Wastewater Testing - Healthy Davis Together

HDT Wastewater region map — Click to Enlarge

Environmental monitoring of wastewater (regularly testing sewage) is another tool Healthy Davis Together is using to learn about COVID-19 and prevent its spread in our community.

The virus that causes COVID-19 (SARS-CoV-2) is found in the excrement (feces and urine) of individuals who have COVID-19, including asymptomatic and pre-symptomatic individuals. This means that wastewater testing can be used as an early-warning detection system for the virus in a regional area (e.g., a neighborhood).

Healthy Davis Together (HDT) wastewater testing is helping to slow and prevent the spread and reemergence of COVID-19 in Davis in two ways. First, wastewater data can be used to strengthen confidence in clinical-testing data. When wastewater and clinical data don’t match, it can be a sign that clinical testing needs to be ramped up. Second, wastewater testing is an inexpensive way to get information on regional infection trends over time. Trend data indicate whether existing efforts to combat COVID-19 are working and whether more needs to be done in certain areas.

As of May 18, 2021, this dashboard includes data for an expanded number of sampling locations. Additional locations sample from smaller geographic regions, enabling the HDT wastewater-monitoring team to continue monitoring small changes in SARS-CoV-2 concentrations even as COVID-19 levels in the City overall remain low.

Davis Regional Testing Graphs

Wastewater sampling week of June 7

How to read regional wastewater graphs

After collection, wastewater samples are analyzed in a UC Davis lab to determine the concentration of virus copies present. Concentration data are normalized (to adjust for things like rainfall and other factors that could affect the sample dilution) and plotted against the City’s Wastewater Treatment Plant numbers (the city-wide average). A two-week moving average is calculated to smooth out noise in the data, Finally, a linear fit is applied to data from the past two weeks to determine whether the amount of virus in a region is going up, going down, or staying about the same.

By comparing regional data (in blue) to the city-wide average (in green), we can see how trends compare.

These regional wastewater graphs are updated every Saturday.

Variability is expected in samples collected from sewer systems. Environmental factors and the timing of sample collection occasionally yield unusually high results. For this reason, wastewater data is best used to assess trends: that is, to monitor whether concentrations of SARS-CoV-2 in a particular area are persistently low, high, increasing, or decreasing over multiple days. The City of Davis does not make public-health decisions based on isolated (i.e., single-day) sampling results.

Healthy Davis Together encourages everyone to remain vigilant and follow local guidance to prevent the spread of COVID-19 and all its variants, including:

Getting vaccinated

Wearing a mask/face covering

Keeping a physical distance

Practicing good hand hygiene

Getting tested for COVID-19

Avoiding large crowds, opt for outdoors

Staying home if you feel sick

Wastewater testing is just one of the many tools and resources that can help keep Davis healthy. To learn more about wastewater surveillance as a strategy for containing COVID-19, please visit the Centers for Disease Control and Prevention (CDC)’s webpage on the topic.

If you have questions, please email wastewater@HealthyDavisTogether.org.