The contaminated site consultancy

Thursday, 31 May 2018 14:21

Turbidity as a Proxy for Suspended Dioxins

Kopeopeo Canal, Whakatane Kopeopeo Canal, Whakatane

One of the key challenges of the Kopeopeo Canal remediation project is keeping dioxins from moving around the broader drainage network. The dioxins are adsorbed to fine sediment particles in the canal bed. If the sediment is disturbed, the dioxins can be resuspended into the overlying water column, and transported wherever the water goes next. Even in this flat landscape, flow velocities can exceed silt curtain specifications, so it is not feasible just to enclose the work area in silt curtains.

But the canal sediment can’t be left completely undisturbed. The canal is an integral part of the Rangitaiki Plains flood management system, it needs to be able to convey floodwater during high rainfall events – even during remedial works, if necessary. So how does the project avoid redistributing dioxins back into remediated sections – or, worse still, out onto adjoining land? A mixture of hard controls, and practices that minimise resuspension, is needed.

One key control is the dredging method, which inherently minimises resuspension of sediment – that topic needs an article all to itself. Another is silt curtains around the dredge zone, which are effective at no-low flow. Yet another is canal level management using flood and sediment control structures – earthen/sheetpile walls equipped with floodgates and spillways.

How do we know that these controls are working? Water samples can readily be collected and analysed to see how much sediment and dioxin is in the water column at any given time. But this approach isn’t adequate for real time control. Even under urgency, it takes days to get samples collected, transported to the certified laboratory, and processed. That’s just not fast enough to inform management decisions when the rain is heavy and water levels are rising by the hour. What needs to be done to comply with consent conditions – is there enough storage in the dredge zone? Is it time to stop dredging and let everything settle?

Kopeopeo Canal project manager, HAIL Environmental’s Brendon Love, needed a better management tool. He and the project team could monitor some parameters in real time, such as turbidity – how murky the water is, roughly speaking. Could turbidity be used as a proxy for suspended dioxin loads? Simulation trials showed that they could indeed construct a traffic-light system, setting turbidity levels that give them confidence suspended dioxins are within target levels. With turbidity and water level monitors at ten locations throughout the canal, Brendon and the stakeholders can now be confident that the remedial works aren’t transporting dioxin where it shouldn’t be. Now, with this reliable and well maintained monitoring network, the project team can be confident that compliance can be achieved and remedial works can continue even during rainfall events.