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Causes of Contamination

Satellite image of Lake Atitlán showing a cyanobacteria algae bloom.

Satellite image of Lake Atitlán showing a cyanobacteria algae bloom.

The Atitlán basin is volcanic in origin; it is surrounded by rock escarpments and volcanoes and has no outflow to the ocean, meaning that contamination that goes into the lake remains in the lake.

Nutrient overloading has caused severe deterioration to the quality of the water in Lake Atitlán. The lake’s zero oxygen layer is rising by hundreds of feet every year, and its clear water is now only a few hundred feet deep. E. coli levels are currently tens of thousands of times over the safe limit.

Contamination of Lake Atitlán due to human activity has increased exponentially in recent decades, as the surrounding towns and villages have grown along with the lake’s thriving tourism industry.

Untreated Sewage

Sewage in Lake Atitlán.

Sewage in Lake Atitlán.


Thousands of gallons of sewage flow into the lake every day.

There are not enough wastewater treatment plants around Lake Atitlan to treat the ever-increasing amount of raw sewage produced by the area’s growing population. Most of the existing plants operate well below full capacity. They simply remove solids and do little to reduce the volume of pathogenic bacteria entering the lake. Only one of many planned new facilities has been built so far, and municipal authorities do not have the resources they need for maintenance and upgrading.

As a result, thousands of metric tons of raw sewage or improperly treated wastewater from households and businesses in more than 20 towns enter the lake each year. The ecosystem is under constant siege from putrefactive bacteria and other pathogens. At the San Bartolo wastewater treatment plant in Sololá, the outflow contains over 1,000 times the accepted level of coliforms (such as e-coli). This water is actually used for direct irrigation onto vegetable crops.

Chemical Agriculture

Tens of thousands of gallons of contaminants flow into Lake Atitlán every day.

Tens of thousands of gallons of contaminants flow into Lake Atitlán every day.


Chemical agricultural runoff is destroying Lake Atitlán’s ecosystem.

Just over a third of the land in the Lake Atitlán basin is used for agriculture (mainly coffee, vegetables, and fruit). Since the 1950s, local farmers have been pressured to abandon natural farming and instead use chemical fertilizers, pesticides, fungicides, and herbicides. These toxic products are highly subsidized, and poor farmers are strongly encouraged to use them copiously to boost their production.

The fertilizers, which are particularly high in phosphorous, run off into the lake during the rainy season. Waste water from coffee processing is also acidic and high in natural effluents.

Soil fertility and sustainable agriculture depend on a balanced supply of essential nutrients and minerals, and the continuous and excessive use of chemical products destroys this balance. They replenish only nitrogen, phosphorous, and potassium, while killing soil-friendly microorganisms such as insects, fungus, and bacteria. Over time, the soil becomes depleted of trace minerals and essential micronutrients. It hardens and ultimately the microorganisms that the soil needs to live will die, and nothing will grow any more.

Commercial Tilapia Farming

Tilapia farm near Santiago Atitlán, Guatemala.

Tilapia farm near Santiago Atitlán, Guatemala.


Why are tilapia farms allowed to contaminate the lake?

Lake Atitlán is a ‘Protected Area’ under Guatemalan law, but commercial tilapia farms are being allowed to operate on a massive scale, even though all the nutrients that they leave in the lake are accelerating the destruction of the lake’s ecosystem. This one activity has destroyed other lakes all by itself.

There are six commercial tilapia farms in Santiago Bay alone. Like all intensively fed fish, farmed tilapia produce fecal waste and other waste such as uneaten food, causing nutrient enrichment in the water. Exactly how many excess nutrients are added to Lake Atitlan every day by these operations, and how quickly are they accelerating the collapse of the lake’s ecosystem?  It’s impossible to know exactly. We also don’t know who owns the farms and what powerful (dangerous) connections they have.

We do know that there are tens of thousands of tilapia and that tons – literally boatloads – of commercial fish food are taken to the farms each week, and that these tons of fish food are then converted into highly bioavailable nutrients (phosphorus). And we know that the farms are operating with impunity in a so-called Protected Area.

The tilapia farm in the photograph above is in an unusually shallow area. The lake has responded with an immense hydrilla bed to consume the nutrients, but it will only consume a small fraction and the rest will flow into the open water on the other side of the farm. Most of the farms are in water that is too deep for weeds to grow.