Regional patterns in the isotopic composition of natural and anthropogenic nitrate in groundwater, High Plains, U.S.A

Mobilization of natural nitrate (NO3-) deposits in the subsoil by irrigation water in arid and semiarid regions has the potential to produce large groundwater NO3-concentrations. The use of isotopes to distinguish between natural and anthropogenic NO3- sources in these settings could be complicated by the wide range in delta15N values of natural NO3-. An approximately 10 000 year record of paleorecharge from the regionally extensive High Plains aquifer indicates that delta15N values for NO3- derived from natural sources ranged from 1.3 to 12.3 per thousand and increased systematically from the northern to the southern High Plains. This collective range in delta15N values spans the range that might be interpreted as evidence for fertilizer and animal-waste sources of NO3-; however, the delta15N values for NO3- in modern recharge (< 50 years) under irrigated fields were, for the most part, distinctly different from those of paleorecharge when viewed in the overall regional context. An inverse relation was observed between the delta15N[NO3-] values and the NO3-/Cl- ratios in paleorecharge that is qualitatively consistent with fractionating losses of N increasing from north to south in the High Plains. N and O isotope data for NO3- are consistent with both NH3 volatilization and denitrification, having contributed to fractionating losses of N prior to recharge. The relative importance of different isotope fractionating processes may be influenced by regional climate patterns as well as by local variation in soils, vegetation, topography, and moisture conditions.     

Isotopic apportionment of atmospheric and sewage nitrogen sources in two Connecticut rivers

We used the dual isotope approach to identify sources of nitrate (NO3-) to two mixed land-use watersheds draining to Long Island Sound. In contrastto previous work, we found that sewage effluent NO3- was not consistently enriched in 15N. However, these effluents followed a characteristic denitrification line in delta15N-delta18O space, which could be used as a source signature. We used this signature, together with those of atmospheric deposition and microbial nitrification, to calculate ranges of possible contributions from each of these sources. These estimates are unaffected by any denitrification that may have taken place in soils or streams. Our estimates for atmospheric nitrogen only include unprocessed atmospheric deposition, i.e., NO3-that is not taken up in watershed soils before being delivered to rivers. Using this method, the contribution of atmospheric NO3- could be assessed with good precision and was found to be very low at all our sampling sites during baseflow. During a moderate storm event, atmospheric deposition contributed up to approximately 50% of stream NO3-, depending on the site, with the sites that experienced more stormflow showing a greater contribution of atmospheric NO3-. Our estimates of sewage contribution generally had too large a range to be useful.     

Tracking sources of unsaturated zone and groundwater nitrate contamination using nitrogen and oxygen stable isotopes at the Hanford site, Washington

The nitrogen and oxygen isotopic compositions of nitrate in pore water extracts from unsaturated zone (UZ) core samples and groundwater samples indicate at least four potential sources of nitrate in groundwaters at the U.S. DOE Hanford Site in south-central Washington. Natural sources of nitrate identified include microbially produced nitrate from the soil column (delta15N of 4 - 8 per thousand, delta18O of -9 to 2 per thousand) and nitrate in buried caliche layers (delta15N of 0-8 per thousand, delta 18O of -6to 42 per thousand). Isotopically distinctindustrial sources of nitrate include nitric acid in low-level disposal waters (delta15N approximately per thousand, delta 18O approximately 23%o) per thousandnd co-contaminant nitrate in high-level radioactive waste from plutonium processing (6'5delta1of 8-33 % o, per thousand18delta oO -9 to 7%0). per thousandThe isotopic compositions of nitrate from 97 groundwater wells with concentrations up to 1290 mg/L NO3- have been analyzed. Stable isotope analyses from this study site, which has natural and industrial nitrate sources, provide a tool to distinguish nitrate sources in an unconfined aquiferwhere concentrations alone do not. These data indicate that the most common sources of high nitrate concentrations in groundwater at Hanford are nitric acid and natural nitrate flushed out of the UZ during disposal of low-level wastewater. Nitrate associated with high-level radioactive UZ contamination does not appear to be a major source of groundwater nitrate at this time.     

