The Chicago metropolitan area is one of the most rapidly expanding metropolitan areas in the USA, with a projected population increase of greater than 20% by 2020. In addition to increased population, there are major changes in land use, primarily from rural to urban/residential, expansions in roads and vehicle traffic, and increased water demand. The main sources of water that will be used to meet the increases in water demand are the shallow bedrock and overlying sand and gravel aquifers, which are vulnerable to surface contamination. Temporal changes in water quality in shallow (< 200 ft) unconfined aquifers in the Chicago region were evaluated using archived and published data. The median chloride concentration has steadily increased from 6 mg/L prior to 1950 to almost 20 mg/L in samples from the 1990s and 2000s. The highest chloride concentrations were typically found in the collar counties to the west and northwest of Chicago, and in the shallowest wells (< 100 ft). Municipal wells that have been periodically sampled were statistically evaluated for temporal trends in chloride concentrations. Significant positive trends were calculated for 55% of 240 wells, with 63% having significant positive slope values. Approximately 43% had trends exceeding 1 mg/L/yr, and 12% exceeded 4 mg/L/yr. The increase in chloride concentrations began in the 1960s, when road salt began being used in large amounts. Prior to 1960, about one-third of samples had chloride concentrations < 10 mg/L and less than 10% had concentrations > 40 mg/L. By the 1990s, 80% of samples were > 10 mg/L, almost 50% > 40 mg/L, and 16% > 100 mg/L. The increases in chloride concentrations are most pronounced in the counties west and northwest of Chicago, where land use is changing rapidly from rural to urban, and aquifer material is closer to the land surface.