When I collect groundwater quality samples, temperature is one of the parameters always measured. It is considered a “physical” characteristic as opposed to a “chemical” characteristic of the water. In groundwater studies, temperature is not usually considered a water quality parameter, rather it is used to help understand recharge and discharge processes and determine inputs from deep geologic formations. In surface water, however, temperature is an important water quality parameter affecting aquatic organisms, and a recently published paper has looked at the magnitude of thermal pollution in many river basins throughout the world (Raptis, C.E., et al., 2016. Environmental Research Letters 11:104011; doi:10.1088/1748-9326/11/10/104011).

The main source of thermal pollution in rivers is discharge of warm water from power plants. Power plants in Illinois and elsewhere use huge amounts of water for cooling purposes; they represent by far the largest withdrawals of surface water in Illinois. In 2011, 98% of the surface water withdrawn in Illinois was by the power generation industry, as reported to the Illinois Water Inventory Program (IWIP), housed at the Illinois State Water Survey. However, this is primarily a non-consumptive use of water in that most of the water is returned to the river or reservoir from which it was taken (a small percentage is lost via evaporation). And when it is returned, it has an elevated temperature.

According to the Raptis et al. study, the Mississippi River receives the highest total amount of heat emissions in the world, the majority of which are from coal-fueled power plants. The Mississippi also has the highest number of instances where the temperature increase is greater than 3°C, a typical regulatory standard. However, the Rhine River is the most thermally polluted basin as a percentage of total flow in its watershed, and rivers in Europe are generally the most thermally polluted waters in the world.