Thermal discharge — the river's heat budget

Cooling water leaves a plant warm. The water-balance page asks how much water a campus takes and the toxics report asks what its discharge carries; this page asks the third question — how much heat the Ottawa River can absorb before it breaches Ohio's numeric temperature standard, and who is already spending that budget. Under CWA §316(a), a discharger whose thermal load is more stringent than necessary may seek alternative limits by demonstrating a balanced indigenous community of fish and wildlife in the receiving water — which is why the biological tolerances below sit alongside the temperature numbers.

Ohio sets its numeric temperature criteria by geographic zone, not by aquatic-life-use column — Ottawa River sits in lake_erie_basin_general (OAC 3745-1-35 Table 35-11 (G)), whose peak-summer daily maximum falls in the Jun 16-30 half-month period. The criterion is [reference] (the rule text); the design ambient is [verified] — the corridor's own permit-required in-stream station (OH0026069 outfall 901 (Downstream Monitoring)), a measured receiving-water temperature. It sits downstream of that plant's own outfall, so it is not an undisturbed upstream background; the zone's seasonal-average criterion (27.8 °C) is what the screen falls back to without it.

One load has no bar, and that is the finding. Project BOSC's whole condenser rejection, fully mixed into a 0.2 cfs design flow, would raise the reach past the range water stays liquid in — so the screen reports no temperature at all rather than a literal rise of hundreds of degrees. The magnitude is carried as a ratio instead: see the modeled section below. Its bars above are its partitions — how much of that rejection each cooling archetype actually sends to the water.

The chart is not the whole corridor. OH0095346, OH0146552, OHGC02549 carry no bar because the screen resolves no fully-mixed temperature for them — no reported effluent temperature to screen, so nothing is asserted about their heat either way. They stay in the corridor permit count below.

What the corridor already reports

Before modeling anything, read the record. Of the 6 NPDES permits sharing this receiving water, 3 report an effluent temperature to EPA ECHO over the 2024-05-01..2024-10-31 warm season. Every figure below is the permittee's own submission, reduced to °C by its reported unit — ICIS carries temperature under parameter 00010 (°C) and 00011 (°F), and this corridor uses both. [verified]

OH0002623, OH0002615 report a peak daily-maximum effluent temperature at or over the 29.4 °C criterion — from outfalls their permits require them to measure but do not cap.

Monitor-only is the common case here. OH0002623, OH0002615, OH0026069 monitor effluent temperature under a permit that carries no numeric thermal limit on the discharging outfall. That is a cited absence, not a clean bill of health — and a permit's ceiling and its discharging outfall are often different outfalls, so a limit is reported here for context and only ever compared within the outfall it binds. An exceedance count, where one appears, is ECHO's own determination and is never computed by reading a value against a limit.

The modeled campus heat load — Project BOSC

The campus holds no discharge permit, so there is nothing of its own to read. This row is the condenser heat rejection derived from the disclosed IT load (316.2 ± ~28.8 MW) — an [inference] about a facility that is not yet discharging, screened on exactly the same reach, design flows, and criterion as the measured rows above, and never conflated with them.

Project BOSC: ~316.2 MW condenser rejection vs Ottawa River thermal capacity — 2,470x over at 7Q10 (only 0.04% of the rejection exhausts the capacity) [critical].

Where the heat goes

The scenarios span the heat partition, not uncertainty in the load: once-through sends the whole rejection to the stream by definition, evaporative blowdown sends only the blowdown's sensible heat (the rest leaves as latent heat to the air), and the conservative bound is the ceiling. They sit two orders of magnitude apart — and the point of running all three is that they all exhaust the reach's capacity.

Even the smallest partition — the tower blowdown at 1.19% of the rejection — clears the criterion at the design low flow. That robustness is the claim, and it is why the headline does not rest on the conservative bound alone.

