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Forensic Applications of Nitrogen and Oxygen Isotopes in Tracing Nitrate Sources in Urban Environments
Environmental Forensics (2002) 3, 125±130 doi:10.1006/enfo.2002.0086, available online at http://www.idealibrary.com on
Forensic Applications of Nitrogen and Oxygen Isotopes in Tracing Nitrate Sources in Urban Environments S. R. Silva* U.S. Geological Survey, Menlo Park, CA, U.S.A.
P. B. Ging U.S. Geological Survey, Austin, TX, U.S.A.
R. W. Lee U.S. Geological Survey, Dallas, TX, U.S.A.
J. C. Ebbert U.S. Geological Survey, Tacoma, WA, U.S.A.
A. J. Tesoriero U.S. Geological Survey, Raleigh, NC, U.S.A.
E. L. Inkpen U.S. Geological Survey, Tacoma, WA, U.S.A. (Received 30 June 2001, Revised manuscript accepted 22 December 2001) Ground and surface waters in urban areas are susceptible to nitrate contamination from septic systems, leaking sewer lines, and fertilizer applications. Source identi®cation is a primary step toward a successful remediation plan in aected areas. In this respect, nitrogen and oxygen isotope ratios of nitrate, in conjunction with hydrologic data and water chemistry, have proven valuable in urban studies from Austin, Texas, and Tacoma, Washington. In Austin, stream water was sampled during strem¯ow and base¯ow conditions to assess surface and subsurface sources of nitrate, respectively. In Tacoma, well waters were sampled in adjacent sewered and un-sewered areas to determine if locally high nitrate concentrations were caused by septic systems in the un-sewered areas. In both studies, sewage was identi®ed as a nitrate source and mixing between sewage and other sources of nitrate was apparent. In addition to source identi®cation, combined nitrogen and oxygen isotopes were important in determining the signi®cance of denitri®cation, which can complicate source assessment by reducing nitrate concentrations and increasing d15N values. The two studies illustrate the value of nitrogen and oxygen isotopes of nitrate for forensic applications in urban # Published by Elsevier Science Ltd. on behalf of AEHS. areas. Keywords: isotopes; nitrate; sewage; urban; source identi®cation.
Introduction
sewer systems or perhaps only better management practices in fertilizer applications on public and private lands. It is, therefore, necessary to understand the source and transport of nitrate contamination before an ecient remediation plan can be devised. Many studies have shown that stable isotope techniques are valuable tools for understanding nitrogen sources and cycling. This is true because certain sources of nitrate may be identi®ed by their characteristic or distinctive isotopic compositions. If the source is not distinctive, isotopes may still add unique and useful information in combination with other chemical and hydrologic data.
High concentrations of nitrate pose a direct health risk in drinking water and can lead to eutrophication in surface waters resulting in ecosystem damage (Spalding and Exner, 1993). Urban environments are of particular concern because of the high population density and the immediate proximity of inherent nitrate sources: sewage and fertilizers. Remediation eorts in urban areas may require costly underground repairs to aging *Corresponding author. Tel.: 1-650-329-4558; Fax: 1-650-3295590; E-mail: [email protected]
125 1527-5922/02/$35.00/00
# Published by Elsevier Science Ltd. on behalf of AEHS.
126 S. R. Silva et al.
Isotopes of nitrogen and oxygen are measured as the ratio of 15 N : 14 N and 18 O : 16 O, respectively, and are expressed in units of per mil ( parts per thousand {-}) of 15 N or 18 O higher or lower than that in a reference standard material. The Greek letter delta (d) is used to indicate a value derived by the formula: d15 N or d18 O Rsample =Rstandard
1 1000
where Rsample and Rstandard are the ratios of 15 N : 14 N or 18 O : 16 O in the sample and standard, respectively. The international standard for nitrogen is atmospheric air, and for oxygen is VSMOW (Vienna Standard Mean Ocean Water), which both have de®ned delta values of 0-. The range of common d15 N values for nitrate is between about 10 and 25- (Figure 1). Sewage (including animal waste) can often be distinguished from other nitrate sources on the basis of its relatively high d15 N value, which is achieved largely through ammonia volatilization preceding nitri®cation (Hubner, 1986). Synthetic fertilizers generally have d15 N values within a few per mil of zero because their nitrogen is derived from air. Soil nitrate, formed from the natural oxidation of organic nitrogen has, on average, a d15 N value between that of synthetic fertilizer and sewage/animal waste but its range overlaps both. The d15 N values of nitrate sources may be modi®ed by the eects of denitri®cation and, to a lesser extent, assimilation by plants. Denitri®cation increases the d15 N value of the residual nitrate (Figure 1). It occurs in anoxic environments and is caused by denitrifying bacteria. Terrestrial plants have d15 N values usually within 2- of their nitrogen source and, therefore, uptake of nitrogen has only a small eect on the residual nutrient (ammonium or nitrate) composition (Mariotti et al., 1980; Hubner, 1986). In theory, nitrate formed by nitri®cation (microbially mediated oxidation) derives two of its oxygen atoms from water and one from dissolved oxygen gas (Andersson and Hooper, 1983; Hollocher, 1984; Amberger and Schmidt, 1987; Bottcher et al., 1990). Oxygen in air has a composition of approximately
