Physical, Chemical, and Biological Characteristics
2.1 - Introduction
This section of the report includes a discussion of the physical, chemical, and biological water quality parameters, concentrating on the 1997-1998 Lower Cape Fear River Program 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. The chemical makeup of the Cape Fear River was investigated by measuring the magnitude and composition of nitrogen and phosphorus in the water. Three biological parameters, fecal coliform bacteria, chlorophyll a and biochemical oxygen demand, were examined to assess the effects of point and non-point source loadings of nutrients, organic materials, and other pollutants 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 YSI 6920 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 the surface, mid-depth, and bottom, or until the cable connecting the datalogger with the sonde had been fully extended. The instrument was calibrated prior to and after each sampling trip to ensure accurate measurements.
Chemical Parameters - Nutrients
All nutrient analyses were performed at the Center for Marine
Science Research for samples collected prior to January 1996. A local state-certified
analytical laboratory was contracted to conduct all subsequent analyses (February 1996
through May 1998), except for the orthophosphate procedure which was still performed at
CMSR. The following methods detail the techniques used by CMSR personnel only.
Water samples were collected ca. 0.2 m below the surface in triplicate
in amber 125 mL Nalgene plastic bottles. After collection, the samples were immediately
placed on ice. In the laboratory, 50 mL of each triplicate was filtered through a separate
1.0 micron pre-combusted glass fiber filter, which were frozen and subsequently used for
chlorophyll a analysis. The triplicate filtrates were pooled in a common glass
flask, mixed thoroughly, and approximately 100 mL was poured into one 125 mL plastic
bottle to be used for orthophosphate analyses. Filtered samples were stored in a freezer
until time of analysis.
Orthophosphate (PO4-3)
All orthophosphate analyses were performed in duplicate 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. Standards and spiked samples were analyzed for quality assurance as described in the NO3/NO2 method.
Biological Parameters
Chlorophyll a
The analytical method used to measure chlorophyll a is described in Welschmeyer (1994) and USEPA (1997). Chlorophyll a concentrations were determined directly from the 1.0 micron glass fiber filters used for filtering samples for nitrate+nitrite and 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 concentrations using a Turner AU-10 fluorometer. This method uses an optimal combination of excitation and emission bandwidths which reduces the errors inherent in the acidification technique.
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 (NCF117 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 C. 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 at all stations on the lower Cape Fear River
system averaged approximately 18-20 oC during the 1997-98 monitoring period (Table 2.1). The river water was coolest during February, when
temperatures ranged from 8-10 oC, and warmest during July (29-30 oC).
Temperatures in the shallow tributary stations were generally cooler
than in the river averaging 16.5-19 oC (Table 2.1). These shallow water sites
were much more affected by daily air temperature fluctuations and remained very cool from
November through February. Systemwide, maximum water temperatures did not exceed 31oC
and minimum temperatures never fell below 6 oC.
During the 1997-98 monitoring period (Table
2.2) estuarine salinity values averaged from 1-22 ppt. Salinities were generally
higher during the summer months when low flow periods bring less freshwater flow from
upstream areas. Extremely high rainfall occurred during January - April 1998, bringing
salinity measurements to their lowest levels since the LCFRP began monitoring the estuary
in 1995. Salinities during February 1998 were even lower than measurements shortly
following Hurricane Fran (Mallin et al. 1997). Near the mouth of the estuary at M18, the
salinity during February 1998 was only 6.3 ppt (compared with 9.9 ppt following Fran) and
the river was completely fresh as far downstream as M35.
Although not listed in Table 2.2, measurable salinities (1-2 ppt) were
occasionally observed at two tributary stations, NC403 and Panther Branch. Both sites are
located in freshwater regions, but receive large amounts of salty effluent from upstream
pickle factories.
Unlike 1996-97, when Hurricane Fran caused extended periods of near
anoxia throughout the river and estuary, hypoxic episodes were limited in severity during
this study (Table 2.3). Substandard dissolved oxygen levels
(< 5 ppm) occurred during July - October at some sites, but remained above 3.4 ppm at
the stations on the lower Cape Fear.
During the 1997-98 study (Fig.
2.1), the mean monthly oxygen level in the lower Cape Fear system stayed above the NC
State Standard (5 ppm). The graph also clearly shows the effects of Hurricane Fran during
September 1996 and the persistent hypoxic conditions experienced during the summer of
1995.
The highest average dissolved oxygen levels (Fig. 2.2) have been observed at the upper river
sites, NC11 (7.9 ppm) and AC (7.7 ppm), as well as in the lower estuary at M23 (7.7 ppm)
and M18 (7.8 ppm). Summertime dissolved oxygen concentrations along the Cape Fear River
exhibit a distinct sag downstream of NC11 and AC (Fig.
2.3). Large organic loads from the paper mill just upstream from AC are slowly
degraded down the river, utilizing available oxygen during bacterial respiration.
Additionally, the Black River enters upstream of IC. The combination of hypoxic blackwater
and organic inputs from the paper mill serve to drive oxygen concentrations to their
lowest point near the head of the estuary at NAV and HB. The system recovers downstream at
M42 and M35 as a result of increased tidal flushing and phytoplankton productivity. The
lowest mean DO levels (Fig. 2.2) in the system have occurred in the Northeast Cape Fear
River at NCF117 (5.8 ppm) and NCF6 (6.0 ppm). These stations have experienced severe
hypoxic/anoxic periods over the past three years as a result of Hurricane Fran in 1996 as
well as a chicken waste lagoon spill in 1995. Since the Northeast Cape Fear is
predominantly a blackwater river which experiences natural summertime hypoxic conditions,
the addition of BOD loads from these two events severely augmented the naturally occurring
oxygen problems.
