Physical, Chemical, and Biological Water Parameters
2.1 Introduction
This section of the report includes a discussion of the physical,
chemical, and biological water quality parameters measured as a part of the Cape Fear
River Program, with emphasis on the 1996-1997 monitoring period. These parameters are
interdependent and define the overall condition of the river. Physical parameters measured
during this study included water temperature, dissolved oxygen, turbidity, salinity,
conductivity, and pH (only the first four will be discussed). The chemical makeup of the
Cape Fear River was investigated by measuring the magnitude and composition of nitrogen,
phosphorus, silica and a suite of metals. Three biological parameters, chlorophyll a,
fecal coliform, and biochemical oxygen demand, were examined to assess the effects of
loading of nutrients and organic materials on water quality.
2.2 Materials and Methods
All samples and field parameters collected for the estuarine stations of the Cape Fear River (i.e. NAV down through M18) were gathered on an outgoing tide. This was done so that the data better represented the river water flowing downstream through the system rather than the tidal influx of coastal ocean water.
Physical Parameters
Water Temperature, pH, Dissolved Oxygen, Turbidity, Salinity, and Conductivity
Field parameters were measured at each site using a Solomat 803PS Multiparameter Water Quality Probe (sonde) and displayed on a Solomat 803 datalogger. Individual probes within the instrument measured water temperature, pH, dissolved oxygen, turbidity, salinity, and conductivity. At each station, physical parameters were measured at surface, middle and bottom. The instrument was calibrated regularly to ensure accurate measurements.
Chemical Parameters - Nutrients
All nutrient analyses were performed at the Center for Marine Science Research for samples collected from June 1995 through January 1996. A local state-certified analytical laboratory (Law and Company) was contracted to conduct all subsequent analyses (February 1996 through May 1997), except for the orthophosphate and silicate procedures which were performed at CMSR. The following methods detail the techniques used by CMSR personnel only.
Orthophosphate (PO4-3)
All orthophosphate analyses were performed in duplicate on filtered water samples using an approved EPA method for the Technicon AutoAnalyzer (Method 365.5). In this technique, the orthophosphate in each sample reacts with ammonium molybdate and antimony potassium tartrate in an acidic medium (sulfuric acid) to form an antimony-phospho-molybdate complex. The complex is then reacted with ascorbic acid and forms a deep blue color. The intensity of the color is measured at a wavelength of 880 nm by a colorimeter and displayed on a chart recorder.
Silicate (SiO3-2)
Reactive silicate analyses were performed in triplicate using the method described in Parsons et al. (1984). Reactive silicate, the type utilized by diatoms to produce their frustules, is oxidized to form a silicomolybdate complex. This complex is then reduced with a solution of metol sulfide and oxalic acid to produce a blue color. The resulting extinction is measured at a wavelength of 810 nm on a spectrophotometer.
Biological Parameters
Chlorophyll a and BOD analyses were performed at the UNCW Center for Marine Science Research. Fecal coliform analyes were performed by a state-certified commmercial laboratory.
Chlorophyll a
The analytical method used to measure chlorophyll a was taken from Parsons et al. (1984). Chlorophyll a concentrations were determined directly from the 1.0 micron glass fiber filters used for filtering samples for orthophosphate analyses. All filters were wrapped individually in aluminum foil, placed in an airtight container containing drierite, and stored in the freezer. During the analytical process, the glass filters were separately immersed in 10 ml of a 90% acetone solution for 24 hours. The acetone extracts the chlorophyll a from the glass filters into solution. Each solution was then analyzed for chlorophyll a concentration using a Turner AU-10 fluorometer.
Biochemical Oxygen Demand (BOD)
Five sites were chosen for BOD analysis. Two sites were located near
Riegelwood (Blenon Landing-BLL and Acme-AC), two sites were located upstream in blackwater
regions (N117 and B210), and one site (BBT) was situated in an area influenced by both the
mainstem Cape Fear River and "cleaner" blackwater. The procedure used for BOD
analysis was Method 5210 in Standard Methods (APHA 1995). Samples were analyzed for both
5-day and 20-day BOD. During the analytical period, samples were kept in airtight bottles
and placed in an incubator at 20 oC. All experiments were initiated within 5
hours of sample collection. Bioassays were run in duplicate. Dissolved oxygen measurements
were made using a YSI Model 51 meter which was calibrated using the Winkler titration
method. No adjustments were made for pH since all samples exhibited pH values within or
very close to the desired 6.5-7.5 range.
2.3 Results and Discussion
This section will focus on the physical, biological, and chemical parameters measured in the surface waters for stations on the lower Cape Fear system as well as smaller creeks and tributaries which feed the Cape Fear, Northeast Cape Fear, and Black Rivers.
Physical Parameters
Water temperatures for the lower Cape Fear River sites averaged
approximately 18-19 oC for the 1996-97 monitoring period (Table 2.1). The lowest temperatures were recorded during
February, when the water had cooled to between 8-10 oC. An interesting
observation was that water temperatures were warmer during January than during December
because of unusually warm air temperatures which persisted in mid to late December.
Additionally, water temperatures were cooler during May than during April as a result of
cool spring weather.
The mean water temperatures at the shallow tributary stations were
generally cooler than those on the main river sites. These sites are more influenced by
daily air temperature fluctuations since they are shallow and unaffected by tidal ocean
influxes.
Salinity values recorded at the estuarine stations during the
1996-97 monitoring period are listed in Table 2.2. As
expected, salinity values increase downstream throughout the Cape Fear Estuary ranging
from mean values of 0.1 ppt at NAV to 20.7 ppt at M18. During September 1996, surface
waters were completely fresh down through M42 as the torrential rainfall from Hurricane
Fran and the succeeding continental storms washed through the Cape Fear system. At M18,
near the mouth of the river, the measured surface salinity was only 9.9 ppt after the
hurricane. In May 1997, salinities again were low throughout the estuary following a
period of extensive rainfall.
