H41D-0325 0800h
Identification and Quantification of Diffuse Nitrogen Inputs Into a River System Using Stable Isotopes of Nitrogen and Oxygen in Nitrate
A dual-isotope approach with $\delta^{15}$N and $\delta^{18}$O in nitrate was carried out to identify and quantify the diffuse nitrate inputs into a river system in Mecklenburg-Vorpommern (Germany). The three main nitrate sources (drainage water from agricultural soils, groundwater and atmospheric deposition) and the river were sampled monthly from November 2002 to April 2003. To determine the contribution of each sampled nitrate source to the total river nitrate a standard dual isotope, three source linear mixing-model was used. Drainage water nitrate had a mean $\delta^{15}$N value of 10.1 $\permil$ and a $\delta^{18}$O of 5.5 $\permil$, and was significantly different to groundwater nitrate (mean $\delta^{15}$N = 1.1 $\permil$, $\delta^{18}$O = 1.5 $\permil$) and nitrate from atmospheric deposition (mean $\delta^{15}$N = 1.9 $\permil$, $\delta^{18}$O = 50.9 $\permil$). The low $\delta^{18}$O values of groundwater and drainage water nitrate indicated that it was mainly formed during the nitrification process. Isotope values of the river nitrate were close to the values of drainage water nitrate with a mean $\delta^{15}$N of 9.0 $\permil$ and a $\delta^{18}$O of 6.1 $\permil$. The mixing-model revealed that the nitrate from the drainage water contributed 88 % of the river nitrate. Contribution of nitrate from groundwater and atmospheric deposition was 10 % and 2 %, respectively. These results agree with estimations of nitrate input data for this study area given by a nutrient emissions model.
H41D-0326 0800h
A Dual Isotope Study of Nitrates in Aquifer and Surface Waters: Initial Results at the Watershed Scale
This dual isotope-nitrate study is aimed at contributing to the quantification of the annual N budget of the Wilmot River watershed on Prince-Edward Island (PEI, Canada). Aquifers constitute the only source of freshwater on PEI. In many areas, nitrate concentrations in groundwater (GW) have been increasing over time. It is assumed that mineral fertilization for potato cropping constitutes a major source of nitrates. However, a better understanding of the transfer dynamics of nitrates from soils to GW is required to reduce their detrimental effects. Here we report on N concentrations ([N-NO3-]), nitrate isotope analyses (N and O), and water isotope ratios (H and O) obtained after one year of seasonal sampling of surface water and groundwater (GW). The analyses of the nitrate isotopes were performed on silver nitrates using EA-CF- and Pyrolysis/EA-IRMS systems for N and O, respectively. Water isotopes were analysed with an equilibration system in continuous flow. Our 2003 summer and fall results indicate that 23% of the samples have N-NO3 concentrations above the threshold established for human health (10 mg/L), whereas 10% have concentrations within natural ranges (<1 mg/L). Combined nitrate and water isotope results suggest that during summer and fall most nitrates in the Wilmot River are derived from GW, and that about 75% of the GW samples contain nitrates from chemical fertilizers while the remaining 25% of nitrates are from natural soils, manures or septic wastes. Moreover, the N isotopes-nitrate concentration trend for GW departs significantly from the curve expected to result from microbial denitrification and corresponds better to natural attenuation due to dilution of nitrate-rich waters with water devoid of nitrates. The oxygen isotopes in GW nitrates indicate that summer rain and evaporated vadose water likely represent the O sources involved in soil nitrification. Future work will involve characterization of potential nitrate sources and of N-species from the vadose zone, characterization of vertical distribution of the isotopic tracers following sampling of discrete intervals along wells, as well as 2D and 3D modelling.