When comparing the Cape Fear mainstem with the Black and Northeast Cape
Fear (Fig. 2.4), dissolved oxygen
concentrations are clearly higher on the Cape Fear than on either of the two blackwater
rivers. Substandard levels have rarely been experienced on the mainstem, but have occurred
frequently in the summertime at both B210 and NCF117. Comparing the Black and Northeast
Cape Fear Rivers, it appears that dissolved oxygen levels are routinely higher in the
Black River. This may be due in part to differences in anthropogenic BOD loading from
municipalities and agriculture (i.e. livestock) between the two rivers. As mentioned
previously, all three rivers experienced much higher dissolved oxygen concentrations
during the summer of 1997 than during either of the previous two years, although
substandard dissolved oxygen levels occurred in both blackwater rivers in 1997.
Several tributary stations experienced severe hypoxic conditions during
the summer months (Table 2.3). During the 1997-98 study,
GS, PB, NC403, BCRR, and SR experienced periods when dissolved oxygen concentrations fell
below 2 ppm. At NC403 (upper Northeast Cape Fear River site below Mt. Olive Pickle), D.O.
levels were below 1 ppm for three consecutive months (July - September). The average
dissolved oxygen level at NC403 for the 1997-98 monitoring period was only 4.4 ppm, below
the NC State Standard for acceptable water quality. Severe hypoxia (<2 ppm) persisted
at SR (South River below Dunn) through early October. All of these sites are characterized
by low flow during the summer, and the addition of any BOD load contributes to hypoxic
conditions.
Several estuarine stations exhibited mean turbidities above the NC
State Standard of 25 NTU (tidal saltwater standard) during the 1997-98 study period,
including NAV, HB, BRR, M61, M54, M42, and M35 (Table 2.4).
Turbidity has been a frequent problem in the Cape Fear and results primarily from periodic
dredging activities and the upstream runoff of clay particles from the NC Piedmont (Mallin
et al 1997). Many municipalities rely on the Cape Fear River for drinking water, and it is
costly to remove high sediment loads during the treatment process. During the past year,
turbidity levels reached as high as 157 NTU (station DP in March 1997) in the mainstem
Cape Fear River, considerably above the NC State Freshwater Standard of 50 NTU. Heavy
rains during the winter of 1998 were the primary factor contributing to heavy particle
loads in January - April.
Over the past three monitoring periods (Fig. 2.5), average turbidity levels have exceeded
the tidal saltwater standard of 25 NTU from NAV (31.9 NTU) downstream through M42 (26.5
NTU). Peak estuarine turbidities have been measured at M54 (33.5 NTU). This site is
located in the "turbidity maximum zone", where electrochemical forces cause
charged particles (i.e. clays) to flocculate and subsequently settle out of the water
column. Much of the sediment loading in the lower Cape Fear system is input upstream of
the sampling area as evidenced by average turbidity levels being highest at NC11 (33.0
NTU), DP (40.7 NTU), and AC (33.9 NTU), the most upstream stations. Turbidities have been
substantially lower (Fig. 2.5) at B210 (4.5 NTU) and NCF117 (4.7 NTU), sites located on
the Black and Northeast Cape Fear Rivers. These sites are located in relatively
undisturbed areas and have extensive wetland buffers which help filter any sediment-laden
inputs.
The winter of 1998 exhibited the two highest months in average
turbidity levels since June 1995 (Fig. 2.6), a
result of record rainfall levels which occurred during February and March. Turbidities
have been generally lower during the summer months (June - August) due to increased
evapotranspiration processes which limit river flow, but low sediment loading has also
occurred during dry winter months. Therefore, it is not possible to discern any particular
seasonal pattern. Turbidity values in the Cape Fear system are closely linked to
short-term meteorologic/hydrologic events (see Chapter 3).
Local rains contribute very large sediment loads to the smaller
tributary stations (Table 2.4) as illustrated by the very
high turbidity levels experienced during March and April. Turbidities were greater than
100 NTU (BRN, HAM, LRC, BCRR, and BC117) at several sites during these months.
Interestingly in January, levels were very low, averaging between 1-7 NTU at almost every
tributary station. The two most problematic sites are the stations located on the Burgaw
Canal, BCRR (51.6 NTU) and BC117 (50.1 NTU). These same stations experienced the highest
turbidity levels during the 1996-97 monitoring period as well (Mallin et al 1997). Short
term turbidity fluctuations are substantially greater at these tributary sites than at the
lower Cape Fear River sites and therefore any seasonal pattern would be difficult to
determine. Nonetheless, high sediment loads which enter these small streams will
eventually enter the Cape Fear system and this concept is very important in understanding
the source of particulate matter to the river.
