Median Nitrate Concentrations Research Articles

Groundwater nitrate contamination in China: Spatial distribution, temporal trend, and driver analysis

China's groundwater is facing a significant threat from nitrate pollution. Here we analyzed 2348 regional surveys of groundwater nitrate levels in China from 1990 to 2020, examining distribution, trends, and drivers. This study uncovers a concerning rise in nitrate pollution, with estimated median nitrate levels climbing from 3.84 mg/L in 1990 to 6.94 mg/L in 2020. A stark contrast is observed between regions: the northern areas have a median nitrate concentration of 8.54 mg/L, significantly higher than the southern regions, where the median is just 7.15 mg/L. From 1990 to 2020, agricultural activity consistently emerges as the dominant driver of changes in groundwater nitrate concentrations, while groundwater exploitation, domestic pollution, and industrial production also contribute to varying degrees. This analysis highlights the urgency for region-specific policies and interventions to address the escalating nitrate pollution in China's groundwater.

Nitrate in Maternal Drinking Water during Pregnancy and Measures of Male Fecundity in Adult Sons.

Animal studies indicate deleterious effects of nitrate exposure on fecundity, but effects in humans are unknown, both for the prenatal and postnatal periods. We aimed to investigate if exposure to nitrate in maternal drinking water during the sensitive period of fetal life is associated with measures of fecundity in the adult sons. In a sub-analysis, the potential effects of nitrate exposure in adulthood were investigated. This cohort included 985 young adult men enrolled in The Fetal Programming of Semen Quality Cohort (FEPOS). Semen characteristics, testes volume and reproductive hormones were analyzed in relation to nitrate concentration in maternal drinking water, using a negative binomial regression model. The nitrate concentration in drinking water was obtained from monitoring data from Danish waterworks that were linked with the mothers' residential address during pregnancy. The median nitrate concentration in maternal drinking water was 2 mg/L. At these low exposure levels, which are far below the World Health Organization's (WHO) guideline value of 50 mg/L, we did not find indications of harmful effects of nitrate on the investigated measures of male fecundity.

Nitrate in Danish household tap water and the risk of small-for-gestational-age, 1991-2015

BACKGROUND AND AIM: Findings from studies examining the effects of prenatal exposure to nitrate from drinking water on fetal growth have been inconsistent, small, and mostly ecologic in nature. The aim of this study was to assess maternal exposure to nitrate (NO₃⁻) in drinking water and small-for-gestational-age in a large population-based cohort. METHODS: Exposure estimates from each maternal address during pregnancy were obtained from a national public and private water monitoring database. We linked these individual household nitrate concentrations with individual-level birth registry data to create a cohort of 1,078,892 singleton live births in Denmark to Danish-born parents during 1991–2015. Exposures were modeled as log-transformed continuous estimates and categorically. Small-for-gestational-age was defined as the lightest 10% of infants at birth per sex and gestational week. Multivariable logistic regression with generalized estimating equations was used to assess associations and account for the non-independence of siblings. National register-based data were used to adjust for sex, birth year, birth order, and maternal age, smoking, education, income, and employment status at the individual level. RESULTS:In this cohort, the median nitrate concentration was 1.9 mg/L NO₃⁻. An increased risk of small-for-gestational-age was observed when comparing half the European Union standard (25 mg/L NO₃⁻) to 1 mg/L NO₃⁻ [OR: 1.03 (95% CI: 1.01, 1.05)] and in categorical analyses, except for the highest category (25 mg/L NO₃⁻) compared to the referent (2 mg/L NO₃⁻), possibly due to small numbers. CONCLUSIONS:In this nationwide study spanning 25 years, we found that nitrate in maternal household tap water during pregnancy was associated with an increased risk of her child being born small-for-gestational-age. While the increased risk may be small at an individual level, given the widespread presence of nitrate in drinking water it is important on a global scale. When evaluating nitrate regulations, policymakers should focus on health impacts at levels below current standards. KEYWORDS: Big data, Birth outcomes, Children's environmental health, Environmental epidemiology, International collaboration, Water quality