23- (Kroopnick and Craig, 1972); therefore, nitrate formed by nitri®cation with, for instance, an d18 Owater value of 10- will have a nitrate d18 O value of 1[(232( 10))/3]. Natural samples tend to have nitrate d18 O values between the theoretical value (as calculated above) and a few per mil heavier. Possible reasons for this are discussed in some detail by Kendall (1998) and include eects of evaporation of soil water, transpiration of dissolved oxygen during respiration, and as yet unrecognized nitri®cation pathways. Atmospherically derived nitrate has a large range of relatively high d18 O values, usually between 20 and 70 per mil (Figure 1) (Kendall, 1998). Synthetic nitrate fertilizers are produced from atmospheric nitrogen and oxygen and have d18 O values measured between about 18 and 26-. The range of values is apparently the result of fractionations during the manufacturing process. The d18 O value of naturally occurring soil nitrate may be indistinguishable from contaminant nitrate, such as produced by nitri®cation of ammonium fertilizer, if the contaminant was oxidized in the same environment as the soil nitrate. The use of combined d15 N and d18 O of nitrate may reveal source separation and mixing relationships better than d15 N alone. In addition, the isotopic composition of both nitrogen and oxygen increase during denitri®cation in a ratio of about 2 : 1 (slope 0.5) (Figure 1), which is very helpful in determining if denitri®cation has been a signi®cant factor in setting the isotopic compositions (Bottcher et al., 1990). Two studies, one from Austin Texas (Ging et al., 1996) and the other from Tacoma Washington (Inkpen et al., 2000) are described below to illustrate the utility of the dual isotope technique in forensic investigations of nitrate sources in urban environments. The samples were prepared and analyzed for isotopic composition using the methods described by Silva et al. 2000 (see also Chang et al., 1999). In both studies, samples were ®ltered (0.45 m ®lters) and passed through anion exchange resin columns in the ®eld to extract and preserve nitrate and to facilitate sample transport and storage. The columns were refrigerated
Figure 1. Common ®elds of d15 N and d18 O of nitrate from precipitation, synthetic nitrate fertilizers, ammonium fertilizers (including urea), soil, and sewage/animal waste. Arrow labeled ``denitri®cation'' illustrates the slope of a denitri®cation trend. Figure modi®ed from Kendall (1998).
Forensic Applications of Nitrogen and Oxygen Isotopes in Tracing Nitrate Sources in Urban Environments 127
and returned to the U.S. Geological Survey isotope laboratory in Menlo Park, California for preparation and analysis.
Austin Texas In Austin Texas, nitrate-N concentrations between 0.02 and 1.9 mg/l were measured in Shoal Creek and Waller Creek. The creeks pass through the city and empty into Town Lake, which is used as a local water supply. Although the concentrations were not alarming, the city wanted to know if the source of the nitrate was predominantly from the subsurface, perhaps sewage or naturally occurring soil nitrate, or from surface (or near surface) sources such as fertilizers applied to the local golf course, parklands, and private property. Water samples were collected from both creeks on three occasions during base¯ow conditions when stream water originated mostly from the subsurface, and on four occasions during storm¯ow conditions when much of the streamwater came directly from the surface. Samples were analyzed for a suite of common dissolved ions, conductivity, dissolved solids, pH, alkalinity, fecal bacteria and isotopes of nitrate. Nitrate concentrations were variable with no consistent dierences between base¯ow and storm¯ow. However, on a plot of d15 N vs. d18 O (Figure 2), the ®elds for storm¯ow and base¯ow show nearly complete separation. Storm¯ow values are centered in the ®eld of synthetic nitrate fertilizer and overlap all source ®elds except for sewage/animal waste. The base¯ow samples are centered slightly outside the ®elds of common compositions but overlap both ®elds of soil nitrate and sewage/animal waste. Although there are undoubtedly multiple surface and subsurface nitrate sources, the tighter ranges of d15 N and d18 O values in the base¯ow samples suggests that a single nitrate source is dominant in base¯ow while the greater scatter in the storm¯ow data indicates a mixture of sources. A plot of average d15 N and d18 O values for all samples on both creeks vs. sampling date (Figure 3)
helps explain the distribution of data in Figure 2. The near mirror image of d15 N and d18 O values indicates that the averaged isotopic compositions change in response to varying proportions of surface-(storm¯ow) and subsurface-(base¯ow) derived nitrate. The base¯ow samples are characterized by relatively high d15 N and low d18 O values and vice versa for the storm¯ow samples. This pattern strongly suggests that the base¯ow samples, which are centered in a ®eld above common source values (Figure 2), are actually a mixture of a nitrate derived largely from sewage/animal waste and the storm¯ow component. A positive correlation (r2 0.61) between d15 N and chloride concentrations also indicates mixing between base¯ow and storm¯ow nitrate components. The higher chloride concentration in the base¯ow is expected from a sewage source (Figure 4). The origin of the storm¯ow nitrate is less clear since nitrate concentrations in precipitation are quite low and nitrate fertilizers are not as commonly used as ammonium varieties (Terry and Kirby, 1999). Figure 5, showing d18 O NO3 vs. percentage of impervious cover, suggests one source of storm¯ow nitrate. Waller Creek was chosen because its four sampling sites have a wide range of percent impervious cover as estimated by GIS coverage. The percentage of precipitation reaching the streams via runo should increase with the percentage of impervious cover. Therefore, if atmospheric nitrate is signi®cant, there should be a positive correlation between d18 O NO3 and percent impervious cover. Indeed, Figure 5 shows that the highest average d18 O value is at the 5th street location in downtown Austin and the lowest value is at the golf course, suggesting that the atmosphere may be a signi®cant source of nitrate in runo at least in certain areas. It should be noted, however, that the nitrate concentration in these samples is fairly low (averaging 0.42 + 0.3 mg/l NO3 ±N for Waller Creek) and, therefore, atmospheric input is modest. An interesting note in the Austin study is that bacteria samples ( fecal coliform and fecal streptococci) were also collected as a tracer of septic input. Contrary
Figure 2. d15 N vs. d18 O values of nitrate during storm¯ow and base¯ow conditions from Waller and Shoal Creeks, Austin, Texas superimposed on common ®elds of nitrate from various sources. Ellipses indicate two standard deviations from average values. Arrow points in the direction of the base¯ow (subsurface) nitrate endmember.
