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

2. 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.

3. 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.

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

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

6. 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”

7. Hubbard Brook Experimental Forest

8. Hubbard Brook Experimental Forest

9. Hubbard Brook Experimental Forest

10. Hubbard Brook Experimental Forest

11. Pb in Precipitation:

12. Pb in Precipitation: 1975-2001 (log scale)

13. Pb in Streamwater:

14. Pb in Streamwater: 1975-2001 (log scale)

15. 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.

16. 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?

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

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

19. ‘Fractionation’ of Pb in Soils
‘Labile’

20. Pb Fractions in Hubbard Brook Soils

21. ‘Labile’ Pb Fractions in Hubbard Brook Soils

22. 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).

23. Reconstructing a Pb Deposition Record

24. Pb Deposition:

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

26. 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

27. Pb Content of the O Horizon

28. Pb ‘Budget’ 1926 – 1992 (kg/ha) Atmospheric Deposition, 1926-1997 8.66
Pb in O Horizon,
Pb in O Horizon, 1926 (est.)
Net Accumulation in O Horizon,
Accumulation as % of Deposition 63%
Pb Input to Mineral Soil, (1) – (2)
Pb Export in Stream Water (est.)
Net Accumulation in Mineral Soil, (3) – (4)
‘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

29. 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

30. Estimating Weathering by ‘Depletion’ Analysis

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

32. Estimating Weathering by ‘Depletion’ Analysis

33. Post-Glacial Release of Pb from Hubbard Brook Soils

34. Post-Glacial Pb ‘Budget’ for Hubbard Brook Soils
Weathering Release, years kg ha-1
Atmospheric Deposition, kg ha-1
Soluble Pb Input to Soil (1) + (2) kg ha-1
‘Labile’ Pb in Soil kg ha-1

35. 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 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 post-glacial period.
Together, weathering and anthopogenic Pb almost exactly equal the labile Pb in the soil profile.