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 during the 1995-1996 Cape Fear River 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 River was investigated by measuring the magnitude and composition of nitrogen and phosphorus in the water. Two biological parameters, chlorophyll a and biochemical oxygen demand, were examined to assess the direct effects of nutrient loading and physical variations in water quality. The results indicate that the river experienced no serious phytoplankton blooms during the monitoring period. However, the results will show that the Cape Fear River exhibited substantial turbidity and nutrient loading in addition to seasonal dissolved oxygen problems.
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
WaterTemperature, 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 approximately two meter intervals until either the probe reached the bottom or the cable connecting the datalogger with the sonde had been fully extended. 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 1996), except for the orthophosphate procedure which was still performed at CMSR. The following methods detail the techniques used by CMSR personnel only.
Nitrate+Nitrite (NOx-)
Water samples were collected 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. The filtered triplicates were collected in a common
glass flask, mixed thoroughly, and approximately 100 ml was poured into one 125 ml plastic
bottle to be used for nitrate+nitrite and orthophosphate analyses. Filtered samples were
stored in a freezer until time of analysis.
Samples were analyzed for NO3/NO2 on a Technicon
Auto-Analyzer using an approved EPA method (Method 353.4). All analyses were performed in
duplicate. In the NO3/NO2 method, samples are treated with ammonium
chloride and passed through a copper treated cadmium column which converts the nitrate in
each sample to nitrite. The nitrite then reacts with a color reagent containing
sulfanilimide and N-1 napthylethyleneidiamine dihydrochloride to form a pink colored dye.
The amount of color is directly proportional to the concentration of nitrate + nitrite
originally present in the sample. The intensity of the dye is measured at a wavelength of
550 nm using a colorimeter and peaks are displayed on a chart recorder. Nitrate and
nitrite standards were analyzed after every 10 samples for quality assurance. As part of
the EPA protocol, reagent water blanks and random samples were spiked with a specific
standard and analyzed for additional technique verification.
Orthophosphate (PO4-3)
All orthophosphate analyses were performed in duplicate using an approved EPA method for the Technicon Auto-Analyzer (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.
Ammonium (NH4+)
The method used for ammonium analysis involved a colorimetric technique adapted from Parsons et al. (1984) and Burkholder and Sheath (1985). Since ammonia is a volatile gas and sample contamination occurs easily, all samples were collected in triplicate in 30 ml glass serum vials and sealed with a rubber stopper. All reagents were injected through the rubber stoppers using 3 ml syringes and 20 gauge sterile needles. A new needle was used after each set of triplicates in order to avoid reagent contamination. The method involves treating each sample with sodium hypochlorite (household bleach) and a 10% phenol solution (in ethanol) in an alkaline citrate environment to form a blue indophenol color. Sodium nitroprusside reagent is used as catalyst for this reaction. The phenol solution was added to each sample immediately after collection at the field site in order to fix the ammonia concentration in the sample. After all reagents had been injected, the colorization process was allowed to continue in each glass vial for approximately 1 hour. Ammonium concentrations were then determined using a spectrophotometer at a wavelength of 640 nm. Prepared standards were also analyzed in the same manner.
Total Phosphorus (TP)
Triplicate samples were collected in 60 ml clear plastic bottles and used for both total phosphorus and total nitrogen analyses. The analytical method used in determining total phosphorus was adapted from an oxidation method detailed in Valderamma (1981). In individual test tubes, 36 ml of each sample was treated with an oxidizing reagent composed of potassium persulfate, boric acid, and sodium hydroxide and placed in an autoclave for 15 minutes. The oxidation process converts organic phosphorus to orthophosphate. Samples were not filtered prior to oxidation. After the test tubes had cooled, the samples were colorized manually using the same reagents as used in the orthophosphate analysis. Total phosphorus concentrations were then determined using a spectrophotometer at a wavelength of 885 nm.
Total Nitrogen (TN)
This analysis incorporated the same oxidation process as described in the total phosphorus method. During oxidation, organic nitrogen and ammonia are converted to nitrate. Following oxidation, a portion of each sample was filtered using 1.0 micron glass fiber filters. Triplicate samples were not combined as described in the NO3/NO2 section, but were stored individually. Total nitrogen was then determined by measuring the nitrate concentration of the oxidized filtered sample using the nitrate+nitrite auto-analyzer method described previously.
Biological Parameters
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 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.
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 swamp
water regions (NCF117 and B210), and one site was situated in an area influenced by both
the mainstem Cape Fear River and "cleaner" swamp water. The procedure used for
BOD analysis was method 5210 in the Standard Methods (18th edition). 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 intitiated 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. No sample dilutions were necessary since BOD requirements
were always less than the initial DO of each sample.
2.3 Results and Discussion
This section will focus on the physical, biological, and chemical parameters measured at the surface of the water column. Preliminary statistical tests conducted during the fall of 1995 indicated little difference between surface and bottom parameters. As a result, bottom sampling was then discontinued after November 1995. Those results will be presented later in this report (Chapter 3) and will show that the Cape Fear River is a well mixed system maintaining little physical and chemical stratification. This data has not been checked for quality assurance/quality control. Referenced figures are available in the hard copy of this report.
