RED COLORATION WITH ELEVATED NO3, SE, SO4, TOC, AND U IN MANCOS SHALE SEEPAGE, SOUTHWESTERN UNITED STATES

Surface water in swamps and bogs is often colored yellow or brown from dissolved humic materials. While common in humid regions, such coloration is less common in arid desert areas. However, groundwater seeping from the Mancos Shale throughout much of its depositional basin in the arid southwestern United States has a distinctive yellow-red coloration. Field relationships indicate that the yellow-red color derives from contact with Mancos Shale. The seepage also has elevated concentrations of organic carbon, nitrate, selenium, sulfate, and uranium, but low iron concentrations. The sulfate is characterized by low δ³⁴S values and the uranium by high ²³⁴U/²³⁸U activity ratios; both are consistent with a Mancos Shale origin. We used extraction and chemical oxidation methods to characterize the yellow-red color and to isolate the color-forming constituents. Filtering and dialysis tests indicate that organic carbon moieties smaller than 1000 Da cause the coloration. Tests with peroxide and bleach indicate that a portion of the color-forming constituents are readily oxidized. Adsorption to various resins suggests humic-based compounds. The combined results indicate that the color-forming constituents may be fulvic acid. The color‑forming moieties were isolated by adsorption to an acrylic ester‑based commercial adsorbent (DAX-8) followed by elution with NaOH and pH adjustment with a commercial resin (OnGuard II H). This method caused the color to increase by a factor of 5 and the salt content to decrease nearly 40-fold.