H41D-0327 0800h
Managing Nutrients in two New England Estuaries: The Feasibility of Using Stable Isotopes to Monitor Nitrate Sources
Nitrogen is the limiting nutrient in many estuaries, and the rising availability of nitrogen has led to increased eutrophication in many coastal ecosystems. In order to reduce nitrogen loading to coastal waters it is necessary to know the relative importance of the various sources, both point and non-point. Recent studies have shown that sources of nitrate, the most bioavailable form of nitrogen, can be differentiated based on their isotopic signature using a dual stable isotope approach (\delta$^{15}$N and \delta$^{18}$O). Currently, the range of isotopic values in the literature for nitrate sources (sewage, atmospheric deposition, microbial nitrification, fertilizer and animal waste) across various systems is too large to clearly differentiate between sources. In this study, I will use a stratified sampling plan to assess the feasibility of using this stable isotope approach to determine the relative importance of various nitrogen inputs in a limited geographic region. I will examine how these isotope signatures vary spatially across different watersheds and how much sampling (spatially and temporally) is needed to accurately assess the delivery of nitrogen to a given estuary. In particular, four estuarine catchments within two different New England estuaries will be sampled at three different scales. At the smallest scale, first and second order forested, agricultural, and urban catchments will be sampled to identify source signatures of these landscapes. Tributaries, mostly third order streams, will be sampled to gain additional information about the relationship between land cover and isotopic signatures. At the largest scale, longitudinal sampling will be done along the main stems to assess the relative contributions of different tributaries and to provide further information regarding nitrogen cycling within these systems. Various chemical indicators of nitrogen loading will also be measured to assess their use in conjunction with the stable isotopes. This suite of measurements will help determine the best strategy (in terms of both time and cost) for detecting watershed nitrogen loading to estuaries.
H41D-0328 0800h
Evaluation of Stable Isotope Analysis as a Tool to Determine Nitrate Sources in Irish Groundwaters
Natural abundance of stable isotope ratios of nitrogen (\delta$^{15}$N) and oxygen (\delta$^{18}$O) in NO$_{3}$ were measured in a nitrate-vulnerable aquifer in eastern Ireland underlying an intensive agricultural area. The aim was to determine whether the dual stable isotope approach (\delta$^{15}$N and \delta$^{18}$O) could provide a more power tool than \delta$^{15}$N measurements alone for determining the source of groundwater nitrate. Forty-five private wells and boreholes representing a range of potential N sources (diffuse sources such as synthetic fertiliser spreading and/or farmyard effluents and/or septic tank effluents) were sampled on seven occasions between 2002 and 2004. Additionally, nitrate leachate samples were collected from the unsaturated zone under small-scale experiments simulating different agricultural practices. Nitrate was extracted from each water sample using an anion exchange resin technique (Silva et al., 2000), and \delta$^{15}$N and \delta$^{18}$O values were both measured by Continuous-Flow Isotopic Ratio Mass Spectrometry. The initial hypothesis was that, in a shallow gravel aquifer where nitrate leaching is important and fast, groundwater nitrates are likely to display the isotopic characteristics of their source (low \delta$^{15}$N and high \delta$^{18}$O for diffuse sources, compared to high \delta$^{15}$N and low \delta$^{18}$O for point sources). The first set of results indicates that the \delta$^{15}$N values only follow this trend, although the \delta$^{18}$O values suggest the occurrence of denitrification. The levelling of \delta$^{18}$O values may in fact be caused by the Mineralisation-Immobilisation Turnover processes (MIT) that have previously been found to occur in the unsaturated zone, incorporating new oxygen atoms during the remineralisation of the nitrate (Mengis et al., 2001). References Mengis, M.; Walther, U.; Bernasconi, S.M.; Wehrli, B. (2001) Limitations of using \delta$^{18}$O for the source identification of nitrate in agricultural soils. Environmental Science & Technology, 35, 1840-1844. Silva, S.R.; Kendall, C.; Wilkison, D.H.; Ziegler, A.C.; Chang, C.C.Y.; Avanzino, R.J. (2000) A new method for collection of nitrate from fresh water and the analysis of nitrogen and oxygen isotope ratios. Journal of Hydrology, 228, 22-36.