Total suspended solids and turbidity measurements differ somewhat regarding spatial patterns in the Cape Fear system. The data listed in Table 2.5 reveal that the largest average TSS loads (lower Cape Fear only) during the 1997-98 study were observed at M54 (22 mg/L) and at M23 (21 mg/L), somewhat higher than the sites NC11 (12 mg/L) and AC (13 mg/L) which typically exhibit the highest turbidity concentrations (see previous section). The most obvious explanation for this pattern is that clay particles (very fine particles) dominate the sediment loads at the upstream riverine stations, while the sediment particles in the lower estuary are larger and heavier as a result of flocculation and biochemical processes. The particles in the lower estuary are likely composed of more organic compounds and the inorganic clay particles tend to be removed from the water column near the estuarine turbidity maximum zone discussed in the Turbidity section. TSS concentrations are generally much lower in the tributary stations (Table 2.5) than in the lower Cape Fear. BC117 exhibited the highest mean (25 mg/L) concentration of all sites.
The median pH value increases from the upper river (6.7) down through the estuary (8.0) as organic acids from upstream swamps and blackwater sources mix with high pH seawater, and algal productivity increases (Table 2.6). The tributary station at Colly Creek (COL), a relatively unimpacted blackwater creek, exhibited the lowest pH averaging 4.1 during the 1997-98 monitoring period. The median pH value at LRC was 7.8, which is unusual for a freshwater site. The discharge from the upstream Stevecoknit Mills was apparently very alkaline.
Conductivity measurements are much higher in the estuary than in the upper river because of the seawater salts (Table 2.7). Two tributary sites, PB and LRC, have displayed conductivities similar in magnitude to the upper estuary. Panther Branch (PB) receives salty effluent from Cates Pickle. BC117 (downstream of the Burgaw WWTP) and NC403 (downstream of Mt. Olive Pickle) have also exhibited elevated conductivities.
Nutrients
The primary sources of ammonium in the Cape Fear River include
wastewater treatment plants (WWTP) and numerous agricultural and industrial outputs.
Livingston Creek (LVC) is a major source of ammonium to the Cape Fear system (Fig. 2.7). This creek has exhibited by far the
highest mean concentration of NH4+ over the past few years,
averaging 154 m g/L. Wright Chemical has an outfall located in
Livingston Creek upstream of the sampling site. The second largest ammonium concentrations
have been measured at AC (109 m g/L), located just downstream
of LVC and International Paper. In the Cape Fear Estuary, the data indicate peak NH4+
concentrations at M54 (84 m g/L). Ammonium levels at this site
are likely influenced by the estuarine turbidity maximum zone, which may include some
sediment recycling of this nutrient (Day et al 1989; Stanley 1987). Wilmington’s WWTP
is also located in the vicinity of M54. The blackwater stations NCF117 (42 m g/L) and B210 (32 m g/L) have maintained
the smallest quantities of ammonium. There were no unusual or excessive NH4+
levels uncovered at any lower Cape Fear site during the 1997-98 study (Table 2.8).
There did not appear to be any seasonal trend in ammonium
concentrations (Fig. 2.8), and ammonium levels during the 1997-98 study were not
distinguishable from the previous two years. NH4+ levels were very
high during August 1997, but very low in September 1997, which illustrates the variability
and transience in this inorganic nitrogen compound.
A station located on the Burgaw Canal (BC117) has exhibited the highest
ammonium concentrations of any tributary or riverine site (Table 2.8). During the 1997-98
study, NH4+ levels averaged 1737 m g/L,
and even reached 9900 m g/L during July 1997. This station
receives discharge from the Burgaw WWTP. Other tributary sites displayed periodic
concentrations above 300 m g/L including LRC, NC403, ROC, BCRR,
SR, LCO, and COL. It is interesting that during July and August 1997, NH4+
concentrations were high at COL since this site routinely has very low levels of inorganic
nitrogen. There are some livestock operations located upstream of this station which may
have contributed to the unusually high concentrations.
Most of the inorganic nitrogen which enters the Cape Fear system is
in the form of nitrate+nitrite (henceforth referred to as nitrate only). The source of
much of this nitrate is upstream of our sampling area (Fig. 2.9). The largest concentrations have been
observed at NC11 (621 m g/L) and AC (611 m
g/L ), the two most upstream stations. LVC does not appear to be a significant source of
nitrate as it is for ammonium since concentrations have been lower in this creek (528 m g/L) than in its neighboring river stations. Nitrate concentrations
decrease at each station downstream through the estuary. M23 (213 m
g/L) and M18 (152 m g/L) located near the mouth have exhibited
the lowest mean nitrate levels. NO3- concentrations have also been
low in the Northeast Cape Fear (NCF117 = 283 m g/L, NCF6 = 301 m g/L) and Black Rivers (B210 = 247 m
g/L). These blackwater rivers are bordered by extensive swamplands which help reduce the
quantity of inorganic nitrogen entering these systems.
Mean monthly nitrate levels of all lower Cape Fear sites have revealed
no seasonal pattern since June 1995 (Fig. 2.10).
However, it would be expected that nitrate concentrations would increase during the early
spring coinciding with increasing fertilizer applications. There are nitrate peaks during
winter and early spring, but other peaks occur as well, which implies that nitrate
concentrations are affected by short term meteorologic and hydrologic conditions (see Chapter 3). Concentrations during the 1997-98 monitoring
period (Table 2.9 and Fig. 2.10) were similar to the previous two
studies, with an overall peak occurring in June 1997 (582 m
g/L) and a minimum in April 1998 (253 m g/L). It should be
mentioned that the lowest recorded value was measured in September 1996 (110 m g/L) following Hurricane Fran when much of the river was
hypoxic/anoxic, driving inorganic nitrogen into its reduced ammonium form.
