Microbial nitrogen removal in a developing suburban estuary along the South Carolina coast

The conversion of undisturbed coastal regions to commercial and suburban developments may pose a threat to surface and groundwater quality by introducing nitrate-nitrogen (NO₃ ^?-N) from runoff of land-applied wastewater and fertilizers. Microbial denitrification is an important NO₃ ^?-N removal mechanism in coastal sediments. The objective of this study was to compare denitrification and nitrate conversion rates in coastal sediments from a golf course, suburban site, undeveloped marsh, and nonmarsh area near rapidly developing Hilton Head Island, South Carolina. Nitrous oxide was measured using gas chromatography and nitrate and ammonium concentrations were measured using a flow injection autoanalyzer in microcosms spiked, with 50 μg NO₃ ^?-N gdw^?1. The two marsh sites had the greatest ammonium production, which was correlated with fine sediment particle size and higher background sediment nitrate and surface water sulfate concentrations. The golf course swale had greatest denitrification rates, which were correlated with higher total carbon and organic nitrogen in sediments. Nitrate was consumed in golf course sediments to a greater extent than in the undeveloped marsh and upland freshwater sites, suggesting that the undeveloped sites and receiving estuaries may be more susceptible to nitrate contamination than the golf course swale and marsh under nonstorm conditions. Construction of swales and vegetated buffers using sediments with high organic carbon content as best management practices may aid in removing nitrate and other contaminants from runoff prior to its transport to the receiving marsh and estuary.     

Signatures of Restoration and Management Changes in the Water Quality of a Central California Estuary

Coastal managers and policy-makers are concerned with tracking improvements to water quality linked to management changes. Long-term water quality data acquired from two wetland areas in the upper reaches of the Elkhorn Slough estuary in central California were analyzed for signatures of land restoration or water control structure management. Post-restoration averaged NO₃, NH₃, and PO₄ concentrations were 50–70% less than before-restoration concentrations. Assessment of watershed-scale effects revealed that proximity of restoration to sampling locations had almost as strong an effect on water quality as the percentage of land restored relative to watershed size. Results also suggest that restoration of even 1% of an agriculturally intensive watershed such as that of the Elkhorn Slough may result in improvements to water quality. Finally, results indicate that tide gate function can dominate water quality in managed wetlands and must be carefully tracked and managed in the context of estuarine conservation targets.     

Tidal and Nontidal Oscillations in Elkhorn Slough,CA

Elkhorn Slough is a shallow, tidally forced estuary that is directly connected to Monterey Bay. It is ebb-dominated and, due to continued erosion, the tidal prism has tripled over the past 40 years. Water level measurements at four locations are used to examine tidal and nontidal oscillations in Elkhorn Slough. The tidal response of Elkhorn Slough differs from that of Monterey Bay primarily due to the generation of a relatively large number of shallow-water tidal constituents that are due to tidal distortion caused by friction along the bottom and lateral boundaries, intertidal storage, and nonlinear advection. The shallow-water constituents range from 3 to almost 15 cycles per day (cpd) and include a rich variety of overtides and compound tides, whose amplitudes generally increase toward the head of the slough. The tidal harmonics are seasonally dependent, with lower amplitudes during the fall and winter and higher amplitudes in summer. The tidal constituents were examined using two types of spectral decomposition, the conventional power spectrum and the more recent Hilbert spectrum. Unlike the power spectrum, the Hilbert spectrum does not reveal any harmonic structure in the data. Energy associated with tidal distortion in this case appears to be broadly distributed across the spectral continuum. At least four nontidal oscillations occur in Elkhorn Slough with frequencies of 26.0, 39.7, 52.7, and 66.9 cpd. The Hilbert spectrum reveals maxima at 26, 39.7, and 66.9 cpd, but not at 52.7 cpd, suggesting that it is harmonically related to the oscillation at 26.0 cpd. The nontidal oscillations fall into the range of frequencies associated with the natural oscillations of Monterey Bay. However, evolutionary power spectra indicate that they appear to be permanent features of the system and thus are not necessarily consistent with seiche-like oscillations that are often transient and subject to damping. These oscillations could be caused by several factors including edge waves along the coast of Monterey Bay, long-period surface waves of atmospheric origin that enter the bay from offshore, or breaking internal waves in and around the Monterey Submarine Canyon. In conclusion, detailed hydrodynamic models are needed to provide a better understanding of how tidal harmonics are generated and preserved in Elkhorn Slough, and to determine the origin of the natural oscillations in Monterey Bay.     

