The Impact of Flooding on Water Quality in the Waccamaw...

The Impact of Flooding on Water Quality in the Waccamaw River
Susan M. Libes, Kim Weaver, Matt Cline, Nicole Short, and Misty Frierson
Center for Marine and Wetland Studies
Coastal Carolina University, Conway, SC 29526
Daily Discharge of Waccamaw River at Longs
Introduction
The Waccamaw River is a black water river located in the coastal plain of northern South Carolina and southern North
Carolina (Figure 1). The river water’s dark color reflects high concentrations of humic materials produced by the
decomposition of organic matter derived from terrestrial plants. Black water rivers typically contain a vast expanse of
swamps and are characterized by slow water flow, high levels of dissolved organic matter and low levels of dissolved
oxygen. The latter is caused by aerobic respiration of dissolved organic matter by native microbes. Dissolved oxygen
(DO) levels, pH and alkalinity are low throughout the Waccamaw River. As a result, the South Carolina Department of
Health and Environmental Control (SC DHEC) has set a special minimum DO limit of 4 ppm that applies only to the
Waccamaw River. Nevertheless, DO levels frequently fall below these limits during summer and flood events.
1996
1997
1998
1999
30000
Floyd 9/15/99
(Harvey) 9/20/99
Discharge (cu. ft/s)
25000
Variations in river stage and discharge are driven by seasonal differences in rainfall. The average annual rainfall is
approximately 50 inches. Rainfall is lowest in the summer and highest in the winter. Deviations from this general trend
are caused by flood events usually associated with hurricanes during late summer and fall. As shown in Figure 2, during
the sampling period, discharges were highest as a result of Hurricanes Bonnie, Dennis, Harvey, Floyd and Irene as well
as a stationary cyclonic storm which occurred in April 1999. Rain during the winters of 1998 and 1999 was copious
enough to also cause sustained periods of flooding.
The Waccamaw River is located in a watershed currently under intense development due to the rapid growth occurring in
Horry and Georgetown Counties. Stormwater runoff draining developed areas could have significant effects on water
quality in the river. To test this hypothesis, we are conducting several studies on the Waccamaw River. The results
presented herein are based on samples collected on alternating days since September 1998 at one site located north of
Conway. Also included are some results obtained by synpotically sampling the river and its adjacent tributary creeks
from the NC-SC state line to Bucksport, SC at conditions of low and high flow. Also presented are results of bacterial
contamination measurements collected from tributary creeks draining the city of Conway and environs as part of an
ongoing U.S. EPA funded 319 Program project entitled “Identification and Mitigation of Non-Point Sources of Fecal
Coliform Bacteria and Low Dissolved Oxygen in Kingston Lake and Crabtree Creek (Waccamaw River Watershed)”.
More information about this project is available at: http://www.coastal.edu/science/eql/319/index.html.
2000
Hurricane Floods
Fran
9/5/96
Eduoard
Winter Floods
20000
1998
1999
8/31/96
Stationary
Cyclonic
Storm
4/27/99
15000
Irene
10/17/99
Bonnie
8/26/98,
Frances
9/8/98
& Earl
9/15/98
10000
Bertha
7/12/96
5000
0
1/0
2/29
4/29
6/28
8/27
10/26
12/25
Day/Month
Figure 2. Discharge at USGS Gauging Station located at
Longs, SC in the Waccamaw River.
Methods
Grab samples of surface water were collected from Murrells Landing, which is located 4 miles north of the city of
Conway, on alternating days from September 1998 to present. As shown in Figure 3, daily sampling was done
during flood events. The following analytes were measured in the lab using U.S. EPA methods: true color, pH,
alkalinity, turbidity, and chlorophyll. A Scout 2 Hydrolab was used to make in-situ measurements of temperature,
conductivity and dissolved oxygen. The Hydrolab also generates values of % saturation of dissolved oxygen.
Grab samples of surface water were similarly collected at the heads of 19 tributary creeks during three dates
selected to represent average high flow, low flow and flooding conditions. Also collected were samples from the river
spaced equidistantly between the NC-SC state line and Bucksport. These were analyzed as above. Elemental
analyses of the major ions were also performed. A Hydrolab was used to make in-situ measurements as described
above.
