Chapter 12 - Lead Isotopes as Monitors of Anthropogenic and Natural Sources Affecting the Surficial Environment

Abstract

Abstract Arsenical pesticides and herbicides were used extensively in New England (USA) during the first half of the twentieth century. The pesticides were used on apple, blueberry, and potato crops. The arsenical pesticides (lead arsenate, calcium arsenate, and sodium arsenate) have similar Pb-isotopic compositions: 208 Pb/ 207 Pb = 2.3839–2.4722 and 206 Pb/ 207 Pb = 1.1035–1.2010. Other arsenical pesticides such as copper acetoarsenite (Paris green), as well as arsanilic acid, are widely variable in isotope composition. Common organoarsenical pesticides and herbicides such as monosodium methyl arsonate (MSMA), methyl arsonic acid, and methane arsonic acid have Pb-isotopic signatures that overlap the historical arsenical pesticides, but extend to significantly less radiogenic ratios of 206 Pb/ 207 Pb and 208 Pb/ 207 Pb. This Pb-isotopic difference is notable and suggests that these types of pesticides could be distinguished in Pb-bearing minerals formed in the near-surface environment. A full assessment of the environmental impact of the historical use of arsenical pesticides is not available, but initial studies indicate that arsenic and lead concentrations in stream sediments in New England (northeastern USA) are higher in agricultural areas that intensely used arsenical pesticides than in other areas. The possible lingering effects of arsenical pesticide use were tested in a detailed geochemical and isotopic study of soil profiles from a small watershed containing arsenic-enriched groundwater in coastal Maine. The Pb-isotopic compositions of acid-leach extractions represent lead adsorbed to mineral surfaces or held in soluble minerals (Fe- and Mn-oxyhydroxides, carbonate, and some micaceous minerals) in the soils, whereas residue compositions likely reflect bedrock compositions. Labile Pb-isotopic compositions (acid-leach) show a moderate range in 206 Pb/ 207 Pb (1.1870–1.2069) and 208 Pb/ 207 Pb (2.4519–2.4876). Isotope values vary as a function of depth: the lowest Pb-isotopic ratios (e.g., 208 Pb/ 206 Pb) representing labile lead are in the uppermost soil horizons (also containing highest Pb abundances). A multicomponent mixing scheme that includes predominantly lead from the local parent rock (Penobscot Formation) and lead derived from combustion of fossil fuels could account for the observed Pb-isotopic variations in the soil profiles. In general, however, our preliminary data also show that the extensive use of arsenical pesticides and herbicides in agricultural regions can be a notable anthropogenic source of arsenic and lead to stream sediments and soils.