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. The surprise: at this site, total NPV barely moves. The exceedance footprint does.
Footprint Scales. Cost Saturates.
Footprint scales with the concentration gradient at the plume edge. Near the MCL threshold, small reductions in the allowable concentration capture large additional areas of the aquifer. At 10 ppt, the exceedance area is the dense core: 89,038 m². Drop to 4 ppt and you capture the inner fringe (121,325 m²). At 2 ppt the outer fringe comes in (146,225 m²). By 0.5 ppt you're treating the dispersive halo at 197,775 m² — more than double the 10 ppt area.
Cost behaves differently. Containment NPV ($30M capex + $2M/yr opex over 100 years at 3%) is set by the planning horizon, not by the target concentration. Three extraction wells running for a century cost the same whether they're chasing 10 ppt or 1 ppt — the wells run continuously either way. Only at 0.5 ppt does the P50 cost dip ($81.2M), because in some realizations the model declares the area effectively unmanageable and the optimizer caps treatment volume. The P95 stays at $93.2M.
The May 2025 rollback shifts the ground in both directions. A 4-ppt site that becomes a 2-ppt site doesn't see its bill triple — the bill was already paid. What changes is which sites are on the list. A 2-ppt standard puts ~37% of monitoring wells over the limit. A 0.5-ppt standard puts 53% over. The cost question is national, not site-level.
Cost Across Five MCL Levels
| MCL | Exceedance % | Plume Area (m²) | Cost P50 ($M) | Cost P95 ($M) |
|---|---|---|---|---|
| 10 ppt | 13.3% | 89,038 | 93.2 | 93.2 |
| 4 ppt (current) | 26.7% | 121,325 | 93.2 | 93.2 |
| 2 ppt | 36.7% | 146,225 | 93.2 | 93.2 |
| 1 ppt | 43.3% | 171,612 | 93.2 | 93.2 |
| 0.5 ppt | 53.3% | 197,775 | 81.2 | 93.2 |
Model C: 30 Monte Carlo realizations × 5 MCL levels (150 runs total) at a 50-year horizon. Costs: P&T CAPEX ($30M) + NPV of $2M/yr opex at 3% real discount over 100 years. Exceedance % is the fraction of realizations exceeding the MCL during the simulation.