Non-Point Source Pollution: Types, Sources and Impacts on Virginia's Waters
Wayne S. Teel
James Madison University
Though there is little doubt that the Clean Water Act passed in the early 1970s had a positive impact reducing point source pollution in this country, it is only recently that the importance of non-point source (NPS) pollution was fully acknowledged. NPS stems from a number of sources and, as the name implies, is much harder to trace and quantify than point-source pollution. In general NPS pollution can be placed in one of four categories: toxic chemicals, nutrients, sediment, and bacteria. These can arise from a number of sources including urban and suburban lawns, agricultural activities, logging, construction, mining, and arguably the most insidious form is runoff from impervious layers.
Understanding the details and dynamics of each of these forms of NPS pollution are both simple and complicated. Often the origin of NPS pollution is easily spotted while the political and economic backdrop muddies the waters considerably. This paper is designed to give a general background to each type of NPS pollution, their sources and the difficulties surrounding their reduction. Though important, less time will be spent on bacterial pollution as it is directly related to the discussion on nutrients. More time will be devoted to the problems of impervious surfaces and their impact on flow rates, stream bed and flood plain physical characteristics, and stream pollution loads.
Toxic Chemicals
Ask most people what they think of when the subject of pollution comes up and toxic chemicals rank on or near the top. Toxic chemicals come in many forms, from simple heavy metals like mercury, to the complex organic molecules designed for specialized use that "escape" into the environment. Though point sources serve as the entry point for most toxic chemicals, such as the infamous groundwater contamination by PCBs from Front Royal's Avtex Chemical Plant on the Shenandoah River, a considerable volume of these chemicals reach our waterways from non-point sources.
Among the most notable of these chemicals are the herbicides and pesticides commonly used on farms, though use is not restricted to agriculture. An example of these chemicals is Atrazine, a complex organic molecule persistent in soil, which kills and inhibits the growth of broadleaf weeds in cornfields. This chemical gained widespread acceptance because it limits the need for cultivation, reducing labor of farmers and associated expenses. Because of its persistence in the soil Atrazine poses a pollution problem. Erosion on soils where Atrazine is present carries it to surrounding streams. Even more problematic is its mobility in soils, avoiding capture by clay particles and on some occasions leaching into the water table affecting well-quality.
We know a lot about Atrazine because it has been in use for a number of years and its presence in the environment is well documented. There is less certainty about other agri-chemicals. Many, like DDT, have been banned altogether because of their persistence. Others like Round-Up, a short-half life, broad spectrum plant killer, break-down quickly in sunlight or soil and pose little threat beyond drift during application. The tendency in agriculture to move away from persistent to short duration chemicals has helped, but in many cases the full impact of these chemicals on our waterways is not known.
Use of toxic chemicals in agriculture receives considerable attention, but critics of these practices often fail to recognize that many "agri-chemicals" are used more intensively, and in greater quantity, in urban and suburban settings such as lawns or golf-courses. Regulation of these users is less certain, and the impact of the chemicals less clear, though the potential for environmental entry is high.
Another category of toxic chemicals comes from our transportation system. Oil drips and spills of coolant from cars, trucks and buses are frequent occurrences. Often these fall on impervious surfaces, to be washed into the nearest stream with the next major storm. In addition careless users put these and a whole category of household chemicals down storm sewers, not realizing that these systems empty into streams and are not filtered out by the nearest sewage treatment plant. Reduction of toxic chemicals' entry into the environment will fall when sloppy handling is reduced.
Nutrients
Nutrients are not pollutants. They are vital to plants and the whole cycle of life. Nature created a system of nutrient production and cycling that keeps these vital life ingredients flowing. Nitrogen is captured from the air and "fixed" or converted by certain bacteria to nitrates that are easily used by plants. Legumes have nodules on their roots inhabited by these bacteria, which provide the plant with nitrogen and the plant provides them with energy. This form of nitrogen becomes available to animals when the plant is eaten or to other plants when leaves die and decay.
