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47
I 20) The average seasonal nutrient loadings attributed to
_ land runoff (non-point sources) and the average annual nutrient
• loadings attributed to municipal and industrial wastewater
discharges (point sources) are summarized as follows:
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NUTRIENT LOADINGS
I IN THE
LOWER SUSQUEHANNA RIVER BASIN
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Point Source* , Non_Point source Contributions , Total (Point + Non-Point Sources
Contributions
(Municipal ; -
Parameter Wastewater & June 1971 Nov. 1971 Mar. 1971 June 1971 Nov. 1971 Mar. 1972
Industrial to to to to to to
Discharges) Oct. 1971 Feb. 1972 May 1972 Oct. 1971 Feb. 1972 May 1972
I Ibs/day j
i
- » -
IT.POij as
PO^ 21,800 8,400 20,000 31,300 30,000 42,000 53,000
JTKN as N 17,400 14,000 35,000 54,000 31,500 53,500 71,500
as N 20,100 106,000 198,000 264,000 126,000 218,000 284,000
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* Average annual load applicable to each season
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48
a) Of tKe total phosphorus and nitrogen loadings from
the lower Susquehanna River Basin the percentages attributable
to point source and non-point source discharges are as follows:
1
Parameter
June 1971 - Oct. 1971
; Point Source Source"1
Nov. 1971 - Feb. 19
Point Source N°^
72 JMar. 1972 - May 1972
^nt:! Point Source |N^nt
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T.PO^ as
TKN as N
TN as N
PO
72
55
16
28
45
84
52
33
48
67
91
41
24
59
76
93
As can be seen, non-point source contributions of T.P04 and
TKN predominate when flows increase. Total nitrogen contributions
from non-point sources are most significant in every season. These
differences in percentage signify the increased importance of the
collective load from non-point sources when runoff rates are high.
During the high flow period (March - May 1972) approximately
93 percent of the estimated 284,000 Ibs/day of total nitrogen
(N02 + N03 and TKN) entering the surface waters of the lower basin
was from land runoff (non-point sources) with the remaining 7 percent
from municipal wastewater and industrial discharges (point sources).
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49
•
•
Of the 264,000 Ibs/day of total nitrogen from land runoff,
approximately 229,000 Ibs/day, or 87%, was from agricultural land
areas which comprise only 42% of the total drainage area in the
I lower basin.
b) The average annual yield, Ibs/day/sq. mile, for
each season based on 8,550 square miles in the lower Susquehanna
River Basin (3600 mi2 - agriculture, 4500 mi2 - forest and 450 mi2
B urban) is as follows:
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m Average Annual Nutrient Yield
(Point + Non-Point Sources)
• Ibs/day/sq. mile
Parameter ' June - Oct. 1971 ;Nov. 1971 - Feb. 1972 !Mar. 1972 - May 1972
T.PO,, as P04 3.5 | 4.9 6.2
TKN as N 3.7 6.3 : 8.4
N02N03 as N 11.1 19.3 24.9
TN as N 14.7 25.5 33.2
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• Thus, the average annual yield was directly related to the
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runoff rates.
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21) An attempt was made to mass balance the average
I seasonal phosphorus and nitrogen loads (TPO., TKN and NCL+NO.,)
in each of the tributary basins. The method employed for this
H analysis was to compare measured loads with expected loads in
accordance with the following equation (Total Phosphorus):
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Pt = Pw * Pa * Pf + Pu t Ps
Where:
P. = total measured phosphorus in watershed
P = phosphorus in wastewater discharges
P = phosphorus from agricultural land
a
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Pf = phosphorus from forested land
• P = phosphorus from urban runoff
P = phosphorus lost or released in the stream channel
I through biological utilization, deposition, scouring, etc,
Of particular importance in this analysis is the magnitude and
I sign of the P term. The following tables, which delineate the
_ various components of the mass balance equations, permit several
™ conclusions to be drawn regarding P (or TKN and NO depending on
j o o
the parameter).
The negative signs shown for most of the P terms, regardless
• of flow, indicate that phosphorus was being retained in the stream
channels, bound there by sediments and/or aquatic plants. The
• apparent loss of nitrogen fractions which prevailed during the low
• flow period might be temporary, however, as indicated by the
increased number of positive TKN and NO terms during flood flow
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51
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conditions when a considerable tonnage of sediment is known to be
I transported to the main stem of the Susquehanna River. An
• explanation of why nitrogen and phosphorus recoverability differ so
greatly during periods of high streamflow and extensive scouring
• within these tributary basins may be due to the high solubility of
nitrogen - especially the nitrate form.
