I would be interested in Andrew’s comments or answers to the following.

On page two of his review he writes: ‘Dioxins are strongly hydrophobic and partition from the effluent to the sediment and the water column.’

The assumption in this sentence is that dioxins in the effluent are dissolved and then partition when mixed in the environment. The USEPA function he highlights in figure 3.2 is defined as ‘total water body COPC concentration including water column and bed sediment’.

However, dioxins (PCDD/Fs) are hydrophobic and will adsorb onto particles in the waste-stream. Work done by the Ohio River Valley Water Sanitation Commission [1] has shown that water column concentrations of dioxins can be an order of magnitude greater when sorbed dioxin is measured compared to standard methods that measure only dissolved dioxins.

This implies that the concentration values for the pulp mill effluent, if they are based only on dissolved dioxins rather than ‘dissolved+sorbed’ dioxins, may be much lower than that which will actually occur in the effluent (see Section 6 of the review).

On first reading, the funtions used in the equations quoted in the review do not seem to account for sorbed dioxins. Is this correct?

If so, is the effluent stream likely to carry a higher dioxin load than previously estimated using these equations (either correctly or incorrectly)?

The implication is that the values for dissolved dioxins used in the pulp mill assessment are already a large underestimate, even before the alleged errors in the calculations done by Toxikos that Andrew describes are taken into account…

[1] Dinkins, S.A., and Heath, J.P. (1998). ‘Quantification of Dioxin Concentrations in the Ohio River Using High Volume Water Sampling’. Paper: National Water Quality Monitoring Conference (USA).

Jon,

the emission standard is not clear as to whether the dioxin limit of 13 pg TEQ/L relates to the effluent water including suspended solids (20-30 mg/L) or to the water only (without the suspended solids). In the analysis carried out, the assumption has been that the limit applies to water+suspended solids. So in fact, most of the dioxin present in the effluent will be sorbed to the suspended solids in the effluent stream. The USEPA protocol does include partitioning between the water column, suspended solids and sediment.

Of course, it really should be made clear in the emissions guidelines what the limit applies to.

What was not been taken into account in the analysis was the increase in organic carbon (TOC) due to the dispersion and settling-out of the suspended solids in the area near the outfall. Over several years this leads to a significant increase in sediment TOC with a resulting increase in sorbed dioxins.