Dissolved Nitrate Concentrations Research Articles

Metagenomic insights into the functional microbial diversity of the lower stretch of the River Ganga: mapping antibiotic and metal resistance genes

The banks of the lower stretch of River Ganga are home to several towns and megapolis representing high density of human population. A stretch of 50 km represented by prefixed stations of the lower part of Ganga (Ganga Environmental Time Series- GETS) was monitored spatiotemporally using eDNA metagenome-based Nanopore sequencing to elucidate structure of microbial communities along with mapping of antibiotic-resistant genes (ARGs), metal resistance genes (MRGs) and mobile genetic elements (MGEs). Besides, in situ environmental parameters, concentration of dissolved nutrients, metals and metalloids were measured. The concentration of dissolved oxygen ranged from 3.4–6.2 mg l−1, indicating deteriorating water quality corresponding to high population density. Dissolved nitrate concentrations were higher in some stations reflecting direct release of untreated municipal sewage into the river. In particular, concentration of metals such as Cd (2.34–38.52 ppb) and metalloids such as As (0–218.7 ppb) were found to be alarmingly high in surface water. Gammaproteobacteria was encountered ubiquitously while rare bacterioplankton represented by eleven classes showed site specificity. Several genes belonging to ARGs were identified and multidrug resistance genes (MDR) were found in all the studied stations exhibiting high abundance. The ‘hotspots’ of ARGs were widespread, possibly owing to rampant usage of personal health care products that may have contributed to observed ARG abundances. High abundance of MRGs comprising arsenic (∼12%) and copper (∼12%) were also identified showing strong correlations with the abundance of ARGs. Correlation and network analysis revealed the potential role of MGEs in the dissemination of ARGs. ANOSIM and cluster analyses supported patchy distribution patterns of microbial communities. The RDA plot showed possible influences of environmental parameters in shaping the microbial community structure and towards dissemination of functional genes such as ARGs and MRGs in the lower stretch of the River Ganga.

Carbon monoxide (CO) cycling in the Fram Strait, Arctic Ocean

Abstract. Carbon monoxide (CO) influences the radiative budget and oxidative capacity of the atmosphere over the Arctic Ocean, which is a source of atmospheric CO. Yet, oceanic CO cycling is understudied in this area, particularly in light of the ongoing rapid environmental changes. We present results from incubation experiments conducted in the Fram Strait in August–September 2019 under different environmental conditions: while lower pH did not affect CO production (GPCO) or consumption (kCO) rates, enhanced GPCO and kCO were positively correlated with coloured dissolved organic matter (CDOM) and dissolved nitrate concentrations, respectively, suggesting microbial CO uptake under oligotrophic conditions to be a driving factor for variability in CO surface concentrations. Both production and consumption of CO will likely increase in the future, but it is unknown which process will dominate. Our results will help to improve models predicting future CO concentrations and emissions and their effects on the radiative budget and the oxidative capacity of the Arctic atmosphere.

Co-occurrence of pectenotoxins and Dinophysis miles in an Indonesian semi-enclosed bay

The study aims to unravel the variability of Dinophysis spp. and their alleged toxins in conjunction with environmental drivers in Ambon Bay. Phytoplankton samples, lipophilic toxins and physiochemical water properties were analysed during a 1.5-year period. Three Dinophysis species (D. miles, D. caudata, and D. acuminata) were found in plankton samples, of which D. miles was the most abundant and persistently occurring species. Pectenotoxin-2 (PTX2) and its secoacid (PTX2sa) were detected throughout, and PTX2sa levels strongly correlated with D. miles cell abundance. The toxin showed a positive correlation with temperature, which may suggest that D. miles cells contain rather constant PTX2sa during warmer months. Dissolved nitrate concentrations were found to play a major role in regulating cell abundances and toxin levels. This study adds adequate information regarding marine biotoxins and potentially toxic species for future Harmful Algal Bloom management in Ambon and Indonesia at large.

