SULFUR GEOCHEMISTRY IN AN ALLUVIAL AQUIFER AFFECTED BY LANDFILL LEACHATE—A CASE STUDY FROM NORMAN OK, USA

Sulfate is an important electron acceptor for bacterial degradation of organic matter in landfill leachate, therefore, processes controlling its distribution and availability are important in studying leachate attenuation. These processes are similar in contaminated and uncontaminated ground water, pore water, and alluvium along a transect from the closed Norman, Oklahoma landfill to the nearby Canadian River. Sulfur isotope systematics differentiated complex processes of natural sulfur cycling and the impact of landfill leachate.

Regional ground water and river water contain 100 to 500 mg/L sulfate with d34SO4 of approximately 10 to 12‰. Contaminated ground water contains less sulfate. A Rayleigh fractionation model describes the d34SO4 as groundwater sulfate concentrations decrease. In some contaminated wells, sulfate is <5 mg/L, suggesting extensive sulfate reduction, yet isotopic compositions are near 12‰. We hypothesize that this sulfate is from dissolution of barite (identified by SEM analysis) or sulfate released from organosulfur compounds in within the leachate (sulfur concentration in dissolved organic matter is about 2%).

Natural sulfur cycling in alluvium produces variable isotopic compositions. When porewater sulfate is >20 mg/L, d34S pyrite values are negative (-24 to -9‰); when porewater sulfate is <2 mg/L, d34S pyrite values fall within a greater range (-24 to +12‰). Acid-volatile sulfides (FeS) are always 0 to 15‰ more positive than coexisting pyrite. Isotopically heavier FeS is likely a recent phase, whereas pyrite accumulates throughout burial history.

The amount of pyrite or FeS does not differentiate contaminated from uncontaminated alluvium. However, the isotopic composition of sulfides in uncontaminated alluvium (pyrite median -15‰; FeS median -10‰) is significantly more negative than in contaminated alluvium (pyrite median -5‰; FeS median 0‰). These differences are attributed to availability of organic matter in the leachate, which allows for more complete reduction of sulfate