Grabs samples of surface water are also being collected on alternating weeks from three tributary creeks and the
Waccamaw River adjacent to the City of Conway in conjunction with an ongoing U.S. EPA 319 Program Project. The
following analytes are being measured in the lab using U.S. EPA methods: fecal coliform, Enterococcus, nitrate,
nitrite, phosphate, ammonium, BOD5, true color, pH, alkalinity, total dissolved solids, turbidity, and chlorophyll. A
Hydrolab is also being used to make in-situ measurements.
U.S. EPA 319 Program Project
Stormwater Sampling Site in
Downtown Conway
Figure 1. South Carolina Portion of Waccamaw River
and Pee Dee Watershed
Water Sampling
Flooding at Conway Marina from
Hurricane Floyd
30000
Hurricane
Floyd
25000
•S.C. DHEC 319 Program
•National Science Foundation, and U.S. EPA AIRE Program
•Environmental Quality Lab, Center for Marine and Wetland Studies, Coastal Carolina University
•Department of Marine Science, Coastal Carolina University
•Paul Drewes and Paul Conrads, U.S.G.S.
Discharge (cfs)
Acknowledgements
20000
15000
Winter Flood
of 1998
Cyclonic
Winter
Storm
Flood
of 1999
10000
5000
0
8/96
Hurricane
Fran
Hurricane
Bonnie
Non
flood
Nonflood
12/96
4/97
8/97
12/97
4/98
8/98
12/98
4/99
8/99
12/99
Date
Figure 3. Discharge at USGS Gauging Station located at
Longs, SC in the Waccamaw River on Sampling Dates.
RESULTS
Figure 4.
Figure 5.
Temporal Variation in Turbidity at Murrells Landing
Hurricane
Floyd
Cyclonic
Storm
600
25
Winter Flood
of 1999
Hurricane
Bonnie
20
Nonflood
15
Winter Flood
of 1998
10
Cyclonic
Storm
400
300
4/97
8/97
12/97
Winter Flood
of 1999
Nonflood
Hurricane
Floyd
6.0
4/98
8/98
12/98
4/99
8/99
0
8/96
12/99
Normal DHEC criteria
5.5
Winter Flood
of 1998
Hurricane
Fran
12/96
4/97
8/97
12/97
4/98
Date
8/98
12/98
4/99
8/99
Hurricane
Bonnie
Winter Flood
of 1998
Cyclonic
Storm
Winter Flood
of 1999
4.0
8/96
12/99
Hurricane
Floyd
Blackwater DHEC criteria
4.5
Nonflood
Nonflood
Nonflood
6.5
5.0
100
12/96
7.0
500
Hurricane
Fran
High acidity (low pH) is associated with flooding
events. This is likely due to the production of
acid as the dissolved organic matter decays as
well as to dilution of natural buffers by the large
volume of flood waters.
7.5
Hurricane
Bonnie
200
Hurricane
5
Fran
0
8/96
High color is associated with
flooding events.
This color is
associated with the input of high
molecular weight dissolved organic
compounds such as humic and
tannic materials
700
Color (PtCo)
Turbidity (NTU)
30
8.0
800
High turbidities are associated with flooding
events as well as significant rainfall events.
This is likely due to soil erosion during periods
of high sheet flow. Spikes in turbidity during
nonflood conditions are all associated with rain
events.
pH
40
35
Figure 6.
Temporal Variation in pH at Murrells Landing
Temporal Variation in Color at Murrells Landing
12/96
4/97
8/97
12/97
4/98
8/98
12/98
4/99
8/99
12/99
Date
Date
Dissolved Oxygen
As shown in Figure 7, low dissolved oxygen levels are characteristic of summer conditions
as well as flood events. These levels frequently drop below special criteria established by
S.C. DHEC for blackwater rivers. During the summer, dissolved oxygen levels are
influenced by the effect of temperature, as this gas is less soluble at higher temperatures.
To eliminate this influence, the dissolved oxygen concentrations are also reported as %
saturations. The % saturation of dissolved oxygen is defined as the percentage of
dissolved oxygen that is present relative to the amount which should be in the water if
equilibrium with the atmospheric oxygen reservoir had been attained at the in-situ
temperature. Values less than 100% represent a net deficiency in dissolved oxygen.