128 S. R. Silva et al. 30
δ15N and δ18O – NO3( o /oo)
25
20 δ 18O 15
10
δ15N
5
0 09/07/94
09/09/94 11/05/94
Baseflow
01/11/95
Stormflow 15
03/13/95 04/20/95
Baseflow
04/26/95 Baseflow
Stormflow
18
Figure 3. d N and d O compositions for Waller and Shoal Creeks averaged together by collection date. Plot indicates mixing in various proportions of averaged base¯ow (subsurface) and storm¯ow (surface) compositions.
to expectations, the higher concentrations on average were from the storm¯ow samples; therefore, bacterial concentrations alone were not a reliable indicator of sewage. However, the ratios of the two types of bacteria suggest a predominance of bacteria from animal waste in the storm¯ow samples and a mixture of animal and human waste from the base¯ow samples (American Public Health Association, 1981), consistent with the interpretation from the chemical and isotopic data.
Tacoma Washington In Tacoma Washington, water samples from several city wells had nitrate concentrations approaching or at the limit of the drinking water standard of 10 mg/l. Residential areas of Tacoma that are served by sewer systems are interspersed with areas using septic systems. Both areas have wells with high nitrate concentrations. Samples were collected from six wells in the sewered area and seven wells in the septic area to determine if the high
90 Shoal Creek Stormflow 80
Waller Creek Stormflow Shoal Creek Baseflow Waller Creek Baseflow
Chloride (mg/L)
70 60 50 40 30 20 10 0 0
2
4
6
8
10
12
14
δ15N–NO3(o /oo) Figure 4. d15 N vs. chloride concentration for Shoal and Waller creeks showing a positive correlation among the base¯ow samples, consistent with a sewage source.
Forensic Applications of Nitrogen and Oxygen Isotopes in Tracing Nitrate Sources in Urban Environments 129 Waller Creek 45 Golf Course
40
45th St.
5th St.
38th St.
δ18N–NO3( o /oo)
35 30 25 20 15 10 5 0 0
20
40
60
80
100
Percent Impervious Cover Figure 5. d18 O of nitrate vs. percent impervious cover (ground cover consisting of pavement and buildings) for Waller Creek showing a positive correlation between runo and d18 O values suggestive of an atmospheric source.
concentrations of nitrate were derived from the local or upgradient septic systems or from other sources. In addition to isotopes of nitrate, samples were analyzed for concentrations of nitrate, chloride, and sulfate. On a plot of d15 N vs. d18 O of nitrate (Figure 6), four of ®ve samples from wells in the sewered areas plot within the overlapping areas of soil nitrate and ammonium fertilizers, while a single point plots considerably to the right of the other four with the highest d15 N and d18 O values (discussed below). The seven well samples from the septic system areas have, with one exception, d15 N values higher than those from the sewered areas. Both chloride and d15 N show nearly complete separation between the septic and sewered samples (Figure 7a). Again, higher chloride concentrations might be expected in waters in¯uenced by septic systems. Mixing between septic system-derived nitrate and a ``background'' composition is indicated on a plot of nitrate concentration versus d15 N (Figure 7b), which shows a strong linear trend (r2 0.79) toward high d15 N values
with higher nitrate concentrations. A plot of d15 N vs. oxygen concentration shows a linear trend among the septic samples, with higher d15 N values corresponding to lower oxygen content as expected for increasing input from septic systems (Figure 7c). The samples from the sewered areas have high and similar oxygen concentrations except for the outlier noted on Figure 6. The sample is anoxic, has relatively low nitrate concentrations, high d15 N value, and lies along a reasonable denitri®cation trend on Figure 6, all of which indicate that the sample has experienced signi®cant denitri®cation. The sewered area samples are widely scattered and slightly exceed the range of nitrate concentrations from the septic system areas. Together, the linear trend from the septic system wells and the scatter of values from the sewered areas indicates that the high nitrate concentrations are caused by both septic input and by other sources, and that the septic isotopic signal does not signi®cantly occur outside of the areas served by septic systems in the areas sampled. The source of the highest nitrate concentrations in the
Figure 6. d15 N vs. d18 O values for well waters from areas using septic systems and those served by a municipal sewer system in Tacoma, Washington superimposed on common ®elds of nitrate from various sources. Ellipses indicate two standard deviations from average values. Arrow enclosing the ellipses points toward the endmember composition for wells in the septic system areas. One sewered area sample with an d18 O value of 6.7 does not have a corresponding d15 N value and, therefore, does not appear on the plot.
