Short answers, pulled from the story.
About 30 percent of all readily available fresh water in the world is groundwater. About 99 percent of the world's liquid fresh water is groundwater, making it the dominant form of accessible liquid fresh water on the planet.
Over 2 billion people rely on groundwater as their primary water source worldwide. About 2.5 billion people depend on it solely to meet their basic daily water needs.
Subsidence occurs when groundwater is pumped out of an aquifer, reducing the hydraulic pressure that supports the weight of overlying sediments. The aquifer compresses, and much of that compression is permanent. Mexico City has experienced subsidence at rates of up to 40 centimetres per year, and the San Joaquin Valley in California sank by as much as 8.5 metres during the first half of the 20th century.
Water extracted from deep aquifers in the Great Artesian Basin of Australia can be more than 1 million years old. The basin extends across almost 2 million square kilometres, and water flowing through it travels at an average rate of roughly 1 metre per year.
Groundwater pollution most often results from improper disposal of wastes on land. Major sources include industrial and household chemicals, garbage landfills, excessive fertilizers and pesticides, industrial waste lagoons, mine tailings, industrial fracking, leaking underground oil storage tanks and pipelines, and septic systems. Arsenic and fluoride have been identified as priority contaminants at the global level.
Climate change drives increased irrigation demand through higher evapotranspiration, contributing to groundwater depletion, particularly in drylands. Sea level rise causes seawater intrusion into coastal aquifers. Research from the Karlsruhe Institute of Technology and the University of Vienna projects that by 2100, between 77 million and 188 million people will live in areas where groundwater exceeds the highest drinking-water temperature threshold set by any country, under a medium emissions pathway.