Environ. Sci. Technol. 1999, 33, 21-25
N2O Emissions from a Nitrogen-Enriched River P. B. MCMAHON* AND K. F. DENNEHY U.S. Geological Survey, Denver Federal Center, Mail Stop 415, Denver, Colorado 80225
Nitrous oxide (N2O) emissions from the South Platte River in Colorado were measured using closed chambers in the fall, winter, and summer of 1994-1995. The South Platte River was enriched in inorganic N (9-800 µM) derived from municipal wastewater effluent and groundwater return flows from irrigated agricultural fields. River water was as much as 2500% supersaturated with N2O, and median N2O emission rates from the river surface ranged from less than 90 to 32 600 µg-N m-2 d-1. Seventy-nine percent of the variance in N2O emission rates was explained by concentrations of total inorganic N in river water and by water temperature. The estimated total annual N2O emissions from the South Platte River were 2 × 1013-6 × 1013 µg-N yr-1. This amount of annual N2O emissions was similar to the estimated annual N2O emissions from all primary municipal wastewater treatment processes in the United States (1). Results from this study indicate that N-enriched rivers could be important anthropogenic sources of N2O to the atmosphere. However, N2O emission measurements from other N-enriched rivers are needed to better quantify this source.
Introduction Nitrous oxide (N2O) plays an important role as a greenhouse gas and as a catalyst in the destruction of ozone in the stratosphere (2, 3). Concentrations of N2O in the atmosphere are increasing at a rate of about 0.3% per year (4). Atmospheric N2O is derived from natural and anthropogenic sources, but anthropogenic sources are the only ones that are likely to be reduced through improvements in technology or changes in land use. Several anthropogenic sources of N2O to the atmosphere have been identified, including wastewater treatment plants and agricultural soils (1, 5). Nitrogen (N) from wastewater treatment plants and agricultural fertilizers is a leading pollutant of rivers and lakes in the United States (6), and supersaturated aqueous concentrations of N2O have been reported in these aquatic environments (7, 8). Thus, surface water impacted by these land-use activities could be an important anthropogenic source of N2O to the atmosphere. This paper presents measurements of N2O emissions to the atmosphere from the South Platte River in Colorado. The South Platte River receives treated wastewater effluent from several municipalities (including the city of Denver, CO), and it receives groundwater return flows from irrigated agricultural fields. Few data exist for N2O emissions from rivers (8); therefore, this paper is intended as a first step in understanding the role of N-enriched rivers as N2O sources to the atmosphere. * Corresponding author phone: (303)236-4882, ×286; fax: (303)2364912; e-mail:
[email protected]. 10.1021/es980645n Not subject to U.S. Copyright. Publ. 1999 Am. Chem. Soc. Published on Web 11/14/1998
Experimental Section Site of Investigation. The South Platte River drains an area of about 63 000 km2 in parts of CO, NE, and WY (Figure 1). The annual mean flow in the river is about 11.1 m3 s-1 near Denver, CO and it increases to about 22.9 m3 s-1 at Kersey, CO. These gains result from sewage-treatment effluent, tributary, and groundwater additions. Denver’s largest wastewater treatment plant (serving 1.3 million people) discharges to the South Platte River about 18 km upstream from N2O measurement site no. 8 (Figure 1). Effluent from the plant is a major source of ammonium and nitrate to the river, and the effluent flow can constitute greater than 90% of the river’s discharge at the plant during low flow (9). The annual mean flow in the river decreases downstream from Kersey to about 6.12 m3 s-1 at North Platte, NE because of irrigation diversions. During irrigation season, a substantial portion of the water in the river downstream from Kersey is groundwater return flows from irrigated fields that overlie the alluvial aquifer adjacent to the river. The groundwater return flows are a source of nitrogen (as nitrate) to the river in this reach (10). Based on 10 years of streamflow data (19831993), Sjodin et al. (11) estimated that 21% of the flow in the river between sites 3 and 5 (Figure 1) was from groundwater return flows during the peak irrigation months of July through September. The groundwater contribution to streamflow decreased to about 13% during the rest of the year because of increased additions from snowmelt runoff from the Rocky Mountains, increased rainfall, and because less water is diverted for irrigation. The river is confined to a single channel upstream from N2O measurement site no. 7 (Figure 1), whereas the channel is primarily braided downstream from site no. 7. The total wet channel width ranged from about 20-80 m. The water depth at the N2O measurement sites ranged from 0.03 to 0.82 m. Sample Collection. Measurements of N2O emissions from water surfaces of the South Platte River to the atmosphere were made using closed chambers in October 1994, February 1995, and August 1995 at nine sites along a 733-km reach of river extending from near the mouth of the South Platte River in North Platte, NE to upstream from Denver, CO (Figure 1). Nitrous oxide emission measurements were made at 3-5 locations across the river channel at each site, with the number of locations depending on the channel width. The acrylic chamber (0.0085-m3 volume, 0.06-m2 surface-area of opening) was suspended from a tripod and lowered only 1-2 mm into the water column, resulting in minimal disruption of the water surface. The chamber was tapered at two ends (canoe shaped) to