California Waste Extraction Test (WET): Thresholds and Procedure

The California Waste Extraction Test, known as the WET, is a laboratory procedure that measures how much of a toxic substance leaches out of a solid waste when it sits in a mildly acidic solution for 48 hours. The Department of Toxic Substances Control requires this test to classify waste under Title 22 of the California Code of Regulations, and the results are compared against the state’s Soluble Threshold Limit Concentration values to decide whether the material is hazardous under California law.¹

Why California Uses the WET Instead of the Federal TCLP

California does not accept federal Toxicity Characteristic Leaching Procedure results as a substitute for WET testing when evaluating waste under state law. The two methods differ in three ways that make the California test substantially more likely to classify a material as hazardous.

The WET uses a citric acid extractant buffered with sodium citrate, while the federal TCLP uses acetic acid. The WET extraction runs for 48 continuous hours compared to 18 hours under the federal method. And the WET uses a 10-to-1 dilution ratio of extractant to solid, while the TCLP uses 20-to-1. That lower dilution means the theoretical maximum concentration a substance can reach in the WET extract is double what it can reach in the TCLP extract.²

The practical consequence is straightforward. Waste that passes federal TCLP screening can still fail the California WET. A sample with a borderline metal concentration might stay below the federal threshold after 18 hours in a weaker solution but exceed the California limit after 48 hours in a more concentrated one. Generators who rely solely on federal test results risk shipping improperly classified waste.

California also regulates far more substances than federal RCRA. The federal TCLP covers eight metals, while California’s STLC list includes 19 inorganic substances plus additional organic compounds. Metals like copper, nickel, zinc, and cobalt have no federal TCLP threshold at all but carry California limits that can classify waste as hazardous.³

The Two Thresholds: TTLC and STLC

California evaluates toxicity using two separate benchmarks. The Total Threshold Limit Concentration measures the total mass of a contaminant in a waste sample, expressed in milligrams per kilogram. The Soluble Threshold Limit Concentration measures the amount of a contaminant that leaches into liquid during the WET procedure, expressed in milligrams per liter. A waste that exceeds either threshold is classified as hazardous.¹

The TTLC and STLC values for the metals generators encounter most frequently are: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 is regulated far more tightly than any other metal on the list. Its TTLC of 20 mg/kg is roughly one-fiftieth of lead’s, and its STLC of 0.2 mg/L matches the lowest threshold on the table. Small quantities from dental offices or fluorescent lamp recycling can trigger hazardous classification that surprises generators unfamiliar with how low the threshold sits.³

The 10x Screening Shortcut

Because the WET uses a 10-to-1 dilution ratio, the maximum possible extract concentration is one-tenth of the total concentration in the solid. If a TTLC result for lead is 40 mg/kg, the WET extract can never exceed 4.0 mg/L, which falls below the 5.0 mg/L STLC limit. In that scenario, the waste will pass the WET without running it. Many consultants use this screening step to avoid unnecessary lab costs: divide the TTLC by 10, and if the result falls below the corresponding STLC, the WET extraction is mathematically unnecessary for that analyte.²

How the Laboratory Runs the Test

The extraction begins when the laboratory prepares a 0.2 molar sodium citrate solution buffered to a pH of 5.0. The solid waste sample is combined with this 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 using a table shaker, overhead stirrer, or rotary extractor. The regulation requires that the sample stay in a “vigorously agitated suspension” throughout the extraction period, meaning the fluid must contact every particle of solid waste.⁴

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

Filtration and Analysis

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, though pressure filtration is an acceptable alternative.⁴

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 unique light signature of each element to detect metals at very low concentrations. The WET regulation itself does not mandate a specific analytical instrument, but ICP is the industry standard for metal detection in environmental work because of its accuracy and ability to screen for multiple elements simultaneously.

Sampling, Custody, and Hold Times

WET results are only as reliable as the sample that enters the lab. A representative sample should reflect the chemical makeup of the entire waste stream, which means collecting portions from multiple locations within the waste pile or container rather than one scoop from the top. How you collect depends on the physical form of the waste: liquids, sludges, and solids each require different equipment and techniques.

Every sample needs a Chain of Custody document that tracks who handled the material from the point of collection to the laboratory. This 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 lab results may not hold up in an enforcement action or audit.

Samples should be placed in glass or high-density polyethylene containers and sealed to prevent cross-contamination. Each container needs a clear label with the sample identification number and the specific analyses requested. State-certified laboratories are the only facilities authorized to perform Title 22 analyses and produce legally defensible data; accredited labs can be located through the Environmental Laboratory Accreditation Program maintained by the State Water Resources Control Board.

For metals other than mercury, EPA guidelines allow up to 180 days from field collection to the start of the leaching procedure, 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 these hold times or preservation steps can invalidate the entire analysis and force resampling.⁵

What the Results Mean

The classification outcome 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. Standard disposal methods apply.

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

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

When the WET Does Not Apply

Not every waste stream requires WET testing. Certain materials are excluded from hazardous waste regulations entirely, including industrial wastewater discharges regulated under the Clean Water Act, spent sulfuric acid used to produce virgin sulfuric acid, and secondary materials that are reclaimed and returned to the original production process.⁶

Recycled hazardous materials may also qualify for exclusions, but only if the recycling process meets all five legitimacy factors: the material must contribute a useful ingredient, the process must produce a valuable product, the material must be managed like a raw commodity, the product cannot contain hazardous constituents at elevated levels compared to a legitimate analog, and at least 75% of stored material must be recycled by the end of the calendar year.

Waste held for treatability studies can be exempt from full hazardous waste regulation, but only within strict quantity and time limits. The sample cannot exceed 400 kg for standard hazardous waste or 1 kg for extremely hazardous waste. Generators may keep the sample at the site of generation for no more than one year, and no more than 10 days can pass between shipping and lab receipt. DTSC can grant case-by-case exemptions for additional quantities when equipment failure or study design requires it.⁶

Penalties for Skipping or Mishandling the Test

Misclassifying hazardous waste or failing to test altogether can result in administrative penalties of up to $70,000 per day of noncompliance. These fines apply to a range of 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, which can carry jail time in addition to fines.

The penalties are not limited to large industrial operations. Any generator, regardless of size, that skips required testing or submits inaccurate waste profiles is subject to enforcement. Inspectors from local Certified Unified Program Agencies conduct routine inspections, and they examine waste characterization records as a standard part of the process. Generators who rely on outdated test data, or who fail to recharacterize waste 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 waste 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