Concentrations In Shallow Groundwater Research Articles

Field investigation of the transport and attenuation of fugitive methane in shallow groundwater around an oil and gas well with gas migration

‘Fugitive’ or ‘stray’ gas migration from deeper formations due to well bore integrity failure has prompted concern regarding environmental impacts. Unintended methane (CH4) migration can increase greenhouse gas emissions and affect groundwater quality in the critical zone. Although the CH4 transport in shallow aquifers has been investigated at experimental injection sites, no intensive groundwater studies have been published around an oil and gas well that has been leaking for a significant period of time. In this field study, groundwater samples were collected from sixteen groundwater monitoring wells (1.25 m below ground surface) installed around a suspended oil and gas well with decadal scale gas migration (estimated ~0.2 m3/day). Stray CH4 distribution and preferential pathways in the shallow groundwater zone were evaluated though high-resolution profiling of equivalent concentrations of hydrocarbon gases (C1-C6; >85 % CH4 at the study site) and bulk formation electrical conductivity to 6.0 m below ground surface. The highest dissolved CH4 concentration (0.074 mmol/L or 1.18 mg/L) in groundwater (1.25 m bgs) was observed immediately downgradient (1.25 m) of the oil and gas well head. Similarly, high-resolution profiling data also revealed the occurrence of relatively high CH4 concentrations in shallow groundwater along the groundwater flow direction and below fine-grained layers up to 10 m distance from the well head. Microbial DNA analysis from groundwater showed significant community shifts, with the highest relative abundance and diversity of methanotrophs observed in the vicinity of the oil and gas well. This study supports findings from experimental injection and laboratory studies, which also found that significant CH4 transport i) dominantly occurs in the groundwater flow direction, and ii) laterally as free phase below fine-grained layers. The occurrence of CH4 concentrations below saturation after more than two decades of gas migration suggests limited impacts have occurred in the shallow subsurface investigated.

Phosphorus in shallow and deep groundwater: Importance of P/Fe ratio in Fe(III) oxides in aquifer sediments

The causes of the differential enrichment of dissolved phosphorus (P) in the shallow and deep aquifers remain elusive. In this study, groundwater and aquifer sediments were sampled from the Songnen Basin to explore the behaviour and mobilization mechanism of P in a shallow sandy aquifer and a deep sand-gravel aquifer by means of hydrogeochemical analyses and characterizations of dissolved organic matter (DOM) and sedimentary Fe minerals. The results showed that both aquifers were enriched in dissolved P, being mainly related to the organic matter degradation and Fe(III) oxides reduction. Comparatively, total dissolved phosphorus concentrations in shallow groundwater were significantly higher than those in the deep one (p < 0.01), owing to the stronger extent of reductive dissolution of Fe(III) oxides with higher P/Fe molar ratio in the shallow aquifer. Moreover, groundwater DOM with more humic substances in the shallow aquifer promoted the P mobilization via electron shuttling and/or competitive adsorption. In the deep aquifer, reductive dissolution of Fe(III) oxides with high P content resulted in the enrichment of groundwater P. In the two aquifers, immobilization of P via Fe(II) mineral precipitation played a minor role in the fate of groundwater P. This study underlines that geogenic enrichment of P in groundwater can occur in the sand-gravel aquifers with high contents of P and Fe(III) oxides under reducing conditions, and highlights that P/Fe molar ratio in Fe(III) oxides is the most important factor controlling dissolved P in groundwater systems.

Anthropogenic processes drive heterogeneous distributions of toxic elements in shallow groundwater around a smelting site

Smelting activities have a far-reaching influence on the quality of soil and groundwater, while most studies have neglected the information on the pollution characteristics of groundwater. The hydrochemical parameters of shallow groundwater and the spatial distributions of toxic elements were investigated in this study. Correlations analysis and groundwater evolution revealed that the major ions were primarily determined by silicate weathering and calcite dissolution process, and anthropogenic processes had a significant effect on groundwater hydrochemistry. Almost 79%, 71%, 57%, 89%, 100%, and 78.6% of samples exceeded the standards of Cd, Zn, Pb, As, SO42-, and NO3-, and their distribution is closely related to the production process. Analysis of soil geochemistry indicated that the relatively mobile forms of toxic elements strongly influence the origin and concentration in shallow groundwater. Besides, rainfall with high magnitude would lead to a decrease of toxic elements in shallow groundwater, whereas the area once stacked waste residue was the opposite. It is recommended to strengthen risk management of the limited mobility fraction while devising a plan for waste residue treatment in accordance with the local pollution conditions. The research on controlling the mechanism of toxic elements in shallow groundwater, along with sustainable development in the study area and other smelting zones may benefit from this study.