H41D-0329 0800h
Chemical and Isotopical Patterns of Nitrate Contamination in the Southern Willamette Valley, Oregon
A persistent problem with elevated NO$_{3}$ concentrations in rural drinking wells in the southern Willamette Valley, Oregon has been documented since the 1930's. We explore the origin of this contaminant. The objective of this study was to use isotopes of NO$_{3}$ and other ionic chemical indicators to determine the sources of NO$_{3}$ in drinking water wells in the southern Willamette Valley, OR. Many non-point sources were found to contribute to the elevated levels of NO$_{3}$ in ground water, including high-density residential and high-intensity agricultural. 466 wells met the criteria to be included in the study: (1) less than 75 feet in depth (2) installed after 1960 (3) domestic use and (4) be located in the southern Willamette Valley. 120 wells were sampled during the summer of 2003. Geologic units, dominant land use and soil types were determined for each well in an attempt to determine vulnerability of wells for NO$_{3}$ contamination. 20 drinking water wells were selected to undergo isotopic and further chemical analyses. In order to determine the chemical and isotopic fingerprints of the dominant sources of NO$_{3}$ contamination soil samples were augered from 10 septic drain fields and water samples were collected below 10 agricultural fields. NO$_{3}$-N concentrations in the study area ranged from below detection ($<$0.20 mg/L) to 13.70 mg/L, with a mean concentration of 4.81 mg/L. There was a statistically significant trend (i.e. P $<$ 0.05) in NO$_{3}$-N with well depth, well age, pH and SO$_{4}$. Findings suggest that geologic setting is an important factor in predicting vulnerability, with land use also being important but less so. Determination of septic and agricultural sources of NO$_{3}$ contamination were inconclusive, though various chemical indicators were found to suggest the origin of the NO$_{3}$.
H41D-0330 0800h
Use of Nitrogen-15 Isotope Method in Soils and Ground Water to Determine Potential Nitrogen Sources Affecting a Municipal Water Supply in Kansas, USA
Nitrate-N concentrations have increased to greater than 10 mg/L in a municipal water supply in western Kansas from 1995 to 2002. A study was done by the Kansas Geological Survey using the nitrogen-15 natural abundance isotope method to determine potential sources for the increasing nitrate concentrations. Preliminary results of the isotope analyses on water samples suggest that animal waste and/or denitrification enrichment has affected the water supply. Soil samples from areas near the wells that were not treated with manure show a general increase of nitrogen-15 signature (+9 to +15 \permil) to a depth of 5 m. Soils are silt loams with measurable carbonate (0.8 to 2 % by weight) in the profile, which may permit volatilization enrichment to occur in the soil profile. Wells in the area range from 11 to 20 m in alluvial deposits with depth to water at approximately 9 m). Nitrate-N values range from 8 to 26 mg/L. Nitrogen-15 values range from (+17 to +28 \permil) with no obvious source of animal waste near the well sites. There are potential nearby long-term sources of animal waste - an abandoned sewage treatment plant and an agricultural testing farm. One well has a reducing chemistry with a nitrate value of 0.9 mg/L and a nitrogen-15 value of +17 \permil suggesting that alluvial sediment variation also has an impact on the water quality in the study area. The other wells show values of nitrate and nitrogen-15 that are much greater than the associated soils. The use of nitrogen-15 alone permited limited evaluation of sources of nitrate to ground water particularly in areas with carbonate in the soils. Use of oxygen-18 on nitrate will permit the delineation of the processes affecting the nitrogen in the soil profile and determination of the probable sources and the processes that have affected the nitrogen in the ground water. Final results of the nitrogen-15 and oxygen-18 analyses will be presented.
H41D-0331 0800h
Investigation of Nitrate Loading in Groundwater from Eastern San Joaquin Valley, California Using \delta$^{15}$N Isotopic Ratios
Agricultural activities in the Eastern San Joaquin Valley have affected groundwater quality in the area. The dependence of its population on groundwater for drinking has triggered several water quality studies in the past two decades. A total of 20 wells were sampled during the summer of 2003. These wells are installed to different depths at six clustered locations along the flow-path southeast of Fresno in a 4.6 Km transect agricultural area. The depth of these wells ranges from 70 to 270 feet. Vineyards and orchards are the predominant land use in the area but other crops are also cultivated in a minor scale. On a regional scale the groundwater flows towards southwest but on a local scale the water flows towards WSW because of local pumping. The United States Geological Survey has been monitoring the groundwater in this area as part of their National Water Quality Assessment Program. They have established a series of monitoring wells to detect the concentration of an agricultural fumigant DBCP banned from use since the 1970's and to evaluate trends in nitrate concentrations. Our main purpose in this study is to study the \delta$^{15}$N ratios along with \delta$^{18}$O ratios of dissolved nitrate to investigate the possible denitrification process occurring in the subsurface along the flow-path. The measured \delta$^{15}$N isotopic ratios of dissolved nitrate in groundwater samples from different depths and locations range from 4 to 9.5. The data show that there are strong negative correlations between depths and nitrate concentrations and also depths and TDS concentrations. On the contrary, we found no significant correlation between depths and \delta$^{15}$N or between nitrate concentrations and \delta$^{15}$N. These observations suggest the absence of significant denitrification process. However, the decrease in nitrate concentrations with increasing depths could be caused simply by the process of dilution by fresh ground water as the contaminated irrigation water travels downwards and along the flow-path. This interpretation is supported by Piper diagram plots. Distinct mixing trends are observed pointing towards the HCO$_{3}$+CO$_{3}$ corner as depth increases. This observation is consistent with the fact that the surface infiltration is mixed and diluted with the carbonate-type ground water originated from the Sierra Nevada mountains. Alternatively, these trends may indicate an increase in nitrate loading with time, as the age of water increases with depth. [This study was performed with the collaboration of Karen Burow of USGS, Sacramento.]
