Nitrate Concentrations In Excess Research Articles

Removal of nitrate in raw water using a vertical roughing filter with an external carbon source

AbstractHigh nitrate concentration in water endangers health to both humans and livestock. The aim of this research was to investigate the efficacy of a roughing filter enhanced by an external carbon source in removing nitrate in raw water. Two upward vertical roughing filters in series were used—a vertical roughing filter with ethanol as a carbon source (VRFwt) and a vertical roughing filter without a carbon source (VRFwo). The average nitrate removal efficiency in a VRFwt was 88%, 70% and 83%, at carbon:nitrogen ratios (C/N ratio) of 1.05, 1.08 and 1.1, respectively. The average chemical oxygen demand (COD) removal efficiency in the VRFwt was 75%, 43% and 46% at C/N ratios of 1.05, 1.08 and 1.1, respectively. The total average reduction of dissolved oxygen (DO) in the VRFwt at C/N ratios of 1.05, 1.08 and 1.1 was 42%, 54% and 51%, respectively, while the drop in the VRFwo was 17%, 18% and 17%. The VRFwt showed a high potential for removing nitrate in raw water. Therefore, when the VRFwt is applied at large scale, it would improve access to water sources that were previously declared unacceptable by many water utilities due to excessive nitrate concentrations.

Transcriptomic and fatty acid analyses of Neochloris aquatica grown under different nitrogen concentration.

In this study, we characterized the fatty acid production in Neochloris aquatica at transcriptomics and biochemical levels under limiting, normal, and excess nitrate concentrations in different growth phases. At the stationary phase, N. aquatica mainly produced saturated fatty acids such as stearic acid under the limiting nitrate concentration, which is suitable for biodiesel production. However, it produced polyunsaturated fatty acids such as α-linolenic acid under the excess nitrate concentration, which has nutritional values as food supplements. In addition, RNA-seq was employed to identify genes and pathways that were being affected in N. aquatica for three growth phases in the presence of the different nitrate amounts. Genes that are responsible for the production of saturated fatty acids were upregulated in the cells grown under a limiting nitrogen amount while genes that are responsible for the production of polyunsaturated fatty acid were upregulated in the cells grown under excess nitrogen amount. Further analysis showed more genes differentially expressed (DEGs) at the logarithmic phase in all conditions while a relatively steady trend was observed during the transition from the logarithmic phase to the stationary phase under limiting and excess nitrogen. Our results provide a foundation for identifying developmentally important genes and understanding the biological processes in the different growth phases of the N. aquatica in terms of biomass and lipid production.

An investigation into seasonal variations of groundwater nitrate by spatial modelling strategies at two levels by kriging and co-kriging models

Nitrate pollution of groundwater through spatial models is investigated in this paper by using a sample of nitrate values at monitoring wells using the data from four seasons of a year, in which data are sparse. Two spatial modelling strategies are formulated at two levels, in which Strategy 1 comprises: three variations of kriging-based models (ordinary kriging, simple kriging and universal kriging), which are constructed at Level 1 to predict nitrate concentrations; and a Multiple Co-Kriging (MCoK) model is used at Level 2 to enhance the accuracy of the predictions. Strategy 2 is also at two levels but employs Indicator Kriging (IK) at Level 1 as a probabilistic spatial model to predict areas at risk of exceeding two thresholds of 37.5 mg/L and 50 mg/L of nitrate concentration, and Multiple Co-Indicator Kriging (MCoIK) at Level 2 for a better accuracy. The improvements at Level 2 for both strategies are remarkable and hence they are used to gain an insight into inherent problems. The results of a study delineate areas with excessive nitrate concentrations, which are in the vicinity of urban areas and hence reflect poor planning practices since the 1990s. The results further reveal the patterns on sensitivities to seasonal variations driven by aquifer recharge and strong dilution processes in spring times; and on the role of pumpage impacting aquifers giving rise to possible hotspots of nitrate concentrations.