Seasonal Variability of Phytoplanktons in the Non-Point Source Polluted River Nggada, Maiduguri – Borno State, Nigeria

Borno State is the epicenter of Boko Haram crisis which displaced millions of individuals. Maiduguri metropolis which became the principal refuge centre, has witnessed a rapid demographic change, land use and economic activities resulting into intense irrigational practice along the bank of river Nggada. This may result into changes in the physico – chemical parameters of the aquatic ecosystem and the dynamism may disrupt phytoplantonic community. The study was aimed at assessing the seasonal variability of phytoplanktons in non-point source polluted river Nggada of Maiduguri. Water samples for physico -chemical analysis were collected bi-weekly for a period of 11 months at three sampling stations. Physico-chemical parameters, including Dissolve Oxygen (DO), Dissolved Nitrate concentration, Dissolved Phosphorus Concentration, Total Dissolved Solids (TDS), Total Suspended Solids (TSS), Surface Water Temperature (SWT) and potential Hydrogen (pH) were determined using standard procedures. Results indicated that all physico – chemical parameters except temperature, DO and pH of the river during both irrigational and non-irrigational regimes were slightly above the National Environmental Standards and Regulations Enforcement Agency’s (NESREA) permissible limits. A total of 7 classes (Cyanophyceae Chlorophyceae, Cryptophyceace, Euglenophyceae, Bacillariophyceae, Crysophyceace and Rhodophyceae) and 16 genera of phytoplanktons were identified. The results further revealed that the irrigation activities (dry season) have impacted on the physico – chemical parameters (pH, DO, TDS, nitrate and phosphate concentrations) of the study area. Remarkably, the changes in the physico – chemical parameters during the irrigational regime (dry season) have not impacted on the abundance and distribution of phytoplanktons. Hence, the non-point pollutants influenced the physico – chemical parameters of the river butalgal abundance and distribution were not influenced by the changes in the physico – chemical parameters of the river. The study thus, recommends the continuous monitoring and systematic effort to conserve the bio-resources and water quality of the river

Seasonal dynamicity of environmental variables and water quality index in the lower stretch of the River Ganga

Rapid human pressure in semi-urban and urban areas along with increasing industrial activities has resulted in release of untreated sewage and other forms of pollutants into major rivers globally including in the Ganga. In this study, nine sites represented by 59 stations along the lower stretch of the River Ganga were monitored seasonally to understand the effect of monsoonal precipitation on environmental variables and Water Quality Index (WQI). Sampling was undertaken in pre-monsoon, monsoon and post-monsoon seasons (2019). In situ surface water temperature, dissolved oxygen, pH, total dissolved solids (TDS) and electrical conductivity (EC) were measured along with dissolved nutrients and Chlorophyll-a. Both pH and DO were strongly influenced by monsoon and affected WQI. TDS was higher in point source (PS) stations during pre-monsoon (113–538 ppm) compared to surface water (SW) stations (113–248 ppm) with strong influence of monsoon (PS-27.4-310.3 ppm; SW-27-68.9 ppm). Dissolved nutrients including nitrate and o-phosphate concentration showed significant seasonal variation and influenced monsoonal precipitation. In PS stations across studied seasons dissolved nitrate concentration varied from 26.33–646 μM while in SW the range was from 21.22–148.06 μM. In the studied sites, higher concentration of dissolved nutrients in PS stations reflected the release of untreated municipal and industrial sewage directly into the river. The effect of precipitation and resulting environmental variables was clearly evident on biological variable (concentration of Chl-a) with observed values in PS stations (0–21 mg l−1) which were lesser compared to SW stations (0–89.3 mg l−1) during monsoon. Non-metric multidimensional scaling revealed three distinct clusters with greater overlap between PS and SW stations in monsoon. The WQI values (14–52) determined for lower stretch of Ganga revealed very poor water quality in majority stations and monsoonal precipitation did not have any influence on the observed trends.

Influence of transient porosity in a coupled fracture-skin-matrix system at the scale of a single fracture.

The subsurface leaching of soluble chemicals in a fractured porous medium poses long-term risk of groundwater contamination. Tracing the occurrence, movement and consequences of such hydro-geo-chemical interactions is the fundamental process for an effective remediation plan. However, the complexity of geomorphology and mass transfer mechanisms makes it challenging while addressing these issues in a real field scale. The present study focuses on simulating the concentration profile of nitrate elution in a pseudo two-dimensional coupled fracture-skin-matrix system under active biodegradation using an implicit finite difference numerical technique. The interface between the fracture and rock matrix is assumed to possess a skin with time-varying porosity imitating the effect of bio-clogging. The results indicate that denitrification is significant in reducing the dissolved nitrate concentration for initial skin porosity of 10% in the presence of an unlimited oxygen and primary substrate. When the rate of change of skin porosity remains lower with a minimal variation, the nitrate concentration provided a considerable reduction in the vicinity of the fracture inlet. A similar trend is observed for dissolved oxygen concentration as well. The concentration profile of nitrate showed a higher rate of reduction with an increase in initial skin porosity value from smaller to significantly larger values. The present study clearly indicates the role of skin interface in depicting the solute concentration profile in fracture, especially during the washout of bio-clogged membrane (biofilm) attached to the rock matrix.

