(Published August 2004)
“Out of sight and out of mind” is a very appropriate truism with regard to Chesapeake Bay. One of the Bay’s largest sources of pollution is out of sight, as is one of the major consequences of that pollution. Maybe that is why the Bay is not improving. People only believe what they can see.
The recently released Virginia “Tributary Strategies” attribute approximately 70% of the nitrate and phosphate pollution in the Eastern Shore to agricultural practices. Virginia’s Northern Neck is very similar to Virginia’s Eastern Shore in terms of topography, hydrogeology and land use. The “Neck” is quite unlike the Potomac or Rappahannock watersheds with their intense urbanization, despite the fact that the Northern Neck is formally included in those watersheds.
To date, most attempts to reduce agricultural pollution have focused on runoff from fields, an easily observable process. Conservation tillage, contour plowing and grass buffer strips are all good, albeit not universally applied, practices that can be very effective in reducing loss of soil and its contained fertilizer by runoff to local waterways. But the real culprit is out of sight. Most (about 2/3) of our 42 inches of annual rainfall is evapo-transpired, meaning that the water either evaporates or is transpired by plants. Given our sandy soils and relatively flat topography, most of the remainder of the rainfall, amounting to about a foot of water each year, infiltrates down to the water table. In a recent survey of domestic shallow water wells, I helped measure the depth to water and found that, on average, it is about 1/4 of the topographic elevation. Because the water table is always higher than sea level, gravity guarantees that the water, called groundwater, will flow “downhill” underground toward the nearest waterway. Groundwater flow is “out of sight” but it is immense. The United States Geological Survey estimates that “…of the 50 billion gallons of water that reaches the Chesapeake Bay each day, nearly 27 billion gallons is base flow.“ Base flow is the discharge of groundwater to tidal or non-tidal streams.
If rain falls on over-fertilized agricultural land, lawns or landscaping, excess nitrate, and to a smaller extent, phosphate, is dissolved and carried down to the water table and then underground to the nearest waterway. Septic systems add additional nitrate and phosphate as they convert the fertilizer used to grow the food eaten by the household back into inorganic form. A 1997 study of Northumberland County groundwater by VA Tech found the average shallow groundwater nitrate concentration is 5 mg/l (ppm), similar to the Eastern Shore, and similar to the goal, rarely achieved today, for modern wastewater treatment plants. It is groundwater flow, “out of sight and out of mind,” that accounts for the lower salinity toward the headwaters of all our local waterways, and for most of the massive nutrient load they bear.
The nutrients promote the rapid growth of tiny algae. The algae have short life spans, and since there aren’t enough animals to eat all of them, when they die they settle to the bottom of the waterways and the Bay. In summer, when the warm water can dissolve only small amounts of gasses such as oxygen, microbial respiration (rotting) of the organic material-rich “muck” on the Bay or creek bottom removes dissolved oxygen from the water. In the open Bay east of the Northern Neck, oxygen levels drop below 5 mg/liter, low enough to stress animal life, below about 30 feet, and there is insufficient oxygen to support life deeper than about 50 feet in July and August. It has been estimated that for the entire Bay, nearly 2/3 of the Bay bottom cannot support animal life (worms, clams, oysters, etc.) in mid-summer because of low oxygen concentrations. Local waterways can also be affected. These “dead zones” are “out of sight” as people happily race around in boats and personal watercraft or look out over the water. The surface water bears no witness to the toxic conditions below.
The surface of the land, with the beautiful landscaping and lush crops does not bear witness to the massive subsurface groundwater flow carrying excess fertilizer and septic system effluent toward every waterway. The surface of the waterways or the Bay does not bear witness to the deadly environment below caused by the excess nutrients. These processes are “out of sight” but they must no longer be “out of mind” if we are to have any hope of improving our waterways and the Bay.
NAPS Stewardship Tips addressing many issues of good stewardship, together with links to other sources of information, can be found here at www.napsva.org. Nitrate concentrations can be found under the Stewardship Tip on shallow groundwater and the “Tip” on dead zones provides links to informative graphics.