The SMC’s chemistry laboratory intercalibration exercise was designed to help laboratories that conduct chemistry analyses of runoff samples improve their ability to collect consistent, high-quality data.

The intercalibration was divided into three rounds; the first round was completed in spring 2022, the second round in spring 2023, and the third round in spring 2024.

During each round, all participating laboratories were sent wet-weather samples, dry-weather samples and reference samples, then asked to measure a range of contaminants that fall into three major groups:

• General analytes: Total suspended solids, hardness, ammonia, chloride, nitrate+nitrite silica, sulfate, total nitrogen, total phosphorus, ortho-phosphorus
• Metals: Arsenic, cadmium, copper, lead, nickel, selenium, and zinc
• Organics: DDT compounds, chlordane compounds, PCBs, pyrethroid insecticides, and polycyclic aromatic hydrocarbons (PAHs); PAHs are a class of organic compounds added for the first time to the SMC’s chemistry intercalibration

Laboratories were encouraged to measure as many of the contaminants as they subsequently intended to analyze for SMC member agencies and other partners.

The SMC’s intercalibrations are performance-based quality assurance. With performance-based quality assurance, limits are set for sensitivity (i.e., detection limits), accuracy relative to a known concentration (i.e., custom-designed reference materials), and precision (i.e., comparison to other laboratories).

Letter grades of A through F were assigned based on combined scores for all three of the performance-based criteria across multiple samples. Laboratories needed a minimum score of 70% or a “C” letter grade in each class of chemicals to pass.

All but two laboratories that voluntarily opted not to participate in the final round of the intercalibration scored between 80%-100% – or an “A” or “B” letter grade – for all five classes of chemicals.