Evaluating regional patterns in nitrate sources to watersheds in National Parks of the Rocky Mountains using nitrate isotopes

In the Rocky Mountains, there is uncertainty about the source areas and emission types that contribute to nitrate (NO3) deposition, which can adversely affect sensitive aquatic habitats of high-elevation watersheds. Regional patterns in NO3 deposition sources were evaluated using NO3 isotopes in five National Parks, including 37 lakes and 7 precipitation sites. Results indicate that lake NO3 ranged from detection limit to 38 microeq/L, delta18O (NO3) ranged from -5.7 to +21.3% per thousand, and delta15N (NO3) ranged from -6.6 to +4.6 per thousand. delta18O (NO3) in precipitation ranged from +71 to +78% per thousand. delta15N (NO3) in precipitation and lakes overlap; however, delta15N (NO3) in precipitation is more depleted than delta15N (NO3) in lakes, ranging from -5.5 to -2.0 per thousand. delta15N (NO3) values are significantly related (p < 0.05) to wet deposition of inorganic N, sulfate, and acidity, suggesting that spatial variability of delta15N (NO3) over the Rocky Mountains may be related to source areas of these solutes. Regional patterns show that NO3 and delta15N (NO3) are more enriched in lakes and precipitation from the southern Rockies and at higher elevations compared to the northern Rockies. The correspondence of high NO3 and enriched delta15N (NO3) in precipitation with high NO3 and enriched delta15N (NO3) in lakes, suggests that deposition of inorganic N in wetfall may affect the amount of NO3 in lakes through a combination of direct and indirect processes such as enhanced nitrification.     

Nitrate contamination in groundwater on an urbanized dairy farm

Urbanization of rural farmland is a pervasive trend around the globe, and maintaining and protecting adequate water supplies in suburban areas is a growing problem. Identification of the sources of groundwater contamination in urbanized areas is problematic, but will become important in areas of rapid population growth and development. The isotopic composition of NO3 (delta15N(NO3) and delta18O NO3), NH4 (delta15N(NH4)), groundwater (delta2H(wt) and delta18O(wt)) and chloride/bromide ratios were used to determine the source of nitrate contamination in drinking water wells in a housing development that was built on the site of a dairy farm in the North Carolina Piedmont, U.S. The delta15N(NO3) and delta18O NO3 compositions imply that elevated nitrate levels at this site in drinking well water are the result of waste contamination, and that denitrification has not significantly attenuated the groundwater nitrate concentrations. delta15N(NO3) and delta18O(NO3) compositions in groundwater could not differentiate between septic effluent and animal waste contamination. Chloride/ bromide ratios in the most contaminated drinking water wells were similar to ratios found in animal waste application fields, and were higher than Cl/Br ratios observed in septic drain fields in the area. delta18O(wt) was depleted near the site of a buried waste lagoon without an accompanying shift in delta2H(wt) suggesting water oxygen exchange with CO2. This water-CO2 exchange resulted from the reduction of buried lagoon organic matter, and oxidation of the released gases in aerobic soils. delta18O(wt) is not depleted in the contaminated drinking water wells, indicating that the buried dairy lagoon is not a source of waste contamination. The isotope and Cl/Br ratios indicate that nitrate contamination in these drinking wells are not from septic systems, but are the result of animal waste leached from pastures into groundwater during 35 years of dairy operations which did not violate any existing regulations. Statutes need to be enacted to protect the health of the homeowners that require well water to be tested prior to the sale of homes built on urbanized farmland.     