People have enhanced the nitrogen cycle by production of nitrogen fertilizers using high temperatures and pressures to convert the normally inert nitrogen gas to nitrates. These are given to plants in the form of inorganic fertilizers, often combined with two other major nutrients, potassium and phosphorus. The agricultural production boom of the post World War II period is due primarily to the breeding of plants to respond to increased application of these fertilizers. Other nutrients are added as well, like calcium, magnesium, sulfur, selenium, copper, and boron, complimenting the impact of the major nutrients.
The problem with nutrients, whether they be in inorganic form or organic form, like compost or manure, is that they leak, especially when used at rates that exceed the natural cycling ability of an ecosystem. Nitrates, nitrites, and ammonium compounds are highly soluble, dissolving readily in water and moving with water on or under the soil surface. Potassium moves easily too, but causes fewer problems than nitrogen. Phosphorus is generally thought to be immobile, but recent studies indicate that after phosphorus saturates the soil profile it too becomes mobile, even reaching water tables in some locations.
Nature, when left to operate freely, has ways of capturing nutrients, diffusing them in the environment and holding them in living tissues. Our economic systems like to concentrate nutrients. Take as an example the poultry industry in the Shenandoah Valley. Although it is an agricultural center for Virginia the Shenandoah Valley does not produce enough grain to feed all the turkeys and chicken grown here. Thus some two million tons of corn, soybeans and other grain come into the valley. This grain from Indiana and Illinois was grown using high inputs of chemical fertilizer, much of which is converted to grain. Poultry consumes between two and four times as much grain as is used for growth. Nutrients from the grain either end up in the birds, or in the vast quantities of poultry manure they generate, which is high in ammonia, nitrates, and phosphorus compounds. This manure, combined with some amount of bedding material, is then either spread as poultry litter on the farmers' fields, or sold to others to spread.
Nutrient surpluses in aquatic systems cause a number of problems. Arguably chief among these is algae growth and subsequent decay. Nutrients, after all, are fertilizer, and fertilizer promotes rapid plant growth. Algae in streams responds no differently. Rapid growth also means an increase of organic materials in the water that also die and decay. The decay process uses oxygen at rates greater than the production by new algae or incorporation through flow over rocks. A decrease in oxygen threatens animal life in the stream, from the smallest mayfly to the largest trout or bass. Since some of these creatures consume algae, their demise leads to a greater accumulation of rotting organic material.
Another problem associated with nutrient overload has been widely documented of late. The outbreaks of Pfisteria piscicida have killed fish, caused health problems among fisherman, and have raised concern about the impact of hog and poultry operations since Pfisteria is linked to surplus Phosphorus in brackish water.
Using the Department of Environmental Qualities data on Muddy Creek in Rockingham County, poultry houses in that watershed produce some 63,146 tons of litter each year. In addition dairy and cattle farms produce another 119,093 tons of manure. The watershed only has 11,158 acres of active crop, hay, and grazing land, meaning that nearly 6 tons of poultry litter (30% water by weight) and over 10 tons (wet weight) of cattle manure must be spread on each acre, each year if none is exported. Recommended spreading maximums are about half that amount for cattle manure, and one-third that amount for poultry manure, and the two are normally not spread on the same land. The excess either penetrates to the water table or moves into Muddy Creek during rainfall events, given rise to high nitrogen and phosphorus levels and excessive amounts of fecal coliform bacteria in the creek.
This is where the politics and economics of the nutrient problems becomes complicated. There is no doubt that the nutrient levels in Muddy Creek, and by implication much of the upper Shenandoah Valley, are excessive, but apportioning responsibility for reducing nutrient loads is controversial and figuring out exactly where loading comes from is difficult. Nutrients accumulate through the watershed. Some are found even in waters leaving the forest deposited by wildlife or the natural decay of plants. (The poultry industry claims that a majority of fecal coliform and nutrient loading comes from this source, but that is ridiculous.) As the stream flows from the forest, the pollution load increases cumulatively, eventually impacting the benthic community native to the stream environment.
Who is responsible? No one group is to blame, yet all share responsibility. Consumers who demand cheap poultry, beef and dairy products; corporations that externalize the costs of pollution; government agencies that fail to recognize or regulate obvious sources of pollution; and farmers who fail to buffer streams, spread too much manure and litter, and allow their animals to dwell in the stream bed, all share responsibility for cleaning the mess up. Only with a cooperative effort, and perhaps decentralization of production, will a reduction of nutrient pollution be realized.
