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Perchlorate is a widespread environmental contaminant having both anthropogenic and natural sources. Stable isotope ratios of O and Cl in a given sample of perchlorate may be used to distinguish its source(s). Isotopic ratios may also be useful for identifying the extent of biodegradation of perchlorate, which is critical for assessing natural attenuation of this contaminant in groundwater. For this approach to be useful, however, the kinetic isotopic fractionations of O and Cl during perchlorate biodegradation must first be determined as a function of environmental variables such as temperature and bacterial species. A laboratory study was performed in which the O and Cl isotope ratios of perchlorate were monitored...
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Results of geophysical surveys in Indian River Bay, Delaware, indicate a complex pattern of salinity variation in subestuarine ground water. Fresh ground-water plumes up to about 20 meters thick extending hundreds of meters offshore are interspersed with saline ground water, with varying degrees of mixing along the salinity boundaries. It is possible that these features represent pathways for nutrient transport and interaction with estuarine surface water, but the geophysical data do not indicate rates of movement or nutrient sources and reactions. In the current study, samples of subestuarine ground water from temporary wells with short screens placed 3 to 22 meters below the sediment-water interface were analyzed...
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To better understand large-scale interactions between fresh and saline groundwater beneath an Atlantic coastal estuary, an offshore drilling and sampling study was performed in a large barrier-bounded lagoon, Chincoteague Bay, Maryland, USA. Groundwater that was significantly fresher than overlying bay water was found in shallow plumes up to 8 m thick extending more than 1700 m offshore. Groundwater saltier than bay surface water was found locally beneath the lagoon and the barrier island, indicating recharge by saline water concentrated by evaporation prior to infiltration. Steep salinity and nutrient gradients occur within a few meters of the sediment surface in most locations studied, with buried peats and estuarine...
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Groundwater from remote parts of the Middle Rio Grande Basin in north-central New Mexico has perchlorate (ClO4-) concentrations of 0.12?1.8 ?g/L. Because the water samples are mostly preanthropogenic in age (0?28?000 years) and there are no industrial sources in the study area, a natural source of the ClO4- is likely. Most of the samples have Br-, Cl-, and SO42- concentrations that are similar to those of modern bulk atmospheric deposition with evapotranspiration (ET) factors of about 7?40. Most of the ET values for Pleistocene recharge were nearly twice that for Holocene recharge. The NO3-/Cl- and ClO4-/Cl- ratios are more variable than those of Br-/Cl- or SO42-/Cl-. Samples thought to have recharged under the...
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