Subsidized or stressed? Shifts in freshwater benthic microbial metagenomics along a gradient of alkaline coal mine drainage

AbstractChemical pollution mixtures enter aquatic environments and interact with microorganisms in eclectic ways with disparate consequences for microbial ecosystem services. Can using a thermodynamic framework help to determine the net influence of a chemical mixture on the functional capacity of benthic microbial communities? We examined this question by comparing benthic stream microbial communities exposed to a gradient of neutral‐alkaline coalmine effluent. Using a combination of approaches (metagenomics, quantitative polymerase chain reaction [qPCR], and functional assays), we show that functional genes and pathways of microbial communities growing in mine effluent differed in composition, but not diversity. The majority of functional genes and pathways that changed decreased at sites exposed to mine effluent, resulting in lower abundances of nitrogenase and methanogen genes and fermentation pathways. However, selenate reductase gene abundance increased with water and sediment concentration of an ecologically important contaminant at mined sites: selenium. Denitrification genes nosZ and nirK differed between sites: metagenome‐based nosZ increased with dissolved nitrate concentration and qPCR‐based nirK had a hump‐shaped pattern across the mining gradient. Osmoprotectant gene abundance did not change. Extracellular enzyme assays and alkaline phosphatase gene relative abundance suggested that mined stream microbial communities may be constrained by phosphorus bioavailability. Subsidies and stressors related to changes in a set of functional genes and pathways, but differences were not consistently predictable using thermodynamic expectations. This suggests that pairing hypotheses for expected subsidies and stressors with post hoc explorations can yield valuable directions for future study of how microbial functional capacity responds to pollutant mixtures.

Punching above their weight: Large release of greenhouse gases from small agricultural dams.

Freshwater ecosystems play a major role in global carbon cycling through the breakdown of organic material and release of greenhouse gases (GHGs). Carbon dioxide (CO2 ) and methane (CH4 ) emissions from lakes, wetlands, reservoirs and small natural ponds have been well studied, however, the GHG emissions of highly abundant, small-scale (<0.01km2 ) agricultural dams (small stream and run-off impoundments) are still unknown. Here, we measured the diffusive CO2 and CH4 flux of 77 small agricultural dams within south-east Australia. The GHG emissions from these waterbodies, which are currently unaccounted for in GHG inventories, amounted to 11.12±2.59gCO2 -equivalent m2 /day, a value 3.43 times higher than temperate reservoir emissions. Upscaling these results to the entire state of Victoria, Australia, resulted in a farm dam CO2 -equivalent/day emission rate of 4,853 tons, 3.1 times higher than state-wide reservoir emissions in spite of farm dams covering only 0.94 times the comparative area. We also show that CO2 and CH4 emission rates were both significantly positively correlated with dissolved nitrate concentrations, and significantly higher in livestock rearing farm dams when compared to cropping farm dams. The results from this study demonstrate that small agricultural farm dams can be a major source of greenhouse gas emissions, thereby justifying their inclusion in global carbon budgets.

Does carbon limitation reduce nitrogen retention in soil?