Short-term responses of tropical lagoon sponges to elevated temperature and nitrate

Sponges are often important components of coastal lagoons, however their responses to anthropogenic stressors remain poorly understood. Here, we tested the responses of three lagoon sponges, Neopetrosia exigua, Amphimedon navalis and Spheciospongia vagabunda from Mauritius (Western Indian Ocean), to nine temperature and nitrate combinations for 14 days. We found that elevated seawater temperature resulted in significant physiological responses in all species, but there was generally little negative effect of elevated nitrate. At the end of the experiment, the buoyant weight of all three species were significantly reduced, while for the two chlorophyll a-containing species, N. exigua and S. vagabunda, effective quantum yield (ΔF/Fm’) of photosystem (PS) II, photosynthetic pigment concentrations, gross photosynthetic rate and gross photosynthesis to respiration (P:R) ratio were also reduced. Dark respiration rates were higher in all three species at elevated temperature. While these lagoon sponges appeared to be impacted by elevated temperature, here, we demonstrate that these species are physiologically tolerant to excess nitrate concentrations.

Environmental land use conflicts in catchments: A major cause of amplified nitrate in river water

Environmental land use conflicts are uses of the land that ignore soil capability. In this study, environmental land use conflicts were investigated in mainland Portugal, using Partial Least Squares (PLS) regression combined with GIS modeling and a group of 85 agricultural watersheds (with >50% occupation by agriculture) as work sample. The results indicate a dominance of conflicts in a region where vineyards systematically invaded steep hillsides (the River Douro basin), where forests would be the most appropriate use. As a consequence of the conflicts, nitrate concentrations in rivers and lakes from these areas have increased, sometimes beyond the legal limit of 50mg/L imposed by the European and Portuguese laws. Excessive nitrate concentrations were also observed along the Atlantic coast of continental Portugal, but associated to a combination of other factors: large population densities, and incomplete coverage by sewage systems and inadequate functioning of wastewater treatment plants. Before this study, environmental land use conflicts were never recognized as possible boost of nitrate concentrations in surface water. Bearing in mind the consequences of drinking water nitrate for human health, a number of land use change scenarios were investigated to forecast their impact on freshwater nitrate concentrations. It was seen that an aggravation of the conflicts would duplicate the number of watersheds with maximum nitrate concentrations above 50mg/L (from 11 to 20 watersheds), while the elimination of the conflicts would greatly reduce that number (to 3 watersheds).

Intrusions of excess nitrate in the Kuroshio subsurface layer

Vertical profiles of nutrient concentration were investigated in the southeastern East China Sea and the Kuroshio (30°N–31°N, 128°E–130°E) in June 2013, in order to understand the effects of less-saline water intrusions on nutrient composition. The ratio of nitrate to phosphate is commonly <16; however, values >16 were observed in the northern edge of the Kuroshio at a depth of 50–100m at the beginning of June, associated with a less-saline water intrusion with a density (σt) of 23.5–24.8. This less-saline water was not present at the end of June. A positive excess nitrate concentration was found in water with a salinity of <34.17, revealing a significant negative correlation between salinity and excess nitrate concentrations. This less-saline excess-nitrate water originated from winter mixing of the East China Sea coastal waters, which were affected by the Changjiang River according to a back-trajectory analysis, and was transported to the east by the Kuroshio >500km away from the source. The excess-nitrate water existed below the euphotic layer, and it prevented the utilization of nutrients by the phytoplankton during the transportation. The intrusion of this less-saline water increased the nitrate to phosphate ratio and supplied excess nitrate to the Kuroshio subsurface layer.

Influence of anthropogenic contamination on groundwater chemistry in Mulbagal town, Kolar District, India

Geochemical studies are performed to examine the impact of leachate infiltration from on-site sewage disposal systems on the groundwater chemistry in Mulbagal town, Kolar District, Karnataka State, India. The leachate infiltration imposed nitrate concentrations ranging from 4 mg/L to 388 mg/L in the groundwater samples; it was observed that 79% of the samples exhibited nitrate concentrations in excess of drinking water permissible limit (45 mg/L). The average (of 43 measurements) E. coli levels in the groundwater samples corresponded to 189 MPN/100 mL and 55% of the samples tested exhibit pathogen contamination. Results also showed that the groundwater in the study area is characterized by acidic pH, large calcium + magnesium ion and Na/Cl ratios of < unity causing majority of the ground water samples to classify as Ca-Mg-Cl type and Na-Cl type. Saturation index (SI) computation using Visual MINTEQ program showed that the groundwater samples are under-saturated with respect to calcite. The theoretical SI values (of calcite) however suggested that the groundwater samples ought to be over-saturated with calcite. Under-saturation of the groundwater samples with calcite is attributed to increased dissolution of the mineral in the acidic environment of the groundwater.

