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In the fall of 2002, New Jersey Department of Environmental Protection (NJDEP) began to include Water Quality Based Effluent Limits (WQBELs) for phosphorus in New Jersey Pollutant Discharge Elimination System (NJPDES) permit renewals for municipal discharges to surface waters. Effluent limits of 0.1 mg/l are being imposed on discharges to “impaired” waters (designated by NJDEP for all freshwaters where instream total phosphorus exceeds the numeric stream criterion of 0.1 mg/l); effluent limits for discharges to unimpaired waters are being imposed to ensure that the discharge will not cause the water to exceed the numeric criterion during critical low-flow conditions.

The problem is that the criterion on which the effluent limits are based may not apply to the receiving water of the discharge. New Jersey's freshwater stream criterion for phosphorus reads: “Total phosphorus shall not exceed 0.1 mg/l unless it can be demonstrated that phosphorus is not a limiting nutrient and will not otherwise render the waters unsuitable for designated uses.” [bold added] In order to generate its list of impaired waters as well as to calculate effluent limits, however, the NJDEP has applied the threshold of 0.1 mg/l to all freshwater streams without regard to nutrient limitation or use impairment, essentially assuming that the conditions under which the criterion applies are always satisfied.

It is important to note that the intent of the phosphorus stream criterion is to prevent eutrophication and its consequences. Phosphorus is a required nutrient for plants and algae, but is considered a pollutant because it can stimulate excessive primary production. The conditions in the stream criterion are important because they link phosphorus to use impairment. The criterion does not apply to streams where something other than phosphorus is limiting the growth of primary producers to acceptable levels. In these circumstances, phosphorus has no effect on eutrophication and therefore the phosphorus criterion does not apply.

On behalf of three municipal wastewater authorities who saw the benefit of working together to evaluate a broader portion of their watershed, TRC Omni performed a Phosphorus Evaluation Study of the Lower Millstone / Raritan River watershed. The study was the first of its kind in the State of New Jersey. The study encompassed four major rivers and five sewage treatment plants within a rural and suburban watershed in central New Jersey that is experiencing enormous development pressures. TRC Omni collected extensive data in 2003 under a variety of low conditions for this study. Response indicators of excessive productivity were measured, including diurnal dissolved oxygen, phytoplankton concentration, and periphyton density. The primary purpose of the study was to determine whether the phosphorus criterion applies to the various water segments in their watershed.

The study identified segments of the watershed where phosphorus in high concentrations is not controlling productivity, and where the productivity is adequately restrained by other factors. In these circumstances, phosphorus has no effect on eutrophication, and therefore the phosphorus criterion does not apply. The study also identified a real phosphorus impairment that was previously not assessed. Finally, the study identified waters that experience excessive productivity as indicated by diurnal oxygen measurements, but where the primary producers (rooted plants) may not be dependent on phosphorus in the water column. Changes in the concentration of phosphorus in the water, such as from reduction or elimination of point sources, will therefore have no impact on productivity of producers that do not obtain nutrients from the water column. The study collected information to identify real nutrient impairments where they exist and charted an approach to develop real solutions, including site-specific criteria, to fully restore uses.
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Document Type: Research Article

Publication date: 2004-01-01

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