Sodium is present in most natural waters. High levels can be caused by seawater and road salt runoff.
Although an essential nutrient, high levels of sodium have been linked to hypertension. The guideline of 20 mg/L maximum is important for people on salt restricted diets, however higher levels are generally tolerable for those not on restricted diets.
The EPA recommends consulting a physician to plan a healthy diet that is low in sodium from food intake.