If the PEQ is more than either 1.0 TU[a] or 1.0 TU[c], or as otherwise provided in Section 352.530, the Agency will determine eligibility for a dilution allowance consistent with Section 352.422. The preliminary effluent limitation (PEL) expressed in terms of acute and chronic toxicity units (TU[a] and TU[c]) shall be calculated pursuant to Section 352.423. Unless there is data indicating otherwise, the pollutant concentration in the background water (Cd) will be assumed to be zero.