Drinking Water Sources and Water Quality in the Agricultural Health Study

BACKGROUND AND AIM: Drinking water contaminants have been associated with adverse health effects. However, little is known about exposures in agricultural communities where farm practices and well use may enhance exposures to specific contaminants. We describe the exposure assessment and exposure distributions for common drinking water contaminants by water source in an agricultural population. METHODS: We used questionnaire data from the Agricultural Health Study (AHS), a cohort of licensed pesticide applicators (mostly farmers) and their spouses in Iowa (IA) and North Carolina (NC) to ascertain drinking water source at enrollment (1993-1997). For users of public water supplies (PWS), we linked participants’ geocoded addresses to monitoring data for regulated contaminants, including five haloacetic acids (HAA5), total trihalomethanes (TTHM), and nitrate-nitrogen (NO₃-N). For private well users, we estimated nitrate levels using previously developed random forest models based on well depth, land use, soil characteristics, nitrogen inputs, and other relevant factors. RESULTS:We assigned drinking water source for 84% (N=74,919) of AHS participants; approximately 70% in IA and 75% in NC used private wells. Median nitrate concentrations (mg/L NO₃-N) in PWS were higher in IA (0.8, interquartile range (IQR): 0.2-3.1) than NC (0.5, IQR: 0.5-0.6), while median concentrations of HAA5 (µg/L) and TTHM (µg/L) were higher in NC (HAA5: 12.1, IQR: 3.8-33.4; TTHM: 35.4, IQR: 8.8-54.7) than IA (HAA5: 4.3, IQR: 3.0-10.5; TTHM: 13.0, IQR: 3.7-32.3). Private well nitrate concentrations in IA (1.5, IQR: 0.8-4.9) and NC (1.9, IQR: 1.4-2.5) were higher than in PWS. A larger proportion of private wells exceeded the 10mg/L regulatory limit in IA (12%) than NC (1%). CONCLUSIONS:Drinking water nitrate levels were higher for private well users and in IA PWS, while TTHM and HAA5 levels were higher in NC PWS. This work will facilitate future analyses of the health effects of drinking water contaminants in the AHS. KEYWORDS: Water quality, Exposure assessment, Environmental epidemiology

Spatial and temporal variability in discharge and nitrate in Iowa subsurface drains.

Agricultural subsurface drainage can be an important conduit of nitrate from agricultural fields to streams. This study focused on understanding the variability in nitrate concentrations and loads, exported by subsurface drains, into a small, north-central Iowa stream. Ninety-three subsurface drains in this watershed were sampled up to 5 times between 2006 and 2008. Additionally, 2 subsurface drains and the stream draining the study area (South Fork Iowa River near Blairsburg, IA, USA) were sampled frequently during the growing seasons in 2007 and 2008. Spatial variability analysis revealed no distinct spatial pattern in nitrate concentrations. The median nitrate concentrations were not significantly different when the drain outlets were characterized by diameter (17-23 cm, 27-48 cm, 60-108 cm). The eight large subsurface drains (part of the public drainage network) had less variability in nitrate concentration than the smaller drain sizes and generally contributed 70-87% of the total water and nitrate loads exported by subsurface drains to the stream. During high-discharge events, the medium-sized (27-48 cm) subsurface drains discharging to the stream became more important by contributing a higher discharge and nitrate load. The temporal variability examined in this study found that discharge and nitrate loads were influenced by the amount of precipitation that had occurred over the previous months. This paper demonstrates the spatial and within-season homogeneity of nitrate delivery to a stream from an intensely agricultural landscape that has subsurface drainage.

