Concentrations In Shallow Groundwater Research Articles

Regulatory groundwater monitoring: Realistic residues of pinoxaden and metabolites at vulnerable locations

A bespoke groundwater monitoring programme was designed to generate a database of pinoxaden and metabolite concentrations in shallow groundwater at agricultural locations across Europe. The data generated from this programme represent a higher tier refinement of modelled exposure estimates and provide realistic information on groundwater quality at vulnerable locations which will aid plant protection product (PPP) assessment in Europe in relation to Regulation (EC) No. 1107/2009.The Regulatory GeoPEARL_3.3.3 model developed by RIVM was used to estimate the vulnerability of cereal growing regions to leaching of two pinoxaden metabolites across the entire EU at a 1 km2 level using 20 years of daily weather data (MARS, EU JRC). Seventy sites located within the upper 50th percentile of leaching vulnerability from this modelling exercise, crop density and shallow groundwater were selected for monitoring groundwater.Retrospective and prospective pinoxaden product applications at candidate sites were recorded and these data used to place sites in the distribution for Europe. The 70 sites all fulfil the site assessment criteria and have no confining layers which may prevent or delay leaching. All sites equipped with groundwater wells had a minimum of two pinoxaden applications in the preceding four years to cereal crops.A total of 1326 samples were analysed from up to 90 down hydraulic gradient wells at 70 locations between June 2015 and July 2018. Results indicate that pinoxaden and pinoxaden metabolites are very unlikely to reach shallow groundwater at concentrations greater than 0.1 μg/L for relevant metabolites, or 10 μg/L for non-relevant metabolites, respectively (Sanco/221/2000-rev.10). Over 38 months of groundwater monitoring the annual average and 90th percentile for pinoxaden or its metabolites never exceeded 0.1 μg/L and it is proposed that these data infer that exposure to these metabolites is minimal.

Formation and In Situ Treatment of High Fluoride Concentrations in Shallow Groundwater of a Semi-Arid Region: Jiaolai Basin, China.

Fluorine is an essential nutrient, and excessive or deficient fluoride contents in water can be harmful to human health. The shallow groundwater of the Jiaolai Basin, China has a high fluoride content. This study aimed to (1) investigate the processes responsible for the formation of shallow high-fluoride groundwater (SHFGW); (2) identify appropriate methods for in situ treatment of SHFGW. A field investigation into the formation of SHFGW was conducted, and the results of experiments using soils from high-fluoride areas were examined to investigate the leaching and migration of fluoride. The results showed that the formation of SHFGW in the Jiaolai Basin is due to long-term geological and evaporation processes in the region. Stratums around and inside the basin act as the source of fluoride whereas the terrain promotes groundwater convergence. The hydrodynamic and hydrochemical conditions resulting from slow groundwater flow along with high evaporation and low rainfall all contribute to the enrichment of fluoride in groundwater. In situ treatment of SHFGW may be an effective approach to manage high SHFGW in the Jiaolai Basin. Since soil fluoride in high-fluoride areas can leach into groundwater and migrate with runoff, the construction of ditches can shorten the runoff of shallow groundwater and accelerate groundwater loss, resulting in the loss of SHFGW from high-fluoride areas through river outflow. The groundwater level will be reduced, thereby lowering the influence of evaporation on fluoride enrichment in shallow groundwater. The results of this study can act a reference for further research on in situ treatment for high-fluoride groundwater.

Environmental Isotope of Radon-222 for Ciliwung River and Shallow Deep Water Interaction Study

Aquifer in river bank area is mostly susceptive toward pollution occurring in river. One of parameters to determine the interaction process between groundwater and river is a natural isotope of 222Rn. The significant difference of radon concentration in groundwater and river water can be utilized as a scientific basis for investigating groundwater infiltration in river bank. Those studied parameters are residence time and infiltration rate. The research using 222Rn had been conducted in shallow groundwater of Ciliwung river bank - South Jakarta during rainy and dry season. The range of 222Rn concentration in shallow groundwater monitored in dry season was between 666 - 2590 Bq/m3 which was higher than that of rainy season ranging at 440 to 1546 Bq/m3. Otherwise, concentration of 222Rn in river water could not be detected (its 222Rn concentration = 0 Bq/m3) due to its much lower concentration either rainy or dry season. During dry season monitoring, equilibration between groundwater and river water was reached at the distance approximately 98 - 140 m away from river side. Estimating residence time based on 222Rn concentration at nearest site from the river and at equlibration area was 4.2 days such that the infiltration rate from river water into aquifer might be 7.8 m/day.Keywords: 222Rn, groundwater, residence time, infiltration rate.

