What does "used" mean in this case? Aren't these closed systems?
"When water used as a coolant is returned to the natural environment at a higher temperature, the sudden change in temperature decreases oxygen supply and affects ecosystem composition. Fish and other organisms adapted to particular temperature range can be killed by an abrupt change in water temperature (either a rapid increase or decrease) known as "thermal shock". Warm coolant water can also have long term effects on water temperature, increasing the overall temperature of water bodies, including deep water. Seasonality effects how these temperature increases are distributed throughout the water column. Elevated water temperatures decrease oxygen levels, which can kill fish and alter food chain composition, reduce species biodiversity, and foster invasion by new thermophilic species."
The water still exists yes, but ground water is essentially “used up.”
Other heat sink strategies could involve refrigeration systems, but those are significantly more energy intensive and a lot more complex.