Spatial distribution and habitat suitability of Biomphalaria straminea, intermediate host of Schistosoma mansoni, in Guangdong, China

BackgroundBiomphalaria straminea is an invasive vector in China, posing a significant threat to public health. Understanding the factors affecting the establishment of this snail is crucial to improve our ability to manage its dispersal and potential risk of schistosomiasis transmission. This study sought to determine the spatial distribution of B. straminea in mainland China and whether environmental factors were divergent between places with and without B. straminea.MethodsA malacological survey of B. straminea was conducted in Guangdong Province, China. Snails were identified using anatomical keys. Water and sediment samples were taken, and their physicochemical properties were analyzed using national standard methods. Landscape and climatic variables were also collected for each site. We compared the environmental characteristics between sites with and without B. straminea using Mann-Whitney U test. We further used generalized linear mixed models to account for seasonal effects.ResultsB. straminea was found at six sites, including one in Dongguan and five in Shenzhen. Probability map found a hot spot of B. straminea distribution at Shenzhen and Hong Kong. Sites occupied by B. straminea were characterized by higher median altitude, mean annual precipitation and moderate temperature. Water with snails had higher median concentrations of total nitrogen, nitrate and nitrites, ammoniacal nitrogen, calcium, zinc and manganese but lower dissolved oxygen and magnesium. Sediments with snails had higher median copper, zinc and manganese. B. straminea was associated with maximum temperature of the warmest month (pMCMC < 0.001) and sediment zinc (pMCMC < 0.001).ConclusionsB. straminea is distributed in Shenzhen and its surrounding areas in Guangdong, China. Sites with and without B. straminea differed in the maximum temperature of the warmest month and sediment zinc. Surveillance should be continued to monitor the dispersal of this snail in China.

Occurrence of nitrate in vegetables and dietary exposure assessment for the Cypriot adolescent consumers

In this study, a simple, rapid and sensitive ion chromatography method was used for the determination of mass fraction nitrate (mg kg−1) in 396 samples of vegetables. The method performance characteristics were determined after spiking blank samples. The mean recoveries in vegetables ranged from 94.0 % to 108.7 %. Limits of detection and quantification were 10 mg kg−1 and 30 mg kg−1, respectively. The results showed the mass fraction nitrate in vegetables was in the range of 10 mg kg−1–5619 mg kg−1. Leafy, root and stem vegetables, and herbs were the most contaminated, compared to other vegetables. Approximately 87 % of the samples were contaminated with nitrate; however, none of the samples analyzed was above the maximum limits for nitrate in vegetables. The dietary nitrate intake through consumption of vegetables was estimated for the adolescent population in Cyprus. The mean and 95th percentile intake values, based on median nitrate concentrations, were 1.12 kg d−1 and 3.42 mg kg−1 d−1, respectively. A small percentage (3.3 %) of the adolescent population had a nitrate intake above the Acceptable Daily Intake (3.7 mg kg−1 d−1). Lettuce and other leafy vegetables (spinach, rucola, purslane, beet leaves) contributed nearly 60 % to the overall nitrate exposure (mg kg−1 d−1).

Assessing background values of chloride, sulfate and fluoride in groundwater: A geochemical-statistical approach at a regional scale

