The health ministry said Tuesday that someone poisoned a water tank in a system that supplies about 8,000 persons.
At least five springs are involved, said the Ministerio de Salud.
The discovery generated a major effort by public agencies, including the Caja Costarricense de Seguro Social which put some residents in the San Juan Norte de Desamparados clinic.
The health officials told residents to refrain from drinking the contaminated water. Involved are the communities of Copalchí de Cartago, Higuito and San Miguel de Desamparados.
Residents were told to wash out with soap any containers or tanks that had been filled with water Tuesday.
The symptoms of the poisoning are vomiting, stomach pain and diarrhea, said the health ministry. The residents were urged to obtain their water from the nearest pipes of the Instituto Nacional de Acueductos y Alcantarillados. The water company is expected to be providing tankers, too.
The Laboratorio Nacional de Aguas is testing the water, the health ministry said, and results are expected later today.