In addition to the estuarine stations, two tributary sites (LRC and PB)
also exhibited measurable salinities averaging 0.5-1.0 ppt. However, at both of these
stations, the observed salinities originated from anthropogenic sources. Cates Pickle,
located just above the PB site, releases salty effluent into the creek. At LRC,
Stevecoknit Mills is situated just upstream of our sampling site and their effluent is
apparently responsible for the slightly saline waters.
Mean monthly dissolved oxygen data for all 16 of the lower Cape Fear
River sampling stations from June 1995 - May 1997 are illustrated in Figure 2.1. Wintertime dissolved oxygen concentrations
were much higher than summertime levels (lower water temperature causes increased
dissolution of O2). Additionally, stream ecosystem respiration increases with
warmer temperatures, contributing to lower dissolved oxygen concentratiions. It does
appear that wintertime dissolved oxygen levels were slightly lower during the 1996-97
period than during the preceding year. However, water temperatures were also considerably
warmer during this same period.
In the summer of 1995 (Figure 2.1), hypoxic (<5 mg/L) conditions
persisted for nearly four months (Mallin et al. 1996). It is likely that aquatic organisms
experiencing low oxygen concentrations in this range will become stressed (Boyd 1979).
During this period there was no significant vertical stratification and hypoxic conditions
persisted throughout the water column. During September 1996, a major hurricane (Fran)
struck the Cape Fear region and the substandard mean August DO levels of 4.1 mg/L were
reduced to a dangerously low systemwide average of 1.5 mg/L. At two stations in the
Northeast Cape Fear River, NCF117 and NCF6, near anoxic conditions in the surface and
bottom waters persisted for nearly three weeks. Other North Carolina river systems suffer
from frequent anoxic episodes, but for different reasons. In both the Neuse (Paerl et al
1995) and the Pamlico (Stanley 1987) systems, the bacterial decomposition of dying algal
blooms, stratification, and the insufficient flushing capabilities of each system cause
periodic summertime bottom water anoxia. In the Cape Fear system, where there is high
water flow and little vertical water column stratification, the long period of anoxia
which persisted following Fran was unprecedented. The consequences of the hurricane will
be discussed in detail in Chapter 5.
Mean dissolved oxygen data for the June 1995 - May 1997 monitoring
period (Figure 2.2) revealed an interesting spatial pattern in the lower Cape Fear system
(BLL, LVC, and B210 since February 1996 only). At the upstream stations (NC11-AC) and at
the lower estuarine stations (M18 and M23), concentrations were greatest and averaged
approximately 8 mg/L. Towards the lower riverine and upper estuary region, dissolved
oxygen concentrations averaged only 6.5-7.0 mg/L. This pattern results in part from the
introduction of blackwater from the Northeast Cape Fear River and Black River into the
Cape Fear system, which naturally contains reduced oxygen levels. Low dissolved oxygen
concentrations occur in blackwater rivers where high inputs of swamp-derived organic
matter lead to high levels of heterotrophic activity by the microbial community (Meyer
1990). BOD inputs from International Paper and other industries also help reduce dissolved
oxygen in the lower riverine portion of the Cape Fear near IC and NAV. Near the mouth of
the estuary at M18 and M23, inputs from the coastal ocean, increased wind action, and
elevated phytoplankton productivity add oxygen to the system.
The smaller tributaries and creeks which feed the Cape Fear system also
suffered from periodic hypoxic events (Table 2.3),
including those stations which drain streamside swamps ( ANC and GS) or drain areas
containing swine farming operations (6RC, SAR, and COL). These sites exhibited low
dissolved oxygen concentrations of <1.5 mg/L following the hurricane because of the
flooding, followed by a general recovery in October. The NC state standard for swamp
waters is 4 mg/L (NCDEHNR 1994). Low dissolved oxygen was also measured during April 1997
(1.1 mg/L) for the Burgaw Canal station BC117, which is located downstream of the Burgaw
Wastewater Treatment Facility. For the period from June 1996 through October 1996, the
South River station (SR) remained below 4 mg/L. This site is located downstream of Dunn
and may be heavily influenced by BOD loading from this municipality. In the non-swamp
tributaries included in this study, hypoxia is generally not a problem since most of these
streams are quite shallow (0.5-2 m).
Turbidity is a measure of the amount of suspended organic and
inorganic particulater matter in the water column. Turbidity is deleterious to water
quality because it reduces the amount of light available to rooted aquatic plants,
increases water treatment costs, transports phosphorus downstream (NCRS 1995), and serves
as a refuge and transport mechanism for fecal coliform bacteria (Pommepuy et al. 1992).
The Cape Fear River system regularly experiences high sediment loading
during which turbidity levels exceed the North Carolina state standards of 25 NTU for
estuarine waters and 50 NTU for freshwater. During December 1996 and May 1997, turbidity
levels substantially exceeded the state standards in both the estuary and at the upriver
stations (Table 2.4, Figure
2.3). During December 1996, the highest levels were recorded at NC11 and BLL (56 NTU)
in the mainstem, and at the mid-estuarine site M42 (45 NTU). In May 1997, turbidity levels
throughout the river and estuary averaged 44 NTU, which was the highest observed level
since sampling was begun in June 1995. During this month, measurements at NC11 and BLL
were 63 NTU and reached 65 NTU at the mid-estuarine station M54, considerably above the NC
standard for estuaries.
The data in Figure 2.4 show that the mid-estuarine site M54 has
exhibited the highest mean turbidity level since June 1995, followed by M42. The
explanation for this is likely two-fold. First, this station is located in the turbidity
maximum zone where the suspension of particles is greatest due to the interaction of ions
from seawater and the large load of charged inorganic sediment particles (e.g.. clays)
which have been washed down the river from upstream sources. Much of the particulate
matter reaching these upper stations probably originates in the Piedmont region of North
Carolina from non-point sources (Mallin et al. 1997a). Second, periodic dredging
acitivities near the Wilmington port result in isolated high turbidity levels in the
mid-estuary region. Dredging activities are confined to the winter months from October
through March. The consequences of high turbidity at M54 will be explained further in the
nutrient section. The upstream stations NC11-AC also experienced periods of elevated
sediment loading. Turbidity levels averaged approximately 30 NTU at these sites.