Physical Parameters
The annual mean water temperature for the Cape Fear River was approximately 18.5 C at all sampling sites (Table 2.1). The lowest temperatures were recorded during January ranging from 4-7 C. The warmest water temperatures were recorded during August when readings ranged from 27-30 C. In general, there was little variation among stations during any particular month. However, the range of water temperatures over the entire monitoring period was slightly larger at the upstream riverine sites than at the lower estuarine stations where tidal ocean influxes help moderate water temperatures.
Salinity values measured during the study are displayed in Table 2.2. The lowest estuarine station near Southport (M18) exhibited a considerable surface salinity range of 13.5-30.0 ppt. The minimum salinity at M18 was recorded during July 1995, a period of exceptionally high rainfall. Similarly wide salinity ranges were observed at M42, M35, and M23. Measurable surface salinities reached as far upstream as NAV and NCF6 during January of 1996, but were usually limited to stations below M61 (Wilmington port). Mallin et al. (1995) reported salinity ranges at NAV, M61 (formerly M55), and M54 (formerly M50) during the 1994-1995 monitoring period which were similar to this study. Salt wedges were occasionally discernible as far upstream as M61. However, as mentioned above, the Cape Fear River flushes quickly and water stratification is very ephemeral.
The Cape Fear River experiences wide-ranging levels of dissolved
oxygen. At stations in the mainstem Cape Fear River, annual surface dissolved oxygen
levels averaged 7-8 ppm (1 ppm = 1 mg/l) (Table 2.3 and
Figure 2.1). The greatest mean concentrations were measured in the lower estuary at M18
(8.1 ppm) and M23 (7.9 ppm), and at the most upstream station NC11 (8.0 ppm). Bottom
dissolved oxygen levels were not significantly different from surface measurements since
the Cape Fear River is a high flow and well-mixed system (see Chapter 3). Annual mean
surface concentrations at stations in the Northeast Cape Fear and Black River systems were
slightly lower ranging from 5.6-6.7 ppm. The Northeast Cape Fear River station NCF117 had
the lowest annual mean of 5.6 ppm, although data from March was unavailable and would have
likely increased this number slightly. This station is situated in a distinctly blackwater
region with little anthropogenic influence. However, NCF117 did experience the lowest
dissolved oxygen level of the entire monitoring period when surface concentrations plunged
to 1.9 ppm during July 1995 (Table 2.3). This minimum appears to have been largely
attributable to an oxygen sag associated with a chicken waste lagoon spill in Duplin
County in July 1995 (Mallin et al. 1996a; Chapter 4 - this report).
Dissolved oxygen levels were considerably higher at all stations during
the winter and early spring months than during summer months (Figure 2.2). From January
through April, concentrations typically averaged 9-12 ppm. These winter levels are similar
to those reported in the Pamlico River estuary (Stanley 1986) and in the 1994-1995 Cape
Fear River report (Mallin et al. 1995). During the summer and early fall (June-November),
dissolved oxygen concentrations were considerably lower at all stations. As illustrated in
Figure 2.3 and detailed in Table 2.3, dissolved oxygen levels often fell below the minimum
North Carolina state standards for water quality of 5.0 ppm for estuaries and 4.0 ppm for
blackwater systems (NCDEHNR 1994). Minimum surface and bottom concentrations in the Cape
Fear River reached 2.8 ppm at Navassa (NAV) during August 1995. It is quite likely that
aquatic organisms experiencing low oxygen concentrations in this range (hypoxia) will
become severly stressed. Subsequent sampling during September (3.3 ppm) and October (3.8
ppm) also revealed low dissolved oxygen readings at Navassa (Table 2.3). Data collected
during the previous year did not show similar problems (Mallin et al. 1995) at stations
sampled in that study (NAV, M61, M54).
As seen in Figure 2.3, monthly dissolved oxygen concentrations were
relatively consistent down the length of the river. However, for unknown reasons during
June 1995, the estuary stations exhibited surface and bottom concentrations 3-4 ppm higher
than at the upstream stations. This pattern can not be explained by a phytoplankton bloom
(blooms temporarily increase oxygen levels due to photosynthesis) which occurred in the
estuary during June since the upstream stations exhibited similar chlorophyll a
levels.
The Cape Fear River between NC11 and the mid-estuary never suffered
from anoxia, but maintained an extensive period of substandard dissolved oxygen, or
hypoxia (Figure 2.3). This likely resulted from a combination of BOD loading below NC11
and contributions from the hypoxic blackwater inputs. The lack of stratification in the
Cape Fear River prevents anoxia, but does promote hypoxic conditions throughout the water
column. Bottom water dissolved oxygen problems in the Neuse (Paerl et al. 1995) and
Pamlico (Stanley 1986) estuarine systems result from water column stratification,
insufficient flushing, and algal bloom decomposition. However, the data collected during
this study indicate that even in the well-mixed and high flow Cape Fear system, waste
lagoon spills and increasing urbanization can be detrimental to overall dissolved oxygen
levels. Recently, CMSR has begun BOD (biochemical oxygen demand) sampling which will
increase the current knowledge of what controls dissolved oxygen in the Cape Fear River
(discussed later in Chapter 2).
