Potentially Mobile Lead Fractions in Montane Organic-Rich Soil Horizons

Anthropogenic emissions during the 20th century resulted in global lead (Pb) contamination of soils. Recent studies have demonstrated that the organic horizons of temperate montane Spodosols in the northeastern United States retain Pb on timescales of 50 to 150 years. The precise mechanism(s) of this strong Pb partitioning to organic-rich soil material remain elusive, but a detailed understanding of Pb retention by organic layers and mineral topsoils is critical for predicting the fate of pollutants deposited on ecosystems. Here we use selective extractions to quantify potentially mobile pools of Pb in the surface horizons of relatively remote montane Spodosols from New Hampshire and Vermont. Using 10 consecutive rinses of water, we extracted a total of 1 to 5% of the carbon, and 4 to 12% of the Pb. Dialysis equilibration experiments demonstrate that this Pb is >5,000 molecular weight, and not truly dissolved as Pb²⁺. When soil was extracted with a single rinse of 0.02 M HCl (pH 1.7), 5 to 11% of the Pb was mobilized. However, hydroxylamine hydrochloride in 0.02 M HCl (a reducing agent) extracted 30 to 40% of the Pb. Repeated rinses with sodium hydroxide and sodium pyrophosphate, which target organic matter but may extract other soil phases removed 16 to 75% and 60 to 100% of the Pb, respectively. We show that significant Fe, Pb, and Al can be released from soils under reducing conditions, and that this fraction can be underestimated if sodium pyrophosphate is used in a previous step for leaching the organic-metal phase, as is typically done in sequential extraction schemes. Our results indicate that inorganic phases play an important role in determining Pb mobility and bioavailability, even in surface soil horizons dominated by organic matter.