Comparing isotopic groundwater age measurements with simulated groundwater ages: example of the Abbotsford–Sumas Aquifer (USA and Canada) and application

AbstractElevated nitrate concentrations have been documented since the 1970s within the trans‐national Abbotsford–Sumas aquifer, situated in the central Fraser Valley of southern British Columbia, Canada, and northern Washington State, United States. Nitrate concentrations in excess of 10 mg/L NO3–N are commonly observed in the monitoring wells. A groundwater model is used in this study to conduct a particle tracking analysis to estimate groundwater travel times and compare these with groundwater ages determined from 3H/3He concentrations measured in several monitoring wells. Groundwater ages estimated from particle tracking show excellent agreement with measured ages (slope=1), although there is scatter (R2=0.76), reflecting the complexity of the travel pathways and ambiguities in groundwater ages because of mixing. An assessment of the efficacy of recommended nutrient management practices, which were implemented in 1992 to reduce nitrogen loading, suggests that wells shallower than about 20 m should be monitoring a decrease in nitrate concentration. However, persistent elevated nitrate concentrations even at shallow depths point to the lingering effects of a remnant manure–nitrate signature in combination with continued high loading.

DETERMINANTS OF ENVIRONMENTAL NONCOMPLIANCE BY PUBLIC WATER SYSTEMS

Whereas a large number of empirical studies have been devoted to analyzing determinants of environmental compliance (EC) by firms, less attention has been paid to EC by public water systems (PWS). To address this gap in the literature, this article uses data on compliance with maximum contaminant levels (MCL) under the Safe Drinking Water Act for 971 PWS in Arizona and identifies the characteristics of PWS that are associated with violation of MCL standards. Three main findings emerge from the analysis. First, larger PWS are more likely to violate MCL standards than smaller PWS. Second, publicly owned PWS have slightly higher probability of violating MCL standards than privately owned systems. Finally, PWS serving residential areas are more likely to violate MCL standards, as opposed to PWS serving school districts or office buildings. The results suggest that for ensuring safe water for people, effectiveness of monitoring policy, and an efficient utilization of resources, the environmental agencies may focus their monitoring and enforcement efforts on these water systems.(JEL K32, Q53)

Assessment of the predictive quality of simple indicator approaches for nitrate leaching from agricultural fields

Diffuse N losses from agriculture are a major cause of excessive nitrate concentrations in surface and groundwaters. Leaching through the soil is the main pathway of nitrate loss. For environmental management, an anticipatory assessment and monitoring of nitrate leaching risk by indicator (index) approaches is increasingly being used. Although complex Nitrogen Loss Indicator (NLI) approaches may provide more information, relatively simple NLIs may have advantages in many practical situations, for instance, when data availability is restricted. In this study, we tested four simple NLIs to assess their predictive properties: 1. N balance (Nbal); 2. Exchange frequency of soil solution (EF); 3. Potential nitrate concentration in leachate (PNCL); 4. A composite NLI (balance exchange frequency product, BEP). Field data of nitrate leaching from two sites in northeast Germany along with published data from several sites in Germany, Scotland and the USA were utilized. Nbal proved to be a relatively poor indicator of Nloss for the time frame of one year, whereas its prediction accuracy improved for longterm-averaged data. Correlation between calculated EF and experimental data was high for single-year data, whereas it was lower for longterm-averaged data. PNCL gave no significant correlations with measured data and high deviations. The results for BEP were intermediate between those for Nbal and EF. The results suggest that the use of EF is appropriate for assessing N leaching loss for single-year data and specific sites with comparable N input and management practices, whereas for longterm-averaged data, Nbal is better suited. BEP is an appropriate NLI both for single year and longterm data which accounts for source and transport factors and thus is more flexible than source-based Nbal and transport-based EF. However, such simplified NLIs have limitations: 1. The N cycle is not covered completely; 2. Processes in the vadose zone and the aquifer are neglected, 3. Assessment of management factors is restricted.

