The Water Quality Guidance for the Great Lakes System, also known as the Great Lake Initiative (40 CFR 132), promulgated ambient water quality criteria that applied to waters in the Great lakes basin. The methodologies used to calculate these water quality standards were essentially the same as those used by US EPA since 1994 to calculate National Water Quality Criterion, with the addition of several new types of standards, such as wildlife-based criteria.
[Read more about GLI Wildlife-based Water Quality Criteria]
A minimum data set is required to calculate water quality criteria for both National and GLI (“Tier I”) standards. However, the GLI did provide for methodology States could use to calculate standards for other chemicals which may not have enough data to fulfill these minimum requirements. The resulting standards are referred to as “Tier II” values, or “secondary” values.
Under the GLI, if all minimum data requirements for calculating Tier I are not met, a “safety factor” or “adjustment factor” is applied to the existing data to calculate a water quality standard. All the same requirements for test data acceptability apply equally to Tier I and Tier II standards.
Because the Tier II values use conservative adjustment factors and assumptions, and rely on a limited data set, standards derived using Tier II methodologies will typically result in much lower concentrations compared to Tier I standards.
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