Dissolved phosphate concentrations in Iowa shallow groundwater.

Regional groundwater phosphorus (P) concentrations are rarely reported, and it is important to develop a better understanding of background concentrations in shallow groundwater to help develop strategies to mitigate environmental risks. In this study, results collected from 17 different Iowa-based studies conducted from 2006 to 2019 and a total of 210 discrete locations of water table dissolved phosphate (DPO4 3- ) measurements are summarized (a) to assess the occurrence, range, and statistical distribution of groundwater DPO4 3- concentrations in Iowa and (b) to evaluate statewide patterns of DPO4 3- concentrations related to land use or land cover and landscape position. The DPO4 3- concentrations ranged from 0.02 to 1.56mg L-1 and averaged 0.15 ± 0.19mg L-1 with a median value of 0.10mg L-1 (95% confidence interval of 0.08-0.11mg L-1 ). Although minor variations were observed among land cover class and landscape position, concentrations exhibited uniformity across the state, likely attesting to the legacy of P from historical agricultural management. Median concentrations are higher than typical water quality criteria used to assess risk to surface water systems, implying that simply discharging groundwater DPO4 3- to streams, rivers, and lakes would be sufficient to cause environmental degradation.

Controls on species distribution and biogeochemical cycling in nitrate-contaminated groundwater and surface water, southeastern Australia

A detailed study of groundwater and surface water nitrate over four seasons across an area of varied landuse provided insights into the mechanisms that underlie accumulation and transport of nitrate. High nitrate concentrations found in a significant percentage of surface water and shallow groundwater samples are due to anthropogenic contamination. Statistics (PCA, ANOVA, parsimonious model and general linear regression) were used to explore the relationship between NO3− and land use, and confirmed that areas of high NO3− concentration are associated with dairy pasture and horticulture. Seasonally, NO3− levels are greater during winter, the wettest part of the year. Values of δ15N showed that most nitrate is sourced from livestock waste, with a smaller contribution from synthetic fertilizer. Direct wash-off of animal waste from dairy farms results in higher NO3− concentrations in surface water than in groundwater. Denitrification is an important NO3− attenuation mechanism which reduces NO3− to NH4, as demonstrated by the PCA analysis, which showed positive correlation of NO3− concentrations with dissolved oxygen and negative correlations with NH4+, Fe2+and Mn2+; the latter two species may act as the electron donors necessary for reduction of NO3−. The often high NO3− concentrations in shallow groundwater are decreased by denitrification, which can occur at relatively shallow depths (<3 m). The relatively small NO3− concentrations in deeper groundwater are due partly to denitrification, but more to originally lower NO3− concentrations, as the age of deeper groundwater shows that it was recharged before agriculture was established in the study area. Overall, the study demonstrates the usefulness of hydrogeochemical characterisation and multivariate statistics in the evaluation of impacts of agricultural land-use on regional N cycling. In particular, the results show that efforts to mitigate NO3− pollution from farms should concentrate more on wash-off of animal waste than the contribution of nitrogenous synthetic fertilizer.

Anthropogenic influences on dissolved organic matter transport in high arsenic groundwater: Insights from stable carbon isotope analysis and electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry

In East and Southeast Asia, the health of over 100 million people is threatened by the consumption of groundwater containing high concentrations of arsenic (>10 μg L−1), which is released from sediments through reductive dissolution of arsenic-bearing iron/manganese oxides. Dissolved organic matter (DOM) is known to play a crucial role in the process of arsenic mobilization in shallow aquifers, and its availability and reactivity are key factors controlling the variation of arsenic concentrations in groundwater. However, it is unclear how human activities influence the transport of DOM and how the transportation affects the DOM molecular properties in high arsenic groundwater. This study provides insights on the sources and molecular compositions of DOM in groundwater from the Jianghan Plain, central China, a newly discovered area with seasonal fluctuations in arsenic concentrations in shallow groundwater. Monitoring of water levels and stable carbon isotope compositions in groundwater from different depths and canal water over a year indicated that terrestrial DOM was the dominant source, accounting for 54.2%–85.5% of groundwater DOM. Electrospray ionization combined with ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry revealed that canal water infiltration transferred aliphatic, tannin-like and leached aromatic DOM from sediments into groundwater. Therefore, groundwater recharge through irrigation using canal water not only inputs terrestrial DOM, but also accelerates the release of sedimentary DOM. Furthermore, carboxylic-rich alicyclic molecule (CRAM)-like DOM that is derived from biomolecules has the highest proportion (60.1%–65.5%) among the identified DOM structures. And, it might be reused in biochemical processes during arsenic mobilization, suggesting a third source of groundwater DOM in addition to canal water and sediments. The findings in this study advance the understanding on transport processes and molecular properties of DOM in high arsenic groundwater under extensive anthropogenic influences.