Modelling recovery from soil acidification in European forests under climate change

A simple soil acidification model was applied to evaluate the effects of sulphur and nitrogen emission reductions on the recovery of acidified European forest soils. In addition we included the effects of climate change on soil solution chemistry, by modelling temperature effects on soil chemical processes and including temperature and precipitation effects on nitrogen uptake and on leaching. Model results showed a strong effect of the emission reduction scenarios on soil solution chemistry. Using the Current Legislation (CLE) scenario, the forest area in Europe with soil solution Al/Bc >1 mol mol − 1 (a widely used critical limit) decreased from about 4% in 1990 to about 1.7% in 2050. Under Maximum Feasible Reductions (MFR), the exceeded area will be < 1% in 2050. In addition, the area where limits for the nitrate concentration in soils are violated is predicted to be smaller under MFR than under CLE. Using the most stringent criterion for nitrate ([NO 3] <0.3 mg l − 1 ), the area with nitrate concentrations in excess of the critical limit is about 33% in 2050 under CLE, but only 12% under MFR. Recovery, i.e. attaining non-violation of the criterion, is also much faster under MFR than under CLE. Climate change leads to higher weathering rates and nitrogen uptake in the model, but positive effects on recovery from acidification are limited compared to current climate, and differences between the A1 and B2 climate change scenarios were small. Target loads for 2050 exist for 4% of the area for Al/Bc = 1 and for 12% of the area when using a criterion of ANC = 0 for the soil solution. In about 30% of the area where meaningful target loads exists, the computed target load is lower than the deposition under MFR, and thus cannot be attained with current emission abatement technologies.

Seasonal variation in groundwater levels and quality under intensively drained and grazed pastures in the Montagu catchment, NW Tasmania

Water quality is a significant environmental issue in the Montagu River and its estuary in north-west Tasmania. Groundwater is the major contributor to baseflow for about half of the year. ‘Hump and hollow’ surface drainage is increasingly being used to reduce the effects of seasonal waterlogging on pasture production. However, little is known about the effects of ‘hump and hollow’ structures on watertable levels or intensive grazing on groundwater quality in the catchment. The objectives of this study were to evaluate the impacts of ‘hump and hollow’ drainage by comparing watertable levels in drained and undrained paddocks and to quantify the effects of intensive grazing on groundwater quality underlying pastures. In December 2004, 10 wells and 2 piezometers were installed at depths of 2–6 m at seven sites along two transects across the dairying area of Togari. Water levels were monitored and water samples collected every 2 months were analysed for pH, electrical conductivity, total dissolved solids, ammonium, nitrate, nitrite, total nitrogen, dissolved reactive phosphorous, Ca, Mg, K and Na. Thermotolerant coliforms and Enterococcus were measured when watertable levels were low and high. Watertable levels were within 0.5 m of ground level for over 3 months on undrained sites. ‘Hump and hollow’ surface drainage increased the depth of the unsaturated zone under the ‘humps’ but did not lower the watertable. Watertable levels on the crests of the ‘hump and hollow’ structures rose and fell daily in response to periods of rainfall and drought. Gradients of the groundwater surface, albeit very low, indicated the potential for groundwater flow from the base of the hills to the Montagu River in the centre of the valley. The median nitrate concentration of all samples was 0.018 mg NO 3-N L −1 but one site had nitrate concentrations in excess of that recommended for potable water for a period of 1–2 months. Nitrate concentrations varied seasonally by 20–1000 times with an early winter pulse of nitrate evident both in the groundwater and in the Montagu River. In contrast, the median ammonium concentration in the groundwater was 0.274 mg NH 4-N L −1 which was well above the trigger value for lowland streams. The median concentration of dissolved reactive phosphorus was 0.008 mg P L −1 which was slightly higher than the trigger value. There was some evidence of low levels of faecal bacterial contamination of the shallow aquifers. Transects across the dairying area did not clearly demonstrate increasing concentrations of analytes due to intensive grazing though lower levels of nutrients were generally found at sites adjacent to undisturbed native forest. Variation in water quality parameters along the transects suggested water quality at a site was mostly related to local conditions and hazards.

Nitrate and bacterial contamination in well waters in Vinh Phuc province, Vietnam

Measurements were made of the nitrate concentration and bacterial contamination of groundwater samples taken from both dug wells and bores in the commune of Hop Thinh', in the administrative district of Vinh Phuc, Vietnam. A significant number (18%) of samples had nitrate concentrations in excess of the WHO Guideline value for drinking water of 50 mg L(-1), with a higher proportion in the dug wells (29%) than the bores (3.8%). High concentrations of thermotolerant coliforms were found in many of the dug wells and even in the deeper drilled bores. At the time of the study no Shigella or other infectious organisms were found. There was no correlation between nitrate concentration and bacterial content and it is concluded that nitrate concentration is not a good indicator for bacterial contamination.

