The speed at which the Bay responds to cleanup efforts may have more to do with the trajectory of a falling drop of water than by actions being taken on the ground today, new research suggests.
That drop may land and roll downhill until it, and anything it has picked up on the way, splashes into the nearest stream and begins its journey oward the Chesapeake. It will reach the Bay in a few days.
But suppose that instead of flowing down to the stream, the drop soaks into the ground. Eventually, it reaches the groundwater, at which point it begins - slowly - to work its way toward a stream, pond or wetland.
Someday, it - along with chemicals it has picked up along the way - will reach the Bay. But instead of a matter of days, the journey may take decades.
The time from which a water drop hits the ground to the time it works its way through the groundwater and to the Bay is called "lag time." And, at least as far as nitrogen is concerned, the lag time is dampening expectations for a speedy Bay improvement.
A pair of recent studies from the U.S. Geological Survey's Chesapeake Bay Ecosystem Program show that about half of the water flowing into the Bay originates from groundwater, which carries about half of the nitrogen that enters the Chesapeake.
The studies found that most groundwater appears to take an average of 10 to 20 years to work through soils and aquifers and into waterways. That means the groundwater entering streams today is actually carrying nitrogen it absorbed when rain soaked the groundwater table in the late 1970s and early 1980s - long before anyone emphasized nutrient reductions. Likewise, nutrient reduction efforts taking place today will help - but their full effects may not be realized until after 2010.
"This gives an indication as to why we may not see the rivers or the Bay come back as quickly as we hope," said Scott Phillips, chief of the USGS's Chesapeake effort.
Groundwater lag time is a problem that mainly affects nitrogen. Phosphorus, which is more likely to bind with soil particles, doesn't go through groundwater in significant concentrations. Management practices that control sediment runoff can help keep phosphorus bound up - and out of the water - for decades, if not centuries.
Nitrogen, though, is more soluble and is easily carried away either through surface water runoff, or through the groundwater.
The Bay states have been working since 1987 to reduce the amount of nitrogen and phosphorus entering the Bay. Those two nutrients degrade water quality by spurring algae blooms that block sunlight to critical underwater plants, and - when the algae decompose - deplete the water of oxygen needed by other organisms.
In the last decade, a variety of actions have been taken to reduce the amount of nutrients used in the watershed and to employ techniques that control runoff or help soak up excess nutrients, such as planting forest buffers along streams or winter cover crops on barren farm land.
But because of the lag time issue, scientists say that even if the Bay states enact enough control measures to meet their nutrient reduction goals, the full effect may not be realized in 2000.
In one of the recent USGS studies, scientists examined the "base flow" at 276 sampling sites in the Bay watershed. A stream's base flow is what originates from groundwater - it's what is still flowing in the stream during long dry spells. The scientists found that in a typical year about 54 percent of the streamflow originates from base flow - or groundwater. (In rainy years with high runoff, a greater proportion would come from surface flow.)
Further, the study determined that base flow carries more than half - about 56 percent - of the nitrate (the main form of nitrogen in runoff) that enters the Bay from the watershed.
In the second study, scientists examined existing groundwater data and collected samples from 46 springs throughout the watershed to determine the age of the groundwater flowing out.
Because water itself is ageless, age has to be determined by looking for some trace compound the water picked up in the atmosphere. "Once the water is through the soil and into the groundwater, this atmospheric signature is sort of frozen," Phillips said. Water samples in the study were "aged" by dating the chlorofluorocarbons - the same chemicals which destroy Earth's ozone layer - they carried. Different types of CFCs were used - and therefore entered the atmosphere - at different times, and they have different decay rates. By determining the types of CFCs in the water, and how far they have decayed, scientists can determine what they consider the "apparent" age of the groundwater.
Of 46 samples, they determined the apparent age of water from 33 springs was less than 20 years, including six springs that had "modern" water less than 2 years old. Four samples had apparent ages of 21 to 33 years, and two others from thermal springs had water more than 43 years old. The remaining seven samples could not be dated accurately.
