Publication

Lead is a highly toxic contaminant that has been delivered in large quantities to forested ecosystems worldwide via atmospheric deposition. In the past, it has been well-documented that high concentrations of lead are found in surface soils of even remote ecosystems. Recent long-term measurements have shown a significant decline in lead inventories in the forest floor, and there is the danger that this lead is being released to streams, rivers, or groundwater. However, understanding lead's behavior has been complicated by the extreme difficulty of measuring it at the low levels that occur in natural waters (soil water, groundwater, streams). This study conducted at the Hubbard Brook Experimental Forest (HBEF) provides some of the first reliable data on lead concentrations in streams and surface waters using state-of-the-art clean techniques at all stages of sample collection, processing, and analysis. Results show that dissolved lead in streams and seeps at HBEF is surprisingly low (mostly under 10 parts per trillion), though a significant amount of lead is found to be leached out of the forest floor layer (approximately 5 parts per billion in soil solutions beneath O horizons). Our study also shows that lead concentrations in streams and seeps are at least 10-100 times lower than those measured previously at HBEF without clean techniques. We conclude that mineral soil horizons act as the net sink for atmospheric lead (approximately 0.8 ppb in recent bulk precipitation) as it passes through forested ecosystems. The ecosystem appears to be an excellent ''filter'' that completely retains industrial contaminant lead in its soil profile. In estimates of total Pb outflow in streams, over 80% is found to be associated with particulate matter. Since a large portion of the particulate matter may be derived directly from surface soil debris, the stream Pb contributed by soil percolates is virtually nil. Undisturbed forested ecosystems control soil erosion and particulate matter output, so leakage of Pb from the ecosystem is minimal.