H41D-0332 0800h
A Survey of \delta$^{18}$O and \delta$^{15}$N Ratios in Ground Water from an Agricultural Community in the San Joaquin Valley, California
We studied ground water samples from domestic and monitoring wells in an agricultural community in the eastern side of the San Joaquin Valley, California. The study area is rich in alluvial soils creating an extremely fertile farmland. Livestock farms and agricultural fields are abundant in the area. Fifty-four ground water samples were analyzed for \delta$^{18}$O and \delta$^{15}$N in dissolved nitrate, in addition to nutrients and major minerals. Nitrate concentration levels in groundwater are elevated and affected by agricultural and other activities. Possible sources of nutrients include: a municipal waste-water treatment facility, a raisin processing plant, a meat processing plant, a turkey farm, diary operations, and agricultural fields. However, except for the turkey farm and a diary, we found no statistical significant contribution of nitrate from the other facilities as compared to the rest of the area. The \delta$^{18}$O versus \delta$^{15}$N ratios plot of dissolved ground water nitrate shows most samples clustered around an area consistent with soil organic nitrogen. In addition, the rest of the samples show a trend that is indicative of denitrification process. Generally, high \delta$^{15}$N values are associated with low nitrate concentrations. The isotopic signal of denitrification is particularly pronounced in samples in the vicinity of the waste water treatment facility, where the highest values of \delta$^{15}$N and the lowest nitrate concentrations are observed. However, these samples also have elevated chloride concentrations indicating a waste-water source. These data suggest that the denitrification in the subsurface may have been enhanced by bacteria species introduced by the effluence of the plant. [This study was performed with the collaboration of Steven R Silva of USGS, Menlo Park, and Iris Yamagata and Holly Jo Ferrin of California Department of Water Resources.]
H41D-0333 0800h
Nitrogen and Oxygen Isotopes of Nitrate as Indicators of Nitrogen Utilization in Monterey Bay
As nitrate is utilized by phytoplankton in the euphotic zone, the isotopic compositions of both oxygen and nitrogen in the residual nitrate become enriched in the heavier isotopes ($^{15}$N and $^{18}$O). Recent evidence from phytoplankton culture experiments showed that the enrichments in $^{15}$N and $^{18}$O occur intracellularly during the reduction of nitrate to nitrite. Furthermore, the isotopic composition of the external nitrate pool reflects the balance between intracellular reduction and efflux of the intracellular nitrate. It has also been shown that while $^{15}$N and $^{18}$O fractionation factors ($\epsilon$) can vary widely depending on growth conditions, there is nonetheless a strong coupling (1:1) of these isotope effects during reduction of nitrate by phytoplankton. In this study, the isotopic composition of nitrate in the upper 200m from three stations in Monterey Bay was measured monthly over an annual cycle. Preliminary results reveal relatively invariable isotopic compositions ($\delta^{15}$N = +7 to +8 permil; $\delta^{18}$O = 0 to +1 permil) in the deeper portions of the water column, with strong enrichments generally occurring shallower than 60m ($\delta^{15}$N up to +14 permil; $\delta^{18}$O up to +15 permil). Initial results also suggest some degree of spatial and temporal variability in the relationship between $\delta^{15}$N and $\delta^{18}$O. While it is unclear exactly what may be controlling this relationship, results from the full suite of monthly samples will provide additional insight on nitrogen utilization and its effects on nitrate isotopic composition. We discuss the results in relation to temperature, salinity, oxygen and nutrient concentrations, primary productivity and phytoplankton species.
