4.0  Burnt Mill Creek

            The Burnt Mill Creek watershed was sampled just upstream of Ann McCrary Pond on Randall Parkway (BMC-AP1), about 40 m downstream of the pond outfall (BMC-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 BMC-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 entering and leaving the pond were low to moderate.  Fecal coliform concentrations entering Ann McCrary Pond at BMC-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.  All seven samples collected in 2004 at BMC-AP1 had counts exceeding 200 CFU/100 mL; however, only one sample at BMC-AP3 exceeded the standard.  There were minor algal blooms at BMC-AP1 in May and June and in June at BMC-AP3 (with chlorophyll a concentrations between 30 and 40 mg/L). 

The efficiency of the pond as a pollutant removal device was poor last year.  Fecal coliforms were significantly reduced during passage through the pond (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.  Neither ammonium, nitrate, total nitrogen, orthophosphate nor total phosphorus 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 BMC-AP2 site (Fig. 4.1), short circuiting the ability of the pond to remove nutrients.  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 (BMC-PP) experienced several water quality problems during the sample period (Appendix B).  Dissolved oxygen was substandard (between 2.0 and 5.0 mg/L) from May through September 2004.  The most important issue, from a public health perspective, was the excessive fecal coliform counts, which maintained a geometric mean (639 CFU/100 mL) well 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 five of seven months, or 71% of the time.  It is notable that fecal coliform bacteria, ammonium, nitrate, TP and orthophosphate concentrations all increased along the passage from BMC-AP3 to the Princess Place location, while dissolved oxygen decreased (Table 4.1).  No algal blooms exceeded the State standard for chlorophyll a at Princess Place. 

Table 4.1.  Mean and (standard deviation) of water quality parameters in Burnt Mill Creek, January - September 2004.  Fecal coliforms as geometric mean; N/P as median.

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Parameter                              BMC-AP1         BMC-AP3               BMC-PP
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DO (mg/L)                              6.5 (2.2)             9.5 (2.5)**               4.9 (2.3)
Cond. (mS/cm)                       248 (39)            220 (61)                 459 (389)
pH                                           6.8 (0.2)             7.5 (0.2)**               7.0 (0.2)
Turbidity (NTU)                      5 (2)                    8 (8)                        5 (3)
TSS (mg/L)                            4.7 (2.4)             5.7 (3.7)                  5.0 (2.5)
Nitrate (mg/L)                        0.079 (0.060)   0.056 (0.040)        0.115 (0.086)
Ammonium (mg/L)                0.057 (0.035)   0.081 (0.102)        0.096 (0.075)
TN (mg/L)                              0.787 (0.198)   1.164 (0.556)        1.126 (0.420)
OrthopPhosphate (mg/L)     0.012 (0.010)   0.007 (0.006)        0.021 (0.016)
TP (mg/L)                               0.066 (0.042)   0.053 (0.017)        0.080 (0.025)
N/P molar ratio                      21.0                   22.1                        31.0
Fec. col. (/100 mL)               927                    74**                         639
Chlor. a (mg/L)                       13.3 (14.6)        13.7 (13.3)             8.0 (11.9)
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* Indicates statistically significant difference between AP1 and AP3 at p<0.05
**Indicates statistically significant difference between AP1 and AP3 at p<0.01