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1
22) A seasonal mass balance analysis for the main stem
• Susquehanna River between Northumberland, Pa., and Conowingo Dam,
• Md. was performed based upon all of the regression data previously
presented. The graphs in the Appendix vividly depict the relative
• effects of each tributary's load and the Harrisburg metro area on
the phosphorus and nitrogen balances in the river. In addition,
changes in mass between tributary confluences resulting from
m various physical, chemical and biological reactions occurring
within the stream channel are illustrated. The following
• observations are noteworthy:
a) The impoundments along the lower Susquehanna,
especially Conowingo Dam, had a profound effect on the phosphorus
M load in the river during the low flow periods. As can be seen,
the load decreased from about 17,000 Ibs/day to 9,000 Ibs/day
between Columbia and Conowingo. During high flow-low temperature
periods this decrease diminished because of the reduced rates of
biological utilization and shorter retention times in the impoundments,
§b) During the high flow period a considerable increase
in phosphorus (20,000 Ibs/day) was detected between Penns Creek
V and the Juniata River. Since this area is primarily undeveloped
with the total phosphorus contribution from existing land usage
estimated to be less than 2,000 Ibs/day, it was assumed that scouring
of the bottom sediment and inundation of shoreline marsh and weeds
played an important role in the phosphorus balance. The Susquehanna
channel is very unique in that its width undergoes a much greater
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62
increase than its depth when flows rise. It is also a known fact
that aquatic weeds and other sources of nutrients are prevalent
along the river's shore.
• Allowing for contributions from land runoff, it was estimated
that about 500 Ibs/day/mile of total phosphorus (as PO.) was
J introduced into this reach of the Susquehanna River during the
• maximum flow period. During the mean flow period (Nov., 1971 -
Feb., 1972) this overall scouring rate was computed to be
• approximately 70 Ibs/day/mile.
c) A comparison of wastewater effluents and other urban
9 contributions of phosphorus in the Harrisburg metro area revealed
• the significance of the sewage treatment plants during low-flow
periods and the over-shadowing of this load by non-point source
• loads during high flow periods.
d) Total nitrogen behaved much more conservatively in
y the Susquehanna River than phosphorus, particularly in the area
§of major impoundments. While the phosphorus load was reduced
radically through the impoundments, nitrogen remained essentially
V unchanged regardless of flow.
e) The relative importance of point source and non-
P point source contributions of total nitrogen from the Harrisburg
§area for various flow conditions closely paralleled the findings
presented in the above statement for phosphorus.
M f) Due to excessive stratification it was not possible
to adequately balance the summation of the North Branch and West
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• 63
™ Branch nitrogen load with the measured load at Sunbury. This
• problem became especially acute during the high flow period when
about 25,000 Ibs/day of TKN could not be accounted f0r\
I g) The effects of scouring and inundation of shoreline
vegetation were not restricted to phosphorus. A review of the
• nitrogen data between Penns Creek and the Juniata River indicated
• a significant increase in load during high flow periods (60,000
Ibs/day) which corresponded closely to the phosphorus profile and
• which could not be attributable to normal runoff from the area.
Deducting the appropriate agricultural and forested runoff loads
m from this observed increase yielded a scouring rate of 1,200
• Ibs/day/mile. A rate of about 100 Ibs/day/mile was computed for
the mean flow condition. During low flow - high temperature periods
• both nitrogen and phosphorus loadings were reduced in this stream
reach probably because of a physical deposition process.
| h) The mass balance analysis of the nitrogen fractions
H (TKN and NO.,) generally corroborated the pertinent findings for
total nitrogen. During the low flow period the ratio of TKN to NO.,
IT varied from about 2:1 in the extreme upper reach of the Susquehanna
River to about 1:1 near Conowingo. This increased abundance of nitrate
£ nitrogen may be partly due to nitrification and, more importantly,
I to the relatively greater nitrate loadings contributed by the
various sub-basins. A similar pattern was evidenced during the
I mean flow condition when nitrification was minimal.
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* 65
I
transported by the Susquehanna River, it has been estimated that
P only about 2 million tons actually enters the Chesapeake Bay
_ because much sediment is trapped behind the power dams along the
* lower Susquehanna.
• 25) A summary of annual sediment yields and computed
nutrient yields for a comparable time period are presented in the
• following table for eight stations throughout the lower
Susquehanna River Basin. Except for Conestoga Creek, the data
• revealed a definite relationship between the tons per square mile
• of sediment yield and the phosphorus yield (lbs/mi2) on an annual
basis. The annual TKN yield also appeared to be strongly influenced
• by sediment load. The leaching and general mobility characteristics
of the NCu ion in soil are such that a reliable correlation between
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