Sediment-water oxygen and nutrient exchanges along the longitudinal axis of Chesapeake Bay: Seasonal patterns,controlling factors and ecological significance

Sediment-water oxygen and nutrient (NH₄ ⁺, NO₃ ^?+NO₂ ^?, DON, PO₄ ^3?, and DSi) fluxes were measured in three distinct regions of Chesapeake Bay at monthly intervals during 1 yr and for portions of several additional years. Examination of these data revealed strong spatial and temporal patterns. Most fluxes were greatest in the central bay (station MB), moderate in the high salinity lower bay (station SB) and reduced in the oligohaline upper bay (station NB). Sediment oxygen consumption (SOC) rates generally increased with increasing temperature until bottom water concentrations of dissolved oxygen (DO) fell below 2.5 mg l^?1, apparently limiting SOC rates. Fluxes of NH₄ ⁺ were elevated at temperatures >15°C and, when coupled with low bottom water DO concentrations (<5 mg l^?1), very large releases (>500 μmol N m^?2 h^?1) were observed. Nitrate + nitrite (NO₃ ^?+NO₂ ^?) exchanges were directed into sediments in areas where bottom water NO₃ ^?+NO₂ ^? concentrations were high (>18 μM N); sediment efflux of NO₃ ^?+NO₂ ^? occurred only in areas where bottom water NO₃ ^?+NO₂ ^? concentrations were relatively low (<11 μM N) and bottom waters well oxygenated. Phosphate fluxes were small except in areas of hypoxic and anoxic bottom waters; in those cases releases were high (50–150 μmol P m^?2 h^?1) but of short duration (2 mo). Dissolved silicate (DSi) fluxes were directed out of the sediments at all stations and appeared to be proportional to primary production in overlying waters. Dissolved organic nitrogen (DON) was released from the sediments at stations NB and SB and taken up by the sediments at station MB in summer months; DON fluxes were either small or noninterpretable during cooler months of the year. It appears that the amount and quality of organic matter reaching the sediments is of primary importance in determining the spatial variability and interannual differences in sediment nutrient fluxes along the axis of the bay. Surficial sediment chlorophyll-a, used as an indicator of labile sediment organic matter, was highly correlated with NH₄ ^?, PO₄ ^3?, and DSi fluxes but only after a temporal lag of about 1 mo was added between deposition events and sediment nutrient releases. Sediment O:N flux ratios indicated that substantial sediment nitrification-denitrification probably occurred at all sites during winter-spring but not summer-fall; N:P flux ratios were high in spring but much less than expected during summer, particularly at hypoxic and anoxic sites. Finally, a comparison of seasonal N and P demand by phytoplankton with sediment nutrient releases indicated that the sediments provide a substantial fraction of nutrients required by phytoplankton in summer, but not winter, especially in the mid bay region.     

Fortnightly Tidal Modulations Affect Net Community Production in a Mesotidal Estuary

Optical in situ chemical sensors enable sampling intervals and durations that rival acoustic techniques used for measuring currents. Coupling these high-frequency biogeochemical and physical measurements in estuaries to address ecosystem-scale questions, however, is still comparatively novel. This study investigated how tides affect ecosystem metabolism in a mesotidal estuary in central California (Elkhorn Slough). Dissolved oxygen measurements were used to estimate the terms in a control volume budget for a tidal creek/marsh complex at tidal timescales over several weeks. Respiration rates were 1.6 to 7.3 g O₂ m^?2 day^?1; net community production approached 20 g O₂ m^?2 day^?1. We found that aquatic NCP integrated throughout the creek complex varied significantly over the spring-neap cycle. The intertidal contribution to aquatic metabolism was net heterotrophic during spring tides and generally in balance during neap tides because spring-tide marsh inundation was limited to nighttime, and therefore the marsh could not contribute any primary production to the water column. At the estuary scale, the fortnightly export of oxygen from the main channel to the intertidal was largely balanced by an advective flux up-estuary.     

