Geologic controls on the chemical behaviour of nitrate in riverside alluvial aquifers,Korea

To investigate the origin and behaviour of nitrate in alluvial aquifers adjacent to Nakdong River, Korea, we chose two representative sites (Wolha and Yongdang) having similar land‐use characteristics but different geology. A total of 96 shallow groundwater samples were collected from irrigation and domestic wells tapping alluvial aquifers. About 63% of the samples analysed had nitrate concentrations that exceeded the Korean drinking water limit (44·3 mg l^?1 NO₃^?), and about 35% of the samples had nitrate concentrations that exceeded the Korean groundwater quality standard for agricultural use (88·6 mg l^?1 NO₃^?). Based on nitrogen isotope analysis, two major nitrate sources were identified: synthetic fertilizer (about 4‰ δ¹⁵N) applied to farmland, and animal manure and sewage (15–20‰ δ¹⁵N) originating from upstream residential areas. Shallow groundwater in the farmland generally had higher nitrate concentrations than those in residential areas, due to the influence of synthetic fertilizer. Nitrate concentrations at both study sites were highest near the water table and then progressively decreased with depth. Nitrate concentrations are also closely related to the geologic characteristics of the aquifer. In Yongdang, denitrification is important in regulating nitrate chemistry because of the availability of organic carbon from a silt layer (about 20 m thick) below a thin, sandy surface aquifer. In Wolha, however, conservative mixing between farmland‐recharged water and water coming from a village is suggested as the dominant process. Mixing ratios estimated based on the nitrate concentrations and the δ¹⁵N values indicate that water originating from the village affects the nitrate chemistry of the shallow groundwater underneath the farmland to a large extent. Copyright © 2003 John Wiley & Sons, Ltd.     

Evaluation of nitrate levels in groundwater under agricultural fields in two pilot areas in central Chile: A hydrogeological and geochemical approach

The aim of this study is to evaluate the impact of the application of industrial fertilizers and liquid swine manure in groundwater in two pilot agricultural areas, San Pedro and Pichidegua, which have been under long‐term historic use of fertilizers. A comprehensive hydrogeological investigation was carried out to define the geology and the groundwater flow system. Chemical and isotopic tools were used to evaluate the distribution and behavior of the nitrate in the groundwater. The isotopic tools included δ¹⁸O, δ²H, and ³H, which provide information about the origin and residence time of the groundwater; δ¹⁵N‐NO₃^? and δ¹⁸O‐NO₃^?, which provide information about nitrate sources and processes that can affect nitrate along the groundwater flow system. The application rate of liquid manure and other fertilizers all together with land uses was also evaluated. The hydrogeological investigation identified the presence of a confined aquifer underneath a thick low‐permeability aquitard, whose extension covers most of the two study areas. The nitrate concentration data, excepting a few points in zones located near recharge areas in the upper part of the basins and lower areas at the valley outlets (San Pedro), showed nitrate concentration below 10 mgN/L at the regional scale. The isotope data for nitrate showed no influence of the liquid swine manure in the groundwater at the regional scale, except for the high part of the basins and the outlet of the San Pedro valley, which are areas fertilized by manure. This data showed that the regional aquifer on both pilot study areas is protected by the thick low‐permeability aquitard, which is playing an important role on nitrate attenuation. Evidence of denitrification was also found on both shallow and deep groundwater in the Pichidegua site. This study showed that a comprehensive hydrogeological characterization complemented by chemical and isotope data is key for understanding nitrate distribution and concentration in aquifers from areas with intensive agriculture activities.     

High spatial-resolution monitoring to investigate nitrate export and its drivers in a mesoscale river catchment along an anthropogenic land-cover gradient

