwater to the town of Todos Santos, and is utilized for domestic and agricultural needs crucial to the town's economy. The region is characterized by an arid climate. Major recharge to the aquifer is supplied from intermittent cyclones. Irregular and unpredictable recharge rates combined with population growth resulting from resort development put the Todos Santos aquifer at risk of overexploitation, causing potentially permanent water quality degradation by salinization as a result of seawater intrusion. Understanding the complex response of seawater intrusion to variable pumping rates and sea-level rise is critical to water resource management in Todos Santos. This study utilized numerical simulation of variable-density groundwater flow, using SEAWAT, in conjunction with temporal and spatial hydrochemical analysis, to evaluate the current and future extent of seawater intrusion in the area. Forecasting simulations were run for five, ten, and twenty years following 2017, for five different hydrologic scenarios, which implemented various pumping rates, sea-level rise, and overexploitation of significant surface water resources. Hydrochemical analysis shows an increase in groundwater specific conductance and chloride concentration within two kilometers of the coastline from 2007 to 2017. This combined with the distribution of groundwater samples exhibiting chloride concentration above the permissible limit for potable water (250 mg/L) suggest that the Todos Santos aquifer is experiencing effects of seawater intrusion up to 1.6 kilometers inland as of 2017. Analysis of groundwater cation exchange reactions indicates widening of the freshwater-seawater mixing zone from 2007 to 2017, further suggesting the exacerbation of seawater intrusion over this time span. Forecasting simulation results indicate that the extent of seawater intrusion is exacerbated by increased water withdrawal, overexploitation of surface water resources, the current rate of sea-level rise (~ 4 mm/yr), and an increased rate of sea-level rise of 25 mm/yr.