It may be possible to determine the relative contributions of the major sources of nitrate in river water using the stable isotopic ratios d15N and d18O of the nitrate ion. A few researchers have used the d15N and/or d18O isotope ratios to determine sources of nitrate in ground water, headwater catchments, and small rivers, but little is known about the isotopic composition of nitrate in larger rivers. The objective of this study is to measure the isotopic composition of nitrate and suspended organic matter in the Mississippi River and its major tributaries, in discharge to the Gulf of Mexico, and in streamflow from smaller watersheds that have distinct sources of nitrogen (row crops, animal wastes, and urban effluents) or are minimally impacted by man (undeveloped). Samples from seven sites on the Mississippi River and its tributaries and from 17 sites in smaller watersheds within the Mississippi River basin will be analyzed for d15N and d18O of dissolved nitrate. Suspended sediment collected from these sites will also be analyzed to determine the d15N, d13C, and d34S of the suspended organic material. Six samples will be collected at each site during the winter, spring, and summer of 1996-97. Results from these samples will be used to identify seasonal and flow-related variability in d15N and d18O ratios from each site and may also help determine the principal sources of the nitrate entering the Gulf of Mexico.
Nitrate along with other nutrients are suspected of being responsible for a large zone along the Louisiana-Texas coast where dissolved oxygen (DO) levels in bottom water of the Gulf are seasonally lowered (zone of hypoxia) (Justic and others, 1993; Justic and others, 1994; Turner and Rabalais, 1991; Rabalais and others, 1996). The seasonal reduction in DO occurs each year during late spring and summer following high inflows of freshwater and nutrients to the Gulf. The zone of hypoxia covered nearly 17,000 square kilometers following the 1993 flood, twice the size of Chesapeake Bay. The zone of hypoxia was reported to be as large or larger in 1994 and 1995 (Rabalais and others, 1995). Estimates of the size of the zone of hypoxia prior to the 1993 flood (1985-1992) averaged about 10,000 square kilometers (Rabalais and others, 1995).
While most of the inputs of nitrogen to the Mississippi basin can be estimated and the outputs in surface water can be measured, the actual sources of the nitrate transported by the Mississippi River are unknown. How much is from this year's fertilizer? from last year's fertilizer flushed from the soil zone? from manure? legumes? natural sources? Of an estimated 11.6 million metric tons of nitrogen added annually to the Mississippi and Atchafalaya basins, approximately 51 percent is from commercial fertilizer, 30 percent is from livestock manure, 9 percent is from fixation by legumes, 5 percent is from human domestic waste, and 4 percent is deposited by precipitation. Municipal and industrial point discharges of nitrogen to rivers are estimated to contribute only 2 and 1 percent, respectively, to the total annual loading of nitrogen in the Mississippi basin. Municipal and industrial point discharges of nitrogen are often directly to rivers, whereas the other potential nitrogen sources are applied or generated at the land surface. Municipal and industrial point discharges of nitrogen could be the source of as much as 37 percent of the nitrate discharged to the Gulf of Mexico.
The contribution from nitrification of soil nitrogen to surface water or ground-water nitrate is difficult to estimate. Although the soil is a large reservoir of N, the amount lost or gained by this reservoir on an annual basis is unknown. Contributions of soil organic nitrogen to surface water or ground-water nitrate are difficult to quantify because they are highly variable and dependent on climatic and cultural factors (Cheng and others, 1964; Bremner and Tabatabai, 1973: Edwards, 1973; Heaton, 1986; Hubner, 1986). Results of laboratory analysis of soil samples are often not representative of field conditions (Letolle, 1980). Several researchers have argued that mineralization and nitrification of soil organic nitrogen is the predominant source of nitrate to surface waters (Bremner and Tabatabai, 1973; Edwards, 1973; Heaton, 1986). Nitrogen in mineral soils can be in the inorganic form, but generally most is organic nitrogen in the form of plant debris that must be nitrified before it can be utilized by plants (Buckman and Brady, 1970). If nitrification of soil organic nitrogen was a significant source of nitrate to surface waters, one would expect that substantial concentrations of nitrate would be observed in some surface waters draining undeveloped land. However, elevated nitrate concentrations in water associated with natural organic nitrogen sources in undisturbed environments are documented in only a few exotic settings such as deserts and limestone caves populated by large numbers of bats (Hem, 1985). Mueller and others (1995) reported that concentrations of nitrate in surface waters of the United States draining undeveloped land exceeded 1 mg/L in less than 10 percent of 3,751 samples, whereas nitrate in surface waters draining agricultural and urban land exceeded 1 mg/L in about 50 percent of 7,656 samples. Smith and others (1993) reported that less than 10 percent of 171 sites that drain forest and range land had average nitrate concentrations that exceeded 1 mg/L, whereas average nitrate concentrations at sites that drain agricultural and urban land exceeded 1 mg/L at 35 percent of 112 agricultural and urban sites. Beisecker and Leifeste (1975) observed that nitrate concentrations in samples collected at "hydrologic benchmark stations" had median nitrate concentrations substantially below those in samples from major streams in the same general region. Benchmark stations were selected to represent conditions uninfluenced by human activity. Together, these studies indicate that natural soil organic nitrogen is not the source of elevated nitrate concentrations in rivers of the United States, and that inputs of nitrogen from agricultural and urban sources result in leaching or runoff of excess as nitrate in many watersheds. The bulk of plants grown in agricultural areas use commercial fertilizers or animal manure as a supplement to the nitrogen present in the soil, so in agricultural areas mineralization and nitrification of soil organic nitrogen from plant debris may contribute more significantly to nitrate concentrations in surface waters.
