Potecasi Creek Assessment report_final

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NC Division of Water Quality
Planning Section
Modeling and TMDL Unit

MEMORANDUM

To: Kathy Stecker, Modeling Unit Supervisor
From: Narayan Rajbhandari, Senior Env. Specialist
CC: Alan Clark, Dianne Reid, Jeff Manning, Peter Caldwell, Cam McNutt,
Heather Patt, Sam Whitaker

Date: October 6, 2009
Subject: Assessment of Natural Condition for DO and pH in Potecasi Creek,
Chowan River Basin, NC

Summary
Potecasi Creek impaired segment (AU# 25-4-8), 42.5 miles, exhibits low velocity
and large areas of agricultural and forested lands with swamps and heavy tree canopy.
Decomposition of the large inputs of vegetation from areas of forested land with swamps
and heavy tree canopy throughout the watershed create lower pH and lower DO as they
decay in the creek. There is no discernable anthropogenic impact on the creek. The creek
exhibits low nutrient concentrations near or below national background levels from
undeveloped areas. There is also not a close correlation between precipitation amounts
and field pH at the NC DWQ ambient water quality monitoring station. Based on the
information, the water quality standards for Potecasi Creek and its tributaries have not
been violated. A Total Maximum Daily Load (TMDL) is not required for DO and pH for
the creek.
Introduction
Potecasi Creek was listed as impaired since 1998 on North Carolina’s 303(d)
Report of Impaired Waters due to violations of the State’s water quality standard for DO
and pH. As reported in the list, the impaired segment in the creek due to DO and pH is
located from the source to the Meherrin River (Figure 1). The assessment unit number
for the impaired section of the creek is 25-4-8. The total mileage of impaired section is
42.5 and is designated as Class C NSW. The Division of Water Quality (DWQ) defines
Class C as waters protected for secondary recreation, fishing, wildlife, fish, and aquatic
life propagation and survival, agriculture and other uses suitable for Class C. The
division also defines NSW as supplemental classification intended for waters needing
additional nutrient management due to their being subject to excessive growth of
microscopic or macroscopic vegetation. This report evaluates the DO and pH
impairments by determining if natural conditions are the cause of the apparent
impairments, thus obviating the need for a TMDL.
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Figure 1. Potecasi Creek watershed
General Description of Watershed
Potecasi Creek, located within both Northampton and Hertford counties in North
Carolina, is a tributary to the Meherrin River in the Chowan River Basin. The watershed
has an area of approximately 257.48 square miles. There is a USGS flow gauge station
(USGS 02053200) and an ambient station (D4150000) at NC11 near Union, with a
drainage area of 225 square miles. The USGS gauge station measures flow continuously,
whereas the DWQ collects water samples every month to estimate physical and chemical
constituents. DO, pH, and flow measured at these stations are utilized for this study.
Geology and Soils
Potecasi Creek is in the Coastal Plain Physiographic area in eastern North
Carolina. The Coastal Plain geology consists mostly of marine sedimentary rocks. This
rock is overlain by fluvial (waterborne) deposits. These deposits are primarily sand and
clay from rivers that have been laid down over many thousands of years (Source:
http://www.soil.ncsu.edu/publications/BMPs/physio.html).

Local topographic relief is only 0.8 feet per mile and maximum land surface altitude is
about 50 ft above sea level. This low topographic relief is reflected in low hydraulic
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gradient with less potential to move water to streams. Therefore, the soil characteristics
are predominantly hydric soils in the watershed (Figure 2). These soil types are formed
under conditions of saturation, flooding or ponding long enough during the growing
season to develop anaerobic conditions.
.

Figure 2. Hydric soil distribution in the Potecasi Creek watershed.
Climate
The climate data is acquired from the State Climate Office of North Carolina
(http://www.nc-climate.ncsu.edu) for the study periods, 1998 – 2007. Figure 3 shows
monthly averaged temperature and total rainfall distribution at a climate station
(#315996) located at Murfreesboro, Hertford County. The average annual maximum and
minimum temperature (°F) at the climate station are 69.45 and 47.64, respectively. The
average annual precipitation is 46.74 inches. Amount of precipitation gradually increases
during summer period in the watershed.

