MABTON, Wash. — Mabton residents will learn today if efforts to eradicate E. coli from the city’s water system are working.

The first of two rounds of water samples were sent for testing Thursday, with the results due back today. If those results, and those of today’s samples are returned clean, the boil water order in place since Tuesday will be lifted, Mayor Mario Martinez said.

Meanwhile, officials say efforts to find the source of the contamination have been unsuccessful.

E. coli is a bacteria commonly found in feces. Health officials say its presence can indicate other contaminants in a water system.

The bacteria was first detected when water from Artz-Fox Elementary School was tested Saturday. Residents were told Tuesday to boil water before drinking or cooking with it.

State Department of Health workers inspected the city’s water system and were unable to find a source for the bacteria, said Carolyn Cox, a spokeswoman with the department’s Office of Drinking Water, said Thursday.

Chlorine levels in the city’s water system were increased to kill the bacteria, Martinez said.

The city and the Mabton School District are offering free bottled water for residents.

E. coli can cause nausea, diarrhea, cramps and headaches. Infants, young children, the elderly and people with weakened immune systems are at greater risk, according to the health department.

Martinez said he’s only received one phone call from a resident who thought her child might be sickened by the water, and he advised her to see a doctor to confirm it.

The city also conducted a public question-and-answer session Thursday at Artz-Fox Elementary School regarding the situation.