An important aspect of the data illustrated in Figure 2.4 is the very
low turbidity levels observed at the upstream blackwater stations in the Northeast Cape
Fear River (NCF117) and the Black River (B210). Turbidities averaged <4.5 NTU at both
of these stations. These blackwater tributaries have extensive streamside riverine
wetlands which have been shown to act effectively as sediment and nutrient filters
(Johnston 1991).
Turbidity data for the tributary and creek stations are listed in Table
2.4. The Burgaw Canal which feeds the Northeast Cape Fear receives high volumes of
sediment loading with turbidity levels averaging >34 NTU at its sampling stations BCRR
and BC117. Browns Creek (1996-97 average = 31 NTU) and Hammond Creek (1996-97 average = 20
NTU) both feed the mainstem Cape Fear River and also carry high volumes of particulate
matter. These stations receive drainage from open agricultural acreage with non-point
source runoff. The remaining tributary sites have extensive streamside wetland buffers and
do not receive much sediment loading.
The concentration of total suspended solids at each site for the
1996-97 monitoring period is displayed in Table 2.5. This
parameter is closely related to turbidity and will not be discussed in detail.
Nutrients
Ammonium is an inorganic form of nitrogen which is readily available
as a nutrient source for phytoplankton and bacteria. Two-year mean ammonium concentrations
for individual stations show a distinct peak at Livingston Creek (LVC) which receives
discharge from Wright Chemical (Figure 2.5). It is clear that LVC is a source of ammonium
to the Cape Fear system. The 2-year mean concentration at LVC (Feb 96 - May 97 only) was
157 ug/L, far greater than at any other station. Ammonium levels at this station exceeded
240 ug/L four times in 16 months. Just downstream of LVC at AC, the mean ammonium
concentration was 95 ug/L. Upstream of LVC at BLL, NH4+ levels
averaged only 73 ug/L. The paper mill located upstream of AC may also contribute some
ammonium to the river as has been documented in the Chowan River (Kuenzler et al 1982).
The data also suggest that there may be a smaller source for ammonium near M54 (Table 2.6,
Figure 2.5). M54 is located near the City of Wilmington’s wastewater plant. Since
sediments may contain high reservoirs of ammonium (Day et al 1989; Stanley 1987) and since
this site is characterized by high turbidities some of this NH4+ may
have been recycled from the sediment. The Northeast Cape Fear stations NCF117 (2-year mean
= 41 ug/L) and NCF6 (2-year mean = 43 ug/L) and the upstream Black River station B210
(2-year mean = 29 ug/L) generally contain little ammonium.
Mean monthly ammonium concentrations of all the lower Cape Fear sites
for the previous 2-year period are illustrated in Figure 2.6. The data did not display an
obvious seasonal trend. The greatest monthly mean (137 ug/L systemwide) occurred in
February 1996. Data collected in September 1996 did not reveal an overall systemwide peak
related to the hurricane, although individual stations displayed peaks. For instance,
ammonium concentrations at NCF117 during September 1996 measured 140 ug/L, nearly 100 ug/L
greater than the 2-year average. The implications of this will be discussed in Chapter 6.
In the Burgaw Canal at BC117, mean 1996-97 ammonium concentrations
measured 1067 ug/L, an order of magnitude higher than at any other site in this study (Table 2.6). BC117 receives ammonium loading from the Burgaw
WWTP. During April and May 1997, ammonium levels were greater than 4000 ug/L. Upstream of
the plant at BCRR, ammonium levels averaged only 118 ug/L. There were occasional spikes of
greater than 200 ug/L at LRC, ROC, BCRR, BRN, GS, and 6RC, but no concentrations ever
exceeded 700 ug/L.
Nitrate is the other principal inorganic nitrogen form readily
available for plant uptake. The lower Cape Fear River receives much of its nitrate loading
from sources upstream of the study area. The highest concentrations were measured at N11,
BLL, and AC, where mean nitrate concentrations were approximately 600 ug/L (Figure 2.7).
LVC does not appear to be a source of nitrate since average NO3- measurements
were 481 ug/L, somewhat less than at BLL and AC. Nitrate concentrations decreased steadily
downstream, reaching their lowest level at M18 (2-year mean = 157 ug/L). As the nitrate is
transported downstream, phytoplankton productivity and tidal intrusions transform and
transport nitrate out of the system. The Northeast Cape Fear and Black Rivers are
significantly less industrialized and have extensive streamside wetland buffers which help
reduce nitrate concentrations as evidenced by the lower levels at NCF117 (307 ug/L), NCF6
(311 ug/L), and B210 (259 ug/L).
Mean monthly nitrate levels for the past two years are illustrated in Figure 2.8. Although the data were inconclusive, there
did appear to be some nitrate loading occurring in January and February, followed by
decreases in early spring. During the winter, cold water and short daylight hours inhibit
primary productivity and significantly reduce nitrate uptake. During March and April,
spring phytoplankton blooms result in decreased nitrate concentrations. The two highest
mean monthly nitrate levels were observed during June 1995 (mean = 642 ug/L) and June 1996
(mean = 632 ug/L). These peaks can likely be attributed to high runoff events following
rainstorms. Mallin (1994) discussed how short term hydrological/meteorological conditions
are the dominant factor affecting nitrate variability in other North Carolina systems.
During September 1996, Hurricane Fran and succeeding storms dumped a tremendous amount of
rain on the Cape Fear Basin. However, nitrate levels averaged only 110 ug/L. The river was
suffering from severe hypoxic/anoxic conditions and much of the nitrogen in the system was
in the reduced forms of organic nitrogen or ammonium.
