by Billy Evans and Monika Schneider
Illustration of the Chesapeake Bay (courtesy of the EPA)
Table of Contents
Introduction
The
Chesapeake Bay and its Watershed
Groundwater
Nitrate and its Sources (Eastern Shore
example)
How Groundwater
Nitrates enter the Bay
Effects
Nitrates have on the Bay
Detection,
Prevention, but no Remediation
Conclusion
References
Links to
Interesting and Pertinent Places
The Chesapeake Bay, historically known as one of the most impressive bodies of water in the world, serves many purposes and effects millions of people. It not only provides an economical seafood industry, shipping routes, and recreation, but also an ecological haven for numerous animals and plants. And it is this biotic integrity of the Bay that has been jeopardized over the past decade due to the influx of nutrients entering the watershed. These nutrients originating from natural and anthropogenic sources are heavily responsible for causing eutrophication and a decreased amount of dissolved oxygen in the Bay. Recent evidence shows that the groundwater in several shallow unconfined aquifers that are recharging the Bay are a vector for these nutrients (Reay and Simmons 1992), particularly the nitrogen based ones. However all is not lost, this process of harming the Bay can be slowed down by: monitoring the recharging groundwater, through regulation, and by incorporating best management processes (BMP's),
The Chesapeake
Bay and its watershed
The Chesapeake Bay and its watershed (as shown to
the left in a picture from the EPA) consist of an area covering
some 64,000 square miles in parts of six states; New York,
Pennsylvania, Delaware, Maryland, West Virginia, and Virginia and
Washington D. C. (Cronin 1971). This area encompasses many small
rural agricultural areas, such as Lancaster County, Va., as well
as huge urban economical areas, such as Baltimore, Md. However
the Chesapeake watershed consists mostly of forests (54%), as
compared to 36% for agriculture lands, and 7% for urban areas
(EPA 1988). Joining the land in the watershed are eight major
rivers and about 150 smaller surface waters, all of which flow
into the Bay (Reay and Simmons 1992). Along with the surface
waters there are a series of unconfined and confined aquifers
that recharge the Bay. These aquifers are made up of sand, silt,
clay, and shells and their confining units are silt-clays
(Vroblesky and Fleck 1991). The hydraulic conductivities of the
aquifers range from 3x10-4 to 3x10-8 m per day (Silka 1984). The
large area, the millions of people, the many agricultural
processes, the numerous industries, and all of the water make the
Chesapeake Bay and its watershed a prime place for pollution.
Groundwater
Nitrate and its Sources (Eastern Shore example)
The two pollutants, that make their way to the Bay via groundwater, receiving the most attention are organic-based pesticides and nitrogen-based nutrients (Reay and Simmons 1992). This page deals with the nitrogen, but if you want to know more about the pesticides present in the Bay, the USDA has an excellent site containing valuable information. Of the nitrogen-based nutrients the one that creates the most hysteria is the nitrate ion (NO3-. This highly mobile ion is naturally found in abundance in the atmosphere, in the soil, and in the water. It occurs in the atmosphere as a result of nitrogen oxides (NOx's) reacting with the water and particles in the air. In soils and water where oxygen is present nitrification does the job by converting ammonium (NH4+) to the short-lived nitrite (NO2-) and then to the nitrate ion.
This diagram from
the USGS shows the general routes in which nutrients, nitrates in
our case, can make their way through the environment. The four
most important ways (Zipper 1996) nitrates can make their way to
water bodies are:
The main anthropogenic source of nitrates into the groundwater is agricultural applications in rural areas (Simmons, Reay, Smedley, and Williams 1991). The dominating agents in the agricultural realm are the nitrogen-based fertilizers. They are made up of highly soluble components such as anhydrous ammonia, urea, and ammonium nitrate, that when applied to the land can experience the nitrification process (Hay 1981). Once the nitrate anion is formed it can easily move through either sandy or (negatively charged) clayey environments down to the shallow unconfined aquifers, and eventually to the Chesapeake Bay.
