Environ. Sci. Technol. 1996, 30, 1152-1162
Denitrification and Nitrogen Transport in a Coastal Aquifer Receiving Wastewater Discharge LESLIE A. DESIMONE* Water Resources Division, U.S. Geological Survey, 28 Lord Road, Suite 280, Marlborough, Massachusetts 01752
BRIAN L. HOWES Biology Department, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543
Denitrification and nitrogen transport were quantified in a sandy glacial aquifer receiving wastewater from a septage-treatment facility on Cape Cod, MA. The resulting groundwater plume contained high concentrations of NO3- (32 mg of N L-1), total dissolved nitrogen (40.5 mg of N L-1), and dissolved organic carbon (1.9 mg of C L-1) and developed a central anoxic zone after 17 months of effluent discharge. Denitrifying activity was measured using four approaches throughout the major biogeochemical zones of the plume. Three approaches that maintained the structure of aquifer materials yielded comparable rates: acetylene block in intact sediment cores, 9.6 ng of N cm-3 d-1 (n ) 61); in situ N2 production, 3.0 ng of N cm-3 d-1 (n ) 11); and in situ NO3- depletion, 7.1 ng of N cm-3 d-1 (n ) 3). In contrast, the mixing of aquifer materials using a standard slurry method yielded rates that were more than 15-fold higher (150 ng of N cm-3 d-1, n ) 16) than other methods. Concentrations and δ15N of groundwater and effluent N2, NO3-, and NH4+ were consistent with the lower rates of denitrification determined by the intact-core or in situ methods. These methods and a plumewide survey of excess N2 indicate that 2-9% of the total mass of fixed nitrogen recharged to the anoxic zone of the plume was denitrified during the 34-month study period. Denitrification was limited by organic carbon (not NO3-) concentrations, as evidenced by a nitrate and carbon addition experiment, the correlation of denitrifying activity with in situ concentrations of dissolved organic carbon, and the assessments of available organic carbon in plume sediments. Carbon limitation is consistent with the observed conservative transport of 85-96% of the nitrate in the anoxic zone. Although denitrifying activity removed a significant amount (46250 kg) of fixed nitrogen during transport, the effects of aquifer denitrification on the nitrogen load to receiving ecosystems are likely to be small ( 10 mg of N L-1; 6), but even much lower levels can result in the degradation of receiving waters. Nitrogen limits phytoplankton and macroalgal productivity in many coastal ecosystems (7, 8) and is the primary limiting nutrient for salt marsh vegetation. Because of the increasing levels of nitrogen loading to coastal watersheds, an accurate assessment of NO3- removal during transport through the aquifer is critical to the management and study of nitrogen inputs to coastal waters. The occurrence and potential for denitrification in aquifer sediments and the various factors controlling microbial activity have been demonstrated by numerous recent studies (9-12). Typically, denitrifying bacteria are facultative anaerobes, using NO3- as the terminal electron acceptor in organic carbon (or reduced iron or sulfur) oxidation after O2 is depleted. Rates of denitrification in the subsurface depend on the concentrations and distribution of the primary rate-limiting substances, NO3-, O2, and the electron donor (usually organic carbon) in the aquifer (13). In oxygenated sediments, denitrification is generally limited by energy yield, with aerobic respiration dominating microbial carbon remineralization. In groundwater systems with sufficient organic matter, aerobic respiration can result in zones of O2 depletion (14) and a shift to NO3- as the terminal electron-acceptor pathway. However, in depleting the O2 level, aerobic respiration also may consume much of the labile organic carbon pool and limit the ensuing rates of denitrification (15). Through these recent studies, the potential processes controlling denitrification rates at individual aquifer sites have been detailed. However, few studies have expanded the site-specific, process-level measurements to entire watersheds or large groundwater plumes. Yet the larger areal measure is required to evaluate the role of aquifer denitrification in the interception of groundwater-transported nitrogen, to determine biogeochemical balances, and to make land-use management decisions. Larger spatial-scale measures can be hampered by extremely variable denitrification rates (16, 17) and, in some instances, by methodological problems such as the lack of comparable results from in situ and laboratory approaches (18). In addition, the mass of nitrogen in an aquifer usually is not well constrained due to inadequate data on nitrogen inputs and/or physical transport processes (16). Nitrogen inputs, groundwater distributions, and rates of removal by denitrification are all necessary to assess the effect of denitrification on fixed nitrogen transport in a groundwater system. We studied denitrification in a large nitrogen-enriched groundwater plume in order to determine the quantitative importance of denitrification to nitrogen transport through the aquifer. The groundwater plume resulted from waste* Corresponding author e-mail address:
[email protected]; telephone: (508) 490-5023; fax: (508) 490-5068.
1152
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 30, NO. 4, 1996
0013-936X/96/0930-1152$12.00/0
1996 American Chemical Society
TABLE 1
Dissolved Oxygen, Chloride, Nitrogen Species, and Organic Carbon in Effluent and Groundwater (mg L-1) groundwatera plume constituent
effluentb
dissolved O2 Clparticulate organic nitrogen dissolved organic nitrogen NH4+-N NO2--N NO3--N total nitrogen particulate organic carbon dissolved organic carbon total organic carbon
NDc 950 3.1 4.0 27 0.76 9.4 46.0 9 14 24
anoxic suboxic zone zone ambient 0.05 860 ND 0.5 6.4 0.08 32 40.5 ND 1.9 1.9
3.4 390 ND 0.06 0.02 0.02 12 12.2 ND 0.5 0.5
6.8 21 ND 0.05 0.02