Nitrate concentrations averaged 2263 ug/L in Little Rockfish Creek and
1289 ug/L in the Burgaw Canal at BC117 during the 1996 - 1997 sampling period (Table 2.7). As mentioned earlier, LRC is located just
downstream of Stevecoknit Mills and BC117 is situated downstream of the Burgaw WWTP. PB,
ROC, GCO, and 6RC also received considerable amounts of nitrate. Panther Branch receives
effluent from Cates Pickle. The other three sites (ROC, GCO, 6RC) likely receive nitrate
from agricultural runoff. These tributaries all experience high amounts of inorganic
nutrient loading which ultimately flow into the Cape Fear system. As was observed in the
Cape Fear River, there was little nitrate at most of these tributary stations in September
1996 following the hurricane since many sites were experiencing hypoxic/anoxic conditions.
Total Kjeldahl Nitrogen (TKN) is a measure of the total
concentration of organic nitrogen plus ammonium. TKN concentrations averaged approximately
900 ug/L-1000 ug/L at most riverine and estuarine stations (Figure 2.9). The peaks at LVC (mean = 1144 ug/L) and
AC (mean = 1096 ug/L) were likely a result of ammonium and organic nitrogen loading from
industrial sources. The observed TKN increase near M54 may have been caused by input from
Wilmington’s WWTP or from the recycling of organic nitrogen and ammonium from the
sediments. M54 exhibited the highest turbidity of any estuarine station and experienced
increased turbidity loads from upstream dredging activities which may have enhanced TKN
levels. At the lowest estuarine station M18, TKN values were low (460 ug/L); by the time
river water reaches the lower estuary much of the organic material has either been
consumed by higher organisms or has been diluted with low nutrient ocean water. The three
blackwater stations (NCF117 = 1090 ug/L, N64 = 1002 ug/L, B210 = 1073 ug/L) all exhibited
relatively high TKN concentrations. These sites receive significant organic nitrogen
loading from streamside swamps.
Average monthly TKN concentrations for all lower Cape Fear stations
reveal no seasonal pattern (Figure 2.10). The greatest monthly mean was observed during
September 1996 following Hurricane Fran. TKN concentrations averaged 1371 ug/L during this
month. Post-hurricane rains caused a substantial amount of swamp land to flood, sending
large loads of organic nitrogen into the system. Diverted wastes from inoperative
treatment facilities, livestock lagoons, and septic systems also added organic nitrogen to
the Cape Fear. TKN loading is likely most affected by short-term meteorologic/hydrologic
conditions.
Total Kjeldahl nitrogen concentrations (Table
2.8) were very high at LRC (mean = 3326 ug/L) and BC117 (mean = 2230 ug/L). Since
ammonium levels averaged only 160 ug/L, organic nitrogen loading to LRC must have averaged
greater than 3000 ug/L. At BC117, mean ammonium concentrations (1067 ug/L) accounted for
nearly 50% of the TKN. Both of these sites are heavily influenced by upstream
anthropogenic inputs. Colly Creek and Angola Creek is an undisturbed site located in a
swampy settings. However, TKN concentrations were relatively high (mean of 1488 ug/L) at
this station as a result of the natural loading of organic nitrogen from the extensive
surrounding swamplands. Colly Creek drains an area containing hog farms. Mean 1996-97 TKN
levels there were 1806 ug/L , the highest of all tributary sites except LRC and BC117. TKN
levels at all other tributary stations were comparable to the mainstem Cape Fear sites.
The highest mean total nitrogen concentration for the period
February 1996 through May 1997 was 1684 ug/L at AC (Figure 2.11). Livingstone Creek (LVC),
which feeds into the Cape Fear River just above AC, also exhibited relatively high TN
concentrations measuring 1621 ug/L. Since TN levels upstream of AC at N11 and BLL measured
1486 and 1493 ug/L respectively, the data clearly shows that industrial inputs into LVC
and into the Cape Fear at Rieglewood added significant levels of nitrogen to the system.
As discussed previously, this nitrogen was primarily in the form of ammonium and organic
N. The downstream estuarine stations M23 (mean = 1009 ug/L) and M18 (mean = 626 ug/L) had
the lowest nitrogen levels because of tidal flushing and the transfer of nitrogen to
higher levels in the food chain. TN levels at stations in the Northeast Cape Fear and
Black Rivers were comparable to the Cape Fear concentrations.
The data did not reveal a distinct seasonal pattern of nitrogen loading
in the lower Cape Fear River system (Figure 2.12). The largest monthly average occurred
during June 1996 when TN concentrations averaged 1691 ug/L. Total nitrogen levels averaged
only 958 ug/L during April 1997, the lowest recorded average.
TN values after Hurricane Fran during September 1996 averaged 1476
ug/L, comparable to most other monthly means (Figure 2.12). However, as mentioned above,
TKN concentrations were substantially higher during September 1996 than during any other
month. The data illustrated in Figure 2.13 show that the organic N to total N ratio peaked
at 89% during this month as a result of the natural and anthropogenic loading of organic
nitrogen. The lowest ratio occurred during February 1996 when nitrate+nitrite and ammonium
concentrations were relatively high. Typically, organic nitrogen comprised approximately
60%-70% of the total nitrogen throughout the lower Cape Fear (Figure 2.14). The percentage
of organic nitrogen was substantially higher at the blackwater stations (NCF117 = 74%,
NCF6 = 71%, B210 = 79%) than at the mainstem upriver stations (N11 = 56%, BLL = 55%). The
organic nitrogen in the Northeast Cape Fear and Black Rivers, however, is not primarily in
phytoplankton biomass as evidenced by their very low chlorophyll a concentrations
(Figure 22), but is likely in the form of complex organic molecules such as humic
substances. This data is further evidence of the importance of streamside wetlands as
nutrient filters (Johnston 1991; Mallin et al. 1996). Streamside wetlands remove or alter
inorganic nitrogen by denitrifying it to nitrogen gas, through uptake by plants and
subsequent conversion to organic matter, and filtration and settling of turbidity
particles containing bound ammonia. The lower Cape Fear system receives the majority of
its inorganic nitrogen upstream of the Lock and Dam #1.