A prime example of a rural area that has had its unconfined aquifer effected by the anthropogenic source is the Eastern Shore of Virginia. The Eastern Shore is a small peninsula that is situated between the Atlantic Ocean and the Chesapeake Bay, and atop the sandy sediment laden Columbia unconfined aquifer (Vroblesky and Fleck 1991). In this confined area there are roughly 140,305 acres of farmland (Virginia Agriculture Statistics Service 1991). The result of this (caused by reasons mentioned above) is increased levels of nitrates in the aquifers' groundwater. Studies have shown that the Columbia aquifer has the highest concentrations of nitrates leaving it, 6.9 mg/L, than any of the other unconfined aquifers, which average around .94 mg/L, that empty into the Chesapeake Bay (Reay and Simmons 1992).
How Groundwater
Nitrates Enter the Bay
The two mechanisms in which the groundwater nitrates are
transported to the Bay are by advection and dispersion.
Advection, the main mechanism of transportation, is the bulk
movement of dissolved nitrates flowing with the groundwater from
the aquifer to the Chesapeake (Wood 1984). How much nitrate that
is inputted is dependent on how fast the groundwater is flowing
into the Bay. An aquifer containing nitrates, with a slow
velocity, will transport less of the ions into the Bay than a
high velocity aquifer containing the same amount of nitrates.
On a smaller level there is hydrodynamic dispersion, encompassing both molecular diffusion and mechanical dispersion. Molecular diffusion is "the thermally induced, random movement of solutes (nitrates) across a concentration gradient (fresh to salt water)", while mechanical dispersion is "caused by differential flow velocities (fresh to salt) and directions within sediment media" (Bear 1979). These processes occur in the zone that lies between the the freshwater and sea water where there the greatest mixture of the two waters occur and where the concentraion gradients are the greatest. How much nitrate that makes its way to the Bay via these two processes is mainly dependent on the concentrations that are present in the two clashing water bodies.
Effects Nitrates
have on the Bay
The primary effect that the groundwater carrying the increased amounts of nitrates has on the Chesapeake Bay is eutrophication. Eutrophication is the process by which a body of water becomes enriched in dissolved nutrients, in our case nitrates, and experiences an increase in aquatic plant life (Webster 1993). The effects of this can be seen as algal blooms floating on top of the water and increased amounts of submerged aquatic vegetation (SAV) on the bay floor. This increased aquatic plant life has an inverse effect on the amount of dissolved oxygen, and as a result aquatic organisms are diminishing. Thus the harming of the Bay not only brings about economical repercussions but ecological ones as well.
Detection,
Prevention, but no Remediation
The first step on the road to prevention is the detection of the nitrates in the groundwater. One popular technique used that can do this is the sampling of the water obtained from a seepage meter. A seepage meter consists of a metal rim that ranges in diameter, a fitted top with a bag connector atttached to it, and a detachable bag that will collect the water. The seepage meter is first manually placed in shallow water so that it is submerged (photo 1). Once in place (photo 2), surface water diffuses across the bag and groundwater enters the bag from the bottom. The bag eventually fills up (photo 3) and contained within it is a representative sample of the surrounding water that can be taken back to the lab and analyzed for nitrates.
Photos courtesy of Dr. George Simmons. VPI Departement of
Biology
photos 1-3
The most likely solution to the Chesapeake Bay's nitrate problem seems to be prevention and not remediation. Several things can be done, and are being done, directly and indirectly, in order to prevent the increasing amount of nitrates from entering the groundwater systems. The first indirect step came in 1987 with the signing of the Chesapeake Bay Agreement by Virginia, Maryland, Pennsylvania, Washington D. C., and the EPA. This agreement's objective was and still is to reduce the 1985 controllable nitrogen and phosphorous loads to the Chesapeake Bay by 40 % by the year 2000 (Shuyler 1993). Then in 1988 Virginia passed the Chesapeake Bay Preservation Act. The goal of which is to establish a cooperative program between state and local government aimed at reducing nonpoint source pollution (CBLAD homepage 1997). However these are only acts, and therefore people cannot be forced to comply.
Directly the most common measure employed today is the
application of best management practices (BMP's). BMP's are
procedures and land use practices that minimize nutrient inputs
into adjacent waters (Reay and Simmons 1992). Two procedures that
have been proven to decrease the amount of nitrates entering the
groundwater are fertilizer management and the planting of winter
cover crops (Brinsfield and Staver 1989). The goal of fertilizer
management is to lower application rates and to time applications
in order to optimize crop uptake. 
