It is a challenge for the average layman to decipher the hard figures revealed in technical studies on pollution levels in Lake Chapala, especially when scientists appear to draw conflicting conclusions from the data.

The standard measure of water pollution in Mexico is based on an evaluation of nine measurements called the water quality index (ICA) that are summarized in median figures.

Are pelicans that winter in Lake Chapala suffering from genetic mutations caused by local water contamination? U de G scientists offer conflicting views on the lake’s pollution problems.

In contrast, a 2002 report on a water analysis of Lake Chapala — based on methodology developed in France — concluded that the overall quality rates as mediocre at best. “In August, 1999, measurements of the various parameters were abnormally elevated, particularly in reference to the total suspended solids, chemical oxygen demand and total phosphorus,” the report authored by Chloe Malie stated. These factors signal a lake in a process of eutrophication, an increase in chemical nutrients that can cause excessive plant growth and decay.

And what about those worrisome heavy metals?

Data published in 2006 by the Mexico’s National Ecology Institute (INE) gives values for 10 heavy metal substances measured at 20 points in Lake Chapala’s sediments. The results show that none of the substances appeared in sufficent quantities to qualify in the Severe Effect Level range, which refers to heavily polluted sediments likely to affect the health of sediment-dwelling organisms.

However, arsenic, chromium, copper, magnesium and nickel registered above the parameter of Lowest Effects Level, (LEL), the frame of reference for marginally polluted sediments in which ecotoxic effects become apparent without impact on the majority of sediment-dwelling organisms.

It is worth mentioning that lake scientists tend to agree that the arsenic found in Lake Chapala is a natural element that comes from thermal springs located within the body of the lake, rather than a contaminant introduced from external sources.

What is missing in this picture is data for parameters beyond the realm of standard testing. Most evaluations of water quality in Lake Chapala are based on measurements of inorganic components and bacterial contamination. There is not yet enough data to draw conclusions of the impact of organic components such as pesticides used in farming or the solvents and other chemical compounds produced by industry.

Only recently have scientists started to delve into the direct effects heavy metals have on invertebrates and other living creatures. Additional topics still on the agenda for in-depth research include pollutants that may cause resistance to antibiotics or act as endocrine disruptors