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0.00
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Figure 3. Air temperature and rainfall distribution in the Potecasi Creek watershed,
Murfreesboro, NC (Station #315996).

Land Use
The Potecasi Creek watershed is approximately 257.48 square miles in size, and is
predominantly forested (46%) and agricultural (38%) (Figure 4). The forested area
includes 20% deciduous forest, 25% evergreen forest, and 1% mix forest. The
agricultural area includes 26% cultivated crops and 12% pasture/hay lands. Other uses
are comprised of 12% woody wetlands, 2% barren lands, and 2% developed open spaces.
The woody wetlands are largely concentrated along the creek.

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Figure 4: 2001 NLCD of the Potecasi Creek Watershed
Water Quality Standard
According to North Carolina Water Quality Standards for class C waters (15A
NCAC 02B.0211), DO concentration shall not be less than 6.0 mg/l for trout waters; for
non-trout waters, not less than a daily average of 5.0 mg/l with a minimum instantaneous
value of not less than 4.0 mg/l; swamp waters, lake coves or backwaters, and lake bottom
waters may have lower values if caused by natural conditions. For pH, it shall be normal
for the waters in the area, which generally shall range between 6.0 and 9.0 except that
swamp waters may have a pH as low as 4.3 if it is the result of natural conditions.
Statement of Impairment
The entire length of Potecasi Creek (from source to the Meherrin River) is listed
on the 2006 303(d) list of impaired waters for DO and pH. The DWQ recommended that
the creek be included in a Swamp Waters Study Plan to determine if the low DO and pH
were associated with naturally occurring swamp conditions.

In the Chowan River Basinwide Water Quality Plan (NCDENR, September
2007), it is recommended that the upstream portion of Potecasi Creek [AU# 25-4-8a],
from source to Cutawhiskie Creek (21.5 miles), be removed from the 2008 303(d) list of
impaired waters due to a moderate swamp benthic bioclassification. This upper portion
of the creek exhibits natural channel morphology, intact riparian zones with a mature
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forest on either side, and a large percentage of the reach are available for benthic
colonization.

Lower Potecasi Creek [AU# 25-4-8b], from Cutawhiskie Creek to Meherrin River
(21.0 miles), is recommended to remain on the 303(d) list for water quality standards
violations for DO and pH, until it can be further determined that these conditions
represent natural swamp drainage. Water was sampled 59 times over the course of the
five-year assessment period (2002 – 2007) at the ambient site (D4150000). Over 44
percent of the samples were below 5.0 mg/l and over 25 percent were below the 4.0 mg/l
standard for dissolved oxygen. The pH was below the standard of 6.0 in 22 percent of the
samples.

Natural Condition Assessment
In a water body, oxygen is usually restored through aeration and photosynthesis
processes, whereas oxygen is depleted through decomposition and respiration processes.
Oxygen-depletion processes dominate oxygen-restoration processes in slow-moving,
ripple-less waters. In such waters, the decay of organic matter depletes DO at a faster
rate than it can be replenished and produces organic acids, thereby reducing pH level.
Because swamp waters are characterized by very low velocity, it is common to observe
naturally low DO and/or pH level in swamp waters. The situation will be more
compounded when excessive nutrients or readily available organic matters are discharged
to these waters. Therefore, following five steps are selected to identify natural conditions
that result in low DO and/or pH levels and to determine the likelihood of anthropogenic
impacts that will exacerbate the natural condition in Potecasi Creek: observation of low
velocity, impact from point sources, impact from nonpoint sources, impact from seasonal
fluctuation, and field observation.