Temporal Variation in Dissolved Oxygen
at Murrells Landing
Dissolved Oxygen (ppm)
12
Nonflood
Nonflood
6
Winter Flood
of 1998
4
2
Hurricane
Fran
0
8/96
Winter
Flood
of 1999
Cyclonic
Storm
Hurricane
Bonnie
12/96
4/97
8/97
12/97
4/98
Hurricane Floyd
8/98
12/98
4/99
8/99
% Saturation of DO
% Saturation of DO
Nonflood
r = -0.91
Winter Flood of 1999
r = -0.64
40
30
Hurricane Bonnie
r = -0.94
Hurricane Fran
10
12/96
4/97
8/97
12/97
4/98
8/98
Hurricane Floyd
12/98
4/99
8/99
12/99
Date
Winter Flood
of 1998
Winter Flood
of 1999
70
60
50
Cyclonic
Storm
40
Hurricane Bonnie
30
Hurricane Fran
r = -0.94
Hurricane Floyd
0
5
10
15
20
25
o
30
0
35
100
200
300
400
500
Figure 15.
Temporal Variation in Conductivity at Murrells Landing
Kingston Lake
700
Figure 16.
Fecal Coliforms
Conway Marina
600
Color (PtCo)
Temperature ( C)
140
Enteroccocus
Longs Ave
Conway Marina
Kingston Lake Swamp
Kingston Lake
Longs Ave
Kingston Lake Swamp
2500
80
60
Hurricane Winter
Bonnie
Flood
of 1999 Cyclonic
Storm
Winter Flood
Hurricane
of 1998
Floyd
Hurricane
Fran
20
Fecal Coliforms (CFU/100 mL)
Nonflood
100
Enterococcus (CFU/100 mL)
1800
Nonflood
120
1600
1400
1200
1000
800
600
400
200
0
0
8/96
12/96
4/97
8/97
12/97
4/98
8/98
12/98
4/99
8/99
12/99
08/08/99
2000
1500
1000
500
0
09/07/99
10/07/99
Date
11/06/99
12/06/99
01/05/00
02/04/00
08/08/99
09/07/99
10/07/99
Date
11/06/99
12/06/99
01/05/00
02/04/00
Date
Conductivity
Bacterial Contamination
As shown in Figure 11, low conductivities are associated with flooding suggesting a dilution effect. During periods of low
discharge, significant flood events are associated with high conductivities. During all levels of discharge, the tributary creeks
along the Waccamaw River are sources of high conductivity water (with the exception of Tilly Swamp) (Figure 12, 13 and 14).
This suggests that the impact of these creeks on the chemical composition of the river can be significant during times of low
river flow. The elemental composition of the high conductivity water is consistent with a groundwater and/or sea salt source.
The likely source of the groundwater are irrigation wells, especially those associated with golf courses. The seasalt is likely the
result of atmospheric transport from the sea surface. While conductivity is not a pollutant, it appears to be a tracer of
stormwater runoff via the River’s tributaries. This suggests the potential for significant pollutant transport, such as of nutrients,
heavy metals, pesticides and herbicides via these tributaries. Interestingly, these creeks are not sources of dissolved organic
matter and have higher dissolved oxygen concentrations than the river. This suggests that the major sources of dissolved
organic matter to the river are from adjacent swamps, transported by either sheet flow or shallow groundwater seeps. Current
sampling efforts are directed at characterizing the chemical composition of these flows.
As shown in Figures 15 and 16, high levels of bacterial contamination are consistently observed in several tributaries draining the city of Conway and environs. Flooding associated with
Hurricane Floyd (9/15/99) appears to have had a diluting effect, if any, probably because it was preceded by a lesser event (Hurricane Dennis on 8/29/99) and several very heavy rain
events on 9/5/99 through 9/9/99. This likely flushed the land surfaces prior to Hurricane Floyd. Similar results were observed at other sites on the Waccamaw and the Pee Dee Rivers.
Enteroccocus is being measured as an independent tracer of microbial contamination. We are also adapting a more specific tracer, multiple antibiotic resistance, to further ascertain
sources of E. coli which we are also measuring. This technique enables discrimination of wildlife, livestock, and pet sources from humans.