Investigating rising nitrate concentrations in groundwater in the Permo-Triassic aquifer, Eden Valley, Cumbria, UK

Abstract Groundwater nitrate concentrations in the Permo-Triassic aquifer of the Eden Valley vary from less than 4 mg l −1 to in excess of 100 mg l −1 (as NO 3 ). A significant number of boreholes exhibit rising trends in nitrate concentration that either approach or exceed the CEC Directive 80/778 Maximum Admissible Concentration (MAC) of 50 mg l −1 . The main source of the nitrate is believed to be the nitrogen applied to grassland, both as slurry and as inorganic fertilizers. The variability in groundwater nitrate concentrations is thought to be due in part to land use, particularly where low-yielding boreholes derive their water from a limited/localized area, and in part due to the variability in the travel times for water and solutes to migrate from the soil to the water table and then to the borehole. This variability in travel times is a function of surficial geology, depth to water table, depth of borehole and superficial deposit thickness, amongst other factors. It is surprising, given the considerable storage within the saturated zone of the aquifer and the slow groundwater movement, that some relatively deep boreholes pump groundwater with nitrate concentrations in excess of 20 mgl −1 . Simple numerical modelling suggests that the fraction of modern water pumped is sensitive to the presence of fissures close to the abstraction boreholes and the location of the boreholes relative to superficial deposits. For some scenarios, using realistic superficial deposit geometries and aquifer hydraulic parameters, the proportion of modern water (water that is derived from infiltration that reached the water table since pumping started) could exceed 40% within 15 years of pumping.

Nitrate in groundwater: a water company perspective

More than 100 groundwater sources operated by Anglian Water Services are vulnerable to diffuse and point pollution, and 33 of these sources have nitrate concentrations in excess of 50 mg l −1 as NO 3 . Most time series nitrate data ‘recess’ to a long-term rising baseline, upon which seasonal variations and/or data scatter are superimposed. Investigations at boreholes abstracting from the Chalk aquifer show that concentrations decrease with depth below the water table and with time after pump start-up. Abstraction rates do not affect concentrations but the combination of boreholes in use does. A reliable and robust semi-quantitative method for predicting future nitrate concentrations has been developed. Use of this method has allowed a rigorous definition of capital investment requirements under the AMP4 process to maintain compliance with drinking water standards. The method has been used to predict short-term nitrate concentrations for a variety of recharge scenarios.

Overcoming oxygen limitations in membrane-attached biofilms—investigation of flux and diffusivity in an anoxic biofilm

The possibility of overcoming oxygen limitations in membrane-attached biofilms has been investigated by using nitrate as an electron acceptor instead of oxygen in an extractive membrane bioreactor (EMB) degrading toluene. The effect of nitrate concentration on toluene flux, the effective diffusivity in the biofilm and the biofilm activity has been investigated. A counter-diffusion-reaction model is also presented, describing the pollutant flux versus biofilm thickness. The toluene flux decreased with increasing biofilm thickness under excess nitrate concentrations, similar to the experiment with low nitrate. Mathematical modelling indicated that this was either due to decreasing activity, and/or different diffusivities in the biofilm. The effective diffusivity was investigated by using an inert tracer molecule. It remained constant for biofilm thicknesses up to 1.8 mm, with a value twice that in water. The biofilm activity was investigated by inactivating a mature biofilm using sodium azide. The toluene flux remained the same before and after the addition of sodium azide, suggesting that the activity in the biofilm is very low. We conclude that the decreasing toluene flux with increasing biofilm thickness is due to the diffusional resistance of the inactive biofilm.

Nitrate distributions and source identification in the Abbotsford-Sumas Aquifer, northwestern Washington State.

The Abbotsford-Sumas Aquifer is a shallow, predominantly unconfined aquifer that spans regions in southwestern British Columbia, Canada and northwestern Washington, USA. The aquifer is prone to nitrate contamination because of extensive regional agricultural practices. A 22-month ground water nitrate assessment was performed in a 10-km2 study area adjacent to the international boundary in northwestern Washington to examine nitrate concentrations and nitrogen isotope ratios to characterize local source contributions from up-gradient sources in Canada. Nitrate concentrations in excess of 10 mg nitrate as nitrogen per liter (mg N L(-1)) were observed in ground water from most of the 26 domestic wells sampled in the study area, and in a creek that dissects the study area. The nitrate distribution was characteristic of nonpoint agricultural sources and consistent with the historical documentation of agriculturally related nitrate contamination in many parts of the aquifer. Hydrogeologic information, nitrogen isotope values, and statistical analyses indicated a nitrate concentration stratification in the study area. The highest concentrations (> 20 mg N L(-1)) occurred in shallow regions of the aquifer and were linked to local agricultural practices in northwestern Washington. Nitrate concentrations in excess of 10 mg N L(-1) deeper in the aquifer (> 10 m) were related to agricultural sources in Canada. The identification of two possible sources of ground water nitrate in northwestern Washington adds to the difficulty in assessing and implementing local nutrient management plans for protecting drinking water in the region.