Turbidity
Turbidity is a measure of the amount of suspended organic and
inorganic material 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 mainstem Cape Fear River exhibited high turbidity levels during the
1995-1996 monitoring period. Monthly surface turbidity data is listed in Table 2.4 and the annual mean for each station is illustrated in
Figure 2.4. The highest annual mean was recorded at Navassa where the turbidity averaged
35 NTU. The North Carolina state standard for acceptable water quality is 25 NTU (NCDEHNR
1994). Seven of the sixteen stations monitored in this study exhibited yearly mean
turbidity levels above this standard (Figure 2.4). All seven of these stations were
located along the mainstem of the Cape Fear River. Mean turbidity levels were especially
high near the Wilmington port from NAV down to M42. Sampling sites on the Northeast Cape
Fear River and the Black River received much less sediment loading. The annual mean at the
blackwater site NCF117 was only 4.6 NTU. Although the data set is incomplete for B210, an
environment similar to NCF117, turbidity at this site averaged only 3.6 NTU. The
blackwater tributaries have extensive streamside riverine wetlands which serve as sediment
filters to protect water quality (Johnson 1991). The stations at NCF6 (Northeast Cape
Fear) and BBT (Black River below Thoroughfare) are similar in that both are influenced by
the turbid Cape Fear and also by a clean predominately blackwater river system as
evidenced by their mid-range turbidity levels of 15-16 NTU. Turbidities measured during
the 1994-1995 study (Mallin et al. 1995) averaged approximately 18 NTU for the estuarine
stations located at NAV, M61, and M54. Data collected during the present study reveal a
significant increase at all three stations over the preceding year, where annual means
were 35 NTU, 27 NTU, and 35 NTU for NAV, M61, and M54 respectively.
The highest individual turbidity measurement of 100 NTU was recorded during
February 1996 at M54. Also during February 1996, stations at M61 (73 NTU) and M42 (67 NTU)
experienced very high sediment loads. Since the neighboring upstream and downstream
stations did not show similar elevated turbidities during this month, the exceptionally
high readings at those three sites seem to have been directly attributable to winter
dredging activities just below Wilmington. In October 1995, heavy rains washed a
tremendous sediment load down the length of the Cape Fear River causing elevated surface
turbidities from NC11 down through M54. The upstream stations (NC11 and AC) were again
heavily influenced by rain in May 1996 as turbidity levels were approximately 80 NTU at
each station. It is likely that much of the sediment reaching these upper stations
originates from upstream non-point sources in the Piedmont region of North (Mallin et al.
1996b).
Chemical Parameters
The Cape Fear River receives considerable non-point source nutrient
loading as demonstrated by the nitrate+nitrite data (henceforth referred to only as
nitrate or NO3-) in Table 2.5.
Mainstem nitrate concentrations entering the study area at NC11 averaged 650 ug/l.
The data in Figure 2.5 depict a nearly uniform decrease in NO3-
concentrations down the mainstem of the river. Mean nitrate concentrations were only 140 ug/l
near the mouth of the river at M18, the lowest recorded value in the system. This station
is constantly refreshed with low-nutrient ocean water. Data collected in the Cape Fear
River during the previous 1994-1995 monitoring period displayed mean annual nitrate levels
of 439-514 ug/l at the three middle to upper estuary stations (M54, M61, NAV) which
were almost identical to data from this study (441-545 ug/l). With the exception of
the station at BBT, which is heavily influenced by Thoroughfare and tidal intrusions of
the Cape Fear River, sampling sites on the Black River and Northeast Cape Fear River
systems displayed NO3- levels averaging approximately 300 ug/l,
substantially lower than most mainstem sites.
Monthly nitrate levels varied greatly during this study (Figure 2.6).
The Cape Fear River received the greatest nitrate load during June 1995 averaging 672 ug/l
at all stations (B210 not included since its data set was incomplete). Nitrate levels were
well above 1.0 mg/l at NC11, AC, IC, and NAV. Peak nitrate levels in the 1994-1995 data
set were comparable to the 1995-1996 data, except that on one occasion in August 1994,
bottom water NO3- concentrations were greater than 2.5 mg/l at M50.
In comparison, Stanley (1986) reported peak nitrate levels of approximately 300 ug/l
upstream in the Pamlico River near Washington, N.C., in a 1986 study. Paerl et al. (1995)
reported maximum concentrations near 600 ug/l at an upstream site in the Neuse
River near New Bern, N.C., during a four year period from 1990-1993. Kuenzler et al.
(1982) recorded peak nitrate levels of approximately 450 ug/l in the Chowan River
during a 1981 monitoring period. Interestingly, this maximum in the Chowan River occurred
at a downstream station and was attributed to renitrification processes rather than point
or non-point source runoff. Clearly, mean monthly nitrate levels in the Cape Fear River
system routinely exceeded the maximum concentrations found in those other North Carolina
river systems. During the present study, the lowest average monthly concentration for all
stations was 235 ug/l occurring in July 1995. As illustrated in Figure 2.6, there
did not appear to be any clear seasonal nitrate pattern. Nitrate loading in the Cape Fear
River seems to be affected mostly by short term hydrologic and meteorological conditions.
This finding was also discussed for other North Carolina rivers in Mallin (1994).
The Cape Fear River system displayed no discernible spatial pattern
for ammonium during this study as illustrated in Figure 2.7 (see also Table 2.6). Mean ammonium concentrations ranged from 30-100 ug/l.