Effect of storage on the isotopic composition of nitrate in bulk precipitation

Stable isotopic analysis of atmospheric nitrate is increasingly employed to study nitrate sources and transformations in forested catchments. Large volumes have typically been required for delta18O and delta15N analysis of nitrate in precipitation due to relatively low nitrate concentrations. Having bulk collectors accumulate precipitation over an extended time period allows for collection of the required volume as well as reducing the total number of analyses needed to determine the isotopic composition of mean annual nitrate deposition. However, unfiltered precipitation left in collectors might be subject to microbial reactions that can alter the isotopic signature of nitrate in the sample. Precipitation obtained from the Turkey Lakes Watershed was incubated under conditions designed to mimic unfiltered storage in bulk precipitation collectors and monitored for changes in nitrate concentration, delta15N, and delta18O. Results of this experiment indicated that no detectable nitrate production or assimilation occurred in the samples during a two-week incubation period and that atmospheric nitrate isotopic ratios were preserved. The ability to collect unfiltered precipitation samples for an extended duration without alteration of nitrate isotope ratios is particularly useful at remote study sites where daily retrieval of samples may not be feasible.     

Fertilizer characterization: isotopic data (N, S, O, C, and Sr)

A detailed isotopic characterization (delta15N(Ntotal), delta15N(NO3), delta18O(NO3), delta34S(SO4), delta18O(SO4), (delta13C(Ctotal), and 87Sr/86Sr) of 27 commercial fertilizers used in Spain is presented in this paper. Results together with a compilation of fertilizer isotopic published data are used for two purposes: (i) to identify the origin of the primary constituents and raw materials used in fertilizer manufacture and relate these data with their heavy metals and rare earth elements (REE) contents; (ii) to compare the fertilizer isotopic signatures with natural values and other anthropogenic pollutants and evaluate the usefulness of multi-isotopic analyses to trace fertilizer contaminations in future study cases. Isotope data permit us to know, in most cases, the origin of the primary constituents of fertilizers, and the 87Sr/86Sr ratio distinguishes the origin of the phosphate content--phosphorites or carbonatites--which in turn implies a qualitatively defined and potentially contaminant presence of REE and heavy metals in fertilizers. Delta15N, delta34S, and 87Sr/86Sr have already been used to trace fertilizer contaminations. Their utility can be improved by the coupled use of delta15N(NO3)-delta18O(NO3) and delta34S(SO4)-delta18O(SO4) to evaluate the fractionation processes that can affect contaminants. Moreover, multi-isotopic analyses, using heavy isotopes, allow us to see beyond the fractionation effects to the fertilizer stable isotope signatures and a better distinction from other anthropogenic contaminants.     

Effect of different carbon substrates on nitrate stable isotope fractionation during microbial denitrification

In batch experiments, we studied the isotope fractionation in N and O of dissolved nitrate during dentrification. Denitrifying strains Thauera aromatica and "Aromatoleum aromaticum strain EbN1" were grown under strictly anaerobic conditions with acetate, benzoate, and toluene as carbon sources. (18)O-labeled water and (18)O-labeled nitrite were added to the microcosm experiments to study the effect of putative backward reactions of nitrite to nitrate on the stable isotope fractionation. We found no evidence for a reverse reaction. Significant variations of the stable isotope enrichment factor ε were observed depending on the type of carbon source used. For toluene (ε(15)N, -18.1 ± 0.6‰ to -7.3 ± 1.4‰; ε(18)O, -16.5 ± 0.6‰ to -16.1 ± 1.5‰) and benzoate (ε(15)N, -18.9 ± 1.3‰; ε(18)O, -15.9 ± 1.1‰) less negative isotope enrichment factors were calculated compared to those derived from acetate (ε(15)N, -23.5 ± 1.9‰ to -22.1 ± 0.8‰; ε(18)O, -23.7 ± 1.8‰ to -19.9 ± 0.8‰). The observed isotope effects did not depend on the growth kinetics which were similar for the three types of electron donors. We suggest that different carbon sources change the observed isotope enrichment factors by changing the relative kinetics of nitrate transport across the cell wall compared to the kinetics of the intracellular nitrate reduction step of microbial denitrification.     

