NC0026441_Correspondence_19910820

NPDES DOCYNENT SCANNINS COVER SHEET
NPDES Permit:
NCO026441
Siler City WWTP
Document Type:
Permit Issuance
Wasteload Allocation
Authorization to Construct (AtC)
Permit Modification
Engineering Alternatives Analysis
201 Facilities Plan
Instream Assessment (SOC)
Co�p�arre: algal bloom
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Document Date:
August 20, 1991
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ENVIRONMENTAL SCIENCES BRANCH
BIOLOGICAL ASSESSMENT
Tuesday, August 20,.1991
TO: Tim Donnelly, RRO
FROM: Dianne Reid
THROUGH: Trish MacPherson-F--
RE: Algal Bloom Investigation on Rocky River July 17, 1991
Per your request I met Ms. Connie Allred at the Rocky River to investigate an algal
bloom she reported to your office the week of July 15th. On July 17th, there was no
visible bloom occurring in the River except in some of the pools out of the flow. I sampled
at the NC 902 bridge (R3) and upstream half way between the NC 902 bridge and the SR
2170 bridge (R2B) with Ms. Allred's assistance. I have attached a map noting the
sampling locations and previous sampling locations. Ms. Allred noted that it was much
greener earlier in the week. She had been all the way upstream at the confluence of Love's
Creek and the Rocky River on Monday (July 15). She remembered a strong smell of .
chlorine at the confluence and said that the river had been greener from below the
confluence to the 902 bridge.
Results of the field and laboratory analysis indicate that there was an algal bloom
underway at R2B. The chlorophyll -a concentration at this site was 80 µg/l, which is twice
the state standard. Phytoplankton density and biovolume at this station were 22,651
units/ml and 19,182 mm3/m3, respectively. Both of these estimates are over the accepted
algal bloom levels of 10,000 units/ml density and 5,000 mm3/m3 biovolume. Biovolume
and density at R3 were just at bloom levels. Chrvsochromulina breviturrita, a
prymnesiophyte (golden algae), was among the dominant species at both sites. This genera
has formed blooms in both fresh and salt waters. Samples from both sites had a wide
variety of species with chlorophytes (green algae), diatoms, and prymnesiophytes
dominating density and biovolume at both sites.
Physical and chemical data at both sites are presented below. Dissolved oxygen and
pH were elevated as would be expected during the day in the presence of an algal bloom.
Nutrients at both stations were elevated with a total nitrogen (TN) of 2 mg/l at the 902
bridge (R3) and 0.95 mg/1 at R2B. At present DEM uses an in stream target level of 1
mg/l. For station R2B to have approximately that level of TN in the presence of excessive
phytoplankton populations indicates that there is excessive nutrient loading occurring above
this area.
Results.
STATION TIME mg/1 7c pH µohms mg/1 mg/1 mg/1 µg/l units/ml mm'/m3
R2B 1413 8.76 26.6 8.6 153 0.08 0.95 0.41 80 22,651 19,182
R3 1328 9.9 26.8 8.5 209 0.10 2.0 0.65 25 12,927 4,819
Due to time constraints I was unable to sample further upstream; however, I did do
visual reconnaissance at several bridges above RUB. There were no surface mats evident
and the waters did 'hot appear to be excessively green. Sedimentation was heavy through
the stretch of the Rocky River I sampled indicating possible nonpoint problems upstream.
The weather prior to Ms. Allred's report had been very dry increasing retention time
within the river and allowing phytoplankton populations to reach bloom proportions. Once
a minimum release is set for the upstream dam the increased flow during dry weather

should assist in deterring algal blooms in the Rocky. However, according to work done by
DEM modelers (Rocky River Nutrient Budget (Chatham County), DEM 1991), nonpoint
sources in the upper watershed are the major contributor of nutrients to the system. Any
good control of algal blooms will require control of those nonpoint sources.
Please call if you have questions regarding this information.
cc: Karen Lynch
Connie Allred
Juan Mangles

STUDY: ROCKY:RIUER
RIUER BASIN: CAPE FEAR
COUNTY: CHRTHRM
ALGAL BLOOM INVESTIGATION 910717
ROCKY
RIVER
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