By Terry McLean , Michigan State University Extension
With the news of lead contamination in the City of Flint’s water supply, people who farm and garden in the city are questioning the safety of growing in soil that has been irrigated with contaminated water.
Much has been reported recently about lead contamination of the drinking water in the City of Flint, Michigan, and the danger that represents to its residents. In an effort to apply research based information to the water contamination in Flint and its possible effects on soils, local Michigan State University Extension educators reached out to experts in MSU’s Department of Plant, Soil, and Microbial Sciences. Lead dissolved in water used for irrigation sticks to soil particles, and over time this newly added lead would add to the lead already found in the soils. Lead occurs naturally in soils in a range from ten to 50 parts per million (ppm). Generally, it is considered safe to use garden produce grown in soils with total lead levels less than 300 ppm.
MSU Environmental Chemistry professor Stephen Boyd’s initial thought was that irrigation during the past two growing seasons would not have substantially increased the lead levels in the soils, and he offered to perform calculations to test this idea. Using the edible flint demonstration community garden as an example, located in the city where water lead levels tested high, we were able to provide the information needed to perform the calculations. By calculating the area (square footage) of the garden, the volume of water used over the growing season (we had a reading from the rented city water meter that is hooked up to a fire hydrant), the concentration of the lead in the water used for irrigating, along with the previous lead level of the soil in the garden, he was able to demonstrate that the lead newly added in the irrigation water was minimal compared to typical levels of lead already present in Flint soils.
Our edible flint demonstration garden’s most recent soil lead test (from Spring 2015) was 93pm, and the season’s irrigation water would increase lead in the top six inches of soil by just 0.0025 ppm – or just an 0.0025 percent increase in the soil lead level of that garden. A second MSU soil chemistry professor re-did these calculations independently and came up with the same answer. Thus, it seems unlikely that lead contaminated irrigation water had any significant impact on lead levels in Flint garden soils. This will provide reassurance for Flint’s gardeners/farmers questioning the safety of growing food in soil that has been irrigated with lead contaminated water over the past two growing seasons.
Residual lead in urban soils themselves is more of a concern than additional lead added from the irrigation water, which is why soil testing is recommended for all new food gardens, as well as researching the site’s previous uses.