6.0
Greenfield Lake
One of the major pollution mitigation features in the Greenfield Lake
watershed is an extensive wet detention pond along the Silver Stream branch (Fig.
6.1). The pond drains
approximately 280.5 acres, of which about 43% is impervious surface area.
The pond is divided into a 1.25 acre upper and a 1.48 acre lower basin by
a causeway pierced by three pipes connecting the flow.
In early summer 1998 approximately 70% of the upper pond was covered by a
mixture of floating and emergent aquatic macrophyte vegetation, with about 40%
of the lower pond covered by vegetation. Principal
species were alligatorweed Alternanthera
philoxeroides, pennywort Hydrocotyle
umbellate, water primrose Ludwigia
leptocarpa and cattail Typha
latifolia. This pond’s
performance as a nutrient removal system was mixed last year (Table
6.1). Statistically significant removal of orthophosphate (82%) and
TP (71%) was achieved, while reduction of ammonium was not statistically
significant (10%), nor was reduction of TN (28%). Average reduction of nitrate was high (85%), but the large
variability among concentrations led this to be statistically non-significant
(although not biologically non-significant).
Turbidity and TSS were generally low at both locations this past year
(Table 6.1). Fecal coliform
concentrations at SS2 were not overly high, and there was no significant
reduction through the pond. The
lack of significant nitrogen removal may have resulted from construction
activities near the lower end of the pond.
Dissolved oxygen significantly increased, because of aeration while
passing through the outfall and increased oxygenation through pond
photosynthesis.
Table 6.1.
Comparison of pollutant concentrations in input (SS1) and output (SS2)
waters
of regional wet detention pond on Silver Stream, in Greenfield Lake watershed,
August 2002 –
September 2003. As mean (standard deviation); geometric mean for fecal coliform
bacteria.
_____________________________________________________________________
Parameter
SS1
SS2
_____________________________________________________________________
DO (mg/L) 4.8 (1.5) 7.3 (2.1)**
Cond. (mS/cm) 331 (121) 163 (45)**
pH 6.8 (0.2) 7.0 (0.2)*
Turbidity (NTU) 3 (6) 6 (5)
TSS (mg/L) 7.1 (12.1) 6.1 (6.1)
Nitrate (mg/L) 0.450 (0.690) 0.069 (0.050)
Ammonium (mg/L) 0.232 (0.131) 0.209 (0.169)
TN (mg/L) 1.472 (0.733) 1.073 (0.397)
Phosphate (mg/L) 0.110 (0.117) 0.020 (0.014)*
TP (mg/L) 0.223 (0.154) 0.064 (0.045)**
Chlorophyll a (mg/L) 1.5 (1.5) 17.0 (27.5)
Fecal col. (CFU/100 mL)
285
124
_____________________________________________________________________
* indicates significant difference between input and output concentration at
p<0.05
**Indicates significant difference between input and output concentration at
p<0.01
Three tributaries of Greenfield Lake were sampled for physical, chemical,
and biological parameters (Table 6.2, Fig.
6.1).
All three tributaries suffered from extreme hypoxia, with GL-JRB (Jumping
Run Branch), GL-LB (creek at Lake Branch Drive) and GL-LC (creek beside
Lakeshore Commons) all showing average concentrations below the state standard
(DO < 5.0 mg/L). Dissolved
oxygen levels were substandard eight of 11 times at GL-JRB, 10 of 11 times at
GL-LC, and 10 of 11 times at GL-LB (Appendix B). Turbidity and suspended solids were generally low in the
tributary stations (Table 6.2). Nitrate
concentrations were highest at GL-LC, somewhat lower at GL-LB, and lowest at GL-JRB
(Table 6.2). Ammonium
concentrations were highest at GL-LB, and generally similar across the other two
tributary stations. Orthophosphate
concentrations were highest at GL-LC, and similar at GL-LB and GL-JRB, and
showed a general decrease from the previous year. Overall, GL-LC maintained the highest nutrient concentrations
of any of the input streams tested. All
three of these input streams maintained fecal coliform levels indicative of poor
water quality, with fecal coliform counts exceeding the state standard for human
contact waters (200 CFU/100 mL) eight of 11 times at GL-LB, 9 of 11 times at
GL-LC, and seven of 11 times at GL-JRB. This
represents a worsening over 2001-2002, possibly resulting from the breaking of
the drought conditions that had subsequently reduced non-point source pollution.
Chlorophyll a levels were
generally non-problematic in these streams (Table 6.2).
Table 6.2.
Mean and (standard deviation) of water quality parameters in tributary
stations
of Greenfield Lake, August 2002 - September 2003.
Fecal coliforms as geometric mean;
N/P ratio as median.