Bacteria
As a general rule, if you have nutrient pollution, you probably have bacteria problems as well. The term often used for bacteria in this context is fecal coliform. Fecal coliforms are bacteria that occur naturally in the gut of all animals, from tiny mites to humans. They are not generally regarded as disease causing agents, but they do indicate the possible presence of types that do cause serious disease like hepatitis A, salmonella, E. coli varieties, and even parasites like giardia. Any direct or unfiltered contact between fecal material and the stream is likely to elevate fecal coliform counts in the stream.
Fecal coliform does not directly harm most aquatic life, and if the addition of fecal material is not continuous in a watershed, these bacteria rather quickly die off. However, unbuffered grazing land, poorly managed and spread poultry litter, leaky septic systems, and fecal material from urban pets all enter watershed on a regular basis, especially after a rain. Therefore fecal coliform counts become a useful and relatively easy to measure indicator of stream health, pointing to other problems.
Though at present it is difficult to distinguish between fecal coliforms from cattle, poultry or humans, scientists are working on this problem. Advances in DNA research have enabled scientist to distinguish fecal material from different species. Soon instead of getting a total fecal coliform count, we may be able to get a breakdown of the sources, helping us pinpoint where corrections to management strategies may be most effective.
Sediment
Sediment is mobilized soil particles carried by water or air and re-deposited in another location. It is a product of erosion, both natural and human caused. In Virginia the primary type of erosion is water. Erosion is nature is a slow process, sped by earthquakes and violent storms, and slowed by vegetation. Over millions of years it lowers mountains and builds deltas. Humans, by their activities on the landscape, have become primary agents of erosion. Whenever agriculture, construction, or other activities, leave soil bare, unprotected by the roots, stems, leaves and litter of plants, erosion during rains is common. Raindrops loosen and break apart exposed soil particles, splashing them downhill, and when heavy enough, resulting runoff carries these particles to streams, rivers, and eventually to the ocean.
Sediment is not a poison, and although it can carry poisons as well as nutrients adhering to the clay, silt or organic particles, the damage done by silt is quite different. Sediments destroy habitat. Many of the most productive bottom dwelling macroinvertebrates, such as mayflies, stoneflies, and caddisflies, live on rocks or cobbles that water bubbles over, maximizing oxygen supply. When sediment loads increase spaces between these rocks becomes filled with mud, depriving the critters of habitat, which in turn leads to declines in fish populations dependent on these critters for food. A highly sediment compromised stream has a clay-silt lining that greatly restricts both variety and absolute numbers of animals, many of whom get their food by filtering the water, further reducing stream health in a cascade a problems.
The number one problem in the Chesapeake is probably sediment, not nutrients or toxic chemicals, though the later get more publicity. Not too long ago the Chesapeake produced 100,000 tons of oysters a year. Production is now 1,000 tons, a 99% fall. Oyster beds are clogged with sediment, covering rocks and old oyster shells that once provided a home for one of the most productive and beneficial species in the bay. Oysters filtered the entire bay on a daily basis, now they could not do the entire bay in a year. No other single decline has effected the bay as much as this.
Bare soil is the single greatest source of sediments, and there are three major areas were bare soil is common. The most obvious, but perhaps least important on a per acre basis is the farming of annual crops like corn and soybeans on plowed land. Erosion on these fields is seasonal, occurring mainly in the spring between land preparation and the time when the plants themselves cover 50% of the soil. Farmers have successfully reduced this erosion in many areas by using cover-crops and shifting to no-till land preparation that leaves plant residues on the surface.
The second source of sediment is grazing land, particularly that on or near stream banks. Cattle spend about 8 hours a day eating. The remainder of the day they rest, and on hot days their favorite spots are shade trees and streams. Because of the close access to water, cattle trampling, dusting and sleeping on stream banks is common. The heavy pressure on vegetation near the stream smoothers grasses and herbaceous plants, damages shrubs and prevents regeneration of trees. The relatively simple step of fencing off a riparian (near stream) zone to limit or prevent cattle access, will reverse this problem in a short period, and planting trees, shrubs and grasses in this zone speeds the process.