Artificial soils made from waste materials offer an alternative to imported natural topsoils, notably in large-scale groundwork and reclamation projects. Benefits include diversion of waste from landfill and recycling. Nonetheless, there is limited information on the characteristics needed to support plant growth in the long term, particularly the existence of a sustainable nitrogen reservoir. Therefore, we assessed the efficacy of nitrogen cycling and retention within an artificial soil composed of 25% sand, 10% clay, 32.5% composted bark and 32.5% composted green waste over 52 weeks. Leachate was analysed for nitrogen species and nitrogen concentrations, and two of the soil columns had fertiliser added after 26 and 48 weeks. Results show that nitrate concentrations decreased from 6.73 to 0.36 mg N L−1 after 2 weeks, due to poor retention of this anion in soil, and remained low for 6 months, before increasing up to 5.87 mg N L−1 after week 26. This sharp increase in dissolved nitrate was preceded by a decrease in the ratio of dissolved organic carbon to dissolved organic nitrogen in the soil leachate. This finding indicates that the soil had become carbon-limited, leading to mineralisation of organic nitrogen by soil organisms and excretion of nitrogen. We also found that fertilisation of the soil did not alleviate carbon limitation and nitrogen loss was greater in fertilised soils, indicating nitrogen saturation. After the onset of carbon limitation, the dissolved nitrate concentrations in both the fertilised and unfertilised soils were close to exceeding the European Union threshold of concern for nitrate groundwater and river pollution. Thus, while the deployment of artificial soils is a viable option for landscaping projects, loss of nitrogen may be environmentally significant and soil management protocols must take account of both the carbon and nitrogen status of the substrate.

Patterns of Transcript Abundance of Eukaryotic Biogeochemically-Relevant Genes in the Amazon River Plume.

The Amazon River has the largest discharge of all rivers on Earth, and its complex plume system fuels a wide array of biogeochemical processes, across a large area of the western tropical North Atlantic. The plume thus stimulates microbial processes affecting carbon sequestration and nutrient cycles at a global scale. Chromosomal gene expression patterns of the 2.0 to 156 μm size-fraction eukaryotic microbial community were investigated in the Amazon River Plume, generating a robust dataset (more than 100 million mRNA sequences) that depicts the metabolic capabilities and interactions among the eukaryotic microbes. Combining classical oceanographic field measurements with metatranscriptomics yielded characterization of the hydrographic conditions simultaneous with a quantification of transcriptional activity and identity of the community. We highlight the patterns of eukaryotic gene expression for 31 biogeochemically significant gene targets hypothesized to be valuable within forecasting models. An advantage to this targeted approach is that the database of reference sequences used to identify the target genes was selectively constructed and highly curated optimizing taxonomic coverage, throughput, and the accuracy of annotations. A coastal diatom bloom highly expressed nitrate transporters and carbonic anhydrase presumably to support high growth rates and enhance uptake of low levels of dissolved nitrate and CO2. Diatom-diazotroph association (DDA: diatoms with nitrogen fixing symbionts) blooms were common when surface salinity was mesohaline and dissolved nitrate concentrations were below detection, and hence did not show evidence of nitrate utilization, suggesting they relied on ammonium transporters to aquire recently fixed nitrogen. These DDA blooms in the outer plume had rapid turnover of the photosystem D1 protein presumably caused by photodegradation under increased light penetration in clearer waters, and increased expression of silicon transporters as silicon became limiting. Expression of these genes, including carbonic anhydrase and transporters for nitrate and phosphate, were found to reflect the physiological status and biogeochemistry of river plume environments. These relatively stable patterns of eukaryotic transcript abundance occurred over modest spatiotemporal scales, with similarity observed in sample duplicates collected up to 2.45 km in space and 120 minutes in time. These results confirm the use of metatranscriptomics as a valuable tool to understand and predict microbial community function.

Widespread detection of human- and ruminant-origin Bacteroidales markers in subtidal waters of the Salish Sea in Washington State.

Rising populations around coastal systems are increasing the threats to marine water quality. To assess anthropogenic fecal influence, subtidal waters were examined monthly for human- and ruminant-sourced Bacteroidales markers at 80 sites across six oceanographic basins of the Salish Sea (Washington State) from April through October, 2011. In the basins containing cities with individual populations>190,000, >50% of sites were positive for the human marker, while in the basins with high densities of dairy and cattle operations, ∼30% of sites were positive for the ruminant marker. Marker prevalence was elevated in spring (April and May) and fall (October) and reduced during summer (June through September), corresponding with seasonal precipitation. By logistic regression, the odds of human marker detection increased with percentage of adjacent catchment impervious surface, dissolved nitrate concentration, and abundance of low nucleic acid bacteria, but decreased with salinity and chlorophyll fluorescence. The odds of ruminant marker detection increased with dissolved ammonium concentration, mean flow rate for the nearest river, and adjacent shoreline length. These relationships are consistent with terrestrial to marine water flow as a transport mechanism. Thus, Bacteroidales markers traditionally used for identifying nearby sources can be used for assessing anthropogenic fecal inputs to regional marine ecosystems.