If the soil nitrogen reservoir was in a natural steady-state condition (that is, prior to any agricultural impacts), then the contribution of soil nitrogen to surface and ground water would equal the amount of nitrogen added by precipitation plus the amount fixed by native plants minus that utilized by native plants. The concentration of nitrogen in pre-industrial age precipitation is unknown, but is almost certainly less than the amount in present-day precipitation (Lynch and others, 1996). The amount of nitrogen entering tallgrass prairie streams has been estimated by Tate (1989) and Dodds and others (1996). Tate (1989) observed that nitrogen concentrations in streams increased during storm events, but that overall mean total nitrogen concentrations were small and similar between growing (0.087 mg/L) and dormant seasons (0.082 mg/L). Dodds and others (1996) estimated that the total annual export of nitrogen from a tallgrass prairie watershed via streams ranged from 0.01 to 6 percent of the nitrogen input from precipitation. Ten percent of current nitrogen inputs from precipitation in the Mississippi basin would represent a mass of nitrate equal to about 5 percent of the current flux to the Gulf of Mexico.
Few studies have attempted to identify sources of nitrate in surface water using stable isotopic ratios. Kohl and others (1971) used d15N ratios in a reduced soil sample and in raw fertilizer as end members and compared the values to the d15N in water samples collected from the Sangamon River near Lake Decatur, Ill. A simple mixing model suggested that the fertilizer contribution to nitrate in the river varied seasonally and peaked at 55% during the spring months. Their work was criticized for a lack of detail in the determination of the d15N value for soil N, failure to account for transformation of fertilizer nitrogen in the soil zone, failure to account for variability in the d15N of different fertilizers, and for failure to account for other sources of nitrogen such as precipitation (Edwards, 1973; Hauck and others, 1972, Freyer and Aly, 1974; Bremner and Tabatabai, 1973). Some of these criticisms appear justified, but statements such as "...there is no question that human and animal wastes and soil organic nitrogen all outweigh inorganic fertilizers as contributors to the general nitrate load of surface waters, even in agricultural counties..." (Edwards, 1973) are not substantiated.
Showers and others (1990) used the d15N ratio of nitrate in the Neuse River, N.C., to determine that the relative contributions from point and nonpoint sources varied by season and discharge rate and that the isotopic composition of nitrate was exponentially related to river discharge. They concluded that the mixing of point and non-point source nitrogen reservoirs was not entirely controlled by surface-water runoff of agricultural fertilizer and excess soil nitrate, but that non-point source nitrate passed through a reservoir (either ground water or wetlands) that modulated the mixing. Cravotta (1995) attempted to use the stable isotopes of carbon, nitrogen, and sulfur to identify sources of nitrogen in the Susquehanna River and found that variations in source isotopic compositions and transformation and fractionation during natural cycling of nitrogen prevented the accurate estimation of relative contributions of multiple nitrogen sources to nitrogen loads in streams. Kendall and others (1995b) used d18O and d15N to determine sources of nitrate in snowmelt runoff from three watersheds in the USA. They determined that most of the nitrate in early runoff was derived from the soil, and not from atmospheric nitrate released from the current year's snowpack. Kellman and Hillaire-Marcel (1996) used d15N to determine the importance of in-stream denitrification on the N-budget of a small watershed. They found that denitrification could be identified and was significant during dry conditions in late summer, but that on a yearly basis, in-stream denitrification did not significantly affect the N-budget. Ging and others (1996) used d15N and d18O of nitrate to determine sources of nitrate in two small streams in Austin, Tex. They concluded that the most likely sources were atmospheric nitrate, soil nitrate, and ammonium fertilizer.
