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Dentel, S. K.; Gossett, J. M. J.-Am. Water Works Assoc. 1988,80, 187-198. Matijevic, E.; Mangravite,F. J., Jr.; Cassell,E. A. J. Colloid Interface Sci. 1971, 35, 560-568. Letterman, R. D.; Vanderbrook, S. G.; Sricharoenchaikit, P. J.-Am. Water Works Assoc. 1982, 74, 44-51. Dempsey, B. A.; O’Melia,C. R. In Aquatic and Terrestrial Humic Materials; Christman, R. F., Gjessing, E. T., Eds.; Ann Arbor Science: Ann Arbor, MI, 1983; pp 239-273. Tanford, C. Physical Chemistry of Macromolecules;Wiley: New York, 1961. Stumm, W.; O’Melia, C. R. J.-Am. Water Works Assoc. 1968,60, 514-525.
(26) Bottero, J. Y.; Tchoubar, D.; Cases, J. M.; Fiessinger, F. J. Phys. Chem. 1982,86, 3667. (27) Chern, J.-M. Master’s Thesis, University of Delaware, Newark, DE, 1987. (28) Dentel, S. K. In Proceedings of the AWWA Seminar on Influence of Coagulation on the Selection, Operation, and Performance o f Water Treatment Facilities; American Water Works Association: Denver, CO, 1987; pp 49-88.
Received for review September 17, 1987. Accepted January 26, 1988. This study was funded by the University of Delaware Research Foundation and by National Science Foundation Grant ECE - 8504898.
Potential Denitrification Rates in Deep Sediments from the Southeastern Coastal Plain James T. Morris,*,+Gary J. Whiting,+ and Francis H. Chapelle$ Department of Biology, University of South Carolina, Columbia, South Carolina 29208, arld Water Resources Division, U S . Geological Survey, Columbia, South Carolina 2920 1
rn Measurements of potential denitrification, by the acetylene block technique, and C 0 2 production were made in sediment samples from cores taken to a depth of 185 m on Parris Island, SC. A significant denitrification potential in sediments overlying the Floridan aquifer and in the Floridan aquifer itself was found. Denitrification rates in subsurface sediments (5-185 m) amended with 1mM NO< averaged 1.7 f 1.8 nmol of NzO g-l d-l (fl SD), whereas samples of organic rich surface soils gave mean rates from 117 f 71 to 173 f 39 nmol of NzO 8-l d-l. Depth-integrated rates indicate that potential denitrification in a 150-m column of subsurface sediment, excluding the surface layer, is 253 mmol of N 2 0 m-2 d-l. The surface layer has a potential rate of 74 mmol of NzO m-2 d-l. Denitrification rate was apparently limited by NO3concentration in assays with 1mM NO3-. C 0 2 and N 2 0 production showed parallel trends with depth; mean surface COzproduction was from 104 f 29 to 1034 f 307 nmol of COz g-l d-l and was greater than subsurface rates. The latter averaged 21 f 18 nmol of C 0 2g-l d-l between 5- and 185-m depth.
Introduction There is increasing concern about nitrate contamination of groundwater that results from a variety of activities including agricultural and waste disposal practices. A method of municipal and industrial waste water treatment that involves land irrigation is becoming common (1-3) and is a potential source of nitrate and numerous other groundwater pollutants. Nitrate contamination of groundwater may occur when rates of land application of waste water or fertilizers exceed a site-specific threshold (4, 5). For instance, NO3- concentrations of about 1.5 mmol/L in shallow groundwater beneath agricultural areas of the North Carolina coastal plain can be attributed to nitrogen fertilizer (6). However, the denitrification of NO3to N20 or N2 in subsurface sediments and aquifers could abate NO3- contamination of groundwater. Consequently, there is increasing interest in assessing the potential for denitrification in subsurface sediments (7,8) in order to t University of South Carolina. t U.S. Geological Survey.
832 Environ. Sci. Technol., Vol. 22, No. 7, 1988
design rational waste water disposal schemes, to implement programs for the treatment of contaminated groundwater, and to better understand the basic microbiology of subsurface environments. Our research was designed to measure potential rates of denitrification activity in deep sediments near the site of current land applications of municipal waste water on Hilton Head Island, SC. S t u d y Location
Sediment cores were collected on Parris Island in southeastern South Carolina (Figure 1). Surficial sediments are typically unconsolidated sands (Figure 2) of Pleistocene age that function as a water table aquifer. Underlying the water table aquifer are fine-grained clastic silts and clays of the Hawthorn Formation (Miocene) that function as a semiconfining layer. Immediately below the Hawthorn Formation lies the upper Floridan aquifer (9), which is a weakly cemented bryozoan limestone (biosparite) characterized by extensive secondary porosity and permeability. This unit is a highly productive aquifer and is the principal source of potable water for the communities on Hilton Head Island and Parris Island. The highly permeable upper Floridan aquifer is underlain by lowpermeability Eocene limestones (biomicrite) that function as a lower confining bed. There is presently a significant downward flux of groundwater (10-15 cm/yr) from the water table aquifer through the Hawthorn Formation to the upper Floridan aquifer (10). Furthermore, hydrologic evidence and geochemical evidence have demonstrated that this downward hydrologic flux has been maintained in this area throughout much of recent geologic history (11). Water movement in the Hawthorn semiconfining bed is predominantly vertical (downward). However, in the highly permeable upper Floridan aquifer, water movement is predominantly horizontal. Materials and Methods
Sediment cores were collected in November 1986 with a rotary-type core barrel about 3 m long and 10 cm in diameter. A metal core sleeve was employed that separated the core from the circulating drilling fluid. When the sediment-containing core barrel was removed from the
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Figure 1. Location of the core hole on Parris Island, SC.
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were taken at the surface and a t depths of 5,9, 17,32,47, 88, 125, and 185 m. Denitrificgtion rates were determined by the acetylene block technique (12-14). This technique blocks the microbial reduction of NzO to N2 by appropriate levels of acetylene, and the denitrification rate equals the rate of production of N 2 0 that is quantified by gas chromatography. From each depth sampled, two test tubes containing subsamples, previously purged with N P ,were injected with 2 mL of 1 mM KNO,, plus 1mL of C2H2,and gently mixed. A third was injected only with 2 mL of 1 mM NO
