Geoscience Reference
In-Depth Information
Table 6.5 Estimated total phosphorous load exported by the Saginaw River and load generated by
point sources
CSO/SSO est.
(met. ton)
Total load
a
(met. ton)
WWTP
(met. ton.)
CSO fraction of
load (%)
WWTP fraction of
load (%)
2001 2.43
-
642
0.38
-
2002 3.02
-
513
0.59
-
2003 0.59
-
345
0.17
-
2004 2.98
116
724
0.41
16.0
2005 -
110
288
-
38.2
a
MDEQ (
2003
,
2004
,
2005
,
2006
)
area. The processed data were to be used in the DLRBM to estimate the frequency
and magnitude of the CSOs in the study area. However, the phosphorous load
generated by point sources accounts for ~20 % of the total load exported by the
Saginaw River during wet years and ~40 % of the load during dry years (Table
6.5
),
and CSO's contribution to the total phosphorous load entering the bay is negligible,
indicating CSO's impacts are more localized. Consequently, CSOs were not
modeled at least initially. Loads from other municipal and industrial sources will
vary only at the monthly scale (He and DeMarchi
2010
).
As addition of water quality components to the DLRBM is undergoing, a suite of
regression models were first developed to relate total phosphorous (TP) concentra-
tion to the river discharge using concentration data reported by the MDEQ (
2003
,
2004
,
2005
,
2006
) and river discharge data for the watersheds shown in Fig.
6.1
(He
and DeMarchi
2010
; Tao et al.
2010
). Point sources in Saginaw/Bay contribute
almost one quarter of the TP load reaching the Bay (Table
6.5
). The estimated
annual TP loads by different watersheds are shown in Fig.
6.5
. The more rural
Tittabawasse River Watershed (the largest tributary to the Saginaw River), Cass
River, and Shiawassee River contributed about 31 %, 12 %, and 7 % of the TP load
entering the bay, respectively, and that most of these loads are of agricultural origin.
The two urbanized Saginaw River and Flint River Watersheds contributed 23 % and
20 % of the TP load, respectively (Tao et al.
2010
). On per square kilometer basis,
both the Saginaw and Flint Rivers showed a TP load four times higher than the
average level of all other sub-watersheds, indicating significant contributions from
urban sources as cities of Saginaw and Bay City, and Flint are located in the two
watersheds respectively. A notable importance appears the role of the National
Wildlife Refuge (NWR), an area of wetlands and swamps upstream the city of
Saginaw, which acts as a sink for almost 10 % of the load coming from the upper
part of the watershed (He and DeMarchi
2010
).
6.7 Critical Nonpoint Source Pollution Areas
The Great Lakes Water Quality Agreement sets target total phosphorus (TP) load of
440 metric tons/year for Saginaw Bay. However, as shown in Fig.
6.6
, the TP load
estimates from the Saginaw River Watersheds (not including Au Gres-Riffle,