The tributary stations have received wide-ranging quantities of nitrate
loading (Table 2.9). Little Rockfish Creek (LRC) exhibited
exceptionally high concentrations averaging 3758 m g/L for the
1997-98 study, with a peak of 11000 m g/L during July 1997.
Burgaw Canal (BC117) also received large nitrate loads averaging 1650 m
g/L. The source of nitrate for LRC has likely been Stevecoknit Mills which is located less
than 1 mile upstream. BC117 receives direct inputs from Burgaw WWTP. Other stations
displayed periodic NO3- levels above 1000 m
g/L during the 1997-98 study including GS, PB, SAR, NC403, ROC, and LCO. All of these
tributary sites can be significantly impacted by short term rainfall episodes which
quickly bring inorganic nitrogen into the streams.
Total Kjeldahl Nitrogen (TKN) is a measure of the total
concentration of organic nitrogen plus ammonia nitrogen. Since February 1996 (Fig. 2.11), LVC (mean = 1081 m
g/L) and NCF117 (mean = 1129 m g/L) have exhibited the largest
TKN concentrations. NCF117 is a blackwater site which receives large quantities of organic
nitrogen from surrounding swamps, but has also been subject to organic N and ammonium
loading from malfunctioning sewer systems and livestock waste spills. During October 1997
(Table 2.10), TKN values exceeded 4000 m
g/L for unknown reasons. LVC has exhibited an average TKN value which has been larger than
its nearest Cape Fear River sites because it has an upstream source of ammonium (see NH4+
section). B210, the other predominantly blackwater site, has also exhibited high TKN
concentrations (mean 1011 m g/L) because of its large organic N
reservoir. The lowest TKN levels have been observed near the mouth of the estuary at M23
(715 m g/L) and M18 (524 m g/L)
since February 1996. Organic nitrogen levels are lower at these stations due to the
dilution with seawater and the incorporation of nitrogen by higher organisms further up
the food chain.
TKN values are likely related to short term hydrologic/meteorologic
events and have not displayed an obvious seasonal pattern (Fig. 2.12). Heavy rains can lead to swampland
flooding and the subsequent input of heavy organic loads as was observed after Hurricane
Fran in September 1996.
During the 1997-98 monitoring period, LRC (mean = 3218 m g/L) and BC117 (mean = 3048 m g/L)
received TKN loads nearly 3X greater than any Cape Fear or other tributary site. Both of
these stations are subject to high ammonium and organic nitrogen loading. At LRC, TKN
values reached as high as 6800 m g/L (July 1997). The maximum
concentration at BC117 was 10500 m g/L, also during July 1997.
The source for this pollution has been mentioned previously. Stevecoknit mills closed in
May 1998; thus, sampling during the next year will allow us to determine the effect of the
discharge over "baseline" conditions. Colly Creek (COL - mean = 1532 m g/L) and Angola Creek (ANC - mean = 1648 m
g/L) are located in blackwater areas and have exhibited large organic nitrogen
concentrations.
The highest levels of total nitrogen (Fig. 2.13) have been measured at AC (mean = 1627 m g/L), LVC (mean =1598 m g/L), and NC11
(mean = 1489 m g/L). These data suggest that there is a source
of nitrogen downstream of NC11 which raises total nitrogen levels at AC and LVC, likely
Wright Chemical (primarily ammonium inputs) and International Paper. Total nitrogen
concentrations generally decrease down the river through the estuary reaching minimum
levels at M18 (mean = 679 m g/L). Regular tidal flushing and
the uptake of nitrogen by higher organisms lower the concentrations in the estuary. The
data reveal a slight peak at M54 (mean = 1314 m g/L) relative
to M61 (1268 m g/L). This feature is a result of increased
ammonium and organic N loading at M54, possibly related to the recycling of nitrogen from
sediments (see Chapter 3) or discharge from the WWTP as
discussed previously. The two predominantly blackwater stations B210 (mean = 1254 m g/L) and NCF117 (mean = 1428 m g/L)
receive large organic N loads from surrounding swamp lands.
There is no clear seasonal pattern for nitrogen loading in the Cape
Fear River (Fig. 2.14). Inorganic and organic
nitrogen inputs appear to be more closely correlated with short-term meteorologic and
hydrologic changes.
The composition of nitrogen becomes increasingly more organic in nature
from the river to the estuary (Fig. 2.15).
Upstream at NC11 and AC, organic nitrogen compounds (calculated as TKN - NH4+)
represented 54% and 56 % respectively of the total nitrogen concentrations. Near the upper
and middle estuary, the percentages ranged from 63-65% from NAV - M35. At the mouth near
Southport, 70% of the nitrogen was organic. The reason for this change is likely two-fold.
First, the Black River and Northeast Cape Fear Rivers contain considerably less inorganic
nitrogen than the mainstem. At B210 and NCF117, organic compounds contribute approximately
80% of the total N. These blackwater rivers dilute the inorganic compounds with organic
nitrogen compounds downstream in the Cape Fear. B210 and NCF117 have extensive wetlands
which act as buffer zones where inorganic N is converted to organic N through assimilation
and nitrogen gas is produced through denitrification (NO3- à N2). Secondly, as mentioned above, much of the
inorganic nitrogen from the mainstem is utilized by phytoplankton and subsequently by
higher organisms as illustrated by the higher chlorophyll a levels in the estuary.
