Ohio Surface Water Quality Criteria - Aquatic Life
Ohio water quality standards for the protection of aquatic life, including hardness-dependent metals criteria. OAC 3745-1-35.
Overview
These are Ohio’s water quality criteria for the protection of aquatic life, as established in OAC 3745-1-35. These criteria apply to all surface waters of the state with aquatic life use designations (Warmwater Habitat, Exceptional Warmwater Habitat, Modified Warmwater Habitat, Coldwater Habitat, Seasonal Salmonid Habitat, and Limited Resource Water).
Aquatic life criteria are expressed at three levels:
IMZM (Inside Mixing Zone Maximum) - The maximum concentration allowed within the mixing zone immediately adjacent to a discharge.
OMZM (Outside Mixing Zone Maximum) - The maximum instantaneous concentration allowed outside the mixing zone. This is the acute criterion.
OMZA (Outside Mixing Zone Average) - The maximum average concentration allowed outside the mixing zone. This is the chronic criterion and the value most commonly used for ambient water quality assessment.
Several metals have criteria that vary with water hardness (measured as mg/l CaCO3). Hardness-dependent criteria are calculated using equations provided in Table 35-9. Higher hardness generally means higher (less restrictive) criteria because dissolved metals are less bioavailable in harder water.
Table 35-1: Statewide Aquatic Life Criteria (Non-Hardness-Dependent)
All values in ug/l unless noted. D = dissolved, TR = total recoverable, T = total, R = total residual, free = free (un-complexed) form.
| Chemical | Form | Units | IMZM | OMZM | OMZA |
|---|---|---|---|---|---|
| Arsenic | D | ug/l | 680 | 340 | 150 |
| Arsenic | TR | ug/l | 680 | 340 | 150 |
| Chlorine (WWH, EWH, MWH, CWH) | R | ug/l | - | 19 | 11 |
| Chlorine (LRW) | R | ug/l | - | 19 | - |
| Chromium VI | D | ug/l | 31 | 16 | 11 |
| Cyanide (Lake Erie basin) | free | ug/l | 44 | 22 | 5.2 |
| Cyanide (Ohio River - WWH, EWH, MWH) | free | ug/l | 92 | 46 | 12 |
| Cyanide (Ohio River - LRW) | free | ug/l | 92 | 46 | - |
| Cyanide (Ohio River - SSH, CWH) | free | ug/l | 45 | 22 | 5.2 |
| Dieldrin | T | ug/l | 0.47 | 0.24 | 0.056 |
| Endrin | T | ug/l | 0.17 | 0.086 | 0.036 |
| Lindane | T | ug/l | 1.9 | 0.95 | - |
| Mercury | D | ug/l | 2.9 | 1.4 | 0.77 |
| Mercury | TR | ug/l | 3.4 | 1.7 | 0.91 |
| Parathion | T | ug/l | 0.13 | 0.065 | 0.013 |
| Selenium | D | ug/l | - | - | 4.6 |
| Selenium | TR | ug/l | - | - | 5.0 |
Habitat-specific criteria for ammonia and dissolved oxygen are defined in Tables 35-2 through 35-8 of the rule. Temperature criteria are in Table 35-11. Cadmium, chromium III, copper, lead, nickel, and zinc are hardness-dependent - see Table 35-9 below.
Table 35-9: Hardness-Dependent Aquatic Life Criteria (OMZA)
The table below shows the chronic (OMZA) criteria at four standard water hardness values. Use the equations in the full rule to calculate criteria at site-specific hardness. For hardness above 400 mg/l CaCO3, use the 400 mg/l values.
All values in ug/l.
| Metal | Form | H=100 | H=200 | H=300 | H=400 |
|---|---|---|---|---|---|
| Cadmium | D | e(0.7852[ln H]-2.810) | 2.2 | 3.9 | 5.3 |
| Cadmium | TR | e(0.7852[ln H]-2.715) | 2.5 | 4.2 | 5.8 |
| Chromium III | D | e(0.819[ln H]+0.5340) | 74 | 130 | 180 |
| Chromium III | TR | e(0.819[ln H]+0.6848) | 86 | 150 | 210 |
| Copper | D | e(0.8545[ln H]-1.743) | 9.0 | 16 | 23 |
| Copper | TR | e(0.8545[ln H]-1.702) | 9.3 | 17 | 24 |
| Lead | D | e(1.273[ln H]-4.237) | 5.1 | 12 | 21 |
| Lead | TR | e(1.273[ln H]-4.003) | 6.4 | 16 | 26 |
| Nickel | D | e(0.846[ln H]+0.0554) | 52 | 93 | 130 |
| Nickel | TR | e(0.846[ln H]+0.0584) | 52 | 94 | 130 |
| Zinc | D | e(0.8473[ln H]+0.870) | 120 | 210 | 300 |
| Zinc | TR | e(0.8473[ln H]+0.884) | 120 | 220 | 300 |
Source: OAC 3745-1-35, Table 35-9. D = dissolved (requires approved translator). TR = total recoverable (applies absent an approved translator). H = water hardness as mg/l CaCO3.
Table 35-12: Wildlife Criteria
These criteria protect wildlife (primarily piscivorous birds and mammals) from bioaccumulative contaminants. Expressed as OMZA values.
| Chemical | Form | Units | Lake Erie Basin | Ohio River Basin |
|---|---|---|---|---|
| DDT (sum of DDT and metabolites) | T | ug/l | 0.000011 | - |
| Mercury | TR | ug/l | 0.0013 | - |
| Polychlorinated biphenyls (PCBs) | T | ug/l | 0.00012 | 0.001 |
| 2,3,7,8-TCDD (dioxin) | T | ug/l | 3.1E-9 | - |
Source: OAC 3745-1-35, Table 35-12.
Practical Notes
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Know your hardness. For metals like cadmium, copper, lead, nickel, and zinc, the applicable criterion depends on the receiving water’s hardness. A site with 300 mg/l hardness has very different criteria than one with 100 mg/l. Always calculate site-specific values using the equations in Table 35-9.
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Dissolved vs. total recoverable. Ohio provides criteria in both forms. Dissolved criteria are used when an approved translator exists for the discharge. Otherwise, total recoverable criteria apply. Dissolved criteria are generally less restrictive because they exclude the fraction of metal bound to particles.
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Pentachlorophenol is pH-dependent. Its criteria are calculated using equations in Table 35-10 of the rule, not listed here. Higher pH results in more restrictive criteria because the ionized form is less toxic.
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Habitat designations matter. Some criteria vary by aquatic life use designation (WWH, EWH, MWH, CWH, SSH, LRW). Ammonia, dissolved oxygen, chlorine, and temperature criteria differ significantly between habitat types.
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These are surface water criteria, not cleanup standards. They are used for NPDES (National Pollutant Discharge Elimination System) permit limits, TMDL (Total Maximum Daily Load) development, and ambient water quality assessment - not for soil or groundwater cleanup under the VAP.
Related Resources
- Ohio Surface Water Criteria - Water Supply - Table 33-1 criteria for drinking water
- Ohio Surface Water Criteria - Fish Consumption - Table 34-1 criteria for fish consumption
- Ohio Water Quality Standards (full PDF) - Complete OAC Chapter 3745-1
- EPA National Recommended Water Quality Criteria - Aquatic Life - Federal recommended criteria
- Ohio Sediment Reference Values - Sediment screening values by ecoregion