When a drop of water falls onto the watershed, it often is only beginning a long journey to the Chesapeake Bay—one that may not be completed for years, or decades.
If the water runs off the surface and into a stream, it may end up in the Bay in a matter of hours or days.
Just slightly more than half of the water that ends up in the Chesapeake, though, takes a much longer route. It sinks into the soil and then slowly moves toward rivers and streams through groundwater, a journey that can take anywhere from a matter of weeks, to many decades, to complete.
Which path the water takes has huge implications for the Bay cleanup. Water readily absorbs nitrogen, one of the key nutrients affecting the Chesapeake. The more water—and nitrogen—that takes the slow-moving groundwater route, the longer it will take to clean up the Chesapeake.
That’s because even when nitrogen use is slashed on the soil above, large amounts of the nutrient from past actions are already in the groundwater, where they will slowly bleed into the Bay’s tributaries for years to come.
While scientists have long understood there is a “lag time” between taking some actions and seeing results, new research by the U.S. Geological Survey has begun to spell out the implications in more detail.
In the past, the USGS estimated that the lag time for groundwater, and the nutrients it carries, was about a decade.
New research shows that much of the groundwater moves through the system faster. A quarter of all groundwater is flushed out within 7 years—which means many nutrient control actions launched in the mid-1990s should be reflected in reduced nitrogen concentrations.
“Within a decade, and in some cases sooner, we should be seeing improvements in the stream if conservation actions have been placed on the landscape,” said Scott Phillips, who oversees Chesapeake-related activities for the USGS. “I think we are starting to see some of these reductions taking effect.”
By 10 years, according to the USGS’ new estimates, about half of all the groundwater has moved through the system, and three-quarters is flushed out in 13 years.
But the remaining quarter of the groundwater takes more than 13 years to reach surface streams, and a portion takes many decades, according to the USGS.
Because of the lengthy lag times, it is possible that millions of pounds of nitrogen reduction actions that must be made in the next few years to clean up the Bay will not be felt in the Chesapeake until sometime after the Bay Program’s 2010 cleanup goal.
While the situation would improve annually as the old nitrogen gradually bled out of the system, it would likely take years for some parts of the Bay to reach their water quality standards.
Right now, there’s no strategy about what to do about the millions of pounds of nitrogen that could be in the groundwater “pipeline” for decades.
“I think we are going to need to have that discussion,” said Rich Batiuk, associate director for science with the EPA’s Bay Program Office. “People aren’t going to be that patient.”
Among the possible options, Phillips said, would be reducing nitrogen in discharges from wastewater treatment plants and industries. Because those discharges go straight into rivers and streams—not through groundwater—there is no lag time.
But because those sources account for only 20 percent of all the nitrogen reaching the Bay, focusing on them alone would not achieve the cleanup objectives.
Another option would be overachieving nutrient reductions from nonpoint—or land runoff—sources to compensate for those that may not be seen for years.
“With these lag times, if you really want to make a certain date, it means you have to accelerate the nonpoint source reductions,” Phillips said. “That’s what it comes down to.”
The USGS made its analysis by sampling the age of water coming out of springs throughout the watershed.
Water was aged by examining the concentrations of chlorofluorocarbons in the samples. Since the 1940s, different levels of CFCs have entered the atmosphere each year. That concentration is reflected in the rain which absorbs the chemical, inadvertently giving scientists a way to age the water.
The groundwater tested ranged from less than a year to 60 years in age.
The studies have shown no significant difference in the amount of time it takes water to move through different geological regions. “That was a surprise,” Phillips said. “We thought going into this that the coastal plain residence times would be distinct from the Piedmont or the Valley and Ridge. We didn’t find that.”
The USGS studies do show that, as a rule of thumb, the closer a drop of water falls to a stream, the shorter its journey through groundwater. The farther upslope it falls, the longer its path.
The studies also show that slightly more nitrogen enters streams through surface runoff than through groundwater, but it’s not a huge difference. About 54 percent of the water in streams in the watershed comes from groundwater, and about 48 percent of the nitrogen—in the form of nitrate—passes through groundwater, according to the USGS.
(While the groundwater lag time is a concern for nitrogen, it is not for phosphorus, which is more likely to bind to particles. But phosphorus can also take years, or decades, to reach the Bay as sediment slowly works its way down streams.)
The good news is that some portion of the oldest nitrogen in the groundwater will be denitrified before it reaches the stream. The studies showed that in some, but not all watersheds, the slowest moving water gradually becomes depleted of oxygen. As that happens, microbes begin producing oxygen from the nitrate in the water.
“You can see up to 100 percent denitrification in the older water,” said Bruce Lindsey, a USGS hydrologist. But that doesn’t happen everywhere, he said. In some areas, extensive denitrification may take place in groundwater more than 20 years old, but in others there is no denitrification at all.
For instance, groundwater in limestone areas that underlay agricultural areas in many of the region’s valleys usually has plenty of oxygen and therefore little denitrification takes place, Lindsey said. Fine tuning exactly where denitrification will take place, he said, is something that needs more study.
The bottom line, Lindsey said, is it will take patience to see the full effect of nutrient reduction efforts after they are implemented.
“Eventually, we should be showing a great deal of benefit,” he said. “To look at it in the short term and say we don’t see a response is not a reason to back off from anything. You have to stick with this for a long time.”
The USGS has produced a fact sheet, “The Influence of Ground Water on Nitrogen Delivery to the Chesapeake Bay,” which is available on the USGS Chesapeake Bay web site, http://chesapeake.usgs.gov