The data did not reflect any seasonal change in nitrogen composition in the Cape Fear
River.
Most of the smaller tributary sites maintained nitrogen concentrations
during the 1997-98 (Table 2.11) which were similar to the
river and estuary stations (1100-1700 m g/L). The exceptions
were LRC (mean = 6975 m g/L) and BC117 (mean = 4698 m g/L) which exhibited significantly higher TN levels. TN at LRC
reached 17800 mg/L during July 1997, an order of magnitude greater than any lower Cape
Fear site. BC117 (10750 m g/L) also experienced very high
nitrogen levels during July 1997. Interestingly, these stations exhibit much different
types of nitrogen loading. At LRC, 58% of the total N was in the form of nitrate or
ammonium, while 72% was inorganic at BC117. The two sites which are primarily swamps, COL
and ANC, exhibited relatively high nitrogen concentrations (1547 m
g/L and 1726 m g/L), but contained mostly organic compounds
(81% and 90%).
There are various sources of inorganic phosphorus in the Cape Fear
River Basin. Orthophosphate may be generated by industry, agriculture, and municipal
wastes. It may also have natural origins such as old carbonate deposits and clay
particles, unlike nitrate which is generated principally by agricultural runoff or
industrial and municipal discharges. Since 1995 (Fig.
2.16), the largest orthophosphate concentrations have been observed at NC11 (67 m g/L), AC (68 m g/L), and DP (68 m g/L). Phosphorus binds readily to particles and not coincidentally
these sites have also exhibited the highest turbidities. Inorganic nitrogen levels are
also highest at these stations suggesting that upstream fertilizer runoff may be an
important source of both nutrients. Orthophosphate concentrations decreased steadily
downstream and reached a minimum at M23 (23 m g/L) and M18 (17 m g/L). Inorganic phosphorus loading in the Cape Fear does appear to
be somewhat less than in other NC rivers such as the Pamlico (Stanley 1987) and Neuse
(Paerl et al 1995).
While inorganic nitrogen levels did not show a clear seasonal trend,
orthophosphate concentrations distinctly rose during the summer months (Fig. 2.17). Wintertime (January - April)
orthophosphate levels have averaged approximately 30 m g/L
while summertime (June - September) values have averaged nearly 60 m
g/L. These patterns are not exclusive to the Cape Fear, but represent the naturally
occurring geochemical phosphorus cycle of many river basins. As sediments become anoxic
during the summer months, phosphates can be regenerated into the overlying water column
(Day et al 1989). Additionally, primary productivity becomes more limited by nitrogen in
the Cape Fear system during the summer months allowing PO4-3 pools
to accumulate (Mallin et al 1997a). This concept will be discussed in more detail in Chapter 3.
Inorganic phosphorus concentrations have been extraordinarily high in
the two tributary stations LRC (1997-98 mean = 1133 m g/L) and
BC117 (1997-98 mean = 1024 m g/L), the same sites which receive
heavy inorganic nitrogen loading (Table 2.12).
Orthophosphate levels were an order of magnitude greater at both of these sites than at
any other tributary or river station. Summertime concentrations (June 1997 - September
1997) were above 1800 m g/L at each site during each month. It
is not known whether anoxic sediments, nitrogen limitation, or increased effluent from
upstream sources caused these severely elevated levels. The remaining tributary sites had
mean orthophosphate concentrations ranging from 23-96 m g/L
during the 1997-98 monitoring period.
The lower Cape Fear system not only receives much of its nitrogen
from upstream of NC11, but much of its phosphorus as well. The largest concentrations of
TP (Fig. 2.18) in the system have been
measured at NC11 (mean = 186 m g/L), LVC (mean = 172 m g/L), and AC (193 m g/L). TP
concentrations generally decreased downstream through the estuary. The exception occurred
at M54, where total phosphorus concentrations have averaged 149 m
g/L, 20% higher than its upstream neighbor M61 (125 m g/L). The
source of this phosphorus is not clear, but may be related to sediment recycling (see Chapter 3). Since orthophosphate concentrations at M54
and M61 have been equivalent (43 m g/L), this implies a local
source of organic phosphorus. Unlike total nitrogen concentrations, total phosphorus
concentrations at the blackwater sites B210 (mean = 82 m g/L)
and NCF117 (mean = 104 m g/L) were somewhat less than those on
the mainstem Cape Fear, excluding the lower estuarine stations M35, M23, and M18. These
data indicate that blackwater swamps are not large sources of phosphorus to the Cape Fear
system. Phosphorus is likely removed through biological uptake and settling processes in
the lower estuary.
The data do suggest a seasonality pattern for total phosphorus
concentrations, with higher values occurring during the summer months (Fig. 2.19). The likely cause of this observed
phenomenon was described in the orthophosphate section.
The composition of phosphorus (organic
vs. inorganic) in the Cape Fear River has been remarkably uniform from the upstream
stations NC11 down through the estuary at M18. At each site, organic phosphorus comprised
between 60-68% of the total phosphorus, with the exception of M54 (71%). In sharp contrast
with nitrogen, the two blackwater sites B210 (66%) and NCF117 (60%) did not exhibit a
higher organic/total P ratio. Inorganic compounds comprised a larger percentage of the
total phosphorus during the summer months, directly related to seasonal peaks in
orthophosphate levels (Fig. 2.21).