The Sardinia island (Italy) is one of the European areas least affected by potentially anthropogenic impacts, such as spreading urbanization, intensive agriculture and regional atmospheric contamination. Such characteristics allow to consider Sardinia a good site for testing an approach that integrates geochemical tools, hierarchical cluster and geographical information system, aimed at estimating background concentrations of chloride, sulfate and fluoride at the regional scale.Analytical data were obtained from several hydrogeochemical surveys and from the groundwater-monitoring program established by the Sardinian Regional Government. Groundwater samples were grouped according to their circulation in the predominant hydrogeologic complex: Quaternary sediments, Quaternary basalts, Tertiary sediments, Tertiary volcanic rocks, Mesozoic carbonatic rocks, Paleozoic carbonatic rocks, granitic rocks and metamorphic rocks.Samples surely affected by anthropogenic inputs, thermal waters, waters collected at wells with unknown construction details and poor quality analyses were excluded. The resulting dataset included 1414 groundwater sampling sites distributed over an area of 24,090km2 (All data). Another dataset comprised of 641 sampling sites (Selected data) was derived by All data excluding the groundwater with NO3−>10mg/L. Hierarchical clustering analysis was performed on both datasets considering Ca2+, Mg2+, Na+, K+, Cl−, HCO3−, SO42−, NO3− and F−. The values of total dissolved solids (TDS) were a major distinguishing factor among clusters, but distinct signatures related to the median nitrate and fluoride concentrations were also recognized. The geographic distribution of clusters reflected the role of geological and geographic characteristics on the geochemistry of groundwater.Background ranges of the regulated parameters chloride, sulfate and fluoride in each cluster, identified either using All data or Selected data, were calculated using the median±2MAD. Although results were found in general agreement, the threshold using the median+2MAD was calculated using the Selected data only, because the Selected data better represents near pristine conditions. Chloride threshold values above the drinking water limit were mainly observed in groundwater located in western Sardinia, where sediments and volcanic rocks prevalently outcrop, and also in some coastal areas. Threshold values of sulfate and fluoride above the limit were related to local conditions. Specifically, high threshold values of sulfate were observed in groundwater interacting with the Tertiary volcanic rocks that host known sulfide mineralization and at sites where evaporitic deposits occur. Threshold values of fluoride above the limit were often observed in the areas where fluoride mineralization occurs. High fluoride values may also result from cation exchange and/or supersaturation with respect to calcite.The results of this study indicate that the integration of hierarchical clustering analysis with the geochemical characteristics of groundwater, also taking into account the geological context, allow the repartition of groundwater samples in distinct hydrogeochemical groups, which in turn allow to calculate the background ranges and reliable threshold values in groundwater. This approach can be applied to assess the background concentrations of chemical parameters at a regional scale when a large dataset is available.

Contenido en nitratos de aguas de consumo público españolas

ObjectiveTo determine nitrate concentrations in drinking water in a sample of Spanish cities. Material and methodsWe used ion chromatography to analyze the nitrate concentrations of public drinking water in 108 Spanish municipalities with more than 50,000 inhabitants (supplying 21,290,707 potential individuals). The samples were collected between January and April 2012. The total number of samples tested was 324. ResultsThe median nitrate concentration was 3.47mg/L (range: 0.38-66.76; interquartile range: 4.51). The water from 94% of the municipalities contained less than 15mg/L. The concentration was higher than 25mg/L in only 3 municipalities and was greater than 50mg/L in one. ConclusionsNitrate levels in most public drinking water supplies in municipalities inhabited by almost half of the Spanish population are below 15mg/L.

A Comparison of Wet Deposition Collectors at a Coastal Rural Site

Atmospheric deposition occurs in a variety of forms and is crucial for the evaluation of nutrient budgets, critical loads, and pollution inputs across space and time. Atmospheric wet deposition is typically quantified by analyzing the chemistry of precipitation that is collected in some type of container with a lid that opens in response to precipitation. However, collectors can vary in shape as well as in the sensor that signals when precipitation is occurring. Here, we compare the collectors made by Aerochem Metrics and N-CON Systems Company Inc. The former has been widely used for several decades, while the latter is relatively new and has been used in a variety of configurations depending on the solute of interest. Event-based samples were collected from August 2007 to October 2008 and analyzed for nitrate, ammonium, sulfate, chloride, and dissolved organic carbon (DOC). A variety of approaches were used to assess the comparability of the two collectors. Regressions of concentration versus concentration from the two collectors were strong, and the slope did not differ from 1 for nitrate, ammonium, or sulfate. The median concentrations of nitrate, ammonium, and sulfate were, however, significantly higher in the N-CON collector, while there were no overall differences between collectors for chloride or DOC. Although we have observed some statistically significant differences between solute concentrations of samples collected from the two collectors, our mixed results suggest that these differences are relatively small.

