COMPOSITE LANDFILL AND NATURAL BIOGENIC LEACHATE DISCHARGE FROM THE AUSTIN CHALK, NORTH TEXAS

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.

Strongly Fe-enriched seepage with minor organic content emerges from alluvial paleochannels in the Chalk, exhibiting reducing ORP, sulfurous odor and a surficial organic chemical sheen. Seeps have moderately elevated salinity (Total Dissolved Solid (TDS) of 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). Evidence of anthropogenic origin may be indicated by high Arsenic (105 ppb), Barium (634 ppb), Cobalt (1.1 ppb), Chromium (1.3 ppb), Iron (40 ppm), Mercury (0.513 ppb), Manganese (3.3 ppm), Nickel (3.62 ppb), and Zinc (6.1 ppb) concentrations.

Analysis indicates minimal organic content in the seeps, while ratios of Biological Oxygen Demand (BOD), Chemical Oxygen Demand (COD), and Total Organic Carbon (TOC)) indicate that the landfill is ultra-mature (BOD/TOC 0.83). High levels of pre-emergence microbial activity evident from BOD/COD (0.24) suggest that any anthropogenic hydrocarbons are naturally attenuated. A strong degree of reduction is indicated (COD/TOC 3.49),

SEM results indicate authigenic bacterial reduction of iron to form botryoidal sulfides (forming calc-spheres of biogenic sulfide minerals), and later microbial sulfide dissolution. The presence of an upgradient landfill appears to enhance this later alteration, perhaps via addition of nutrients or bacterial seeding. Elevated iron and arsenic in the seeps appears to be of natural origin (dissolution of sulfides in the chalk), while other metals seem likely to be derived from the landfill. At the seep orifice, As exceeds permissible standards, but rapid precipitation after exposure to air reduces metals concentrations by 50-80%. 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.