Since promulgation of the Clean Water Act in the 1970s and implementation of technology-based treatment requirements, such as secondary treatment for publicly owned treatment works, the nation’s water quality has improved dramatically. Current water quality concerns are no longer attributed to excessive loads of carbonaceous and nitrogenous biochemical oxygen demand. Rather, the largely uncontrolled discharge of nutrients from point and non-point sources has resulted in the anthropogenic eutrophication of lakes and streams, causing designated use impairment due to aesthetics (nuisance algae and excessive periphyton/macrophytes growth) and low dissolved oxygen levels (diurnal D.O. swings associated with the photosynthesis-respiration cycle and decomposition of excess plant growth). The Environmental Protection Agency recognizes such eutrophication problems as the most pervasive water quality problem facing the nation and has started to implement programs to address this issue. Most notably, EPA has developed guidance for the states to develop Nutrient Criteria and is pressing the states to develop TMDLs to achieve designated uses.
The guidance on development of nutrient criteria is unlike any other water quality criteria promulgated by EPA. Nutrients (nitrogen and phosphorus, but mainly phosphorus in freshwater systems) are not toxic. Designated use impairment is a secondary effect that results when a combination of conditions allows aquatic plants to grow out of control. This depends upon a number of variables as discussed by K. Mackenthun:
It is generally conceded that abundant major nutrients in the form of available nitrogen and phosphorus are an important and a necessary component of an environment in which excessive aquatic growths arise. Algae, however, are influenced by many and varied factors. Vitamins, trace metals, hormones and auxins, extracellular metabolites, autointoxicants, viruses, and predation and grazing by aquatic animals are factors that stimulate or reduce algal growths. Some of these may be of equal importance to the major nutrients in influencing nuisance algal bloom production. (Mackenthun, K. The Practice of Water Pollution Biology. 1969. at 41)
Thus, many other factors come into play when considering appropriate levels of nutrients to allow in a receiving water. Even so, we are reasonably able to assess nutrient control requirements when the receiving water is a lake or other impoundment because eutrophication problems in these waters are due primarily to the phosphorus-stimulated growth of phytoplankton, nutrient concentrations remain relatively stable due to the retention time associated with such water bodies, and surface shading is usually limited. As a rule of thumb, lake phosphorus levels must be less than 0.05 mg/l to avoid eutrophication related use impairment.
The control of eutrophication problems or potential problems (as would be the case for developing nutrient limits for a new or expanded discharge) in streams is much more complex than that for lakes. Due to the short residence time associated with flowing water bodies, phytoplankton are not a significant concern. Rather, periphyton (fixed plant growths on submerged surfaces) and rooted aquatic growths pose the greatest potential problems for use impairment. In EPA’s Protocol for Development of Nutrient TMDL, the Agency identifies a single event threshold of 200 mg/m2 and a seasonal average of 150 mg/m2 for periphyton chlorophyll as the concentrations, above which, waters may be unsuitable for designated uses. These threshold periphyton values correspond to dissolved reactive phosphorus concentrations as low as 1-4 ug/l and is at least 10 times more restrictive than the phosphorus target level for lakes. However, periphyton growth in streams is likely to be mitigated by shading and scour (associated with runoff flows). With respect to preemptive nutrient TMDL development, periphyton present a problem because the science is not sufficiently developed to accurately predict periphyton growth, which is a prerequisite to predicting adverse impacts associated with dissolved oxygen. In addition, rooted aquatic vegetation, which may dominate shallow streams and render them useless for recreational activity, may be unrelated to water column nutrient levels and the science behind modeling these plants is rudimentary at best.
The problems associated with developing a nutrient TMDL for streams is exemplified by the situation in Upper East Canyon Creek in Park City, Utah. This stream supported a trout fishery prior to 1990. Subsequent land development in the drainage area resulted in a significant reduction in the trout population that was attributed to extensive growths of rooted macrophytes. This, in turn, caused excessive diurnal D.O. swings that reduced the dissolved oxygen level below the state’s monthly average and daily minimum requirements. Due to the fact that the problem was associated with rooted macrophytes, a model was not developed to predict the nutrient levels necessary to achieve water quality standards. Rather, the treatment facility was required to drastically reduce its phosphorus load so that the stream could meet a phosphorus concentration of 0.05 mg/l during the growing season. A facility expansion achieved this goal in 2003 and the stream has achieved the desired phosphorus goal. However, the use impairment due to rooted macrophytes persists to this day.