Denitrification in marine shales in northeastern Colorado

Parts of the South Platte River alluvial aquifer in northeastern Colorado a re underlain by the Pierre Share, a marine deposit of Late Cretaceous age t hat is <1000 m thick. Ground water in the aquifer is contaminated with NO3- , and the shale contains abundant potential electron donors for denitrifica tion in the forms of organic carbon and sulfide minerals. Nested piezometer s were sampled, pore water was squeezed from cores of shale, and an injecti on test was conducted to determine if denitrification in the share was a si nk for alluvial NO3- and to measure denitrification rates in the shale. Mea sured values of NO3-, N-2, NH4+, delta(15)N[NO3-], delta(15)N[N-2], and del ta(15)[NH4+] in the alluvial and shale pore water indicated that denitrific ation in the shale was a sink for alluvial NO3-. Chemical gradients, reacti on rate constants, and hydraulic head data indicated that denitrification i n the share was limited by the slow rate of NO3- transport (possibly by dif fusion) into the shale. The apparent in situ first-order rate constant for denitrification in the shale based on diffusion calculations was of the ord er of 0.04-0.4 yr(-1), whereas the potential rate constant in the shale bas ed on injection tests was of the order of 60 yr(-1). Chemical data and mass balance calculations indicate that organic carbon was the primary electron donor for denitrification in the shale during the injection test, and ferr ous iron was a minor electron donor in the process. Flux calculations for t he conditions encountered at the site indicate that denitrification in the shale could remove only a small fraction of the annual agricultural NO3- in put to the alluvial aquifer. However, the relatively large potential first- order rate constant for denitrification in the shale indicated that the per centage of NO3- uptake by the shale could be considerably larger in areas w here NO3- is transported more rapidly into the share by advection.