LANDFILL-PROMOTED DISSOLUTION OF PYRITE/MARCASITE (SULFIDES) IN CRETACEOUS CHALK

The origin of noxious iron-rich seepage from the Austin Chalk appears to be combination of anthropogenic and natural processes. Chemical composition of the seeps suggests a natural origin, while spatial correlation with very mature landfills suggests anthropogenic influences. This study assesses bedrock alteration processes using Scanning Electron Microscopy (SEM) with Energy Dispersive X-Ray Spectrometry (EDS), seepage composition via commercial lab analysis, and spatial distribution of old landfill deposits via surface resistivity surveys to investigate the interplay between these two processes.

Textural and elemental distribution maps via SEM/EDS indicate active natural weathering. In North Texas, the Austin Chalk contains abundant pyrite/marcasite grains and sulfide weathering (biogenic and chemical) is indicated in the field by limonite staining along fractures. The seeps exhibit abundant iron precipitation and orange algae formation away from the orifice, as well as an H₂S (sulfurous) odor, consistent with a pervasive bioleaching process.

Chemical analysis of the seeps indicates elevated arsenic (105 ppb), but not other metals typically found in landfill leachate. Elevated Iron (40 ppm) and Manganese (3.3 ppm) concentrations in seep waters may be of anthropogenic or sulfide-weathering origin. Mildly acidic pH (6.1-6.6) and minimal turbidity at the orifice (1.07 NTU) are most consistent with the latter. Moderately elevated salinity (Total Dissolved Solids 1100-1400 ppm compared to 350 ppm in adjacent stream) and spatial proximity to very mature landfills suggest some anthropogenic contribution.

Unknown chemistry of (or microbial seeding by) very mature landfill leachate appears to enhance subsurface alteration of natural sulfides in these areas. The nature of these linked natural and anthropogenic processes will be clarified by planned analysis of dissolved organic compounds and geophysical delineation and characterization of landfill materials. While the seeps are toxic upon emergence, dissolved metals are rapidly precipitated after exposure to air, and are primarily deposited as sediment in the stream. The net effect on surface water quality appears to be minimal at these sites, but may represent a transient risk in similar locales elsewhere in Texas and beyond.