Observation of Low Flow Velocity
Local topographic relief is only 0.8 feet per mile and maximum land surface
altitude is about 50 ft above sea level. Based on the low slope topography, flow velocity
in the creek is expected to be low. The low flow 7Q10 values ranges from 0 to 0.019
ft3/s/mi2, and the median value is 0 (Giese and Manson, 1993). The flow value is
expressed by drainage area.

So as to understand the flow velocity in the creek, the daily discharge rates and
flow depths measured at the USGS gauge station 02053200 from 1998 through 2007 are
obtained from USGS database (Source: http://waterdata.usgs.gov/nc/nwis/rt). The
discharge rate ranges from 0.06 ft3/ sec to 15200 ft3/ sec, and the median discharge is 69
ft3/ sec. Since the USGS does not measure velocity at the site, Equation 1 is used to
estimate flow velocity.

Velocity (ft/sec) = Discharge (ft3/ sec)/ Cross Section (ft2) ----------------------- (1)

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Since cross section varies with flow depth, a statistical relationship between cross
section and flow depth is estimated using bathymetry data measured at the site (Figure 5).
The relationship is quadratic and is given below.

Figure 5. Bathymetry data at USGS 02053200 in Potecasi Creek. (Source: Personal
contact with Dr. Loren Wehmeyer, USGS NC Water Science Center 2.)

Cross Section = 5.0186* Depth * Depth + 50.302 * Depth ------ (2)
(R2= 0.99)

Solving the two equations 1and 2 flow velocities are estimated, which ranges
from 0 to 2.78 ft/ sec, and median value is 0.22 ft/ sec. The monthly percentile
distribution of the flow velocity from 1998 through 2007 is given in Table 1. It appears
that flow velocity in the creek is substantially slow throughout the year. It could be
because the watershed location comprises the near upstream and downstream boundaries
of the low slope segment. Slow velocity enhances decomposition of vegetation inputs
from forested lands with swamps and heavy canopy throughout the watershed, thereby
increasing oxygen demand and lowering DO as they decay. Furthermore, the
decomposition of vegetation produces organic acid, thereby reducing pH in the creek.

Table1. Monthly percentile distribution of estimated flow velocity (ft/s) at the ambient
station (D4150000) at NC11 near Union (1998-2007)
Months Minimum 10% 25% Median 75% 90% Maximum
Jan 0.03 0.19 0.25 0.37 0.53 0.73 1.10
Feb 0.11 0.19 0.30 0.44 0.60 0.81 1.31
Mar 0.12 0.18 0.30 0.45 0.60 0.75 1.20
Apr 0.04 0.12 0.20 0.35 0.52 0.72 1.25
May 0.01 0.05 0.07 0.14 0.29 0.50 1.02
Jun 0.01 0.02 0.04 0.10 0.26 0.68 1.41
Jul 0.01 0.02 0.03 0.06 0.25 0.50 0.93
Aug 0.00 0.01 0.02 0.04 0.14 0.51 1.35
Sept 0.00 0.01 0.01 0.04 0.38 0.85 2.78
Oct 0.00 0.01 0.02 0.05 0.20 0.41 1.46
Nov 0.01 0.02 0.05 0.20 0.35 0.54 1.08
Dec 0.01 0.031 0.21 0.29 0.50 0.63 1.14
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Impact from Point Sources
There are no point sources in Potecasi Creek watershed.
Impact from Nonpoint Sources
Land Cover
Residential and other developed areas are not significant contributors of the land
base composition, an insignificant portion of the watershed. The watershed is
predominantly forest (46 %), agricultural (38 %), and wetland (12 %). Forested lands and
wetlands are considered background sources. These lands are not indicative of human
impact. However, agriculture lands are indicative of human source; therefore, they are
considered nonpoint sources.

