Breton W. Bruce

Denitrification in marine shales in northeastern Colorado

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 <1000 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+, δ15N[NO3−], δ15N[N2], and δ15N[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 of the order of 0.04–0.4 yr−1, whereas the potential rate constant in the shale based 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 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− advection.

Shallow ground-water quality beneath a major urban center: Denver, Colorado, USA

Abstract A survey of the chemical quality of ground water in the unconsolidated alluvial aquifer beneath a major urban center (Denver, Colorado, USA) was performed in 1993 with the objective of characterizing the quality of shallow ground-water in the urban area and relating water quality to land use. Thirty randomly selected alluvial wells were each sampled once for a broad range of dissolved constituents. The urban land use at each well site was sub-classified into one of three land-use settings: residential, commercial, and industrial. Shallow ground-water quality was highly variable in the urban area and the variability could be related to these land-use setting classifications. Sulfate (SO 4 ) was the predominant anion in most samples from the residential and commercial land-use settings, whereas bicarbonate (HCO 3 ) was the predominant anion in samples from the industrial land-use setting, indicating a possible shift in redox conditions associated with land use. Only three of 30 samples had nitrate concentrations that exceeded the US national drinking-water standard of 10 mg l −1 as nitrogen, indicating that nitrate contamination of shallow ground water may not be a serious problem in this urban area. However, the highest median nitrate concentration (4.2 mg l −1 ) was in samples from the residential setting, where fertilizer application is assumed to be most intense. Twenty-seven of 30 samples had detectable pesticides and nine of 82 analyzed pesticide compounds were detected at low concentrations, indicating that pesticides are widely distributed in shallow ground water in this urban area. Although the highest median total pesticide concentration (0.17 μg l − ) was in the commercial setting, the herbicides prometon and atrazine were found in each land-use setting. Similarly, 25 of 29 samples analyzed had detectable volatile organic compounds (VOCs) indicating these compounds are also widely distributed in this urban area. The total VOC concentrations in sampled wells ranged from nondetectable to 23 442 μg l − . Widespread detections and occasionally high concentrations point to VOCs as the major anthropogenic ground-water impact in this urban environment. Generally, the highest VOC concentrations occurred in samples from the industrial setting. The most frequently detected VOC was the gasoline additive methyl tert -butyl ether (MTBE, in 23 of 29 wells). Results from this study indicate that the quality of shallow ground water in major urban areas can be related to land-use settings. Moreover, some VOCs and pesitides may be widely distributed at low concentrations in shallow ground water throughout major urban areas. As a result, the differentiation between point and non-point sources for these compounds in urban areas may be difficult.