Toxic Units (TU) can be TUa or TUc

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Toxic Units (TU) can be TUa or TUc
Assume you have measured the following
concentrations of metals in the water below a
series of industrial dischargers. The Tua Is
defined as the concentration in the water divided
by the LC50. 100 ug/L zinc, 200 ug/L lead, 60
ug/L copper, 30 ug/l silver The LC50 for zinc is
200 ug/L, lead 300 ug/L, copper 80 ug/L silver
100 ug/L. The TUa 100 ug/L Zn 200 ug/L Pb
60 ug/L Cu 30 ug/L Ag 200 ug/L Zn 300 ug/L
Pb 80 ug/L Cu 100 ug/L Ag The TUa for zinc
is 0.5 The Tua for lead is 0.67 The TUa for
copper is 0.75 The TUa for silver is 0.30 The
sum of the TUa is 2.3 if you assume that the
toxicity of these metals is strictly additive,
and they are bioavailable you should see acute
toxicity. The job of the regulator is to reduce
the concentrations so the sum is lt1.
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Current technical guidelines recommend that no
discharge should exceed 0.3 TUa at the edge of
an acute mixing zone and 1.0 TUc at the edge of a
chronic mixing zone. When a WET test is
performed by a discharger it is usually performed
on dilutions of 100 effluent. So at the end of
the pipe before the effluent is mixed with the
receiving water the regulator knows what the TUa
is for that effluent 100/LC50 of the effluent.
The regulator knows that the TUc is
100/NOEC. Therefore it is the regulators job to
make sure that the dilution In the receiving
system is sufficient to meet the 0.3 TUa and the
1.0 TUc. The regulator must also be concerned
with the loading (WLA) from other dischargers and
non-point source pollution so that the combined
effects after mixing do not exceed the
established limits. If the limits are exceeded
the regulator has To along with the dischargers
do a total maximum daily load or TMDL to reduce
the TUs to appropriate levels with a MOS that
allows for future uses.