Grading the model against the record

The verdict is about the model, not the facility. The derived once-through effluent (34.0 °C) is read against the warmest effluent temperature actually reported on this reach (32.2 °C , NPDES OH0002623 outfall 001 (LIMA REFINERY)) — an [inference] by analogy, never this facility's own figure. Conservative means the screen runs hotter than the record, which is the defensible direction; understated would mean the screen needs revisiting.

The closed-cycle off-ramp. OAC 3745-1-06 (O)(5): closed-cycle blowdown 2.5 MGD (3.868 cfs) vs 5% of the 0.2 cfs 7Q10 = 0.010 cfs — NOT exempt from the thermal-mixing-zone rule

The balanced indigenous community

A §316(a) demonstration turns on whether the receiving water still supports a balanced indigenous community. These are the Great Lakes representative-important-species thermal limits the screened mixed temperature is read against — federal guidance (EPA-833-F-23-007 Table 3-5), [reference] , not law.

The screen evaluates them at this reach's binding design flow, which here is the 1Q10 = 0 cfs — a dry channel, where every limit is crossed by construction and the honest answer is no temperature at all rather than a fabricated one. So the Crossed column below is read at the warmest temperature this facility does resolve at the 7Q10 — 34.0 °C, its hottest heat partition.

How to read these numbers

• A MODELLED (data-center) row's heat load is the CONDENSER heat rejection (IT x cooling overhead) — an inference about a facility that is not yet discharging. An INDUSTRIAL row's is the permittee's own reported effluent temperature x reported flow — a measurement. They are screened identically from there on but never conflated; read `kind` before quoting a number.
• Fully-mixed, design-low-flow, order-of-magnitude: no CORMIX plume model, no mixing-zone credit, no decay. T_mixed above the daily-max criterion flags the need for a permit-level thermal / CWA §316(a) analysis, NOT an automatic violation.
• The thermal assimilative capacity is rho*cp*Q*(daily_max - ambient). At a 0 cfs design flow (the Ottawa 1Q10) or an ambient already at the criterion the capacity is 0 — any heat load exceeds by construction (no Inf ΔT, mirroring the toxics screen).
• The design ambient is a live NWIS 00010 reading where the gage carries one, else the reach's own reported in-stream (upstream/downstream) DMR monitoring, else the zone's seasonal-average temperature criterion as a stated design ambient. An in-stream station sits downstream of that plant's own outfall, so it is a measured in-stream temperature, not an undisturbed upstream background.
• Cooling scenarios span the heat PARTITION, not uncertainty in the load: `once_through` sends the whole rejection to the stream by definition, `evaporative_blowdown` sends only the blowdown's sensible heat (the rest leaves as latent heat to the air) at a temperature CALIBRATED to an observed corridor analog — an [inference] by analogy, never this facility's own figure. `conservative_bound` is the Phase-2 ceiling.
• Reported DMR values are verbatim from the permittee's submissions via ECHO and reduced to degC by their REPORTED unit (00011 is Fahrenheit, 00010 Celsius); an exceedance count is ECHO's own determination, never computed here by comparing a value to a limit. A permit with no numeric thermal limit is recorded as monitor-only — a cited absence, not a clean bill of health.
• The OAC 3745-1-06 (O)(5) closed-cycle-blowdown exemption (blowdown < 5% of the 7Q10) is evaluated and surfaced whether or not it applies. RIS tolerances are the Great Lakes biological limits for a §316(a) balanced-indigenous-community read, [reference] (federal guidance, not law).

The volume side of the same cooling load is on the water balance page ; the chemistry side is in toxics and the corridor . Source: watermark.hydrology.cooling_models (condenser heat rejection) x EPA ECHO DMR effluent temperature (parameters 00010/00011) + flow (50050) x Ohio EPA cited design low flows (data/reference/hydrology/low-flow-7q10.yaml) x Ohio temperature criteria (data/reference/wqs/ohio-temperature-criteria.yaml) x Great Lakes RIS tolerances (data/reference/thermal/great-lakes-ris-thermal-tolerances.yaml, EPA-833-F-23-007)