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the discharge was 1542 mg/L, with a median concentration of approximately 500 mg/L.
The large difference between the two values is the result of a single sample with an
extremely high concentration, so the median is probably more representative of the water
quality normally discharged from these structures.
The efficiency of the geotextile silt fences was based upon a comparison of the
particle loading of the upstream pond and the effluent downstream. This procedure
allowed the determination of the removal efficiency of the silt fence alone and ignored
removal attributed to sedimentation. The TSS removal efficiency was calculated by:
TSS reduction % =
Upstream TSS (mg/L)- Downstream TSS (mg/L)
Upstream TSS (mg/L)
x 100
The median removal efficiency determined in this manner was 0%, with a
standard deviation of ±26%. The range in calculated efficiencies was -61% to 54%. A
negative reduction signifies an observed increase in TSS downstream of the silt fence.
Minor errors for in-situ sampling at construction sites are typical. Other sources of error
which could result in negative removal efficiencies include: disturbance of bottom
sediments during sample collection and commingling of filtered and unfiltered flows
below the silt fence. The TSS removal efficiencies for sample pairs are presented in Table
B3 in Appendix B.
The highest removal rate calculated was 54%. This removal corresponds to
samples collected for a non-woven fabric, inlet perimeter protection silt fence at Site 4.
The higher efficiency may be attributed to shallow depth of the ponded water. The
maximum depth at this location was estimated to be only 15 centimeters. Using Stokes'
Law, and assuming:
kinematic viscosity = 0.01 cm 2 /s,
particulate specific gravity = 2.65, and
median particle size = 9.5 µm (small silt),
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