Evaluation of Trends for Iron and Manganese Concentrations in Shallow Groundwater, Jakarta City

Groundwater is one of the sources of clean water in DKI Jakarta. However, it was found that there is a decrease in groundwater quality. The objective of this study is to evaluate trends for iron and manganese concentration in shallow groundwater in Jakarta City from 2017-2020. Based on groundwater quality testing data, analysis and evaluation of groundwater causation were carried out by a spatial map of the distribution of DKI Jakarta groundwater quality. The results of the analysis show that some groundwater quality parameters experience fluctuations in the concentration of contaminants. Spatial maps produced through geographic information systems represent the distribution of patterns of groundwater quality distribution that occurred in the period 2017-2020 and 2022 using interpolation techniques. The amounts of manganese and iron in the groundwater of Jakarta are slightly higher than the permissible limits of national standards guidelines.

A parsimonious model for predicting the NO3−-N concentration in shallow groundwater in intensive agricultural areas using few easily accessible indicators and small datasets based on machine learning

Groundwater nitrate poses a health risk to humans, and monitoring groundwater quality is time-consuming and expensive. To improve the efficiency and reduce the costs of monitoring groundwater nitrate, a parsimonious model was developed using easily accessible predictors and small datasets based on random forest (RF), neural network (NN), and support vector machine (SVM) algorithms. Groundwater NO3−-N concentrations of 1196 groundwater samples from intensive agricultural areas along with data on 13 predictors were obtained for the construction of prediction models. R2 and RMSE metrics were used to evaluate the models, with an emphasis on a minimal number of predictors and a low data size. The results showed that even with few readily available predictors (conductivity, EC; chemical nitrogen fertilization rate, NF; and precipitation, PPT) and a small dataset (260–700 observations), the three models achieved good precision, with R2 and RMSE values of 0.70–0.80 and 6.87–8.13, respectively. In these prediction models, EC was the primary predictor acting as an intrinsic factor, NF acted as a loading factor to supplement the blind area of EC for NO3−-N prediction, and PPT was introduced because of its accessibility to improve predictive performance. The final predictors applied in this study link the two currently dominant prediction methods by combining the high predictive accuracy of the intrinsic predictor with the accessibility of the extrinsic predictor, and the results demonstrate the practicability and applicability advantages of this model in groundwater NO3−-N prediction. This model provides technical support for the rapid and low-cost realization of the online monitoring of NO3−-N pollution in shallow groundwater in intensive agricultural areas.

Prediction of phosphorus concentrations in shallow groundwater in intensive agricultural regions based on machine learning

Excessive accumulation of phosphorus in soil profiles has become the main source of phosphorus in groundwater due to the application of phosphorus fertilizers in intensive agricultural regions (IARs). Elevated phosphorus concentrations in groundwater have become a global phenomenon, which places enormous pressure on the safe use of water resources and the safety of the aquatic environment. Currently, the prediction of pollutant concentrations in groundwater mainly focuses on nitrate nitrogen, while research on phosphorus prediction is limited. Taking the IARs approximately 8 plateau lakes in the Yunnan-Guizhou Plateau as an example, 570 shallow groundwater samples and 28 predictor variables were collected and measured, and a machine learning approach was used to predict phosphorus concentrations in groundwater. The performance of three machine learning algorithms and different sets of variables for predicting phosphorus concentrations in shallow groundwater was evaluated. The results showed that after all variables were introduced into the model, the R2, RMSE and MAE of support vector machine (SVM), random forest (RF) and neural network (NN) were 0.52–0.60, 0.101–0.108 and 0.074–0.081, respectively. Among them, the SVM model had the best prediction effect. The clay content and water-soluble phosphorus in soil and soluble organic carbon in groundwater had a high contribution to the prediction accuracy of the model. The prediction accuracy of the model with reduced number of variables showed that when the number of variables was equal to 6, the RF model had R2, RMSE and MAE values of 0.53, 0.108 and 0.074, respectively, and the number of variables increased again; there were small changes in R2, RMSE and MAE. Compared with the SVM and NN models, the RF model can achieve higher accuracy by inputting fewer variables.

