A Natural Origin for Arsenic and Iron Rich Seeps from a Chalk in the Vicinity of Unlined Landfills, North-Central Texas, USA
Seepage of suspicious iron, arsenic and sulfur rich waters from streambanks down-gradient from unlined landfills in North Texas has led to sharp public concern. Seep waters exhibit somewhat low pH (6.3), reduced state (Eh = -100 mV), high iron (40 ppm), elevated arsenic (105 ppb), barium (634 ppb), manganese (3 ppm), bromine (9 ppm), and salinity (TDS = 1380 ppm), an iridescent sheen and sulfurous odor. Bright orange colloids precipitate soon after the seeps emerge from the streambank, reminiscent of acid mine drainage. Other indicators of landfill leachate are absent, however, e.g., elevated Cr-Pb-Zn-Cu, PAHs. Ample natural sources of iron and arsenic are present. The bedrock is Cretaceous chalk, and SEM/microprobe analysis reveals considerable pyritization of foraminifera tests, with framboidal texture indicating microbial deposition. These diagenetic sulfide grains were subsequently altered to iron oxyhydroxides (FeOOHₓ). Electron microprobe profiles reveal primarily iron and sulfur in the original grains, and primarily iron in the weathered rinds. Arsenic is distributed evenly in both phases, but is absent in surrounding carbonate. Presumably, dissolution of the FeOOHₓ simultaneously releases iron and arsenic to form the seep discharge. Reducing acidic conditions (e.g., acid mine drainage or landfill leachate contribution from up-gradient landfills) allow this breakdown and transport. As a result, further sulfide minerals may breakdown as conditions transition from a reducing to oxidizing environment (e.g., shallow aquifer and surficial weathering) creating a positive feedback loop to sulfide decay. Some type of microbial seeding from the landfills may explain their spatial association with apparently natural Fe-rich seeps.