NATURAL OR ANTHROPOGENIC ORIGIN FOR URBAN IRON-RICH SEEPAGE FROM THE AUSTIN CHALK? EVIDENCE FROM SEM/EDS

Noxious iron-rich seepage from the Austin Chalk in proximity to solid waste storage facilities in North Texas has been alleged to be anthropogenic. Preliminary chemical analyses suggest the origin is natural, raising the question of how concentration and acidic transport of Fe and other metals is possible in this fractured carbonate formation. This study utilizes Scanning Electron Microscopy (SEM) with Energy Dispersive X-Ray Spectrometry (EDS) to evaluate natural sources for this iron (alteration of sulfide) in the bedrock surrounding the seepage.

In North Texas the Austin Chalk contains abundant small marcasite/pyrite grains and sulfide weathering (biogenic and chemical) indicated 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. A natural origin for the seeps is suggested by their moderately elevated salinity (Total Dissolved Solid 1100-1400 ppm compared to 350 ppm in adjacent stream), mildly acidic pH (6.1-6.6), and minimal turbidity at the orifice (1.07 NTU). Anthropogenic origin may be indicated by high As (105 ppb), Fe (40 ppm) and Mn (3.3 ppm) concentrations in seep waters.

SEM-EDS analysis supports a natural source for the iron, revealing dissolution of biogenic frambodial pyrite formed inside foraminiferal tests. Pyrite cast structures are abundant within foraminifera, and the removed pyrite is a likely source of dissolved Fe. Growth layers of the original pyrite are evident, highlighted by the dissolution process.

The use of SEM-EDS has provided identification of FeS bearing minerals, a quantitative geochemical analysis of Cretaceous calcium carbonate limestone, and favors FeS bearing minerals as one of the sources of bio-leachate emerging from the exposed Austin Chalk. Results of planned ICP analysis of trace metals in the seepage waters will be required to authenticate the determination of a natural weathering origin for the waters.