130 S. R. Silva et al.
oxygen and nitrogen isotopes of nitrate in both studies suggests that denitri®cation was insigni®cant with the exception of one sample in the Tacoma study where it was clearly indicated. The oxygen isotopes from the base¯ow samples in the Austin study showed a much narrower range of values than the storm¯ow samples. This consistency in composition of subsurface-derived nitrate oxygen may be due to the relatively uniform conditions under which sewage N was nitri®ed. This characteristic may be a useful indicator of nitri®cation under such conditions. Although the non-sewage derived nitrate was less de®nitely identi®ed, the isotopes indicated measurable concentrations of atmospheric nitrate in the Austin study and suggested that fertilizer was an important nitrate source in the Tacoma study. Isotopic analysis seems particularly well suited to forensic investigations of nitrate sources in urban environments due to the distinctive isotopic signatures of potential urban nitrate sources namely sewage and fertilizers.
References
Figure 7. (a) d15 N vs. chloride concentration for sewered and septic areas in Tacoma showing a moderate positive correlation indicative of mixing between a probable septic source and background compositions. (b) d15 N vs. nitrate concentration from the sewered and septic areas in Tacoma showing a strong positive correlation among the septic area samples indicating mixing between a septic source and a background composition. (c) d15 N vs. oxygen concentration for sewered and septic areas in Tacoma showing a negative correlation among the septic system area samples, consistent with a sewage source. Note the single anoxic sample from a sewered area well with the highest d15 N value indicative of denitri®cation.
sewered areas is unknown but the two samples of highest concentration have the lowest d15 N values, which would be consistent with a fertilizer source.
Conclusions At both sites, isotopes of nitrate clearly dierentiate sewage from other sources of nitrate. Although concentration analyses alone suggest separate sources and mixing trends, the isotopes enhanced assessment of the mixing relationships and helped to identify the sources. Analyses of additional substances associated with domestic use such as caeine, phosphate, boron, or CFCs may be of further help. The presence or absence of denitri®cation is an important factor to consider when evaluating nitrate concentrations and isotopes. The combined use of
Amberger, A. and Schmidt, H.L. 1987. Naturliche isotopengehalte von nitrat als indikatoren fur dessen herkunft. Geochim. Cosmochim. Acta 51, 2699±2705. Andersson, K.K. and Hooper, A.B. 1983. O2 and H2O are each the source of one O in NO2-produced from NH3 by NitrosomonasÐ 15N-NMR evidence. FEBS Lett. 64, 236±240. Bottcher, J., Strebel, O., Voerkelius, S. and Schmidt, H.L. 1990. Using isotope fractionation of nitrate±nitrogen and nitrate±oxygen for evaluation of microbial denitri®cation. J. Hydrol. 114, 413±424. Chang, C.C.Y., Langston, J., Riggs, M., Campbell, D.H., Silva, S.R. and Kendall, C. 1999. A method for nitrate collection for d15 N and d18 O analysis from waters with low nitrate concentrations. Can. J. Fish. Aquat. Sci. 56, 1856±1864. Ging, P.B., Lee, R.W. and Silva, S.R. 1996. Water chemistry of Shoal Creek and Waller Creek, Austin Texas, and potential sources of nitrate. U.S. Geol. Surv. Water-Resour. Invest. Rep. 96±4167. Hollocher, T.C. 1984. Source of oxygen atoms in nitrate in the oxidation of nitrite by Nitrobacter agilis and evidence against a P±O±N anhydride mechanism in oxidative phosphorylation. Arch. Biochem. Biophys. 233, 721±727. Hubner, H. 1986. Isotope eects of nitrogen in the soil and biosphere. In: Handbook of Environmental Isotope Geochemistry, 2b. The Terrestrial Environment, pp. 361±425. (Fritz, P. and Fontes, J.C., Eds). Amsterdam, Elsevier. Inkpen, E.L., Tesoriero, A.J., Ebbert, J.C., Silva, S.R. and Sandstrom, M.W. 2000. Ground-water quality in regional agricultural and urban settings in the Puget Sound Basin, Washington and British Columbia, 1996±1998. U.S. Geol Surv waterResources Invest Rep. 00±4100. Kendall, C. 1998. Tracing sources and cycling of nitrate in catchments. In: Isotope Tracers in Catchment Hydrology, pp. 519±576. (Kendall, C. and McDonnell, J.J., Eds). Amsterdam, Elsevier. Kroopnick, P.M. and Craig, H. 1972. Atmospheric oxygen: isotopic composition and solubility fractionation. Science 175, 54±55. Mariotti, A., Pierre, D., Vedy, J.C., Bruckert, S. and Guillemot, J. 1980. The abundance of natural nitrogen 15 in the organic matter of soils along an altitudinal gradient (Chablais, Haute Savoie, France). Catena 7, 293±300. Silva, S.R., Kendall, C., Wilkison, D.H., Ziegler, A.C., Chang, C.C.Y. and Avanzino, R.J. 2000. A new method for collection of nitrate from fresh water and the analysis of the nitrogen and oxygen isotope ratios. J. of Hydrol. 228, 22±36. Spalding, R.F. and Exner, M.E. 1993. Occurrence of nitrate in groundwaterÐa review. J. Environ. Qual. 22p 392±402. Terry, D.L. and Kirby, B.J. 1999. Commercial Fertilizers 1999ÐA Summary of Fertilizer Use in the United States. Association of American Plant Food Control Ocials, Inc. and The Fertilizer Institute, Lexington, KY, 41 p.