Low δ(18)O Values of Nitrate Produced from Nitrification in Temperate Forest Soils

Analyses of δ(18)O of nitrate (NO(3)(-)) have been widely used in partitioning NO(3)(-) sources. However the δ(18)O value of NO(3)(-) produced from nitrification (microbial NO(3)(-)) is commonly estimated using the δ(18)O of environmental water and molecular oxygen in a 2:1 ratio. Here our laboratory incubation of nine temperate forest soils across a 1500 m elevation gradient demonstrates that microbial NO(3)(-) has lower δ(18)O values than the predicted using the 2:1 ratio (by 5.2-9.5‰ at low elevation sites), in contrast to previous reports showing higher δ(18)O values (up to +15‰) than their predicted values. Elevated δ(18)O values of microbial NO(3)(-) were observed at high elevation sites where soil was more acidic, perhaps due to accelerated O-exchange between nitrite, an intermediate product of nitrification, and water. Lower δ(18)O of microbial NO(3)(-) than the predicted and from previous observations suggests that the contribution of anthropogenic N inputs, such as fertilizer and atmospheric deposition, to a given ecosystem and the progress of denitrification in nitrogen removal are greater than we know. More than half of the δ(18)O of stream NO(3)(-) lower than the predicted value along the elevation gradient also indicate the impropriety using the 2:1 ratio for differentiating NO(3)(-) sources.     

Hydrodynamic and microbial processes controlling nitrate in a fissured-porous karst aquifer of the Franconian Alb, southern Germany

Concentrations and stable isotope compositions of nitrate from 11 karst springs in the Franconian Alb (southern Germany) were determined during low flow and high flow conditions to assess sources and processes affecting groundwater nitrate. During low flow, nitrate concentrations in groundwater were around 0.10 mM in springs draining forested catchments, whereas in agricultural areas nitrate concentrations were typically higher reaching up to 0.93 mM. The isotopic composition of groundwater nitrate during low flow (delta15N values of -3.1 to 6.7% per hundred, delta180 values of +2.1 to 4.0% per hundred) in concert with concentration data suggests that nitrate is formed by nitrification in forest and agricultural soils. In addition, synthetic fertilizer N that has undergone immobilization and subsequent remineralization likely constitutes an additional nitrate source in agriculturally used catchments. During recharge conditions, concentrations and delta15N values of groundwater nitrate changed little, but delta18O values were significantly elevated (up to 24.5%o per hundred suggesting that around 25% of the nitrate was directly derived from atmospheric deposition. Groundwater dating revealed that low nitrate concentrations in groundwater (_> or =0 years) are consistent with a mixture of old low nitrate-containing and young water, the latter being affected by anthropogenic N inputs predominantly in the agriculturally used catchment areas during the last few decades. Thermodynamic and hydrogeological evidence also suggests that denitrification may have occurred in the porous rock matrix of the karst aquifer. This study demonstrates that a combination of hydrodynamic, chemical, and isotopic approaches provides unique insights into the sources and the biogeochemical history of nitrate in karst aquifers, and therefore constitutes a valuable tool for assessing the vulnerability of karst aquifers to nitrate pollution in dependence on land use and assessing their self-purification capacity.     

Using delta15N- and delta18O-values to identify nitrate sources in karst ground water, Guiyang, southwest China

Nitrate pollution of the karstic groundwater is an increasingly serious problem with the development of Guiyang, the capital city of Guizhou Province, southwest China. The higher content of NO3- in groundwater compared to surface water during both summer and winter seasons indicates that the karstic groundwater system cannot easily recover once contaminated with nitrate. In order to assess the sources and conversion of nitrate in the groundwater of Guiyang, we analyzed the major ions, delta(15)N-NH4+, delta(15)N-NO3-, and delta(18)O-NO3- in surface and groundwater samples collected during both summer and winter seasons. The results show that nitrate is the major dominant species of nitrogen in most water samples and there is a big variation of nitrate sources in groundwater between winter and summer season, due to fast response of groundwater to rain or surface water in the karst area. Combined with information on NO3- /Cl-, the variations of the isotope values of nitrate in the groundwater show a mixing process of multiple sources of nitrate, especially in the summer season. Chemical fertilizer and nitrification of nitrogen-containing organic materials contribute nitrate to suburban groundwater, while the sewage effluents and denitrification mainly control the nitrate distribution in urban groundwater.     

