Tue, Aug 03, 2021:On Demand
Background/Question/Methods
Scientists with the USDA Natural Resources Conservation Service have in recent years completed a major effort to describe and map the soils of the greater Los Angeles Area, which had previously been broadly categorized as “urban land.” This mapping effort resulted in the collection of a valuable set of soils data but was focused on a set of properties used for the classification and specific interpretations of soils. Measurements of some chemical properties highly relevant to urban soil use such as concentrations of heavy metals were not determined. The research presented here begins to fill in these knowledge gaps by measuring a variety of soil physical and chemical properties at 39 random points across the city of Los Angeles. Soil properties measured were heavy metals (including arsenic, lead, and others) using pXRF total elemental analysis, soil pH by 1:1 DI H20 and 1:2 0. 1M CaCl2, soil particle size analysis by hydrometer, total carbon and nitrogen by combustion, and extractable cations and salinity (as measured by electrical conductivity) from saturated paste extract.
Results/Conclusions Results of these measurements established a range of values for each parameter. For the 39 samples, total As values ranged from 0-137.5 mg/kg with a mean concentration of 33.18 mg/kg. 26 of the soils exceeded California and US EPA thresholds for human health. Pb values ranged from 0-667 mg/kg with a mean concentration of 77 mg/kg, with 3 soils exceeding US EPA screening levels and 9 exceeding CA screening levels. The pH values ranged from 3.28-10.38, (mean 6.48) in 1:1 DI water, and from 3.40-10.28, (mean 6.46) in 2:1 0.1 M CaCl₂. Soil textural classes were dominantly sandy loam and loam textures. Total carbon values range from 0.665 to 6.125%, with a mean of 2.65%, and total nitrogen values range from 0.034 to 0.576%, with a mean of 0.201%. Electrical conductivity values range from 0.73-15.3 mS/cm (mean 3.2 mS/cm), with 10 of 39 samples considered saline with EC greater than 4.0 mS/cm. Saturated paste extractable cation values ranged from 3.50-89.13 meq/L (mean 21.14 meq/L) for Ca, 0.05-7.33 meq/L (mean 1.95 meq/L) for K, 0.04-872.19 meq/L (mean 29.58 meq/L) for Mg, and 0.53-86.23 meq/L (mean 8.57 meq/L) for Na. Overall, these values indicate some parameters of potential concern for management of the city’s soils, including heavy metal contamination and salinity.
Results/Conclusions Results of these measurements established a range of values for each parameter. For the 39 samples, total As values ranged from 0-137.5 mg/kg with a mean concentration of 33.18 mg/kg. 26 of the soils exceeded California and US EPA thresholds for human health. Pb values ranged from 0-667 mg/kg with a mean concentration of 77 mg/kg, with 3 soils exceeding US EPA screening levels and 9 exceeding CA screening levels. The pH values ranged from 3.28-10.38, (mean 6.48) in 1:1 DI water, and from 3.40-10.28, (mean 6.46) in 2:1 0.1 M CaCl₂. Soil textural classes were dominantly sandy loam and loam textures. Total carbon values range from 0.665 to 6.125%, with a mean of 2.65%, and total nitrogen values range from 0.034 to 0.576%, with a mean of 0.201%. Electrical conductivity values range from 0.73-15.3 mS/cm (mean 3.2 mS/cm), with 10 of 39 samples considered saline with EC greater than 4.0 mS/cm. Saturated paste extractable cation values ranged from 3.50-89.13 meq/L (mean 21.14 meq/L) for Ca, 0.05-7.33 meq/L (mean 1.95 meq/L) for K, 0.04-872.19 meq/L (mean 29.58 meq/L) for Mg, and 0.53-86.23 meq/L (mean 8.57 meq/L) for Na. Overall, these values indicate some parameters of potential concern for management of the city’s soils, including heavy metal contamination and salinity.