Nitrate monitoring is commonly conducted with low-spatial resolution, only at the outlet or at a small number of selected locations. As a result, the information about spatial variations in nitrate export and its drivers is scarce. In this study, we present results of high-spatial resolution monitoring conducted between 2012 and 2017 in 65 sub-catchments in an Alpine mesoscale river catchment characterized by a land-use gradient. We combined stable isotope techniques with Bayesian mixing models and geostatistical methods to investigate nitrate export and its main drivers, namely, microbial N turnover processes, land use and hydrological conditions. In the investigated sub-catchments, mean values of NO₃⁻ concentrations and its isotope signatures (δ¹⁵N_NO3 and δ¹⁸O_NO3) varied from 2.6 to 26.7 mg L⁻¹, from −1.3‰ to 13.1‰, and from −0.4‰ to 10.1‰, respectively. In this study, land use was an important driver for nitrate export. Very strong and strong positive correlations were found between percentages of agricultural land cover and δ¹⁵N_NO3, and NO₃⁻ concentration, respectively. Mean proportional contributions of NO₃⁻ sources varied spatially and seasonally, and followed land-use patterns. The mean contribution of manure and sewage was much higher in the catchments characterized by a high percentage of agricultural and urban land cover comparing to forested sub-catchments. Specific NO₃⁻ loads were strongly correlated with specific discharge and moderately correlated with NO₃⁻ concentrations. The nitrate isotope and concentration analysis results suggest that nitrate from external sources is stored and accumulated in soil storage pools. Nitrification of reduced nitrogen species in those pools plays the most important role for the N-dynamics in the Erlauf river catchment. Consequently, nitrification of reduced N sources was the main nitrate source except for a number of sub-catchments dominated by agricultural land use. In the Erlauf catchment, denitrification plays only a minor role in controlling NO₃⁻ export on a regional scale.     

Isotopic signature of nitrate in river waters of the lower Mississippi and its distributary,the Atchafalaya

Periodic summer hypoxia occurring in the Northern Gulf of Mexico has been attributed to large nutrient inputs, especially nitrate‐nitrogen, from the Mississippi–Atchafalaya River system. The 2008 Gulf Hypoxia Action Plan calls for river corridor wetland restoration to reduce nitrate loads, but it is largely unknown how effective riverine wetland systems in the lower Mississippi River are for nitrate removal. We carried out an intensive isotope study to address this question by comparing nitrate isotopic signatures of the well‐channelized Mississippi River with those of the Atchafalaya River, which has extensive floodplains and backwater swamps. We investigated changes in δ¹⁵N_NO3 and δ¹⁸O_NO3 for water samples collected biweekly to monthly over a 2‐year period at the Atchafalaya River outlets (Morgan City and Wax Lake) and on the Mississippi River at Baton Rouge. In addition, in situ water quality parameters including temperature, dissolved oxygen and pH were recorded for each sampling date. Waters from both rivers showed moderately high nitrate concentration (>1 mg l^?1) and undetectable (< 0.01 mg l^?1) nitrite throughout the study period. The Mississippi River had slightly, but significantly higher (p=0.01) mean nitrate concentrations (1.5 mg l^?1) and higher δ¹⁵N_NO3 (7.7‰) than the Atchafalaya (1.1 mg l^?1, 7.0‰); while no difference in δ¹⁸O_NO3 (4.6‰) was found between the rivers. Flux‐weighted mean isotope values were overall lower than mean values for both the Mississippi and Atchafalaya Rivers, with a greater difference between the two rivers (7.4‰ versus 6.5‰, respectively). River flooding and hurricane storm surge also appeared to affect nitrate isotopic values. The lack of large difference in isotopic values between the Atchafalaya and Mississippi Rivers suggests that the majority of nitrate is transported through the Atchafalaya River with relatively little processing, and that riverine floodplains and wetlands are not effective sinks for nitrate, as previously assumed, because of insufficient residence time and well‐oxygenated river waters. Copyright © 2012 John Wiley & Sons, Ltd.     

Tracing groundwater nitrate sources in the Dakar suburban area: an isotopic multi‐tracer approach

The Dakar region is a mega city with multiple contaminant sources from urban expansion as well as industrial and agricultural activities. The major part of the region is underlain by unconfined sandy aquifer, which is vulnerable to contaminants derived from human land use. At present, the contaminated groundwater which extends over a large area in the suburban zone of Thiaroye poses a threat to the future of this valuable resource, and more specifically, a health threat. This study focuses on nitrate pollution occurrences and associated processes using nitrate isotope data (¹⁵N_NO3, ¹⁸O_NO3) combined with environmental isotopic tracers (¹⁸O, ²H, and ³H). Samples from 36 wells were collected to determine the level, distribution, and sources of contamination in relation to land use. Results indicate that shallow groundwater in the urbanized area of Thiaroye shows distinct evidence of surface contamination with nitrate as much as 300 mg/l NO₃^?. In rural area not serviced by water supply distribution network, much higher NO₃^? contents were found in few wells due to household and livestock feedlots. In most groundwater samples δ¹⁵N values ranged from + 10 to + 22‰, indicative of predominantly human and animal wastes. This was confirmed by environmental isotope data which suggest a mixture of polluted recharge waters. By using the dual δ¹⁵N vs δ¹⁸O as well as δ¹⁵N vs NO₃^? approach, denitrification may occur to some extent but it is blurred by mixing with new infiltrated nitrates and cycling derived from continuous leaky septic system. Results suggest that nitrate contamination of the aquifer is a consequence of unregulated urbanisation (homemade latrines), continuing contaminant transfer in shallow water depth where aerobic conditions prevail. Copyright © 2011 John Wiley & Sons, Ltd.     