Identification of high nitrate concentration in shallow groundwater of an arid region: a case study of South Kuwait's Bay.

Coastal aquifer is a fragile environment due to the interaction of groundwater with seawater, especially in arid environments. Groundwater along Kuwait's Bay is polluted due to discharge of waste from desalination plants, power plants, and other anthropogenic activities. Earlier studies on submarine groundwater discharge in Kuwait's Bay region have reported the transfer of nutrient flux from the groundwater to Kuwait's Bay. The current study focused on nitrate sources and processes governing their distribution in groundwater samples collected from the southern part of Kuwait's Bay. The concentration of nitrate in the samples ranged from 22.7 to 803.9mg/L. Higher values were noted in the samples collected inland and a few samples adjacent to the Bay. Spearman's correlation analysis of the data indicated that NO3- has a strong positive correlation with SO42- and moderate positive correlation with Na + , TDS/EC. The PCA analysis and factor scores revealed the different sources for groundwater nitrate contamination as follows: leakage of sewer lines in the urban region has led to the infiltration of contaminated sewage, high saline environment due to seawater intrusion, chemical weathering, and influence of denitrifying bacteria. The health risk has resulted due to the NO3- concentration being above the standard limit for adults. Furthermore, the nitrate concentration was higher in the region adjoining the landfills. In addition, the discharge of groundwater with higher nitrate to the adjacent open water in the Bay may lead to eutrophication. Hence, proper management strategies are to be adopted to control the nitrate pollution in groundwater.

Groundwater depth alters soil nutrient concentrations in different environments in an arid desert

Soil nutrients are vital for plant growth and survival and present a crucial role in terrestrial function and productivity. However, little is known about the effect mechanism of groundwater table on soil nutrients in an arid desert ecological system. This study investigated the impacts of groundwater depth on the concentrations of soil organic carbon (C), available nitrogen (N), phosphorus (P), and potassium (K) at shallow groundwater depths (0.4, 0.8, 1.2, 1.8, and 2.2 m) and field deep groundwater depths (2.5, 4.5, and 11.0 m) in a desert-oasis ecotone in Central Asia in 2015 and 2016. Soil nitrate-N, inorganic-N, soil available P, and K concentrations were significantly affected by shallow and field deep groundwater. Groundwater depths did not alter soil ammonium-N concentration. Soil organic C concentration was influenced by field deep groundwater depth. Structural equation model showed that groundwater depth directly affected soil nitrate-N and K concentrations and indirectly altered the soil inorganic-N, soil organic C and available P concentrations in shallow groundwater. Moreover, groundwater depth directly influenced soil nitrate-N and soil organic C, available P and K concentrations and indirectly affected soil inorganic-N concentration in deep groundwater. Hence, groundwater depth should be considered one of the most critical environmental factors affecting soil nutrient variation in an arid desert. This study provides new insights into the soil nutrient variation under a declining groundwater depth in a hyper-arid ecosystem.

Shallow Groundwater Around Plateau Lakes: Spatiotemporal Distribution of Phosphorus and Its Driving Factors

The extensive application of phosphorus fertilizers to croplands and the aggregation of towns and villages around plateau lakes has resulted in the continuous accumulation of phosphorus in the soil profile and the discharge of phosphorus pollutants, which causes phosphorus pollution in shallow groundwater around the lakes. The phosphorus entering the lake with shallow underground runoff in the region around the lake also affects the water quality safety of plateau lakes. The spatiotemporal differences in phosphorus concentrations in 452 shallow groundwater samples and the driving factors were analyzed by monitoring wells in croplands and residential areas around the eight lakes in Yunnan province during the rainy and dry seasons from 2019 to 2021. The results showed that seasonal changes and land use influenced phosphorus concentrations and their composition in shallow groundwater. The concentration of phosphorus in shallow groundwater in the rainy season was higher than that in the dry season, and it was also greater in cropland than that in residential areas. DTP was the dominant form of TP, accounting for 75%-81%, and DIP was the dominant form of DTP, accounting for 74%-80%. Nearly 30% of the samples around the eight lakes had TP concentrations exceeding the surface water Class Ⅲ standard (GB 3838); the exceeded rates of phosphorus in groundwater around the Erhai Lake (52%), Qiluhu Lake (45%), Xingyun Lake (42%), and Dianchi Lake (29%) were far higher than those of Yangzonghai Lake (16%), Fuxianhu Lake (13%), Chenghai Lake (6%), and Yilonghu Lake (5%). The key driving factors of phosphorus concentrations in shallow groundwater were water-soluble phosphorus (WEP), water content (MWC), soil organic matter (SOM), total nitrogen (TN), pH in the soil profile, and pH and groundwater level in the shallow groundwater (P<0.05). The increases in WEP, SOM, TN, and MWC in the soil and pH in groundwater significantly increased the concentrations of DIP and DTP in shallow groundwater, whereas the decrease in groundwater level significantly reduced the concentrations of DTP and DIP in the groundwater.

