What Does “10,000 mg/L TDS” Mean? The USDW Bright Line in U.S. Groundwater Regulation
The exact definition — 40 CFR 144.3
“Underground source of drinking water (USDW) means an aquifer or its portion: (a)(1) Which supplies any public water system; or (2) Which contains a sufficient quantity of ground water to supply a public water system; and (i) Currently supplies drinking water for human consumption; or (ii) Contains fewer than 10,000 mg/l total dissolved solids; and (b) Which is not an exempted aquifer.”
Statutory basis: SDWA §1421(d)(2), which directs EPA’s underground injection control (UIC) regulations to prevent injection that endangers “drinking water sources” as defined there.
Why 10,000 mg/L — the logic
EPA’s position is forward-looking: water under 10,000 mg/L TDS could reasonably be expected to supply a public water system in the future, because it is feasible to treat it down to drinking-water standards. Above that line, treatment is considered impractical, so the water is not a reasonably expected drinking-water source — and falls outside USDW protection.
Do not confuse this number with drinking-water quality benchmarks:
| Number | What it is |
|---|---|
| 500 mg/L TDS | EPA secondary (aesthetic, non-enforceable) drinking-water standard |
| 1,000 mg/L TDS | Common “fresh water” convention used by several states |
| 10,000 mg/L TDS | USDW boundary — a jurisdictional line, not a health standard |
Brackish water at 5,000 mg/L TDS fails every palatability test yet is still a fully protected USDW.
What it decides in practice
- UIC well siting: Class I hazardous-waste injection wells must inject below the lowermost USDW and demonstrate confinement so injected fluids cannot migrate upward into any USDW (40 CFR Part 146, Subpart G).
- Aquifer exemptions: An aquifer portion that would otherwise be a USDW can be exempted — and thus lose USDW status — only through the formal exemption process (40 CFR 144.7, 146.4), which requires showing it does not currently and will not in the future serve as a drinking-water source.
- Area of review and corrective action: UIC permit reviews map the vertical extent of USDWs to set the area of review; every improperly abandoned well penetrating a USDW within it needs corrective action.
- State overlays: States may protect more (e.g., lower TDS cutoffs for “usable quality water”), but none may protect less than the federal USDW floor.
Common misunderstandings to avoid
- “Above 10,000 mg/L means safe to pollute.” No — it means outside USDW jurisdiction. Other programs (state groundwater rules, CERCLA, RCRA corrective action) can still apply.
- “A USDW must be currently used for drinking.” No — clause (a)(2)+(ii) covers water nobody drinks today if it is under 10,000 mg/L TDS and could supply a system.
- “Exempted = worthless water.” Exemption is a regulatory finding about future drinking-water use, often granted for mineral/energy-producing zones — not a statement about the water’s chemistry alone.
References (cite CFR first)
- 40 CFR 144.3 — definition of “underground source of drinking water”
- SDWA §1421 (42 U.S.C. §300h); §1421(d)(2) — endangerment of drinking water sources
- 40 CFR 144.7, 146.4 — aquifer exemption criteria and process
- EPA, “General Information About Injection Wells” (UIC program page)
This article summarizes the regulatory definition for reference. It is not legal advice; consult the cited CFR sections and EPA guidance for authoritative text.