During the 1994-1995 monitoring period, NH4+ levels averaged nearly
60 ug/l at NAV, M61, and M54 (Mallin et al. 1995), considerably lower than at those
same sites (85-99 ug/l) in the present study. The greatest annual mean ammonium
concentrations were measured at the upstream station AC (101 ug/l) and at the
mid-estuary site M54 (99 ug/l). Average ammonium concentrations at AC were nearly
double those at the upstream station NC11 (55 ug/l). AC also exhibited the highest
individual ammonium concentration of 282 ug/l during August 1995. The AC site is
located just downstream of a large paper processor in Rieglewood. Kuenzler et al. (1982)
reported a significant increase in ammonium levels in the Chowan River directly
attributable to discharge from an upstream pulp mill. However, recent investigations have
shown that Livingston Creek, just upstream of AC and near the paper industry, also has
high ammonium concentrations and may be the direct source of this compound for the river.
Livingston Creek receives discharges from the Wright Chemical Company.
The M54 site, located just downstream of the Wilmington port, receives
heavy recycled sediment loading (see turbidity results) from dredging activities which may
have significantly affected ammonium concentrations in the water column. Sediments may
contain a significant reservoir of ammonium which can be recycled into the water column
through various mechanisms (Day et al. 1989; Stanley 1986). Additionally, effluent from
the city of Wilmington’s wastewater treatment plant may influence water quality at
this station. As observed for nitrate and chlorophyll a, the blackwater stations at
B210 (28 ug/l), NCF117 (35 ug/l), and NCF6 (38 ug/l) exhibited
relatively low mean NH4+ levels compared with the mainstem Cape Fear
River.
The data depicted in Figure 2.8 does not indicate any seasonal pattern
in ammonium concentrations. Data collected at B210 was not included in monthly
calculations because of an incomplete data set. Samples analyzed during February 1996
displayed the highest monthly mean of 126 ug/l. This peak did coincide with a
similar peak in nitrate (Figure 2.6) and chlorophyll a (Figure 2.19)
concentrations, as well as turbidity levels (Table 2.4). It is
quite possible that the February NH4+ maximum could have resulted
from a combination of nutrient loading, phytoplankton excretion, and sediment recycling of
reduced nitrogen. During June 1995 and October 1995, ammonium concentrations were at a
minimum, averaging less than 40 ug/l.
One interesting data point is the July 1995 ammonium measurement at
NCF117. Throughout the rest of the year, NH4+ levels were nearly
always substantially less than the overall average. However, during the July 1995 sampling
period, the peak ammonium concentration for the entire system was measured at this site.
As will be discussed in Chapter 4, it appears that this unusual peak can be attributed to
a plume from a chicken waste lagoon spill which occurred in Duplin County a week prior to
sampling.
Annual mean total nitrogen concentrations ranged from 498-1221 ug/l
as the data illustrate in Figure 2.9 and Table 2.7. Total
nitrogen values were consistently in the range of 1.0-1.2 mg/l at all stations, except
where concentrations showed declining values in the lower estuary sites M35 (909 ug/l),
M23 (711ug/l), and M18(498 ug/l). Total nitrogen is low at these three
stations probably because of routine tidal flushing from the low nutrient coastal ocean
and transfer of nitrogen into higher levels of the estuarine food chain, but may also
involve more complex sediment-water column interactions. The stations located on the Black
and Northeast Cape Fear rivers exhibited TN means ranging from 900-1127 ug/l,
comparable to the mainstem Cape Fear River. Samples collected in the estuary during
September 1995 were lost and were not included in annual mean calculations. Data from the
previous monitoring period in 1994-1995 revealed TN concentrations in the Cape Fear River
ranging from 850-981 ug/l at NAV, M61, and M54 (Mallin et al. 1995), slightly lower
than data from this study.
The data displayed in Figure 2.10 represent the mean monthly TN
concentrations of all stations except B210 (insufficient data). Data collected during June
1995 and July 1995 exhibited higher TN concentrations than the following six month period
from August 1995-January 1996. The following four month period from February 1996-May 1996
seems to indicate increased total nitrogen levels in the river. However, it should be
mentioned that beginning in February 1996, all TN analyses were performed by Law and Co.
and not by CMSR personnel. Law and Co. calculated TN values using the formula TN = total
kjeldahl nitrogen (org N + NH4+) + nitrate+nitrite. The TKN analysis
(Standard Methods 4500 NH3) is slightly more rigorous than is the TN method
performed by CMSR staff (Valderama 1981), and the observed increase in TN may simply
represent better organic N yields in the method used by Law and Co. Therefore, any further
discussion of temporal TN data patterns will not be presented.
There was a clear difference in nitrogen composition between upstream,
estuarine, and blackwater stations as indicated in Figure 2.11. The nitrogen that entered
the upstream stations NC11 and AC was approximately 60% inorganic (nitrate+nitrite +
ammonium) and 40% organic (calculated as TN - inorganic N). An important note is that most
of the inorganic N in the Cape Fear River system exists in the form of nitrate+nitrite,
entering the river primarily from agricultural runoff, and only a small inorganic fraction
is ammonium. The data in Figure 2.11 indicate that as water flows downstream towards the
estuary, an increasing amount of the inorganic nitrogen is converted to organic N. In the
estuarine region from NAV-M35, organic N composition ranged from 47-54%. In comparison,
results from Mallin et al. (1995) show estuarine organic N ratios of only 42-43%,
suggesting some small-scale annual variablity. At stations near Southport, M23 and M18,
organic N comprised 60% and 65% of the total N respectively. Chlorophyll a levels
were at a maximum at M23 and M18 suggesting that much of the inorganic N had been
converted to phytoplankton biomass.
