- water
- water-quality
- permitting
A total maximum daily load is the pollutant budget for an impaired water body: the maximum daily load of a given pollutant the water can absorb and still meet its water-quality standard, plus a margin of safety. The Clean Water Act requires a state to write one for each impaired water body–pollutant combination on its list.
The budget is then divided. Point sources — permitted dischargers — receive a wasteload allocation, which a state implements by writing that share into each facility's NPDES permit; nonpoint sources (runoff) receive a load allocation. In this watershed the binding TMDLs are nutrient budgets for total phosphorus and total nitrogen, the loads that drive the Lake Erie / Maumee algae problem. A TMDL tightens the dilution math: it can leave a reach with little or no headroom for a new load, regardless of that reach's flow.