Sediment-water oxygen and nutrient fluxes in a river-dominated estuary

Sediment oxygen uptake and net sediment-water fluxes of dissolved inorganic and organic nitrogen and phosphorus were measured at two sites in Fourleague Bay, Louisiana, from August 1981, through May 1982. This estuary is an extension of Atchafalaya Bay which receives high discharge and nutrient loading from the Atchafalaya River. Sediment O₂ uptake averaged 49 mg m^?2 h^?1. On the average, ammonium (NH₄ ⁺) was released from the sediments (mean flux =+129 μmol m^?2 h^?1), and NO₃ ^? was taken up (mean flux =?19 μmol m^?2h^?1). However, very different NO₃ ^? fluxes were observed at the two sites, with sediment uptake at the upper, river-influenced, high NO₃ ^? site (mean flux =?112 μmol m^?2 h^?1) and release at the lower, marine-influenced low NO₃ ^? site (mean flux =+79 μmol m^?2 h^?1). PO₄ ^3? fluxes were low and often negative (mean flux =?8 μmol m^?2 h^?1), while dissolved organic phosphorus fluxes were high and positive (mean flux =+124 μmol m^?2 h^?1). Dissolved organic nitrogen fluxes varied greatly, ranging from a mean of +305 μmol m^?2 h^?1 at the lower bay, to ?710 μmol m^?2 h^?1 at the upper bay. Total dissolved nitrogen and phosphorus fluxes indicated the sediments were a nitrogen (mean flux =+543 μmol m^?2 h^?1) and phosphorus source (mean flux =+30 μmol m^?2 h^?1) at the lower bay, and a nitrogen sink (mean flux =?553 μmol m^?2 h^?1) and phosphorus source (mean flux =+17 μmol m^?2 h^?1) in the upper bay. Mean annual O∶N ration of the positive inorganic sediment fluxes were 27∶1 at the upper bay and 18∶1 at the lower bay. Based on these data we hypothesize that nitrification and denitrification are important sediment processes in the upper bay. We further hypothesize that Atchafalaya River discharge affects sediment-water fluxes through seasonally high nutrient loading which leads to net nutrient uptake by sediments in the upper bay and release in the lower bay, where there is less river influnces.     

Early oxidation of organic matter in pelagic sediments of the eastern equatorial Atlantic: suboxic diagenesis

Pore water profiles of total-CO₂, pH, PO^3?₄, NO^?₃ plus NO^?₂, SO^2?₄, S^2?, Fe²⁺ and Mn²⁺ have been obtained in cores from pelagic sediments of the eastern equatorial Atlantic under waters of moderate to high productivity. These profiles reveal that oxidants are consumed in order of decreasing energy production per mole of organic carbon oxidized (O₂ > manganese oxides ~ nitrate > iron oxides > sulfate). Total CO₂ concentrations reflect organic regeneration and calcite dissolution. Phosphate profiles are consistent with organic regeneration and with the effects of release and uptake during inorganic reactions. Nitrate profiles reflect organic regeneration and nitrate reduction, while dissolved iron and manganese profiles suggest reduction of the solid oxide phases, upward fluxes of dissolved metals and subsequent entrapment in the sediment column. Sulfate values are constant and sulfide is absent, reflecting the absence of strongly anoxic conditions.     

