Penobscot formation

Foley and Ayuso (2008) suggest that typical processes that could explain the release of arsenic from minerals in bedrock include oxidation of arsenian pyrite or arsenopyrite, or carbonation of As-sulfides, and these in general rely on discrete minerals or on a fairly limited series of minerals. In contrast, in the Penobscot Formation and other metasedimentary rocks of coastal Maine, oxidation of arsenic-bearing iron—cobalt— nickel-sulfide minerals, dissolution (by reduction) of arsenic-bearing secondary arsenic and iron hydroxide and sulfate minerals, carbonation and/or oxidation of As-sulfide minerals, and desorption of arsenic from Fe-hydroxide mineral surfaces are all thought to be implicated. All of these processes contribute to the occurrence of arsenic in groundwaters in coastal Maine, as a result of the variability in composition and overlap in stability of the arsenic source minerals. Also, Lipfert et al. (2007) concluded that as sea level rose, environmental conditions favored reduction of bedrock minerals, and that under the current anaerobic conditions in the bedrock, bacteria reduction of the Fe-and Mn-oxyhydroxides are implicated with arsenic releases. 

Soil samples were air dried and prepared for lead isotope analysis using 50-100 mg of the <0.2-mm size fraction. Sulfides, Fe-hydroxides, and other secondary minerals in the Penobscot Formation can be used to monitor the composition of labile Pb and provide the means to discriminate labile (anthropogenic) lead from lead inherited from the parent rocks and sulfides (e.g., Ayuso and Foley, 2008). A cool mild acid leach (1,5N HC1 + 3N HNO3) was used to attack the secondary minerals. This solution likely reflects the labile Pb (e.g., Erel et al., 1997) captured in the Fe-hydroxide, carbonate, or organic materials, or other secondary minerals (clays). These minerals can contain lead, arsenic, and other elements derived from outside of the watershed. Mixed solutions of HF—HN03 were used for final dissolution of the residual fractions. Table 12.2 summarizes the Pb-isotopic data for the leach fractions of the soil horizons (together with Pb and As contents) Table 12.3 shows equivalent data for the residues. 

Trace element compositions and mineralogy of the soils and Penobscot Formation were obtained by various analytical techniques including XRD, XRF, ICP-MS, and INAA. Table 12.4 is a summary of the trace element data for the leach aliquots obtained on the soils in the Pb isotope study, and for the Penobscot Formation. 

Table 12.4 Average of selected trace element compositions of the soil horizons (leach fractions), and the Penobscot Formation (whole rocks), Northport area... 

---