Seasonal Variability of Flow and Nitrate Flux in Gilgel Gibe River, South West, Ethiopia

Nitrate is a limiting nutrient for plant growth and vital for crop production to increase agricultural productivity. However, its excessive presence in aquatic environment poses risk turning in to major aquatic ecosystem perturbation. Dissolved nitrate from lands to waterways is mainly exported via runoff and leaching. The transport of nitrate in a river is a function of the streamflow rate and its concentration, which render to the seasonal variation. Therefore, the aim of this study was to assess the seasonal variation of flow and nitrate flux. Streamflow data of the Gilgel Gibe River for the period of two years (2013-2014) were used. The baseflow is separated using Web-based Hydrograph Analysis Tool (WHAT). Dissolved nitrate concentrations measured on daily basis were also used. The effects of storm events on flow and nitrate transport were examined during dry and wet seasons. The river had shown high discharge rate from the mid June to late October. The baseflow index (BFI) was found 0.76 indicating that the streamflow is mainly controlled by groundwater discharge. Similarly, during the dry seasons, total nitrate input was considerably less than in wetter seasons. Nitrate concentrations, however, were unusually high in the first summer storm runoff after the dry season. The average annual nitrate loads varied from 13761.2 ton to 156.45 ton during the wet and dry seasons respectively. The regression curve for the nitrate load against streamflow (r2=0.88) has shown significant relationship (p-value=0.000). In the time interval studied, over 95% of the nitrate was transported from the watershed during the wet seasons of greater rainfall, which disclosed that seasonality and river flow are primary forcing functions when considering nitrate loadings in this watershed

Fire retardant and post-fire nutrient mobility in a mountain surface water—karst groundwater system: the Hidden Fire, Sequoia National Park, California, USA

Nutrient mobility in strongly coupled surface water–groundwater systems is not well studied in fire dominated ecosystems. In 2008, the Hidden Fire in the Kaweah River basin in the Sierra Nevada, CA, USA provided an unexpected opportunity to document how nutrient concentrations change post-fire in a karstic groundwater—surface water system. The results from 4 years of sampling and water quality data (2009–2012) suggest that nutrient byproducts from 94 Phos-Check D75 R fire retardant that was dropped to combat the fire were mobilized into the aquatic system. Dissolved nitrate concentrations sharply increased at most monitoring sites with the onset of winter precipitation and seasonal snowmelt in the spring of 2009, remained elevated during seasonal sampling at some sites in 2010, and were significantly lower in 2011 and 2012. Average nitrate concentrations in sub-basins during the 2009 high-discharge period are strongly correlated with the amount of retardant applied in each sub-basin, but not correlated with the sub-basin area. Dissolved phosphate concentrations were highest upstream of sink points where streams directly recharge marble karst aquifers, and decreased between recharge sites and springs; indicating organic and/or inorganic uptake of P in the subterranean system. Although measured nutrient concentrations were within the range documented post-fire in other aquatic systems that did not receive fire retardant, evidence from the Hidden Fire strongly suggests that fire retardant derived nutrients entered and flushed through the Yucca Creek system, and that managers should consider the potential impacts of retardant application on aquatic ecosystems even when retardant is not applied directly on waterways .

Assessment of denitrification process in lower Ishikari river system, Japan

Sediment denitrification rate and its role in removal of dissolved nitrate load in lower Ishikari river system were examined. Denitrification rate were measured using acetylene inhibition technique on the sediment samples collected during August 2009–July 2010. The denitrification rate varied from 0.001 to 1.9μgNg−1DMh−1 with an average value of 0.21μgNg−1DMh−1 in lower Ishikari river system. Denitrification rate showed positive correlation with dissolved nitrate concentration in the river basin, indicating overlying water column supplied nitrate for the sediment denitrification processes. Nutrient enrichment experiments result showed that denitrification rate increased significantly with addition of nitrate in case of samples collected from Barato Lake however no such increase was observed in the samples collected from Ishikari river main channel and its major tributaries indicating that factors other than substrate concentration such as population of denitrifier and hydrological properties of stream channel including channel depth and flow velocity may affects the denitrification rate in lower Ishikari river system. Denitrification rate showed no significant increase with the addition of labile carbon (glucose), indicating that sediment samples had sufficient organic matter to sustain denitrification activity. The result of nutrient spiraling model indicates that in- stream denitrification process removes on an average 5%d−1 of dissolve nitrate load in Ishikari river. This study was carried out to fill the gap present in the availability of riverine denitrification rate measurement and its role in nitrogen budget from Japanese rivers characterize by small river length and high flow rate.