Rates of nitrification along an estuarine gradient in Narragansett Bay

Rates of pelagic nitrification, measured using N-Serve-sensitive [¹⁴C]bicarbonate uptake, varied by as much as an order-of-magnitude among three sites along the salinity gradient of Narragansett Bay (Rhode Island, United States). Rates were always higher at the Providence River estuary site (0.04–11.2 μmol N I^?1 d^?1) than at either the lower Narragansett Bay site (0.02–0.98 μmol N I^?1d^?1) or the freshwater Blackstone River site (0.04–1.7 μmol N I^?1d^?1). Although temperature was the most important variable regulating the annual cycle of nitrification, ammonium concentrations were most likely responsible for the large differences in rates among the three sites in summer. At the levels found in this estuarine system, salinity and concentrations of oxygen or total suspended matter did not appear to have a direct measurable effect on nitrification and pH did only occasionally. Nitrification played an important role in the nitrogen cycle at all three sites. In Narragansett Bay, nitrification contributed 55% of the NO₂ ^? and NO₃ ^? entering annually, and was the major source during spring and summer. Water from offshore was the only other large source of NO₂ ^? and NO₃ ^?, contributing 34%. High summer rates of nitrification could support much of the phytoplankton uptake of NO₂ ^? and NO₃ ^?. In the Providence River estuary, the largest annual input of NO₂ ^? and NO₃ ^? was from rivers (54%), although nitrification (28%) and water from lower portions of the bay (11%) also made large contributions. Again, nitrification was most important in the summer. The high rates of nitrification in the Providence River estuary during summer were also likely to be important in terms of oxygen demand, and the production of nitric and nitrous oxides. In the Blackstone River, NO₂ ^? and NO₃ ^? concentrations increased as the river flowed through Rhode Island, and nitrification was a possible source.     

A poly-ε-caprolactone based biofilm carrier for nitrate removal from water

Nitrate removal from water has been accomplished by heterotrophic biofilms using organic carbon as a source of reducing power. To overcome the natural limitation in organic carbon in water, a poly-ε-caprolactone based biofilm carrier that serves simultaneously as a biofilm carrier and as a source of organic carbon was developed and tested in the present work. The feasibility of the new biofilm carrier for nitrate removal from water was evaluated in a packed bed reactor. The combination of size and structure provided a carrier element having high surface area and void volume, 1,170 m²/m³ and 67 %, respectively. A maximum denitrification rate of 4.4 mg N–NO₃ ^?/(L.h) (9.2 mg N–NO₃ ^?/(m².h)) was achieved in the packed bed reactor at 20 °C and pH 7.0. Main advantages of the biofilm carrier developed in the present work are its mechanical stability in water even after biofilm formation and controlled release of organic carbon by enzymatic reactions. The proposed biotechnology to remove nitrate from groundwater is robust and easy to operate.     

Ammonium excretion by benthic invertebrates and sediment-water nitrogen flux in the Gulf of Mexico near the Mississippi River outflow

Benthic macroinvertebrate biomass and ammonium excretion rates were measured at four stations in the Gulf of Mexico near the Mississippi River mouth. Calculated areal excretion rates were then compared to sediment-water nitrogen fluxes measured in benthic bottom lander chambers at similar stations to estimate the potential importance of macroinvertebrate excretion to sediment nitrogen mineralization. Excretion rates for individual crustaceans (amphipods and decapods) was 2–21 nmoles NH₄ ⁺ (mg dry weight)^?1 h^?1. The mean excretion rates for the polychaetes, Paraprionaspio pinnata [6–12 nmoles NH₄ ⁺ (mg dry weight)^?1h^?1] and Magelona sp. [27–53 nmoles NH₄ ⁺ (mg dry weight)^?1h^?1], were comparable or higher than previous measurements for similar size benthic or pelagic invertebrates incubated at the same temperature (22±1°C). Although the relatively high rates of excretion by these selective feeders may have been partially caused by experimental handling effects (e.g., removal from sediment substrates), they probably reflected the availability of nitrogen-rich food supplies in the Mississippi River plume. When the measured weight-specific rates were extrapolated to total areal biomass, areal macroinvertebrate excretion estimates ranged from 7 μmole NH₄ ⁺ m^?2h^?1 at a 40-m deep station near the river mouth to 18 μmole NH₄ ⁺ m^?2h^?1 at a shallower (28-m deep) station further from the river mouth. The net flux of ammonium and nitrate from the sediments to the water measured in bottom lander chambers in the same region were 15–53 μmole NH₄ ⁺ m^?2h^?1 and ?25–21 μmole NO₃ ^? m^?2h^?1. These results suggest that excretion of NH₄ ⁺ by macroinvertebrates could be a potentially important component of benthic nitrogen regeneration in the Mississippi River plume-Gulf shelf region.     