Blackwater stations located at B210, NCF117, and NCF6 exhibited the
greatest percentage of organic N/total nitrogen, averaging near 70% at all three stations
(Figure 2.11). Since TN concentrations were comparable to the upstream and mid-estuarine
values and chlorophyll a levels were relatively low at these three stations it may
be concluded that much of the N entering these blackwater stations is bound by refractory
organic material such as humic acids. The swamps and wetlands surrounding these regions
apparently act as effective buffers from nutrient loading (Mallin et al. 1996b). The
station BBT, located in the Black River a few miles upstream of the junction with the Cape
Fear River, exhibited a nitrogen composition (43% organic) similar to its nearest Cape
Fear River neighbors (AC and IC), but maintained a substantially lower average chlorophyll
a level. These data indicate that downstream flow from the swampy headlands, tidal
flow upstream from the mainstem of the Cape Fear, and mainstem water routed by
Thoroughfare all affect water quality at BBT. The total nitrogen data described in this
section clearly stresses the importance of the swamps and riparian wetlands surrounding
the Cape Fear River and its tributaries in maintaining overall water quality.
Annual mean orthophosphate concentrations calculated for each
station are illustrated in Figure 2.12 and listed in Table 2.8.
The data indicate that orthophosphate levels generally decrease as the river moves
downstream towards the estuary. The pattern is very similar to the nitrate+nitrite pattern
illustrated in Figure 2.5. Orthophosphate levels entering the system at NC11 averaged 61 ug/l.
The greatest yearly average was 66 ug/l measured at AC, which is located just
downstream from a large paper processing plant, possibly indicating orthophosphate
contributions from the plant. Kuenzler et al. (1982) reported significant increased
orthophosphate loading in the Chowan River from an upstream pulp mill where PO4-3 exceeded 300 ug/l. The highest PO4-3
measured during this study was 134 ug/l measured at NC11 during June 1995. For the
mid-estuary stations M54, M61, and NAV, concentrations were considerably higher (47-60 ug/l)
during this study than during the 1994-1995 monitoring period (27-49 ug/l),
suggesting interannual variability in P loading to the Cape Fear River. Lebo and Sharp
(1993) reported mean phosphate ranges of approximately 60-125 ug/l in the heavily
industrialized Delaware River estuary.
The lowest orthophosphate levels (13 ug/l) were found near
Southport at M18 which experiences regular tidal flushing from the low nutrient water of
the ocean. The Black River station BBT and the two Northeast Cape Fear River stations
NCF117 and NCF6, displayed orthophosphate concentrations of 49,44, and 38 ug/l
respectively, which were considerably lower than the upstream stations on the Cape Fear
River. Not all phosphate loading is anthropogenic in origin, but may be naturally derived
from the sediments (Day et al. 1989). Since NCF117 and NCF6 displayed relatively low
nitrate concentrations, which are often a result of riverine fertilizer pollution, it
appears that these stations may experience some natural phosphate loading. Also, these
stations receive drainage from swine farming areas, and swine waste is rich in phosphate
(Westerman et al. 1990; Mallin et al. 1996a). As mentioned above, the BBT site is
significantly influenced by the Black River and the Cape Fear River. The data collected
for B210 indicate low PO4-3 levels, but the data set is incomplete.
Mean orthophosphate levels (Figure 2.13) were much higher during the
summer-early fall months (47-67 ug/l) than during the winter-spring months (26-35 ug/l).
The peak occurred during September 1995 when PO4-3 concentrations
reached 67 ug/l. The minimum was observed during February 1996 (26 ug/l). A
similar seasonal trend has been reported in the Pamlico River (Stanley 1986) and Neuse
River (Paerl et al. 1995) where mean annual orthophosphate levels were reported to average
50-150 ug/l. Day et al. (1989) discussed how significant amounts of phosphate can
be regenerated from reduced sediments into the overlying water column during the summer
months. This process may be important in the Cape Fear River system. Additionally, Mallin
et al. (1995) reported that nitrogen limitation occurs in the Cape Fear estuary during the
summer months. A combination of these two factors, as well as reduced summertime flow from
increased evapotranspiration, are likely responsible for the seasonal orthophosphate trend
in the Cape Fear River system.
One especially interesting data point is the July 1995 orthophosphate
concentration measured at NCF117. In general, orthophosphate levels measured at this
station were approximately equal to or below the mean of all 16 stations. However, during
this sampling period, PO4-3 concentrations were nearly twice the
monthly mean. This situation was likely a consequence of the chicken waste lagoon spill
mentioned earlier. The orthophosphate data, along with the low dissolved oxygen and the
elevated ammonium data at NCF117, almost certainly pinpoint the lingering effects of the
waste plume.