Elevated manganese concentrations in shallow groundwater of various aquifers in a rapidly urbanized delta, south China

High concentration of manganese (Mn) in groundwater is a major concern because of its harmful to human health, and the origin of which in urbanized areas is often complicated. The present study aims to delineate spatial distributions of groundwater Mn in various aquifers and in areas with different urbanization levels in the Pearl River Delta (PRD), and to identify the origins of groundwater Mn in this region. Nearly 400 groundwater samples collected, and 14 chemicals were analyzed. The results show that approximately 20% groundwater in granular aquifers showed elevated-Mn (>0.4 mg/L), and was more than two times of that in fissured aquifers, while that in karst aquifers was absent. The proportions of elevated-Mn groundwater in urbanized areas and peri-urban areas were higher than that in non-urbanized areas. The decomposition of organic matter and reduction of Fe (hydr)oxides in sediments with reducing condition was likely to be the main factor controlling elevated-Mn groundwater in granular aquifers at a regional scale. By contrast, elevated-Mn groundwater in fissured aquifers was likely mainly affected by the urbanization accompanied with the leakage of low-oxygen domestic sewage and the industrialization accompanied by the leakage of industrial wastewater. In addition, Mn-rich surface water was also probably an important source for groundwater Mn in river network areas. Therefore, it is necessary to make a long-term monitoring for groundwater Mn in granular aquifers, especially in urbanized areas and river network areas, because of the high proportion of elevated-Mn.

Monitoring of nitrate contamination in groundwater: case study of the campus of UNESP, Rio Claro/SP

This study presents the results of the monitoring of nitrate concentrations in shallow groundwater at the UNESP Campus of Rio Claro/SP assumed to be sourced by septic tank leakage, which were discontinued in October 2014. The distribution of nitrate concentrations provides support to a conceptual model of contamination by multiple sources, since the concentration gradients are not observed along the flowpaths. The results of the monitoring indicate that in some monitored wells, the nitrate concentrations remain stable, while in other wells minor to strong fall trends were observed. These results provide support to the presence of other active sources, such as sewage leakage in the external and internal area of the campus. This scenario perfect fit with the maintenance of recorded high nitrate concentrations over the time. Despite the nitrate concentrations are below potability limit, additional investigations will be conducted to identify sources of contamination to ensure water quality in the future.

Endobenthic Organisms Exposed to Chronically High Chloride from Groundwater Discharging along Freshwater Urban Streams and Lakeshores.

Chloride, especially from road salt, is one of the most ubiquitous contaminants of urban groundwater in temperate climates. The discharge of chloride-laden groundwater to freshwater ecosystems may pose a heightened risk to endobenthic organisms (buried in sediments), which may experience high concentrations prior to dilution from the overlying water. However, available exposure data is limited. Presented here are 22 chloride data sets from 15 urban sites across Canada, encompassing >1300 samples of shallow discharging groundwater collected principally during summer through autumn. Over half of the sites had an average chloride concentration above the long-term aquatic life guideline (120 mg/L), while 14% of each site's samples, on average, surpassed the short-term guideline (640 mg/L). Chloride concentrations frequently varied substantially (even >1000 mg/L) between adjacent locations (mostly <20 m separation), indicating patchy exposure. Chloride/bromide ratios, artificial sweeteners, and other tracers suggest a predominant contribution from road salt, with wastewater and landfill leachate important at some sites. Overall, these concentrations exceed those typically reported for urban wells and streams (even during the snowmelt period) in similar climates. These findings suggest that high chloride concentrations in shallow groundwater, largely from road salt, present a long-term threat to endobenthic organisms of urban surface waters in cold-region countries.