Temporal variability of nitrate concentration in a schist aquifer and transfer to surface waters

Nitrate concentrations monitored for 2.5 a in the stream water and groundwater of a small catchment, 86.5% of which is devoted to intensive agriculture, show temporal variations with a maximum during winter (as much as 200 mg l⁻¹ in groundwater and 100 mg l⁻¹ in stream water) and a minimum at the end of summer/beginning of autumn. Variations were also observed in the stream water and shallow groundwater after rainfall. The processes involved to explain these variations, determined mainly from NO₃⁻ Cl⁻, SO₄²⁻, piezometric and streamflow data, are: (a) variability of the relative contributions to stream water and shallow groundwater by upward fluxes of deeper groundwater which, as demonstrated previously, is denitrified mainly as a result of reaction with pyrite. (b) Denitrification of shallow groundwater during summer with organic matter acting as the electron donor. (c) Dilution by rain water. Nitrate concentrations in both stream water and shallow groundwater depend on the amount of precipitation, with an increased contribution from deep denitrified groundwater during dry periods. The temporal variations in NO₃⁻ concentration observed several metres below the water table are related to the preferential and rapid movement of NO₃⁻-polluted water through fractures and large fissures, which has been estimated at 1 m day⁻¹. Nitrate pollution in the catchment, because of the interaction with pyrite, also increases the net chemical weathering rate to values exceeding the world average.     

The influence of tidal action on porewater nitrate concentration and dynamics in intertidal sediments of the Sado estuary

The change in porewater nitrate (NO₂ ^? + NO₃ ^?) concentrations during exposure of intertidal sediment was studied at a fixed location in the Sado estuary, southwest Portugal, in November 1994. In order to follow nitrate concentration and dynamics from pre-ebb to post flood, during the day, high vertical resolution profiles (0.2 cm) were studied. As a complement, in February 1995, potential nitrification rates in the sediment were measured by laboratory incubations, with high vertical resolution (0.2 cm) up to 3 cm depth. Oxygen penetration was measured with polarographic mini-electrodes. The sediment’s texture as well as the organic matter composition in carbon and nitrogen were studied in deeper (30 cm) cores. In February 1993,²¹⁰Pb activity depth profiles were measured in a core sampled at the beginning of exposure, in order to evaluate the possibility of nonlocal particle exchange. C:N ratios and²¹⁰Pb activity profiles show evidence of nonlocal exchange of solid phase particles between the surface and deeper sediment, most likely due to macrofaunal activity. As a consequence, fresh organic matter is brought from the surface to 7–9 cm depth, causing enhancement of nutrient concentrations. Results of this study suggest nitrate dynamics in intertidal sediments of the Sado estuary are strongly influenced by tidal action. Periodic submersion and exposure allow for the diversification of pathways of oxygen supply to the sediment. Tidal stress at the sediment-water interface during the arrival (flooding) and departure (exposure) of the tidal front at the site has an important bearing on the effective depth of the nitrification zone. A denitrification rate of 2.16 μmol N dm^?5 h^?1 was measured directly from the nitrate inventory in the 1.5–6 cm depth layer. The schematic model of N cycling in these sediments suggests that 20% of the N pool is denitrified during exposure, and that this process is limited by O₂ availability for nitrification.     

Temporal and spatial patterns in abundance and diversity of fish assemblages in Elkhorn Slough,California

