Environmental Engineering Reference
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data on ammonia toxicity, which implies that it is a very conservative number for
toxicants that are not as fast-acting as ammonia. While the supporting data fail to
support the 1 hr averaging period, the importance of exposure duration to toxicity
is well documented (Newman and Crane 2002). Alternative methods of addressing
exposure duration in criteria derivation will be discussed later.
Finally, the frequency of one excursion every 3 yr is intended to allow ecosys-
tem recovery (USEPA 1985). Again, this number appears arbitrary, because it is
based on studies that show ecosystems require from 6 week to10 yr to recover from
toxicant-induced damage (USEPA 1985). However, the TSD (USEPA 1991) indi-
cates that, although data were lacking in 1985 to relate criteria excursions to eco-
logical effects, the criteria are designed such that a single marginal excursion should
cause little-to-no ecological effect. It goes on to argue that if marginal excursions
are rare, then high-stress events that require recovery time would be extremely rare,
and the 3-yr interval should be very protective.
The UK methodology states criteria in terms of magnitude and duration, but
not frequency. An AA concentration is intended to protect ecosystems against
long-term exposure, whereas, a MAC is meant to protect against transient con-
centrations that may cause acute toxicity (Zabel and Cole 1999). Although the
AA and MAC are intended to protect against different exposure durations, they
generally do not include specific statements regarding duration (such as the 1 hr
and 4 d averaging periods stated in USEPA criteria). Defined this way, determi-
nations of whether the AA and MAC are being met or not is dependent on monitoring
program design.
In Australia/New Zealand, Canada, EU member nations, and the state of North
Carolina, criteria are expressed in terms of magnitude only, and are designed to
protect against long-term exposure (ANZECC and ARMCANZ 2000; CCME
1999; BMU 2001; Lepper 2002; Irmer et al. 1995; ECB 2003; Bro-Rasmussen
et al. 1994; North Carolina Department of Environment and Natural Resources 2003).
In these cases, the values derived are intended to be used by water quality managers
to develop enforceable standards (which take into account factors such as use
designations and economic considerations), or to trigger further data collection.
Thus, allowable frequency and duration of exceedances are part of the management
process, rather than the criteria derivation process.
For countries that follow EU guidance, the pesticide criteria reflect values not to
be exceeded by the 90th percentile of the levels monitored in water (Lepper 2002);
hence, duration and frequency of exceedances depend entirely on monitoring
design. An EU Expert Advisory Forum, convened in 2001 and 2002, considered
alternatives to analyzing monitoring data for determining WFD goal compliance
(Lepper 2002). Possible alternatives included use of annual arithmetic mean,
geometric mean, median, 90th percentile, and a maximum never to be exceeded.
The Forum concluded that, from a scientific standpoint, either the arithmetic mean
or the 90th percentile would be the best measure to determine a reference condition,
but the choice between those two was political. In his report, Lepper (2002) pro-
poses that the EU should consider the use of a maximum acceptable concentration
value, in addition to quality standards designed to assess annual reference conditions.
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