The effect of changes in land use on nitrate concentration in water supply wells in southern Chester County, Pennsylvania

An analysis of private potable water well data was conducted for seven single family residential developments in southern Chester County, Pennsylvania. Background data were available for 165 wells within the communities when the wells were first drilled in the 1980s and early 1990s. Sampling of 75 wells within these same communities was performed in 2006 to determine whether conversion of the land to residential housing along with the use of conventional on-lot septic systems had resulted in elevated concentration of nitrate-nitrogen in the drinking water aquifer. The data indicate that prior land use influenced the occurrence of nitrate-nitrogen in the drinking water aquifer. The median nitrate-nitrogen concentration for the 165 wells in the background dataset was 2.9 mg/L. One hundred-seven of those wells were drilled on land previously used for active agricultural purposes. The median nitrate concentration in these wells was 3.8 mg/L. Of 48 wells drilled on forested land, the median nitrate concentration was 1.1 mg/L, approximately 3.5 times lower than those drilled on active agricultural land. The median nitrate concentration in the 2006 sampling dataset was 3.6 mg/L, an increase of 0.7 mg/L. The data indicate that conversion of the land has not resulted in contamination of the drinking water aquifer with respect to nitrate-nitrogen. Likewise, the data suggest that the conversion has not resulted in significant improvements to overall water quality.

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.

Temporal Trends in Nitrate and Selected Pesticides in Mid‐Atlantic Ground Water

Evaluating long-term temporal trends in regional ground-water quality is complicated by variable hydrogeologic conditions and typically slow flow, and such trends have rarely been directly measured. Ground-water samples were collected over near-decadal and annual intervals from unconfined aquifers in agricultural areas of the Mid-Atlantic region, including fractured carbonate rocks in the Great Valley, Potomac River Basin, and unconsolidated sediments on the Delmarva Peninsula. Concentrations of nitrate and selected pesticides and degradates were compared among sampling events and to apparent recharge dates. Observed temporal trends are related to changes in land use and chemical applications, and to hydrogeology and climate. Insignificant differences in nitrate concentrations in the Great Valley between 1993 and 2002 are consistent with relatively steady fertilizer application during respective recharge periods and are likely related to drought conditions in the later sampling period. Detecting trends in Great Valley ground water is complicated by long open boreholes characteristic of wells sampled in this setting which facilitate significant ground-water mixing. Decreasing atrazine and prometon concentrations, however, reflect reported changes in usage. On the Delmarva Peninsula between 1988 and 2001, median nitrate concentrations increased 2 mg per liter in aerobic ground water, reflecting increasing fertilizer applications. Correlations between selected pesticide compounds and apparent recharge date are similarly related to changing land use and chemical application. Observed trends in the two settings demonstrate the importance of considering hydrogeology and recharge date along with changing land and chemical uses when interpreting trends in regional ground-water quality.

Decadal‐Scale Changes of Nitrate in Ground Water of the United States, 1988–2004

This study evaluated decadal-scale changes of nitrate concentrations in ground water samples collected by the USGS National Water-Quality Assessment Program from 495 wells in 24 well networks across the USA in predominantly agricultural areas. Each well network was sampled once during 1988-1995 and resampled once during 2000-2004. Statistical tests of decadal-scale changes of nitrate concentrations in water from all 495 wells combined indicate there is a significant increase in nitrate concentrations in the data set as a whole. Eight out of the 24 well networks, or about 33%, had significant changes of nitrate concentrations. Of the eight well networks with significant decadal-scale changes of nitrate, all except one, the Willamette Valley of Oregon, had increasing nitrate concentrations. Median nitrate concentrations of three of those eight well networks increased above the USEPA maximum contaminant level of 10 mg L(-1). Nitrate in water from wells with reduced conditions had significantly smaller decadal-scale changes in nitrate concentrations than oxidized and mixed waters. A subset of wells had data on ground water recharge date; nitrate concentrations increased in response to the increase of N fertilizer use since about 1950. Determining ground water recharge dates is an important component of a ground water trends investigation because recharge dates provide a link between changes in ground water quality and changes in land-use practices.

