PHOSPHORUS SOURCES AND UPPER MISSISSIPPI RIVER WATER QUALITY.  

Michael L. Meyer and Scott Schellhaass.   Environmental
Monitoring and Assessment Section, Metropolitan Council
Environmental Services, Mears Park Centre, 230 East Fifth Street,
St. Paul, MN 55101.  

The Metropolitan Council Environmental Services (MCES) River
Monitoring Program, established in 1976, monitors the water
quality of the Mississippi, Minnesota, and St. Croix Rivers in
the Twin Cities Metropolitan Area.  For the period 1976 to 1996,
total phosphorus (TP), soluble reactive phosphorus (SRP), total
suspended solids, (TSS), and chlorophyll-a were examined at three
key locations: the Mississippi River at Lock and Dam No. 1 (Pool
1), and near the mouths of the Minnesota and St. Croix Rivers. 
FLUX was used to estimate annual and monthly TP and TSS loads. 
Information on phosphorus loads from point sources was compiled
from the MCES, Minnesota Pollution Control Agency, and the
Wisconsin Department of Natural Resources.  Landuse varies
greatly among the three river basins.  The Minnesota River Basin
(16,988 mi 2 ) is dominated by row crops with extensive tile
drainage; the Mississippi River Basin (19,884 mi 2 ) is a mix of
forest, row crops, and pasture/grass; and the St. Croix River
Basin (7,722 mi 2 ) is dominated by forest and pasture/grass.  The
Mississippi River had the highest mean annual flow (9,248 cfs),
followed by the Minnesota River (6,274 cfs) and the St. Croix
River (5,531 cfs).  However, mean annual runoff was highest in
the St. Croix Basin (9.67 in), followed by the Mississippi Basin
(6.31 in) and the Minnesota Basin (5.04 in).  The percentage of
mean annual runoff was very high in the St. Croix Basin (31.0%),
followed by the Mississippi Basin (22.4%) and the Minnesota Basin
(16.8%).  In all three rivers, mean monthly flow is highest in
April, with flows declining steadily through the summer. 
However, Minnesota River summer flows do not decline as rapidly
as summer flows in the other two rivers, possibly due to the
displacement of perennial vegetation with annual row crops. 
Flow-weighted mean annual TSS concentrations and loads were very
high in the Minnesota River (93.3 mg/L, and 623 million metric
tons/yr, respectively), lower in the Mississippi River (19.4
mg/L, and 155 million metric tons/yr, respectively), and very low
in the St. Croix River (5.4 mg/L, and 24 million metric tons/yr,
respectively).  Annual TSS yields from the Minnesota,
Mississippi, and St. Croix River Basins were 134, 28, and 13
lb/ac/yr, respectively.  Inorganic content of the TSS in the
Minnesota, Mississippi, and St. Croix Rivers was 89, 69, and 50
percent, respectively.  Rankings of the three rivers were similar
for average TP concentrations and loads: 1) Minnesota River (0.32
mg/L and 1,565 metric tons/yr); 2) Mississippi River (0.10 mg/L
and 889 metric tons/yr); and 3) St. Croix River (0.05 mg/L and
262 metric tons/yr).  In the same order, phosphorus yields for
the three were 0.33, 0.16, and 0.14 lb/ac/yr.  Monthly TSS and TP
loads from the St. Croix River were highest in April, following
the pattern of an undisturbed, natural system.  In the Minnesota
and Mississippi Rivers, monthly loads peaked in June as well as
April, possibly another outcome of the displacement of perennial
vegetation with annual row crops.  In the Minnesota River Basin,
monthly precipitation and rainfall erosivity indices are
relatively constant from June through August; however, as the row
crop canopy closes and evapotranspiration rates increase in late
summer, flows and loads decrease dramatically.  During the near-
normal flow years of 1994-1996, nonpoint sources contributed 51%
of the TP load and point sources contributed 49% of the TP load
upstream of Lock and Dam No. 3 (Pool 3).  The two largest sources
of TP were the Minnesota River Basin (35%) and Metropolitan
Wastewater Treatment Plant (28%).  Wastewater treatment plants
(WWTPs) discharge phosphorus at a relatively constant rate
throughout the year, while phosphorus loads from nonpoint sources
vary directly with precipitation and runoff.  As a consequence,
point sources dominate TP loads during low river flows, and
nonpoint sources dominate during high river flows.  

Keywords:  Water quality, suspended solids, phosphorus, landuse,
point source, nonpoint source