Shallow Groundwater Around Plateau Lakes: Spatiotemporal Distribution of Nitrogen and Its Driving Factors

Shallow groundwater around plateau lakes is one of the important sources of production and potable water. Shallow groundwater NO3--N pollution driven by factors such as surface nitrogen input load, rainfall, and irrigation is serious and threatens the water quality of plateau lakes. In order to identify the characteristics of nitrogen pollution and its driving factors in shallow groundwater, 463 shallow groundwater samples were collected from wells in farmland and residential areas around eight plateau lakes of Yunnan in the rainy and dry seasons in 2020 and 2021. The results showed that the average values of ρ(TN), ρ(NO3--N), ρ(ON), and ρ(NH4+-N) in shallow groundwater were 24.35, 15.15, 8.41, and 0.79 mg·L-1, respectively. Nearly 32% of the shallow groundwater samples around the eight lakes failed to meet the groundwater Class Ⅲ water quality requirements (GB/T 14848) of 20 mg·L-1 for NO3--N. Among them, the NO3--N concentration in the shallow groundwater around Erhai Lake, Qiluhu Lake, and Dianchi Lake had the highest rate of exceeding the standard, followed by that around Xingyunhu Lake, Yangzonghai Lake, Yilonghu Lake, Fuxianhu Lake, and Chenghai Lake as the smallest. Land use and seasonal changes affected the concentration and composition of various forms of nitrogen in shallow groundwater. The concentration of various forms of nitrogen in shallow groundwater in the farmland area was higher than that in the residential area. The nitrogen concentration in shallow groundwater in farmland was higher than that in residential areas. Except for NH4+-N, the concentration of various forms of nitrogen in shallow groundwater in the rainy season was higher than that in the dry season. NO3--N was the main nitrogen form in shallow groundwater; the fraction of TN was 57%-68%, and the fraction of ON was 27%-38%. The EC, DO, ORP, and T in shallow groundwater were the key factors reflecting or affecting the concentration of various forms of nitrogen in shallow groundwater, whereas soil factors had a weak impact on the concentration of various forms of nitrogen in shallow groundwater.

Genesis of high hexavalent chromium groundwater in deep aquifers from loess plateau of Northern Shaanxi, China

Hexavalent chromium (Cr(VI)) groundwater usually exists in shallow aquifers related to ultramafic and serpentine formations, but knowledge of the genesis of dissolved Cr(VI) in deep sandstone aquifers is limited. Both groundwater and aquifer sediments were taken from the Jingbian County in the Loess Plateau of Northwestern Shaanxi to investigate distribution and genesis mechanism of high Cr(VI) groundwater in deep sandstone aquifers. Results showed that the Cr concentrations (median 142 μg/L) in groundwater from deep aquifers (>100 m) were relatively high, while the Cr concentrations in shallow groundwater were low (median 33.8 μg/L). Dissolved Cr mainly existed in the species of Cr(VI) (average, 90%). Deep groundwater with higher Cr(VI) concentrations generally had higher pH, Eh, and DO than shallow groundwater, indicating that the high Cr(VI) groundwater existed in relatively oxic environment. Cretaceous sandstones in deep aquifers had anomalously high contents of total Cr (average 115 mg/kg), where Cr was mainly present in silicates-bound form, and secondly in strongly adsorbed form. There were positive correlations between Mn and Cr in the unweathered silicate-bound form and adsorbed form, which were conducive to Cr(III) oxidation into Cr(VI) in an alkaline-oxic environment. The different ionic ratios (i.e. (Ca2+ + Mg2+)/(HCO3− + SO42−)) also supported silicate weathering as the dominant rock-water interactions in the deep groundwater, which enhanced the release of the unweathered silicate-bound Cr. Relatively high pH and ionic strength mobilized the adsorbed Cr(VI) into groundwater. This investigation emphasizes the geological origin of high Cr(VI) groundwater in deep sandstone aquifers containing Mn oxides, which deserves more concerns for the purpose of drinking water supply.

