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Article published Jan 29, 2006 in The Fort Collins Coloradoan
Horsetooth heads to impaired list
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Horsetooth Reservoir could wind up on the state's list of the most impaired waters because of threats to fish and other aquatic life.

The U.S. Environmental Protection Agency says Horsetooth, Fort Collins' primary drinking water supply, should be placed on the list - called the 303d list - because levels of dissolved oxygen in the reservoir have failed to meet standards for aquatic life.

Dissolved oxygen, the amount of oxygen dissolved in water, is essential for aquatic life. When it gets too low, fish die or their populations struggle.

The EPA had also pursued the listing based on what it said were 21 failures in the last seven years by the reservoir - based on data provided by the city of Fort Collins - to meet state drinking water standards, but an EPA official said Friday the agency will drop that argument for now.

State water quality leaders say Horsetooth should be monitored for two more years because they don't have enough data to justify a spot on the 303d list, the same recommendation they made in 2004.

The Northern Colorado Water Conservancy District, which operates the reservoir for the U.S. Bureau of Reclamation, says Horsetooth doesn't belong on the list because low dissolved oxygen, or D.O., is only a problem in some parts of the reservoir.

"Low D.O., if in a reservoir at all depths, is a serious issue," said Don Carlson, assistant manager of the NCWCD.

But Carlson said dissolved oxygen levels at Horsetooth are "only a problem at a certain depth."

The state's Water Quality Control Commission will hear the case Feb. 13 in Denver.

Are fish threatened?
That depth is a transition zone about 30 feet below the surface that forms in the late summer and early fall. As the water warms through the summer, the less dense warm water and more dense cold water separate. The transition zone in the middle contains much of the reservoir's aquatic life, which needs dissolved oxygen to survive.

One question the state and EPA are debating is how often dissolved oxygen in that zone has fallen below the federal standard. Another is how often the reservoir can fail to meet the standard before it's considered a violation.

State regulations say "standards for dissolved oxygen are 1-day minima, unless specified otherwise. For the purposes of permitting, dissolved oxygen may be modeled for average conditions of temperature and flow for the worst case time period."

State and federal officials say that's unclear; but the EPA says that, even by calculating various averages, Horsetooth doesn't meet dissolved oxygen standards for aquatic life.

Fish in Horsetooth haven't suffered from the low dissolved oxygen levels because they're able to swim to depths with better levels, said Ken Kehmeier, an aquatic biologist for the Colorado Division of Wildlife and the fisheries manager for Horsetooth.

"From a fisheries standpoint, water quality is not an issue," Kehmeier said, adding there's a difference between "what water quality (managers) like to see and what I like to see."

The state aquatic life standard for dissolved oxygen in the reservoir is 7 milligrams per liter during spawning seasons and 6 milligrams per liter at other times. Dissolved oxygen levels at Horsetooth have often been lower than that in late summer and fall.

But Kehmeier said he doesn't "start to worry" until levels drop below 5 milligrams per liter. Even then, fish survive - they just don't thrive like the otherwise might. He said die-offs don't occur until levels drop below 2 milligrams per liter.

"I have water by Walden in the threes and fours and trout survive," Kehmeier said. "They don't prosper, but they survive."

Is drinking water threatened?
But another question is how often dissolved oxygen levels at lower depths near the Soldier Canyon Dam - where the city of Fort Collins draws water for its drinking water treatment plant - have violated state standards. The EPA says it's happened 21 times in the last seven years.

Brian Janonis, Fort Collins water utility's water resources and treatment department manager, first said the utility "is not all at all that concerned about water quality" but later said it's taking a wait-and-see approach, noting that a 303d listing "would be a red flag for us."

"We are concerned about it," Janonis said. "But we're waiting on all these studies by the experts to tell us what this means and what's really happening."

Janonis said the utility doesn't question the data but hasn't taken an official position on the EPA's recommendation.

Prolonged low levels of dissolved oxygen near the bottom of the reservoir can cause the release of manganese from nutrients in the water. High levels of manganese can be difficult to treat out of drinking water but are seen primarily as an aesthetic problem.

Fort Collins residents in September 1989 turned on their taps to find water with an orange hue, the result of elevated manganese. The city later added steps to the treatment process to solve the manganese problem.

Low dissolved oxygen levels are the result of high nutrient levels. Those nutrients, including phosphorus and nitrogen, trigger algae growth and drive up total organic carbon in the water, not all of which can be eliminated by the city's drinking water treatment facility.

The remaining TOC, when mixed with the chlorine used to treat drinking water, produces chloroform, a known carcinogen tightly regulated in drinking water by the EPA. City officials say TOC is not a problem for the city's water supply.

Nitrogen and phosphorus enter the Colorado-Big Thompson, of CB-T, system and Horsetooth through wastewater discharges, urban development, rain (which carries nitrogen) and C-BT's own pipelines, where algae grows.

Controlling phosphorus at sites that discharge water into the C-BT system is one way to reduce nutrients in the reservoir and raise dissolved oxygen levels.

If Horsetooth goes on the 303d list, it could jumpstart a process to limit the amount of phosphorus that upstream dischargers can release daily. That could mean expensive new equipment for NCWCD and upstream treatment systems.

A similar situation unfolded in the 1970s and '80s at Dillon Reservoir, which serves Denver Water, leading to state-imposed daily discharge limits for phosphorus into the reservoir.

Since guidelines were unclear, NCWCD's Carlson said EPA "switched the rules in the middle a little bit" when it proposed Horsetooth for the 303d list.

"The criteria question has more to do with how many violations and at what depth those need to occur to have impaired water," Carlson said.

The district is in the first of a three-year study of nutrients in the Colorado-Big Thompson Project, which includes Horsetooth Reservoir. The C-BT Project brings water from the state's Western Slope to the Front Range, where it's used for drinking, irrigation and industry.

"We hope to learn a lot in a few short years," Carlson said.

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Discussion Starter · #3 ·
I agree with you completely that Horsetooth is primarily an artificial body of water used primarily for drinking water, however, so doesn't really warrant as much attention for failing to meet fish standards.

However, if you poor money into creating a fishery to further justify a reservoir as a recreation center, you should be aware of how the DO in the res affects the growth and survival of the fish if you want to manage the recreational fishery wisely. Lakes or reservoirs like Horsetooth don't end up just having low Dissolved Oxygen (DO) in the hypolymnion (below the thermocline), and plenty above it. During the daytime, oxygen is plentiful above the thermocline while the algea photosynthesizes (using CO2 and releasing O2). During night, while no photosynthesis occurs, the algae in the upper strata of the lake respire and in many cases can use up most of the O2, leaving the DO levels even in the upper strata low as standards for aquatic life go, and really putting the sqeeze on fish and other aquatic life that contribute to the value of the lake as something more than just a drinking water resource.
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