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 <1.
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.