Distribution of Nitrate Household Waste and Groundwater Flow Direction around Code River, Yogyakarta, Indonesia

The nitrate concentrations in shallow groundwater of the Yogyakarta City and its surroundings has increased to about twenty times in the period 1985-2018. The objective of this study was to analyse the distribution of nitrate concentrations in water well around the Code River, Yogyakarta. Flownets mapping was performed to find out the distribution and direction of potential groundwater pollution. Nitrate concentration was analysed by taking 18 groundwater samples scattered in the upstream, midstream and downstream areas of the Code River. The results of this study indicate that nitrate concentrations in the water well of upstream and downstream areas could still be used as a source of drinking water and recreation-irrigation-livestock. Meanwhile, the nitrate concentration in water well of the midstream area of the Code River, Yogyakarta City, mostly (80% of the sample) did not meet all classes of water quality standards. Human activities in the densely populated settlements were the main factors that influence nitrate pollution. Furthermore, groundwater flow in the study area leads from north to south and towards the Code River. This condition indicates that the nitrate concentrations in the groundwater can be a source of a pollutant for the Code River.

Hydrogeochemical characteristics and water quality assessment of shallow groundwater: a case study from Linhuan coal-mining district in northern Anhui Province, China

Abstract Major ion and trace element concentrations in shallow groundwater of Linhuan coal-mining district, Northern Anhui Province, China, were analyzed to determine its hydrogeochemical characteristics and to assess drinking and irrigation water quality. The relative abundance of cations and anions was Na+ &amp;gt; Mg2+ &amp;gt; Ca2+ &amp;gt; K+, and HCO3− &amp;gt; SO42− &amp;gt; Cl− &amp;gt; NO3−, respectively. The concentrations of Na+, HCO3−, NO3−, and total dissolved solids (TDS), and the electric conductivity (EC) values in some samples were higher than the permissible limits of the Water Health Organization (WHO). Gibbs diagrams showed that rock weathering mainly controlled the major ion chemistry of the groundwater, and the first aquifer of this study area had a weak hydraulic connection with atmospheric precipitation. The calculated sodium percentage (%Na) and sodium adsorption ratio (SAR) revealed that the slight sodium and high salinity hazards needed to be controlled before irrigation. According to the fuzzy comprehensive assessment, the groundwater samples were classified into four categories. The results showed that 92.86% of the groundwater samples were suitable for drinking use. For human health, the NO3− and Mn levels in the groundwater should be reduced before drinking, and treatment of the high salinity hazards is required before irrigation.

Importance of the vegetation-groundwater-stream continuum to understand transformation of biogenic carbon in aquatic systems – A case study based on a pine-maize comparison in a lowland sandy watershed (Landes de Gascogne, SW France)

During land-aquatic transfer, carbon (C) and inorganic nutrients (IN) are transformed in soils, groundwater, and at the groundwater-surface water interface as well as in stream channels and stream sediments. However, processes and factors controlling these transfers and transformations are not well constrained, particularly with respect to land use effect. We compared C and IN concentrations in shallow groundwater and first-order streams of a sandy lowland catchment dominated by two types of land use: pine forest and maize cropland. Contrary to forest groundwater, crop groundwater exhibited oxic conditions all-year round as a result of higher evapotranspiration and better lateral drainage that decreased the water table below the organic-rich soil horizon, prevented the leaching of soil-generated dissolved organic carbon (DOC) in groundwater, and thus limited consumption of dissolved oxygen (O2). In crop groundwater, oxic conditions inhibited denitrification and methanogenesis resulting in high nitrate (NO3−; on average 1140 ± 485 μmol L−1) and low methane (CH4; 40 ± 25 nmol L−1) concentrations. Conversely, anoxic conditions in forest groundwater led to lower NO3− (25 ± 40 μmol L−1) and higher CH4 (1770 ± 1830 nmol L−1) concentrations. The partial pressure of carbon dioxide (pCO2; 30,650 ± 11,590 ppmv) in crop groundwater was significantly lower than in forest groundwater (50,630 ± 26,070 ppmv), and was apparently caused by the deeper water table delaying downward diffusion of soil CO2 to the water table. In contrast, pCO2 was not significantly different in crop (4480 ± 2680 ppmv) and forest (4900 ± 4500 ppmv) streams, suggesting faster degassing in forest streams resulting from greater water turbulence. Although NO3−concentrations indicated that denitrification occurred in riparian-forest groundwater, crop streams nevertheless exhibited important signs of spring and summer eutrophication such as the development of macrophytes. Stream eutrophication favored development of anaerobic conditions in crop stream sediments, as evidenced by increased ammonia (NH4+) and CH4 in stream waters and concomitant decreased in NO3− concentrations as a result of sediment denitrification. In crop streams, dredging and erosion of streambed sediments during winter sustained high concentration of particulate organic C, NH4+ and CH4. In forest streams, dissolved iron (Fe2+), NH4+ and CH4 were negatively correlated with O2 reflecting the gradual oxygenation of stream water and associated oxidations of Fe2+, NH4+ and CH4. The results overall showed that forest groundwater behaved as source of CO2 and CH4 to streams, the intensity depending on the hydrological connectivity among soils, groundwater, and streams. CH4 production was prevented in cropland in soils and groundwater, however crop groundwater acted as a source of CO2 to streams (but less so than forest groundwater). Conversely, in streams, pCO2 was not significantly affected by land use while CH4 production was enhanced by cropland. At the catchment scale, this study found substantial biogeochemical heterogeneity in C and IN concentrations between forest and crop waters, demonstrating the importance of including the full vegetation-groundwater-stream continuum when estimating land-water fluxes of C (and nitrogen) and attempting to understand their spatial and temporal dynamics.

