ORIGIN OF ELEVATED ARSENIC IN POSSIBLE LANDFILL LEACHATE IN THE AUSTIN CHALK, NORTH TEXAS: BIOGENIC REDUCTION OF IN SITU IRON SULFIDES?

Seepage of a noxious iron (Fe) and arsenic (As) enriched discharge from streambanks in the Austin Chalk near abandoned landfills has raised significant local concern. Our detailed study of the seepage and host rock indicates a combination of anthropogenic and natural processes are responsible for this alarming phenomenon.

An anthropogenic origin is supported by close spatial correlation between seeps and abandoned (I.e. ultra-mature) landfills and As concentrations (105 ppb) above EPA limits (10 ppb). Chemical composition of the seeps suggests a natural origin (e.g. low concentrations of other RCRA-8 metals). Finally, temporal variation in metals concentration indicates diverse contaminant sources for metals in these seeps. This study assesses the nature of a likely natural source for As using Scanning Electron Microscopy (SEM) with Energy Dispersive X-Ray Spectrometry (EDS), Microprobe analysis, and Magnetic Separation.

Abundant microscopic Fe oxides and sulfides fill pore space within the biogenic sediments of the Austin Chalk and are the likely natural source for Fe and trace metals in the seeps. Evidence from SEM-EDS analysis confirms different morphologies, which are the source of Fe in the area. Fe in the chalk is found with framboidal texture, in single or multidomain crystals, and in conglomerates of minerals (oxides and sulfides) replacing Fe-carbonates with other minerals (magnetite and maghemite). The levels of As vary from (0.1 to 0.27 wt%) in the Fe sources, and are typically higher in oxides than sulfides. The conversion of Fe oxides to ferrihydrites releases and mobilizes it together with Fe. The presence of extra Fe and reduced pH (7.9 to 6.1), due to sulfuric acid released from sulfide decay, gives a favorable environment for microbial populations to consume the sulfides and to coprecipitate biogenic minerals. Microprobe data suggest that 80% of the Fe comes from oxides and the remaining 20% are from the authigenic and biogenic growth of sulfide.

Seep fluids are exclusively calcium bicarbonate waters with a strong Fe component (40 ppm), and show elevated metals of mercury (5 ppb) and manganese (3 ppm). This implies that the metals are not sourced from the landfill, yet spatial correlation between seeps and landfill suggests that nutrient or other contributions enhance the microbial action at these sites.