Assemblages of ichthyofauna of shallow inshore habitats along Californía’s central coast are described in terms of species composition, abundance, and life-style categories. A total of 22,334 fishes from 65 species and 27 families was collected with otter trawls at six sites in the main channel and tidal creeks of Elkhorn Slough, a tidal embayment and seasonal estuary, and two nearshore ocean stations in Monterey Bay during 44 months between August 1974 and June 1980. Greater than 90% of the catch comprised 10 species. The four dominant species,Cymatogaster aggregata, Leptocottus armatus, Phanerodon furcatus, andEmbiotoca jacksoni, occurred during most or all seasons and were classified as residents or partial residents. Several abundant species were marine immigrants that seasonally use the slough as spawning and nursery grounds; this resulted in higher abundance and species richness during summer. Species collected during winter largely were slough residents. Species compsosition and richness varied with distance from the slough entrance. The ocean assemblage was most different, and its similarity to other stations decreased progressively with distance inland and into the tidal creeks. During our study, 5,074 fishes were collected by beach seine in Bennett Slough, a remote shallow marsh basin adjacent to the entrance of Elkhorn Slough. Species richness was relatively low and three euryhaline species accounted for >80% of the total catch. The species assemblage was most similar to those at the tidal creek and most shallow stations of Elkhorn Slough. Resident species numerically dominated assemblages in Bennett Slough and the most inland areas of Elkhorn Slough. The high relative abundance of marine-related fishes (classified as marine, marine immigrant, and partial resident), entering Elkhorn Slough early in life or as spawning adults indicates the importance of this habitat to nearshore fish assemblages.     

The influence of subsurface hydrology on nutrient supply and smooth cordgrass (Spartina alterniflora) production in a developing barrier island marsh

The supply of nutrients from surface and subsurface water flow into the root zone was measured in a developing barrier island marsh in Virginia. We hypothesize that high production of tall-formSpartina alterniflora in the lower intertidal zone is due to a greater nitrogen input supplied by a larger subsurface flux. Individual nitrogen inputs to the tall-form and short-formS. alterniflora root zones were calculated from water flow rates into the root zone and the nutrient concentration corresponding to the source of the flow. Total dissolved inorganic nitrogen (DIN) input (as ammonium and nitrate) was then calculated using a summation of the hourly nutrient inputs to the root zone over the entire tidal cycle based on hydrologic and nutrient data collected throughout the growing season (April–August) of 1993 and 1994. Additionally, horizontal water flow into the lower intertidal marsh was reduced experimentally to determine its effects on nutrient input and plant growth. Total ammonium (NH₄ ⁺) input to the tall-formS. alterniflora root zone (168 μmoles 6 h^?1) was significantly greater relative to the short-form (45 μmoles 6 h^?1) during flood tide. Total NH₄ ⁺ input was not significantly different between growth forms during ebb tide, and total nitrate (NO₃ ^?) and total DIN input were not significantly different between growth forms during either tidal stage. During tidal flooding, vertical flow from below the root zone accounted for 71% and horizontal flow from the adjacent mudflat accounted for 19% of the total NH₄ ⁺ input to the tall-formS. alterniflora root zone. Infiltration of flooding water accounted for 15% more of the total NO₃ ^? input relative to the total NH₄ ⁺ input at both zones on flood tide. During ebb tide, vertical flow from below the root zone still accounted for the majority of NH₄ ⁺ and NO₃ ^? input to both growth forms. After vertical flow, horizontal subsurface flow from upgradient accounted for the next largest percentages of NH₄ ⁺ and NO₃ ^? input to both growth forms during ebb tide. After 2 yr of interrupted subsurface horizontal flow to the tall-formS. alterniflora root zone, height and nitrogen content of leaf tissue of treatment plants were only slightly, but significantly, lower than control plants. The results suggest that a dynamic supply of DIN (as influenced by subsurface water flows) is a more accurate depiction of nutrient supply to macrophytes in this developing marsh, relative to standing stock nutrient concentrations. The dynamic subsurface supply of DIN may play a role in spatial patterns of abovegroundS. alterniflora production, but determination of additional nitrogen inputs and the role of belowground production on nitrogen demand need to also be considered.     

The distribution of nitrogen speciation and sources of nitrate in the north of Taihu Lake

