Around the world, each region faces unique water challenges. In California, freshwater flows into the northern part of the state when the Sierra Nevada Mountain snowpack melts in the spring. This snowmelt provides as much as one-third of California’s water. But as Earth’s climate changes, the state could lose much of its snowpack. Indeed, in the 2009 “water year” (California tracks its yearly totals from July of one year to June of the next), California’s precipitation was 20% below average, and the snowpack was 40% below its average size. The 2014 water year was the driest on record in California; precipitation was 55% below normal, and the spring snowpack was 97% below normal.

Even in a good snowpack year, the state faces major water issues, explains Deshmukh, now working at the West Basin Municipal Water District in Carson, California. Two-thirds of California’s water is located in the northern part of the state, but two-thirds of the state’s residents live in the south, he explains. At the moment, the state ships some of the northern water to the south via pipes and canals, which costs money and uses a great deal of electricity. And if there is an earthquake or another natural disaster, that transport system could be cut off.

Many people (not just in California) draw their water from an underground region of permeable soil or porous rock saturated with water, called an aquifer. These stores receive water from rainfall and snowmelt that soaks into the ground through infiltration. Plant roots take up some of the water along the way, but much of the water continues to move downward, filling every available space in the aquifer. As the water trickles down, it becomes naturally filtered by rocks and soil, which trap bacteria and other contaminants as the water passes by. The top of this water-saturated region, referred to as the water table, rises and falls due to seasonal weather changes.

The depth of Orange County’s groundwater varies, says Deshmukh. At the coast, the aquifer is 6 to 90 meters (20 to 300 feet) deep, but further inland, at its deepest, the groundwater extends about 900 meters (3,000 feet) deep. Water quantity is often measured in terms of acre-feet—the amount needed to cover an acre in water to a depth of 1 foot (30.5 centimeters), which is equivalent to more than 1.1 million liters (300,000 gallons). One acre-foot of water is enough for two American families for 1 year. Deshmukh estimates that nearly 5,000 acre-feet of water is accessible from the deepest part of the aquifer. But that deep subterranean water is harder to get, and it costs more to pump it out of the ground than it costs to remove the groundwater closer to the surface.

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In addition to withdrawals for agriculture, industry, and personal use, anything that reduces infiltration will reduce the rate at which the aquifer refills and thus decreases the amount of water we can sustainably remove. Infiltration is hampered in urban and suburban settings because of all the hard surfaces, such as roads and buildings; even a typical suburban lawn is so compacted from the home construction process that very little water infiltrates the ground. Urban and suburban designs that provide ways for water to soak into the ground—such as permeable pavement and rain gardens—can help refill aquifers as well as help prevent flooding events after heavy rainfalls. (For more on rain gardens, see Chapter 15.)

But in California, excess withdrawals from the aquifer were not leading to depleted aquifers and dry wells. Instead, the proximity to saltwater was actually threatening the freshwater supply.

Decades ago, Orange County officials discovered to their dismay that saltwater was seeping into some of the region’s aquifers, putting those precious freshwater stores in jeopardy. Groundwater levels are typically higher than sea level, so saltwater doesn’t infiltrate aquifers. But as freshwater was pumped out of the county’s aquifer inland, the groundwater level dropped, so salty ocean water had started to enter the coastal edge of the aquifer to the west, where it bordered the Pacific Ocean. In Orange County, some aquifers are confined by geologic faults, which prevent ocean water from entering at some points—but not everywhere. It was in these unconfined coastal aquifers that saltwater intrusion was becoming a problem. INFOGRAPHIC 14.4

To stem the influx of saltwater, in 1975 the Orange County Water District started pumping highly treated (purified) sewage wastewater into injection wells. At about 19 million liters (5 million gallons) a day, the underground injection created a curtain of freshwater along the California coast that prevented salty ocean water from seeping into the county’s aquifer. This water management program was the first to pump treated wastewater into the ground, says Deshmukh.

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Most residents—even those who thought about water every day—had no idea that water that had recently been flushed down someone’s toilet was being cleaned and then pumped into the area’s groundwater supply.