California Updates Groundwater Protection List Using New Method

The California Department of Pesticide Regulation (DPR) recently finalized regulations to fundamentally overhaul how the state identifies pesticides with the potential to pollute groundwater. The new regulations (DPR Regulation No. 25-002) taking effect July 1 represent the most significant update to California's Groundwater Protection List (GWPL) since its introduction in 1989.

The new regulations replace the specific numerical value (SNV) procedure that has been used since 1991 with a multivariate leaching value (MLV) method for identifying which pesticide active ingredients and their degradation products may have the potential to leach into groundwater. The updated GWPL now lists 119 pesticides, a net increase from the 105 previously listed, with 52 new additions, 32 removals, and 6 consolidations of salts and esters under their parent compound.

These changes significantly impact pesticide registration and have the potential to drastically alter pesticide use in California, which is the single largest producer of vegetables, fruits, and nuts in the U.S. The new regulations directly affect pesticide manufacturers, registrants, distributors, and agricultural operators in California, with implications to product registration pathways, groundwater monitoring obligations, and long-term product stewardship strategies, among others.

MLVs replace SNVs, pesticide degradation products trigger GWPL listing 

Under California's Pesticide Contamination Prevention Act (PCPA), DPR initially established the GWPL using SNVs to screen pesticides for leaching potential. The SNV approach uses a univariate method comparing known "leachers" to "non-leachers" across physicochemical properties based on soil mobility and persistence parameters, including water solubility, soil adsorption, and hydrolysis as well as aerobic and anaerobic soil metabolism rates. In contrast, the MLV procedure combines environmental fate data with quantum molecular variables derived from computational modeling of chemical structure.

In 2015, Senate Bill 1117 (SB 1117) amended California's Food and Agricultural Code (FAC) Section 13145, directing DPR to develop a new method for the GWPL. SB 1117 also expanded the scope of the PCPA to include pesticide degradation products, allowing pesticides to be listed on the GWPL due to their active ingredients and/or degradation products.

What DPR Regulation No. 25-002 changes

Adoption of the MLV procedure. The regulation replaces the SNV procedure with the MLV approach and codifies the multivariate leaching threshold (MLT) of 14.4706. If a currently registered active ingredient or its degradation product has an MLV greater than or equal to the MLT, the active ingredient will be placed on the GWPL. 

Inclusion of salts, esters, and degradates. The regulation clarifies that salts and esters of listed active ingredients and their degradation products are expressly covered under the GWPL. Previously, some salts and esters were individually listed as separate entries; they are now consolidated under their parent compound. Additionally, this regulation codifies that degradation products with an MLV equal to or greater than the MLT warrant addition of the pesticide to the GWPL.

Reorganization of the GWPL into three subsections. The three categories of the GWPL, which will be codified into the California Code of Regulations (CCR), are as follows:

• Section 6800(a): Pesticides detected in groundwater pursuant to FAC Section 13149(a) and subject to use modifications under FAC Section 13150(d)(2).
• Section 6800(b): This is a new section that encompasses pesticides detected in groundwater pursuant to FAC Section 13149(a) that have gone through the PCPA process and were determined not likely to pollute groundwater. Therefore, detections of these pesticides have not warranted use modifications pursuant to FAC Section 13150(d)(1).
• Section 6800(c): Pesticides identified by the MLV procedure that have potential to pollute groundwater but have not yet entered the PCPA hearing process under FAC Section 13150.

Net changes to the GWPL. A total of 52 pesticides are newly added to the GWPL: Additions consist of 48 new pesticides whose active ingredient was identified by the MLV procedure and 3 new pesticides whose degradation product(s) were identified by the MLV procedure, all listed under new Section 6800(c), as well as 1 pesticide listed under new Section 6800(b). Deletions consist of 32 pesticides that were removed, including 4 due to inactive California registrations and 28 because the active ingredient or degradation product(s) passed the MLV procedure. Additionally, 6 individually listed salt and ester forms were consolidated under their respective parent compounds.