H41D-0334 0800h
Characteristics of the Nitrogen Cycle in Southern Everglades Marshes: 15N Tracer and Natural Abundance Experiments
An {\it in situ} mesocosm experiment was conducted in the freshwater marshes of the Southern Everglades. The object of this experiment was to compare 15N natural abundances to N-cycling in mesocosms amended with 15N tracer. The 15N tracer technique allowed us to isolate the flows of N among various ecosystem components. Six mesocosms (2 m$^{2}$) were deployed in the oligotrophic P-limited marshes of the C-111 basin. The experiment ran for a period of 21 days with sampling occurring at t=0, 5, 10, 20, 30min, 1, 3, and 6hr on the first day and subsequently on days 2, 3, 5, 9, 15, and 21. Periphyton, soil, macrophytes ({\it Cladium jamaicense}), and consumers ({\it Gambusia holbrooki}) were collected. The isotopic tracer added to the enriched mesocosms possessed a \delta 15N value of 300\permil. Isotopic analysis was completed on a Finnigan Delta C EA-IRMS. All ecosystem components showed tracer uptake except for the soil, which remained at natural abundance levels throughout the experiment. Periphyton showed both the most rapid and the greatest N tracer uptake with an increase of 8.7\permil, 5 minutes after tracer addition and a maximum \delta 15N value of 197.93\permil at day 3. This experiment demonstrated that at study sites with low nitrate concentrations (\approx3\muM) a 300 permil dosing rate of 15N labled Ca(NO$_{3}$)$_{2}$ (\+98% 15N) would not increase mesocosm nitrate above ambient concentrations and would provide a significant isotopic signal to track fluxes of N among ecosystem components. In addition, an experiment was conducted in the same region to determine natural abundance values for ecosystem components at both near-canal and downstream sites. All components sampled at the near-canal site had significantly heavier nitrogen isotopic values than did the downstream components (\delta 15N 7.44\permil \pm2.11 and 2.01\permil \pm1.44 respectively); suggesting the Southern Everglades marsh is acting as a sink for canal-borne DIN and a source for "new" marsh derived DON.
H41D-0335 0800h
Investigations into the \delta $^{15}$N and \delta $^{13}$C of POM, Primary Producers, and Filter Feeders in the Florida Keys Coral Reef Tract
In order to examine the possible influences of anthropogenic wastes on the Florida Keys coral reef tract, water column particulate organic matter \(POM\), primary producers, and filter feeders were sampled from within the coastal waters of the reef tract, and analyzed for \delta $^{15}$N and \delta $^{13}$C isotopic composition. The POM was found to have a mean \delta $^{15}$N value of +4.13\permil (\pm 1.03\permil\) and a mean \delta $^{13}$C value of -19.93\permil (\pm 0.58\permil). The \delta $^{13}$C values for POM across the entire reef tract were heavier inshore \(avg \delta $^{13}$C = -18.29\permil \(\pm 1.04\permil\)\) and became lighter offshore \(avg \delta $^{13}$C = -21.41\permil \(\pm 0.89\permil\)\), which is consistent with influences from benthic algae/seagrass closer to shore in contrast to open-marine influences offshore. The \delta $^{15}$N values for POM were neither temporally nor spatially interrelated, as the data showed no clear trend moving in either the inshore or offshore direction. Instead, our \delta $^{15}$N values for POM reflect a combined influence from nitrogen fixation \(\approx0\permil\), zooplankton and other suspended particles, and not anthropogenic wastes \($>$+10\permil\), as other authors claim. The mean isotopic composition of various primary producers suggests a nitrogen fixation source of nutrients to the reef system ({\it Halimeda sp.} (\delta $^{15}$N = 2.43\permil; \delta $^{13}$C = -18.04\permill), {\it Dictyota sp.} (\delta $^{15}$N = +2.86\permil; \delta $^{13}$C = -16.30\permil), {\it Thalassia sp.} (\delta $^{15}$N = 2.39\permil; \delta $^{13}$C = -10.32\permil)), while filter feeders were found to have a mean isotopic composition more similar to POM values (sponge \delta $^{15}$N = 4.95\permil; \delta $^{13}$C = -18.02\permil). Our data simply do not appear to show evidence of anthropogenic influence on the Florida Keys reef tract. Further more, we suggest that \delta $^{15}$N values of +4\permil, which have recently been suggested to indicate anthropogenic influences are not, in fact, indicative of sewage.