Total phosphorus concentrations at LRC (1997-98 mean = 1506 m g/L) and BC117 (1997-98 mean = 1127 m
g/L) were an order of magnitude greater than at any other tributary, river, or estuarine
station (Table 2.13). Organic phosphorus comprised only 25%
and 10% respectively of the total P concentrations. NC403 also exhibited high TP levels
averaging 230 m g/L. This site experienced frequent
hypoxic/anoxic water column conditions, implicating sediment conditions which would have
resulted in the recycling of phosphorus similar to the lower Cape Fear stations.
Phosphorus concentrations at the remaining tributary stations were similar to the river
and estuarine sites.
The general concept of nutrient limitation is that algal tissues contain nitrogen and phosphorus in a molar ratio of 16:1 (Redfield ratio). Water column increases in this ratio (>16) suggest that there is excess nitrogen available, while a ratio of <16 suggests that phosphorus concentrations are in excess and will limit primary productivity. A seasonal pattern of N/P ratios exists in the Cape Fear River (Fig. 2.22). These data represent the mean monthly N/P molar ratio of all sites on the lower Cape Fear River, including the Northeast and Black Rivers. N/P ratios peaked during the winter months (>30) and dropped substantially during the summer (<20). These data suggest that strong phosphorus limitation occurs during the winter months and that the system approaches nitrogen limitation during the spring and summer (see Chapter 3). As discussed above, inorganic phosphorus concentrations are much higher during the summer than during the winter due in part to the regeneration of orthophosphate from anoxic/hypoxic sediments. Therefore, it appears as if seasonal phosphorus concentration patterns dominate the overall N/P pattern. Phosphorus cycles in the Cape Fear are similar to those in other North Carolina estuaries (Mallin et al 1997b). During September 1996, N/P ratios plunged to 5.6 following Hurricane Fran. Redfield ratios below 16, which would implicate nitrogen limitation, are not common in the Cape Fear system since it receives such large inorganic nitrogen loads primarily in the form of nitrate (see NO3- section). Experiments conducted by Mallin et al (1997b and 1998a) support these findings. It should be noted that the implications of the N/P ratio are theoretical and that other factors, including light limitation from high turbidities, also limit primary productivity in the Cape Fear system (Mallin et al. 1997b).
Biological Parameters
The Cape Fear River does not experience extended algal blooms as do
many of the other rivers in North Carolina (Mallin 1994) because of its swift currents and
efficient flushing ability. High turbidity levels also cause the system to be light
limited during periods of high flow. These turbidity inputs have been discussed previously
and are believed to be the major limitation factor of primary productivity at various
times during the year. During the 1997-98 monitoring period, there were only two instances
(July at AC and September at NC11) when river or estuarine chlorophyll a concentrations
exceeded the North Carolina State Standard of 40 m g/L (Table
2.14). The highest mean chlorophyll a levels since June 1995 (Fig. 2.23) have occurred in the estuary at
M35 (7.4 m g/L), M23 (7.1 m g/L),
and M18 (7.3 m g/L), with a maxima M42 (8.1 m
g/L), just below the turbidity maximum (Table 2.14). The lowest mean phytoplankton biomass
concentrations have been observed at the two blackwater stations, B210 (0.9 m g/L) and NCF117 (1.6 m g/L). Both of
these sites are characterized by low nutrient concentrations and darkly colored water,
which inhibit primary productivity.
Phytoplankton biomass levels have been considerably lower during the
fall and early winter than during the summer months since June 1995 (Fig.2.24). Decreased sunlight, colder water,
increased turbidity, and increased flow all limit primary productivity during this period.
A strong spring bloom occurred during 1996 and 1997, but was not as evident during 1998.
Spring blooms result from warmer water temperatures, increasing sunlight, and increased
nutrient loads from spring fertilizer applications. The lowest monthly average of
chlorophyll a was measured during December 1997, when the systemwide median
concentration was only 0.6 m g/L.
As with the river and estuary stations, chlorophyll a
concentrations at most of the tributary sites were also very low during the past
monitoring period (Table 2.14). However, scattered
blooms were evident at NC403, GS, PB, BCRR, SR, and COL, usually during low flow periods.
Additionally, surface macrophytes have been observed to cover much of the water surface at
stream sites such as PB and NC403 during the summer months.
The Cape Fear River experiences periods of hypoxia during the summer
which result from a combination of warm water temperatures and increased biological
activity. BOD loads, both natural (from swampy wetlands) and anthropogenic (industrial and
agricultural waste), can significantly affect dissolved oxygen concentrations. During the
1997-98 monitoring period there were only a few occasions during which there were elevated
amounts of 5-day and 20-day BOD (Table 2.15). Focusing first
on the short-term BOD5 data, which is in effect a measure of the labile (easily consumed
by organisms) organic compounds, only during July 1997 at AC (3.5 mg/L) was there a
significant amount of BOD. Mean BOD5 levels were higher at AC than at any of the other
stations. The probable source of this BOD load is organic inputs from the paper mill
located just upstream at Rieglewood since BOD5 measured only 1.3 mg/L at NC11, which is
upstream of the plant. On several occasions, BOD5 was considerably higher at AC than at
NC11 (July, November, and January). BOD5 concentrations at B210 (Black River) and NCF117
(Northeast Cape Fear River) were virtually identical over the past year. During the
previous 1995-96 and 1996-97 studies, NCF117 had received heavy BOD loads from a chicken
waste lagoon spill and from the after-effects of Hurricane Fran (Mallin et al. 1996;
Mallin et al. 1997c; Mallin et al. 1998b).
