RFI - Attachment F - Engineering Report

                                                                                                                                             3
                                                                                                                 Technical Papers


measurement equipment were secured                efficiency as the average event mean           Results and Discussion
to the invert of the influent and effluent        concentration of pollutants over some          Monitoring 13 storm events over
pipes using stainless steel spring rings.         time period.                                   a 20-month period resulted in the
   Following a precipitation event,                                                              collection of cumulative rainfall
composite samples were submitted                                                                 representing 704mm. Comparison of
for analysis according to accepted,                 The ER method assumes: 1) The weight         the measured rainfall intensities is not
relevant EPA, ASTM (Suspended                     of all storm events is equal, regardless of    wholly representative of that found in
Sediment Concentration) and SM254D                the relative magnitude of the storm event;     Australia due to varying climatic regions.
(TSS) methods. The field monitoring               and 2) that if all storm events at the site    The presence, however, of upstream
methods used for this study represent             had been monitored, the average inlet          storage attenuates flows from all storms
the current state-of-the-art practice and         and outlet EMCs would be similar to those      such that differences in rainfall intensities
are similar to those used by researchers          that were monitored (URS/ EPA, 1999). ER       are not a significant factor on treatment
in North Carolina to evaluate vegetated           results for each parameter in the 13 events    performance and pollutant wash-off. Full
stormwater treatment systems.                     sampled are summarised in Table 1.             treatment flow through each RCFS is not
                                                                                                 achieved until the water level inside the
  To obtain a better understanding of                Performance was also calculated using
                                                                                                 chamber reaches a height of 460mm. At
RCFS performance with respect to solids,          the Summation of Loads (SOL) efficiency
                                                                                                 this water elevation a siphon is activated
the portion of SSC (Suspended Sediment            calculation method. The SOL method
                                                                                                 and the cartridge throughput of 0.5L/s
Concentration) particles smaller than             defines the efficiency as a percentage
                                                                                                 per cartridge is achieved, regardless of
500µm and 2000µm was also determined.             based on the ratio of the summation of
                                                                                                 the rainfall intensity.
                                                  all influent loads to the summation of all
  For each of the 13 qualifying storm             effluent loads.                                   Non-parametric statistical methods
events sampled between November                                                                  were used to evaluate correlations and
of 2010 and June of 2012:                                                                        differences between non-transformed
1. The total rainfall was greater than
                                                                                                 influent and effluent event mean
  2.5mm for each event sampled;                                                                  concentrations (EMCs), since influent
                                                    The SOL method assumes: 1)
                                                                                                 and effluent EMCs were generally not
2. The minimum inter-event period was
                                                  Monitoring data accurately represents
                                                                                                 from the same statistical distribution
  greater than six hours for all storm            the actual entire total loads in and out
                                                                                                 due the complex nature and variability
  events sampled;                                 of the BMP for a period long enough
                                                                                                 of stormwater monitoring. To test for
                                                  to overshadow any temporary storage
3. The minimum number of influent and
                                                                                                 positive correlations between influent
                                                  or export of pollutants; and 2) Any
                                                                                                 and effluent EMCs, the Spearman Rank
  effluent aliquots collected per storm           significant storm events that were not
                                                                                                 Order Correlation test was used (USGS,
  event was five;                                 monitored had a ratio of inlet to effluent
                                                                                                 1991). To evaluate the significance of
                                                  loads similar to the storms events that
4. Seven influent flow-weighted                                                                  differences between influent and effluent
                                                  were monitored (URS/EPA, 1999). In an
  composite samples covered ≥ 50% of                                                             EMCs, the Mann-Whitney Rank Sum Test
                                                  effort to eliminate the introduction of
  the total storm flow, while six covered                                                        was used (USGS, 1991).
                                                  potential bias associated with observed
  between 34% and 49%; effluent flow-
                                                  discrepancies between influent and               For the Mann-Whitney Rank Sum
  weighted composite samples covered
                                                  effluent measured volumes, it was              Test the null hypothesis was that the
  ≥ 50% of the total storm flow for all
                                                  assumed that the influent volume was           two samples were not drawn from
  storm events sampled.
                                                  equal to the effluent volume. Measured         populations with different medians. A
  Performance was calculated using the            effluent volume was used to calculate          significant difference between influent
Efficiency Ratio (ER) efficiency calculation      loads for both the influent and effluent       and effluent EMCs was concluded
method. The ER method defines the                 sample locations.                              when P<0.05. Based on the use of the

 Table 1. Summary of results.
                           Range of       Median          Mean           Range of       Median        Mean        Mean Removal     Efficiency
               no. of
 Analyte                Influent EMCs     Influent       Influent     Effluent EMCs     Effluent     Effluent        Efficiency      Ratio
               events
                             (mg/L)      EMC (mg/L)     EMC (mg/L)         (mg/L)      EMC (mg/L)   EMC (mg/L)    (Sum of Loads)      (ER)
 SSC < 2000
                 12     17.7 - 2080.0          53.4         231          1.9 - 7.2        3.4           3.9           98.3%          98.3%
 micron
 SSC < 500
                 12       9.0 - 393.0          28.6         66.1        1.7 - 10.0        2.8           4.4           93.7%          93.4%
 micron
 TP              11       0.065 - 0.9          0.14        0.223       0.025 - 0.058     0.025         0.031          87.1%          86.1%
 PP              9      0.019 - 0.225          0.06        0.083        0.0 - 0.033       0.0          0.007          96.4%          91.3%
 DP              9      0.025 - 0.850        0.054         0.155       0.025 - 0.160     0.025          0.04          67.3%          74.2%
 TN              10      0.035 - 2.95          0.85         1.00        0.35 - 0.82       0.35          0.44          50.2%          55.9%
 TKN             11       0.25 - 2.70          0.72         0.94        0.25 - 0.58       0.25          0.28          60.9%          70.2%
 NH3-N           11       0.05 - 0.72          0.21         0.27        0.05 - 0.24       0.05          0.10          60.0%          62.8%
 NOx             10       0.10 - 0.35          0.16         0.17        0.10 - 0.35       0.10          0.16          11.2%          9.8%




                                                                                                                 SEPTEMBER 2014 water