Reliable identification of nitrogen source contributions in both two- and three-component mixing models require that the isotopic composition of the sources are stable and distinctive from one another. Various researchers (Aravena and others, 1993; Brandes and others, 1996; Heaton, 1986; Hubner, 1986; Kendall and others, 1995a; 1995b; and Letolle, 1980) have identified ranges for isotopic compositions of potential sources of nitrate (table 2; fig. 3). Nitrogen isotope values (d15N) are reported in per mil relative to the standard air (atmospheric nitrogen isotopic ratio) defined as 0 per mil; oxygen isotope values (d18O) are reported relative to the standard V-SMOW (Vienna Standard Mean Ocean Water) defined as 0 per mil. The d18O and d15N of nitrate from samples collected in several previous investigations are plotted on figure 3 . Values from surface water and precipitation samples are shown on figure 3a, and values from groundwater samples are shown on figure 3b. The ranges of isotopic ratios for the sources of nitrogen in table 2 and figure 3 show some overlap that might prevent accurate modeling of nitrogen sources.
The extent to which in-stream processes such as nitrification and denitrification affect the d18O and d15N of nitrate in large rivers like the Mississippi has not been determined. Analysis of data collecting in previous investigations suggests that there is not a significant loss in NO3 in river water as it flows down stream. Results from six Mississippi River cruises that employed a Lagrangian sampling strategy showed little change in nitrate concentrations in the Mississippi River from its confluence with the Ohio River to New Orleans (Moody, 1993; Brinton et al., 1995; Garbarino et al., 1995). The transport (flux) of nitrate continued to increase down river. These results suggest that denitrification or other nitrogen transformations do not significantly alter the mass of nitrate as it is transported through the lower Mississippi River.
One way to assess whether the isotopic composition of nitrate has been affected by in-stream transformations is to analyze the isotopic composition of the suspended organic matter. The d15N values of organic matter from sewage, fertilizer, and soils show about the same values as the nitrate from these sources. However, since the particulate and dissolved organic matter that washes off the landscape is less biologically labile than the nitrate, the isotopic compositions of this material is less likely to be affected by the above processes. Hence, the d15N values of the organic matter can provide additional data about nitrate sources. The organic matter associated with different land uses can have very distinct isotopic compositions. Analysis of the carbon, nitrogen, and sulfur isotopic compositions of the organic matter may provide a more precise characterization of the land use where the material originated than can be obtained using the d18O and d15N of dissolved nitrate. For example, organic matter associated with corn fields has a d13C different from organic matter from wheat fields, legumes have a lower d15N value than other crops, and rice fields probably have lower d34S values than crops grown under drier conditions.
In the proposed study, both water samples and suspended organic matter will be analyzed for stable isotopes. Water samples will be analyzed for d18O and d15N in the nitrate ion. Suspended sediment samples will be analyzed for d15N, d13C, and d34S in the organic matter. Information on the stable isotopic composition of the organic matter will allow an independent assessment of the extent of biological transformations, and will provide additional information on the sources of the suspended organic loads that are transported to the Gulf along with nitrate. These data will also provide an important link between the USGS study of nutrient sources in the Mississippi River Basin and the study proposed by Dr. Cifuentes at Texas A&M, for identifying sources of nutrients responsible for oxygen consumption in the Gulf itself.
1. There are significant temporal and spatial variations in observed d15N and d18O of nitrate in water and the d15N, d13C, and d34S of suspended organic matter from the Mississippi River and its major tributaries.
2. Small streams draining areas of distinctly different land use (corn and soybean production, livestock production, urban land, or undeveloped land) will have distinctly different isotopic ratios.
3. The d15N and d18O ratios of nitrate
in water and the d15N, d13C, and d34S
ratios of suspended organic matter from the Mississippi River can be used to
determine the principal sources of the nitrate entering the Gulf of Mexico.
