WATERSHEDS - Water Quality Engineering in Natural Systems - page 207

TABLE 6.2. Average Water Quality of Combined Sewer
Overflows

concentration of Pb in stormwater runoff has decreased
by at least a factor of 4 due to the shift from leaded
to unleaded gasoline (NRC, 2009).
The NURP study indicated that there is no consistent
pattern of concentration within runoff events, and the
study focused on the statistical evaluation of the event
mean concentration (EMC), defined as

Parameter Average Concentration

BOD 5 (mg/L) 115

TSS (mg/L) 370

Total N (mg/L) 9-10

PO 4 -P (mg/L) 1.9

Pb (mg/L) 0.37

10 2 -10 4 Total coliform (MPN/100 mL)

mass of pollutant contained in runoff event
total volum

EMC =

e of runoff in the event

Source of data : USEPA (1978).

= ∑
∑

QC

i i

water-quality characteristics of CSOs are shown in Table
6.2. Organic solids accumulate and sewer slime grows
during dry periods, and hence far more pollutants accu-
mulate in combined sewers than in storm sewers, which
are idle and have relatively low flows between rainfall
events.

Q

i

(6.1)

where Q i and C i are the flow and pollutant concentra-
tion, respectively, at time step i within a runoff event.
The EMC is equal to the flow-weighted mean concen-
tration. The NURP study found that geographical loca-
tion, land use category, runoff volume, and other factors
are statistically unrelated to EMCs and do not explain
site-to-site or event-to-event variability. A possible
reason for this is that it is not the land or land use that
causes pollution, but pollution is more related to inputs
and polluting activities that occur on the land. In most
cases of practical interest, the total mass and EMC of a
pollutant are far more important than the individual
concentration distributions within individual runoff
events. A useful finding of the NURP study was that the
EMCs for most pollutants follow a log-normal distribu-
tion, and median values and coefficients of variation
(COVs) of EMCs found by NURP for the the major
urban land use categories are shown in Table 6.3. The
COV is defined as the standard deviation of the obser-
vations divided by the mean. At any particular site,
EMCs are assumed to vary between storm events, and
the values shown in Table 6.3 reflect this variability. It
should be noted the COV ≈ 1 for many contaminants,
which indicates that EMCs can have order-of-magni-
tude variability. The site mean concentration (SMC) is
the characteristic runoff concentration at the site and
can be taken as the median value of the EMCs at the
site (hvitved-Jacobsen et al., 2010). In U.S. practice, it is
common to use the acronyms EMC and SMC inter-
changeably to represent the median EMC at a site.
Subsequent to the NURP study, a National Stormwa-
ter Quality Database (NSQD) has been developed in
the United States to compile the results from required
regulatory monitoring of stormwater discharges. The
EMCs compiled from this data set are given in Table 6.4
(NRC, 2009). It is apparent from these data that the
medians of the NSQD data are relatively close to those
of the NURP data; however, the COV of the NSQD

6.2.2 Fate and Transport Processes

The fate and transport of contaminants in urban water-
sheds is very complex, contaminant-specific, and influ-
enced by numerous factors. In contrast to this complexity,
and as a matter of practicality, relatively simple lumped-
parameter models are commonly used to describe con-
taminant fluxes from urban watersheds into receiving
water bodies. The most commonly used models are the
event mean concentration (EMC) model and the
buildup-wash-off model. These models are described in
detail in the following sections.

6.2.2.1 Event Mean Concentration Model. A seminal
study called the National Urban Runoff Program
(NURP; USEPA, 1983a) was conducted in the United
States between 1978 and 1983 to investigate the rela-
tionship between urban land uses and pollutant load-
ings resulting from surface runoff from urban areas. The
NURP study included analyses of thousands of storms
in 28 experimental watersheds located in a wide variety
of locations in the United States. A key finding from the
NURP study was that uncontrolled discharges from
storm sewer systems that drain runoff from residential,
commercial, and light industrial areas carried more than
10 times the annual loading of total suspended solids
(TSS) than did discharges from municipal wastewater
treatment plants that provide secondary treatment
(USEPA, 1999). The NURP study also showed that
urban stormwater runoff carries higher annual loadings
than effluent from secondary treatment plants for:
chemical oxidation demand (COD), lead (Pb), copper
(Cu), oil and grease, and polyaromatic hydrocarbons
(PAhs). The results of the NURP study are still consid-
ered applicable today, with the exception that the typical