RFI - Attachment F - Engineering Report

    3. EMC Comparison with Local Data
Very little published data exists on pollutants in runoff from carparks worldwide, and in
particular in Australia. The very nature of the carpark and its use can vary both the amount
and type of pollutants present in stormwater and the high impervious area and small
catchment area generally favours a “first flush” effect, especially in regions with high rainfall
intensities and defined wet and dry seasons. Further, maintenance/cleaning procedures e.g.
mechanical sweeping, can artificially alter the pollutant species and their concentrations, in
runoff from carparks.

Morison P (2001) reported on a study that categorised pollutant loads from a commercial
carpark in western Sydney using a rainfall simulator calibrated for a 1 in six month storm of 15
minutes duration. The results of the study demonstrate a marked “first flush” effect for 10
minutes with an approximate Event Mean Concentrations (EMCs) for a duration of 15
minutes of:
     Suspended Solids 95 mg/L;
     Total Phosphorus 0.15 mg/L;
     Total Nitrogen 1.85 mg/L.

The EMCs, determined by this study, approximate Typical Wet Weather Concentrations for
Roads provided in Tables 2.43, 2.44 and 2.45, by Fletcher T et al (2004), which are generally
accepted as typical EMCs for various land uses in Australia:
    Suspended Solids range 90 to 800 with a typical value of 270 mg/L;
    Total Phosphorus range 0.15 to 1.5 with a typical value of 0.5 mg/L;
    Total Nitrogen range 1 to 5 with a typical value of 2.2 mg/L.

The EMCs reported in the StormFilter® Product Evaluation carried out over twenty (20)
months within the Mitchell Community College carpark are:
    Suspended Solids range 10.3 to 98.2 with a mean value of 34.6 mg/L;
    Total Phosphorus range 0.07 to 0.90 with a mean value of 0.22 mg/L;
    Total Nitrogen range 0.35 to 2.95 with a mean value of 1.00 mg/L.

          Comparison of Event Mean Concentration (mg/L) for Various Locations
Variable                       St Martins Carpark    CRCCH TR04/8          Mitchell Community
                                      NSW              (Australia)             College NC
Suspended Solids                   95 (approx)           90 to 800             10.3 to 98.2
                                                       typical 270              mean 34.6
Total; Phosphorus                  0.15 (approx)       0.15 to 1.5             0.07 to 0.90
                                                        typical 0.5             mean 0.22
Total Nitrogen                     1.85 (approx)          1 to 5               0.35 to 2.95
                                                        typical 2.2             mean 1.00

The above comparison identifies significantly lower EMCs measured in runoff from the
Mitchell Community College, Mooresville NC than are generally accepted as typical for car
parks in NSW and Australia. With respect to the determination of an Efficiency Ratio (ER) for
a stormwater treatment system, the higher the EMC in the influent the easier it is to achieve a
particular removal efficiency. Consequently the ER determined for the Mitchell Community
College system could be expected to be much higher if the influent EMCs approached those
reported in the Australian literature.


HES – StormFilter® System Peer Review                                                        Page 3