A new study shows some Chesapeake Bay rivers have gotten cleaner over the past three decades, while others are getting worse.
The analysis, released Wednesday by the U.S. Geological Survey, suggests costly upgrades of sewage plants have helped, scientists say, but it raises questions about the effectiveness of efforts to date to curb polluted runoff, particularly from farms on Maryland's Eastern Shore.
"We're going in the wrong direction in some places, and the right direction in others," said William Dennison, vice president for science applications of the University of Maryland Center for Environmental Science. He called the USGS analysis a breakthrough in tracking where the 27-year-old bay restoration effort is making progress — and where it's falling short.
Using a new statistical technique to factor out weather's variable impact on water quality, USGS researchers say sampling of the bay's rivers indicates that nutrient pollution has declined in the Patuxent and Potomac since 1978, and in the Susquehanna as well, though there are signs of backsliding there in the past decade.
Pollution levels have either increased or remained unchanged in Virginia's major rivers, the study says. And in Maryland's Choptank, levels of nitrogen — one of the two main nutrients fouling the bay — have increased 53 percent, with no sign of letting up.
The USGS calculations "confirm the success we're having with sewage upgrades and the big challenges we're having with diffuse runoff," said Dennison, whose UM scientists draw up annual "report cards" on the health of the bay and its tributaries. That's particularly true in the Patuxent and Potomac, he noted, where municipal sewage treatment plants have been overhauled at great expense in the past three decades to remove more phosphorus and nitrogen from wastewater before it gets discharged into the rivers.
Dennison said he was concerned to see levels of phosphorus in the Susquehanna have started to climb again in the past decade. The Susquehanna furnishes half of all the fresh water getting into the bay and a major portion of its nutrient pollution as well.
Nitrogen and phosphorus — from human and animal waste, chemical fertilizer and fossil-fuel burning power plants and motor vehicles — are the main nutrients responsible for fouling the bay's waters. They trigger massive growth of algae in the water, which then deplete the oxygen that fish need to breathe, creating a large "dead zone" in the bay every summer.
USGS scientists crunched numbers from hundreds of water samples taken from 1978 to 2008 on nine major rivers feeding into the bay. While those results have long been available, researchers say it's been hard to spot the trends in them because wet or dry weather can cause wide swings in water quality, as rainfall and snow melt tends to wash more nutrient pollution into rivers.
The new analysis allows researchers "to get a much clearer picture of the effect of human activities," according to Robert Hirsch, the USGS research hydrologist who led the effort.
Now, Hirsch said, "I think we're seeing a somewhat different picture than we've seen before."
The USGS analysis shows significant improvements in water quality in rivers where sewage plants were a major source of pollution. But little or no progress is apparent in rivers where nutrients more likely washed off the land or seeped into ground water from fertilized farm fields or household septic systems.
On the Choptank, the Shore's largest bay tributary, farming remains the dominant land use, and many homes are on septic systems. USGS sampling indicates much of the increase there is seeping into the river from ground water.
Ground water is so slow-moving that nitrogen that seeps into it from the land's surface can take years or even decades to find its way into the river. Some have suggested that pollution-control efforts won't show results there right away. But Hirsch said the lack of any easing of the increase in nitrogen levels measured up through 2008 suggests that whatever's been done to try to limit the loss of fertilizer from farm fields hasn't been particularly effective.
"At least in this one watershed, we would expect, if things were really improving at the land surface, to see it begin to come out, and it's not," the USGS scientist said.
"This is very troubling for the restoration effort on the Eastern Shore," said Dennison. The septic [systems] plus agriculture, we're just not getting a handle on that. We're not getting the progress we should."
Others say it's too soon to judge farm pollution control efforts to date on the Shore a failure.
"If we hadn't done what we've done, the trends may have been higher," said Russell B. Brinsfield, director of the Maryland agricultural experiment station at Wye.
Brinsfield said he was not surprised to see that nitrogen levels in the Choptank are not going down, and there's no clear trend on phosphorus, either.