Long term BOD (20-day) data are indicative of the more recalcitrant
organic compounds which take longer for bacteria to consume. Station AC also exhibited the
highest 20-day BOD concentrations (Table 2.15). During the 1997-1998 period, BOD5 at AC
averaged about 33% higher than at NC11, and BOD20 at AC averaged about 20% higher than at
NC11 (Table 2.15). BOD20 at AC may also be increased due to nitrogenous long-term BOD from
elevated ammonium loading at LVC. There appeared to be two months, July and November, when
our sampling revealed especially large BOD20 loads, possibly attributable to the paper
mill and elevated ammonium loading at LVC. During both of these months, BOD20
concentrations were more than double those at NC11. As with BOD5, the blackwater river
sites at B210 and NCF117 exhibited similar long-term BOD levels. The Black River site at
BBT, which receives water from the Cape Fear and Black Rivers, had BOD concentrations
which reflect the mixing of these two rivers.
Fecal coliform bacteria counts are used as indicators of possible
microbial pathogens which can be harmful to human health. The current North Carolina State
Standard for recreational waters is 200 colony-forming units (CFU) per 100 ml. In the
lower Cape Fear River, fecal coliform bacteria counts are usually below this standard.
However, there were instances during the 1997-98 monitoring period when coliform levels
greatly exceeded 200 CFU/100 ml (Table 2.16), primarily
following heavy rains which bring increased volumes of suburban and urban runoff. In
January 1998 (Table 2.16), bacteria counts were high at NAV, BRR, and HB. During March,
following a period of very heavy rain, fecal coliform levels were excessively high
(910-1800 CFU/100 ml) at the upstream stations NC11, AC, DP, and IC. When bacteria levels
reach this magnitude over such a long stretch of the river, the source is more likely a
malfunctioning sewage plant or confined animal installation rather than simple land
runoff. However, it was not clear what the source was for this episode. A comparison of
the two blackwater stations B210 and NCF117 reveals geometric means of similar magnitudes,
but that NCF117 is more prone to periodic fecal coliform loading. There is a sewage
outfall near our sampling site which likely contributes bacteria to the system. Fecal
coliform bacteria counts at NCF117 reached 200 CFU/100 ml in July 1996 and 244 CFU/100 ml
during December 1996. There were no instances at B210 when coliform levels were above 78
CFU/100 ml.
The head of the estuary including NAV (46 CFU/100 ml), HB (46 CFU/100
ml), BRR (57 CFU/100 ml), and M61 (50 CFU/100 ml) has been most susceptible to bacterial
loading since February 1996 (Fig. 2.25). The
source of this contamination is not known, but may include a wastewater treatment plant in
Belville (outfall near BRR). Fecal coliforms have been much less numerous near the mouth
of the estuary at M35, M23, and M18 since these sites are flushed regularly with seawater.
In general, fecal coliform concentrations were generally not a human health problem in the
lower Cape Fear River system during the 1997-98 monitoring study (Table 2.16). However,
the NC standard for safe shellfishing waters is only 14 CFU/100 ml, and stations in the
lower estuary occasionally violated this standard.
The tributary stations are particularly susceptible to fecal coliform
loading since many of them lie directly adjacent to housing or agricultural properties. A
moderate rainfall will elevate bacterial counts. Fecal coliform levels were above 200
CFU/100 ml at some point at almost every tributary site during the 1997-98 monitoring
period (Table 2.16) . The sites with the most frequent problems are LRC (geomean = 452
CFU/100 ml), BCRR (geomean = 202 CFU/100 ml), BC117 (geomean = 92 CFU/100 ml), BRN
(geomean =137 CFU/100 ml), and HAM (127 CFU/100 ml). Fecal coliform levels at LRC, BC117,
and BCRR often exceeded 1000 CFU/100 ml, a dangerous level of contamination. The same
observations were made during the 1996-97 monitoring year (Mallin et al 1997). Local point
sources upstream of LRC and BC117 are easily identified, while the source(s) of coliform
bacteria at BCRR are unknown. Non-point source runoff from agricultural operations
apparently cause the elevated readings at BRN and HAM.
Metals
The sources of metals in the Cape Fear River likely include industry, agriculture, and suburban and urban waste. Metals can accumulate in the tissue of marine and riverine organisms and eventually become toxic to that organism. Humans may then become affected through the consumption of fish or shellfish. There does not seem to be widespread current health problems related to metal contamination in the Cape Fear River. However, future problems may arise since metals are known to accumulate in the sediments through settling processes, and can be reintroduced into the food chain as a result of dredging or natural geochemical processes. Metals are believed to be toxic primarily in their free ionic state (i.e. as a positively charged ion). Although total metal concentrations are measured for this program, the data presented below should be used as an indicator of future contamination problems that may arise from the input of potentially toxic metals into the Cape Fear system.
