Trout are among the most highly prized of freshwater fish; their presence in a stream is a sign that the water is clean, cold and rich in all the things fish need to survive, grow and reproduce.

So, perhaps it’s no surprise that these pollution-sensitive fish are at the center of a debate in Maryland about how best to sustain them amid the sprawling development that threatens their survival in the central part of the state.

Carroll County plans to upgrade an aging, poorly performing sewage treatment plant serving the town of Hampstead in the northwestern suburbs of Baltimore. In an effort to reduce pollution to Piney Run, a trout stream into which the plant discharges, the county wants to split the wastewater flow and pipe a portion over to another stream.

But the other stream, Deep Run, also has trout. Now there’s a dispute over how much protection each stream should receive.

Anglers, environmental groups and at least one streamside landowner are voicing concern about the state Department of the Environment’s tentative decision to permit the Hampstead plant to discharge into both streams. They say they’re worried that the treated wastewater, particularly its temperature, may make the streams untenable for the brown trout found in each.

“See this big mayfly (nymph)?” asked Theaux Le Gardeur, the Gunpowder Riverkeeper, as he picked up a rock from a clear, fast-flowing stretch of Deep Run. A dark, cricket-like bug — choice trout food — clung to the bottom. “If that water warms up, he’s not going to be here.”

County officials counter that their plan poses no threat to the trout, and that both streams’ water quality should improve as a result.

“We have voluntarily agreed to do some actions we think will help mitigate any temperature concerns people have,” said Tom Devilbiss, Carroll’s director of land and resource management.

The Hampstead wastewater plant discharges about 550,000 gallons of treated sewage daily into Piney Run, a branch of the Gunpowder River watershed that ultimately flows into Loch Raven reservoir, part of the Baltimore area’s drinking water supply.

The plant has violated the pollution limits in its discharge permit in 11 of the last 12 quarters, according to the U.S. Environmental Protection Agency’s ECHO compliance database. Patrick DeArmey, a lawyer for the Gunpowder Riverkeeper, one of the groups opposing the county’s plan, said the plant racked up 50 violations last year alone. Over at least the last three years, he added, it has been discharging excessive phosphorous, nitrogen and total suspended solids, among other pollutants.

County officials say the pollution violations should go away under a planned $16 million overhaul that would install state-of-the-art, enhanced nutrient-removal technology at the plant. The state is financing half of the upgrade cost, with county ratepayers covering the rest.

But the temperature of the plant’s treated wastewater discharge has been a bone of contention — and litigation — for years. Piney Run is officially classified as a trout stream, meaning that under state water quality standards, no discharge into it should raise the water temperature above 68 degrees Fahrenheit. That’s the maximum that native Eastern brook trout can tolerate. Brown trout, the fish found in both Piney and Deep runs, are a nonnative species introduced to state waters more than a century ago; they can handle somewhat warmer temperatures, but not much.

Citing a consultant’s study, county officials have argued that the plant’s discharge temperature isn’t harming the trout in Piney Run, and that runoff from development makes the water warmer in summer anyway. (The springs that are Piney Run’s source begin beneath a parking lot.) Under a 2006 consent judgment with the MDE, the plant has been allowed to discharge effluent warmed by the sun to more than 68 degrees, though it was also required to restore tree cover along the stream.

But now, seeking to accommodate Hampstead’s anticipated population growth, county officials say they want to avoid any further issues with temperature in Piney Run and split the discharge as part of the upgrade project. They propose to pipe treated effluent 8,900 feet to an existing outfall on the Deep Run tributary, which is now being used by an industrial park. In turn, the county will take over treatment of the business facility’s waste, piping it to the Hampstead municipal plant for enhanced processing.

For the wastewater facility’s discharge into Piney Run, the MDE has proposed an alternate temperature limit. Its effluent there cannot exceed 75.9 degrees F at any time, regulators say. Nor could it be warmer than 75.2 degrees for more than three consecutive days, they add, or for any five days in a month.

Trout Unlimited and environmental advocates say they’re not convinced the alternate temperature limit for Piney Run is protective enough. Like nearly all streams in Maryland, they say the water warms up to what’s likely an intolerable level for trout in the dog days of summer. But the fish apparently can find enough deep pools or shady overhangs to ride out the heat wave. Not doing anything to ease the heat risks their continued survival, they contend.

