But many places don’t have the same resources to set up wastewater surveillance — or peg public health actions to data — as San Diego does. When asked if he knew of other institutions using wastewater to guide their safety measures, UCSD Health chief medical officer Dr. Christopher Longhurst said no: “I couldn’t point you to one.”

In recent months, scientists in California have expanded wastewater surveillance to more rural parts of the state, with support from the state’s Department of Public Health. This poses new challenges: Public health officials often aren’t used to looking at data from outside hospitals or health clinics, and there are many logistical hurdles to setting up sampling in new locations, combined with the complications of interpreting data from less populous areas, where wastewater surveillance is highly sensitive to changes in COVID-19 spread.

One document from Stanford University’s Sewer Coronavirus Alert Network (SCAN) describes how the genetic material shed by someone with COVID-19 may change: over the course of their infection, from person to person and depending on measurement techniques, the weather, an influx of spring breakers, or even local business practices. For example, Modesto — a city in California’s Central Valley — had a lot of fruit cannery waste in its sewage. This industrial flow may have blocked the signal of the coronavirus’s genetic material, impacting scientists’ ability to isolate it in PCR testing, said Colleen Naughton, an environmental engineering professor at the University of California, Merced, who works on wastewater monitoring in this region.