Influence of redox gradients on nitrate transport from the landscape to groundwater and streams

Increases in nitrogen applications to the land surface since the 1950s have led to a cascade of negative environmental impacts, including degradation of drinking water supplies, nutrient enrichment of aquatic ecosystems and contributions to global climate change. In this study, groundwater, streambed porewater, and stream sampling were used to establish trends in nitrate concentrations and how redox gradients influence nitrate transport across diverse glacial terranes. Decadal sampling has found that elevated nitrate concentrations in shallow groundwater beneath cropland have been sustained for decades. Redox gradients established in the saturated zone using dissolved O2, iron, nitrate and excess N2 from denitrification suggest that nitrate-bearing zones are thin in glacial terranes dominated by fine materials. These thin nitrate-bearing zones lead to suboxic, low nitrate streambed porewater and limit the contributions of nitrate to streams from slow-flow groundwater. In contrast, thick oxic zones in more coarse-grained glacial terranes allow nitrate to reach deeper groundwater, resulting in streambed porewater with elevated nitrate concentrations and causing a large portion of stream nitrate to be derived from slow-flow groundwater. Groundwater age tracer data indicate that denitrification occurs more quickly in the terrane dominated by fine material than in the more coarse-grained terrane. The quicker depletion of nitrate in the more fine-grained terrane suggests that the thinner oxic zone in this terrane is due, in part, to the greater availability and reactivity of electron donors in this terrane than in the more coarse-grained terrane. Groundwater age tracer data and hydrograph separation analysis suggest that saturated zone lag times between when changes in land use practices occur and when changes in stream water are fully observed may vary widely across hydrogeologic settings.

Electrical conductivity and dissolved oxygen as predictors of nitrate concentrations in shallow groundwater in Erhai Lake region

Elevated nitrogen (N) concentration in shallow groundwater is becoming increasingly problematic, putting water resources under pressure. For more effective management of such a resource, more precise predictors of N level in groundwater using smart monitoring networks are needed. However, external factors such as land use type, rainfall, and N loads from multiple sources (residential and agricultural) make it difficult to accurately predict the spatial and temporal variations of N concentration. In order to identify the key factors affecting spatial and temporal N concentration in shallow groundwater and develop a predictive model, 635 groundwater samples from drinking wells in residential areas and agricultural wells in croplands of a typical agricultural watershed in the Erhai Lake Basin, southwest China, in the period from 2018 to 2020, were collected and analyzed. The results showed that the type of land use and seasonal variations significantly affected the N forms and their concentrations in the shallow groundwater, as the ratios of ON and NO3−-N to TN were 30%–39% and 52%–59% for the two land uses and 25%–44% and 46%–66% for seasonal changes. Their variations were reflected by electrical conductivity (EC) and redox environment. EC and dissolved oxygen (DO) had a positive non-linear relationship with the concentrations of total nitrogen (TN) and nitrate (NO3−-N). The fitted non-linear quantitative models were established separately to predict TN and NO3−-N concentrations in groundwater using easily available indictors (EC and DO). The high accuracy and performance of the models were investigated and approved by rRMSE, MAE, and 1:1 line. These findings can provide technical support for the rapid prediction and evaluation of N pollution in shallow groundwater through easily available indicators.

Influence of the shallow groundwater table on the groundwater N2O and direct N2O emissions in summer maize field in the North China Plain