Forensic Applications of Nitrogen and Oxygen Isotopes in Tracing Nitrate Sources in Urban Environments S. R. Silva* U.S. Geological Survey, Menlo Park, CA, U.S.A.
P. B. Ging U.S. Geological Survey, Austin, TX, U.S.A.
R. W. Lee U.S. Geological Survey, Dallas, TX, U.S.A.
J. C. Ebbert U.S. Geological Survey, Tacoma, WA, U.S.A.
A. J. Tesoriero U.S. Geological Survey, Raleigh, NC, U.S.A.
E. L. Inkpen U.S. Geological Survey, Tacoma, WA, U.S.A. (Received 30 June 2001, Revised manuscript accepted 22 December 2001) Ground and surface waters in urban areas are susceptible to nitrate contamination from septic systems, leaking sewer lines, and fertilizer applications. Source identi®cation is a primary step toward a successful remediation plan in aected areas. In this respect, nitrogen and oxygen isotope ratios of nitrate, in conjunction with hydrologic data and water chemistry, have proven valuable in urban studies from Austin, Texas, and Tacoma, Washington. In Austin, stream water was sampled during strem¯ow and base¯ow conditions to assess surface and subsurface sources of nitrate, respectively. In Tacoma, well waters were sampled in adjacent sewered and un-sewered areas to determine if locally high nitrate concentrations were caused by septic systems in the un-sewered areas. In both studies, sewage was identi®ed as a nitrate source and mixing between sewage and other sources of nitrate was apparent. In addition to source identi®cation, combined nitrogen and oxygen isotopes were important in determining the signi®cance of denitri®cation, which can complicate source assessment by reducing nitrate concentrations and increasing d15N values. The two studies illustrate the value of nitrogen and oxygen isotopes of nitrate for forensic applications in urban # Published by Elsevier Science Ltd. on behalf of AEHS. areas. Keywords: isotopes; nitrate; sewage; urban; source identi®cation.
Introduction
sewer systems or perhaps only better management practices in fertilizer applications on public and private lands. It is, therefore, necessary to understand the source and transport of nitrate contamination before an ecient remediation plan can be devised. Many studies have shown that stable isotope techniques are valuable tools for understanding nitrogen sources and cycling. This is true because certain sources of nitrate may be identi®ed by their characteristic or distinctive isotopic compositions. If the source is not distinctive, isotopes may still add unique and useful information in combination with other chemical and hydrologic data.
High concentrations of nitrate pose a direct health risk in drinking water and can lead to eutrophication in surface waters resulting in ecosystem damage (Spalding and Exner, 1993). Urban environments are of particular concern because of the high population density and the immediate proximity of inherent nitrate sources: sewage and fertilizers. Remediation eorts in urban areas may require costly underground repairs to aging *Corresponding author. Tel.: 1-650-329-4558; Fax: 1-650-3295590; E-mail: [email protected]
125 1527-5922/02/$35.00/00
# Published by Elsevier Science Ltd. on behalf of AEHS.
126 S. R. Silva et al.
Isotopes of nitrogen and oxygen are measured as the ratio of 15 N : 14 N and 18 O : 16 O, respectively, and are expressed in units of per mil ( parts per thousand {-}) of 15 N or 18 O higher or lower than that in a reference standard material. The Greek letter delta (d) is used to indicate a value derived by the formula: d15 N or d18 O Rsample =Rstandard
1 1000
where Rsample and Rstandard are the ratios of 15 N : 14 N or 18 O : 16 O in the sample and standard, respectively. The international standard for nitrogen is atmospheric air, and for oxygen is VSMOW (Vienna Standard Mean Ocean Water), which both have de®ned delta values of 0-. The range of common d15 N values for nitrate is between about 10 and 25- (Figure 1). Sewage (including animal waste) can often be distinguished from other nitrate sources on the basis of its relatively high d15 N value, which is achieved largely through ammonia volatilization preceding nitri®cation (Hubner, 1986). Synthetic fertilizers generally have d15 N values within a few per mil of zero because their nitrogen is derived from air. Soil nitrate, formed from the natural oxidation of organic nitrogen has, on average, a d15 N value between that of synthetic fertilizer and sewage/animal waste but its range overlaps both. The d15 N values of nitrate sources may be modi®ed by the eects of denitri®cation and, to a lesser extent, assimilation by plants. Denitri®cation increases the d15 N value of the residual nitrate (Figure 1). It occurs in anoxic environments and is caused by denitrifying bacteria. Terrestrial plants have d15 N values usually within 2- of their nitrogen source and, therefore, uptake of nitrogen has only a small eect on the residual nutrient (ammonium or nitrate) composition (Mariotti et al., 1980; Hubner, 1986). In theory, nitrate formed by nitri®cation (microbially mediated oxidation) derives two of its oxygen atoms from water and one from dissolved oxygen gas (Andersson and Hooper, 1983; Hollocher, 1984; Amberger and Schmidt, 1987; Bottcher et al., 1990). Oxygen in air has a composition of approximately