Characterization of Flow Dynamics and Vulnerability in a Coastal Aquifer System

Traditional aquifer vulnerability techniques primarily rely on spatial property data for a region and are limited by their ability to directly or indirectly assess flow and transport processes occurring from the surface to depth within an aquifer system. The main objective of this study was to investigate groundwater vulnerability in terms of aquifer interconnectivity and flow dynamics. A combination of stable isotopes, groundwater age‐dating (radiocarbon), and geomorphic/geogenic spatial analyses was applied to a regional, highly developed coastal aquifer to explain the presence of nitrate at depth. The average δ¹³C value (?17.3 ± 2‰ VPDB, n = 27) is characteristic of groundwater originating from locally infiltrated precipitation through extensively cultivated soils. The average δ¹⁸O and δD values (?4.0 ± 0.1‰ VSMOW, n = 27; δD: ?19.3 ± 1‰ VSMOW, n = 27, respectively) are similar to precipitation water derived from maritime sources feeding the region's surface water and groundwater. Stable and radioactive isotopes reveal significant mixing between shallow and deep aquifers due to high velocities, hydraulic connection, and input of local recharge water to depths. Groundwater overdevelopment enhances deeper and faster modern water downward flux, amplifying aquifer vulnerability. Therefore, aquifer vulnerability is a variable, dependent on the type and degree of stress conditions experienced by a groundwater system as well as the geospatial properties at the near surface.     

Hydrological flowpaths and nitrate removal rates within a riparian floodplain along a fourth‐order stream in Brittany (France)

Three main reservoirs were identified that contribute to the shallow subsurface flow regime of a valley drained by a fourth‐order stream in Brittany (western France). (i) An upland flow that supplied a wetland area, mainly during the high‐water period. It has high N‐NO₃^? and average Cl^? concentrations. (ii) A deep confined aquifer characterized by low nitrate and low chloride concentrations that supplied the floodplain via flow upwelling. (iii) An unconfined aquifer under the riparian zone with high Cl^? and low N‐NO₃^? concentrations where biological processes removed groundwater nitrate. This aquifer collected the upland flow and supplied a relict channel that controlled drainage from the whole riparian zone. Patterns of N‐NO₃^? and Cl^? concentrations along riparian transects, together with calculated high nitrate removal, indicate that removal occurred mainly at the hillslope–riparian zone interface (i.e. first few metres of wetland), whereas dilution occurred in lower parts of the transects, especially during low‐water periods and at the beginning of recharge periods. Stream flow was modelled as a mixture of water from the three reservoirs. An estimation of these contributions revealed that the deep aquifer contribution to stream flow averaged 37% throughout the study period, while the contribution of the unconfined reservoir below the riparian zone and hillslope flow was more variable (from ca 6 to 85%) relative to rainfall events and the level of the riparian water table. At the entire riparian zone scale, NO₃^? removal (probably from denitrification) appeared most effective in winter, despite higher estimated upland NO₃^? fluxes entering the riparian zone during this period. Copyright © 2003 John Wiley & Sons, Ltd.     

