ANALYSIS OF TRACE METAL CONCENTRATIONS IN FLOODED URBAN GARDEN SOILS

The growing trend of urban agriculture has spread to smaller cities with industrial legacies that pose similar challenges to gardening in larger cities. In this study, we examined a garden in downtown Mount Pleasant, Michigan, USA situated in the Chippewa River floodplain in an area formerly used as a railroad yard. The garden was constructed with raised beds containing non-native soils. However, an extreme flood event in June, 2017 inundated the entire garden and surrounding area for several days. We hypothesized that the floodwaters may have remobilized trace metals from floodplain sediments and deposited them in the garden soils. To test this hypothesis, we measured concentrations and mapped spatial relationships of Co, Cr, Fe, Mn, Pb, Ti, Zn, and Zr in the garden soils and adjacent floodplain. We collected 79 samples (including replicates) at 2m intervals from the raised garden beds and 75 samples (including replicates) from a 20mx20m plot directly north of the garden. Each ~100g composited soil sample was air dried and sieved to <250 μm prior to analysis using a calibrated Bruker Tracer 5i portable x-ray fluorescence (pXRF) elemental analyzer.

With the exception of Fe and Pb, concentrations of all elements in the garden soils followed a normal distribution. In contrast, concentrations in the 20mx20m flood plain plot were highly non-normal. Non-parametric statistical tests confirmed that the median values of all elements analyzed were different at > 99% confidence and median values of Cr, Fe, Pb, Ti, and Zr were greater in the flood plain sediments. With the exception of Mn, maximum concentrations for all elements were also higher in the 20mx20m plot than in the garden soils, by as much as two orders of magnitude. Moreover, none of the metal concentrations in garden soils exceeded Michigan’s residential standards for direct soil contact; however, exceedances were observed for Pb and Fe in several floodplain soil samples. In the raised garden beds, we found strong statistical relationships between Fe and Pb, Mn and Zr, and Ti and Zr. Soils from the 20mx20m plot displayed strong correlations between Co and Ti, Fe and Cr, and Fe and Ti, which were also evident on interpolated concentration maps. Overall, the results of our analysis suggest that the influence of the 2017 flood event on garden soil metal concentrations was small.