California Waste Extraction Test (WET): Thresholds and Penalties

The California Waste Extraction Test (WET) is a laboratory procedure that measures how much of a regulated substance leaches from a waste sample into a mildly acidic citrate solution over 48 hours. The Department of Toxic Substances Control requires the test under Title 22 of the California Code of Regulations to decide whether a waste is hazardous under state law. If the extract concentration exceeds the Soluble Threshold Limit Concentration (STLC) for any listed substance, or the total concentration in the solid exceeds the Total Threshold Limit Concentration (TTLC), the waste is classified as California hazardous waste and must be managed accordingly.¹

Why Federal TCLP Results Don’t Satisfy California

California does not accept federal Toxicity Characteristic Leaching Procedure (TCLP) results as a substitute for the WET. The two methods differ in three ways that make the state test more aggressive. The WET uses a citric acid extractant buffered with sodium citrate; the TCLP uses acetic acid. The WET runs for 48 continuous hours; the TCLP runs for 18. And the WET uses a 10-to-1 dilution ratio of extractant to solid, compared with 20-to-1 for the TCLP, so the theoretical maximum concentration a substance can reach in the WET extract is double what it can reach in a TCLP extract.²

The practical consequence is simple. A sample with a borderline metal concentration can stay below the federal threshold after 18 hours in a weaker solution and still exceed the California limit after 48 hours in a stronger one. California also regulates more substances. The federal TCLP covers eight metals; California’s STLC list includes 19 inorganic substances plus additional organic compounds. Copper, nickel, zinc, and cobalt carry no federal TCLP threshold at all but do carry California limits that can classify waste as hazardous.³

The Thresholds Your Results Are Measured Against

California uses two separate benchmarks. The TTLC measures the total mass of a contaminant in the waste, expressed in milligrams per kilogram. The STLC measures what leaches out during the WET procedure, expressed in milligrams per liter. A waste that exceeds either threshold is hazardous.¹

The TTLC and STLC values for the metals generators encounter most often:³

Arsenic: TTLC 500 mg/kg, STLC 5.0 mg/L
Cadmium: TTLC 100 mg/kg, STLC 1.0 mg/L
Chromium (III): TTLC 2,500 mg/kg, STLC 5.0 mg/L
Copper: TTLC 2,500 mg/kg, STLC 25 mg/L
Lead: TTLC 1,000 mg/kg, STLC 5.0 mg/L
Mercury: TTLC 20 mg/kg, STLC 0.2 mg/L
Nickel: TTLC 2,000 mg/kg, STLC 20 mg/L
Silver: TTLC 500 mg/kg, STLC 5.0 mg/L
Zinc: TTLC 5,000 mg/kg, STLC 250 mg/L

Mercury sits notably lower than the rest. The 20 mg/kg TTLC is a fraction of lead’s 1,000 mg/kg, and the 0.2 mg/L STLC is the lowest on this list. Operations that produce even small mercury-bearing streams, including dental offices and fluorescent lamp recyclers, often cross the hazardous threshold before generators realize how little mercury it takes.³

When You Can Skip the WET

Because the WET uses a 10-to-1 dilution ratio, the extract concentration can never exceed one-tenth of the total concentration in the solid. If the TTLC result for lead is 40 mg/kg, the WET extract cannot exceed 4.0 mg/L, which is below the 5.0 mg/L STLC. The waste will pass the WET for lead without the test being run. Many consultants use this as a screening step: divide the TTLC by 10, and if the result falls below the matching STLC, the extraction is mathematically unnecessary for that analyte.²

How the Lab Runs the Test

The extraction begins when the laboratory prepares a 0.2 molar sodium citrate solution buffered to pH 5.0. The solid sample is combined with the solution at a 10-to-1 ratio by weight: 10 parts extraction fluid to 1 part solid.⁴

After the containers are deaerated and sealed, the mixture undergoes continuous agitation for 48 hours on a table shaker, overhead stirrer, or rotary extractor. The regulation requires the sample to stay in a “vigorously agitated suspension” throughout, so the fluid contacts every particle of solid. The longer extraction time and more concentrated solution are what pull more contaminants out of the matrix than the federal method does.⁴

One exception matters. When testing for hexavalent chromium, the lab must use deionized water as the extraction solution instead of sodium citrate, because the citrate buffer would reduce chromium (VI) to a trivalent state and produce inaccurate results.⁴

