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Analysis

BOSC water-balance run — assimilative screen + the new sanitary figures

Run: watermark hydro (live) + the committed buildout scenario, 2026-06-08. Engine: Tier-0 water balance + 7Q10 low-flow assimilative screen (watermark.hydrology). Tier-0 is an auditable screen, not a SWMM/HEC-RAS result. Live streamflow: Ottawa River at Lima, NWIS gage 04187100 (today ~90.5 cfs / 58.5 MGD — ambient flow, well above the regulatory low-flow design condition).

This memo records the run and folds in two newly document-sourced figures from the commissioners’ minutes (sanitary-economics.yaml) that the model did not previously carry.

Headline

All three County receiving streams already FAIL the 7Q10 low-flow dilution screen at their design flows — before any data-center load. The Ottawa River’s regulatory 7Q10 is 0.20 cfs (≈0.13 MGD); the BOSC campus’s consumptive cooling draw alone is modeled at 3.14–3.84 MGD — i.e. the data center’s evaporative loss is 24–30× the receiving river’s 7Q10 low flow.

The assimilative screen (Tier-0, 7Q10 dilution)

Source: NPDES fact sheets (2IG00001 Ottawa/Lima Refining; 2PH00007 American Bath; 2PH00006 American II) [verified]. 3 checks, 3 violations — the receiving waters are effluent-dominated at low flow.

The BOSC cooling load (buildout scenario, sourced)

• IT load 275 MW [document — OEPA Air PTI P0138965, committed permits/4132514.epa.yaml (final, 2026-05-28); 114 gensets × 2.75 ekW ≈ 313 MW backup (per-engine ekW from the draft public notice — CBI-redacted in the final permit); IT 250–300 MW N+1].
• Makeup demand ≈ 3.92 MGD. The consumptive draw brackets two distinct WUE inputs, not a verified range: the central 3.14 MGD is power × WUE at the 1.8 L/kWh assumption [assumption]; the upper 3.84 MGD is power × the ~2.2 L/kWh physical evaporative-WUE ceiling [derived] — the FM-2 blowdown method (2.5 MGD × (CoC−1) = 10 MGD) implies ~5.7 L/kWh, unreachable for cooling, so it is capped at that ceiling (FM-2 is not purely cooling blowdown). The WUE inputs driving both bounds are the model’s dominant uncertainty.
• Drawn on Lima’s Ottawa/Auglaize supply (7Q10 0.20 cfs). A 3–4 MGD consumptive loss on a river whose low-flow design condition is 0.13 MGD is the core assimilative concern.

New from the minutes (folded in here)

1. Interim BOSC sanitary discharge: 0.13 MGD at 83 °F (sanitary-economics.yaml; minutes 2025-07-29) [document]. Two notes:
   • Thermal. 83 °F is a warm return; the model carries flow but not temperature. A warm discharge into a low-flow stream is an additional (unmodeled) assimilative stressor — flagged for a thermal screen.
   • Scale coincidence. The interim sanitary flow (0.13 MGD) happens to equal the Ottawa 7Q10 (0.13 MGD) — i.e. even the small early-phase return matches the river’s entire 7-day-10-year low flow.
   • The permanent return is the FM-2 2.5 MGD industrial discharge already in the model.
2. The Cridersville / Shawnee Oaks reroute loads the worst-violating node. The County is rerouting ~200+ homes + the Shawnee Oaks domestic flow onto Shawnee II (the $1M 0% loan, #137-26) — the same plant that already screens at 0.04:1 on the Ottawa. The reroute adds domestic flow on top of the BOSC industrial return to the most over-allocated receiving water. (Not yet quantified in MGD in the record — followup.)

What the run shows

• The municipal loop’s WWTPs are already effluent-dominated at low flow (2–25× over a 1:1 dilution); BOSC is added on top of a failing baseline, not a clean one.
• BOSC’s consumptive cooling draw (3–4 MGD) is large relative to the Ottawa’s low-flow supply — a withdrawal concern distinct from the discharge concern, and consistent with the residents’ well-drawdown questions in the PAAC minutes.
• The County’s own reroute concentrates more flow on Shawnee II → Ottawa, the worst-screening node.

Caveats

Tier-0 screen only. The 7Q10 is the cited regulatory low-flow (NPDES fact sheets); the live NWIS reading only sanity-checks ambient flow. WUE and cycles-of-concentration are assumptions (low confidence); the 3.14–3.84 MGD consumptive range is the headline uncertainty. The 83 °F thermal effect is noted, not modeled. Figures from the minutes are OCR-approximate discussion items — verify against the source PDFs.

Cross-references

• sanitary-economics.yaml — the 0.13 MGD @ 83 °F, the reroute, the fees
• bosc-resolution-ledger.yaml — #469-25/#713-25/#137-26 (PS+FM, CMAR)
• data/scenarios/buildout.scenario.yaml — the sourced cooling basis run
• data/reference/hydrology/ — 7Q10 (low-flow-7q10), TMDL WLAs, sanitary basis
• docs/HYDROLOGY.md — the full generated hydrology dossier
• data/extracted/regulatory/wastewater-enforcement-history.yaml — Shawnee II capacity history