Samples of wastewater taken from in and around Boston show levels of COVID-19 near the city at its highest point since the first week of February.

By testing for COVID in the water coming from toilets, showers and other drains across Greater Boston, authorities have tracked the spread of the pandemic without relying on local residents to take a viral test themselves.

After falling off from the height of the pandemic in January, coronavirus levels in the Boston area’s wastewater have surged significantly in recent weeks.

The results align with a pattern of spiking cases across the state. Half of Massachusett counties are now rated at the highest possible risk by the Centers for Disease Control and Prevention, even as much of the country remains at a lower risk.

Some schools have responded by reinstating masking guidelines, while Springfield and other cities have advised high-risk residents to begin masking again.

In samples of wastewater taken the Deer Island Treatment Plant in Boston Harbor, officials from the Massachusetts Water Resource Authority noticed a slight uptick in COVID in mid-March. At the same time, cases were beginning to jump across the state.

Wastewater samples taken north of Boston show a rise in COVID-19 levels near the city, according to the Massachusetts Water Resource Authority.

The presence of COVID in the wastewater samples have more than quadrupled.

While the level of the virus in the water remains far lower than its pandemic peak during the omicron variant wave at the beginning of the year, it is higher than at any point since the beginning of February.

Other metrics for tracking COVID, such as the test positivity rate, are now at their highest point since late January.

Wastewater samples taken south of Boston show a rise in COVID-19 levels near the city, according to the Massachusetts Water Resource Authority.

Untreated water comes from several dozen communities in Greater Boston, stretching west to Ashland and Framingham, south to Walpole and Stoughton, and north to Wilmington and Reading. The data is reported in two groups — the MWRA’s North System and South System, divided roughly by the Massachusetts Turnpike.

The samples are collected at the Deer Island plant between three and seven times a week and analyzed by the Cambridge-based company Biobot Analytics.

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