Long‐term assessment of nutrient flow pathway dynamics and in‐stream fate in a temperate karst agroecosystem watershed

Nutrient dynamics in karst agroecosystems remain poorly understood, in part due to limited long‐term nested datasets that can discriminate upland and in‐stream processes. We present a 10‐year dataset from a karst watershed in the Inner‐Bluegrass Region of central Kentucky, consisting of nitrate (nitrate‐N [NO₃^?]), dissolved reactive phosphorus (DRP), total organic carbon (TOC), and total ammoniacal‐N (TAN) measurements at nested spring and stream sites as well as flowrate at the watershed outlet. Hydrograph separation techniques were coupled with multiple linear regression and Empirical Mode Decomposition time‐series analysis to determine significance of seasonal processes and to generate continuous estimates of nutrient pathway loadings. Further, we used model results of benthic algae growth and decomposition dynamics from a nearby watershed to assess if transient storage in algal biomass could explain differences in spring and downstream watershed nutrient loading. Results highlight statistically significant seasonality for all nutrients at stream sites, but only for NO₃^? at springs with longitudinal variability showing significant decreases occurring from spring to stream sites for NO₃^? and DRP, and significant increases for TOC and TAN. Pathway loading analysis highlighted the importance of slow flow pathways to source approximately 70% of DRP and 80% of NO₃^?. Results for in‐stream dynamics suggest that benthic autotroph dynamics can explain summer deviations for TOC, TAN, and DRP but not NO₃^?. Regarding upland dynamics, our findings agree well with existing perceptions in karst for N pathways and upland source seasonality but deviate from perceptions that karst conduits are retentive of P, reflecting the limited buffering capacity of the soil profile and conduit sediments in the Inner‐Bluegrass. Regarding in‐stream fate, our findings highlighted the significance of seasonally driven nutrient processing in the bedrock‐controlled streambed to influence nutrient fluxes at the watershed outlet. Contrary to existing perceptions, we found high N attenuation and an unexplained NO₃^? sink in the bedrock stream, leading us to postulate that floating macrophytes facilitate high rates of denitrification.     

Effect of variable‐density groundwater flow on nitrate flux to coastal waters

A two‐dimensional variable‐density groundwater flow and transport model was developed to provide a conceptual understanding of past and future conditions of nitrate (NO₃) transport and estimate groundwater nitrate flux to the Gulf of Mexico. Simulation results show that contaminant discharge to the coast decreases as the extent of saltwater intrusion increases. Other natural and/or artificial surface waters such as navigation channels may serve as major sinks for contaminant loading and act to alter expected transport pathways discharging contaminants to other areas. Concentrations of NO₃ in the saturated zone were estimated to range between 30 and 160 mg?L^?1 as NO₃. Relatively high hydraulic vertical gradients and mixing likely play a significant role in the transport processes, enhancing dilution and contaminant migration to depth. Residence times of NO₃ in the deeper aquifers vary from 100 (locally) to about 300 years through the investigated aquifer system. NO₃ mass fluxes from the shallow aquifers (0 to 5.7 × 10⁴ mg?m^?2?day^?1) were primarily directed towards the navigation channel, which intersects and captures a portion of the shallow groundwater flow/discharge. Direct NO₃ discharge to the sea (i.e. Gulf of Mexico) from the shallow aquifer was very low (0 to 9.0 × 10¹ mg · m^?2?day^?1) compared with discharge from the deeper aquifer system (0 to 8.2 × 10³ mg?m^?2?day^?1). Both model‐calibrated and radiocarbon tracer‐determined contaminant flux estimates reveal similar discharge trends, validating the use of the model for density‐dependent flow conditions. The modelling approach shows promise to evaluate contaminant and nutrient loading for similar coastal regions worldwide. Copyright © 2015 John Wiley & Sons, Ltd.     

The impact of pipe flow in riparian peat deposits on nitrate transport and removal

The effect of preferential flow in soil pipes on nitrate retention in riparian zones is poorly understood. The characteristics of soil pipes and their influence on patterns of groundwater transport and nitrate dynamics were studied along four transects in a 1‐ to >3‐m deep layer of peat and marl overlying an oxic sand aquifer in a riparian zone in southern Ontario, Canada. The peat‐marl deposit, which consisted of several horizontal layers with large differences in bulk density, contained soil pipes that were generally 0.1 to 0.2 m in diameter and often extended vertically for 1 to >2 m. Springs that produced overland flow across the riparian area occurred at some sites where pipes extended to the peat surface. Concentrations of NO₃^?–N (20–30 mg L^?1) and dissolved oxygen (DO) (4–6 mg L^?1) observed in peat pipe systems and surface springs were similar to values in the underlying sand aquifer, indicating that preferential flow transported groundwater with limited nitrate depletion. Low NO₃^?–N concentrations of <5 mg L^?1 and enriched δ¹⁵N values indicated that denitrification was restricted to small areas of the peat where pipes were absent. Groundwater DO concentrations declined rapidly to <2 mg L^?1 in the peat matrix adjacent to pipes, whereas high NO₃^?–N concentrations of >15 mg L^?1 extended over a larger zone. Low dissolved organic carbon values at these locations suggest that supplies of organic carbon were not sufficient to support high rates of denitrification, despite low DO conditions. These data indicate that it is important to develop a greater understanding of pipes in peat deposits, which function as sites where the transport of large fluxes of water with low biogeochemical reaction rates can limit the nitrate removal capacity of riparian zones. Copyright © 2011 John Wiley & Sons, Ltd.     

