Microbial Water Quality Profile (MWQP) for Agricultural Water

The U.S. Food and Drug Administration (FDA) published revised Food Safety Modernization Act Produce Safety Rule (PSR) requirements for pre-harvest agricultural water quality in 2024. Produce growers who are subject to the PSR must understand the quality of their agricultural water as part of an agricultural water assessment. As part of this assessment, growers may choose to test their water. These test results may help to determine the appropriate use for that water based, in part, on comparing the data against scientifically valid water quality metrics. One option supported by the FDA for this purpose (see preamble, 2024 revision to the PSR, comment 93, page 37493) is to develop a microbial water quality profile (MWQP). The MWQP uses generic Escherichia coli concentration criteria that were established by FDA in the 2015 PSR (then removed as a requirement in the 2024 revision). These criteria are: an E. coli geometric mean (GM) of 126 CFU/100 mL or less and a statistical threshold value (STV) of 410 CFU/100 mL or less. Test results should be considered as part of an agricultural water assessment and should not be used on a standalone basis for decision-making.

Tools to calculate Geometric Mean and Statistical Threshold Value

Several tools were developed to make it easy to calculate the GM and STV that are part of the MWQP. These tools can also assist growers with making water management decisions if their GM or STV are higher than the FDA-supported criteria (e.g., extending time between last direct application of water and harvest).

The following descriptions provide the sampling frequencies that were established by FDA in the 2015 PSR to establish a MWQP using generic Escherichia coli concentration criteria. Under the 2024 final rule, farms have the flexibility to use any sampling frequency, as long as it is scientifically valid and appropriate to the intended use.

Untreated Surface Water (2015 PSR)

The initial MWQP must be established with a minimum of 20 water samples that are representative of use and collected as close to harvest as possible over a period of at least 2 years to a maximum of 4 years. Geometric mean (GM) and Statistical Threshold Value (STV) are calculated from these 20 samples (minimum). The GM and STV are your MWQP and should be compared to the microbial quality criteria provided in the Produce Safety Rule. After the initial MWQP has been established, GM and STV values must be updated annually based on a minimum of 5 new samples. “Rolling” GM and STV values are calculated by combining the old and new sample data. For example, for an MWQP established with 20 samples, five new samples would be combined with the most recent 15 samples from the previous MWQP to update the MWQP and confirm that the water is still being used appropriately.

Untreated Ground Water (2015 PSR)

The initial MWQP must be established with a minimum of 4 water samples collected as close to harvest as possible over a growing season or over a period of one year. Geometric mean (GM) and Statistical Threshold Value (STV) are calculated from these 4 samples. The GM and STV are your MWQP and should be compared to the microbial quality criteria provided in the Produce Safety Rule.  After the initial MWQP has been established, GM and STV values must be updated annually based a minimum of 1 new sample. “Rolling” GM and STV values are calculated by combining the old and new sample data. The one new sample combined with the most recent 3 samples from the previous MWQP are used to update the MWQP and confirm that the water is still being used appropriately.

Equivalent Testing Methodology for Agricultural Water (FDA, July 2018) (as PDF 57 KB)

Additional information:

Interim guidelines on health-related water quality standards for water actually used tor irrigation on Quality control of wastewater for irrigated crop production (FAO, United Nations)