Samples will also be collected from smaller basins most of which are within NAWQA study units in the Mississippi River basin. These sites were selected to distinctly represent one of four land-use classes: land in rowcrop production; land in hog, cattle, or poultry (livestock) production; urban land; or undeveloped land. Isotopic ratios from samples collected at these sites will define the ranges of isotopic ratios expected in smaller rivers dominated by a single source of nitrate. Sites that represent each of the four land-use classes will be distributed among the Ohio, Missouri, and Mississippi River basins. Sites to be sampled are listed in table 4 and shown in figure 4.
Dissolved Nitrate: Samples will be collected with a depth integrating sampler (where conditions are appropriate for this collection method) from three or more verticals using NASQAN/NAWQA protocols (Shelton, 1994). Samples from the vertical profiles will be composited in a glass, polyethylene, or Teflon container. All sampling equipment will be cleaned with non-phosphate detergent, rinsed thoroughly with tap water, and then rinsed with distilled/deionized water. Samples will be filtered through a 0.45-micron cartridge filter into 1-liter or 1-gallon pre-cleaned polyethylene bottles, chilled without preservative, and sent on ice to the USGS laboratory in Missouri. If the filters are clogging, samples can be pre-filtered using a glass-fiber filter. All sample bottles will be labeled "ISO", and should include the site name, site id, date, and time of sample collection.
About 200 µmol of nitrogen as N2 (about 5 mg of nitrogen) are required for the isotopic analysis. Most of this is needed for determining the d18O of nitrate. Table 5 indicates the volume of sample to be collected and shipped to the Missouri laboratory for various expected concentrations of nitrate. This table should be used to determine sample volumes required for the sites listed in table 4, whereas table 6 indicates the volume of sample to be collected at the sites listed in table 3, by sampling set. Sample collection will not be concurrent with NAWQA or NASQAN activities at a few sites; therefore, other water-quality data will not be available for these samples. Samples will be collected for nutrient analysis (schedule 2702, requires two 125-ml bottles, one filtered and one unfiltered), major ions (schedule 2701, requires one 250-ml filtered acidified bottle, one 250-ml raw untreated bottle, and one 500-ml filtered untreated bottle), and dissolved organic carbon (schedule 2085, requires one 100-ml bottle) at these sites. Sample bottles will be labeled "ISO_NUT", "ISO_ION, and "ISO_DOC", respectively, and include the site name, site id, date, and time of sample collection. Samples will be sent to the USGS National Water Quality Lab (NWQL) using the standard procedure.
Suspended Organic Material: Suspended sediment for isotopic analysis of d15N, d13C, and d34S of the suspended organic material will be collected at each site. Approximately 1 liter of water will be filtered through a 0.7 micrometer heat-cleaned glass-fiber filter (142 mm diameter) using a peristaltic pump and an aluminum plate filter. After filtration of the sample the glass fiber filter will be placed on a small sheet of clean aluminum foil using tweezers. The filter will be folded in half, and then into quarters using the tweezers, keeping the sediment on the inside. The filter will be wrapped in the aluminum foil, and labeled with the station name, ID number, sampling date, and time. The filter will be placed in a ziplock bag, chilled after collection, and frozen upon returning to the office. After several filters have been collected, they will be shipped to the USGS Regional Office in Denver, Colo. for cataloging, and then on to the National Research Program laboratory in Menlo Park, Calif. for isotopic analysis.
Quality Assurance: Quality assurance and quality control
(QA/QC) samples will be collected at selected sites to provide information
on the variability and bias of the measured isotopic ratios. These samples
will consist exclusively of concurrent replicates, which are two or more samples
that are collected as closely as possible in time and space, but processed,
handled, and analyzed separately. Collection of concurrent replicate samples
requires two separate passes at each vertical in the cross section, to be composited
in separate vessels. Table 7 gives the schedule for collection of QA/QC samples.
QA/QC samples should be labeled with sampling times that are later than the
primary sample in order to distinguish them from each other. 
A Geographic Information System (GIS) will be used to manage, analyze, and display data on site locations, isotopic ratios, and nitrogen sources. Information on several nitrogen sources will be updated, with newly available data in an effort to improve the nitrogen loading estimates given in table 1.
The results of the analyses and evaluations of their interpretative value will be made available to the public. Results from the first phase of this project will determine if a second phase will be undertaken.
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William A. Battaglin
U.S. Geological Survey, WRD
P.O. Box 25046, MS 406
Denver, CO 80225
wbattagl@usgs.gov
