STORMWATER HYDROLOGY AND QUALITY - Low Impact Development and Sustainable Stormwater Management - page 47

Some NPS pollutants, such as oxides of sulfur and nitrogen, are even airborne
(dust fall), deposited onto the land (or water) surface and then carried into receiv-
ing waters, although airborne pollutants in most cases comprise a relatively small
fraction of the total NPS pollutant load. Once in a stream, the increased volume
and rate of runoff produce additional sediment pollution from streambank erosion
by undercutting and resuspension of sediment.
The two physical forms of NPS pollutants are particulates and solutes .This
very important distinction for NPS pollutants is the extent to which pollutants
are particulate in form, or dissolved in the runoff as solutes. The best examples
of this comparison are the two common fertilizers phosphorus (TP) and nitrate
(NO 3 -N). Phosphorus typically occurs in particulate form, usually bound to
colloidal soil particles. Because of this physical form, stormwater management
practices that rely on physical filtering and/or settling out of sediment particles
can be quite successful for phosphorus removal, although the phosphorus is
bound to the smallest particles (colloids) and may require more extensive
removal measures. In stark contrast is nitrate, which tends to occur in highly
soluble forms and is unaffected by many of the structural BMPs discussed
later. As a consequence, stormwater management approaches for nitrate must
be quite different in approach, with wetlands or wet ponds and other biological
approaches being more effective, especially where anaerobic conditions can be
achieved and where denitrification can occur. Nonstructural BMPs are in fact
the best approach for nitrate reduction in runoff, and the easiest BMP is where
the surface application of nitrate fertilizer can be reduced or avoided.
Particulates
NPS pollutants that move in association with or attached to particles include total
suspended solids (TSS), phosphorus (TP), most organic matter (as estimated
by COD), metals, and some herbicides and pesticides. Kinetic energy keeps
particulates in suspension, and they do not settle out easily. For example, an
extended detention basin is a good way to reduce total suspended solids, but is
less successful with TP, because much of the TP load is attached to these small
colloids which remain in suspension and pass through detention structures.
Because a major fraction of particulate-associated pollutants is transported
with and loosely bound to colloids, their removal by BMPs is especially diffi-
cult. These colloids are so small that they do not settle out in a quiescent pool
or basin, and remain in suspension for days at a time, passing through a deten-
tion basin with the outlet discharge. It is possible to add chemicals to a detention
basin to coagulate these colloids to promote settling, but this chemical use turns a
natural stream channel or pond into a treatment unit, and subsequent removal of
sludge is required. A variety of BMPs have been developed that serve as runoff
filters and are designed for installation in storm sewer elements, such as inlets,
manholes, or boxes. The potential problem with all measures that attempt to fil-
ter stormwater is that they quickly become clogged, especially during a major
event. Of course, one could argue that if the filter systems become clogged, they