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The rapid expansion of major cities throughout the world is resulting in the degradation of water quality in local aquifers. Increased use of road deicers since the middle of the 20th century in cities in the northern United States, Canada, and Europe has been linked to degraded ground water quality. In this article, Chicago, Illinois, and its outlying suburban areas are used as an example of the effects of urbanization in a historical context. A statistical study of historical water quality data was undertaken to determine how urbanization activities have affected shallow (<60 m) ground water quality. Chloride (Cl(-)) concentrations have been increasing, particularly in counties west and south of Chicago. In the majority of shallow public supply wells in the western and southern counties, Cl(-) concentrations have been increasing since the 1960s. About 43% of the wells in these counties have rate increases greater than 1 mg/L/year, and 15% have increases greater than 4 mg/L/year. Approximately 24% of the samples collected from public supply wells in the Chicago area in the 1990s had Cl(-) concentrations greater than 100 mg/L (35% in the western and southern counties); median values were less than 10 mg/L before 1960. The greater increase in Cl(-) concentrations in the outer counties is most likely due to both natural and anthropogenic factors, including the presence of more significant and shallower sand and gravel deposits, less curbing of major highways and streets, and less development in some parts of these counties.

Winter deicers are a major source of chloride contamination to shallow aquifers in northern latitudes. In the greater Chicago area of northeast Illinois, chloride has been accumulating for decades and in many places now exceeds the United States Environmental Protection Agency secondary standard of 250 mg/L. MODFLOW‐NWT and MT3D‐USGS were used to simulate the accumulation of chloride with 30 years of data in the shallow aquifer of Will County in northeast Illinois. This aquifer is composed of unconsolidated sediments, largely from glacial deposits, overlying a fractured dolomite bedrock. To calibrate to observed heads and chloride, the model needed refined geologic features, higher chloride concentrations on cells representing commercial or industrial lands, lower chloride concentrations on cells representing roads, and point source areas to speed chloride's emergence in the aquifer. These point sources are locally significant and could represent different anthropogenic or geologic features, such as municipal stormwater infrastructure. Future simulations indicate that chloride is not at steady state in the shallow aquifer and wells are at risk of exceeding the secondary standard if winter deicing applications are not reduced. It may take decades for the full impacts of reduced deicing rates to be observed in wells, owing to the long residence time of water in the aquifer. This transient model calibration was possible because of the 30‐year dataset collected by communities and government agencies. [ABSTRACT FROM AUTHOR]

Abstract: Chloride concentrations in waterways of northern USA are increasing at alarming rates and road salt is commonly assumed to be the cause. However, there are additional sources of Cl− in metropolitan areas, such as treated wastewater (TWW) and water conditioning salts, which may be contributing to Cl− loads entering surface waters. In this study, the potential sources of Cl− and Cl− loads in the Illinois River Basin from the Chicago area to the Illinois River’s confluence with the Mississippi River were investigated using halide data in stream samples and published Cl− and river discharge data. The investigation showed that road salt runoff and TWW from the Chicago region dominate Cl− loads in the Illinois Waterway, defined as the navigable sections of the Illinois River and two major tributaries in the Chicago region. Treated wastewater discharges at a relatively constant rate throughout the year and is the primary source of Cl− and other elements such as F− and B. Chloride loads are highest in the winter and early spring as a result of road salt runoff which can increase Cl− concentrations by up to several hundred mg/L. Chloride concentrations decrease downstream in the Illinois Waterway due to dilution, but are always elevated relative to tributaries downriver from Chicago. The TWW component is especially noticeable downstream under low discharge conditions during summer and early autumn when surface drainage is at a minimum and agricultural drain tiles are not flowing. Increases in population, urban and residential areas, and roadways in the Chicago area have caused an increase in the flux of Cl− from both road salt and TWW. Chloride concentrations have been increasing in the Illinois Waterway since around 1960 at a rate of about 1mg/L/a. The increase is largest in the winter months due to road salt runoff. Shallow groundwater Cl− concentrations are also increasing, potentially producing higher base flow concentrations. Projected increases in population and urbanization over the next several decades suggest that the trend of increasing Cl− concentrations and loads will continue. Given the susceptibility of aquatic ecosystems to increasing Cl− concentrations, especially short-term spikes following snow melts, deleterious effects on riverine ecosystems would be expected. [Copyright &y& Elsevier]