The water supplied to at least 129.000 people in the Chorotega Region may not be fit for human consumption, according to a warning issued by the Public Services Regulatory Authority (Aresep).
The presence of fecal coliforms and Escherichia coli bacteria, excess chlorine, or lack of water sterilization are three common problems found in at least 41 aqueducts in the region, according to the results of the 2025 Water Quality Report that Aresep has been producing annually since 2013.
“The presence of these microorganisms (fecal coliforms and Escherichia coli bacteria) indicates fecal contamination and a high probability of transmission of waterborne diseases,” the report details.
This latest report analyzed 243 aqueducts nationwide belonging to the Costa Rican Institute of Aqueducts and Sewers (AyA), the Heredia Public Services Company (ESPH), and the Communal Aqueduct and Sewer System Management Associations (Asadas).
Of the total sample evaluated by Aresep, 52 aqueducts supply the Chorotega Region, and of those, only six were unaffected.
For Germán Rojas, coordinator of Aresep’s Quality Regulation Process, the results are a wake-up call for aqueducts to monitor the procedures they apply to ensure a safe water supply more closely.
“The overall conclusion is that operators need to be more vigilant about their routine processes. Disinfection is our main defense against the spread of bacteria (…) Aqueducts need to be more careful about how they operate on a daily basis,” said Rojas.
One of the concerns emphasized by the official is that most Asadas in the region are small and do not have sufficient resources to address the problems.
“Due to organizational and structural issues, ASADAS often only have one plumber, who works part-time, but in addition to repairing breakdowns, he has to read meters and cannot always be on the lookout to ensure that the chlorination equipment is working properly,” said Rojas.
The study also revealed that the four water treatment plants in Guanacaste could be discharging contaminated water into rivers.
Which Asadas had problems?
Among the ASADAS and aqueducts that presented deficiencies in Guanacaste, there are three that “are in intensive care,” said Rojas. The ASADAS of San Juan de Quebrada Honda, Cañal de Pozo de Agua, and Loma Bonita—all located in Nicoya—were the ones with the most deficiencies.
According to María Laura Arias, director of the Water Microbiology Laboratory at the University of Costa Rica (UCR), water with fecal coliforms, or water that is not clarified, is not drinkable and the population is at risk of becoming ill.
“Coliforms are an indicator of the possible presence of microorganisms that will affect the intestine. Bacteria such as Salmonella (…) and I could go on with a whole list. It can also cause viruses such as rotavirus, hepatitis A, and all viruses that are transmitted through feces,” explained Arias.
The UCR microbiologist believes that water without chlorine “is a time bomb” because it lends itself to the spread of disease. Meanwhile, excess chlorine “causes gastritis. It also causes clothes to fade, hair to lose its color, and in some people, it can cause allergies.”
The water resource analyses were taken between October and November 2025. Both Rojas and Arias emphasized that these results are a snapshot of the moment; this means that water quality is variable.
“Water quality can change in a matter of minutes. For example, we have chlorinated water, but then a truck arrives that discharges sewage into the water. At that moment, we would once again have contaminated water. The important thing here is that at the moment Aresep took the snapshot, it was contaminated, and it should not be contaminated at any time; so the situation could happen again,” said the microbiologist.
Treatment plants cannot keep up
Aresep included 36 wastewater treatment plants (WWTPs) nationwide in the study. There are four public plants in the province, and all were included in the analysis.
The plants are located in Liberia, Nicoya, Cañas, and Santa Cruz. In this section, the Chorotega region was once again the worst rated: it was the only one in the entire country where all the plants analyzed had at least one deficiency.
According to Rojas, these treatment plants “cannot ensure that the water they discharge into rivers is free of organic contamination.”
“This condition requires special attention from operators and regulatory agencies, given the potential environmental and health impact associated with inadequate wastewater treatment,” the report says.
Chemical oxygen demand (COD) and biochemical oxygen demand (BOD) were the deficiencies present in most plants. These parameters describe how much organic matter remains in the water after treatment. The higher these numbers are, the more polluted the water is.
The maximum concentrations are 50 milligrams per liter (mg/L) of BOD and 150 mg/L of COD, as stipulated in Article 20 of the Wastewater Discharge and Reuse Regulations. However, the results revealed that levels in the region are up to three times higher than the values established in the decree.
According to Rojas, the age of the plants is the main problem preventing them from cleaning all the water.
“These are lagoons that date back to the mid-1900s (…) many of the treatment systems in these cantons require very large investment projects,” explained Rojas.
Aresep demands improvements
Given the results of the study, Aresep asked the Asadas, AyA, and ESPH for action plans with specific measures to correct the deficiencies in the aqueducts.
As Rojas confirmed to La Voz, AyA has already sent a report that includes the action plans of the institution and all the Asadas in the region.
However, the three Asadas with the worst results in the Chorotega region have 40 business days, starting on January 26, 2026—that is, expiring on March 20—to submit a specific technical study on their situation to AyA.
To date, the regulatory institution has not received any of these technical studies.
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