Agriculture is an important N2O emissions source. Water cycle and nitrogen cycles have important effects on N2O in farmland ecosystems. The changes in the groundwater table can lead to changes in farmland the water and nitrogen cycle processes. However, how this such changes will affect N2O emissions from farmland remains unclear. In this study, a two-year volume lysimeter experiment (2019–2020), including four controlled groundwater tables (i.e., 40, 70, 110, and 150 cm), was performed to monitor the variations in the NO3− and N2O concentrations in shallow groundwater as well as the direct N2O emissions due to surface soil and groundwater evaporation. Our results showed that N2O emissions during fertilization accounted for 80%–90% of the total N2O emissions throughout the maize growing period. Direct N2O emissions increase with the increase in the groundwater table. The total N2O emissions in 2020 were 96.44, 9.75, 6.46, and 6.22 kg ha−1 y−1 at a groundwater table of 40, 70, 110, and 150 cm, respectively. The high water-filled pore space (WFPS) value resulting from the elevated groundwater table increased the groundwater–atmosphere connectivity, leading to significantly increased N2O emissions after fertilization. Increased precipitation (454.90 mm in 2020 vs. 180.30 mm in 2019) accelerated the hydrological processes in agroecology, reducing the retention time of N2O (6 weeks in 2020 vs. 7.5 weeks in 2019) and NO3− (6.75 weeks in 2020 vs. 7.25 weeks in 2019) in shallow groundwater. Studying the influence of shallow groundwater tables on direct N2O emissions will provide insights into the interaction between the water and nitrogen cycles in agroecosystems. The results of this study suggest that direct N2O emissions can be effectively reduced by controlling the groundwater table in agricultural fields in the North China Plain.

Deterioration of groundwater quality along an increasing intensive land use pattern in a small catchment

Land use change has greatly influenced groundwater quality worldwide. Identifying the effects of different intensive land uses on the groundwater quality is the first step in taking proper action to solve the problem. In this study, we compared the effects of different intensive land uses (region A, natural vegetation; region B, cereal fields; region C, kiwifruit orchards) in the Yujiahe catchment between 2015 and 2017 in Shaanxi, China, on the major ions and stable isotopes of nitrate (δ15N–NO3– and δ18O–NO3–). The NO3- groundwater concentrations increased from region A to region B and region C; NO3- concentrations in shallow groundwater were higher than those of deep groundwater in region C (55.3 vs. 28.9 mg/L, respectively). The NO3- concentrations in region A and region B did not exceed the WHO standard of 50 mg/L. However, 56.3% and 22.2% of the shallow and deep groundwater samples have NO3- concentrations exceeding the standard in region C, respectively. The average electrical conductivity (EC) values of springs in region A and shallow groundwater in regions B and C were 438, 525, and 753 µs/cm, respectively. Concentrations of Ca2+, Mg2+, Na+, Cl-, and HCO3- ions and nitrogen isotope values increased from region A to region C, indicating that intensive land use change has modified groundwater hydrochemical composition, and deteriorated groundwater quality. This study has highlighted the significant effect of intensive land use of orchards at the small catchment scale on the groundwater quality.

LINKING DISSOLVED ORGANIC CARBON SPATIAL HETEROGENEITY TO GROUNDWATER DYNAMICS AND SOIL ORGANIC CARBON CONTENT IN A SUBTROPICAL HEADWATER CATCHMENT

Storage and fluxes of dissolved organic carbon (DOC) are triggered by interactions between hydrological and biogeochemical processes in the landscape. However, the relationship between DOC sources and catchment water storage is poorly understood due to uncertainties about the interactions between shallow groundwater (GW) dynamics and soil organic carbon (SOC). To investigate the relationships between catchment functioning and the spatiotemporal variability of DOC concentrations in shallow GW, we monitored a network of 12 shallow GW wells in a 0.24 km² catchment in Southern Brazil using DOC analysis and SOC measurements together with GW and stream gauging and statistical analyses. The greatest DOC concentration was observed in wells in the zero-order basin with highly dynamic saturation of the soil profile (50% with water table level below 0.5 m, mean DOC = 4.83 ± 2.53 mg L-1), where GW was characterized by a fast rise and slow decay during storm events. In these areas, the relationship between DOC concentration and GW level followed a decay with soil depth and higher DOC concentrations were found when GW level was within 0.1 m of the soil surface (ranging from 2.00 to 14.8 mg L-1). Wells in mid-slope locations were characterized by the lowest SOC content and DOC concentration in GW (1.96 ± 0.91 mg L-1). During the driest period, lower slopes showed higher DOC concentrations than riparian zones due to the persistent near saturation of GW and the occurrence of DOC-enriched water due to the high SOC content in superficial layers. When the riparian zone and hillslopes showed a low water storage deficit and a quasi-permanent hydrological connectivity, DOC produced between rainfall events was transported continuously from saturated areas in hillslopes to riparian zones, with a higher DOC concentration in wells near streams. This study demonstrates that spatial sources of DOC in a subtropical headwater system are linked to streamflow generation processes and GW dynamics.

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.