Total phosphorus concentrations were highest at the upstream
stations NC11 and AC, averaging 118 and 128 ug/l respectively (Table 2.9 and Figure 2.14). This is likely a result of high
anthropogenic loading upstream, possibly augmented by the paper industry’s outfall
located between these two stations. The lowest annual TP mean was measured at M18 (48 ug/l).
The data in Figure 2.14 indicate a general trend of declining total phosphorus levels as
the river moves towards the lower estuary. However, an interesting observation illustrated
clearly in Figure 2.14 reveals that station M61 (74 ug/l) exhibited a notably lower
total phosphorus concentration than did its downstream neighbors at M54 (91 ug/l)
and M42 (86 ug/l). Data reported in Mallin et al. (1995) displayed lower overall TP
values (75-97 ug/l) than found during the present study, but did not reveal a
similar phosphorus sink at M61. Results from Lebo and Sharp (1993) show TP levels in the
urbanized Delaware River estuary ranging from 60-180 ug/l, very comparable to the
increasingly industrialized Cape Fear River. Total phosphorus concentrations for sites on
the Black River and Northeast Cape Fear River ranged from 56-75 ug/l, much lower
than levels in the mainstem Cape Fear River.
The data depicted in Figure 2.15 indicate higher TP levels during late
spring-early summer for the present study, corresponding to periods of increased
orthophosphate levels. The highest monthly mean was 156 ug/l, measured in May 1996.
During this month, total phosphorus levels measured 330 ug/l at both the NC11 and
AC stations, by far the peak values observed during the entire study. Again, these sites
may be influenced by the paper mill. TP concentrations during May 1996 greatly exceeded
the second largest P influx which was 114 ug/l measured during June 1995. Minimum
total phosphorus levels were observed during December 1995 averaging only 51 ug/l.
Data collected in the Pamlico River by Stanley (1986) exhibited peak TP levels of greater
than 600 ug/l during the summer and minimum levels of less than 50 ug/l in
the winter.
Inorganic dissolved phosphorus (orthophosphate) comprised approximately
45-55% of the total phosphorus at all stations in the mainstem of the Cape Fear River from
NC11 to M23, except for M61 (Figure 2.16). The data depicts a slightly decreasing
estuarine ratio of inorganic P downstream towards M23. At M61, orthophosphate
concentrations accounted for 64% of the measured total phosphorus. It was mentioned above
that there was a significant drop in overall TP levels at M61 compared with adjacent
stations even though orthophosphate levels did not display a similar drop (see Figure
2.13). A possible explanation may be that at M61, where fresh water from the Cape Fear
River and Northeast Cape Fear River combine with pulses of incoming saline waters from the
lower estuary, this mixing process is removing a considerable amount of organic phosphorus
to the sediments through floccualtion or some other physical process.
Stations located at M18 (27%) and B210 (34%) exhibited the smallest
relative percentages of orthophosphate. The relative fractions of inorganic nitrogen
constituents were also small at both of these stations (Figure 2.11). Interestingly, the
two stations located on the Northeast Cape Fear River, NCF117 and NCF6, displayed quite
different phosphorus constituents. Orthophophate comprised 67% of the total phosphorus
concentration at NCF117, but only 51% at NCF6, typical of values on the mainstem Cape Fear
River. At NCF6, it appears likely that tidal currents bring substantial amounts of
mainstem Cape Fear water to mix with the Northeast Cape Fear water. Also, phytoplankton
biomass more than doubles between NCF117 and NCF6 (Table
2.11) with subsequent conversion of inorganic to organic P. Another interesting note
is that the blackwater stations NCF117 and B210 displayed similar inorganic N/total N
ratios (30%) and chlorophyll a levels (1.5 ug/l), but very different
inorganic P/total P ratios. Therefore, it appears that there maybe a natural source of
orthophosphate near NCF117, either from the underlying sediments or from a nearby creek.
Additionally, P loading from upstream swine and poultry operations may differ between the
Black and NECF watersheds.
Figure 2.17 reveals that in June 1995 and during February 1996-May
1996, organic P (calculated as TP-OrthoP) levels were very high ranging from 46-69% of
total P and Figure 2.13 showed that orthophophate concentrations were generally low during
these same months. This combined data suggests that phytoplankton uptake of orthophosphate
occurred mostly in the spring and that phosphorus may be a significant limiting factor
during this period. Mallin et al. (1995) reported spring P limitation in the Cape Fear
River during 1994-1995. The Neuse (Paerl et al. 1995) and the Delaware River (Lebo and
Sharp 1993) also exhibit P limitation in the spring.
In summary, the composition of phosphorus along the Cape Fear River and
its tributaries ranged from 27-67% orthophosphate, with the mainstem stations consistently
averaging near 50%. Lebo and Sharp (1993) reported inorganic P/total P ratios of 50-65%
for the urbanized Delaware River estuary. Data collected during the previous Cape Fear
estuary monitioring period exhibited phosporus compositions ranging from 51%-36%
orthophosphate with a more pronounced decrease in inorganic P downriver than was observed
during 1995-1996.
Biological Paramters
Biochemical oxygen demand primarily represents the amount of oxygen
utilized in the biodegradation of organic material over a given time period (5-day or
20-day). Oxygen is consumed by microbial activity in the water column when labile organic
carbon is oxidized to carbon dioxide. The amount of oxygen depletion that occurs is a good
indication of the amount of "juicy" organic carbon available for bacterial
degradation. High BOD loading may come from industrial wastes, urban runoff, and sediments
eroded from upstream wetlands where swampy areas have been cleared for development (Mitsch
and Gosselink, 1986).