23- (Kroopnick and Craig, 1972); therefore, nitrate formed by nitri®cation with, for instance, an d18 Owater value of 10- will have a nitrate d18 O value of 1[(232( 10))/3]. Natural samples tend to have nitrate d18 O values between the theoretical value (as calculated above) and a few per mil heavier. Possible reasons for this are discussed in some detail by Kendall (1998) and include eects of evaporation of soil water, transpiration of dissolved oxygen during respiration, and as yet unrecognized nitri®cation pathways. Atmospherically derived nitrate has a large range of relatively high d18 O values, usually between 20 and 70 per mil (Figure 1) (Kendall, 1998). Synthetic nitrate fertilizers are produced from atmospheric nitrogen and oxygen and have d18 O values measured between about 18 and 26-. The range of values is apparently the result of fractionations during the manufacturing process. The d18 O value of naturally occurring soil nitrate may be indistinguishable from contaminant nitrate, such as produced by nitri®cation of ammonium fertilizer, if the contaminant was oxidized in the same environment as the soil nitrate. The use of combined d15 N and d18 O of nitrate may reveal source separation and mixing relationships better than d15 N alone. In addition, the isotopic composition of both nitrogen and oxygen increase during denitri®cation in a ratio of about 2 : 1 (slope 0.5) (Figure 1), which is very helpful in determining if denitri®cation has been a signi®cant factor in setting the isotopic compositions (Bottcher et al., 1990). Two studies, one from Austin Texas (Ging et al., 1996) and the other from Tacoma Washington (Inkpen et al., 2000) are described below to illustrate the utility of the dual isotope technique in forensic investigations of nitrate sources in urban environments. The samples were prepared and analyzed for isotopic composition using the methods described by Silva et al. 2000 (see also Chang et al., 1999). In both studies, samples were ®ltered (0.45 m ®lters) and passed through anion exchange resin columns in the ®eld to extract and preserve nitrate and to facilitate sample transport and storage. The columns were refrigerated
Figure 1. Common ®elds of d15 N and d18 O of nitrate from precipitation, synthetic nitrate fertilizers, ammonium fertilizers (including urea), soil, and sewage/animal waste. Arrow labeled ``denitri®cation'' illustrates the slope of a denitri®cation trend. Figure modi®ed from Kendall (1998).
Forensic Applications of Nitrogen and Oxygen Isotopes in Tracing Nitrate Sources in Urban Environments 127
and returned to the U.S. Geological Survey isotope laboratory in Menlo Park, California for preparation and analysis.
Austin Texas In Austin Texas, nitrate-N concentrations between 0.02 and 1.9 mg/l were measured in Shoal Creek and Waller Creek. The creeks pass through the city and empty into Town Lake, which is used as a local water supply. Although the concentrations were not alarming, the city wanted to know if the source of the nitrate was predominantly from the subsurface, perhaps sewage or naturally occurring soil nitrate, or from surface (or near surface) sources such as fertilizers applied to the local golf course, parklands, and private property. Water samples were collected from both creeks on three occasions during base¯ow conditions when stream water originated mostly from the subsurface, and on four occasions during storm¯ow conditions when much of the streamwater came directly from the surface. Samples were analyzed for a suite of common dissolved ions, conductivity, dissolved solids, pH, alkalinity, fecal bacteria and isotopes of nitrate. Nitrate concentrations were variable with no consistent dierences between base¯ow and storm¯ow. However, on a plot of d15 N vs. d18 O (Figure 2), the ®elds for storm¯ow and base¯ow show nearly complete separation. Storm¯ow values are centered in the ®eld of synthetic nitrate fertilizer and overlap all source ®elds except for sewage/animal waste. The base¯ow samples are centered slightly outside the ®elds of common compositions but overlap both ®elds of soil nitrate and sewage/animal waste. Although there are undoubtedly multiple surface and subsurface nitrate sources, the tighter ranges of d15 N and d18 O values in the base¯ow samples suggests that a single nitrate source is dominant in base¯ow while the greater scatter in the storm¯ow data indicates a mixture of sources. A plot of average d15 N and d18 O values for all samples on both creeks vs. sampling date (Figure 3)
helps explain the distribution of data in Figure 2. The near mirror image of d15 N and d18 O values indicates that the averaged isotopic compositions change in response to varying proportions of surface-(storm¯ow) and subsurface-(base¯ow) derived nitrate. The base¯ow samples are characterized by relatively high d15 N and low d18 O values and vice versa for the storm¯ow samples. This pattern strongly suggests that the base¯ow samples, which are centered in a ®eld above common source values (Figure 2), are actually a mixture of a nitrate derived largely from sewage/animal waste and the storm¯ow component. A positive correlation (r2 0.61) between d15 N and chloride concentrations also indicates mixing between base¯ow and storm¯ow nitrate components. The higher chloride concentration in the base¯ow is expected from a sewage source (Figure 4). The origin of the storm¯ow nitrate is less clear since nitrate concentrations in precipitation are quite low and nitrate fertilizers are not as commonly used as ammonium varieties (Terry and Kirby, 1999). Figure 5, showing d18 O NO3 vs. percentage of impervious cover, suggests one source of storm¯ow nitrate. Waller Creek was chosen because its four sampling sites have a wide range of percent impervious cover as estimated by GIS coverage. The percentage of precipitation reaching the streams via runo should increase with the percentage of impervious cover. Therefore, if atmospheric nitrate is signi®cant, there should be a positive correlation between d18 O NO3 and percent impervious cover. Indeed, Figure 5 shows that the highest average d18 O value is at the 5th street location in downtown Austin and the lowest value is at the golf course, suggesting that the atmosphere may be a signi®cant source of nitrate in runo at least in certain areas. It should be noted, however, that the nitrate concentration in these samples is fairly low (averaging 0.42 + 0.3 mg/l NO3 ±N for Waller Creek) and, therefore, atmospheric input is modest. An interesting note in the Austin study is that bacteria samples ( fecal coliform and fecal streptococci) were also collected as a tracer of septic input. Contrary
Figure 2. d15 N vs. d18 O values of nitrate during storm¯ow and base¯ow conditions from Waller and Shoal Creeks, Austin, Texas superimposed on common ®elds of nitrate from various sources. Ellipses indicate two standard deviations from average values. Arrow points in the direction of the base¯ow (subsurface) nitrate endmember.
