A TECHNIQUE FOR IDENTIFYING THE SOURCES OF NA AND CL CONTAMINATION IN SURFACE WATER AND GROUND WATER

Elevated concentrations of Na⁺ and Cl^- in natural waters are common in the U.S. and other countries, and can serve as indicators of, or may themselves constitute a water quality problem. We have characterized the most prevalent natural and anthropogenic sources of Na⁺ and Cl^- in groundwater and surface water, and explored techniques that could be used to identify each source in waters in which the sources of Na⁺ and Cl^- may be unknown. We considered seven potential sources including agricultural chemicals, septic effluent, animal waste, municipal landfill leachate, seawater, basin brines, and road deicers.

The halides Cl^->, Br^- and I^- were the most useful indicators of the sources of Na⁺ and Cl^- contamination. Iodide enrichment (relative to Cl^-) was greatest in precipitation, followed by uncontaminated ground water, and landfill leachate. The mass ratios of the halides among themselves, with total nitrogen (N), and with Na⁺ and B provided the most diagnostic methods for graphically distinguishing among sources of Na⁺ and Cl^- in contaminated waters. Cl/Br ratios plotted against Cl^- revealed a clear separation of sample groups. Samples of landfill leachate and ground water known to be contaminated by leachate were enriched in I^- and Br^-; this provided an excellent, fingerprint for identifying leachate contamination. In addition, total N, when plotted against Cl/Br ratios, successfully separated waters contaminated by road salt from waters contaminated by other sources. These graphical techniques provide an improved means of identifying the sources of Na⁺ and Cl^- contamination in natural waters.