Natural Attenuation of Perchlorate in Denitrified Groundwater

Monitoring of a well‐defined septic system groundwater plume and groundwater discharging to two urban streams located in southern Ontario, Canada, provided evidence of natural attenuation of background low level (ng/L) perchlorate (ClO₄^?) under denitrifying conditions in the field. The septic system site at Long Point contains ClO₄^? from a mix of waste water, atmospheric deposition, and periodic use of fireworks, while the nitrate plume indicates active denitrification. Plume nitrate (NO₃^?‐N) concentrations of up to 103 mg/L declined with depth and downgradient of the tile bed due to denitrification and anammox activity, and the plume was almost completely denitrified beyond 35 m from the tile bed. The ClO₄^? natural attenuation occurs at the site only when NO₃^?‐N concentrations are <0.3 mg/L, after which ClO₄^? concentrations decline abruptly from 187 ± 202 to 11 ± 15 ng/L. A similar pattern between NO₃^?‐N and ClO₄^? was found in groundwater discharging to the two urban streams. These findings suggest that natural attenuation (i.e., biodegradation) of ClO₄^? may be commonplace in denitrified aquifers with appropriate electron donors present, and thus, should be considered as a remediation option for ClO₄^? contaminated groundwater.     

Diffusive equilibrium in thin films provides evidence of suppression of hyporheic exchange and large‐scale nitrate transformation in a groundwater‐fed river

The hyporheic zone of riverbed sediments has the potential to attenuate nitrate from upwelling, polluted groundwater. However, the coarse‐scale (5–10 cm) measurement of nitrogen biogeochemistry in the hyporheic zone can often mask fine‐scale (<1 cm) biogeochemical patterns, especially in near‐surface sediments, leading to incomplete or inaccurate representation of the capacity of the hyporheic zone to transform upwelling NO₃^?. In this study, we utilised diffusive equilibrium in thin‐films samplers to capture high resolution (cm‐scale) vertical concentration profiles of NO₃^?, SO₄^2?, Fe and Mn in the upper 15 cm of armoured and permeable riverbed sediments. The goal was to test whether nitrate attenuation was occurring in a sub‐reach characterised by strong vertical (upwelling) water fluxes. The vertical concentration profiles obtained from diffusive equilibrium in thin‐films samplers indicate considerable cm‐scale variability in NO₃^? (4.4 ± 2.9 mg N/L), SO₄^2? (9.9 ± 3.1 mg/l) and dissolved Fe (1.6 ± 2.1 mg/l) and Mn (0.2 ± 0.2 mg/l). However, the overall trend suggests the absence of substantial net chemical transformations and surface‐subsurface water mixing in the shallow sediments of our sub‐reach under baseflow conditions. The significance of this is that upwelling NO₃^?‐rich groundwater does not appear to be attenuated in the riverbed sediments at <15 cm depth as might occur where hyporheic exchange flows deliver organic matter to the sediments for metabolic processes. It would appear that the chemical patterns observed in the shallow sediments of our sub‐reach are not controlled exclusively by redox processes and/or hyporheic exchange flows. Deeper‐seated groundwater fluxes and hydro‐stratigraphy may be additional important drivers of chemical patterns in the shallow sediments of our study sub‐reach. © 2015 The Authors. Hydrological Processes Published by John Wiley & Sons Ltd.     

