Bacterial indicators have a tough job consistently signaling the presence of one or more microbial pathogens without causing false alarms. For additional information on strengths and weaknesses of bacterial indicators, refer to the responses for these questions:

• How do I know if I am the cause of a pathogen problem? What do I monitor? How do I know if I have solved my pathogen problem? FAQ 26432

• What pathogens should I test for to determine if manure has contaminated my well? FAQ 26431

As such, the use of bacterial indicators for irrigation water is difficult at best but better than not monitoring irrigation water for microbial contaminants. For more information on bacterial indicators in well water, see the "wellowner.org" Web site at http://www.wellowner.org/awaterquality/coliform.shtml.

There are no regulations that require specific water quality targets for irrigation water used on food crops. Hence, from a purely regulatory perspective, the presence of indicator pathogens in irrigation water for food crops does not trigger a mandate for treatment. From a business perspective, it may therefore be prudent to take seriously the occurrence of indicator bacteria, especially if irrigation water is likely to come into direct contact with fresh leafy food crops (such as spinach). What would be a good standard to use, given that we do not know how effective bacterial indicators will be in signaling the presence of a pathogen? Water quality standards published by the U.S. Environmental Protection Agency for either swimming, shellfish growing waters, or drinking water may serve as a guideline, depending on how strict public health officials or growers, shippers, and retailers of leafy green vegetables want to be.

For drinking water, for example, the presence of indicator bacteria at any non-zero level is considered to exceed health standards. Older standards allowed the presence of up to 5 colony-forming units (CFUs) per liter, recognizing that most surface waters carried a small background load of coliform bacteria regardless of the presence of actual pathogens. While these standards are widely used throughout the United States, outbreaks of human illness still occur despite the use of these standards. Finally, the rate at which irrigation water is monitored is very important to the success of using bacterial indicators. Pathogens may enter water wells or streams on a continuous or on an infrequent, sporadic basis. For surface water sources, frequent testing on a weekly or more frequent basis from multiple points along the distribution chain of irrigation water is needed in order to have high confidence in the monitoring data.

In contrast, monthly or quarterly testing might be adequate for well water. If irrigation water exceeds the agreed-upon standard, there are many technical options for inactivating microbial pathogens. For treatment, most pathogens, except the protozoa Giardia and Cryptosporidium, can be effectively treated by disinfection with chlorine (a chlorine level below 4 mg/l is considered safe for drinking water consumption). For water that will be used to irrigate crops, chemical disinfection methods cannot have excessive impacts on soil fertility and crop productivity or else farmers will likely struggle to comply. Newer methods such as UV or ozone disinfection can be effective in killing a large number of microbial pathogens, but they can be expensive technologies given the large volume of water that will be treated. Filtration can remove many of the larger microbial pathogens, but the least expensive filtration methods such as sand filtration may not consistently remove some of the smaller bacteria and viruses. Filtration of irrigation water to remove protozoa is a relatively expensive investment. Other treatment methods include iodination and pasteurization.