assessing the potential impact of the antisapstain chemicals ddac ...

4.6 ASSESSING THE POTENTIAL IMPACT OF THE ANTISAPSTAIN CHEMICALS DDAC AND IPBC
IPBC is not considered a persistent substance because soil studies have found that degradation is rapid
under aerobic conditions. Although there are no studies available for biodegradation in water, its behaviour
is expected to be similar to that in soil. Its half-life in soil is two hours, producing the major degradation
product, propargyl butyl carbamate (PBC), which in turn, has a half-life of 4.3 days. Carbamates are not
generally considered persistent in water (Menzer 1991). In addition, PBC is 1,000 times less toxic than
IPBC to both O. mykiss (80 mg/L versus 100 μg/l) and D. magna (60 mg/L versus 40 μg/L) (Springborn
Laboratories Inc. 1992a, b).
The most sensitive endpoint reported for fish was a 35-day LOEL of 19 μg/L for reduced weight and length
of larval P. promelas exposed to IPBC as embryos (Springborn Laboratories Inc. 1992c). Toxicity levels
ranged up to a 96-hour LC 50 of 1.90 mg/L in coho salmon embryo (Farrell and Kennedy 1999).
Invertebrate toxicity ranged from a 48-hour LC 50 of 40 μg/L for adult D. magna (Farrell and Kennedy
1999) to a 24-hour LC 50 of 1.42 mg/L for D. magna juveniles less than 24-hours old (Inveresk Research
International 1989).
DDAC AND IPBC CONCENTRATIONS IN FRASER RIVER WATER
The survey of DDAC and IPBC in the Fraser River was divided into two components, a plume dispersion
study and an on-site dilution study. The objective of the plume dispersion study was to determine the
concentrations of DDAC in the effluent plume from the mill outfall as it dispersed in the river during a
rainstorm event. The purpose of the dilution study was to investigate the partitioning of the two chemicals—when
released during a rainfall event—between river water and suspended sediment.
The survey was undertaken between April and August 1997 during rainfall events. Sampling was conducted
downstream of stormwater discharges at selected sawmills in the Vancouver area. Two types of mills
were sampled, those using the F2 (DDAC/borate) formulation and others using the NP1 (DDAC/IPBC)
formulation. As the survey was conducted during a period of relatively high flow and suspended sediment
content, the results are applicable to those river conditions. Further, because the overseas lumber market
demand had declined during this period and less wood was being treated, the volume of chemicals used
wasn’t representative of typical volumes used.
Methodology details are described in Szenasy et al. (1999).
Results
Plume dispersion study
At the F2 mill site on May 27, the effluent contained an average of 446 μg/L DDAC, of which approximately
half was in the decanted fraction (water with most of the solids removed). In the river, five metres
from the outfall, total DDAC was at the analytical detection limit of 10 μg/L. Boron was an adequately
conservative element which remained in the decanted fraction and hence was used to trace the plume in the
river and to calculate DDAC recovery. DDAC recovery decreased more rapidly with distance from the
outfall than boron. The boron concentrations indicated that the plume had mixed to greater than a 10:1
dilution within 10 m. Results are summarized in Table 2 (see end of chapter) and the dilution of DDAC
and boron in the plume, relative to distance from the discharge, are shown in Figure 1. These results show
that DDAC readily associates with particulate matter (about half the detectable DDAC appears to be in the
particulate phase) and the dilution capacity of the Fraser River is very high in the summer months.
The total suspended solids (TSS) concentration in the plume increased almost four-fold (relative to both
the plume and the river water upstream [TSS of river water=142 mg/L]) at a distance of about four metres
from the outfall (Table 2). While some of this increase could be due to flocculation (Krishnappan and
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