I have recently received calls from growers who use surface water to irrigate produce crops. Their concern was that the results of their current water tests were very different from their last tests and that levels of indicator organisms appeared to have increased dramatically.
Ponds and lakes that are used for irrigation can be very dynamic. Research has shown that levels of indicator organisms can change dramatically in a very short time. Because surface water is open to the environment and unprotected, changes in temperature and weather can affect ponds and lakes. During the spring and autumn seasons, ponds and lakes undergo inversions, stirring up sediments that have settled on the bottom of the pond. Rainfall can also cause bottom sediments to be stirred up. Bottom sediments may contain any number of materials. Soil particles from runoff are one of the primary components. Decaying bits of plant and animal debris may also be found in bottom sediments. These sediments can also contain indicator organisms such as coliforms or generic E. coli. When ponds and lakes experience a change in climate or large amounts of rainfall, all the material found in the bottom sediments, including indicator organisms, is stirred up. Water samples collected during these time periods may have test results that indicate elevated levels of indicator organisms.
Increases in levels of indicator organisms generally follow a large rainfall event. Not only are bottom sediments stirred up by the rainfall, but runoff from neighboring farms, fields, and pastures can also introduce additional material into the water. Fortunately, we seldom need to irrigate immediately after a large rainfall event, as soils are usually saturated. Over time, sediments will settle back to the bottom of the pond and the water will clarify. Growers who are using surface water and see elevated levels of indicator organisms in their water test results should do the following:
- Immediately survey the watershed surrounding your irrigation water source. In particular, look for new potential sources for contamination. Has livestock been pastured for the summer in areas that drain into the pond? Has anything changed in the watershed since the last test? These are questions to ask as you survey the areas surrounding your irrigation water source.
- If no new sources of potential contamination can be found, check to see if the water source experienced any rainfall event just prior to testing.
- Make sure irrigation intakes are elevated. Drawing water from the bottom of the water source will stir up sediments and cause them to be taken into the irrigation system. Ideally, intake lines should be located in deeper water and at least 18 inches off the bottom of the water source. If shallower water is used, intakes should be between the surface of the water and the bottom.
- Wait at least 2-3 days after a rainfall event before irrigating. This will give the water time to clarify and allow sediments to start to settle back to the bottom.
- Collect an additional water sample and submit for testing as close to irrigating as possible. This will give a more accurate assessment of the water that is going to the crop after sediments have started to settle.
- Drip irrigation will prevent splashing and, in many cases, will prevent water from contacting the edible portion of the crop. This will help to reduce the risk of contamination from irrigation water regardless of test results.
- If additional test results indicate that levels of indicator organisms are not decreasing, growers should try to locate an alternative source of irrigation water.
If you have questions or issues concerning your irrigation water, feel free to contact me at the Southwest Purdue Agricultural Center at (812) 886-0198 or on my mobile phone at (765) 427-9910.