A case study is presented to demonstrate the development of a watershed-wide Total Maximum Daily Load (TMDL) to establish nutrient load reductions that translate into real environmental benefits. Extensive monitoring was conducted throughout the Passaic River Basin from May through
November of 2003 and from August to September of 2004 in order to: 1) identify locations where high phosphorus concentrations may be impairing uses; 2) characterize nutrient sources; and 3) obtain adequate data to develop and calibrate a comprehensive watershed model. A total of 70 locations
were sampled, including streams, stormwater drainages, sewage treatment plant effluents, and baseflow locations. Sampling regimes varied by station type and included diurnal dissolved oxygen events, weekly grab sampling, 2-day high and low-flow events, and stormwater sampling. Data collected
for this study were evaluated to determine whether water column phosphorus is controlling productivity and whether phosphorus is causing, or has the potential to cause, instream impairment. Nutrient chemistry was used to evaluate nutrient limitation, and phytoplankton, periphyton, and dissolved
oxygen were used to evaluate stream impairment. Stream segments were identified where the instream phosphorus criterion does not apply because phosphorus is not limiting, algae concentrations are below threshold values, and no dissolved oxygen criteria are violated due to productivity impacts.
Other stream segments were identified as critical locations for dissolved oxygen impacts due to nutrients. Water quality targets (e.g. dissolved oxygen criteria) were defined to solve or prevent nutrient impairments at critical locations in the streams. Additional water quality targets were
defined at potable water intake locations that provide water to reservoirs or directly to water treatment plants based on acceptable loading rates to those water bodies and facilities. Using a watershed model that relates nutrients from various sources to water quality targets at critical
locations throughout the watershed, a number of different nutrient reduction scenarios will be tested under a variety of conditions during the simulation period. Wasteload allocations for point sources and load allocations for nonpoint sources will be established to satisfy water quality targets
at all critical locations and will form the basis for the watershed-specific criteria and TMDL for the non-tidal Passaic River basin.
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