Using ¹⁷O to investigate nitrate sources and sinks in a semi-arid groundwater system

We apply a triple isotope approach for nitrate that utilizes Δ(17)O as a conservative tracer, in combination with δ(18)O and δ(15)N, to assess source/sink dynamics of groundwater nitrate beneath alluvial washes in a semiarid urban setting. Other studies have used δ(18)O and δ(15)N to determine nitrate sources and cycling, but the atmospheric δ(18)O signature can be overprinted by biogeochemical processes. In this study, δ(18)O and δ(15)N values of nitrate were coupled with δ(17)O values of nitrate to quantify atmospheric nitrate inputs and denitrification amounts. Results show generally low groundwater nitrate concentrations (<0.2 mmol/L) throughout the basin; high nitrate concentrations (up to 1 mmol/L) with evidence for some denitrification were detected in areas where effluent was the predominant source of recharge to groundwater. Furthermore, the denitrification was inferred from elevated δ(18)O and δ(15)N values which were reinforced by increases in observed δ(17)O values. Finally, relatively low, but significant atmospheric nitrate concentrations were measured in groundwater (up to 6% of total nitrate). This study concludes that the triple isotope approach improves determination of the proportion of atmospheric nitrate and the significance of denitrification in natural waters, allowing us to develop a conceptual model of the biogeochemical processes controlling nitrogen in an urban setting.     

Saturated zone denitrification: potential for natural attenuation of nitrate contamination in shallow groundwater under dairy operations

We present results from field studies at two central California dairies that demonstrate the prevalence of saturated-zone denitrification in shallow groundwater with 3H/ 3He apparent ages of < 35 years. Concentrated animal feeding operations are suspected to be major contributors of nitrate to groundwater, but saturated zone denitrification could mitigate their impact to groundwater quality. Denitrification is identified and quantified using N and O stable isotope compositions of nitrate coupled with measurements of excess N2 and residual NO3(-) concentrations. Nitrate in dairy groundwater from this study has delta15N values (4.3-61 per thousand), and delta18O values (-4.5-24.5 per thousand) that plot with delta18O/delta15N slopes of 0.47-0.66, consistent with denitrification. Noble gas mass spectrometry is used to quantify recharge temperature and excess air content. Dissolved N2 is found at concentrations well above those expected for equilibrium with air or incorporation of excess air, consistent with reduction of nitrate to N2. Fractionation factors for nitrogen and oxygen isotopes in nitrate appear to be highly variable at a dairy site where denitrification is found in a laterally extensive anoxic zone 5 m below the water table, and at a second dairy site where denitrification occurs near the water table and is strongly influenced by localized lagoon seepage.     

Decadal geochemical and isotopic trends for nitrate in a transboundary aquifer and implications for agricultural beneficial management practices

Nitrate contamination of aquifers is a global agricultural problem. Agricultural beneficial management practices (BMPs) are often promoted as a means to reduce nitrate contamination in aquifers through producer optimized management of inorganic fertilizer and animal manure inputs. In this study, decadal trends (1991-2004) in nitrate concentrations in conjunction with 3H/3He groundwater ages and nitrate stable isotopes (delta15N, delta18O) were examined to determine whether BMPs aimed at reducing aquifer-scale nitrate contamination in the transboundary Abbotsford-Sumas aquifer were effective. A general trend of increasing nitrate concentrations in young groundwater (< approximately 5 yr) suggested that voluntary BMPs were not having a positive impact in achieving groundwater quality targets. While the stable isotope data showed that animal manure was and still is the prevalent source of nitrate in the aquifer, a recent decrease in delta15N in nitrate suggests a BMP driven shift away from animal wastes toward inorganic fertilizers. The coupling of long-term monitoring of nitrate concentrations, nitrate isotopes, and 3H/3He age dating proved to be invaluable, and they should be considered in future assessments of the impact of BMPs on nutrients in groundwaters. The findings reveal that BMPs should be better linked to groundwater nutrient monitoring programs in order to more quickly identify BMP deficiencies, and to dynamically adjust nutrient loadings to help achieve water quality objectives.     