There is no North Carolina state standard for aluminum (Al) concentrations so assessing toxicity problems which may arise from this metal is difficult. The data presented for the 1997-98 monitoring period (Table 2.17) do reveal some interesting patterns. Mean concentrations were greatest in the upper river at NC11 (1047 m g/L), AC (994 m g/L), and DP (900 m g/L) suggesting a source in the North Carolina Piedmont, possibly from high turbidity loads which carry aluminum-laden clay particles. There was a slight peak in Al levels at M54 (700 m g/L) relative to its upstream neighbors HB (545 m g/L), and M61 (487 m g/L), which may be related to the estuarine zone of turbidity maximum or to a possible source from the nearby WWTP. Aluminum salts are used as a coagulant to remove organic matter at some treatment plants. Concentrations at the three tributary sites (SAR, LRC, BC117) were similar to those at the lower Cape Fear stations.
The lower Cape Fear River experienced only a few instances where arsenic (As) was present in measurable quantities during the 1997-98 monitoring period (Table 2.18). All of these occurrences were in the lower estuary at M23 and M18. The source of arsenic at these sites is unknown, but levels did not exceed the NC State Standard of 50 m g/L. For the tributary stations, there was one instance of measurable As concentration which was observed at BC117 (downstream of Burgaw WWTP) during October and measured 460 m g/L, considerably higher than the state standard. The cause of this spike is unknown and occurred only once.
None of the Cape Fear River or tributary stations received a measurable quantity of cadmium during the 1997-98 study period (Table 2.19). The NC State Standard for this metal is 2 m g/L for freshwater and 5 m g/L for saltwater.
There were scattered occurrences of measurable chromium concentrations during the past year (Table 2.20), none of which exceeded the NC State Standards of 20 m g/L for saltwater and 50 m g/L for freshwater. Chromium sources include a variety of industrial wastes, as well as shipping activities. This metal was present on three occasions at M18, possibly from heavy shipping traffic or anchored barges. Chromium levels were negligible at the three tributary stations.
Studies have shown that copper can be toxic to certain estuarine organisms (Sunda et al, 1990), and the Cape Fear River does experience periodic concentrations above the North Carolina State Standards of 7 m g/L for freshwater and 3 m g/L for saltwater. The estuarine stations M54, M23, and M18 exhibited copper levels above 20 m g/L during the course of the 1997-98 study (Table 2.21). The possible sources of this contamination are numerous, including industrial and municipal waste and shipping activities. The tributary station LRC received the most frequent and highest levels of copper. The now-closed Stevecoknit Mills was located upstream of this station and was the likely source for the observed pollution.
Iron is an important metal in many biological and geochemical processes in rivers and estuaries. This metal may enter the system through natural and anthropogenic sources. Blackwater humic substances effectively transport iron to the estuary from upland sources. The North Carolina State Standard for iron is 1000 m g/L for freshwater. There is no saltwater standard. During the 1997-98 study, the average iron concentration in the river and estuary was above 1000 m g/L at all stations except NCF117 (Table 2.22). The largest concentrations occurred at M23 (1356 m g/L) and M18 (1396 m g/L). Possible sources are shipping activities and the military port at Sunny Point. Since iron is typically removed from the water column through estuarine flocculation processes (Day et al 1989), there does appear to be a lower estuarine source of this metal. Iron levels at the three tributary stations were very similar to the river and estuary.
The quantity of lead in the waters of the Cape Fear River appears to be minimal (Table 2.23). There were only three instances of measurable lead quantities during the 1997-98 monitoring period, none above the NC State standard of 25 m g/L. Lead concentrations are likely somewhat higher in the sediments where this metal has been shown to accumulate. The tributary sites exhibited no measurable lead levels.
Mercury is an extremely toxic metal in several forms and originates from industrial sources such as wastewater effluent and airborne particle deposition. There have been reported cases of mercury contamination in fish tissue in the Cape Fear River Basin (NCDEHNR 1996a). However, this study found no measurable quantities of mercury during the 1997-98 period at any river, estuary, or tributary station (Table 2.24). The North Carolina State Standard is only 0.012 m g/L for freshwater and 0.025 m g/L for saltwater.
The mouth of the estuary near M23 (15 m g/L) and M18 (18 m g/L) received large amounts of nickel relative to the saltwater state standard of 8.3 m g/L (Table 2.25). Nickel concentrations were above this level at these sites on 8 of 12 sampling trips during this past monitoring period. The magnitude of nickel contamination in the upper river was negligible and increased downstream through the estuary, suggesting a lower estuarine source. Again, this source is unknown but may include Sunny Point Military Terminal and shipping activities. The freshwater standard is somewhat higher at 88 m g/L and was not exceeded at any upper river station. LRC and BC117 experienced sporadic measurable quantities of nickel, but none above the state standard.
Sources of zinc in the Cape River are numerous and measurable concentrations of this metal are typical (Table 2.26). Sunda et al (1990) has also found estuarine toxicity problems related to zinc. The NC State Standard for freshwater is 50 m g/L and 86 m g/L for saltwater. There was only one observation during the past monitoring study which exceeded these benchmarks and occurred at M61 during July (600 m g/L). There is no clear explanation for this spike in Zn levels and was probably a sample contamination problem. Little Rockfish Creek (LRC) exhibited the highest mean zinc concentrations (39 m g/L) of all stations. The likely source of zinc at LRC was Stevecoknit Mills.
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