Environmental Impacts of Chloride Contamination

Chloride contamination of surface water and groundwater is a growing problem in northeastern Illinois. Elevated chloride concentrations can severely impact freshwater ecosystems and aquatic habitats, increase corrosivity of water, and make drinking water taste salty. We will discuss the major sources of chloride and typical patterns and pathways of chloride to the environment.

Natural sources of chloride mainly include the oceans, atmospheric deposition, and weathering of rocks and minerals. Becasue the impacts of natural sources of chloride on the environment are not significant in northeastern Illinois, we will focus on three major anthropogenic sources of chloride: winter deicers, fertilizers, and water treatment.

Various deicers are used to ensure safe driving in Illinois winters. Among them, rock salt (NaCl), due to its convenience, effectiveness, and low cost, is the most preferred deicing agent and is widely applied to roads, parking lots, and sidewalks. Calcium chloride (CaCl2), which melts more quickly than NaCl but costs more, serves as an alternative for NaCl. Calcium magnesium acetate (CMA) is a more benign deicing compound, but because of its higher cost is used only in some specific cases. It's also less effective in freezing rain, drier snowstorms and light-traffic conditions. Other materials used for deicing include aircraft deicers (primarily glycol), sand, beet juice, etc.

Fertilizer applied to farmland is another source of chloride that may enter aquifers. While its contribution is not as great as winter deicers, it still serves as a very important secondary source. The primary agricultural source of chloride is potassium chloride (KCl). Other fertilizers containing chloride include: calcium chloride (CaCl2), ammonium chloride (NH4Cl) and magnesium chloride (MgCl2).  Scientists from a bioproducts research laboratory  pointed out that nitrification inhibitors that are applied to fields is another potential agricultural chloride source.

Water considered “hard” is high in dissolved minerals, specifically calcium and magnesium. Water softening used to treat hard water commonly use salt brine (NaCl solution) to regenerate resin in the treatment systems, replacing calcium and magnesium as well as some other heavy metals with sodium. The remaining brine is typically discharged into septic or wastewater systems and can potentially reach shallow groundwater. Since groundwater in many other parts of Illinois is hard,  researchers from Illinois State Water Survey  concluded that water treatment or housing with private septic systems can potentially provide significant amounts of chloride.

While large variability exists in sources of chloride inputs, patterns of chloride pathways to the environment are similar and can be roughly summarized as follows:

• Chloride flows into surface water through snowmelt runoff
• Chloride enters underlying aquifers through infiltration
• Chloride enters surface water through groundwater discharge

Note: based on  a review conducted by researchers from University of Regina , chloride concentrations tend to be lower in large lakes compared to smaller lakes as the greater volume and larger flow rate of water in larger basins usually dilutes chloride concentrations. Many lakes in Lake County, Illinois experienced a slight drop in chloride concentrations from 2005-2010 due to the dilution during relatively wet summers during that period. (M. Adam, Lake Co. Health Dept. pers. comm. 2011)

The figure below shows the chloride pathways to water bodies given different sources of chloride input.

Several water quality standards have been set by USEPA (U.S. Environmental Protection Agency) for chloride.  The secondary drinking water standard (non-enforceable)  for chloride in the United States is 250 milligrams per liter (mg/L). Above this level water begins to taste salty.  Some other criteria  focus on toxic effects of chemicals on aquatic species. USEPA recommends a chronic criterion for aquatic life of a four-day average chloride concentration of 230 mg/L (with an occurrence interval of once every three years) and a recommended acute criterion of 860 mg/L.

Long-term chloride inputs from different sources can increase chloride concentrations. The major effect of elevated chloride in surface water is chronic toxicity, which harms aquatic organisms by interfering with their balance of body fluids.  Researchers from USGS  found chloride tolerance levels for some brook trout species to be as low as 3.1 mg/L.  Canadian Scientists  found that glochidia (mollusk larva) are more sensitive to chloride than most other aquatic organisms and hence are more likely to face risks of acute chloride toxicity. The table below shows the effects of chloride on different kinds of aquatic species:

In addition to increasing chloride concentrations, the influx of runoff from deicing salt application also has two other important impacts on surface water: (1) incomplete mixing of salt ions entering lakes creates layers of different densities and can cause depletion of oxygen since  dissolved oxygen is reported to be lower in deeper waters ; and (2) high sodium concentrations might increase the growth of blue-green algae, thereby triggering nuisance algal blooms.

While chloride is non-toxic to humans, high chloride levels can trigger aesthetic and safety concerns about drinking water quality, thus there is a secondary standard of 250 mg/L. While this is a non-enforceable standard, elevated chloride levels can affect taste and decrease public confidence in their water supply. Elevated sodium concentrations are commonly associated with high chloride levels. Sodium levels greater than 20 mg/L are not recommended for people with hypertension.

Dissolved chloride ions increase the corrosivity of the water, which might result in increased corrosion of pipes in water infrastructures. Several studies have shown that elevated chloride concentrations could promote the release of several metals including lead (Pb), copper (Cu) and iron (Fe). Examples include:  A 2005 study  stated that elevated chloride concentrations increased corrosion of iron pipes, releasing Fe and reducing the life span of plumbing.  Researchers from University of Florida  showed that increased chloride forms soluble lead complexes, leading to increasing lead concentration in water;  Scientists from USEPA  found that high chloride concentrations result in pitting corrosion of copper;  Swedish scientists  suggested that chloride complexation with certain metals is one of the major mechanisms of metal mobilization due to elevated salt concentrations.

While public supply wells are regularly monitored and tested, the water quality of private wells is typically not monitored. Corrosion-related damages in households are estimated to cost 2-20 times more than that of public water supplies based on  a study conducted by American Water Works Association . According to  a 2018 study 's testing results, both old and new houses are potentially at-risk as the impact of chloride contamination is dependent on the plumbing materials present in the drinking water infrastructure. Increased education efforts are needed to reach private well owners and residents who rely on groundwater that might be elevated in chloride. Well owners should be encouraged to monitor the water quality of their well water.

If you are curious about the chloride concentrations in your private or domestic well, look into the  free private wells class  offered by groundwater specialists at the Illinois State Water Survey. The Water Survey also has a  public service laboratory  that provides a low-cost water analysis (including chloride) for private well owners.