Nitrogen loading is a severe problem at two tributary sites, BC117 and
especially LRC (Table 2.9). Total nitrogen concentrations
at BC117 averaged 3519 ug/L for the 1996-97 monitoring period. The main source of this
nitrogen was the Burgaw WWTP. It was mentioned earlier that this site experienced ammonium
levels of greater than 1000 ug/L. At BCRR, upstream of the plant, TN concentrations
averaged only 1378 ug/L. Little Rockfish Creek received nearly four times as much nitrogen
as any tributary site other than BC117. TN concentrations averaged 5589 ug/L at LRC.
During April 1997, total nitrogen was measured at 14900 ug/L, a very high level. The
source of this contamination was apparently Stevecoknit Mills located just upstream of the
study station. This mill has no TN or TP discharge limits in its permit (NCDWQ, pers.
com.). Total nitrogen concentrations at the other tributary sites were comparable to the
lower Cape Fear.
Mean orthophosphate concentrations calculated for each lower Cape
Fear station are illustrated in Figure 2.15. Orthophosphate levels generally decreased
downstream toward the lower estuary. The highest average concentrations were observed at
AC (61 ug/L) and at NC11 (59 ug/L). Non-point source runoff is the likely source of PO4-3
at the upper stations. There does not appear to be any substantial inorganic phosphorus
loading from the paper mill which is located between NC11 and AC. There did appear to be a
small peak in orthophosphate concentrations at M54 in the mid-estuary which may be related
to turbidity episodes or sediment nutrient recycling (see NH4 section). Mean
orthophosphate levels dropped steadily throughout the estuary (except at M54) averaging
only 15 ug/L at M18. Even though the Cape Fear is a heavily industrialized system,
inorganic phosphorus levels in the Cape Fear Estuary are generally lower than those
reported in the Delaware (Lebo and Sharp 1993), Pamlico (Stanley 1987), and Neuse (Paerl
et al 1995) estuarine systems. Inorganic phosphorus concentrations in these estuaries
reportedly average 40-150 ug/L.
Orthophosphate concentrations in the Northeast Cape Fear at NCF117
(mean = 43 ug/L) and NCF6 (mean = 40 ug/L) were higher than at the upstream Black River
station B210 (mean = 27 ug/L). As reported in Mallin et al (1996), the Northeast Cape Fear
may receive some naturally derived phosphate loading from sediments. Additionally, there
are numerous livestock operations in the NECF floodplain. Livestock waste spills have
affected the NECF in 1995 (Mallin et al. 1997b) and likely affected water quality
following Hurricane Fran in 1996 (Chapter 5). Studies have
shown that swine waste is very high in phosphate (Westerman et al 1990). However, these
blackwater sites do maintain considerably lower orthophosphate concentrations than the
upstream Cape Fear River sites, again suggesting the importance of the extensive
streamside wetlands as filters of sediment-bound phosphate and transformers of
orthophosphate into organic material via plant uptake.
While there was no clear seasonal pattern for nitrate/nitrite loading
to the Cape Fear system, the data do suggest orthophosphate seasonality patterns (Figure
2.16). During the past two years for the months of December through April, orthophosphate
concentrations have not averaged more than 40 ug/L in the lower Cape Fear. In the months
of May through November, orthophosphate levels averaged 45 ug/L or greater. The highest
monthly means were recorded during August 1996 (71 ug/L) and September 1995 (66 ug/L). The
lowest measured concentrations occurred during February (23 ug/L) and March 1997 (20
ug/L). Similar seasonal patterns have been reported by Paerl et al. (1995) in the Neuse
Estuary. Phosphate can be regenerated from sediments into the overlying water column
during periods of low dissolved oxygen in the summer months (Day et al 1989). As reported
in the first CFR report (Mallin et al 1996), the combination of P regeneration and
summertime nitrogen limitation of phytoplankton productivity were probably responsible for
the observed seasonality.
The two tributary sites which received heavy nitrogen loading also
received tremendous amounts of orthophosphate (Table 2.10).
Mean 1996-97 orthophosphate levels were 953 ug/L at LRC and 473 ug/L at BC117. Inputs of
inorganic phosphorus from sediments cannot account for this heavy loading. Effluent from
the Burgaw WWTP has caused orthophosphate concentrations to exceed 1000 ug/L on a few
occasions. At LRC, orthophosphate levels are routinely above 1000 ug/L and have surpassed
2000 ug/L. These sites appear to be areas of excessive anthropogenic loading of nitrogen
and phosphorus. Angola Creek (ANC) apparently experiences natural loading of inorganic
phosphorus. Mean orthophosphate concentrations at ANC were 89 ug/L for the 1996-97
monitoring period. During June through October 1996, measured values at ANC ranged from
88-234 ug/L, substantially higher than during the winter months. Other than BC117 and LRC,
all other tributary stations experienced orthophosphate concentrations comparable to the
lower Cape Fear sites.
The data in Figure 2.17 imply that the lower Cape Fear River system
receives most of its phosphorus loading from sources upstream of NC11. The highest
concentrations of total phosphorus were observed at NC11 (186 ug/L), BLL (184 ug/L), and
AC (186 ug/L), followed by a steady decline downstream towards the estuary. At M54, there
was a definitive peak in total phosphorus levels (171 ug/L). As mentioned earlier,
phosphorus can be recycled from sediments and is frequently associated with high
turbidities. Particulate matter (e.g. clays) has the ability to bind and transport
phosphorus (Lebo 1991). The upstream riverine stations NC11, BLL, and AC also experience
high turbidity levels. Total phosphorus concentrations for the Northeast Cape Fear
stations over the past two years at NCF117 (111 ug/L) and NCF6 (115 ug/L) were higher than
at B210 (88 ug/L) on the Black River for reasons discussed in Chapter 5.