Results from the 5-day and 20-day BOD analyses are shown in Table 2.10. The procedure outlined in Standard Methods gives a
minimum oxygen depletion level of 2 mg/l for 5-day BOD as a reasonable minimum detection
limit for this method. 5-day oxygen utilization of at least 2 mg/l was observed at only
one station during the entire study reaching 2.1 mg/l at BLL during January. Since no
other sites displayed the minimum depletion level, no further discussion of the 5-day
analysis will be presented.
The maximum 20-day BOD oxygen depletion was 4.5 mg/l measured for the
BLL and B210 samples during January. BOD levels were generally lowest during February and
March at all stations. The sites located on the Cape Fear River at BLL and AC exhibited
the greatest 20-day BOD averaging more than 3 mg/l. None of the samples for any month
approached anoxia during the bioassay’s 20 day incubation period.
Unfortunately, the data was collected only during the winter and spring
so that seasonal and long term assessments of BOD loading are not possible. Since it can
be assumed that there is a significant source of natural organic carbon from the swampy
black waters which feed the system, preliminary results from these five sites suggest that
much of this carbon must be unavailable for bacterial degradation. Industrial inputs of
BOD may be important near Rieglewood, but the data is again inconclusive. However, as
described earlier in Chapter 2, the Cape Fear River does experience extensive periods of
hypoxia during the summer and early fall which are not caused by water column
stratification and large algal blooms. It must be assumed that BOD loading helps to create
these low dissolved oxygen conditions. 5-day and 20-day BOD analyses will continue during
the 1996-1997 monitoring period which will generate more information about the seasonal
and geographical variations along the Cape Fear River system.
The data illustrated in Figure 2.18 clearly depict increasing
surface chlorophyll a levels as the Cape Fear River flows downstream from NC11 (6.0
ug/l) to M18 (11.6 ug/l). Monthly data for each station is listed in Table 2.11. Average yearly chlorophyll a
concentrations were fairly uniform at sampling sites from NC11 down to M61 ranging from
4.7-6.1 ug/l but increased steadily down through the lower estuary toward
Southport. Mallin et al. (1995) reported a similar downstream increase, though data from
that study was limited to three mid-estuary stations (NAV, M61, M54). Each of the Black
River and Northeast Cape Fear River stations exhibited much lower annual mean chlorophyll a
concentrations than the mainstem Cape Fear River (Figure 2.18). The sampling site near
Castle Hayne at NCF117 displayed the lowest chlorophyll a levels averaging only 1.4
ug/l. Low inorganic nutrient loading probably helps to keep this station free from
phytoplankton blooms. None of the sixteen stations had annual means approaching the North
Carolina state standard for eutrophic waters which is 40 ug/l.
Distinct seasonal patterns in chlorophyll a concentrations were
detected (Figure 2.19). The fall-early winter period from September 1995 through January
1996 exhibited relatively low productivity, while the early spring-summer months displayed
considerably higher phytoplankton biomass. Each station recorded its minimum chlorophyll a
level during either November or December, when colder water, reduced daylight, and low
nutrient flow limited productivity. Sampling during June 1995 uncovered a phytoplankton
bloom at nearly all sites. The mean chlorophyll a concentration for all stations
during this month was 15.0 ug/l, with a maximum of 29.1 ug/l at M35, the
highest recorded at any site during the entire study. During the 1994-1995 monitoring
period, Mallin et al. (1995) reported a maximum chlorophyll a level of 30 ug/l,
nearly matching data from the present study. Two other significant phytoplankton blooms
occurred during August 1995 and February 1996, where mean chlorophyll a levels were
10.9 ug/l and 11.3 ug/l respectively. In August 1995, the bloom was limited
primarily to the estuarine stations, while in February 1996, the upper river stations NC11
and AC experienced high phytoplankton biomass. Silicate data collected during June 1995
and August 1995 at the estuarine stations suggest the blooms consisted primarily of
diatoms. Low chlorophyll a biomass was recorded systemwide during July 1995. During
this period, the river was characterized by high flow and turbidity caused by heavy
rainfall (North Carolina Climatology Office, pers. comm.).
The Cape Fear River system experiences small phytoplankton blooms
relative to many other rivers in North Carolina. Serious problems have been reported in
the Tar-Pamlico River system (Stanley 1986), Chowan River (Kuenzler et al. 1982), and
recently in the Neuse River (Paerl et al. 1995) where nutrient loading, low flushing, and
summer water column stratification allow thick algal blooms on surface waters. Peak
chlorophyll a levels in these estuaries routinely exceeded 100 ug/l,
considerably above the North Carolina state standard of 40 ug/l. Consequently,
bottom-water dissolved oxygen depletion has become a normal summertime condition in many
areas of these rivers. Mallin (1994) reported periodic late winter algal blooms in North
Carolina rivers resulting from wintertime nutrient loading and recycling.
The amount of nitrate that enters the Cape Fear River is an important
factor influencing primary productivity in this system. Mallin et al. (1995) reported that
nitrogen was a limiting factor controlling productivity at various times during the year.