Identification of groundwater recharge sources and processes in a heterogeneous alluvial aquifer: results from multi‐level monitoring of hydrochemistry and environmental isotopes in a riverside agricultural area in Korea

We evaluated sources and pathways of groundwater recharge for a heterogeneous alluvial aquifer beneath an agricultural field, based on multi‐level monitoring of hydrochemistry and environmental isotopes of a riverside groundwater system at Buyeo, Korea. Two distinct groundwater zones were identified with depth: (1) a shallow oxic groundwater zone, characterized by elevated concentrations of NO₃^? and (2) a deeper (>10–14 m from the ground surface) sub‐oxic groundwater zone with high concentrations of dissolved Fe, silica, and HCO₃^?, but little nitrate. The change of redox zones occurred at a depth where the aquifer sediments change from an upper sandy stratum to a silty stratum with mud caps. The δ¹⁸O and δ²H values of groundwater were also different between the two zones. Hydrochemical and δ¹⁸O? δ²H data of oxic groundwater are similar to those of soil water. This illustrates that recharge of oxic groundwater mainly occurs through direct infiltration of rain and irrigation water in the sandy soil area where vegetable cropping with abundant fertilizer use is predominant. Oxic groundwater is therefore severely contaminated by agrochemical pollutants such as nitrate. In contrast, deeper sub‐oxic groundwater contains only small amounts of dissolved oxygen (DO) and NO₃^?. The ³H contents and elevated silica concentrations in sub‐oxic groundwater indicate a somewhat longer mean residence time of groundwater within this part of the aquifer. Sub‐oxic groundwater was also characterized by higher δ¹⁸O and δ²H values and lower d‐excess values, indicating significant evaporation during recharge. We suggest that recharge of sub‐oxic groundwater occurs in the areas of paddy rice fields where standing irrigation and rain water are affected by strong evaporation, and that reducing conditions develop during subsequent sub‐surface infiltration. This study illustrates the existence of two groundwater bodies with different recharge processes within an alluvial aquifer. Copyright © 2009 John Wiley & Sons, Ltd.     

Soil NO3− storage from oasis development in deserts: Implications for the prevention and control of groundwater pollution

The sources and storage of soil NO₃⁻ in the western Tengger Desert, Northwest China, were explored using water chemistry analysis and stable isotope techniques. In line with the expansion and development of oases, part of the desert has been transformed into cultivated land and artificial forest land. The mean soil NO₃⁻ contents found in areas of cultivated land and artificial forest were 123.06 mg kg⁻¹ and 1.26 mg kg⁻¹, far higher and slightly lower than the background desert soil values, respectively. The δ¹⁵N-NO₃⁻ and δ¹⁸O-NO₃⁻ values in cultivated soils ranged from 1.00 to 11.81 ‰, and from −1.85 to 8.99 ‰, respectively, and the mean mNO₃⁻/Cl⁻ value in cultivated soils was 2.3. These figures would appear to demonstrate that the rapid increase in the nitrate content in soils is principally due to the use of nitrogen fertilizer. Such increases in soil NO₃⁻ storage is likely to promote the leaching of nitrogen into the groundwater where coarsely textured soils exist, the pollution of water sources used for irrigation water, and extreme precipitation events. The δ¹⁵N-NO₃⁻ and δ¹⁸O-NO₃⁻ values in groundwater ranged from 3.72 to 6.54 ‰, and from −0.19 to 12.06 ‰, respectively, mainly reflecting the nitrification of soil nitrogen. These values appeared similar to those measured in the soil water in adjacent areas of cultivated land and vegetated desert, indicating that the groundwater has been affected by both natural and artificial NO₃⁻. Artificial afforestation of desert regions would therefore seem to be a useful way of reducing the threat posed by anthropogenic sources to the circulation of NO₃⁻-N within arid regions, as well as promoting wind sheltering and sand fixation. This study explored the NO₃⁻ storage and groundwater quality responses to oasis development in arid areas in an attempt to provide effective information for local agricultural organizations and agricultural nitrogen management models.     

Isotope hydrology of a tropical coffee agroforestry watershed: Seasonal and event‐based analyses