After 48 hours, the lab separates the liquid from the remaining solids by filtering through a medium-porosity prefilter and then a 0.45-micron membrane filter. The primary method uses vacuum filtration with a suction flask; pressure filtration is an acceptable alternative. The filtered extract then goes through determinative analysis to measure the concentration of each target analyte. Most labs use Inductively Coupled Plasma spectroscopy, which excites atoms with high-temperature plasma and reads the characteristic light signature of each element. The regulation itself does not mandate a specific instrument.⁴

Sampling, Hold Times, and Chain of Custody

The result is only as reliable as the sample. A representative sample reflects the chemistry of the entire waste stream, which means collecting portions from multiple locations in the container or pile rather than one scoop from the top. Collection equipment depends on physical form: liquids, sludges, and solids each require different tools.

Every sample needs a Chain of Custody document that tracks handling from collection to the laboratory. The form records the collection date, the sampler’s name, any preservatives added, and the physical state and estimated volume of the waste. Without a complete chain of custody, the results may not hold up in an enforcement action or audit. Samples should go into glass or high-density polyethylene containers, sealed to prevent cross-contamination, with a clear label showing the sample identification number and the analyses requested. Only state-certified laboratories are authorized to perform Title 22 analyses and produce legally defensible data; accredited labs can be found through the Environmental Laboratory Accreditation Program maintained by the State Water Resources Control Board.

Hold Times for Metals

Samples do not last indefinitely. For metals other than mercury, EPA guidelines allow up to 180 days from field collection to the start of leaching, and up to 360 days total from collection to final analysis. Mercury has a much tighter window: 28 days to the start of extraction and 56 days total.⁵

Chemical preservatives should not be added before leaching. Once the extraction is complete, the portion of the leachate intended for metals analysis must be acidified with nitric acid to a pH below 2. Missing a hold time or a preservation step can invalidate the analysis and force resampling.⁵

What the Classification Outcome Means

The result determines how the waste is stored, transported, and disposed of. Three outcomes are possible.

If both the TTLC and STLC results fall below every applicable threshold, the material is not classified as hazardous under either California or federal law, and standard disposal applies.

If results exceed a California STLC or TTLC but stay below all federal RCRA limits, the waste is non-RCRA California hazardous waste. It is subject to the California Health and Safety Code and Title 22, but not to federal RCRA manifesting and disposal requirements.¹

If results exceed a federal RCRA threshold, the waste is RCRA hazardous and falls under both federal and California programs. Generators use the test data to complete a waste profile that goes to the intended disposal facility, which then decides whether it can accept the material.

Wastes That Don’t Need the Test

Not every waste stream requires WET testing. Certain materials are excluded from hazardous waste regulation entirely, including industrial wastewater discharges regulated under the Clean Water Act, spent sulfuric acid used to produce virgin sulfuric acid, and secondary materials reclaimed and returned to the original production process. Recycled hazardous materials may also qualify, but only if the recycling meets all five legitimacy factors in the regulation.⁶

Penalties for Skipping or Misclassifying

Misclassifying hazardous waste or failing to test can result in administrative penalties of up to $70,000 per day of noncompliance. The same range applies to related violations, including making false statements for compliance purposes, disposing of hazardous waste at unauthorized sites, and treating or storing hazardous waste without proper authorization. The Health and Safety Code also allows criminal prosecution for intentional violations, with the possibility of jail time in addition to fines.

Enforcement is not reserved for large industrial sites. Any generator that skips required testing or submits an inaccurate waste profile is subject to penalty. Inspectors from local Certified Unified Program Agencies examine waste characterization records as a standard part of routine inspections, and generators relying on outdated test data or failing to recharacterize after a process change are among the most common enforcement targets. Records of waste characterization, manifests, and test results must be retained for at least three years, and gaps in documentation are treated as compliance violations even when the underlying handling was done correctly.

1
Department of Toxic Substances Control. Defining Hazardous Waste
2
Department of Toxic Substances Control. TCLP and WET Test Methods
3
Legal Information Institute. California Code of Regulations Title 22, 66261.24 – Characteristic of Toxicity
4
Legal Information Institute. California Code of Regulations Title 22, Appendix II – Waste Extraction Test (WET) Procedures
5
U.S. Environmental Protection Agency. Holding Time and Preservation
6
Legal Information Institute. California Code of Regulations Title 22, 66261.4 – Exclusions