Manly Lagoon Dredging Project - Manly Council - NSW Government

Manly Lagoon Dredging Project – Review of Environmental FactorsPrepared for Manly and Warringah CouncilsTable 3.2: Summary of Responses to Second Round of ConsultationStakeholderLPMAIssues Raised/CommentsHave at least one full-scale trial before the proposedsequence of dredging and dosing begins to confirm trialresults with actual scale project.Location of the proposed bunded area on Nolan Reservemust be shown precisely. Are there level differences whichwould affect the bunded area?oAny damage to the vegetated banks of Nolan Reservealong Manly Lagoon should be kept to a minimum.LPMA will seek advice from I&I NSW regarding elevatedlevels of some metals, enterococci and nutrients in theoverburden or runoff water, exceeding both relevantguideline values and background levels of lagoon water.Residual metal levels exceeding ANZECC guidelines forchromium, copper, lead and zinc in the second flush ofrunoff water would best be addressed by some mitigationmeasures.While surface sediments may likely present a “worst case”scenario for contaminants, it is possible that deepersediments in Manly Lagoon could contain contaminatedstrata, depending on what chemical matter entered ManlyLagoon at intervals over time e.g. spills, DDT herbicide useetc in years past.Deeper sediments may contain yet unknown local orpatches of variants and it may be presumptive to take it thatall overburden or runoff water will be similar to Lagoonwater.Addressedin REFSections2.5.1 and 5.3Appendix MNotedSection 5.3Sections 3.3and 5.3 andAppendix CSections 3.1and 5.1 andAppendix J3.3 Trial DredgingTwo preliminary dredging trials were undertaken at Manly Lagoon to assess performance ofthe proposed dredging methods and different polymers during dewatering. During the trialsthe composition and quality of the sediment and extracted water (runoff) was assessed todetermine whether the proposal to apply the dredge spoil to open space and reserves wasfeasible, and whether runoff water could be returned to the lagoon. Both trials wereundertaken in accordance with the mitigation measures described in Manly Lagoon DredgingTrial – Review of Environmental Factors (Cardno, 2009). This document was approved byManly Council and LPMA prior to commencement of the trials.Both dredging trials were undertaken at the downstream end of the proposed dredging area,near the end of Riverview Parade. This trial site was selected as previous sediment analysisshowed it to contain very high acid sulfate potential in the surface sediments (PBP, 2002).Observations at the lagoon also indicated that due to the regular feeding of ducks at thislocation, the levels of fine, surface organic material, which are closely associated withcontaminants such as metals, nutrients and bacteria were expected to be higher at thislocation than elsewhere in the lagoon. Consequently, this site was considered to berepresentative of the ‘worst case conditions’ likely to be encountered.2 July 2010 Cardno (NSW/ACT) Pty Ltd 24H:\Doc\2010\Reports.2010\Rep2580v4.doc