Preferential Flow Enhances Pumping-Induced Saltwater Intrusion in Volcanic Aquifers

Abstract

Preferential flow can result in rapid contamination of groundwater resources. This is particularly true in aquifers with connected, high permeability geologic structures and in coastal systems where the oceanic source of contamination is ubiquitous. We consider saltwater intrusion due to pumping in volcanic aquifers with lava tubes represented as connected high-K structures and compare salinization responses to those of heterogeneous aquifers with different structure and equivalent homogeneous systems. Three-dimensional simulations of variable-density groundwater flow and salt transport show that conduits formed by lava flows create preferential groundwater flow in volcanic aquifers. These conduits allow fresh groundwater to extend further offshore than in other systems. However, onshore pumping causes saltwater to migrate landward quickly through the conduits relative to the other models, resulting in more severe saltwater intrusion, particularly at shallow depths. The geometry of geologic heterogeneity in volcanic aquifers leads to increased risk of salinization of fresh groundwater as well as substantial uncertainty due to significant spatial variation in saltwater intrusion. The findings illustrate the importance of considering geologic heterogeneity in assessing the vulnerability of coastal freshwater resources in volcanic and other aquifers with connected high-permeability geologic structures.