Too much sodium in our diets has long been known to raise blood pressure and increase the risk of heart disease and stroke. A modeling study published in the journal Hypertension (Coxson, P.G., et al. 2013. Mortality Benefits From US Population-wide Reduction in Sodium Consumption: Projections From 3 Modeling Approaches) suggests that even a small reduction in sodium consumption could save hundreds of thousands of lives. I’ve been involved with a lot of research on the contamination of shallow aquifers from road salt (sodium chloride) runoff, but we typically focus on chloride and not sodium. Chloride is a conservative ion, so it travels in groundwater basically at the speed of the water, whereas sodium is more reactive and thus more difficult to predict in the subsurface. But sodium is definitely increasing in these aquifers. This sodium study got me wondering how much sodium we ingest through drinking water.

Sodium is an essential element in our diets, but we only need 200 milligrams (mg) per day, and the suggested limit is 2,300 mg and 1,500 mg for people with high blood pressure. The recommended volume of water we should drink daily is about 2 liters or so. The median value of sodium in wells in the Mahomet Aquifer system in my neck of the woods (east-central Illinois) is about 40 mg/L. So if we drink 2 liters of water a day, that’s only 80 mg of sodium. However, a lot of groundwater in Illinois, including the Mahomet Aquifer, is hard, and thus gets softened before it reaches our taps. The main way water is softened is through ion exchange using sodium chloride, with sodium replacing calcium and magnesium. In some treatment plants that we have sampled around here, softening increased the sodium concentration by over 100 mg/L. So that could add another 200 mg of sodium to our diets.

In the Chicago region, road salt runoff is increasing sodium in some shallow aquifers to levels greater than 100 mg/L or even 200 mg/L. Drinking water with a sodium concentration of 200 mg/L would provide you double the amount of sodium you need daily, and almost 20% of the recommended maximum. While it’s not the major source of sodium in our diets in Illinois, it might be significant, and people with hypertension should definitely know how much is in their drinking water.