IDEM is in the process of trying to develop a rule for how to do this. It has asked the Water Quality Advisory Group to make a recommendation. The WQAG is the usual assortment of business people, lobbyists, academics and environmentalists that IDEM puts together t argue over things like this. I don't know about you, but 20 percent doesn't seem very small to me, certainly not insignificantly small. Especially when we are talking about pollution.
Permits only enforce pollution controls designed to protect uses.
So using permit limits to determine a significant lowering of water quality might be okay for Tier 1 anti-degradation, but they should not be used for Tier 2. Therefore, for purposes of Tier 2 anti-degradation, I would suggest defining a significant lowering of water quality as any measurable addition of a pollutant. Industry might howl at this, but it is consistent with the goal of protecting existing wate quality and with the purpose of anti-degradation, which is to involve the public in decisions concerning water quality. Unfortunately, the regulated community lives in terror of such a public process, called an anti-degradation demonstration. It wants to avoid them at any cost.