Parts of the South Platte River alluvial aquifer in northeastern Colorado are underlain by the Pierre Shale, a marine deposit of Late Cretaceous age that is more than 1,000 m thick. Ground water in the aquifer is contaminated with NO3-, and the shale contains abundant potential electron donors for denitrification in the forms of organic carbon and sulfide minerals. Nested piezometers were sampled, pore water was squeezed from cores of shale, and an injection test was conducted to determine if denitrification in the shale was a sink for alluvial NO3- and to measure denitrification rates in the shale. Measured values of NO3-, N2, NH4+, delta-15N[NO3-], delta-15N[N2], and delta-15N[NH4+] in the alluvial and shale pore water indicated that denitrification in the shale was a sink for alluvial NO3-. Chemical gradients, reaction-rate constants, and hydraulic-head data indicated that denitrification in the shale was limited by the slow rate of NO3- transport (possibly by diffusion) into the shale. The apparent in situ first-order rate constant for denitrification in the shale based on diffusion calculations was on the order of 0.04 to 0.4 yr-1, whereas the potential rate constant in the shale based on injection tests was on 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 ferrous iron was a minor electron donor in the process. Flux calculations for the conditions encountered at the site indicate that denitrification in the shale could remove only a small fraction of the annual agricultural NO3- input to the alluvial aquifer. However, the relatively large potential first-order rate constant for denitrification in the shale indicated that the percentage of NO3- uptake by the shale could be considerably larger in areas where NO3- is transported more rapidly into the shale by advection.
1 U.S. Geological Survey, Box 25046, MS 415, Denver Federal Center, Lakewood, CO 80225
2 U.S. Geological Survey, Reston, Virginia