The data displayed in Figure 2.20 reveal that chlorophyll a and nitrate
concentrations peaked during June 1995 and again during February 1996. During July 1995,
November 1995, and April 1996, both nitrate and chlorophyll a levels dipped to
relatively low values.
The data collected during this study demonstrates that present nutrient
loading on the Cape Fear River system is not sufficient to overcome the high flushing
ability of the river in order to sustain heavy algal blooms. Additionally, the
river’s extremely high turbidity levels may considerably limit productivity. Mallin
et al. (1995) reported the likelihood of light limitation in the Cape Fear River. However,
the data clearly indicates that phytoplankton biomass is directly related to nutrient
concentrations and any substantial increase in nutrient loading from non-point source
runoff or waste effluent could result in increased algal blooms throughout the Cape Fear
River system.
2.4 Preliminary Results from New Sites
Physical Parameters
Mean turbidity and dissolved oxygen data for the new sampling sites are listed in Table 2.12. Average turbidity values at these stations for the period from February 1996 - May 1996 ranged from a low of 2.5 NTU at the swampy and undisturbed Colly Creek site to 52.5 NTU at Browns Creek near Elizabethtown. The turbidity was also noticeably high at the Burgaw Canal sites BCRR (45.0 NTU) and BC117 (49.5 NTU). Heavy rain events substantially elevated turbidities at most of the smaller creeks during the sampling period. Mainstem Cape Fear River sites BLL (37.8 NTU) and LVC (33.0 NTU) displayed turbidities typical of the nearby CFR riverine sites NC11 and AC, described in a previous section. Dissolved oxygen concentrations ranged from 7.5 ppm at Colly Creek to 10.1 ppm at Blenon Landing. Since water temperatures were cool during the months sampled, it was expected that dissolved oxygen levels would be well above the NC state standard of 5.0 mg/L.
Chemical Parameters
Nitrogen and phosphorus concentrations were very high
at many of the new sampling sites (Table 2.12). The highest mean ammonia concentration was
measured at BC117 (485 mg/L), which is located just downstream of a wastewater treatment
plant in Burgaw. The ammonia level at Panther Creek averaged 398 ug/L. Nitrate +
nitrite concentrations were also greatest at these two stations, averaging 1763 ug/L
at PC and 1408 ug/L at BC117. Panther Creek is adjacent to a cow pasture and
likely receives heavy loading from this farm. At the sampling station BCRR, located
upstream from the wastewater treatment plant in Burgaw, ammonium (85 ug/L) and
nitrate + nitrite (320 ug/L) concentrations were considerably lower than at BC117
downstream from the facility.
Total nitrogen samples were not processed during May 1996 for the
following sites: GS, PC, SAR, N41, LRC, ROC, BCRR, BC117, and ANC. Mean TN values listed
in Table 2.12 may be somewhat different because of this data
gap. However, the data does indicate that PC receives the greatest amount of total
nitrogen (3463 ug/L), mostly in the inorganic form. LRC, located downstream from
Guilford Mills, also receives heavy TN loading averaging 2697 ug/L. Stations
located in pristine blackwater areas, especially ANC and COL, receive most of their
nitrogen in the form of organic substances draining the swampy headlands.
Orthophosphate concentrations were substantially higher at LRC and at
BC117 than at all other stations, averaging 459 ug/L and 552 ug/L
respectively. As mentioned above, the sources of inorganic phosphorus for these stations
are likely Guilford Mills and Burgaw's wastewater treatment facility. Orthophosphate
levels at all other stations averaged 100 ug/L or less. Most of the total
phosphorus at LRC and BC117 was in the inorganic form, accounting for greater than 70% at
both stations.
Biological Parameters
The data in Table 2.12
indicates that chlorophyll a concentrations have been low at all new stations
thus far. No substantial phytoplankton blooms have been detected at any of the new sites
since sampling began.
The data in Table 2.12 represents the geometric means of the February
1996 - May 1996 fecal coliform data at the new sites. Fecal coliform levels at many of the
new stations were substantially above the state standard of 200 CFU/100 mL on several
occasions. There was a large amount of rainfall during some of the sampling trips which
probably elevated background levels. However, these data show that fecal coliform levels
were elevated at PC (447/100 mL), LRC (307/100 mL), and BC117 (381/100 mL), the same sites
which exhibited the highest nutrient levels. While high bacteria concentrations at PC and
BC117 can be easily traced to a farm and a sewage outfall, Guilford Mills is not
necessarily the source of fecal bacteria as it is the source of nitrogen at LRC. Coliform
counts averaged 220/100 mL at BRN, also above the state standard for safe water. No other
sampling sites exhibited mean coliform levels above 200.
Summary
These preliminary results clearly indicate that some of these new sampling sites received a large amount of turbidity, nutrient, and bacterial loading, especially after heavy rain events. The stations exhibiting the most severe nitrogen loading are PC, LRC, and BC117. LRC and BC117 also receive the largest phosphorus load. Fecal coliform concentrations have been above the state standard at PC, LRC, BCRR, BC117, and BRN. Turbidity, nutrient, and fecal coliform levels were very low at ANC and COL, two relatively unimpacted blackwater sites. In summary, the above data indicates that PC, LRC, and BC117 are the most anthropogenically-impacted of the newest sampling stations.
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