_____________________________________________________________________
Parameter
GL-JRB GL-LB
GL-LC
_____________________________________________________________________
DO (mg/L) 3.8 (2.4) 2.4 (2.0) 2.7 (1.7)
Turbidity (NTU) 6 (5) 5 (2) 4 (4)
TSS (mg/L) 4.4 (3.1) 3.7 (1.3) 13.6 (22.7)
Nitrate (mg/L) 0.110 (0.095) 0.179 (0.215) 0.341 (0.321)
Ammonium (mg/L) 0.155 (0.099) 0.222 (0.123) 0.163 (0.090)
TN (mg/L) 1.076 (0.439) 1.204 (0.725) 1.337 (0.685)
Phosphate (mg/L) 0.027 (0.016) 0.029 (0.019) 0.066 (0.054)
TP (mg/L) 0.060 (0.019) 0.084 (0.081) 0.127 (0.080)
N/P molar ratio 17.0 35.4 15.8
Fec. col. (/100 mL) 357 287 835
Chlor. a
(mg/L)
7.4 (5.4)
4.0 (6.8)
5.9 (7.9)
_____________________________________________________________________
Three in-lake stations were sampled (Table 6.3).
Station GL-2340 represents an area receiving a considerable influx of
urban/suburban runoff, GL-YD is downstream and receives some outside impacts,
and GL-P is at Greenfield Lake Park, away from inflowing streams but in a
high-use waterfowl area (Fig. 6.1). Low
dissolved oxygen affected GL-2340, GL-YD, and GL-P, with 45%, 50%, and 45% of
the samples below the state standard, respectively (Appendix B).
Turbidity and suspended solids were low to moderate at the three sites.
Fecal coliform concentrations were problematic at all three stations.
At GL-2340 the state standard was exceeded on four of 11 occasions, at
GL-YD it was exceeded on five of 10 occasions, and at GL-P it was exceeded on
seven of 11 occasions in 2002-2003.
Nitrate
concentrations were highest at GL-2340, reflecting the proximity of three
tributary streams. Nitrate levels
decreased considerably toward the park (Table 6.3).
Total nitrogen, ammonium, and total phosphorus were highest at GL-YD
(Table 6.3). Inorganic N/P molar
ratios can be computed from ammonium, nitrate, and orthophosphate data and can
help determine what the potential limiting nutrient can be in a water body. Ratios well below 16 (the Redfield ratio) can indicate
potential nitrogen limitation, and ratios well above 16 can indicate potential
phosphorus limitation (Hecky and Kilham 1988).
Based on the median N/P ratios (Table 6.3), phytoplankton growth in
Greenfield Lake at GL-YD and GL-P should be primarily nitrogen-limited.
Our previous bioassay work indicated that this was indeed the case (Mallin
et al. 1999). However, N/P ratios
were high at GL-2340 in 2002-2003, possibly a result of less phosphorus inputs;
thus, phytoplankton growth at GL-2340 should have been mainly P-limited last
year.
Phytoplankton blooms are periodically problematic in Greenfield Lake, and usually consist of green or blue-green algal species, or both together. These blooms have occurred during all seasons, but are primarily a problem in spring and summer. However, algal blooms exceeding the state standard of 40 mg/L were not recorded in our sampling during 2002-2003. Thus, during 2002-2003 Greenfield Lake proper was impaired by high fecal coliform counts and low dissolved oxygen concentrations; its tributary stations were also impaired by high fecal coliform counts and low dissolved oxygen. The lake in general and its tributaries had lower P inputs and less algal blooms as compared with 2001-2002, but worse dissolved oxygen concentrations. Fecal coliforms problems increased somewhat compared with the previous year.
Table 6.3.
Mean and (standard deviation) of water quality parameters in Greenfield
Lake
sampling stations, August 2002 - September 2003.
Fecal coliforms given as geometric mean,
N/P ratio as median.
_____________________________________________________________________
Parameter
GL-2340 GL-YD
GL-P
_____________________________________________________________________
DO (mg/L) 5.0 (1.6) 5.6 (3.5) 6.1 (4.4)
Turbidity (NTU) 2 (1) 4 (4) 3 (3)
TSS (mg/L) 4.0 (3.4) 14.2 (21.7) 4.9 (5.0)
Nitrate (mg/L) 0.098 (0.086) 0.054 (0.054) 0.041 (0.024)
Ammonium (mg/L) 0.119 (0.093) 0.271 (0.498) 0.087 (0.062)
TN (mg/L) 0.926 (0.278) 2.086 (2.730) 1.268 (0.723)
Phosphate (mg/L) 0.013 (0.008) 0.028 (0.023) 0.033 (0.038)
TP (mg/L) 0.065 (0.081) 0.118 (0.139) 0.085 (0.070)
N/P molar ratio 29.5 14.4 10.7
Fec. col. (/100 mL) 147 182 429
Chlor. a
(mg/L)
7.2 (10.1)
15.1 (8.5)
11.6 (6.8)
____________________________________________________________________