Quantifying an early signature of the industrial revolution from lead concentrations and isotopes in soils of Pennsylvania, USA

Abstract

During the early industrial revolution, mining and smelting of ores and coal combustion released significant amounts of lead (Pb) into the atmosphere. While many researchers have documented high Pb concentrations in topsoils due to gasoline combustion between 1940s and 1980s, little work has focused on the extent of Pb and other heavy metal deposition into soils during the early industrial period. Here, we report Pb, cadmium (Cd), and zinc (Zn) concentrations and Pb isotope ratios of soils, sediments, parent bedrock, and waters collected from a small, currently pristine watershed (Shale Hills Critical Zone Observatory) in Pennsylvania (United States of America). Our results show that Pb in the soil comprises an addition profile, i.e. more Pb is present in the soil than is present in the equivalent parent bedrock. All three investigated soil profiles at Shale Hills on the same hillslope have Pb inventories (∼400–600 μg cm⁻²) attributed to atmospheric deposition. Cd and Zn concentrations in these soils show similar addition profiles due to atmospheric deposition. Based on Pb isotopic ratios, the most likely source of the added Pb is coal burning and ore smelting during local iron production in the early 19th century, roughly coincident with the construction of the U.S. transcontinental railroad. Mass balance and diffusive transport modeling were used to quantify Pb deposition rates and redistribution. These model results are consistent with the hypothesis that from ∼1850s to 1920s, coal burning and ore smelting in local iron blasting furnaces significantly increased the local Pb emissions so that Pb deposition rates in soils were in the range of 6–10 μg cm⁻² yr⁻¹. These values are comparable to Pb deposition rates found in other areas with early and intensive industrial activities (e.g. since the ∼1860s in Australia). Our new Pb, Cd, and Zn concentrations and Pb isotope results, in combination with the previously observed manganese (Mn) enrichment at Shale Hills, document that early industrial point sources contaminated local soils with metals that remain even today in topsoils with large sorption capacities. Where these metals are retained, their depth profiles provide a mean to infer the history of metal additions and redistributions.

Introduction

Since the industrial revolution, anthropogenic factors have been dominant in a way that humans are changing the Earth's surface at much faster rates than many natural processes (Caldeira et al., 2004). For example, human activities have significantly increased the metal emissions of lead (Pb), copper (Cu), zinc (Zn), cadmium (Cd) and manganese (Mn) to the atmosphere, and these metals are subsequently deposited back to Earth's surface, disrupting their natural biogeochemical cycles (Lantzy and Mackenzie, 1979, Galloway et al., 1982, Nriagu and Pacyna, 1988, Dudka and Adriano, 1997, Reuer and Weiss, 2002, Taylor et al., 2010, Cloquet et al., 2006, Komarek et al., 2008, Rauch and Pacyna, 2009).

Perhaps the best studied of these metals is Pb – cycling of this element in terrestrial and marine environments has been dominated by anthropogenic factors. In fact, metal production from Pb-rich sulfides has released anthropogenic Pb to the environment since ∼5000 yr BP (Reuer and Weiss, 2002). During the industrial revolution, the extent of atmospheric Pb emission increased significantly due to increasing demands for metals and energy (Edgington and Robbins, 1976). During the early period of the industrial revolution (i.e. in the 1800s), activities such as mining and smelting of ores and coal combustion became major sources of Pb into the environment (Reuer and Weiss, 2002). The dominant source of anthropogenic Pb to the atmosphere from the 1940s to 1980s was leaded gasoline, which caused widespread emission of Pb into the environment (Nriagu and Pacyna, 1988). Lake and estuary sediments, ice cores, lichens, and corals have all been used to reconstruct the history of industrial Pb deposition. These types of samples generally can store metals derived from atmospheric inputs (e.g., Edgington and Robbins, 1976, Graney et al., 1995, Reuer and Weiss, 2002, Marcantonio et al., 2002, Lima et al., 2005, Hissler et al., 2008, Geagea et al., 2008, Kelly et al., 2009). Several dating techniques (e.g., using sedimentation rates, coral growth layers, and radioactive isotope chronometers) have also been applied to such samples to establish the time frames of metal deposition and to quantify the deposition rates (e.g., Graney et al., 1995, Lima et al., 2005, Kelly et al., 2009).

Soils are also sinks for atmospheric metal deposition. However, the geographic extent and pollutant sources of industrial metal loading to soils have not been evaluated extensively (e.g., Markus and McBratney, 2001). One of the reasons is that soils, as open systems, receive metals from atmospheric deposition and from weathering of bedrock while losing these metals through chemical weathering and physical erosion processes (e.g., Watmough et al., 2004, Steinnes and Friedland, 2005). Furthermore, metals such as Pb can be redistributed within the soil column. It has thus been rare to reconstruct the history of metal deposition from soil records. Only a few such attempted reconstructions appear in the literature. In these, models that calculate Pb mass balance and incorporate Pb transport in soil profiles have been used to understand the history of Pb addition and redistribution in the uppermost layers of soils (e.g., Miller and Friedland, 1994, He and Walling, 1997, Kaste et al., 2003, Kaste et al., 2007, Brantley and Lebedeva, 2011, Drivas et al., 2011). Such soil studies should be most successful in illuminating our understanding of metal deposition in field sites where the important hydrologic, geomorphologic, and biogeochemical processes have been investigated.

In this study, we investigate local Pb loading history in soils from a well-studied, relatively pristine, small temperate watershed in Pennsylvania by combining Pb concentration and isotope measurements with mass balance models. To decipher processes related to soil formation and disturbance, it is necessary to integrate soil, geologic, ecologic, and hydrologic data. We therefore focus our study on the Susquehanna Shale Hills Critical Zone Observatory (or Shale Hills), one of ten Critical Zone Observatories (CZOs) in the United States (www.criticalzone.org). This site offers multidisciplinary data encompassing many of the relevant processes within the critical zone, i.e., the layer that extends from the top of the vegetation canopy to the depths of ground water (e.g. Brantley et al., 2007).