Integration of HYDRUS-1D and MODFLOW for evaluating the dynamics of salts and nitrogen in groundwater under long-term reclaimed water irrigation

Reclaimed water has been extensively used as an alternative resource for irrigation, but can affect groundwater quality due to salt and nitrogen leaching. We conducted field investigations of a shallow groundwater monitoring well at the Research Center for Eco-Environmental Sciences, China, and irrigated sample sites of turf grass with reclaimed water for 8 years. The HYDRUS-1D and MODFLOW models were integrated to study the transport and distribution of electrical conductivity (ECgw) and nitrate–N (N–NO3−) in the shallow groundwater under long-term reclaimed water irrigation. Model calibration and validation showed that the integrated model could simulate the fates of ECgw and N–NO3− in the shallow groundwater. Field experiments and the model simulation showed that reclaimed water irrigation can increase salinity and N–NO3− concentration in shallow groundwater and predicted, assuming the continuation of current irrigation practices, that the annual average ECgw and N–NO3− would reach a steady level of 0.72 dS m−1 and 2.18 mg L−1, respectively. Because ECgw increased with increasing irrigation water salinity and amount, there is a risk of increased salinity in the shallow groundwater under long-term reclaimed water irrigation. Under all simulation scenarios, annual average N–NO3− concentrations in the shallow groundwater at an equilibrium state did not exceed the class II groundwater quality standard (2–5 mg L−1). After proper calibration and validation, the integration of HYDRUS-1D and MODFLOW models offers an effective tool for analyzing irrigation management of low-quality water in water-scarce regions.

Risk assessment of arsenic from contaminated soils to shallow groundwater in Ong Phra Sub-District, Suphan Buri Province, Thailand

Study regionThe Arsenic (As) concentrations found in Ong Phra sub-district, Dan Chang district, and Suphan Buri province are approx. 20 times higher than the average concentration of As in agricultural soils of Thailand. Study focusThe objectives of this current study were to investigate the As contamination in soil and groundwater and further develop the risk assessment map of As from contaminated soils releasing to shallow groundwater by combining DRASTIC method with contamination factor (CF) and mobility factor (MF). New hydrological insights for the regionAccording to the sequential extraction result, the concentrations of total As in 39 soil samples ranges from 4.8 to 1070.4 mg/kg. The highest As concentration in shallow groundwater was found at well number 3, located within <0.5 km. downstream from the old tin mine. The Index of DRASTIC groundwater vulnerability, is in the range of 59–147, which is categorized into three vulnerability zones as follows: low (<100), moderate (100–130) and high (>130). The high vulnerability area, found along the waterway, is approximately 35 km2 (18.65% of the study area). Interestingly, the DRASTIC index of the old tin mine is indicated as the zone of moderate and high vulnerability. The findings in this study reveal that the integration of the DRASTIC risk map with CF can well elucidate As contaminated in shallow groundwater.