Effects of nitrate enrichment on leaf litter decomposition

AIM: This study aimed to determine the effects of nitrate enrichment on leaf decomposition process and the kinetic parameters of decomposition model; METHODS: Samples of water from a first-order stream and senescent leaves of the native tree species Campomanesia xanthocarpa O. Berg (Myrtaceae) were collected in South of Brazil. The leaves were oven-dried, grounded and for each experimental condition (control and enriched) 20 decomposition chambers were prepared with leaf fragments and unfiltered stream water (with and without nitrate addition), maintained under low and high oxygen conditions. In sampling days the particulate and dissolved organic carbon and total inorganic carbon concentrations were evaluated; RESULTS: The decomposition of particulate and dissolved organic carbon (POC and DOC) was faster in nitrate enriched treatment under high dissolved oxygen condition. The DOC mineralization coefficients (k3) were in average 283-fold higher than the rate constants for refractory POC (RPOC) mineralization, being the enriched k3 2.3-fold higher than the control k3; CONCLUSIONS: The leaf litter decomposition was affected by dissolved nitrate concentration in the water and RPOC and DOC decomposition was faster with nitrate enrichment than in reference natural conditions (without nitrate enrichment). Thus, dissolved nitrate seems to be an important factor in controlling litter decomposition and its increase affects the leaf carbon processing in stream ecosystems.

Assessment of Microcystis growth rate potential and nutrient status across a trophic gradient in western Lake Erie

Plankton tow samples collected from 2002 through 2009 indicate that Microcystis biovolume in western Lake Erie is often most dense in transition zone (TZ) waters between Maumee Bay and the center of the western basin. TZ waters are generally high in nutrients and turbidity, and concentrations of each decrease with distance from Maumee Bay. High Microcystis biovolume in the TZ suggests the possibility that the conditions in these waters support a greater Microcystis growth rate relative to the open lake. To test this hypothesis, during the 2008 bloom, Microcystis was collected from western Lake Erie for measurements of total protein content (TPC) as an indicator of growth rate potential and cellular nutrient content to indicate nutrient deficiencies. TPC results indicate that Microcystis in the TZ had a higher potential growth rate compared to offshore waters. TPC values in Maumee Bay were intermediate but not significantly different from the TZ and offshore. Nitrogen content of Microcystis remained high over the summer at all sites, despite very low dissolved nitrate concentrations and low total nitrogen-to-total phosphorus ratio in late summer in the lake. Ammonium level in the lake was constant during the summer, and likely provided the nitrogen source for Microcystis. Cellular phosphorus content varied between site and sample date suggesting that Microcystis was moderately phosphorus deficient. Quotas of micronutrient indicated that Microcystis was not deficient of micronutrients. Results of this study suggest the waters in and adjacent to Maumee Bay provide more favorable growth conditions for Microcystis than offshore waters.

Distribution of Dissolved Inorganic and Organic Nitrogen in River Water and Groundwater in an Agriculturally-Dominated Catchment, South-East Spain

This study examines the relationship between dissolved inorganic nitrogen (DIN: NH4 + + NO2 − + NO3 −) and dissolved organic nitrogen (DON) in river water and groundwater in an agriculturally-dominated catchment in south-east Spain, and estimates the contribution of DON to the total dissolved nitrogen (TDN) concentrations. The studied aquifer-dependent river system consists of Quaternary alluvial sediments deposited by the Guadalquivir River and its tributaries, with both river water and groundwater in the catchment being aerobic. DON is the predominant form of nitrogen in river water (72−97% of the TDN), whereas its proportion to TDN varies considerably in groundwater (<1−99%). A seasonal pattern in the concentration of dissolved nitrate in river water was observed, whereas DON concentrations showed no significant change during the study period. The export of DON from the Guadalquivir River is approximately 2 kg N ha−1 year−1 and is an order of magnitude higher than the export of DON from pristine catchments. Dissolved nitrate concentrations in groundwater were slightly higher in winter and DON concentrations were significantly lower (p < 0.05) in summer. It is found that agricultural soils constitute the main storage of organic nitrogen in the catchment with a steady leaching of high DON concentrations into the alluvial aquifer system (mean value 19.1 mg N L−1), and so indicating that DON should not be overlooked in the nitrogen budgets of agriculturally-dominated catchments.