Last year, the Hampstead plant’s discharge twice exceeded the MDE’s proposed limit — topping 76 degrees Fahrenheit one day in July and nearly hitting 77 degrees in August, according to EPA data. Trout are likely to die outright if exposed to 80-degree water for any length of time, experts say, but they stop feeding and growing at temperatures well below that.

For Deep Run, meanwhile, the state is not proposing any temperature limit on the plant’s discharge. That’s because it is not officially classified as a trout stream, though the state’s Department of Natural Resources has recommended it. Instead, it is designated for recreational use, a stream category where the maximum allowable temperature is 90 F.

“This is foreclosing the future of Deep Run,” warned Jim Gracie, an Annapolis environmental consultant and longtime member and former officer of Trout Unlimited. State biologists first spotted brown trout near the stream’s mouth almost a decade ago. But lately, anglers say, they’ve been seen much farther upstream.

Maryland has just 800 miles of self-sustaining trout streams, Gracie noted, a fraction of the 14,000 miles the state once had. “So, we’ve already greatly decimated this resource,” he told state regulators at a hearing on the discharge permits earlier this year. “It’s very scarce, and to us, very precious.”

MDE officials said they can’t change Deep Run’s classification just because trout are there, because the water is already too warm to meet the 68-degree temperature standard. Though advocates point out that the state has reclassified other streams in the past when trout showed up, MDE officials say that they’ve since been apprised of EPA guidance against doing that.

County officials said that although they’re not required to, they’re planning to take steps to cool off the Hampstead plant’s wastewater before it gets discharged into Deep Run. The pipe carrying the effluent will be buried underground, to shield it from the sun’s warming rays. And near the outfall, the wastewater will be mixed with cooler groundwater that’s been pumped up and treated to remove contaminants beneath the industrial park.

Those measures, according to county calculations recently presented to the MDE, should result in a discharge temperature of around 68 F. But county officials say they don’t want to be held to that in an enforceable permit. State regulators say they’re confident the underground piping and mixing with groundwater should cool off the wastewater before it’s discharged into Deep Run.

“Our goal has been to find the best solution to protect the environment, the citizens and the feasibility for taxpayers,” said Ed Stone, the MDE’s chief of wastewater permits.

But even if it doesn’t, officials said that if trout in Piney Run can survive the plant’s discharge now, the Deep Run fish can, too.

“I view that we’re protecting them,” said Lee Currey, the MDE’s acting water management administrator. “We have a fairly detailed study — a natural experiment in Piney Run — that showed we’re supporting that trout population.”

Art Senkel, resource vice chair of Trout Unlimited’s Mid-Atlantic Council, takes little assurance from that. He said he’s seen no studies to back up the county’s temperature-reduction claims. And Deep Run is a more degraded stream than Piney Run, he contended, meaning the fish may already be stressed, so there’s less margin for error in maintaining suitable water quality.

“Temperature is definitely an issue,” said Raymond Morgan, an aquatic ecologist with the Appalachian Laboratory of the University of Maryland Center for Environmental Science. While 68 F is the state’s standard for coldwater trout streams, Morgan said, some research is suggesting that at that temperature or even less, trout stop growing.

Morgan also suggested regulators should be careful about exposing trout to domestic wastewater because of pharmaceuticals and chemicals in personal care products that aren’t fully removed by the treatment process. Some of those compounds mimic hormones and can disrupt fish growth and reproduction, he said.

MDE spokesman Jay Apperson said that neither the state “nor EPA has criteria or data for potentially endocrine disrupting compounds.” A final decision on the permits is pending.

Dave Hudgins and family have lived for more than 30 years in a log home along a woodsy stretch of Deep Run near Hampstead. He’s an avid birder, not a fish expert, he notes, but he’s still concerned about the impact of the discharge. He said anglers have spotted brown trout in his stretch of the stream.

“I know that the temperature does make a difference,” he said during a public hearing in April on the plant permits. “I’m a pretty simple guy. I know what the temperature does when our fish tank temperature goes up and down, and we have fish floating.”