128 S. R. Silva et al. 30
δ15N and δ18O – NO3( o /oo)
25
20 δ 18O 15
10
δ15N
5
0 09/07/94
09/09/94 11/05/94
Baseflow
01/11/95
Stormflow 15
03/13/95 04/20/95
Baseflow
04/26/95 Baseflow
Stormflow
18
Figure 3. d N and d O compositions for Waller and Shoal Creeks averaged together by collection date. Plot indicates mixing in various proportions of averaged base¯ow (subsurface) and storm¯ow (surface) compositions.
to expectations, the higher concentrations on average were from the storm¯ow samples; therefore, bacterial concentrations alone were not a reliable indicator of sewage. However, the ratios of the two types of bacteria suggest a predominance of bacteria from animal waste in the storm¯ow samples and a mixture of animal and human waste from the base¯ow samples (American Public Health Association, 1981), consistent with the interpretation from the chemical and isotopic data.
Tacoma Washington In Tacoma Washington, water samples from several city wells had nitrate concentrations approaching or at the limit of the drinking water standard of 10 mg/l. Residential areas of Tacoma that are served by sewer systems are interspersed with areas using septic systems. Both areas have wells with high nitrate concentrations. Samples were collected from six wells in the sewered area and seven wells in the septic area to determine if the high
90 Shoal Creek Stormflow 80
Waller Creek Stormflow Shoal Creek Baseflow Waller Creek Baseflow
Chloride (mg/L)
70 60 50 40 30 20 10 0 0
2
4
6
8
10
12
14
δ15N–NO3(o /oo) Figure 4. d15 N vs. chloride concentration for Shoal and Waller creeks showing a positive correlation among the base¯ow samples, consistent with a sewage source.
Forensic Applications of Nitrogen and Oxygen Isotopes in Tracing Nitrate Sources in Urban Environments 129 Waller Creek 45 Golf Course
40
45th St.
5th St.
38th St.
δ18N–NO3( o /oo)
35 30 25 20 15 10 5 0 0
20
40
60
80
100
Percent Impervious Cover Figure 5. d18 O of nitrate vs. percent impervious cover (ground cover consisting of pavement and buildings) for Waller Creek showing a positive correlation between runo and d18 O values suggestive of an atmospheric source.
concentrations of nitrate were derived from the local or upgradient septic systems or from other sources. In addition to isotopes of nitrate, samples were analyzed for concentrations of nitrate, chloride, and sulfate. On a plot of d15 N vs. d18 O of nitrate (Figure 6), four of ®ve samples from wells in the sewered areas plot within the overlapping areas of soil nitrate and ammonium fertilizers, while a single point plots considerably to the right of the other four with the highest d15 N and d18 O values (discussed below). The seven well samples from the septic system areas have, with one exception, d15 N values higher than those from the sewered areas. Both chloride and d15 N show nearly complete separation between the septic and sewered samples (Figure 7a). Again, higher chloride concentrations might be expected in waters in¯uenced by septic systems. Mixing between septic system-derived nitrate and a ``background'' composition is indicated on a plot of nitrate concentration versus d15 N (Figure 7b), which shows a strong linear trend (r2 0.79) toward high d15 N values
with higher nitrate concentrations. A plot of d15 N vs. oxygen concentration shows a linear trend among the septic samples, with higher d15 N values corresponding to lower oxygen content as expected for increasing input from septic systems (Figure 7c). The samples from the sewered areas have high and similar oxygen concentrations except for the outlier noted on Figure 6. The sample is anoxic, has relatively low nitrate concentrations, high d15 N value, and lies along a reasonable denitri®cation trend on Figure 6, all of which indicate that the sample has experienced signi®cant denitri®cation. The sewered area samples are widely scattered and slightly exceed the range of nitrate concentrations from the septic system areas. Together, the linear trend from the septic system wells and the scatter of values from the sewered areas indicates that the high nitrate concentrations are caused by both septic input and by other sources, and that the septic isotopic signal does not signi®cantly occur outside of the areas served by septic systems in the areas sampled. The source of the highest nitrate concentrations in the
Figure 6. d15 N vs. d18 O values for well waters from areas using septic systems and those served by a municipal sewer system in Tacoma, Washington superimposed on common ®elds of nitrate from various sources. Ellipses indicate two standard deviations from average values. Arrow enclosing the ellipses points toward the endmember composition for wells in the septic system areas. One sewered area sample with an d18 O value of 6.7 does not have a corresponding d15 N value and, therefore, does not appear on the plot.
