Executive Summary

Polluted runoff, according to the USEPA, is the leading cause of impairment in rivers and streams in the United States.  Many existing stormwater dry detention basins designed solely for stormwater runoff peak discharge magnitude abatement as well as detention basins designed both for water quality and quantity control fail to achieve necessary levels of non-point source pollutant loading reduction.  This correctable situation has necessitated the demonstration of various retrofitting techniques on existing systems.  Once it can be demonstrated that a retrofit technology is efficient in both economic and scientific terms, local watershed groups, private entities, and federal, state, and local officials can pursue implementation. 

The area chosen for such a project was a single-family home development community located on Manette Road off Harter Road in Morris Township, New Jersey, with a 23.1 m wide, 65.3 m long, and 1.5 m deep (2,253.5 m³ or 79,569.9 ft³) dry detention basin.  The contributing area, typical of suburban developments, was approximately 25 percent impervious and equaled a magnitude of 0.08 km².  Runoff was collected by a stormwater sewer system to a 76-cm diameter concrete pipe inlet, entered the rectangular dry detention basin that had not been maintained in 18 years of service and later discharged through a single 38-cm diameter concrete pipe outlet. 

The outfall from the studied basin discharged to a second dry detention basin from which water enters the Great Brook via Pine Brook (also referred to as Great Brook). The second basin is marginally operational due to a large level of sedimentation.  The Great Brook passes through the 29.8 km² (7,375-acre) Great Swamp National Wildlife Refuge and then empties into the Passaic River, a water quality success story of its own.  The Refuge, established in 1960, had part of its 28 km² (6,818 acre) preserved area designated a Wilderness Area by Congress in 1968 and contains various threatened and endangered species.

The New Jersey Department of Environmental Protection recognizes the basin’s collection area as within Watershed Management Area (WMA) 6, Upper Passaic, Whippany and Rockaway, a 1,077 km² (266,240 acre) section of the Hackensack-Passaic watershed (Figure 3-5).  The NJDEP and the USGS monitor water quality in seven locations within WMA 6 and three such stations are located on the Passaic River downstream from the detention basin. According to water quality data collected at the three aforementioned stations from 1994 to 1997, the level of total phosphorus within the river routinely exceeds the NJ SWQS maximum level of 0.1 mg/l for FW2-NT waterbodies and has, on occasion, exceeded the maximum allowable concentration of suspended solids.  

In order to properly characterize the potential benefits to Watershed Management Area 6 of various retrofitting technologies, twelve storms were monitored for levels of ammonia, BOD, COD, NO₂ (nitrite), NO₃ (nitrate), ortho-P, TKN, TP, TSS.  In order to create a meaningful analysis of projected benefits from each retrofit, four storms were monitored before the first retrofit was implemented. 

The first retrofit to the existing dry detention basin consisted of the reduction of the pipe outlet diameter from 38-cm to 8-cm to increase detention time.  Increased detention time was sought as to improve remove through settling.  Three storms were monitored after the first modification.  No conclusive correlation between pollutant removal efficiency and the resulting increased detention time was determined. 

The second modification, the floating riser, a buoyant outlet device that removes only the surface layer of an impounded storm, demonstrated high levels of removal for storms with relatively high initial pollutant concentrations after installation in April, 1998.  Maximum mean removal efficiencies occurred during the last three sampled storms, potentially due to a mixture of high inlet concentrations and repairs made to the floating riser to ensure no leakage.  The outflow concentration data, a more appropriate measure by which to determine effectiveness than percent removal, indicated that the second modification may not be necessary for the removal of TSS, TKN, and nitrate as increases in removal were negligible.  BOD and COD levels were greatly decreased below the outlet concentration of the first modification, -71% and -52% respectively. With regard to the removal of phosphorus and its forms, the second retrofit made a significant improvement over the first modification (-21%) and the original condition (-32%).  However, the mean concentration of TP at the outfall for both storms exceeded the New Jersey Surface Water Quality Standards.  For this reason, a biological, as well as physical, system of removal was pursued.

A third modification, consisting of a subsurface wetland coupled with a dry detention basin, has been designed and will be on-line by December, 1999.  The layout consists of the original 76-cm inlet leading to an in-line baffle box for solids removal.  The first-flush of the design storm will be directed to a subsurface wetland area for biological and physical pollutant removal while additional runoff enters the dry detention area through an overflow.  Based on the collected data, the first-flush, characterized as the initial 30 minutes of inflow, contains the majority of the pollutants and is therefore to be treated to a greater extent.  The floating riser, with design improvements incorporated to ensure longevity, will remain at the outlet of the detention basin.

The floating riser demonstrated large-scale benefits with respect to cost.  Hydraulic characteristics, such as a simulated elevation-discharge curve, can be calculated using the basic orifice equation with the appropriate energy-loss coefficient.  The ease of installation and assembly from common materials coupled with the inexpensive construction costs of the prototype riser further enhanced the possibility of widespread implementation of the riser for both permanent (dry and wet detention basins, artificial wetlands) and temporary (construction sediment control ponds) water quality applications.  As the riser is a prototype, long-term monitoring of the device is warranted.

The proposed third modification will undergo a similar monitoring scheme.  The inflow and outflow from the subsurface wetland, as well as the outflow from the receiving dry detention basin will be analyzed for pollutant constituent levels.  Such data will be used to determine the actual benefit of each component of the dual-approach system.  Flow data will aid in gaining a better understanding of the hydraulic characteristics of this dual-structure system as well as subsurface wetlands in general.  Finally, the continued monitoring of the biological success of placed vegetation will serve as a guide for future wetland projects.  An operation and maintenance schedule with assignment to the appropriate parties will serve to overcome a common shortfall of best management practices, proper upkeep.