The Burnt Mill Creek watershed was sampled just upstream of Ann McCrary
Pond on Randall Parkway (AP1), about 40 m downstream of the pond outfall (AP3),
and in the creek from the bridge at Princess Place (BMC-PP - Fig.
4.1).
Ann McCrary Pond is a large (28.8 acres) regional wet detention pond
draining 1,785 acres, with an apartment complex at the upper end near AP1.
The pond itself usually maintains a thick growth of submersed aquatic
vegetation, particularly Hydrilla
verticillata, Egeria densa, Alternanthera philoxeroides, Ceratophyllum demersum and Valliseneria americana. A
survey in late summer 1998 indicated that approximately 70% of the pond area was
vegetated. There have been efforts
to control this growth, including addition of triploid grass carp as grazers.
Our survey also found that this pond is host to Lilaeopsis
carolinensis, which is a threatened plant species in North Carolina.
The ability of this detention pond to reduce suspended sediments and
fecal coliform bacteria, and its failure to reduce nutrient concentrations, was
detailed in a scientific journal article (Mallin et al. 2002b).
Turbidity and suspended solids concentrations were low to moderate.
Fecal coliform concentrations entering Ann McCrary Pond at AP1 were very
high, however (Table 4.1), possibly a result of pet waste runoff from the
apartment complex and runoff from urban upstream areas.
Ten out of 11 samples at AP1 had counts exceeding 200 CFU/100 mL.
There was an October 2002 algal bloom at AP1 with a chlorophyll a concentration of 54 mg/L
and an August 2002 bloom at AP3 of 65 mg/L. No blooms exceeded the State standard for chlorophyll a
at Princess Place.
The efficacy of the pond as a pollutant removal device was not good last year. Average fecal coliforms were reduced during passage through the pond, but this difference was not statistically significant (Table 4.1). Total suspended solids and turbidity were low entering the pond this year and there was no significant difference in removal of these two parameters. None of the nitrogen or phosphorus species were significantly reduced during passage through the pond this year (Table 4.1). As in previous years, it is likely that inputs of nutrients have entered the pond from a suburban drainage stream midway down the pond across from our former AP2 site (Fig. 4.1). Also, intensive waterfowl use of the pond, particularly at a tributary near the outfall, may have contributed to nutrient loading in the pond and along its shoreline. There was no significant decrease in conductivity through the pond. Dissolved oxygen significantly increased through the pond, probably because of in-pond photosynthesis and aeration by passage over the final dam at the outfall. There was a significant increase in pH, probably due to utilization of CO2 during photosynthesis in the pond.
As in previous studies, the Princess Place location experienced several water quality problems during the sample period (Appendix B). Dissolved oxygen was substandard on six of 11 sampling trips, for a non-compliance rate of 55%. The most important issue, from a public health perspective, was the excessive fecal coliform counts, which maintained a geometric mean (1162 CFU/100 mL) far in excess of the state standard for human contact waters (200 CFU/100 mL). Fecal coliform counts were greater than 200 CFU/100 mL in 9 of 11 samples, or 82% of the time. It is notable that fecal coliform, suspended solids, phosphorus and nitrogen concentrations all increased considerably along the passage from BMC-AP3 to the Princess Place location, while dissolved oxygen decreased (Table 4.1).
Table 4.1.
Mean and (standard deviation) of water quality parameters in Burnt Mill
Creek,
August 2002 - September 2003. Fecal
coliforms as geometric mean; N/P as median.
_____________________________________________________________________
Parameter
BMC-AP1
BMC-AP3
BMC-PP
_____________________________________________________________________
DO (mg/L)
6.6 (1.7)
9.1 (1.8)**
5.7 (2.3)
Cond. (mS/cm) 226 (54) 219 (38) 303 (55)
pH 6.7 (0.3) 7.3 (0.6)** 7.1 (0.3)
Turbidity (NTU) 9 (6) 9 (8) 7 (4)
TSS (mg/L) 8.9 (7.1) 6.4 (3.2) 7.3 (2.8)
Nitrate (mg/L) 0.104 (0.091) 0.061 (0.046) 0.113 (0.088)
Ammonium (mg/L) 0.094 (0.050) 0.081 (0.042) 0.121 (0.157)
TN (mg/L) 0.875 (0.154) 0.945 (0.307) 0.869 (0.214)
Phosphate (mg/L) 0.013 (0.010) 0.026 (0.041) 0.042 (0.058)
TP (mg/L) 0.082 (0.104) 0.069 (0.066) 0.089 (0.080)
N/P molar ratio 48.4 17.7 21.0
Fec. col. (/100 mL) 1162 285 914
Chlor. a
(mg/L)
12.2 (14.8)
16.3 (18.5)
7.5 (9.4)
_____________________________________________________________________
* Indicates statistically significant difference between AP1 and AP3 at
p<0.05
**Indicates
statistically significant difference between AP1 and AP3 at p<0.01