Total phosphorus concentrations were higher during the summer than
during the winter for the lower Cape Fear system (Figure
2.18). As mentioned in the orthophosphate discussion, this seasonal pattern is
probably caused by summertime increased nitrogen limitation and phosphorus recycling. The
highest monthly mean occurred during September 1996 following Hurricane Fran when TP
levels averaged 201 ug/L. The highest phosphorus concentrations during September 1996 were
measured at NCF117 (380 ug/L) and NCF6 (360 ug/L), where TP levels were usually relatively
low. B210 (200 ug/L) also experienced much higher than average phosphorus loading during
September 1996, but significantly less than the NECF sites.
The mean ratio of organic phosphorus to total phosphorus in the lower
Cape Fear system ranged from 74% at M54 to 64% at NCF117 (Figure 2.19). Following Fran,
the mean percentage of organic P was not significantly different from other months during
the monitoring period (Figure 2.20), unlike nitrogen which exhibited a substantial peak in
the organic N/total N ratio. After Hurricane Fran, organic phosphorus (estimated as
TP-orthoP) comprised approximately 77% of the total phosphorus at NCF117 and NCF6,
compared with only 58% at B210. The difference in species composition between the Black
and Northeast Cape Fear Rivers in addition to the high TP levels at NCF117 and NCF6
indicate that some source other than flooded swamplands contributed to organic phosphorus
loading in the Northeast Cape Fear after the hurricane (see Chapter 5).
While inorganic phosphorus comprised approximately 30% of the total
phosphorus in the lower Cape Fear system, it accounted for 78% of the total phosphorus at
BC117 which averaged 603 ug/L (Table 2.11). Total phosphorus
concentrations at LRC averaged 1088 ug/L, nearly 88% existing in the orthophosphate form.
Total phosphorus concentrations elsewhere in the tributaries were comparable to the lower
Cape Fear system, and had organic P/TP ratios ranging from 47% to 91%.
Most of the silicate in the lower Cape Fear River system appears to
be imported from areas upstream of NC11 (Fig. 2.21).
The clay soils in the Piedmont region of North Carolina are the likely source of this
compound, releasing silica into the system following heavy rain events. Concentrations
were usually higher at NC11 (2500-3800 ug/L) than at NAV (2000-3400 ug/L) as increased
diatom abundances in the upper estuary assimilate the silicate. The stations on the
Northeast Cape Fear River at NCF117 (900-2300 ug/L) and Black River at B210 (500-2700
ug/L) exhibited substantially lower silicate levels. These sites are located among
extensive riparian swamplands which have high organic laden sediments rather than clay
soils and less substantial upstream sources. Silicate concentrations were higher during
the winter months at NC11, but much lower at B210 and N117 during the same period. The
reason for this pattern is unclear but may have been caused by isolated precipitation
episodes which could have occurred upstream of NC11 but not in the vicinity of B210 and
NCF117. Additionally, increased wintertime river flow through the swampy riverbanks along
the Northeast and Black Rivers will not add silicate to the system in the same manner as
would the flooding of upriver clay soils along the mainstem Cape Fear.
Silica limitation does occur in some North Carolina estuaries (Mallin
et al. 1997a). However, since the Cape Fear system experiences relatively high silicate
loads, high turbidities, and low to moderate chlorophyll a levels, silica
limitation has only experimentally been detected once in the Cape Fear Estuary (Mallin et
al. 1997a).
Biological Parameters
The two-year mean chlorophyll a data for the lower Cape Fear River
and Estuary are displayed in Figure 2.22. The data reveal a trend of decreasing
chlorophyll a from N11 (5.9 ug/L) to NAV (3.9 ug/L) followed by increasing
chlorophyll a concentrations down to the mouth of the estuary at M18 (9.1 ug/L).
The likely reason for the declining levels from the riverine to the head of the estuary is
that the Black River joins the Cape Fear upstream of Indian Creek bringing water with low
levels of phytoplankton into the system. Station BBT (3.3 ug/L), which is influenced by
both the Cape Fear and Black Rivers, exhibited lower chlorophyll a levels than AC (5.1
ug/L) and IC (4.7 ug/L), its closest Cape Fear neighbors. At B210, an upstream Black River
station, mean chlorophyll a values averaged only 1.0 ug/L which was the lowest in
the whole system. The two Northeast Cape Fear stations, NCF6 (2.8 ug/L) and NCF117 (1.5
ug/L), also exhibited very low chlorophyll a levels. Low inorganic nutrient loading
in the Black and Northeast Cape Fear Rivers, along with light limitation in these
blackwater systems inhibits significant phytoplankton bloom formation at these sites. None
of the sixteen lower river and estuary stations had two-year means approaching the North
Carolina state standard for eutrophic waters (40 ug/L).
Chlorophyll a concentrations were slightly lower during the
1996-97 monitoring period than during the previous year (Figure 2.23). However, the
seasonal patterns were very similar. During both years, chlorophyll a
concentrations were relatively low during the late fall and early winter, followed by
early spring blooms. Colder water, reduced daylight, high turbidity from dredging, and
increased water flow tend to limit productivity in the fall/winter. In the late winter and
early spring the river experiences warming water temperatures, increasing sunlight, and
increased nutrient inputs due to fertilizer applications. Mallin (1994) discussed the
occurrence of late winter algal blooms in various North Carolina estuarine systems which
result from wintertime nutrient loading and recycling. Periodic phytoplankton blooms did
occur during the 1996-97 period, however, most were isolated and occurred in the mid to
lower estuary (Table 2.12). Chlorophyll a was
much lower during August - October 1996 compared with the same period in 1995 (Figure
2.23). This was likely a result of high flow and light attenuation caused by Hurricane
Fran (Chapter 5).
