All possible sources of chloride were analyzed for major ion, minor ion, and trace-metals in the Toronto region. These tests included fertilizers, manure piles, septic tanks, landfill leachates, saline waters form the underlying Paleozoic bedrock formations, and road salt. Researchers had to distinguish between these potential sources of chloride contamination. Interestingly, they found the absence of certain ions, not the presence, indicates NaCl road salt from other sources. Natural saline from bedrock carries much more fluoride and iodide than road salt. Analysis of the 5% NaCl solution showed a high degree of chemical purity. It was clear that road salt's major inorganic components of sodium and chloride harbored the greatest threat to water quality.
A huge contamination warning is the increase in sodium with respect to chlorine in the ground water. In an area where road salt is continually dumped, sodium levels are lower due to an ion exchange between Na+ ions and ions such as Ca+ present in the water-bearing subsurface. A false sense of security is created in these areas, whereas in urbanized areas the ground water does not show any sign of losing sodium because of the lack of exchangeable ions. These indicators alone cannot be the only factors taken into account when examining salt entering the water system.
An important technique in identifying the amount of road salt present below the ground is mass balance. The amount of road salt input to an area over a certain amount of time is measured in comparison to the output in the exiting stream. The difference between these figures gives the amount of salt which is trapped within the ground. A specific example of the use of mass balance is in highly developed Highland Creek basin of the Metropolitan Toronto region watershed. This basin is 104 km2 in area, receiving about 17,000 Mg of NaCl de-icing chemicals each year. That is equivalent to about 200 g of NaCl for every square meter of land. The amounts of chloride leaving the basin from stream flow was carefully monitored, with electrical conductivity measurements recorded at 15-minute intervals over a two-year period. The amount of chloride leaving through stream sediment was extremely small. These results revealed that only 45% of salt applied is removed via surface runoff, the rest lingering in subsurface waters. (GSA TODAY, December 1993, P319) Since the addition of chloride is greater than the mass of chloride leaving the stream, the chloride builds up in ground water. Over time, the input at the subsurface will equal the output in base flow. At this steady-state, no further damage to water quality will occur. Predictions suggest if salt is added at its present rate and 45% enters the subsurface, steady state discharge of chloride concentrations will exceed will exceed 400 mg/L. This is three times greater than the present baseflow concentrations and is highly unsuitable for drinking water. Sodium's impact will be lessened due to ion exchange, depending on the local exchangeable ions present, but it is also predicted to reach an unacceptable 250 mg/L. (GSA TODAY, December 1993, P319)