Stable isotope variations are extremely useful for flow partitioning within the hydrologic cycle but remain poorly understood throughout the tropics, particularly in watersheds with rapidly infiltrating soils, such as Andisols in Central America. This study examines the fluctuations of stable isotope ratios (δ¹⁸O and δ²H) in the hydrologic components of a tropical coffee agroforestry watershed (~1 km²) with Andisol soils in Costa Rica. Samples were collected in precipitation, groundwater, springs, and stream water over 2 years. The local meteoric water line for the study site was δ²H = 8.5 δ¹⁸O + 18.02 (r² = 0.97, n = 198). The isotope ratios in precipitation exhibited an enriched trend during the dry season and a notable depletion at the beginning of the wet season. The δ¹⁸O compositions in groundwater (average = ?6.4‰, σ = 0.7) and stream water (average = ?6.7‰, σ = 0.6) were relatively stable over time, and both components exhibited more enriched values in 2013, which was the drier year. No strong correlation was observed between the isotope ratios and the precipitation amount at the event or daily time‐step, but a correlation was observed on a monthly scale. Stream water and base flow hydrograph separations based on isotope end‐member estimations showed that pre‐event water originating from base flow was prevalent. However, isotope data indicate that event water originating from springs appears to have been the primary driver of initial rises in stream flow and peak flows. These results indicate that isotope sampling improves the understanding of water balance components, even in a tropical humid location, where significant variations in rainfall challenge current modelling efforts. Further research using fine‐scale hydrometric and isotopic data would enhance understanding the processes driving spring flow generation in watersheds.     

Use of stable isotopes to understand run‐off generation processes in the Red River Delta

This paper presents the use of stable isotopes of water for hydrological characterization and flow component partitioning in the Red River Delta (RRD), the downstream section of the Red River. Water samples were collected monthly during 2015 from the mainstream section of the river and its right bank tributaries flowing through the RRD. In general, δ¹⁸O and δ²H river signatures were depleted in summer–autumn (May–October) and elevated in winter–spring (November–April), displaying seasonal variation in response to regional monsoon air mass contest. The Pacific equatorial–maritime air mass dominates in summer and the northern Asia continental air mass controls in winter. Results show that water of the RRD tributaries stems solely from local sources and is completely separated from water arriving from upstream subbasins. This separation is due to the extensive management of the RRD (e.g., dykes and dams) for the purposes of irrigation and inundation prevention. Mainstream river section δ¹⁸O and δ²H compositions range from ?10.58 and ?73.74‰ to ?6.80 and ?43.40‰, respectively, and the corresponding ranges inside the RRD were from ?9.35 and ?64.27‰ to ?2.09 and ?15.80‰. A combination of data analysis and hydrological simulation confirms the role of upstream hydropower reservoirs in retaining and mixing upstream water. River water inside the RRD experienced strong evaporation characterized by depleted d‐excess values, becoming negative in summer. On the other hand, the main stream of the Red River has d‐excess values around 10‰, indicating moderate evaporation. Hydrograph separation shows that in upstream subbasins, the groundwater fraction dominates the river flow composition, especially during low flow regimes. Inside the RRD, the river receives groundwater during the dry season, whereas groundwater replenishment occurs in the rainy season. Annual evaporation obtained from this hydrograph separation computation was about 6.3% of catchment discharge, the same order as deduced from the difference between subbasin precipitation and discharge values. This study shows the necessity to re‐evaluate empirical approaches in large river hydrology assessment schemes, especially in the context of climate change.     

Hydrologic and biogeochemical controls on the spatial and temporal patterns of nitrogen and phosphorus in the Kuparuk River,arctic Alaska

Nitrogen (N) and phosphorus (P) dynamics in the Kuparuk River in arctic Alaska were characterized in a 3‐year study using routine samples near the mouth of the river at the Arctic Ocean, synoptic whole‐river surveys, and temporally intense sampling during storms in three headwater basins. The Lower Kuparuk River has low nitrate concentrations (mean [NO₃]‐N] = 17 µg l^?1 ± 1·6 SE) and dissolved inorganic N (DIN, mean [N] = 31 µg l^?1 ± 1·2 SE) compared with rivers in more temperate environments. Organic forms constituted on average 90% of the N exported to the Arctic Ocean, and high ratios of dissolved organic N (DON) to total dissolved N (TDN) concentrations (mean 0·92) likely result from waterlogged soils formed by reduced infiltration due to permafrost and low hydrologic gradients. Annual export of TDN, DON, and particulate N averaged 52 kg km^?2, 48 kg km^?2, and 4·1 kg km^?2 respectively. During snowmelt, the high volume of runoff typically results in the highest nutrient loads of the year, although high discharge during summer storms can result in substantial nutrient loading over short periods of time. Differences in seasonal flow regime (snowmelt versus rain) and storm‐driven variation in discharge appear to be more important for determining nutrient concentrations than is the spatial variation in processes along the transect from headwaters towards the ocean. Both the temporal variation in nitrate:DIN ratios of headwater streams and the spatial variation in nitrate:DIN between larger sub‐basins and smaller headwater catchments is likely controlled by shifts in nitrification and soil anoxia. Copyright © 2008 John Wiley & Sons, Ltd.     

