|Abstract:||Urban soils can harbor elevated concentrations of trace metal elements, in particular, lead (Pb), that present potentially severe risks to human health. Urbanized regions such as Chicago, in the north-central U.S., harbor soil total Pb that exceeds geogenic concentrations, often by orders of magnitude. Total Pb was quantified in Chicago soils obtained by grid sampling (n=995) parkways and residential properties (n=156). Due to anthropogenic modification of soils, total Pb concentrations may also correlate with other soil properties. In parkways, the mean concentration of Olsen P was 15.1 mg kg-1, while the soil pH mean was 7.7. In residential soils, the mean pH was 7.4 and the Olsen P concentration was 46.1 mg kg-1. The median total Pb for parkway soils was 220 mg kg-1 and 146 mg kg-1 for residential soils, while the mean soil total Pb for parkways was 282 mg kg-1 and 352 mg kg-1 for residential soils. Hotspots of soil Pb exceeding the U.S. EPA inhalation risk threshold of 400 mg kg-1 were estimated for an area of 38 km2. When the difference between soil Pb in parkways and residential soils was calculated, 23% of the city area was overestimating soil Pb in parkways by at least 100 mg kg-1 compared to residential soils. A significant negative linear relationship between Olsen P and soil pH was found in 13% of soil sampling locations when parkway and residential soils were combined. In Chicago, 93% of the area mapped using parkway soils exceeded the conservative CA EPA threshold of 80 mg kg-1, but 94% of the area was below the U.S. EPA threshold of 400 mg kg-1.
Due to the enrichment of soil Pb in Chicago, the growth of urban agriculture entails concerns about soil Pb contamination. There is an increased risk of Pb contamination in commonly consumed urban agricultural products. A frequently recommended practice to decrease the risk of Pb contamination in the soil is to add soluble phosphate (PO43--P). Across replicate urban agriculture sites in Chicago, we employed a randomized complete block design to determine if three PO43- based amendments (triple superphosphate, composted biosolids, and air-dried biosolids) could be used to decrease tomato fruit Pb. Total tomato fruit yield did not vary significantly among treatments but ranged 0.02 - 13.38 kg m-2. Fruit Pb concentrations (mg Pb kg-1 dry mass) were unaffected by mitigation treatments, as were fruit Pb loads (mg Pb m-2.) (i.e., no biomass dilution). However, both fruit Pb concentrations and Pb loads were affected by site and year. In the second year, fruit Pb concentrations increased by an order of magnitude from 0.01 mg kg-1 in 2019 to ≥ 0.13 mg kg-1 in 2020 for two of the three sites. This indicated that where and when the tomato plants were grown played a bigger role in determining the concentration of Pb compared to the amendments. Therefore, to control Pb contamination in tomatoes grown in urban soils, more focus should be placed on when and where tomato plants are being grown.