Long term water supplies in California are quite limited if we continue to use water at present rates and the population grows. The engineers and regulators know this, and theoretically so do the farmers and politicians. We’ve already overallocated the Colorado River, such that it vanishes into the sediments at its mouth, no longer flowing directly into the Gulf of California. And wet years for the Sacramento-San Joaquin River watershed doesn’t mean as much for the river flowrate as it does for the downstream farmers in the Central Valley who want to irrigate as much of their land as possible, or for the southern California utilities who want to store the extra water for drought conditions.
On top of expected in trends in population growth, climate change could potentially decrease the precipitation and/or streamflow in most of the southwest. Furthermore, environmental concerns are pushing water regulators to scale back the allotted flows in the California and Colorado River aqueducts. If you were the water manager for the city of Los Angeles, what would you do? The net balance of your inflows is likely to decrease, while the delivery people demand is increasing…
There are a couple of options for new water supplies. One is to build massive desalination plants, at a high energy cost and a high environmental cost for the disposal of brine waste. In California you also run into the problem that necessary locations on the shoreline have really high property values and a bunch of neighbors screaming NIMBY. Another is to look for temporary solutions like underused groundwater basins or watersheds. Las Vegas is in the process of shipping groundwater from a remote valley of ranchers along the Nevada-Utah border to its water supply. There’s also a project to mine a groundwater basin in the Mojave Desert in California, between Joshua Tree and Death Valley, for water supply to the L.A. basin.
But to me, the most reliable long-term option is indirect water reuse, which is already in place in Orange County, a relatively conservative portion of southern California. I am slightly skeptical of direct reuse of wastewater effluent, even when treated beyond normal potable water standards, just because of the lack of a buffer or factor of safety if anything goes awry at the wastewater/water treatment plant. But indirect potable reuse makes a lot of sense: treated wastewater is injected into a storage basin, most often an aquifer, and then after some flow distance, it is re-extracted as raw water for drinking water supply (i.e., it gets treated again after extraction). During its travel in the subsurface, the water is filtered naturally by the soil and mixes a little bit with the native groundwater. Quite a few cities employ this system with lakes, notably Las Vegas, Milwaukee, and Berlin (Germany), but there’s a lot more mixing and dilution in lakes than in groundwater.
To be honest, the indirect reuse in Europe, which has been working in some cases for over 120 years (see Berlin), is very convincing to me. Water recycling could regenerate roughly 50% of the water supply of the entire L.A. basin (a lot of water is lost to outdoor uses like pools and lawns), and would make water supply as reliable as the wastewater supply. An odd concept, perhaps, but in the case of Los Angeles this would be less wasteful than all the treated wastewater currently discharged to the ocean. Given the right geology, water recycling would also require far less infrastructure than desalination plants or large water pipelines.
Would the public go along with this, if properly informed? Sydney, Australia provides an example of the PR gone wrong – after a contentious public debate, they built a massive desalination plant. But again, Orange County is already employing this successfully, so we should be able to point to them as a model. As Orange County goes, so goes the country? It’s not something I would normally expect to hold.