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sp
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300
200
Conductivity (µS/cm)
50
River
400
300
250
Figure 14.
Tributary Creeks
River
200
150
100
50
100
0
0
or
th
am
Bu 's
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ry
's
Cr
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Sim
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Pit ely p
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La y
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Bu din
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sp
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100
350
W
150
Conductivity (µS/cm)
200
Geographic Variation in Conductivity in the Waccamaw River
and Tributary Creeks during Winter Flood 1999
2/9/99 Discharge 4758 ft3/s
Tributary Creeks
500
or
th
am
Bu 's
ck Fer
ry
's
Cr
ee
k
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on ute
's
9
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S t e ek
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Kin e S mp
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M nC p
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Co ina ek
nw Cr
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Be Ma
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Gr Sw a
a v am
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Pit lly p
ch Gu
La lly
n
Bu din
ck g
sp
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River
250
or
th
am
's
Figure 13.
Tributary Creeks
W
Figure 12.
Sim
300
The following are observed impacts of flooding on the water
quality of the Waccamaw River:
600
400
350
Conclusions
Geographic Variation in Conductivity
in the Waccamaw River and Tributary Creeks (Low water)
11/13/98
Discharge 81 ft3/s
Geographic Variation in Conductivity in the Waccamaw River
and Tributary Creeks after Hurricane Bonnie
9/12/98 Discharge 4587 ft3/s
Conductivity (µS/cm)
Hurricane
Bonnie
Hurricane
Fran
0
8/96
10
Hurricane Floyd
0
Conductivity (µS/cm)
20
20
0
W
Winter
Flood
of 1999
Cyclonic
Storm
30
80
Winter Flood
of 1998
Figure 11.
40
Winter Flood
of 1998
40
Nonflood
r = -0.84
90
10
50
Effect of Color on % Saturation of DO
at Murrells Landing
Cyclonic Storm
r = -0.95
20
60
100
90
50
Nonflood
70
Figure 10.
Effect of Temperature on % Saturation of DO
at Murrells Landing
80
Nonflood
80
Figure 9.
100
60
90
As shown in Figure 9, the % saturation of dissolved oxygen is also inversely related to
temperature. This is likely the result of enhanced microbial activity at higher temperatures
as well as the occurrence of organic matter loading during warm weather, I.e. hurricane
season. It is interesting to note that even during the winter, flooding is associated with
marked oxygen deficiencies relative to non-flooding conditions during this time of year.
12/99
Date
70
100
The waters in the Wacccamaw River are always deficient in dissolved oxygen relative to the
atmosphere (Figure 8). Even during the winter, the surface waters never exceed 80%
saturation. This suggests the presence of a very large and persistent dissolved oxygen
sink. This sink is likely associated with the microbially mediated decomposition of dissolved
organic matter. Hence, as shown in Figure 10, high loads of dissolved organic matter (as
represented by color) are associated with low % saturations of dissolved oxygen. As
flooding intensifies this effect, fresh organic matter appears to be delivered into the river
during these events. This is likely the result of high winds which create new leaf litter by
stripping trees as well as by inducing wind mixing. The latter can churn up swamp
sediments and increase the leaching of dissolved organic matter from these particles.
10
8
Figure 8.
Temporal Variation in % Saturation of Dissolved Oxygen
at Murrells Landing
% Saturation of DO
Figure 7.
•Increased turbidity and color (dissolved organic matter)
•Decreased
conductivity
pH,
alkalinity,
dissolved
oxygen
and
Rain events which occur during periods of low discharge are
associated with periods of high conductivity in the river. This is
thought to reflect a significant contribution of tributary creek
water to the total river flow. During these periods, transport of
pollutants, such as nutrients, heavy metals, pesticides and
herbicides, could result in high river water concentrations.
All rain events are associated with surges in turbidity
suggesting particle loading as a result of soil erosion.
The Waccamaw River is always deficient in dissolved oxygen.
The intensity of this deficiency is intensified by high
temperatures as well as increased loading of dissolved organic
matter. Flooding appears to bring a different type of dissolved
organic matter into the river which has a high oxygen demand.
Flooding was not observed to cause sustained levels of
bacterial contamination probably due to the dilution of sources
by the large volume of water in the river as well as prior
flushing.