However, residual lead in urban soils themselves is more of a concern than additional lead added from the irrigation water, Dr. Boyd says, explaining that residual lead is the same as the total lead in the soil, which, in urban settings, could be a combination of natural background lead in soils plus any other lead added, such as from past use of leaded gasoline or other man-made sources like paint chips.
“That’s why soil testing, including testing for environmental contaminants such as lead, is recommended for all new food gardens, as is researching the site’s previous uses,” Dr. Boyd advises.
Earlier this year, Dr. Boyd performed calculations to determine if irrigation of Flint gardens during the 2014 and 2015 growing seasons could have substantially increased the lead levels in the garden soils.
Using the edible flint Demonstration Garden as an example, Dr. Boyd calculated the area (square footage) of the garden, the volume of water used over the growing season (they 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.
“I did a simple calculation based on the assumptions that 2,250 gallons of water with a lead concentration in the water of 15 parts per billion were added by irrigation to 2,407 square feet of garden space,” Dr. Boyd explains, adding that “one acre of dry soil 6″ deep weighs approximately 2,000,000 pounds.”
“Based on these assumptions, if the contaminated water added was distributed in the top 6 inches of soil, the background lead level of 10 ppm would have increased by only 0.00256 ppm,” Dr. Boyd relates. “So the total lead concentration in soil in this example went from 10 ppm to 10.0025 ppm. If the lead stayed in the top 1 inch, then the soil lead concentration would go from 10 ppm to 10.015 ppm.”
Thus, in the example where the lead was added to the top 6 inches of soil via irrigation water, the level of lead in those soils would have increased by 0.025 percent if the background lead in the soil was 10 ppm, Dr. Boyd elaborates. “If the soil had a higher background lead level, for example 100 ppm as has been demonstrated in Delaware, the percent increase would be even smaller,” he says. “Either way, the increase in soil lead levels due to irrigation for a year, with water containing 15 ppm of lead, is very small. That would be true if the same irrigation protocol was used for two years.”
“Our edible flint demonstration garden’s most recent soil lead test from Spring 2015 was 93 ppm, and the season’s irrigation water would increase lead in the top 6 inches of soil by just 0.0025 ppm, or just an 0.0025 percent increase in the soil lead level of that garden,” Dr. Boyd reports.
A second MSU soil chemistry professor did these calculations independently and came up with the same answer. “It seems unlikely that lead contaminated irrigation water used for one or two growing seasons had any significant impact on the total lead levels in Flint garden soils,” Dr. Boyd concludes.
Choose Low Risk Vegetables
“In situations where heavy metals like lead and arsenic may be high in the soil, selecting certain vegetables to grow can lower the risk of heavy metal exposure,” Tocco says.
“In some cases, consuming the vegetables grown can be less than healthful,” he notes. “In situations where vegetables are grown in environments high in heavy metals, like lead and arsenic, they can actually concentrate these metals in their tissues. When humans eat these tissues, we further concentrate these chemicals, causing us harm.”
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