The MLV procedure

The DPR stated the new MLV procedure reduces the overall misclassification error rate from 36% under the original SNV approach to approximately 5%. The final MLV model incorporates five variables to assess a pesticide's leaching potential; if a pesticide's MLV is greater than or equal to the MLT, it is identified as having the potential to pollute groundwater and placed on the GWPL. The MLT is set at 14.4706, which was derived by DPR from the distribution of MLV scores for a group of known leaching pesticides during model development.

The MLV formula will be codified as follows:

MLV = 1.9349 × Log₁₀(ASM) − 1.2197 × Log₁₀(Koc) + 0.1562 × Dipole Moment 

−0.914 × EHOMO + 2.2728 × Log₁₀(MaxElPot)

• Median soil aerobic metabolism half-life (ASM) — The median time required for 50% of the pesticide to degrade in soil under aerobic conditions.
• Median soil adsorption coefficient (Koc) — The soil organic carbon — normalized adsorption coefficient, which quantifies the tendency of a pesticide to bind to soil organic matter relative to remaining dissolved in the soil solution.
• Dipole moment (Debye) — A molecular property reflecting the magnitude of charge separation within a molecule and its overall polarity.
• Energy of the highest occupied molecular orbital (EHOMO) (eV) — A molecular property reflecting the energy of the highest-energy electron-occupied orbital in the molecule, derived from quantum mechanical modeling.
• Maximum electrostatic potential (MaxElPot) (kJ/mol) — A molecular property reflecting the most positive value on the molecule's electrostatic potential surface, representing the site of greatest electron deficiency. 

Implications for pesticide manufacturers and registrants

The adoption of the MLV procedure has broad implications for the pesticide industry, and there is still significant uncertainty in certain aspects of implementation:

• New registrations and pending applications. Effective July 1, relevant registration applications submitted to or pending with DPR will now be screened using the MLV procedure instead of the SNV approach. Pesticides, including the active ingredient and its degradates, with MLVs at or above the MLT may trigger GWPL listing under Section 6800(c) and subsequent regulatory action.
• Pesticides newly listed on the GWPL. There are 51 pesticides that are newly added to FAC Section 6800(c) as having the potential to contaminate groundwater, with 1 pesticide being newly added to Section 6800(b). Registrants can review their product portfolios for California-registered products that are affected by the updated GWPL and prepare for DPR to begin monitoring for these pesticides, together with publicly reported monitoring results. DPR has not identified the basis for the new listings to registrations (i.e., active ingredient or degradation product(s)), so registrants will need to confirm whether the active ingredient and/or degradation product(s) will be monitored.
• Degradation product considerations. The regulation underscores that an active ingredient can be listed on the GWPL based on the leaching potential of its degradation products, even if the parent compound itself does not exceed the MLT. Out of 107 pesticides listed under Section 6800(c), 24 pesticides are included because one or more of their degradates have MLVs at or above the MLT. DPR has confirmed that pesticides can be removed from the GWPL through future rulemaking if new environmental fate data results in an MLV to below the MLT. Registrants, particularly those with pesticides newly listed based on degradate MLVs, can proactively evaluate the underlying data to assess whether additional environmental fate studies could materially affect the MLVs of their pesticides, including the active ingredient and its degradates, and actively engage with DPR to explore pathways for potential delisting. Additionally, it is critical that registrants understand the scrutiny on degradation products before submitting registration applications to strategically approach the registration process with respect to DPR's groundwater assessment.
• Strategic regulatory engagement. Given the novelty and complexity of the MLV procedure and heightened emphasis on degradation products, registrants are encouraged to proactively seek expert guidance to evaluate the impact of the updated framework on new and existing pesticides. Understanding how the five MLV variables interact in addition to how data quality, data gaps, or the leaching potential of degradation products may influence MLVs, is critical for anticipating potential regulatory actions and developing effective registration and stewardship strategies in California.