Dynamic evolution of nutrient discharge under stormflow and baseflow conditions in a coastal agricultural watershed in Ishigaki Island,Okinawa, Japan

Excessive terrestrial nutrient loadings adversely impact coral reefs by primarily enhancing growth of macroalgae, potentially leading to a phase‐shift phenomenon. Hydrological processes and other spatial and temporal factors affecting nutrient discharge must be examined to be able to formulate effective measures for reducing nutrient export to adjacent reefs. During storm events and baseflow periods, water samples were obtained from the tropical Todoroki River, which drains an intensively agricultural watershed into Shiraho coral reef. In situ nutrient analyzers were deployed for 6 months to hourly measure dissolved nutrient (NO₃⁻‐N and PO₄³⁻‐P) concentrations. Total phosphorus (TP) and suspended solid concentration (TSS) were increased by higher rainfall intensity (r = 0·94, p < 0·01) and river discharge Q (r = 0·88, p < 0·01). In contrast, NO₃⁻‐N concentration tends to decrease drastically (e.g. from 3 to 1 mg l⁻¹) during flood events. When base flow starts to dominate afterwards, NO₃⁻‐N manifested an increasing trend, but decreases when baseflow discharge becomes low. This counter‐clockwise hysteresis for NO₃⁻‐N highlights the significant influence of groundwater discharge. N delivery can therefore be considered a persistent process compared to sediment and P discharge, which are highly episodic in nature. Based on GIS analysis, nutrient concentration along the Todoroki River was largely affected by the percentage of sugarcane/bare areas and bedrock type. The spatial distribution of N concentration in the river reflects the considerable influence of subsurface geology—higher N levels in limestone‐dominated areas. P concentrations were directly related to the total length of artificial drainage, which enhances sediment transport. The use of high‐resolution monitoring data coupled with GIS‐based spatial analysis therefore enabled the clarification of control factors and the difference in the spatio‐temporal discharge characteristics between N and P. Thus, although erosion‐reduction schemes would reduce P discharge, other approaches (e.g. minimize fertilizer) are needed to reduce N discharge. Copyright © 2010 John Wiley & Sons, Ltd.     

Groundwater and surface water connectivity within the recharge area of Guarani aquifer system during El Niño 2014–2016

Recharge areas of the Guarani Aquifer System (GAS) are particularly sensitive and vulnerable to climate variability; therefore, the understanding of infiltration mechanisms for aquifer recharge and surface run‐off generation represent a relevant issue for water resources management in the southeastern portion of the Brazilian territory, particularly in the Jacaré‐Pepira River watershed. The main purpose of this study is to understand the interactions between precipitation, surface water, and groundwater using stable isotopes during the strong 2014–2016 El Niño Southern Oscillation event. The large variation in the isotopic composition of precipitation (from ?9.26‰ to +0.02‰ for δ¹⁸O and from ?63.3‰ to +17.6‰ for δ²H), mainly associated with regional climatic features, was not reflected in the isotopic composition of surface water (from ?7.84‰ to ?5.83‰ for δ¹⁸O and from ?49.7‰ to +33.6‰ for δ²H), mainly due to the monthly sampling frequency, and groundwater (from ?7.04‰ to ?7.76‰ for δ¹⁸O and from ?49.5‰ to ?44.7‰ for δ²H), which exhibited less variation throughout the year. However, variations in deuterium excess (d‐excess) in groundwater and surface water suggest the occurrence of strong secondary evaporation during the infiltration process, corresponding with groundwater level recovery. Similar isotopic composition in groundwater and surface water, as well as the same temporal variations in d‐excess and line‐conditioned excess denote the strong connectivity between these two reservoirs during baseflow recession periods. Isotopic mass balance modelling and hydrograph separation estimate that the groundwater contribution varied between 70% and 80%, however, during peak flows, the isotopic mass balance tends to overestimate the groundwater contribution when compared with the other hydrograph separation methods. Our findings indicate that the application of isotopic mass balance methods for ungauged rivers draining large groundwater reservoirs, such as the GAS outcrop, could provide a powerful tool for hydrological studies in the future, helping in the identification of flow contributions to river discharge draining these areas.