Groundwater nitrogen composition and transformation within a moorland catchment, mid-Wales

The importance of upland groundwater systems in providing a medium for nitrogen transformations and processes along flow paths is investigated within the Afon Gwy moorland catchment, Plynlimon, mid-Wales. Dissolved organic nitrogen (DON) was found to be the most abundant form of dissolved nitrogen (N) in most soils and groundwaters, accounting for between 47 and 72% of total dissolved nitrogen in shallow groundwater samples and up to 80% in deeper groundwaters. Groundwater DON may also be an important source of bio-available N in surface waters and marine systems fed by upland catchments. A conceptual model of N processes is proposed based on a detailed study along a transect of nested boreholes and soil suction samplers within the interfluve zone. Shallow groundwater N speciation reflects the soilwater N speciation implying a rapid transport mechanism and good connectivity between the soil and groundwater systems. Median nitrate concentrations were an order of magnitude lower within the soil zone (<5–31 μg/L) than in the shallow groundwaters (86–746 μg/L). Given the rapid hydrostatic response of the groundwater level within the soil zone, the shallow groundwater system is both a source and sink for dissolved N. Results from dissolved N2O, N2/Ar ratios and dissolved N chemistry suggests that microbial N transformations (denitrification and nitrification) may play an important role in controlling the spatial variation in soil and groundwater N speciation. Reducing conditions within the groundwater and saturated soils of the wet-flush zones on the lower hillslopes, a result of relatively impermeable drift deposits, are also important in controlling N speciation and transformation processes.

Constructed wetland attenuation of nitrogen exported in subsurface drainage from irrigated and rain-fed dairy pastures

Nitrogen removal performance is reported for constructed wetlands treating subsurface drainage from irrigated and rain-fed dairy pastures in North Island, New Zealand. Flow-proportional sampling of inflow and outflow concentrations were combined with continuous flow records to calculate mass balances for the wetlands. Drainage flows from the irrigated catchment were 2.5-4 fold higher and N exports up to 5 fold higher per unit area than for the rain-fed catchment. Hydraulic and associated N loadings to the wetlands were highly pulsed, associated with rainfall, soil water status, and irrigation events. Transient pulses of organic nitrogen were an important form of N loss from the rain-fed landscape in the first year, and were very effectively removed in the wetland (> 90%). Median nitrate concentrations of approximately 10 g m(-3) in the drainage inflows were reduced by 15-67% during passage through the wetlands and annual nitrate-N loads by 16-61% (38-31 7 g N m(-2)y(-1)). Generation in the wetlands of net ammoniacal-N and organic-N (irrigated site) partially negated reduction in nitrate-N loads. The results show that constructed wetlands comprising 1-2% of catchment area can provide moderate reductions in TN export via pastoral drainage, but performance is markedly influenced by variations in seasonal loading and establishment/maturation factors.

EVALUATION OF NITRATE DATA IN THE OCKLAWAHA RIVER BASIN, FLORIDA (1953–2002)1

ABSTRACT: Nitrate levels in the Ocklawaha River Basin in north central Florida were reviewed over a 50‐year period. Data were obtained from the literature, U.S. Environmental Protection Agency (USEPA) STOrage and RETrieval (STORET), and U.S. Geological Survey (USGS) databases. The study objective was to determine whether nitrate concentrations are increasing and if so, whether this increase is linked to land use changes. Increasing nitrate levels were seen at 5 of the 14 stations, while other stations showed no trend or a decreasing trend. Median nitrate concentrations in the Ocklawaha River increased from 0.07 mg‐N/L to 0.78 mg‐N/L at sites downstream from the Silver River. Throughout the Rodman Reservoir, median nitrate concentrations decreased from 0.48 mg‐N/L to 0.01 mg‐N/L and increased to 0.04 mg‐N/L after the Kirkpatrick Dam. Flow and concentration relationships were correlated for five stations. At four of the five stations nitrate concentrations decreased in response to increasing flow, likely the result of dilution with nitrate poor water. Changes in land use over a 20‐ year period (1970 to 1990) also were monitored. Sources of nitrate have been linked by isotopic analysis to organic and inorganic fertilizers, which appear to be related to increased urbanization and an increase in lawns that require nutrient fertilization.