Nitrogen isotopes as indicators of NO(x) source contributions to atmospheric nitrate deposition across the midwestern and northeastern United States

Global inputs of NO(x) are dominated by fossil fuel combustion from both stationary and vehicular sources and far exceed natural NO(x) sources. However, elucidating NO(x) sources to any given location remains a difficult challenge, despite the need for this information to develop sound regulatory and mitigation strategies. We present results from a regional-scale study of nitrogen isotopes (delta15N) in wet nitrate deposition across 33 sites in the midwestern and northeastern U.S. We demonstrate that spatial variations in delta15N are strongly correlated with NO(x) emissions from surrounding stationary sources and additionally that delta15N is more strongly correlated with surrounding stationary source NO(x) emissions than pH, SO4(2-), or NO3- concentrations. Although emission inventories indicate that vehicle emissions are the dominant NO(x) source in the eastern U.S., our results suggest that wet NO3- deposition at sites in this study is strongly associated with NO(x) emissions from stationary sources. This suggests that large areas of the landscape potentially receive atmospheric NO(y) deposition inputs in excess of what one would infer from existing monitoring data alone. Moreover, we determined that spatial patterns in delta15N values are a robust indicator of stationary NO(x) contributions to wet NO3- deposition and hence a valuable complement to existing tools for assessing relationships between NO3- deposition, regional emission inventories, and for evaluating progress toward NO(x) reduction goals.     

Stable nitrogen isotopes in waterfowl feathers reflect agricultural land use in western Canada

Stable nitrogen and carbon isotope analysis was performed on secondary feathers collected from flightless, mallard ducklings (Anas platyrhynchos) from 17 locations across western Canada. The delta15N values of mallard feathers ranged from +6.1 to +23.7 per thousand (AIR). Mean delta15N feather values from the 17 locations were strongly correlated with the percentage of land under agricultural development. Higher delta15N values in waterfowl feathers collected from agricultural areas possibly reflected the entry of excess fertilizer nitrogen into local water bodies. However, other processes may have also been important. These results provide evidence that nitrogen isotope values in avian feathers may reflect long-term nitrogen additions to surface waters in agricultural areas and may also provide important clues in elucidating the origin of nonpoint source nitrogen inputs.     

Impacts of land use change on nitrogen cycling archived in semiarid unsaturated zone nitrate profiles, southern High Plains, Texas

Nitrate (NO3) profiles in semiarid unsaturated zones archive land use change (LUC) impacts on nitrogen (N) cycling with implications for agricultural N management and groundwater quality. This study quantified LUC impacts on NO3 inventories and fluxes by measuring NO3 profiles beneath natural and rainfed (nonirrigated) agricultural ecosystems in the southern High Plains (SHP). Inventories of NO3-N under natural ecosystems in the SHP normalized by profile depth are extremely low (2-10 kg NO3-N/ha/m), in contrast to those in many semiarid regions in the southwestern U.S. Many profiles beneath cropland (9 of 19 profiles) have inventories at depth that range from 28-580 kg NO3--N/ha/m (median 135 kg/ha/m) that correspond to initial cultivation, dated using soil water Cl. These inventories represent 74% (median) of the total inventories in these profiles. This NO3 most likely originated from cultivation causing mineralization and nitrification of soil organic nitrogen (SON) in old soil water (precultivation) and is attributed to enhanced microbial activity caused by increased soil wetness beneath cropland (median matric potential -42 m) relative to that beneath natural ecosystems (median -211 m). The SON source is supported by isotopes of NO3 (delta15N: +5.3 to +11.6; delta18O: +3.6 to +12.1). Limited data in South Australia suggest similar processes beneath cropland. Mobilization of the total inventories in these profiles caused by increased drainage/ recharge related to cultivation in the SHP could increase current NO3-N levels in the underlying Ogallala aquifer by an additional 2-26 mg/L (median 17 mg/L).     

