Microbial water testing results

Published: December 24, 2019

Submitted by Londa Nwadike, KSU/MU Extension Food Safety Specialist and Joshua Maher, KSU food science graduate student

As many of you know, the microbial quality of the water that you are using pre-harvest, as well as post-harvest is very important to the safety of your produce. In order to know the quality of the water you are using, the water must be tested regularly. Growers using municipal (rural or city) water can utilize the annual test results from the municipal water source, but for growers using ground or surface water, testing is essential. The Food Safety Modernization Act (FSMA) Produce Safety Rule (PSR) requires produce growers to develop a microbial water quality profile (MWQP) of their agricultural water source(s) over time and Good Agricultural Practices (GAPs) auditors also require water testing. Importantly, knowing the quality of your water can help to provide a safer product to customers.

Therefore, the University of Missouri and Kansas State University Extension have been providing free microbial water testing to produce growers at our laboratory in Olathe, Kansas, through a grant from USDA. Missouri Department of Agriculture Produce Safety personnel and University of Missouri Extension personnel and others have generously helped to coordinate the pick-up and delivery of these samples to the laboratory as the samples need to arrive on ice at the lab within 24 hours to maximize accuracy.

So far, our laboratory has tested 378 water samples from 158 Kansas and Missouri produce farms. The samples were tested for generic Escherichia coli, an indicator of fecal contamination of water, using one of the methods allowed by both the FSMA regulations and GAPs auditors.

According to the FSMA Produce Safety Rule, the generic E. coli population in an agricultural water source that will likely touch the harvestable portion of the crop BEFORE harvest (for mixing chemicals, irrigation, etc.) must be less than a geometric mean (~average) of 126 CFU/100 mL over time and a standard threshold value (~range) over time of 410 CFU/100 mL. For POST harvest use (washing produce, washing hands, washing packing tables, etc), the water must have no detectable generic E. coli/100 mL water (generally considered to be potable water).

The complete results of our water testing are included in Table 1 below. Overall, 48% of all samples tested had no detectable generic E. coli, and therefore would be acceptable for post-harvest use (although any surface water must be treated before using post-harvest). Ninety-one (91%) of all samples were below the geometric mean level and EPA recreational water standard (which is what GAPs audits also require for pre-harvest water) of 126 CFU generic E. coli/100mL, which means that the water is readily acceptable for pre-harvest use. Notably, 6 (1.5%) of samples (all were surface water samples) exceeded 2420 CFU/100 mL, the maximum reportable value of the laboratory test. Many of these very high results were from samples taken in May 2019, when rainfall levels were extremely high, and therefore runoff into surface water sources was also high.

Table 1. Generic E. coli prevalence data by agricultural water source.

Surface water (pond, stream, river) samples (n=185) contained the highest percentage (85%) of positive generic E. coli tests, with 15% of surface water samples above the geometric mean level, 11% above the standard threshold value, and 3% above the test's maximum reportable value.

MU and KSU Extension have received another USDA grant to continue to provide free microbial water testing to MO and KS produce growers through the fall of 2022. Through this new grant, starting on January 1, 2020, MO produce growers can get their water sources used for produce (both pre- and post-harvest) tested through your local public health department for free. More details on how to complete the required submission form and other details of the free water testing are still being determined, so more information will be provided to all produce growers in the upcoming months.

We will also be providing more information on this new USDA water quality project, including additional trainings and resources related to water quality, in the upcoming months.