The winter
crops (picture: courtesy of the EPA)
are planted in order to provide a continuous source of nitrogen
uptake throughout the year. Although these practices are proven
to work there is little motivation for farmers to practice them,
especially if, "A little extra nitrogen may increase crop
yields in a good rainfall year by 10 to 20 percent."
(Russell and Shogren 1991).
The journey that nitrates follow in going from the land to the Bay is not a straight one. There are many processes that can occur over the vast watershed that can effect how the nutrients are transported to the Bay. And yes they do make it to the Bay fore there is evidence; eutrophication and decreased numbers of aquatic organisms. However throughout all of this one thing remains constant: a vast majority of the nitrates are transported by groundwater. This has been recognized and the wheels have been put in motion in order to save the Chesapeake Bay, the grandest estuary in the world.
Bear, J. 1979. Hydraulics of Groundwater. New York: McGraw-Hill Book Company, 569 pp.
Brinsfield, R. B., and K. W. Staver. 1989. "Cover crops: A paragon for nitrogen management". Proc. ground water issues and solutions in the Potomac River Basin/Chesapeake Bay region. Washington, DC, pp. 271-285.
Chesapeake Bay Local Assistance Department's (CBLAD) homepage. 1997. "Virginia's Bay Act Program".
Cronin, W. B. 1971. "Volumetric and areal statistics of the Chesapeake Bay and its tributaries". Baltimore, MD: the Johns Hopkins University Chesapeake Bay Institute Special Rep. 20.
Hay, R. K. M. 1981. Chemistry for agriculture and ecology. London: Blackwell Scientific Publ., 243 pp.
Merriam-Webster's Collegiate Dictionary. 1993. 10th ed. Massachusetts, U. S. A. p. 401.
Princeton University Water Resources Program, Dir. Wood, E. F. 1984. Groundwater Contamination from Hazardous Waste. New Jersey: Prentice-Hall Inc., p. 35.
Reay, W. G. and Simmons, G. M., Jr. 1992. "Groundwater Discharge in Coastal Systems: Implications for Chesapeake Bay". Perspectives on Chesapeake Bay, 1992: Advances in Estuarine Sciences. pp. 17-44.
Russell and Shogren. 1991. "Theory, Modeling and Experience in the Management of Nonpoint-Source Pollution".
Shuyler, L. R. 1993. "Nonpoint source programs and progress in the Chesapeake Bay". Agriculture, Ecosystems and Environment, vol 46, no. 1-4, pp. 217-222, Amsterdam: Elsevier Science Publishers B. V.
Silka, L. R. 1984. "Understanding the problem of groundwater contamination". Virgiania's groundwater: Proceedings of environmental defense fund symposium, 9-10 November 1983. Blacksburg VA, pp. 5-17.
Simmons, G. M., Jr.; W. G. Reay, S. Smedley and M. T. Williams. 1991. "Assessing submarine groundwater discharge in relation to land use". New perspectives in the Chesapeake system. No. 137.
U. S. Environmental Protection Agency. 1988. Chesapeake Bay nonpoint source programs. Annapolis, MD: U. S. EPA Chesapeake Bay Program.
U.S Envioronmental Protection Agency. Chesapeake Bay Watershed Project. http://www.eps.gov/vislab/SVC/projects/CBAY/watershed.html
"Virginia Agricultural Statistics Service cooperating with Virginia Department of Agriculture and Consumer Services and Devision of Marketing", Sept. 1991. Bulletin No. 61.
Vroblesky, D. A. and W. B. Fleck. 1991. Hydrogeologic framework of the coastal plain of Maryland, Delaware, and the District of Columbia. USGS Professional Paper 1404-E.
Zipper, C. E. 1996. CSES/ENSC 3604 Class Notes. pp. II25-II28.
Links to
Interesting and Pertinent Places
Student authors: Billy Evans and Monika Schneider
Faculty Advisor: Daniel Gallagher, dang@vt.edu
Copyright © 1998 Daniel Gallagher
Last Modified: June 7, 1998
If you have any comments e-mail Billy Evans or Monika Schneider
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References