130 S. R. Silva et al.
oxygen and nitrogen isotopes of nitrate in both studies suggests that denitri®cation was insigni®cant with the exception of one sample in the Tacoma study where it was clearly indicated. The oxygen isotopes from the base¯ow samples in the Austin study showed a much narrower range of values than the storm¯ow samples. This consistency in composition of subsurface-derived nitrate oxygen may be due to the relatively uniform conditions under which sewage N was nitri®ed. This characteristic may be a useful indicator of nitri®cation under such conditions. Although the non-sewage derived nitrate was less de®nitely identi®ed, the isotopes indicated measurable concentrations of atmospheric nitrate in the Austin study and suggested that fertilizer was an important nitrate source in the Tacoma study. Isotopic analysis seems particularly well suited to forensic investigations of nitrate sources in urban environments due to the distinctive isotopic signatures of potential urban nitrate sources namely sewage and fertilizers.
References
Figure 7. (a) d15 N vs. chloride concentration for sewered and septic areas in Tacoma showing a moderate positive correlation indicative of mixing between a probable septic source and background compositions. (b) d15 N vs. nitrate concentration from the sewered and septic areas in Tacoma showing a strong positive correlation among the septic area samples indicating mixing between a septic source and a background composition. (c) d15 N vs. oxygen concentration for sewered and septic areas in Tacoma showing a negative correlation among the septic system area samples, consistent with a sewage source. Note the single anoxic sample from a sewered area well with the highest d15 N value indicative of denitri®cation.
sewered areas is unknown but the two samples of highest concentration have the lowest d15 N values, which would be consistent with a fertilizer source.
Conclusions At both sites, isotopes of nitrate clearly dierentiate sewage from other sources of nitrate. Although concentration analyses alone suggest separate sources and mixing trends, the isotopes enhanced assessment of the mixing relationships and helped to identify the sources. Analyses of additional substances associated with domestic use such as caeine, phosphate, boron, or CFCs may be of further help. The presence or absence of denitri®cation is an important factor to consider when evaluating nitrate concentrations and isotopes. The combined use of
Amberger, A. and Schmidt, H.L. 1987. Naturliche isotopengehalte von nitrat als indikatoren fur dessen herkunft. Geochim. Cosmochim. Acta 51, 2699±2705. Andersson, K.K. and Hooper, A.B. 1983. O2 and H2O are each the source of one O in NO2-produced from NH3 by NitrosomonasÐ 15N-NMR evidence. FEBS Lett. 64, 236±240. Bottcher, J., Strebel, O., Voerkelius, S. and Schmidt, H.L. 1990. Using isotope fractionation of nitrate±nitrogen and nitrate±oxygen for evaluation of microbial denitri®cation. J. Hydrol. 114, 413±424. Chang, C.C.Y., Langston, J., Riggs, M., Campbell, D.H., Silva, S.R. and Kendall, C. 1999. A method for nitrate collection for d15 N and d18 O analysis from waters with low nitrate concentrations. Can. J. Fish. Aquat. Sci. 56, 1856±1864. Ging, P.B., Lee, R.W. and Silva, S.R. 1996. Water chemistry of Shoal Creek and Waller Creek, Austin Texas, and potential sources of nitrate. U.S. Geol. Surv. Water-Resour. Invest. Rep. 96±4167. Hollocher, T.C. 1984. Source of oxygen atoms in nitrate in the oxidation of nitrite by Nitrobacter agilis and evidence against a P±O±N anhydride mechanism in oxidative phosphorylation. Arch. Biochem. Biophys. 233, 721±727. Hubner, H. 1986. Isotope eects of nitrogen in the soil and biosphere. In: Handbook of Environmental Isotope Geochemistry, 2b. The Terrestrial Environment, pp. 361±425. (Fritz, P. and Fontes, J.C., Eds). Amsterdam, Elsevier. Inkpen, E.L., Tesoriero, A.J., Ebbert, J.C., Silva, S.R. and Sandstrom, M.W. 2000. Ground-water quality in regional agricultural and urban settings in the Puget Sound Basin, Washington and British Columbia, 1996±1998. U.S. Geol Surv waterResources Invest Rep. 00±4100. Kendall, C. 1998. Tracing sources and cycling of nitrate in catchments. In: Isotope Tracers in Catchment Hydrology, pp. 519±576. (Kendall, C. and McDonnell, J.J., Eds). Amsterdam, Elsevier. Kroopnick, P.M. and Craig, H. 1972. Atmospheric oxygen: isotopic composition and solubility fractionation. Science 175, 54±55. Mariotti, A., Pierre, D., Vedy, J.C., Bruckert, S. and Guillemot, J. 1980. The abundance of natural nitrogen 15 in the organic matter of soils along an altitudinal gradient (Chablais, Haute Savoie, France). Catena 7, 293±300. Silva, S.R., Kendall, C., Wilkison, D.H., Ziegler, A.C., Chang, C.C.Y. and Avanzino, R.J. 2000. A new method for collection of nitrate from fresh water and the analysis of the nitrogen and oxygen isotope ratios. J. of Hydrol. 228, 22±36. Spalding, R.F. and Exner, M.E. 1993. Occurrence of nitrate in groundwaterÐa review. J. Environ. Qual. 22p 392±402. Terry, D.L. and Kirby, B.J. 1999. Commercial Fertilizers 1999ÐA Summary of Fertilizer Use in the United States. Association of American Plant Food Control Ocials, Inc. and The Fertilizer Institute, Lexington, KY, 41 p.