Nitrate concentrations in water from many springs were similar to concentrations found under fertilized fields - an indication that the groundwater was moving nitrogen from distant sources to its point of discharge. The study also found that proximity to the Bay has little relationship to how long it takes groundwater to reach the Chesapeake. Groundwater from distant mountains - such as the Blue Ridge and the Alleghenies in Central and Northern Pennsylvania - tended to be "younger" than groundwater from the Delmarva Peninsula and the Piedmont.
Phillips attributes this to the fracturing of the rocks in mountainous areas - and the force of gravity on the steep slopes - which allow the water to flow more quickly through the ground. In general, areas with coarse soils or steep slopes pass groundwater more quickly than areas with fine grain soils or flatter topography.
But the news is not all bad.
With further analysis, Phillips said, USGS scientists hope to more precisely determine the groundwater lag times for different geologic settings. That would help managers target nutrient control efforts toward areas where they may see results in a matter of years, rather than decades.
In addition, the work showed that some soil types, under the right conditions, can remove nitrogen through natural biological processes. If those areas can be more precisely identified, it could improve decision making. For example, efforts to restore forest buffers - which can be highly effective at removing nitrogen from very shallow groundwater in certain settings - may be given lower priority in areas where soils are already doing that job.
"When we look where to target buffers, it would be nice to get a good idea of the amount of nutrients from both groundwater and overland runoff," Phillips said.
In addition, Phillips said areas faced with a "constant bleeding" of nitrogen into rivers from groundwater for the foreseeable future may be targeted for enhanced nitrogen-control efforts at wastewater treatment plants and industries to help compensate for the nitrogen from groundwater.
Because those discharges go directly into rivers and streams, there is no groundwater lag time and nutrient reductions to the Bay occur almost immediately. About 20 percent of the nitrogen that enters the Bay originates from such "point source" discharges.
"The Bay's Recovery: How Long Will It Take?" is a new fact sheet from the Alliance for the Chesapeake Bay and the U.S. Geological Survey that explains various types of "lag time," including groundwater, sediment transport and recovery of living resources. Copies are available from the Alliance's Chesapeake Regional Information Service Hotline, 1-800-662-CRIS.
To obtain copies of the following reports, contact the U.S. Geological Survey at 410-238-4202.
"Preliminary Estimates of Residence Times and Apparent Ages of Ground Water in the Chesapeake Bay Watershed and Water-Quality Data from a Survey of Springs," by Michael J. Focazio, L. Joseph Bachman, Johnkarl F. Bohike, Eurybiades Busenberg, L. Niel Plummer, and David S. Powars.
"Relation of Hydrogeomorphic Setting, Ground Water Discharge, and Base-Flow Nitrate Loads from Non-Tidal Streams in the Chesapeake Bay Watershed, Middle Atlantic Coast," by L. Joseph Bachman, Bruce Lindsey, John Brakebill, and David Powars.
- Groundwater is stored primarily in aquifers - geologic zones of unconsolidated rock, sand or gravel which have tiny spaces between the sediment particles. Contrary to common belief, it does not form underground rivers, lakes or oceans (except in some rare cave environments). Layers of aquifers can be separated from each other by layers of clay or other dense materials called "confining units." The aquifer above the uppermost confining unit is known as the water table aquifer. Deeper aquifers are known as confined aquifers.
- In general, groundwater moves very slowly. In formations containing layers of consolidated clay with little fracturing, groundwater may move as slowly as a few inches per year. In strata containing unconsolidated sand and gravel, groundwater can move hundreds of feet or more a year. It can move rapidly through cavernous limestone formations.
- Aquifers are recharged (replenished) primarily by precipitation seeping through the ground. Water seeps to the aquifers and ultimately into low-lying areas where it discharges into wetlands, streams, agricultural ditches, ponds, bays or the ocean.
- Once contaminated, groundwater is difficult to clean because of the long period of time it takes for water to be flushed out. This is true not only for nitrogen, but also for other contaminants, such as pesticides or other toxics, which can continue to flow from groundwater to surface water decades after their use has been discontinued.