MINERALOGICAL AND PB-ISOTOPE APPLICATIONS FOR TRACING NATURAL VERSUS ANTHROPOGENIC SOURCES OF AS, PB, AND OTHER METALS: AN EXAMPLE FROM A COASTAL MAINE WATERSHED

Detailed petrography and mineral chemistry coupled with Pb isotope tracer studies constrain the transport pathways of As and metals as they are liberated from bedrock and anthropogenic materials, captured in secondary minerals, and eventually released into soils and water. Bedrock and mine sources of metals in Coastal Maine are dominated by a diverse suite of arsenic-bearing minerals: pyrite (≤4 wt. % As), pyrrhotite (≤0.5 wt. % As), arsenopyrite, löllingite, cobaltite, glaucodot, and gersdorffite. Supergene minerals in the rocks and soils act as intermediate As sources; these include secondary arsenopyrite, orpiment, arsenolite, jarosite, scorodite, natrojarosite, rosenite, melanterite, ferrihydrite, goethite (≤0.4 wt. % As), and Mn-hydroxide minerals. Some soils also contain Co-Ni-arsenate and Ca-arsenate minerals. Weathering of the sulfide and sulfosalt minerals generates acid and releases metals (e.g., Pb, Cu, As, Co, Ni) that are then sequestered in secondary minerals by sorption or incorporation in crystal structures. The Pb isotope signatures of bedrock sulfides (galena, pyrite, pyrrhotite, löllingite, cobaltite, and arsenopyrite: ²⁰⁶Pb/²⁰⁴Pb= 18.073-19.489, ²⁰⁷Pb/²⁰⁴Pb=15.539-15.675, and ²⁰⁸Pb/²⁰⁴Pb=37.947-39.102) and secondary products (e.g., goethite, jarosite, and natrojarosite: ²⁰⁶Pb/²⁰⁴Pb=18.356-21.945, ²⁰⁷Pb/²⁰⁴Pb=15.595-15.839, ²⁰⁸Pb/²⁰⁴Pb=38.169-39.162)overlap. This suggests that the isotope compositions of the secondary minerals generally reflect the compositions of the sulfides from which they are derived. Other possible contributions of Pb to the secondary minerals include arsenical pesticides: Pb-arsenate, Ca-arsenate, and Na-arsenate (²⁰⁸Pb/²⁰⁷Pb = 2.3839-2.4721, ²⁰⁶Pb/²⁰⁷Pb = 1.1035-1.2010, and ²⁰⁶Pb/²⁰⁴Pb = 17.070-18.759). Isotope compositions of labile Pb extracted from soil profiles in the watershed (²⁰⁸Pb/²⁰⁷Pb = 2.4519-2.4876 and ²⁰⁶Pb/²⁰⁷Pb = 1.1870-1.2069) allow for various Pb sources, including Pb from the primary sulfides and in the secondary minerals, anthropogenic Pb from the historic use of arsenical pesticides, and atmospheric deposition of Pb from combustion of fossil fuels and other emission sources.