None of the tributary stations displayed significant phytoplankton
blooms during the 1996-97 study period (Table 2.12). Both the summers of 1995 and 1996 had
unusually high rainfall and streamflow, which impedes the formation of phytoplankton
blooms in running waters. On occasion, submersed, floating, and emergent macrophytes were
abundant in selected areas.
The Cape Fear River system maintains lower phytoplankton biomass
relative to many other rivers in North Carolina. Stations in the lower estuary (M18 and
M23) display average chlorophyll concentrations similar to those of the lower Neuse
Estuary (Mallin 1994), however, the upper Cape Fear Estuary stations maintain much lower
average chlorophyll levels than at similar upstream Neuse Estuary stations (Christian
1991; Paerl et al. 1995). Serious environmental problems have been reported in the
Tar-Pamlico River system (Stanley 1987), Chowan River (Kuenzler et al. 1982), and recently
in the Neuse River Estuary (Paerl et al. 1995) where nutrient loading, low flushing, and
summer water column stratification allow thick algal blooms to form in surface waters,
which ultimately result in bottom water hypoxic or anoxic conditions. The Cape Fear system
is characterized by high flow and little vertical stratification, which inhibits surface
bloom formation. The Cape Fear system’s high turbidity levels (Table 2.4) and organic color inputs from blackwater tributaries
also limit primary productivity (Mallin et al. 1997a). The data collected during the past
two years has demonstrated that nutrient loading to the Cape Fear River system was not
sufficient to sustain heavy algal blooms under the high flow - high turbidity conditions
present during 1995 - 1997.
The amount of oxygen utilized in the biodegradation of organic
material over a given time period is known as biochemical oxygen demand. Oxygen becomes
depleted as labile organic carbon is oxidized to carbon dioxide during respiration.
Five-day BOD (BOD5) analysis addresses the organic matter which is labile, or readily
available for consumption. Sources of labile carbon include human, animal, and some
industrial wastes. Natural sources of organic carbon from blackwater swamps will also
elevate BOD levels (Meyer 1990). However, a good portion of these blackwater compounds are
considered to be recalcitrant (less reactive as organic substrates for biodegradation) and
may not significantly affect short-term BOD results.
Results from the BOD5 and 20-day BOD (BOD20) analyses are summarized in
Table 2.13 and Figure 2.24. In the current study, BOD5 was
almost always less than 2 mg/L which is considered the minimum level of significant BOD
for this method given by Standard Methods (APHA 1995). The data shows that the Cape Fear,
Northeast Cape Fear, and the Black River stations all exhibited similarly low BOD5 and
BOD20. However, after Hurricane Fran and the subsequent continental storms, BOD
concentrations were elevated throughout the system, especially at NCF117. Measured BOD5 at
NCF117 in September was at least 8.2 mg/L (original 8.2 mg/L depleted at end of 5 day
period), while BOD5 at all other stations was 2.1 mg/L or less. BOD20 concentrations were
also elevated during September. The sources of the organic loading to the system caused by
the hurricane will be discussed in detail in Chapter 5.
Two sites, BLL and AC, were chosen for BOD analysis to investigate
organic loading from the International Paper mill in Rieglewood. BLL is located
approximately 3 miles upstream of the plant and AC approximately 3 miles downstream. Mean
BOD5 and BOD20 concentrations were 15% and 17% higher at AC than BLL (Table 2.13). Since the Cape Fear system experiences prolonged
periods of summertime hypoxia (see dissolved oxygen section), additional BOD loading from
anthropogenic sources will likely exacerbate this problem.
Fecal coliform bacteria can be harmful to humans in excessive
amounts, but are more frequently used as an indicator of other microbial pathogens. For
the lower Cape Fear system, there were only a few instances in which fecal contamination
was above the NC state standard of 200 CFU/100 mL for recreational human contact (Table 2.14). During February 1997 coliform counts were well
above the standard at NAV (1200/100 mL), BRR (880/100 mL), and HB (800/100 mL). The
specific cause of this contamination is unknown, but may be partly attributable to
wastewater discharges near Leland. The Cape Fear also experienced high fecal coliform
levels at M54 (228/100 mL) during February 1997. This site is located near a wastewater
treatment plant on the south side of Wilmington. M54 is also a station with elevated
turbidity levels due to local dredging (see turbidity section). Therefore, since fecal
coliform bacteria may live for extended periods in the sediments, the dredging near M54
may help recycle these bacteria into the water column.
The highest geometric mean concentrations of fecal coliform bacteria
were observed at NAV, BRR, and HB, due primarily to the February 1997 counts (Table 2.14).
The upstream Cape Fear River stations as well as the Black and Northeast Cape Fear
exhibited geometric mean coliform levels of 28-39 CFU/100 mL, well below state standards.
Fecal coliform concentrations in the lower estuary (M35, M23, M18) often exceeded levels
considered safe for shellfishing (14 CFU/100 mL), however.
Some of the CFR tributaries exhibit very high bacterial levels (Table
2.14). At Little Rockfish Creek (LRC) on the border of Duplin and Pender Counties,
geometric mean coliform bacteria levels were 227 CFU/100 mL. On more than one occasion,
fecal coliform concentrations exceeded 1000 CFU/mL. The Burgaw Canal is also periodically
subjected to fecal coliform loading. At BCRR (above Burgaw WWTP) and BC117 (below Burgaw
WWTP), bacterial levels were routinely greater than the state standard (Table 2.14). The
source of this contamination cannot solely be the treatment facility since fecal levels
were often higher upstream of the plant. Brown’s and Hammond’s Creeks, draining
agricultural areas, had periodic elevated fecal coliform concentrations (Table 2.14). Some
of the other tributary stations experienced occasional bacterial contamination problems
after heavy rain events.
Metals
The potential toxicity of metals to marine organisms is not fully understood. However, it has been shown repeatedly that complexes of certain metals are extremely dangerous to particular marine and riverine life (e.g. Sunda 1990) and ultimately to humans. These elements are especially harmful because of the process called bioaccumulation where particular metals accumulate in tissue cells of organisms at the bottom of the food chain and get transferred in large dosages to higher organisms. Metals are susceptible to flocculation processes because of their charged nature and tend to accumulate in sediments. The results described in this study represent water column concentrations only.