Nitrate Distributions and Source Identification in the Abbotsford–Sumas Aquifer, Northwestern Washington State

The Abbotsford–Sumas Aquifer is a shallow, predominantly unconfined aquifer that spans regions in southwestern British Columbia, Canada and northwestern Washington, USA. The aquifer is prone to nitrate contamination because of extensive regional agricultural practices. A 22-month ground water nitrate assessment was performed in a 10-km2 study area adjacent to the international boundary in northwestern Washington to examine nitrate concentrations and nitrogen isotope ratios to characterize local source contributions from up-gradient sources in Canada. Nitrate concentrations in excess of 10 mg nitrate as nitrogen per liter (mg N L−1) were observed in ground water from most of the 26 domestic wells sampled in the study area, and in a creek that dissects the study area. The nitrate distribution was characteristic of nonpoint agricultural sources and consistent with the historical documentation of agriculturally related nitrate contamination in many parts of the aquifer. Hydrogeologic information, nitrogen isotope values, and statistical analyses indicated a nitrate concentration stratification in the study area. The highest concentrations (>20 mg N L−1) occurred in shallow regions of the aquifer and were linked to local agricultural practices in northwestern Washington. Nitrate concentrations in excess of 10 mg N L−1 deeper in the aquifer (>10 m) were related to agricultural sources in Canada. The identification of two possible sources of ground water nitrate in northwestern Washington adds to the difficulty in assessing and implementing local nutrient management plans for protecting drinking water in the region.

Denitrification of drinking water by the association of an electrodialysis process and a membrane bioreactor: feasibility and application

A hybrid process combining electrodialysis and a membrane bioreactor was investigated to treat ground waters with excessive nitrate concentrations. Electrodialysis (ED) allowed the nitrate separation producing, on one hand, partially demineralized waters whose ion concentrations were in agreement with the norm, and on the other hand, brines that were treated by a membrane bioreactor. Experiments performed at laboratory scale with synthetic solutions and pre-industrial scale to treat a ground water contaminated by nitrates showed high efficiency of the hybrid process. The nitrate concentration of the treated water remained below the acceptable value (50 mg/l−1) and even below the recommended value (25 mg/l−1) for drinking water. Moreover, the ED treatment induced a softening of the treated water. The biological denitrification allowed the almost total removal of nitrates (99%) with kinetics close to 0.3kgNNO3/kg MVS/d and a limited sludge production of 0.5 gV.S.S./gNNO3.

Plasma nitrate concentrations in children with infectious and noninfectious diarrhea.

In patients with intact renal function and low dietary nitrate intake, plasma nitrate concentrations reflect endogenous nitric oxide production and are shown to be increased during inflammatory processes. The aim of this study was to compare plasma nitrate concentrations and hence endogenous nitric oxide production in children with infectious and noninfectious diarrhea and to determine whether plasma nitrate concentrations could serve as a discriminant test between acute and chronic diarrhea in children. Three groups of patients were identified: 14 patients with acute gastroenteritis, 13 patients with chronic noninfectious diarrhea, and 14 patients with no evidence of gastrointestinal pathology and no underlying infectious process, who served as control subjects. Plasma nitrate concentrations were determined spectrophotometrically using the Greiss reaction before reduction to nitrite with a copper-coated cadmium column. Mean plasma nitrate concentrations were 405.3 micromol/L +/- 281.6 micromol/L (standard deviation) in patients with infectious diarrhea, 134.7 micromol/L +/- 77.0 micromol/L in patients with chronic diarrhea, and 54.1 micromol/L +/- 20.1 micromol/L in control subjects (F = 42.6, P < 0.0001; analysis of variance). Plasma nitrate concentrations were significantly higher in the infectious diarrhea group compared with the noninfectious diarrhea and control groups (Student-Newman-Keuls test, P < 0.5). Although an optimal cutoff concentration cannot be defined, plasma nitrate concentrations in excess of 300 micromol/L are suggestive of an infectious process whereas values less than 100 micromol/L are indicative of noninfectious diarrhea.