Water Quality Modeling of an Urban Stream Using LSPC

Saddle River is a 16.3 miles long major tributary of the Passaic River in New Jersey. The watershed consists of approximately 60 square miles of developed urban area with extensive residential land use, and two wastewater treatment plants (WWTPs) discharging into Hohokus Brook, a large tributary. New Jersey DEP (Department of Environmental Protection) and USGS (United States Geological Survey) have been monitoring the water quality in Saddle River and Hohokus Brook using both grab samples and continuous monitoring. High nitrogen and phosphorus concentrations have been observed in the reaches below the WWTPs.

A watershed model for Saddle River watershed was developed using the USEPA Load Simulation Program C++ (LSPC) model. The LSPC model is a simplified version of the widely used Hydrologic Simulation Program Fortran (HSPF) model with the same algorithm for routing and reach water quality simulation.

The Saddle River Watershed was sub-delineated to 54 subwatersheds with an average of subwatershed size of 720 acres. Seventeen types of land uses were used in the model, including low density, medium density, and high density residential land uses featuring both pervious and impervious surfaces. Hourly weather data from the Teterboro Airport were used to generate the LSPC weather input file that includes parameters for precipitation, potential evapotranspiration, air temperature, dew point temperature, wind speed, solar radiation, and cloud cover. Non-point source loadings of CBOD, ammonia, nitrite/nitrate, organic nitrogen, orthophosphate, and organic phosphorus from the seventeen modeled land uses were simulated using build-up and wash-off methods. Point source loadings from the WWTPs were directly assigned to the corresponding reaches.

The model simulated hydrology, upland pollutant yield, water temperature,  phytoplankton and benthic algae dynamics in the Saddle River and it’s tributaries for the period from 2000 to 2017. Flow data from three USGS gages were used to calibrate the hydrology parameters. Water quality monitoring data from both the grab sample and continuous sonde sites were used to calibrate the parameters of heat transfer and eutrophication process. The model was able to capture the timing and magnitude of the increase in nitrogen and phosphorus concentrations below the WWTPs, and it also reproduced the diurnal fluctuation of water temperature and DO in both magnitude and timing relative to the continuous monitoring observations.