further minimize disruption of the water surface. The chamber-tripod assembly was covered with a tarp during the period of sample collection to minimize heating inside the chamber. The headspace temperature in the chamber was monitored to verify that heating above ambient temperatures did not occur. The chamber headspace was continuously mixed by the natural flow of the river and by manually pumping with a syringe prior to sampling. At each measurement location, a 10-mL gas sample was collected using a syringe from a port located at the top center of the chamber every 6 min during a 24-min period (total of five gas samples per location). The samples were stored at 4 °C in 10-mL glass vials that were preevacuated and sealed with thick rubber stoppers. Samples were analyzed within 1 week of collection. Time-series measurements indicated that N2O samples remained stable in the vials for at least 3 weeks. VOL. 33, NO. 1, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Map showing the location of the South Platte River Basin, generalized land use in the basin, and N2O sampling sites along the South Platte River. Sampling site numbers increase in an upstream direction. Water for dissolved N2O analyses was collected about 5 mm below the river surface in a syringe, injected into 25-mL glass vials that were preflushed with helium, the vials were sealed with thick rubber stoppers and stored at 4 °C until analysis. Ten milliliters of helium was removed from the vial with a syringe prior to injecting a 10-mL water sample in the vial. Measurements of air temperature, water temperature, water velocity, and water depth were made at each N2O emission-measurement location in the channel. Measurements of water temperature in the river and dissolved nitrous oxide concentrations were used to determine the degree of N2O saturation of the water (12). A grab sample of water for the analysis of dissolved ammonium, nitrite, and nitrate was collected at mid channel at each site using a syringe, filtered (0.2 µm syringe filter), and stored at 4 °C in 125-mL plastic bottles. Sample Analysis. The N2O concentration in the gas collected from the chambers was determined by gas chromatography/63Ni-electron capture detection (Hewlett-Packard 5890 GC, 183 × 0.32-cm stainless steel column packed with 80/100 Porapak Q). (The use of name brands is for identification purposes and does not constitute endorsement by the U.S. Geological Survey.) After bringing the sample to room temperature, a gastight syringe was used to remove 250 µL of sample from the glass vial for direct injection in the GC. The precision of the N2O analyses was (2.5%, based on replicate analysis of samples and standards. Dissolved N2O concentrations were determined by equilibrating the liquid and headspace N2O concentrations at room temperature, analyzing N2O in the headspace, and back calculating the aqueous N2O concentrations (12). Water samples were analyzed for ammonium, nitrite, and nitrate at the U.S. Geological Survey National Water Quality Laboratory (13). Calculation of N2O Emission Rates. The N2O emission rate from the river surface to the closed chamber, Ec (µg-N m-2 d-1), was calculated using eq 1 and the appropriate 22
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conversion factors
Ec ) (∆c/∆t)(V/A)(P/RT)
(1)
where ∆c/∆t is the linear increase in N2O concentration in the chamber during the sampling period (ppmv d-1), V is the chamber volume (m3), A is the enclosed surface area (m2), P is the atmospheric pressure (atm), R is the universal gas constant (0.0821 L‚atm‚K-1‚mol-1), and T is the air temperature (K). Values of the coefficient of determination (r2) for linear regression of the concentration-versus-time data were greater than 0.95 for most data sets. Slopes of the linear regression fits having p-values > 0.05 were not considered to be significantly different from zero. Nitrous oxide emissions from the water surface also were estimated at a subset of sites using N2O saturation data and a generalized two-layer diffusion model (14). Nitrous oxide emission estimates made using the diffusion model were used to qualitatively evaluate the emission measurements made with the chamber. Statistical Analysis. A multiple regression analysis of N2O fluxes measured with the chamber versus total inorganic nitrogen (ammonium + nitrite + nitrate) in the river water, water temperature, water velocity, and water depth was done using the SAS statistical package (15).
Results Rates of N2O emission to the atmosphere from the South Platte River generally increased with the degree of N2O saturation of the river water (Figure 2a). River water at the sites having the largest N2O emission rates was as much as 2500% supersaturated with N2O. Small positive emission rates associated with some N2O-undersaturated water probably resulted from ebullition within the chamber. The chamber measurements were within a factor of 5 of the diffusionmodel estimates at small emission rates and within a factor
FIGURE 2. (a) N2O emission rates measured using the closed chamber versus percent N2O saturation of river water. MDL refers to the method detection level for N2O emission measurements. (b) N2O emission rates measured using the closed chamber versus N2O emission rates calculated using a two-layer diffusion model (14). The lines labeled 1:1, 2:1, and 5:1 define three chamber:diffusionmodel emission rate ratios. of 2 at larger emission rates (Figure 2b). Overall, the emission rates determined using the two methods compared favorably, with 60% of the paired measurements being within a factor of 2 of each other. Median N2O emission rates at the nine channel cross sections ranged from
