Saltwater intrusion refers to the landward movement of seawater into freshwater systems. It’s a game of density: saltwater is denser than freshwater, so it remains underneath the freshwater in water tables. When the freshwater volume decreases, saltwater begins to take its place. Mixing then occurs, contaminating the whole supply. At high concentrations of salt in drinking water, people can develop hypertension, which exacerbates pre-existing health conditions and puts the elderly at higher risk of stroke.