Aluminum levels were consistent from NC11 down through M61 measuring 380-430 ug/L (Table 2.15). In the middle estuary at M54, there was a distinct peak (530 ug/L) which is likely associated with the zone of maximum turbidity. Aluminum concentrations decreased downstream of M54. Of the three tributary stations sampled for metals, the site downstream of the Burgaw WWTP (BC117) experienced the highest aluminum concentrations. This site also experienced high turbidities. The source of the aluminum at both M54 and BC117 may be natural and originate from suspended clay particles. However, some treatment plants use aluminum salts as a coagulant for the removal of organic matter (Clark et al. 1977) and both BC117 and M54 are located near WWTP outfalls. There is no NC state standard for this metal.
The North Carolina state standard for arsenic toxicity in natural waters is 50 ug/L (NCDEHNR 1994). There were no instances at any site where arsenic concentrations reached these levels (Table 2.16). The data did not reveal any significant spatial or temporal trend in arsenic levels in the Cape Fear or its tributaries.
During the entire 1996-97 monitoring period, there was no detectable cadmium at any site (Table 2.17). The NC standard for cadmium is 2 ug/L for freshwater and 5 ug/L for tidal saltwater (NCDEHNR 1994).
Chromium may originate from a variety of industrial wastes (Clark et al. 1977). In the Cape Fear River, chromium contamination is usually not a problem. However, in the lower estuary, especially at M18, measured levels were above the NC state standard of 20 ug/L (NCDEHNR 1994) for tidal saltwater on three occasions (Table 2.18), possibly originating from ships or anchored barges. The standard for freshwater is 50 ug/L. Chromium levels did not exceed this standard at any upstream river or tributary station.
Two stations on the Cape Fear River, M18 (37 ug/L - Jan 97) and AC (44 ug/L - Nov 96), exhibited substantial peaks in copper which were considerably above the NC standard of 7 ug/L for freshwater and 3 ug/L for tidal saltwater (Table 2.19). M18 also experienced several other smaller copper contamination episodes. The source of the copper at these two stations is not known. Sunda et al (1990) reported the deleterious effects of copper pollution on estuarine organisms. At the small tributary station LRC, there does appear to be a significant and recurring problem. Copper concentrations at this site average 25 ug/L and have reached as high as 91 ug/L (Jun 96) and 56 ug/L (Apr 97). LRC is located downstream of Stevecoknit mills on the border of Duplin and Pender Counties.
Iron occurs naturally in river water and seawater and is important in many biological processes in the water column and in marine sediments. Iron is transported into the system as part of larger organic molecules (e.g. humics from blackwater) or adsorbed onto inorganic particles. The data show that in the Cape Fear River the highest concentrations of iron were observed near the mouth of the estuary at M18 and M23 (Table 2.20). In a typical estuarine system, much of the iron is removed from the water column through flocculation processes and the incoming seawater has significantly less iron than does the river water (Day et al. 1989). Therefore, there may be a downstream source of iron in the Cape Fear River. Possible sources include the military port at Sunny Point and large metal barges anchored near Southport. The tributary stations revealed similar iron concentrations ranging from 1.3-1.7 mg/L, somewhat higher than the N.C. freshwater standard of 1000 ug/L.
The amount of lead measured in the water at each station in the Cape Fear River system and its tributaries was below detection limit for the entire 1996-97 monitoring period (Table 2.21). The NC state aquatic standard for lead is 25 ug/L (NCDEHNR 1994). Lead pollution, if present, would more likely be found in the sediments since this metal adsorbs readily onto particles.
Mercury pollution may originate from a number of industrial sources including airborne particulate deposition and wastewater effluent as well as from unknown natural sources. There have been a number of reported cases of excessive mercury in fish in the Cape Fear River Basin (NCDEHNR 1996a). Our data (Table 2.22) suggest that in some instances, there may have been mercury contamination in the Cape Fear River. It should be mentioned that the data listed from August - December was not performed by a NC state certified lab. However, since there is prior evidence of mercury pollution in the Cape Fear River and since mercury may be hazardous to aquatic life and humans at low concentrations, this situation should be monitored closely.
North Carolina state standards for nickel are very different for freshwater and saltwater. The standard for aquatic life in tidal saltwater is 8.3 ug/L but increases to 88 ug/L for freshwater organisms (NCDEHNR 1994). In the Cape Fear Estuary, there were numerous occasions at M23 and M18 where nickel concentrations exceeded the saltwater standard (Table 2.23). Mean concentrations at these two sites (M18 = 13 ug/L, M23 = 11 ug/L) were above the designated minimum level. There were only periodic spikes at the other estuarine stations. The source of the nickel contamination at M18 and M23 is unknown. There were no measurable nickel concentrations at any of the other riverine sites on the lower Cape Fear. LRC displayed the only occurrence (May 1997) of measurable nickel levels of the three tributary stations, but it was well below the freshwater standard.
The mean concentration of zinc at all study sites (Table 2.24) was below the state standards of 50 ug/L for freshwater and 86 ug/L for saltwater (NCDEHNR 1994). However, there were occasions when the estuarine stations exceeded the saltwater limit (June 1996 and January 1997). M54 displayed the greatest mean zinc concentration of 37 ug/L for the 1996-97 monitoring period. Sunda et al (1990) reported that zinc toxicity can be a problem to certain estuarine organisms. Additionally, there were scattered spikes (>50 ug/L) in zinc concentrations at all of the freshwater sites in the riverine portion of the system. Two of the tributary stations, LRC (mean = 61 ug/L) and BC117 (52 ug/L), exceeded the state freshwater standard on several occasions.
|
Back to Table of Contents for 1996-1997 Annual Report |
| Back to Lower Cape Fear River Program Homepage |