Characteristics and compound-specific carbon isotope compositions of sedimentary lipids in high arsenic aquifers in the Hetao basin, Inner Mongolia

Organic matter, as an electron donor, plays a vital role in As mobilization mediated by microorganisms during reductive dissolution of Fe/Mn oxides in shallow aquifers. However, the specific types and sources of organic matter involved in biogeochemical processes accelerating As mobilization are still controversial. Both sediment and groundwater samples were collected at different depths from aquifers of the Hetao Basin, a typical inland basin hosting high As groundwater. Sedimentary lipids and their compound-specific carbon isotope ratios were analyzed to evaluate characteristics and sources of organic matter. Results show that sedimentary As were well correlated with Fe and Mn oxides, suggesting that As exist as Fe/Mn oxide bound forms. Groundwater As far exceeded the drinking water guide value of 10 μg/L. Moreover, As concentrations in shallow groundwater were relatively higher. Lipids in clay were mainly originated from terrestrial higher plants, while that in fine sand samples were derived from terrestrial higher plants, microorganism and petroleum. Shallow fine sand samples were also characterized by evident in-situ biodegradation. Compound-specific carbon isotope compositions of sedimentary lipids showed that short-chain n-alkanes and n-alkanoic acids had more positive δ13C values compared to long-chain compounds, especially in shallow fine sand samples. δ13CTOC were also low in shallow fine sand samples. These results jointly indicate that these lipids in shallow fine sand samples acted as carbon source for indigenous microorganism and the short-chain components were particularly more vulnerable to biodegradation, which may contribute to high As concentrations in shallow groundwater. The new findings provide the first evidence that short chain length n-alkyl compounds afforded a source of potential electron donors for microbially mediated As mobilization process in the shallow aquifers.

Methane in groundwater from a leaking gas well, Piceance Basin, Colorado, USA

Site-specific and regional analysis of time-series hydrologic and geochemical data collected from 15 monitoring wells in the Piceance Basin indicated that a leaking gas well contaminated shallow groundwater with thermogenic methane. The gas well was drilled in 1956 and plugged and abandoned in 1990. Chemical and isotopic data showed the thermogenic methane was not from mixing of gas-rich formation water with shallow groundwater or natural migration of a free-gas phase. Water-level and methane-isotopic data, and video logs from a deep monitoring well, indicated that a shale confining layer ~125m below the zone of contamination was an effective barrier to upward migration of water and gas. The gas well, located 27m from the contaminated monitoring well, had ~1000m of uncemented annular space behind production casing that was the likely pathway through which deep gas migrated into the shallow aquifer. Measurements of soil gas near the gas well showed no evidence of methane emissions from the soil to the atmosphere even though methane concentrations in shallow groundwater (16 to 20mg/L) were above air-saturation levels. Methane degassing from the water table was likely oxidized in the relatively thick unsaturated zone (~18m), thus rendering the leak undetectable at land surface. Drilling and plugging records for oil and gas wells in Colorado and proxies for depth to groundwater indicated thousands of oil and gas wells were drilled and plugged in the same timeframe as the implicated gas well, and the majority of those wells were in areas with relatively large depths to groundwater. This study represents one of the few detailed subsurface investigations of methane leakage from a plugged and abandoned gas well. As such, it could provide a useful template for prioritizing and assessing potentially leaking wells, particularly in cases where the leakage does not manifest itself at land surface.

Temporal-spatial variations and influencing factors of nitrogen in the shallow groundwater of the nearshore vegetable field of Erhai Lake, China.

Nitrogen export from the nearshore vegetable field of Erhai Lake seriously threatens the water quality of Erhai Lake, which is the second largest highland freshwater lake in Yunnan Province, China. Among the nitrogen flows into Erhai Lake, shallow groundwater migration is a major pathway. The nitrogen variation and influencing factors in the shallow groundwater of the nearshore vegetable field of Erhai Lake are not well documented. A 2-year field experiment was conducted to determine the concentrations of nitrogen species in the shallow groundwater and their influencing factors in the nearshore vegetable field of Erhai Lake. The results showed that concentrations of TN, NO3--N, and NO2--N gradually increased with increasing elevation and distance from Erhai Lake, but the opposite was observed for NH4+-N in the shallow groundwater. The concentrations of nitrogen species in the rainy season were greater than those in the dry season. NO3--N accounted for more than 79% of total nitrogen in shallow groundwater. Redundancy analysis showed that more than 70% of the temporal and spatial variations of nitrogen concentrations in the shallow groundwater were explained by shallow groundwater depth, and only approximately 10% of variation was explained by the factors of soil porosity, silt clay content of soil, and NH4+-N and NO3--N concentrations of soil (p < 0.05). The shallow groundwater depth had more notable effects on nitrogen concentrations in the shallow groundwater than other factors. This result will strongly support the need for further research regarding the management practices for reducing nitrogen concentrations in shallow groundwater.

