Louisville and the state’s Bluegrass region are among the likely sources of pollution runoff that have marked Kentucky as one of the top contributors to the Gulf of Mexico’s oxygen-depleted “dead zone,” according to a new federal study.

Building on work released last year that placed Kentucky and Indiana among nine states contributing 75 percent of excess nutrients into the Gulf, a new report by the U.S. Geological Survey identifies watersheds that are most likely to blame.

Areas that drain into the Salt River, the lower Kentucky River — and even Beargrass Creek — are likely among the top 150 contributing to the dead zone, according to the USGS study.

In the Gulf, an overabundance of nutrients has led to an oxygen-depleted area that has grown to the size of New Jersey. Fish and other aquatic life suffocate if they can’t reach better water, threatening the valuable Gulf fishery that supplies many restaurants and kitchens.

The new computer modeling blames sewage that doesn’t get fully treated, lawn fertilizers and runoff tainted by agricultural manure — and which also accounts for the nutrients and phosphorus that are staying in Kentucky and Indiana waterways and can cause water quality problems locally.

Environmentalists said the study points to the need for limits on the levels of nutrients allowed in waterways.

“Until you have a goal in mind, it’s pretty difficult to work toward solving the problem,” said Judy Peterson, executive director of the Kentucky Waterways Alliance.

Developing standards

Regardless of the USGS findings, officials in Kentucky and Indiana and at the multistate Ohio River Valley Water Sanitation Commission said their offices were developing such standards, and it’s uncertain what impact any new rules might have on businesses, cities or farms. Officials also said they are uncertain how the Obama administration plans to address the “dead zone” issues.

Peter Goodmann, assistant director of the Kentucky Division of Water, said Kentucky officials understand they need to better curb the flow of nutrients into waterways — and not just to help solve a problem several states away.

“We are a little more focused on resolving our own water quality issues in Kentucky,” he said. “If we do that appropriately, we will begin to resolve the issues in the Gulf, too.”

Brian Bingham, a senior engineer with Metropolitan Sewer District, said Louisvillians can expect to hear more from MSD in the coming years on how they can help improve water quality.

MSD expects the state will impose more stringent storm-water standards that curb polluted runoff, Bingham said. And residents likely can expect a public education campaign aimed at the dangers of too much fertilizer on their lawns and not picking up their pets’ waste, Bingham said.

“We all know we are part of the problem,” he said.

Pollution controls

The study, published in this month’s Journal of the American Water Resources Association, is intended to identify areas within the Mississippi River Basin where additional water pollution controls could be most effective, said its lead author, Dale M. Robertson.

The basin extends from Montana to New York, funneling water south to Louisiana in such major rivers as the Missouri and Ohio.

Robertson, who works in the USGS office in Middleton, Wisc., said Louisville and north central Kentucky “ranked fairly high” compared to others for phosphorus, which along with nitrogen has been largely blamed for causing the Gulf dead zone.

The study concluded there’s a 75 percent certainty the region is in the top 150 watersheds for phosphorus, he said.

While the study determined the Louisville area and most of Kentucky did not fall into the likely top 150 watersheds for nitrogen, parts of Western Kentucky and swaths of Indiana, Illinois, Ohio and Iowa were identified as likely nitrogen sources.

Chicago, the biggest city in the 31-state Mississippi River basin, ranked worst for both nutrients.

Whether in the Gulf of Mexico, or Beargrass Creek, too many nutrients can set off rampant growth of algae. When the algae dies and decomposes, it creates a condition called hypoxia that can suffocate aquatic life. Certain types of algal blooms can leave an odor and taste that drinking water utilities try to remove. The USGS study considers which watersheds are most likely to pass the nutrients on to the Gulf or keep them closer to their source.

Once they get into a large river like the Ohio, “it’s pretty much a straight pipeline,” said Gregory E. Schwarz, a study co-author.

But Robertson said the study leaves room for uncertainty.

For example, Robertson said the modeling was based on general assumptions of phosphorus and nitrogen coming from urban areas, and did not rely on actual discharge reports from facilities such as wastewater treatment plants. And Bingham, the MSD engineer, said some MSD wastewater treatment plants have already begun to limit phosphorus discharges.

State officials said most in Kentucky don’t limit phosphorus, but may need to in the future.

The study also didn’t consider that limestone rock underlying the Bluegrass region more readily releases phosphorus into waterways, Robertson acknowledged, adding that researchers are working on more refined follow-up studies.

Identify watersheds

Nevertheless, he said the report should help state and federal officials throughout the Mississippi River Basin better identify those watersheds that might need greater attention.

Indiana’s water quality chief said he thought the report could be misleading. Bruno Pigott, the assistant commissioner for Indiana’s Office of Water Quality, said its uncertainties mean it’s premature to draw any conclusions about the areas contributing the most nutrients.

Robertson noted that even Chicago, with its high ranking, was found to be responsible for only about 0.5 percent of the excess nutrients getting into the Gulf.

“The final result is saying you have to do a lot of work throughout the basin,” Robertson said.