Contact Person : Dr. D. J. Mulla
Monthly median loadings from March to August for total suspended
solids (TSS) and total phosphorus (TP) in the 12 major watersheds
of the Minnesota River basin generally increased from west to
east along a gradient of increasing precipitation and more highly
erodible land. The Lower Minnesota watershed produced the greatest
loads for TSS and TP of any watershed, with 26% of the total basin-wide
load of TSS and 32% of the TP. Loadings from the Lower Minnesota
watershed alone were comparable to the combined loadings from
the three next greatest contributing watersheds. The Le Sueur,
Blue Earth and Watonwan watersheds combined produced 27% of the
total basin-wide load of TSS and 37% of the TP load. The Lower
Minnesota, Le Sueur, and Blue Earth watersheds combined produced
64% of the total basin-wide load of TSS and 64% of the TP. The
analysis showed significant loadings from the less intensively
farmed urban edge portion of the Lower Minnesota watershed downstream
of Jordan. This urban-edge region produced 50% of the TSS and
71% of the TP generated within the Lower Minnesota watershed.
Only about 26% of the TP loadings in the Lower Minnesota watershed
were attributable to the Blue Lake and Seneca wastewater treatment
plants.
In this study, estimates were made for monthly median loadings
from March to August of TSS and TP in the 12 major watersheds
of the Minnesota River basin (Fig. 1).
Water quality monitoring and stream flow records for the last
13-21 years were obtained from the Minnesota Pollution Control
Agency, the U.S. Geological Survey, and the Metropolitan Waste
Control Commission. These data were used in a log-linear regression,
stratified according to flow regime, to establish the relationship
between flow and concentration. Loadings were obtained from the
product of flow records and the flow weighted mean concentrations
using the FLUX model and the regression equation for the appropriate
flow regime.
Monitoring data were not available in sufficient detail to obtain
FLUX estimates for loadings in the Upper Minnesota, Lac Qui Parle,
Hawk Creek, Middle Minnesota or Le Sueur watersheds . Data from
the Minnesota River Assessment Project (MnRAP) for the Le Sueur
and Blue Earth watersheds were used to estimate the fraction of
TSS and TP produced by the Le Sueur watershed relative to the
Blue Earth watershed. In March and April the Le Sueur produced
33% less TSS than the Blue Earth, while from May to August the
Le Sueur produced 50% more TSS than the Blue Earth. These percentages
were used in estimating the loads of TSS and TP for the Le Sueur
watershed using loading rates in the Blue Earth river after its
confluence with the Le Sueur and Watonwan, and after subtracting
the loads estimated for the Watonwan river.
In the Upper Minnesota, Lac Qui Parle, Hawk Creek, and Middle
Minnesota watersheds, the loadings of TSS and TP were estimated
using the area of the watershed and the yields in tons/mi2
for the appropriate water quality parameter in the watershed deemed
to be most similar. For example, a TSS load of 6,136 tons/mo was
estimated for the Middle Minnesota watershed by multiplying the
area of the watershed by the TSS yield for the Cottonwood watershed.
Total Suspended Solids
Loading rates for TSS ranged from 18,825 metric tons/month for
the growing season in the Lower Minnesota River to 471 tons/month
in the Pomme de Terre watershed (Fig. 2).
The second and third highest loading rates were 14,804 and 13,595
tons/mo in the Le Sueur and Blue Earth watersheds, respectively.
The fourth highest loading rate was 5,816 tons/mo in the Cottonwood
watershed. Using monitoring data for the Minnesota River at St.
Peter and Jordan, the TSS loads from the urban edge area downstream
of Jordan were estimated at 9,385 tons/mo. Thus, 50% of the TSS
loading for the entire Lower Minnesota watershed originates in
the less intensively farmed urban edge portion of the Lower Minnesota
watershed (Fig. 3).
The TSS loads from the Lower Minnesota, Blue Earth, and Le Sueur watersheds represent approximately 66% of all the TSS in the Minnesota River at Fort Snelling (Fig. 4). The loading of TSS from the Lower Minnesota watershed alone accounts for roughly 26% of all TSS in the Minnesota River at Fort Snelling. The TSS loading from the Le Sueur, Blue Earth, and Watonwan watersheds represents 63% of the TSS load in the Minnesota River at Mankato, and 27% of the loading in the Minnesota River at Fort Snelling. A 40% reduction in TSS for the Lower Minnesota, Le Sueur, and Blue Earth watersheds would result in 18,890 tons/mo of TSS prevented from entering the Minnesota River, which is a 26% reduction in TSS loading in the Minnesota River at Fort Snelling.
