RFI - Attachment F - Engineering Report

Phosphorous
Table 6 details the concentrations of phosphorous and nitrogen in the retained material.

                 Commercial Area                Urban Areas              Bottom of Catchment
             Sub1200      Over 1200       Sub1200        Over 1200      Sub1200       Over 1200
 Clean       N      P      N       P      N          P    N      P      N       P      N       P
 Event
 April      1600    553   2800    492    1700    421     2100   434    1400    859    2400    734

  June      958     524   1762    623     710    662     1680   630    815     355    467     782

 August     2840    692   1120    354    1760    445     2260   1060   1700    428    748     458

October     4520    735   6370    879    3930    694     7180   861    1410    452    10500   1390

December    746     47    1390    683    2800    738     4100   766    1330    550    1610    526

Average     2132    410   2688    517    2380    516     3464   672    1331    374    3145    645

              Table 6 Concentrations of total nitrogen and total phosphorus (mg/kg)


Total phosphorus was, on average, greater in samples above 1200 microns, however the
values were inconsistent with research by the CRC that’s assumes 90% of Total
Phosphorous is associated with particles under 1000 microns. Kinglet Mitchell and
Allison found that dissolved and particulate concentration of nutrients was elevated from
wet sump GPT’s and sumped gully pits. This could be because of biological and
mechanical breakdown in the body of water during prolonged dry periods and turbulence/
re-suspension from a storm event. The Enviropod holds the material dry preventing
biological breakdown. This may be preventing the breakdown into suspended solid size
particles through the treatment device and or reticulation system. Further investigation is
warranted as this suggests that the nature of nutrients may change as it travels through the
stormwater system.

Analysis of nutrient concentration, organic matter concentration and particle size groups
revealed there is little relationship between concentration of phosphorous in the retained
sediment and organic concentration of the retained sediment Figure 15.




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