Lead Accumulation and Loss in the Hubbard Brook (USA) Ecosystem Chris E. Johnson Syracuse University

Background Lead (Pb) is toxic to humans at low concentrations. Historically – two principal paths of exposure: Paint Drinking Water Atmospheric Pb deposition believed to be a significant source of Pb to the environment.

Lead as a Petrol Additive Use of Pb as an ‘anti-knock’ additive in petrol began ca. 1920’s. Pb consumption increased with petrol use during the 20th century. Concerns about Pb pollution grew in 1960’s. Mid-1970’s: U.S. Congress mandated that all new cars must use unleaded petrol.

Questions Was legislation effective in reducing environmental Pb pollution? What is the fate of Pb deposited to the environment during the 20th century?

Lead in Forest Ecosystems Precipitation Forest Floor Soil Solution Uptake Mineral Soil Stream Water

Geochemistry of Pb in Soils Non-Specific: Electrostatic attraction; relatively weak; nature of binding “sites” not important; competition among cations leads to “exchange” Specific: Thermodynamically efficient “fit” between ion and binding site; relatively strong; nature of binding sites crucial; examples shown are “bidentate”

Hubbard Brook Experimental Forest

Hubbard Brook Experimental Forest

Hubbard Brook Experimental Forest

Hubbard Brook Experimental Forest

Pb in Precipitation: 1975-2001

Pb in Precipitation: 1975-2001 (log scale)

Pb in Streamwater: 1975-2001

Pb in Streamwater: 1975-2001 (log scale)

Conclusions Concentration of Pb in precipitation declined > 97% after legislation restricting the use of Pb in petrol. (Political solutions can work!) Typical concentrations in 2000: Precipitation: 0.6 ug/L Streamwater: 0.1 ug/L Despite massive reductions in Pb inputs, the ecosystem remains a sink for Pb.

Questions Was legislation effective in reducing environmental Pb pollution? What is the fate of Pb deposited to the environment during the 20th century? Is Pb moving in the forest soils? In what chemical forms is the Pb found in the soil? Does the Pb deposited in the 20th Century represent a large pool in the soil?

“Podzolic” Soils at Hubbard Brook Oa Horizon: 30-40 % Carbon E Horizon: 1-2 % Carbon Bh Horizon: 4-10 % Carbon Bs1 Horizon: 3-10 % Carbon Bs2 Horizon: 2-5 % Carbon

Pb in the O Horizon (‘Forest Floor’): 1977-1997

‘Fractionation’ of Pb in Soils ‘Labile’

Pb Fractions in Hubbard Brook Soils

‘Labile’ Pb Fractions in Hubbard Brook Soils

Reconstructing a Pb Deposition Record We can estimate Pb input (atmospheric deposition) for 1975 onward because we know Pb concentration and amount of precipitation: We want to know Pb inputs dating back to the beginning of the use of Pb in petrol (1920’s).

Reconstructing a Pb Deposition Record

Pb Deposition: 1926 - 1989

Can Historic Pb Deposition Explain O Horizon Pb Pool? Precipitation (Modeled) FF Pool k.(FF Pool) Uptake ~ 0 Mineral Soil Stream Loss ~ 0

Can Historic Pb Deposition Explain O Horizon Pb Pool? Precipitation Input (Modeled) Model: (FF Pool)t+1 = (FF Pool)t + Inputt+1 – k.(FF Pool)t FF Pool k.(FF Pool) Uptake ~ 0 Mineral Soil Stream Loss ~ 0

Pb Content of the O Horizon

Pb ‘Budget’ 1926 – 1992 (kg/ha) Atmospheric Deposition, 1926-1997 8.66 Pb in O Horizon, 1997 6.84 Pb in O Horizon, 1926 (est.) 1.35 Net Accumulation in O Horizon, 1926-1997 5.49 Accumulation as % of Deposition 63% Pb Input to Mineral Soil, (1) – (2) 3.17 Pb Export in Stream Water (est.) 0.33 Net Accumulation in Mineral Soil, (3) – (4) 2.84 ‘Exchangeable’ Pb in Mineral Soil 0.1 EX + Inorganically Bound Pb in Mineral Soil 0.5 EX + IB + Org. Bound Pb in Mineral Soil 8.4 ‘Labile’ Pb in Mineral Soil 18.3

Long-Term Pb Release “Weathering” – Release of Pb to solution by dissolution of minerals. Soil Development – Glacier retreated ca. 14,000 years ago, leaving behind a relatively uniform till. Present-day soils represent the modification of this more-or-less homogeneous parent material

Estimating Weathering by ‘Depletion’ Analysis

Estimating Weathering by ‘Depletion’ Analysis Before Weathering Depleted Present-Day

Estimating Weathering by ‘Depletion’ Analysis

Post-Glacial Release of Pb from Hubbard Brook Soils

Post-Glacial Pb ‘Budget’ for Hubbard Brook Soils Weathering Release, 14 000 years 14.1 kg ha-1 Atmospheric Deposition, 1926-1997 8.66 kg ha-1 Soluble Pb Input to Soil (1) + (2) 22.8 kg ha-1 ‘Labile’ Pb in Soil 22.1 kg ha-1

Summary Reduced Pb consumption in petrol resulted in a decrease of >98% in atmospheric Pb deposition at Hubbard Brook. Despite the considerable decrease in Pb input, forest soils at Hubbard Brook continue to accumulate Pb. About 2/3 of the anthropogenic Pb inputs in 1926-1997 remain in the O horizon; about 1/3 has migrated to the mineral soil. Anthropogenic Pb inputs in the 20th century equaled about 60% of the weathering release during the entire 14 000 post-glacial period. Together, weathering and anthopogenic Pb almost exactly equal the labile Pb in the soil profile.