Page 425 - The Engineering Guide to LEED-New Construction Sustainable Construction for Engineers

Low-Impact Development and Stormwater Issues 385

C Average concentration of a pollutant in rainfall
PI
C Average concentration of a pollutant coming from miscellaneous flows into a
Pmisc
BMP
C Average concentration of a pollutant flowing out of BMP
Pout
C Average concentration of a pollutant in upslope runoff
Pup
Note that the concentration of a pollutant has been ignored in the evaporation
portion, but this may need to be included as a pollutant evaporation rate if the contaminant
in question is volatile. Also, many pollutants may have reactions internal to the BMP,
such as microbial degradation of organics to inorganic compounds, and the various
internal reaction rates would need to be added. However, for cases where neither internal
reactions nor pollutant evaporation is important, the stormwater equations can be
rewritten as pollutant mass balances as in Eqs. (10.2.9) and (10.2.10).

Accumulation rate of pollutants at time t = Q C + Q C + IA C − FA C − Q C
misc Pmisc up Pup BMP PI BMP PF out Pout
pollutant dynamic mass balance, no internal reactions or evaporation (10.2.9)
0 = Q C + Q C + IA C − FA C − Q C
misc Pmisc up Pup BMP PI BMP PF out Pout
pollutant steady-state mass balance, no internal reactions or
evaporation (10.2.10)

The storage volume is then used along with the time and the equations to calculate the
concentration within the BMP (C ).
PBMP
There are additional models that are needed to aid in this estimation, and these are
models of how the pollutant is distributed within the BMP so that it can be determined
what the pollutant concentrations might be of the flows out of the system. The two
extremes are usually the completely mixed assumption (the concentration is equal
throughout the BMP) and the plug flow assumption. Plug flow means that there is no
mixing in the direction of flow within the BMP so that there is a pollutant concentration
gradient in this direction. If it is assumed that the pollutant is evenly distributed within
the BMP (completely mixed model), then it can be assumed that C is equal to both
PBMP
C and C . Plug flow and other models are beyond the scope of this text and are better
PF Pout
presented in environmental modeling or chemical operation texts, but may be important
in some BMP modeling, particularly in systems such as bioretention cells where there
may be nutrient uptake within some layers of the cell.
If the completely mixed model is assumed for the pollutant mass balances, then
Eqs. (10.2.9) and (10.2.10) simplify to

Accumulation rate of pollutants at time t = Q C + Q C + IA C − (FA + Q )C
misc Pmisc up Pup BMP PI BMP out PBMP
pollutant dynamic completely mixed model, no internal reactions/evaporation
(10.2.11)
0 = Q C + Q C + IA C − (FA + Q )C
misc Pmisc up Pup BMP PI BMP out PBMP
pollutant steady-state completely mixed, no internal reactions/evaporation (10.2.12)
Pervious Concrete
Pervious concrete is an alternative paving surface that allows infiltration into the
subbase and ground below. It is a novel paving material that is being developed to aid
in maintaining preexisting hydrologic conditions while still giving a structural base for