Nitrites are a salt or ester anion of nitrous acid, which can be naturally or artificially occurring in groundwater. Nitrites come from fertilizers through run-off water, sewage, and mineral deposits. Nitrite is used in food production for the curing of meat products due to it inhibiting the growth of bacteria. Unfortunately it can also stimulate the grown of bacteria when introduced in high levels into a body of water.
This USGS map shows how chloride pollution in groundwater is getting worse in many parts of the U.S. "Of the 56 well networks that were analyzed for changes in chloride concentrations using the Wilcoxon-Pratt signed-rank test (1959), 24 had statistically significant increases in concentrations."