Excessive nutrient input can stimulate plant growth, and the resulting die-off and
decay of excessive plankton or macrophytes can decrease DO levels in the creek where
flow is relatively slow and aeration is low. So as to understand the nutrient levels in
Potecasi Creek, the ambient monitoring data collected at D4150000 from 1998 through
2007 are obtained from the EPA’s Storet database (Source: http://www.epa.gov/storet/).
The non-detected values are replaced with half of the practical quantitation limit values
specified by the DWQ (Source: http://h2o.enr.state.nc.us/lab/qa/pqlinorg.htm). Monthly
averaged nutrient concentrations are given in Table 2.

Table 2. Averaged instream nutrient concentration (mg/L) in Potecasi Creek at the
ambient station (D4150000) at NC11 near Union (1998-2007)
Months TKN NOx TN TP
(mg/L) (mg/L) (mg/L) (mg/L)
Jan 0.64 0.08 0.70 0.09
Feb 0.51 0.13 0.62 0.09
Mar 0.73 0.08 0.79 0.09
Apr 0.75 0.09 0.83 0.12
May 0.80 0.26 1.06 0.14
Jun 0.79 0.23 0.99 0.19
Jul 0.67 0.19 0.81 0.13
Aug 0.65 0.14 0.79 0.15
Sept 0.63 0.10 0.71 0.14
Oct 0.60 0.08 0.65 0.13
Nov 0.62 0.05 0.60 0.10
Dec 0.68 0.10 0.65 0.11
Average 0.67 0.13 0.77 0.12

On average TN concentrations in the creek remained lower than or near to 1.0
mg/L. Except in June, TP concentrations remained below or near to 0.1 mg/L. These
averaged nutrient concentrations are near or below the USGS national background
averages (Source: http://pubs.usgs.gov/circ/circ1225/index.html). These averages are
developed from nutrient concentrations in streams from undeveloped areas with typical
concentrations of TN ≤ 1.0 mg/L and TP ≤ 0.1 mg/L. Overall, TN varies from 0.10 mg/L
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to 2.6 mg/L and TP varies from <0.01 mg/L to 0.2 mg/L in undeveloped stream basins in
the USA (Clark et. al., August 2000). Therefore, the low nutrient concentration levels in
Table 2 indicate that Potecasi Creek is not significantly affected by anthropogenic inputs
of nitrogen and phosphorus.

Acid Deposition
Acid rains are also indicative of human sources, because the principal causes of
acid rain are sulfur, carbon, and nitrogen compounds from human sources, such as
electricity generation, factories, and motor vehicles. Coal power plants are one of the
most significant contributors. Gases from these plants can travel hundreds of kilometers
in the atmosphere, thereby reacting with water molecules in the atmosphere to produce
acids. During precipitation the acids are deposited, causing ecological damage. So as to
understand its impact in Potecasi Creek, the rainfall pH data are obtained from the nearby
National Atmospheric Deposition Program/NTN station in Bertie County, NC (Station
Lewiston NC03) (Source: http://nadp.sws.uiuc.edu). Acid deposition occurred in the
Bertie dataset, with weekly rainfall pH during the period from 1998-2007 averaging 4.72
SU with a minimum of 3.93 SU and maximum of 6.40 SU. According to an EPA website,
(Source: http://www.epa.gov/airmarkets/acidrain/index.html) the natural pH of rain is
approximately 5.5 SU.

One method to assess whether acid deposition adversely impacts low pH in the
creek is to observe close relationship between rain deposition and the DWQ ambient
water quality monitoring field pH data. The relationship was estimated to be weak (r =
0.37), suggesting that the extent to which stream pH is decreased by acid deposition in
the creek could not be decisively established. Therefore, impact from acid rain is
uncertain.

