KIN Kin quarry, with its current set-up, would discharge untreated water downstream about seven times a year, according to preliminary calculations made by Sunshine Coast Council officers.
In a report to go to the council on Thursday, staff recommend a limited approval of the Neilsen Quality Gravels amended quarry management plan "for the initial quarry development phase only".
Council officers have estimated that, under the existing quarry water capture and treatment set-up, "15% of the average annual run-off volume" would not be treated.
"Given the topography of the site and size of the contributing catchment it is not considered possible to have a requirement for no discharge of untreated run-off from the site", the council report states.
Pollution of Wahpunga Creek and downstream spoiling of Noosa River due to the inadequacy of the quarry's sediment-holding basin to contain intense prolonged rainfall has been a major concern for Kin Kin Community Group.
"The above predictions demonstrate the system as proposed will not result in rare untreated discharges and, in fact, untreated discharges are likely to occur relatively frequently," the report states.
"These predictions appear to be consistent with community observations."
Council officers maintain the key to holding basin and onsite treatment systems from a steep Sheppersons Hill depends on the volume of the holding area and "the speed with which the basin can be emptied, ready for the next run-off event".
"During (quarry) stages 1-3, additional sediment basin storage will be required. It is also questionable whether such large sediment basins can be constructed on such steep terrain and prior to the benching operations occurring in that section of the quarry.
"Essentially, an innovative and as-yet-unknown system for stormwater treatment will be required if the quarry is to progress with the extent of disturbed areas indicated in the amended management plan."
Neilsen has indicated to the council that its treatment set-up takes 40 days to empty the holding basin. The report states:
"By reducing the time required to empty the basin to 10 days, water balance calculations indicate the average frequency of discharge can be reduced to less than once per year, which would reduce the long-term annual average untreated run-off from the site to 2%.
"With regard to stormwater management, only the initial quarry development phase currently has a feasible conceptual stormwater management plan in place, and a further update would be required before further quarrying stages proceed.
"Stormwater management will be an on-going issue as the quarry progresses."