A study last year found that the level of E. coli bacteria in the Animas River just north of the New Mexico state line met water-quality standards but exceeded them in the New Mexico stretch of the river.
E. coli levels in the San Juan River above its confluence with the Animas at Farmington also were above the limit.
The E. coli limit in New Mexico for a single sample is 410 colony-forming units or a monthly average of 126 CFU, said Melissa May with the San Juan Soil and Water Conservation District. Colorado uses only the second criterium, she said. The CFU is measured by placing bacteria and an algae extract in a petri dish and counting the number of colonies.
But the results of the survey should be considered preliminary until a follow-up study this year is completed, a report by the San Juan Watershed Group says.
The new round of testing, scheduled to get underway in April and run through October, also will look at the level of nutrients – nitrogen and phosphorus – in the rivers.
Levels of E. coli and nutrients are important: E. coli is found in the lower intestine of warm-blooded animals, but a certain strain can cause diarrhea, nausea, fever and vomiting. Nutrients are needed for plant health, but too much increases algae bloom that can rob water of the oxygen required by aquatic life.
Colorado has an interim standard for nutrients, but it has been applied only on the upper reaches of the Rio Grande and Arkansas rivers, said Peter Butler a past member of the Colorado Water Quality Control Commission.
In New Mexico, limits are water-body specific, May said.
Testing was done last year at four sites, two each on the Animas and San Juan rivers, she said. BHP Billiton paid for the study, which consisted of 40 samples in all.
Discrepancies in laboratory analysis of the source of E. coli require a second year of testing, May said. A different laboratory than the one used in previous years did DNA analysis in 2013, she said.
DNA analysis can indicate if the source of E. coli is avian, ruminant (cattle, sheep, deer and elk), equine, canine or human, May said.
The absence of equine samples, the low number of cattle samples and a high number of human samples call into question the sensitivity of the probes and the accuracy of overall results, the watershed group report said.
Bacteria levels increased the further downstream that samples were taken, both in the San Juan and Animas rivers. The highest level of bacteria was found in the San Juan River at the Hogback Canal, the beginning of the Navajo Nation near Waterflow.
Preliminary results of testing at Farmington found that fecal pathogen levels in the San Juan River exceeded the New Mexico standard. A predominant source of the pathogens was human.
The finding of human pathogens was unexpected and not consistent with other studies in New Mexico, the report said.
David Tomko, retired from the New Mexico Environment Department and coordinator of the San Juan Watershed Group, said two tests on the Rio Grande River and one on the Cimarron River have shown little presence of human fecal matter.
The San Juan River upstream of its confluence with the Animas was taken off the “non-attainment” list in 2012, Tomko said. But it could go back on the list of places that exceed E. coli levels if new tests show it exceeds water-quality standards, he said.
The Animas River is complicated because it flows through three political jurisdictions – Colorado, the Southern Ute Indian Tribe reservation and New Mexico – and numerous land uses, Ann Oliver, a spokeswoman for Animas Watershed Partnership, said.
The three political jurisdictions answer to different regions of the Environmental Protection Agency, she said.
“While preliminary data indicates a link between high E. coli from human sources in the Animas in New Mexico, this has not been documented by any study of the Animas near the state line,” Oliver said.