Impact from Seasonal Fluctuation
Figure 6 shows a relationship among averaged monthly DO concentration, water
temperature, and flow velocity in Potecasi Creek. As temperature increases DO
decreases along with flow velocity. This is indeed a natural phenomenon where flow
velocity is lower, because still water is affected by air temperature more rapidly and then
facilitates decomposition of plant materials. This process increases oxygen demand and
lowers DO as plants decay. Therefore, summer periods seem to be critical for low DO in
the creek.
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Dissolved Oxygen Water Temperature Flow
Figure 6. Monthly distribution of median DO, water temperature, and velocity in
Potecasi Creek at the ambient station (D4150000) at NC11 near Union (1998-2007)

Field Observation
On May 27, 2009, DWQ staff visited Potecasi Creek to measure DO, water
temperature, pH, and conductivity as well as to collect water samples upstream at
SR1504, midstream at SR35, and downstream at NC11. The staff also collected water
samples from an unnamed tributary in the upper watershed at SR1500 and from Miller
Branch in the lower watershed at SR1175. These samples were collected to estimate DO
consuming chemical parameters – five-day biochemical oxygen demand (BOD5) and
total organic carbon (TOC). TOC was further broken down to labile organic carbon
(LOC) and refractory organic carbon (ROC) by using equations 3 and 4. These equations
were derived by Hendrickson et al., 2007, considering that LOC and ROC decompose
simultaneously, albeit at different rates. Their first-order decay rates were 0.075 day -1
and 0.001 day -1, respectively. The derivation of the equations is well documented in
Hendrickson et al., 2007.

LOC (mg/L) = BOD5*74.906 – TOC)/61.54 --------------------- (3)
ROC (mg/L) = TOC-LOC --------------------------------------------- (4)

Equations 3 and 4 represent the St. Johns River, which is one of the largest blackwater
rivers of the southeast U.S., draining a 24,765 km2 area in Atlantic coastal plain river
estuary in northeast Florida. The river is slow moving and receives nutrients from
adjoining swamp water. Because of its swampy characteristics, it could be assumed that
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any information drawn from the river would be applicable to Potecasi Creek as well,
although there are some differences in physiological characteristics of the two water
body.
Hendrickson et al found lowest level of LOC in urban and forested watershed runoff and
highest level of ROC in dairy, row crop, and forested watershed runoff. As per their
findings, Potecasi Creek, where forested lands with swamps and heavy tree canopy
dominate the landscape, would have relatively more refractory organic carbon.

Table 3 reveals that DO concentration was very low upstream in the creek. Its
concentration was only 1.6 mg/L. The concentration gradually improves as it goes down
stream. The concentration increased to 2.4 mg/L at the midstream (SR35) and 3.4 mg/L
at the downstream (NC11). However, the observed concentrations remained below the
standard concentration, 4 mg/L, throughout the creek segment.

Table 3. Water sampling results for Potecasi Creek on May 27, 2009
Sample Site Date
yy/mm/dd
Temp
˚C
DO
(mg/L)
pH Cond. BOD 5day
(mg/L)
TOC
(mg/L)
LOC
(mg/L
ROC
(mg/L)
Potecasi @ SR1504 09/05/27 20.5 1.6 6.1 73 1.76* 24 1.75 22.25
UT @ SR1500 09/05/27 21.9 1.1 6.1 69
Potecasi @ SR35 09/05/27 21 2.4 6.2 89 1.77* 29 1.68 27.3
Potecasi @ NC11 09/05/27 22.9 3.4 6.3 113 1.08* 16 1.05 14.95
Mill Branch @ SR1175 09/05/27 22.5 3.5 6.1 73 2.10 19 2.25 16.75
* Concentration is below the DWQ’s Practical Quantitation Limits (2 mg/L). It is a
provisional data

Because ROC concentration was substantially higher than LOC concentration throughout
the Potecasi Creek (Table 3), it appears that the low DO concentration was due to the
presence of forested lands with swamps and heavy tree canopy. Furthermore, the low
DO measurements at the two tributaries, UT (SR1500) and Mill Branch (SR1175)
suggest that the watershed is comprised of swamp areas. Visual inspection from
locations in the watershed also showed large swamp areas with heavy tree canopy
(Figures 7 to 10). There are large inputs of decaying vegetation from areas of forested
land with heavy tree canopy throughout the watershed that increase oxygen demand and
lower DO as they decay.