Amelioration of groundwater nitrate contamination following installation of a central sewage system in two Israeli villages

This study traces both the long-term deterioration of the ground water supply in two neighboring villages that had relied upon cesspits/cesspools for waste disposal, as well as the subsequent progressive improvement to original water quality levels. The rapid improvement is attributed to the replacement of the cesspits by a central sewage disposal network. In each of the villages of Kefar Bara and Kefar Kassem, a single, relatively deep, community well supplies drinking water. These wells were drilled into the underlying carbonate Judea Group aquifer that initially provided very high quality potable water. Over time, large increases in the nitrate contamination, reaching to as high as 67 mg/L nitrate, paralleled the population growth. The higher dissolved nitrate concentrations were also marked by enrichments in the δ15 N (approximately +8 ‰(air)) values above those of the surrounding and regional uncontaminated background δ15 N values (in the range of +3 to +6 ‰ (air)). Within several years after the cesspit disposal was terminated the nitrate values declined to concentrations that were reported (approximately 25 mg/L-NO3) decades prior, when the water quality monitoring had just commenced. This study demonstrates not only how water quality can degrade but also how it can be restored once the problem is identified and countered. This simple method of ameliorating a water quality problem that was tending towards reaching serious proportions would seem to be quite efficacious for any area lacking economic alternative water resources.

Tracing the sources of nitrate in the Han River watershed in Korea, using δ15N-NO 3− and δ18O-NO 3− values

The dissolved nitrate concentrations and their nitrogen and oxygen isotopic ratios were analyzed in seasonal samples from Korea's Han River to ascertain the seasonal and spatial variations of dissolved nitrate and its possible sources. Nitrate concentrations in the South Han River (SHR) were much higher than those in the North Han River (NHR), probably because of the more extensive distribution of agricultural fields, residential areas and animal farms in the SHR drainage basin. The nitrogen isotopic composition of dissolved nitrate indicates that nitrate-nitrogen (NO 3 −-N) is derived mainly from atmospheric deposition and/or soil organic matter in the NHR but comes principally from manure or sewage, with only a minor contribution from atmospheric deposition or soil organic matter, in the SHR. The oxygen isotopic compositions of dissolved nitrate suggest that most atmospheric nitrate undergoes microbial nitrification before entering the river.

Seasonal variations in river discharge and nutrient export to a Northeastern Pacific estuary

Seasonal variations in dissolved nitrogen and silica loadings were related to seasonal variability in river discharge. Dissolved nutrient concentrations measured weekly at three stations in the Yaquina River, Oregon from 1999 through 2001, and then monthly in 2002 were used as the basis for developing a nutrient loading regression as part of a larger agency program for evaluating nutrient processes. Because realistic models of nutrient transport require dense data sets to capture both long and short term fluctuations in nutrient concentrations, data at one freshwater station also were collected hourly for the same years using an in-stream monitor. The effects of storm events on dissolved nutrient transport were examined during three storms, including one in a high rainfall-discharge year, and two in average years, one of which followed a drought year. During the drought year (WY2001), total dissolved nitrate input was considerably less than in wetter years. Dissolved nitrate concentrations, however, were unusually high in the first winter storm runoff after the drought. The freshwater dissolved nitrate nitrogen loads varied from 40,380 kg day −1 during a high-flow storm event to 0.11 kg day −1 during late summer, low flow conditions. Dissolved silica dynamics differed from those of nitrate because during storm events, silica concentrations in the Yaquina River decreased to near zero at the storm height, probably due to dilution by near surface or overland flow, and later recovered. During the time interval studied, over 94% of the dissolved nitrate and silica were transported from the watershed during the winter months of greater rainfall, indicating that seasonality and river flow are primary factors when considering nutrient loadings from this watershed system.