Major issues regarding the efficiency of monitoring programs for nitrate contaminated groundwater

Major issues regarding the efficiency of moni toring programs for nitrate contaminated groundwater are analyzed in this paper: (i) representativeness of monitoring networks; (ii) correct interpretation of the monitoring data and resulting time series and trends; and (iii) differentiation among the different sources of nitrates in groundwater. Following an overview of the nitrate contamination problem and possible solutions, as well as some of the difficulties found, a relatively straightforward method for assessing monitoring network representativity is presented, namely interpolation standard error assessment. It is shown how nitrate-concentration time series resulting from periodic observations can be corrected with a conservative tracer, in order to avoid misinterpretation and confirm or correct apparent trends. Finally, coupled 1?N and 1?O isotope signatures of nitrate (NO??) in groundwater are used to differentiate among nitrogen (N) sources, to ensure correct targeting of restoration measures. The case study regards a Nitrate Vulnerable Zone in the south of Portugal, designated in compliance with the European Nitrates Directive, where coastal discharge of nutrient-rich groundwater threatens the good qualitative and ecological status of the Ria Formosa coastal lagoon. Results show that mineral fertilizer is the main source of N in groundwater, and that increases in N load can be masked by dilution phenomena.     

Identifying sources of nitrogen to Hanalei Bay, Kauai, utilizing the nitrogen isotope signature of macroalgae

Sewage effluent, storm runoff, discharge from polluted rivers, and inputs of groundwater have all been suggested as potential sources of land derived nutrients into Hanalei Bay, Kauai. We determined the nitrogen isotopic signatures (delta(15)N) of different nitrate sources to Hanalei Bay along with the isotopic signature recorded by 11 species of macroalgal collected in the Bay. The macroalgae integrate the isotopic signatures of the nitrate sources over time, thus these data along with the nitrate to dissolved inorganic phosphate molar ratios (N:P) of the macroalgae were used to determine the major nitrate source to the bay ecosystem and which of the macro-nutrients is limiting algae growth, respectively. Relatively low delta(15)N values (average -0.5% per hundred) were observed in all algae collected throughout the Bay; implicating fertilizer, rather than domestic sewage, as an important external source of nitrogen to the coastal water around Hanalei. The N:P ratio in the algae compared to the ratio in the Bay waters imply that the Hanalei Bay coastal ecosystem is nitrogen limited and thus, increased nitrogen input may potentially impact this coastal ecosystem and specifically the coral reefs in the Bay. Identifying the major source of nutrient loading to the Bay is important for risk assessment and potential remediation plans.     

Indirect emissions of nitrous oxide from regional aquifers in the United Kingdom

Diffuse pollution of groundwater by agriculture has caused elevated concentrations of nitrate (NO3-) and nitrous oxide (N2O) in regional aquifers. N2O is an important "greenhouse" gas, yet there are few estimates of indirect emissions of N2O from regional aquifers. In this study, high concentrations of N2O (mean 602 nM) were measured in the unconfined Chalk aquifer of eastern England, in an area of intensive agriculture. In contrast, pristine groundwaters from upland regions of England and Scotland, with predominantly natural vegetation cover, were found to have much lower concentrations of N2O (mean 27 nM). A positive relationship between N2O and NO3- concentrations and delta18O-NO3 values of between 3.36 and 16.00/1000 suggest that nitrification is the principal source of N2O. A calculated emission factor (EF5-g) of 0.0019 for indirect losses of N2O from Chalk groundwater is an order of magnitude lower than the value of 0.015 currently used in the Intergovernmental Panel on Climate Change (IPCC) methodology for assessing agricultural emissions. A flux of N2O from the major UK aquifers of 0.04 kg N2O-N ha(-1) a(-1) has been calculated using two approaches and suggests that indirect losses of N2O from regional aquifers are much less significant (<1%) than direct emissions from agricultural soils.