Because each watershed differs in size, it is useful to estimate
the production of TSS per square mile. The TSS production per
square mile ranged from 13.4 tons/mi2 in the Le Sueur
watershed to 0.52 tons/mi2 in the Pomme de Terre watershed
(Fig. 5). Production of TSS
was 10.3 tons/mi2 in the Lower Minnesota, 8.5 tons/mi2
in the Blue Earth, 4.4 tons/mi2 in the Cottonwood,
3.6 tons/mi2 in the Watonwan, and between 1.0 and 1.5
tons/mi2 in the Chippewa, Yellow Medicine, and Redwood
watersheds. Thus, the watersheds with the highest TSS loads also
had the highest production of TSS per square mile.
Total Phosphorus
Loading rates for TP ranged from 58 metric tons/month in the Lower
Minnesota watershed to 2.4 tons/mo in the Pomme de Terre watershed
(Fig. 2). The second and third
highest loading rates for phosphorus were 29.8 and 26.0 tons/mo
in the Le Sueur and Blue Earth watersheds, respectively. The Chippewa,
Watonwan, and Cottonwood watersheds were tied for fourth highest
loading rates at from 9.3 to 9.9 tons/mo. Phosphorus loads from
the less intensively farmed urban edge portion of the Lower Minnesota
watershed downstream of Jordan were 41.0 tons/mo, which is 71%
of the total phosphorus load produced by the Lower Minnesota watershed
(Fig. 3). Based upon data in
MnRAP, the Blue Lake and Seneca wastewater treatment plants produced
approximately 15.3 tons total phosphorus per month, which on average
is 26% of the phosphorus loadings in the Lower Minnesota watershed.
In recent years, new treatment technologies have significantly
reduced P loadings from the Blue Lake and Seneca wastewater treatment
plants.
The Lower Minnesota, Le Sueur, and Blue Earth watersheds produced
64% of the total phosphorus in the Minnesota River at Fort Snelling
(Fig. 6). The Lower Minnesota
watershed alone produces 32% of the total phosphorus load at Fort
Snelling. The Le Sueur, Blue Earth, and Watonwan watersheds produce
54% of the total phosphorus load in the Minnesota River at Mankato,
and 37% of the loading in the Minnesota River at Fort Snelling.
A 40% reduction in phosphorus loads from the Lower Minnesota,
Le Sueur, and Blue Earth watersheds would prevent 45.5 tons P/mo
from entering the Minnesota River, which is 64% of the reduction
needed to reduce total phosphorus loads at Fort Snelling by 40%.
Implications
The results of this study can be used to indicate which watersheds
are the highest sources of pollutant loading to the Minnesota
River basin. Loading rates for TSS and TP were always higher in
the Lower Minnesota watershed than in any other watershed. In
fact, the loading from the Lower Minnesota watershed was comparable
in magnitude to the total loading produced by the sum of loadings
from the Le Sueur, Blue Earth, and Watonwan watersheds. The Lower
Minnesota watershed is unique in that it is characterized by significant
acreage of 1) very steep topography (Fig. 7) with
high annual precipitation, 2) approximately one quarter of the
land is within a quarter mile of streams, and 3) a very high rate
of urbanization.
Loading rates were also high in the Le Sueur and Blue Earth watersheds, which contributes the second or third highest loading rates for TSS and TP to the Minnesota River. Based upon these results, as well as those in MnRAP, we recommend focussing strategies for reducing point and non-point source pollution loads for sediment and phosphorus in the Lower Minnesota, Le Sueur, and Blue Earth watersheds. Significant attention within the Lower Minnesota watershed should be focussed on reducing pollution loads from the less intensively farmed urban edge portion of the Lower Minnesota watershed downstream of Jordan. After addressing sediment and phosphorus loading from the Lower Minnesota, Le Sueur, and Blue Earth watersheds, attention should be given to the Cottonwood, Middle Minnesota, Watonwan, and Chippewa watersheds. The lowest contributions to non-point source loadings in the Minnesota River occur from the Pomme de Terre, Upper Minnesota, Lac Qui Parle, Hawk Creek, Yellow Medicine, and Redwood watersheds.