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Figure 7. Potecasi Creek at SR 1504, May 27, 2009

Figure 8. Wetland feeding upper Potecasi Creek at SR1504, May 27, 2009

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Figure 9. Measuring flow velocity at Potecasi Creek at SR35, May 27, 2009.
The DWQ staff from left Christopher S. Whitaker and Peter Caldwell.

Figure 10. Potecasi Creek at NC11, May 27, 2009

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The DWQ staff also measured flow velocity in the mid stream of Potecasi at SR35 using
Marsh McBirney Magnetic Flow Meter (Model #201). The velocity was below its
detection limit.

As per the USGS gauge measurement at NC11 near Union, down stream from SR35,
average flow was 9.1 cfs and average water depth was 2.71 ft in Potecasi Creek on May
27, 2009. Based on the measurements, velocity was estimated at 0.05 ft/s using equations
3 and 4, which falls on the 10th percentile for May, as reported in Table 1.
Conclusion
Potecasi Creek impaired segment (AU# 25-4-8), 42.5 miles, shows evidence of
low velocity and low slope. The watershed comprises predominantly hydric soil and
large areas of agricultural and forested lands with swamps and heavy tree canopy.
Decomposition of the large inputs of vegetation from areas of forested land with swamps
and heavy tree canopy throughout the watershed not only produce organic acids and
lower pH but also increase oxygen demand and lower DO as they decay in the creek.
Decomposition of vegetation seems more critical during summer period when water
temperature reaches 25 degrees C (77 degrees F). These are not considered
anthropogenic impacts.

Potecasi Creek exhibits low nutrient concentrations near or below national
background levels from undeveloped areas, which is not indicative of human impact.
Further more, ROC concentration was substantially higher than LOC concentration,
suggesting that low DO concentration in the creek was due to the presence of swamps
around the creek.

There is no active permitted NPDES discharger in the Potecasi Creek Watershed.

There is not a close correlation between precipitation amounts and field pH at the
NC DWQ ambient water quality monitoring station. The r-value was well below 0.5,
which does not indicate a close correlation between the variables. However the extent to
which stream pH is decreased by acid deposition cannot be conclusively determined.
Swamp waters may have a pH as low as 4.3 if it is the result of natural conditions.

Based on the above information, the water quality in Potecasi Creek and its
tributaries is due to natural conditions. For the next 305(b)/303(d) assessment, Potecasi
Creek should be assessed as category 2: natural conditions, no TMDL needed.

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References
Clark , Gregory M, David K. Mueller, and M. Alisa Mast. August 2000. Nutrient
Concentrations and Yields in Undeveloped Stream Basins of the United States. Journal
of the American Water Resources Association. 36(4): 849-860.

Giese, G.L. and Robert R. Mason, Jr. 1993. Low-Flow Characteristics of Streams in
North Carolina. United State Geological Survey Water-Supply Paper 2403. USGS Map
Distribution, Box 25286, MS 306, Federal Center, Denver, CO – 80225.

Hendrickson, John, Nadine Trahan, Emily Gordon, and Ying Ouyang. February 2007.
Estimating Relevance of Organic Carbon, Nitrogen, and Phosphorus Loads to a Black
Water River Estuary. Journal of the American Water Resources Association. 43(1): 264-
279.

NCDENR (North Carolina Department of Environment and Natural Resources). May
2007. Surface Waters and Wetlands Standards. NC Administrative Code 15A NCAC
02B.0100, .0200 & 0.300. 1617 Mail Service Center, Raleigh, NC – 27699-1617.

NCDENR (North Carolina Department of Environment and Natural Resources).
September 2007. Chowan River Basin Water quality Plan. 1617 Mail Service Center,
Raleigh, NC – 27699-1617.

USGS (US Geological Survey). 1999. The Quality of Our Nation’s Waters: Nutrients and
Pesticides. U.S. Geological Survey Circular 1225.