Meiliang Bay and Gonghu Bay, in the north of Taihu Lake, are important water sources for the city of Wuxi, and increased eutrophication now threatens the safety of drinking water. The distribution of nitrogen (N) speciation and source of N in the surface waters in the north of Taihu Lake is studied, which was an important first step in controlling N pollution. The result shows that the average concentration of ammonia (NH₄ ⁺) and nitrate (NO₃ ^?) of surface water in Meiliang Bay was 0.32 and 0.35 mg/L, while 0.21 and 0.74 mg/L of Gonghu Bay, in which both bays had serious nitrate pollution. The concentrations of NH₄ ⁺ and NO₃ ^? in the surface water of the two bays had a trend of gradual decrease from north to south. The maximum concentrations of NH₄ ⁺ and NO₃ ^? of two bays were observed near the inflowing rivers, and the maximum concentrations of NH₄ ⁺ in surface water of two bays were 0.49 and 0.61, and 0.77 and 1.38 mg/L of NO₃ ^?. The concentration of NH₄ ⁺ in the interstitial water of the two bays had a trend of gradual decrease from west to east, but NO₃ ^? had the opposite tendency. The maximum concentrations of NH₄ ⁺ in the interstitial water of the two bays were 5.88 and 4.64, and 3.58 and 7.18 mg/L of NO₃ ^?. The exchangeable NH₄ ⁺ content in the sediment of Meiliang Bay had a trend of gradual decrease from north to south, but Gonghu Bay showed the reverse. The exchangeable NO₃ ^? content in the sediment of Meiliang Bay had a trend of gradual decrease from east to west, but a decreasing trend from north to south was observed in Gonghu Bay. The maximum concentrations of exchangeable NH₄ ⁺ were determined, and the values were 96.25 and 74.90 mg/kg, as well as NO₃ ^? with the values of 12.06 and 7.08 mg/kg. Chemical fertilizer and domestic sewage were the major sources of nitrate in surface water of Gonghu Bay, contributing 39.16 and 47.79%, respectively. Domestic sewage was the major source of nitrate in Meiliang Bay, contributing 84.79%. The denitrification process in Gonghu Bay was more apparent than in Meiliang Bay. Mixing and dilution processes had important effects on changing the concentration of nitrate transportation in the two bays.     

Sources and Processes Affecting Nitrate in a Dam-Controlled Subtropical River,Southwest China

Excess nutrient (N and P) loads are recognized as the major cause of serious water quality problems in China. River systems play a very important role in nitrate (NO₃ ^?) transportation and transformation in the aquatic environment. To understand and clarify the sources and processes affecting NO₃ ^? in river basins, we have examined spatial and temporal variations of concentration and dual-isotopic composition of NO₃ ^? in the dam-controlled Jialing River, a major tributary of the Yangtze River where land use is dominated by agriculture. Water samples were collected in July 2008 and February 2009 from the main channel of the Jialing River and its major tributaries. The δ¹⁵N and δ¹⁸O of NO₃ ^? range from 1.5 to 11.0 ‰ (average 6.2 ‰) and ?5.0 to 11.1 ‰ (average, 1.6 ‰), respectively. NO₃ ^? isotope data and δ¹⁸O of water interpreted in combination with hydrological and chemical data suggest that most of the NO₃ ^? input is from nitrification during the rainy season, and discharge of sewage and manure in the upper course and from cities accounts for much of the NO₃ ^? load during the dry season. The construction of cascade dams has led to retention of Si and a decrease in the Si/N ratio, implying that assimilation and/or denitrification may significantly affect NO₃ ^? in the dam area, as demonstrated by NO₃ ^? and dissolved Si concentrations, and \(\updelta^{ 1 5} {\text{N}}_{{{\text{NO}}_{3} }}\) and \(\updelta^{ 1 8} {\text{O}}_{{{\text{NO}}_{3} }}\) values. This study indicates that dual-isotopic data can be used to identify NO₃ ^? pollution sources and the processes NO₃ ^? has undergone during its retention and transport in the watershed of the dam-controlled Jialing River.     

Sources of dissolved ions revealed by chemical and isotopic tracers in the Geum River,South Korea

Water samples were collected in the main channel of the Geum River, South Korea, and measured dissolved elemental concentrations and isotopic compositions of nitrate in order to identify the factors controlling water chemistry. Elemental concentrations significantly increased location-wise after the confluence from urban areas, indicating the changes in solute sources from chemical weathering to anthropogenic inputs such as manure, fertilizers, and sewage. In particular, the effect of sewage input is manifested in the concentrations of Cl^?, SO₄ ^2?, and Na⁺, while the NO₃ ^? concentration is influenced mainly by soil inputs with minor contributions from manure and fertilizer because both δ¹⁵N–NO₃ and δ¹⁸O–NO₃ indicate NO₃ ^? mostly consists of soil-derived nitrates in the upper reaches but manure/sewage nitrates in the lower reaches. The relative proportion of three factors, Cl^?, Ca²⁺+Mg²⁺, and NO₃ ^?, indicates that water chemistry in the upstream is controlled by the soil weathering but that in the downstream by the sewage. Seasonally, water chemistry during summer is dominated by the soil weathering due to the flushing effect but that during winter by the sewage. This study suggests that the relative proportion of three factors can be used for tracing natural and anthropogenic sources in water chemistry.     