The final source, and in many instances the greatest source of erosion, is construction. Building highways, housing, malls, offices, and parking lots involves exposing soil to the elements. Though guidelines exist to limit soil movement during construction, it seems that these regulations lack rigorous enforcement especially as the size of the project increases. Sugarland Run in Fairfax and Loudoun Counties of Northern Virginia is an example of a stream severely compromised by erosion from construction. Even in excellent water conditions, habitat damage from sediment reduces macroinvertebrate population significantly. Though this problem is as easy to solve as the two agricultural sources of sediment, the will and skills needed to do so are lacking. Unfortunately the impact of sediment on a stream is long term, even if the soil exposure is relatively brief. It can take 30 years for sediment to move out of a watershed, and even longer in a bay like the Chesapeake.
Impervious surfaces
What is an impervious surface? A road, a roof, a driveway, cul-de-sac, sidewalk, plastic tarp, or any layer of material that water cannot penetrate is an impervious surface. At first glance an impervious surface is inert. A cement roadway does not do anything or react with anything, it just sits. But the impact of impervious surfaces may be more damaging than any other aspect of human activity in a watershed.
A forest is a highly absorbent sponge of rainfall, capturing it in organic material, soil strata, and trees. At the same time it filters out and sometimes even neutralizes the airborne particulates, nutrients (in the form of nitrates) and other pollutants. Grassland is less effective, but also absorbent and very effective and stabilizing runoff in the face of excessive rains. There is little or no erosion on a golf course, even if runoff is relatively high. Pavement absorbs nothing. In fact it prevents nearly 100% of the potential absorbency of the soil on which it lies. All the water falling on an impervious layer must either run-off or sit there until evaporated by the sun. In most cases engineers design impervious surfaces to shed water.
This water has to go somewhere. Cities, where the percentage of impervious layer is the highest, channel their storm water through culverts either into the nearest stream or, in rarer cases, to their wastewater treatment facility. During a rainfall event, the higher the amount of impervious surface in a watershed, the greater the maximum pulse of high water will be. Natural streams tend to be sinuous, twisting courses that have floodplains to handle surplus water. City streams are often re-engineered to speed up to flow of water, restricting access to the floodplain by excess water. This increases the energy level of water flow, scouring the stream bed and banks, destroying stream habitat and eroding banks. Streams below cities and other sites with a high percentage of impervious surfaces often take on a U-shape, with steep, undercut banks, and stretches of exposed bedrock in their beds.
This problem by itself is serious enough but it is compounded by another factor, impervious surfaces are not clean. A vast majority of impervious surfaces exists for the sake of our transport sector. Cars, trucks, and buses use fossil fuels, drip oily residue, and spew out varying quantities of hydrocarbons, nitrates and other pollutants. These go into the air but fall later in dust or raindrops. Without rainfall this material builds up on the surfaces, unfiltered by plants or soil, waiting for the first rain to wash them into a stream and ultimately into the ocean. The Center for Watershed Protection estimates that 20 to 25% of the nutrients entering Chesapeake Bay come unfiltered from impervious surfaces. This says little about other items washed from our streets and rooftops, but illustrates something of the problem.
City-based pollution, and other areas of impervious surface, are perhaps the toughest single water-pollution problem today. Bad farm management is correctable. Use of chemicals on lawns and golf courses can be reduced or eliminated. Landowners can plant more vegetation, or even simply allow nature to do it for them. But a parking lot, a driveway, a new road, or even a new house, adds a new layer to the water pollution problem that is not easily reduced or corrected.
Virginia, like elsewhere in this country, has major water problems. Some of them are easily correctable, some will take considerable effort, and still others are nearly insoluble. In order to clean up our waters a better understanding of the nature of the problems is needed. Then we must act. Reducing toxic chemical use, fencing and planting stream buffer zones, determining better methods to manage the presently over-concentrated nutrient loads, and stopping expansion of impervious surfaces are good starting points. The only way to do it is together.
November 1, 1998