JAWRA - Stormwater Runoff

• Project Type: General OWC
• Title: Stormwater Runoff Quality and Quantity from Traditional & Low Impact Development Watersheds
• Date: 12/15/2009
• Water Conservation - Doc Type: Reports

BEDAN AND CLAUSEN
equation (Table 4). The mass export of Cu did not
change significantly in the postconstruction period.
Metals exports of Pb and Zn most likely decreased
because storm flow from this watershed declined dur-
ing the postconstruction period. Studies of individual
BMPs have shown reduced loadings of heavy metals
as well. Grassed swales are reported to reduce Pb
and Zn loadings in stormwater runoff by 60 %, and
permeable pavement can reduce these metals by 90%
(USEPA, 1993a). Rushton (2001) also reported that
pervious pavement with grassed swales reduced Pb
and Zn loadings from parking lot runoff by at least
75 %.
Total Suspended Solids
Traditional Watershed. Mean TSS concentra-
tions in runoff from the traditional watershed signifi-
cantly decreased (p = 0.001) by 79% in the
postconstruction period (Table 3). Decreased TSS con-
centrations may be due to the primary conveyance of
stormwater across the asphalt road and the curb and
gutter system in the postconstruction period. A lawn
area was the primary conveyer of stormwater in pre -
development conditions and soils were a likely source
of TSS. Dilution could also explain lower TSS concen-
trations. Median TSS EMCs from the NURP study
were higher compared with this study (Table 3)
(USEPA, 1983b). Total suspended solids concentra-
tions observed in this study (Table 3) were at the
lower end of the range (4 to 1,223 mgA) reported by
Makepeace et al. (1995) in their review of urban
stormwater quality.
The mass export of TSS from the traditional
watershed significantly increased (p < 0.001) by 32
times compared with the values predicted by the cali-
bration equation (Table 3). Since the TSS concentra-
tions decreased, the increased storm flow in the
postconstruction period presumably caused the
increase in the mass export of TSS. Increased runoff
from urbanization is expected to yield higher loads
of sediment compared with nonurbanized areas
(Leopold, 1968).
Low Impact Development Watershed. Total
suspended solids concentrations in runoff from the
LID watershed significantly increased (p < 0.001) by
almost three times in the postconstruction period
(Table 4). Increased TSS concentrations were likely
due to the grassed swales because stormwater was
primarily directed through them. The mass export of
TSS increased in the postconstruction period as well
(Table 4). However, observed TSS concentrations
were low. The NURP study (USEPA, 1983b) reported
TSS concentrations in residential runoff that were
ten times greater than the concentrations observed in
runoff from the LID watershed (Table 4).
Biochemical Oxygen Demand
Traditional Watershed. The mean BOD concen-
tration in runoff from the traditional watershed sig-
nificantly decreased (p = 0.009) during the
postconstruction period when compared with values
predicted using; the calibration equation (Table 3).
Low Impact Development Watershed. In the
LID watershed, the mean BOD concentration in runoff
did not change significantly (p = 0.693) during the
postconstruction period when compared with values
predicted using the calibration equation. Mean BOD
concentrations in runoff from the LID and traditional
watersheds were not significantly different in the post-
construction period based on a paired t -test (t = — 0.39,
p = 0.708). In this study, LIDs did not effectively
reduce BOD concentrations. Mean BOD concentra-
tions from this study (Tables 3 and 4) were generally
lower than the median EMC (10 mg/1) for residential
areas reported in the NURP study (USEPA, 1983b).
Fecal Coliform Bacteria
Traditional Watershed. Fecal coliform bacteria
abundance in runoff from the traditional watershed
did not significantly change from the calibration to
postconstruction period.
Low Impact Development Watershed. Fecal
coliform bacteria in runoff from the LID watershed
decreased significantly from the calibration to post-
construction period (Table 4). The mean FC concen-
trations of the study watersheds (Tables 3 and 4)
were low when compared with the FC concentrations
reported by the NURP study (USEPA, 1983b). Addi-
tionally, FC concentrations in this study were some-
what lower than the reported concentrations
( >800 fcu/100 ml) in residential runoff from else-
where in Connecticut (Dietz and Clausen, 2004).
CONCLUSIONS
Storm flow from the traditional development
increased dramatically during postdevelopment
conditions compared with predevelopment. The
increased runoff resulted in increased mass exports
of NO + NO NH TKN, TP, TSS, Cu, and Zn.
JAWRA
1006 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION