Stege Marsh is a natural wetland located along the east central San Francisco Bay, CA, USA. Pyrite “cinders”(pyritic mine tailings heated to high temperatures to leach sulfuric acid) produced by mine tailings processing industries were periodically dumped on nearby tidal flats prior to ∼1950. These cinders (Figure 1), mostly iron oxides such as hematite in composition after processing, were dispersed by tidal action and deposited thinly across Stege Marsh. Surface and near-surface cinders subsequently oxidized to generate AMD for almost a century. Additional mercury fulminate contamination from an explosive blasting cap manufacturing plant nearby also washed into the marsh for some unknown period in the mid-to-late twentieth century. Prior to 2003, the marsh contained a large central AMD pond formed over and adjacent to large buried and surficial cinder deposits. Surrounding this acid pond was a broad acidic zone of reddish-orange and yellow oxidized sediments with sparse or no vegetation. Extending outward from the acid pond were tidal sloughs that traversed the remaining area of Stege Marsh, and allowed for the mixing of AMD generated in the acid pond with brackish tidal waters from the San Francisco Bay.

URS Corporation was contracted to study the Stege Marsh by the University of California-Berkeley in 2001 (before the timing of this study). Grab samples of sediments were obtained and homogenized at 10 cm intervals from the cinders, acid pond, and upper tidal slough, which were then acid-digested and analyzed via ICP-MS for a suite of metal(loid)s. These data are shown for As, Cd, Cr, Cu, Hg, Pb, Se, and Zn in Table 1, and represent concentrations as ppm (1 wt% = 10⁴ ppm for comparison to the in situ data obtained by our study).

Field measurements and sampling were performed during low tides. Sampling sites were chosen to represent (1) cinders deposits, (2) sediments impacted directly by the AMD source (“acid pond”), and (3) sediments located proximal to the AMD source in a nearby (but not acidic) major tidal slough that would have been receiving AMD mixed with seawater for a significant period of time (“upper slough”). Measurements of surface or pore water pH were obtained in each site (Table 1) with a field meter/probe (Thermo Orion 5-Star Plus meter with 9107WMMD pH probe).

Grab samples were taken from cinders upland of the acid pond for mineralogical analysis by X-ray diffraction (XRD). Cores from the upper 30–40 cm of each sampling site (“acid pond” and “tidal slough”) were obtained using a stainless-steel push-coring device. Sleeves were cut to accept sediments up to ∼40 cm deep, but recovered cores were generally slightly shorter. At least three cores were obtained per site, and these were subsampled into 4 and 10 cm intervals, for tidal sloughs and acid pond sediments, respectively, which were subsequently homogenized within these intervals to generate representative materials for EMPA analysis. One lengthwise split of each core was used for a microbial diversity study, as well as for sulfur isotopic analyses of sedimentary metal-sulfides (Moreau et al., 2010). The other split was used to obtain sufficient pore waters via centrifugation for analyses of metal(loid) concentrations (Figure 2).

Cinder samples were crushed to a fine powder and resuspended in water. A small amount of cinders was pipetted onto a low-background quartz plate for XRD analysis. XRD was performed on a Bruker-AXS D8 general area detector diffraction system (GADDS) using Co K_α radiation (1.79026 Å). Two-dimensional diffraction patterns were angle integrated to obtain patterns for analysis. The instrument resolution is 0.07° in 2θ space. All X-ray spectra were processed using the software package EVA (Bruker-AXS).

Polished ∼1 cm thick disks of epoxy-impregnated, crushed sediments from each depth of each core were analyzed with EMPA. EMPA was performed with the Cameca SX51 electron probe in the Department of Geoscience, University of Wisconsin-Madison, using wavelength dispersive spectrometry (WDS). Operating conditions were a focused beam at 20 keV and 20 nA, with a 10 s counting time on peak and background positions, with the exceptions of 30 s for As L_α, and 15 s for Zn K_α and Pb M_α. The probe was operated using Probe for EMPA software for both automation and data reduction (Donovan et al., 2012). The x-ray intensity measurements were corrected with the phi-rho-z matrix correction of Armstrong (1988). Elements, lines and respective crystal assignments used were as follows: TAP, Si K_α, As L_α; PET, S K_α, Cr K_α, Cd L_α and Pb M_α; LIF, Mn, Fe, Cu, Zn, and Se K_α and Hg L_α. All reported measurements are >3× the uncertainty of background counts (detection limits).

With regard to interference corrections and pulse height conditions, because of the many higher order interferences, all elements except Zn were acquired with pulse height windows set in differential mode. Peak interference corrections were implemented for interference of As on Pb M_α. Natural and synthetic mineral standards were utilized. Samples were carbon-coated with an approximate thickness of 200 Å, as determined by the interference color on polished brass.

Sediment splits from each depth and each core were dried overnight at ∼60°C, and crushed to a fine powder. Between 1–2 g of each powdered sample was processed by the acid-volatile sulfides (AVS) and chromium-reducible sulfides (CRS) extraction/distillation method of Canfield et al. (1986) for monosulfide and disulfide minerals, respectively. Sulfur recovered from each fraction (as Ag₂S) was weighed in comparison to initial powder weight, to estimate the specific concentration of AVS and CRS sulfides recovered from each depth. A small amount of NIST (http://www.nist.gov/) standard NBS123 Balmat sphalerite was also subjected to this extraction process to test the efficiency of sulfur recovery.

X-ray diffraction patterns of cinders showed a mixture of the iron-sulfoxyhydroxide, iron-hydroxide, and iron-oxide phases jarosite [KFe₃(SO₄)₂(OH)₆], lepidocrocite (γ-FeOOH), and hematite (Fe₂O₃), respectively (Figure 3). Pyrite or marcasite (FeS₂) was also present, although in minor quantities relative to hematite. Minor elemental sulfur was also detected. The remaining diffraction peaks in the pattern could be assigned to halite and barite. Both of these phases were likely to be present, as they are salts of the two most concentrated seawater anions.

Sediment cores from each sampling site varied greatly in lithology (Figure 4). Some roots, smaller plant detritus, pebbles and gravel, invertebrate shells, and burrows could be identified in the acid pond and tidal slough cores.

Some acid pond cores contained larger stones and cinder aggregates throughout. The uppermost sediments of the acid pond were comprised mainly of interlayered red coarse-grained hematite and gray specular hematite, with intermixed yellowish-brown iron (oxyhydr)oxide phases. Below the surface, the core also contained very small amounts of black fine-grained sulfides. Other acid pond cores contained yellowish iron-oxyhydroxides and elemental sulfur mixed with coarse-grained and specular hematite and fine-grained black iron-sulfides throughout the uppermost 8–10 cm. Below this depth, only coarse-grained and specular hematite was intermixed with sulfides in variable proportions, forming layers that appeared as reddish-brown and purple lenses within the otherwise black sediments.

Tidal slough cores were entirely comprised of well-consolidated black muds that were enriched in iron sulfides (most likely pyrite but possibly some marcasite, see below) below the upper few centimeters. The surface layer of each slough core consisted of a water-saturated greenish-black organic-sediment slurry with streaks of bright yellow elemental sulfur.

Sediments from the acid pond cores consisted primarily of cinders intermixed with coarse-grained, irregularly shaped iron oxides and sulfides (mostly hematite and FeS₂, but also some sphalerite), with partly weathered textures. No spheroidal or framboidal metal-sulfides were observed in acid pond core sediments. The concentrations of CRS increased significantly with depth, from ∼5 wt% at 5 cm to a maximum of about 21 wt% at 20 cm (Figure 5). Little (<0.1 wt%) to no AVS was recovered, however.

Tidal slough sediments, in contrast to the acid pond, contained sulfides present mostly as spheroidal and framboidal aggregates of very fine-grained FeS₂, and abundant iron oxides with textural evidence for vigorous weathering by dissolution and reprecipitation as metal-sulfides (e.g., pitting, extremely fine-grained zinc- and iron-sulfide in-fills and overgrowths; Figures 6A,B). Slough sediments also contained extremely fine-grained AVS, likely iron-monosulfides, as evidenced by the strong smell of sulfide released upon acidification with a few drops of 6N HCl. However, relatively small quantities of AVS were recovered by acid distillation from either slough core at any depth (<2 wt%).

Most of the sulfides present in slough sediments were pyrite or its polymorph marcasite (CRS), as determined by analysis with EMPA (i.e., Fe to S elemental weight percentage ratios of 0.8–0.9). Normally, EMPA analytical totals on large flat polished phases, where all elements are accounted for, should total at least 98–99 wt%. However, here, due to observed presence of abundant organic matter (C was not measured directly) and small sample sizes, analytical totals down to 90 wt% were accepted. In the upper slough, the concentration of CRS increased with depth to a maximum of ∼17 wt% at 16 cm (Figure 5).

Sulfur isotopic analyses (Moreau et al., 2010), along with the very fine grain sizes and framboidal or spheroidal character (Figures 6C,D) were used to recognize the biogenic origin of metal-sulfides in slough sediments. These minerals exhibited very different morphological and isotopic properties in comparison to tailings-derived primary sulfides that were present in very small quantities in acid pond sediments. The primary sulfides consisted of larger clasts with no recognizable subgrain particulate structure, whereas biogenic framboids/spheroids clearly consisted of submicron-scale sulfide particles tightly aggregated. However, EMPA scans suggested that most primary sulfides were completely oxidized to hematite during leaching of the sulfur for sulfuric acid manufacturing (e.g., Figure 6B). In some cases, the hematite hosted infill mineralization by zinc-sulfide, which preserved nearly the original shape of the hematite clasts (Figure 6A).

Concentration ranges (as wt%) of contaminant metal(loid)s in authigenic iron and zinc-sulfides are shown in Table 2. Measured concentrations of arsenic, copper, and lead associated with biogenic iron sulfides showed that up to ∼2 wt% of arsenic and copper and ∼0.25 wt% of lead was sequestered into framboidal FeS₂. The presence of trace zinc in iron sulfides consisted of up to ∼0.5 wt%. Nearly all of the remaining zinc found in the slough sediments was present as a zinc-sulfide phase (probably sphalerite but possibly some biogenic wurtzite; Moreau et al., 2004). Where cadmium was present, it was always observed in association with zinc-sulfide infill (up to ∼0.5 wt%). Interestingly, almost no mercury, chromium, or selenium were observed in association with either biogenic iron or zinc-sulfides. These data may be compared to whole sediment acid-digestion based analyses of Stege Marsh sediments (Table 1) and pore water metal(loid) concentrations (Figure 2) to consider metal(loid) bioremediation efficiency.