Temporal variations of methane concentration and isotopic composition in groundwater of the St. Lawrence Lowlands, eastern Canada

Dissolved methane concentrations in shallow groundwater are known to vary both spatially and temporally. The extent of these variations is poorly documented although this knowledge is critical for distinguishing natural fluctuations from anthropogenic impacts stemming from oil and gas activities. This issue was addressed as part of a groundwater research project aiming to assess the risk of shale gas development for groundwater quality over a 500-km2 area in the St. Lawrence Lowlands (Quebec, Canada). A specific study was carried out to define the natural variability of methane concentrations and carbon and hydrogen isotope ratios in groundwater, as dissolved methane is naturally ubiquitous in aquifers of this area. Monitoring was carried out over a period of up to 2.5 years in seven monitoring wells. Results showed that for a given well, using the same sampling depth and technique, methane concentrations can vary over time from 2.5 to 6 times relative to the lowest recorded value. Methane isotopic composition, which is a useful tool to distinguish gas origin, was found to be stable for most wells, but varied significantly over time in the two wells where methane concentrations are the lowest. The use of concentration ratios, as well as isotopic composition of methane and dissolved inorganic carbon (DIC), helped unravel the processes responsible for these variations. This study indicates that both methane concentrations and isotopic composition, as well as DIC isotopes, should be regularly monitored over at least 1 year to establish their potential natural variations prior to hydrocarbon development.

Water quality changes in a short-rotation woody crop riparian buffer

Converting riparian buffers in agricultural areas from annual row crops to short rotation woody crops (SRWCs) grown for biofuel can provide both water quality benefits and a financial incentive for buffer adoption among agricultural producers. A randomized complete block design was used to determine water quality changes resulting from converting plots previously cultivated in corn to SRWC willow (Salix. spp) adjacent to a stream in Storrs, CT. Both overland flow and ground water samples were analyzed for total nitrogen (TN) and total phosphorus (TP). Additionally, overland flow was analyzed for suspended solids concentration (SSC) and ground water samples were analyzed for nitrate + nitrite (NO2+NO3-N). Lower (p = 0.05) concentrations of TN (41%) and TP (53%) were observed in overland flow from willow plots than from corn plots. Shallow ground water concentrations at the edge of willow plots were lower in TN (56%) and NO3+NO2-N (64%), but 35% higher in TP, than at the edge of corn plots. SSC was also lower (71%) in overland flow associated with willow compared to corn. The treatment had no effect on the depth or volume of overland flow or mass export. The changes in concentrations resulting from conversion of corn to SRWC in a riparian buffer are similar to those observed in restored and established buffers.

Dynamics of nitrate and methane in shallow groundwater following land use conversion from agricultural grain production to conservation easement

Abstract Fertilizer applications on agricultural fields lead to elevated nitrate concentrations in groundwater. This increases nitrate concentrations in the baseflow of streams, enhancing downstream eutrophication. Conservation practices reduce the impacts from agriculture, but little is documented on the recovery time of shallow groundwater after agriculture ceases and conservation practices are applied. Although conservation practices may reduce groundwater nitrate, they may also lead to the production of the greenhouse gas methane. This study investigated the temporal sequence of applying post-agricultural conservation practices and the effects on nitrate and methane concentrations in shallow groundwater. Harleigh Farms is a complex of fields near Oxford, MD (USA) that have been taken out of crop production and placed in conservation programs at various times after 1997. Groundwater nitrate and dissolved methane were sampled monthly from Nov 2012-Nov 2013 using age of the conservation practice as a proxy for time since fertilization. In this chronosequence study, an exponential decline in groundwater nitrate levels was found over the 16 year time period since last fertilization. Within 3–5 years after the cessation of intensive grain production, groundwater nitrate concentrations in the top of the surface unconfined aquifer dropped from 11 mg NO3−-N L−1 to 0.5 mg NO3−-N L−1. Methane only accumulated to high concentrations (2–60 μM CH4) in hydric soils with low nitrate concentrations (≪ 0.1 mg NO3−-N L−1). Our results indicate rapid loss of nitrate in the top of the surficial aquifer after the cessation of intensive agriculture and seasonal accumulations of methane in wetland-based conservation practices. These data indicate that time series of groundwater nitrate concentrations at the top of the unconfined aquifer can be used to evaluate the effectiveness of agricultural conservation practices.