Effects of land use on ground water quality in the Anoka Sand Plain Aquifer of Minnesota.

We began a study, in 1996, to compare ground water quality under irrigated and nonirrigated agriculture, sewered and nonsewered residential developments, industrial, and nondeveloped land uses. Twenty-three monitoring wells were completed in the upper meter of an unconfined sand aquifer. Between 1997 and 2000, sampling occurred quarterly for major ions, trace inorganic chemicals, volatile organic compounds (VOCs), herbicides, and herbicide degradates. On single occasions, we collected samples for polynuclear aromatic hydrocarbons (PAHs), perchlorate, and coliform bacteria. We observed significant differences in water chemistry beneath different land uses. Concentrations of several trace inorganic chemicals were greatest under sewered urban areas. VOC detection frequencies were 100% in commercial areas, 52% in sewered residential areas, and <10% for other land uses. Median nitrate concentrations were greatest under irrigated agriculture (15,350 microg/L) and nonsewered residential areas (6080 microg/L). Herbicides and degradates of acetanilide and triazine herbicides were detected in 86% of samples from irrigated agricultural areas, 68% of samples from nonirrigated areas, and <10% of samples from other land uses. Degradates accounted for 96% of the reported herbicide mass. We did not observe seasonal differences in water chemistry, but observed trends in water chemistry when land use changes occurred. Our results show land use is the dominant factor affecting shallow ground water quality. Trend monitoring programs should focus on areas where land use is changing, while resource managers and planners must consider potential impacts of land use changes on ground water quality.

Probability of nitrate contamination of recently recharged groundwaters in the conterminous United States.

A new logistic regression (LR) model was used to predict the probability of nitrate contamination exceeding 4 mg/L in predominantly shallow, recently recharged groundwaters of the United States. The new model contains variables representing (1) N fertilizer loading (p < 0.001), (2) percent cropland-pasture (p < 0.001), (3) natural log of human population density (p < 0.001), (4) percent well-drained soils (p < 0.001), (5) depth to the seasonally high water table (p < 0.001), and (6) presence or absence of unconsolidated sand and gravel aquifers (p = 0.002). Observed and average predicted probabilities associated with deciles of risk are well correlated (r2 = 0.875), indicating that the LR model fits the data well. The likelihood of nitrate contamination is greater in areas with high N loading and well-drained surficial soils over unconsolidated sand and gravels. The LR model correctly predicted the status of nitrate contamination in 75% of wells in a validation data set. Considering all wells used in both calibration and validation, observed median nitrate concentration increased from 0.24 to 8.30 mg/L as the mapped probability of nitrate exceeding 4 mg/L increased from < or =0.17 to >0.83.

Acute toxicity of nitrate on Penaeus monodon juveniles at different salinity levels

Penaeus monodon juveniles (total length 28.4±6.2 mm) were exposed to different concentrations of nitrate–N (nitrate as nitrogen) using the static renewal method at different salinity levels. The 48-, 72-, 96-h LC50s (median lethal concentrations) of nitrate–N for P. monodon juveniles were 2876, 1723, 1449 mg/l in seawater of 15‰ (g/kg); 3894, 2506, 1575 mg/l in 25‰; 4970, 3525, 2316 mg/l in 35‰, respectively. The “safe level” for rearing juvenile P. monodon was estimated to be 145, 158 and 232 mg/l nitrate–N in 15‰, 25‰ and 35‰, respectively. The relationship between LC50 of nitrate–N (mg/l), salinity ( S), exposure time ( t) and the interaction between salinity and exposure time (St) is as follows: LC50=3917.467+102.456S−42.634t−0.342 St (R 2=0.775)