Chesapeake Bay’s abysmal water quality is caused by too much nitrogen and phosphorus being discharged into the water. There is no question as to the correctness of that statement, which is not to say that other problems do not exist, or may become more important than we currently realize. Pharmaceuticals, including antibiotics, dissolved in the water discharged from wastewater plants, or in the sludge, are examples of very worrisome practices that have only recently caused alarm.
Chesapeake Bay is no longer a balanced ecosystem. Nutrients were once recycled efficiently, but human activity has unbalanced the system by discharging huge amounts of nutrients into the water. The excess “fertilizer” causes the prolific growth of tiny suspended plants, called phytoplankton. There are not enough animals to eat all the plants, so the water becomes turbid as the plants grow rapidly. When the short-lived plants die, they settle to the bottom. Microbes decompose (oxidize or respire) the organic material on and below the sediment-water interface, consume oxygen, and return the nitrogen and phosphorus back up into the water. The system spirals out of control as more nutrients continue to be discharged into the water from the land. Most of the nutrients being added to the ecosystem are not being removed, so they accumulate in the sediment and perpetuate the problem. Turbid water restricts light penetration and prevents the growth of bottom-dwelling plants (Submerged Aquatic Vegetation or SAV) that provide critical habitat. Microbial respiration depletes the overlying water of dissolved oxygen. If the water column is stratified or layered because of temperature- and/or salinity-induced changes in water density, the denser water near the bottom becomes depleted in dissolved oxygen to the point that animal life is reduced or prohibited. Zones of hypoxia (low dissolved oxygen) or anoxia (no dissolved oxygen) are commonly called “Dead Zones” although the word “dead” applies only to megascopic, not microscopic life. Dead Zones are alive with microbial activity, especially within the sediment.
The sources of nutrients are well known and are now much better quantified than was true in 1973 when Chesapeake Bay’s water quality problem was elucidated by a massive Army Corps of Engineers study. EPA repeated the study beginning in 1976 and has been the lead agency in Bay restoration ever since. Failure of Bay water quality to improve after 1/3 century attests to EPA’s incompetence. Only one new source of nutrients has been identified since those studies, namely the nitric acid that is a component of acid rain. Only one meaningful action has been taken to significantly reduce nutrient pollution, namely the banning of phosphate detergents. Agricultural fertilization practices are the major source of nutrient pollution, and contamination of shallow groundwater by nitrate is a major, and often ignored, vector of pollution. Agricultural fertilization is not as efficient as it could be, and an understanding of Nitrogen Use Efficiency is important.
Phosphorus (P) discharge in the water from wastewater treatment plants causes immediate pollution, but P is not very soluble. It is important to understand that “not very soluble” does not mean “insoluble” and the word “insoluble” should never be used. Even though there cannot be much P dissolved in water, low concentrations of P in large volumes of water still leads to significant P pollution. P “bonds” or attaches to soil and iron oxide particles, so that much of the P is retained, at least temporarily, in the soil or sediment. There is no such thing as “fixed” P on a long time scale, even though P can be sufficiently immobile as not to be immediately available to crops. The only way to “fix” P is to precipitate the mineral apatite, but of course even apatite has a measurable solubility. When P is over-applied, as is the case with the land application of animal waste, to quote from the Mid-Atlantic Nutrient Management Handbook (p. 82) “… if the level of residual soil P is allowed to build up by repeated application of P in excess of crop needs, a soil can become saturated with P and the potential for soluble P loses in surface runoff will increase significantly.” The P we have banked up in Bay watershed soils because of the land application of P-rich animal waste will take decades to dissipate. To quote A. N. Sharpley (Ed., Agriculture and Phosphorus Management: The Chesapeake Bay, 1999, CRC Press, p. 66.) “… much of the crop land in the Chesapeake Bay watershed is now considered “optimum” or “excessive” in phosphorus from an agricultural perspective and hence needs little additional phosphorus, from any source, to ensure that economically optimum crop yields are attained.” Yet we continue to pile P on fields in the name of “free fertilizer.” Virginia continues to avoid using the common sense “soil test P” method (analyze the soil for P and apply only the amount of P the crop requires), violating the law, and favoring a scientifically flawed methodology, the “P-index,” that keeps special interests pacified.
Second to agricultural nutrient pollution are urban practices. Point source discharges from wastewater treatment plants and storm water discharge, especially Combined Sewage Overflow, are very easily addressed, albeit expensive. There are no other meaningful and actionable sources of Bay nutrient pollution. Still, some agencies tout some of these distractions, mainly to divert public attention from agricultural pollution.