Storm water and Sediment 



Sufficient information now exists, however, to support the assertion that the Bay acts as 

 an effective pollutant trap via sedimentation processes. Several water column processes act 

 on chemicals introduced to the Bay, including photodegradation, uptake by organisms, 

 biological degradation, volatilization, dilution by mixing with more seaward waters, 

 aerosol formation, and sorption to suspended particles with removal to sediments. 



Of these processes, biochemical scavenging and retention in sediments tends to trap 

 pollutants within the Bay. In an effort to learn to what extent sediment scavenging limits 

 the effects of dilution, Hinga (1987) is developing a model of Bay scavenging which treats 

 scavenging and dilution as competing processes, and omits consideration of other removal 

 processes. Given the fact that seasonal trends in scavenging rates and water residence time 

 in the Bay interact to maximize warm-weather retention and minimize cold-weather 

 retention, Hinga postulates that more than 50 percent of all pollutants with a partitioning 

 coefficient exceeding 500,000 would be retained in summer. Since many or most of the 

 pollutants affecting Narragansett Bay have partitioning coefficients within one to two 

 orders of magnitude of the latter threshold, Hinga asserts that significant retention is 

 occurring, according to the model's prediction. He warns that mixing and dilution with 

 offshore waters should not be relied upon to reduce chemical contaminant inputs to trivial 

 concentrations in the Bay. 



In coastal areas generally, sediment frequentiy reaches wetiands, estuaries, and harbors. 

 Sedimentation has caused alteration of circulation patterns in small embayments within the 

 Bay system. Severe habitat alterations have resulted from changes in circulation patterns, 

 increased turbidity, loss of light penetration, shifts in temperature and salinity regimes, and 

 other factors attributable to sedimentation. Alterations documented in the Rhode Island salt 

 ponds include: shift in dominant vegetation type; loss of range of available habitat for fish 

 and shellfish, and consequent shift of species assemblages; and loss of spawning and 

 nursery habitat areas supporting commercial fishery species (CRMC, 1985). 



Siltation, clogging of slips, and related impacts of sedimentation have become major 

 sources of controversy in several harbors and marinas around the Bay, where maintenance 

 dredging is advocated by property owners and the marine and recreation industries. 

 Concerns regarding resuspension of sediment- bound contaminants, and dredge spoil 

 disposal, have been raised by opponents of routine dredging. 



Use of lower salt to sand ratio mixes in deicing has affected sediment loading, in ways 

 which should be accounted for in sediment management. In an effort to determine the role 

 which deicing sand plays in sediment accumulation in detention basins, DEM conducted a 

 survey of towns in 1986/1987 to determine sand application rates. An average application 

 rate was determined to be 28.7 tons of sand/mile/year based on responses of six towns 

 (Stormwater Management and Erosion Control Advisory Committee Notice and Minutes, 

 11/19/87). 



URBAN RUNOFF 



Urban runoff is the major source of non-point source pollutant input to Narragansett 

 Bay, contributing, for example, 48 percent of the petroleum hydrocarbons, 3 percent of tiie 

 lower molecular weight PAH, 44 percent of the higher molecular weight PAH, 65 percent 

 of the lead, 56 percent of the zinc, and 5 percent of the copper entering the Bay watershed 

 annually (Hoffman et al., 1984). Runoff characteristics and loadings vary widely, among 

 source areas and within the same area through rime. Lxjadings are closely related to land 



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