Nitrogen and oxygen isotopic compositions of water-soluble nitrate in Taihu Lake water system, China: implication for nitrate sources and biogeochemical process

The stable isotope nitrogen-15 (¹⁵N) is a robust indicator of nitrogen (N) source, and the joint use of δ¹⁵N and δ¹⁸O–NO₃ ^? values can provide more useful information about nitrate source discrimination and N cycle process. The δ¹⁵N and δ¹⁸O–NO₃ ^? values, as well as major ion tracers, from Taihu Lake in east China were investigated to identify the primary nitrate sources and assess nitrate biogeochemical process in the present study. The results show that the nitrate concentration in West Taihu Lake (WTL) was generally higher than those in East Taihu Lake (ETL) and its upstream inflow rivers. The NO₃ ^?/Cl^? value combined with mapping of δ¹⁵N–NO₃ ^? and NO₃ ^? concentration suggest that the mixing process should play a major effect in WTL, and denitrification was the dominant N transformation process in WTL. A linear relationship of close to ~1: 2 was observed between δ¹⁵N–NO₃ ^? and δ¹⁸O–NO₃ ^? values in WTL, confirming the occurrence of denitrification in WTL. The δ¹⁵N–NO₃ ^? data imply that sewage and manure were the principal nitrate sources in WTL and its feeder rivers, while the nitrate in ETL might derive from soil organic nitrogen and atmospheric deposition. The δ¹⁸O–NO₃ ^? data indicate most of nitrate from microbial nitrification of organic nitrogen matter possibly make a significant contribution to the lake.     

Nitrogen circulation and nitrate in groundwater in an agricultural catchment in Southern India

High contents of nitrate in groundwater, ranging up to 1,500 mg/l, have been found. High concentrations are more common in village wells than in irrigation wells situated in the fields. The losses of nitrogen from the soil zone through deep leaching into the groundwater are small (260 kg N/km²); however, due to a small net infiltration (29 mm/year), the median content in groundwater still approaches the permissible limit of 50 mg/l NO ₃ ^? . In villages about 10–20% of the nitrogen from excreta are leached into the groundwater. Mineralization of soil nitrogen during a dry period, followed by heavy rains, caused extremely high contents of nitrate in groundwater.     

Tracking sources of groundwater nitrate contamination using nitrogen and oxygen stable isotopes at Beijing area,China

The identification of sources and behavior of contaminants is important to control and manage groundwater quality of aquifer systems in urban areas. In this study, hydrogeochemistry of major constituents and stable isotope ratios of nitrate in groundwater were determined to identify contamination sources and transformation processes occurring in soils and deeper groundwater of Beijing with intense human activities. The nitrogen and oxygen isotopic compositions of nitrate in pore water extracts from groundwater samples indicate at least three potential sources of nitrate in groundwaters at Beijing. Stable isotope analyses from this study site, which has atmospheric, chemical fertilizer and human waste nitrate sources, provide a tool to distinguish nitrate sources in a confined aquifer where concentrations alone do not. These data indicate that the most common sources of high nitrate concentrations in groundwater at Beijing are wastewater and denitrification process occurred specially in the Central area. NO₃–N and cation and anion concentrations (Ca²⁺, Mg²⁺ Cl^? and SO ₄ ² ) showed strong correlations indicating that they originated from the same sources. This study demonstrates that a thorough evaluation of hydrodynamic and hydrochemical parameters with dual isotopes of NO₃ ^? constitutes an effective approach for identifying sources and transformation processes of NO₃ ^? in deeper groundwater systems.