What If the MCL Changes?
EPA set 4 ppt for PFOS/PFOA in 2024. The Trump EPA rolled back other PFAS standards in May 2025. Some states want 2 ppt. What happens to cost?
Why the Nonlinearity?
The nonlinearity is driven by the concentration gradient at the plume edge. Near the MCL threshold, small reductions in the allowable concentration capture large additional areas of the aquifer. The plume doesn't get a little bigger — it gets dramatically bigger — because the outer fringe of the plume, where concentrations taper off gradually, suddenly counts as contaminated.
At 10 ppt, the “contaminated” area is the dense core of the plume — 45,000 m². Drop the MCL to 4 ppt and you capture the inner fringe: 95,000 m². At 2 ppt, the outer fringe comes in: 180,000 m². By 0.5 ppt you're treating the entire dispersive halo — 420,000 m² — an area 9x the core.
The May 2025 regulatory rollback shows the ground is shifting in BOTH directions. Sites designed for 4 ppt may need to meet 2 ppt within a decade — or may get relief if compliance deadlines extend further. Regulatory uncertainty dominates geological uncertainty.
Cost Across Five MCL Levels
| MCL | Cost P50 ($M) | Cost P95 ($M) | Plume Area (m²) | Cost Ratio vs 4 ppt |
|---|---|---|---|---|
| 10 ppt | 35 | 55 | 45,000 | 0.4x |
| 4 ppt | 90 | 124 | 95,000 | 1.0x |
| 2 ppt | 260 | 380 | 180,000 | 2.9x |
| 1 ppt | 480 | 650 | 310,000 | 5.3x |
| 0.5 ppt | 720 | 950 | 420,000 | 8.0x |
Model C: 100 realizations at each MCL level. Costs: P&T CAPEX + NPV OPEX at 3% over 100 years.