Use of isotopic tracers to characterize the interaction of water components and nitrate contamination in the arid Rasafeh area (Syria)

Chemical and isotopic data in atmospheric precipitation, surface water, and groundwater in arid Rasafeh area, northeast Syria, are used to clarify the status of groundwater quality, the interaction of water components, groundwater dating, and vulnerability to anthropogenic contamination. Interpretation of chemical data with thermodynamic calculation reveals that the dissolution of evaporate mineral is the main factor of high salinity. The δ¹⁸O and δ²H relationships indicate that the groundwater is fed by mixing water from Euphrates River and precipitation and the isotope balance equation were used to estimate the contribution of the Euphrates River to the aquifers recharge. High tritium content, together with high ¹⁴C activity in the majority of groundwater samples, indicate shorter residence times and consequently potentially greater recharge. The presence of high nitrate concentration associated with high tritium concentration in both shallow and deep aquifer units indicates the presence of high permeability, so that groundwater is highly susceptible to anthropogenic contamination. Nitrate seems to derive exclusively from the application of N fertilizers. The high nitrate values are characteristic of the areas with intensive agricultural activity, indicating the importance of irrigated return flow on the groundwater.     

Assessing nitrate origin in a volcanic aquifer using a dual isotope approach

Identifying the origin of nitrate is important for the control and management of groundwater quality in aquifer systems. In the southern Apennines (Italy), the Mount Vulture volcanic aquifer is a large and valuable resource of potable and mineral water supply. Unfortunately, signs of anthropogenic impact, especially nitrogen contamination, have recently become evident. In this study, and for the first time, stable isotope ratios (δ¹⁵N and δ¹⁸O) of NO₃ ^? were determined in groundwater to identify their origins and evaluate the presence of transformation processes. The Mount Vulture groundwaters are meteoric in origin, as demonstrated by measurements of δD and δ¹⁸O, and can be divided into two distinct areas based on their NO₃ ^? content. In the southeastern area, characterized by active agricultural land use, the high NO₃ ^? content and the δ¹⁵N–NO₃ isotopic values are due to anthropogenic contamination (inorganic fertilizer). In groundwaters from the western area, the NO₃ ^? contents below 4 mg/L and the δ¹⁵N–NO₃ values can be associated at organic soil N. Evidence for local denitrification may be assumed in a few groundwater samples of the western area showing relatively heavy δ¹⁵N values and low concentrations of nitrate. Finally, the low measured δ¹⁸O values indicate that nitrification occurred in both investigated areas.     

The fate and transport of nitrate in shallow groundwater in northwestern Mississippi, USA

Agricultural contamination of groundwater in northwestern Mississippi, USA, has not been studied extensively, and subsurface fluxes of agricultural chemicals have been presumed minimal. To determine the factors controlling transport of nitrate-N into the Mississippi River Valley alluvial aquifer, a study was conducted from 2006 to 2008 to estimate fluxes of water and solutes for a site in the Bogue Phalia basin (1,250 km²). Water-quality data were collected from a shallow water-table well, a vertical profile of temporary sampling points, and a nearby irrigation well. Nitrate was detected within 4.4 m of the water table but was absent in deeper waters with evidence of reducing conditions and denitrification. Recharge estimates from 6.2 to 10.9 cm/year were quantified using water-table fluctuations, a Cl^– tracer method, and atmospheric age-tracers. A mathematical advection-reaction model predicted similar recharge to the aquifer, and also predicted that 15% of applied nitrogen is leached into the saturated zone. With current denitrification and application rates, the nitrate-N front is expected to remain in shallow groundwater, less than 6–9 m deep. Increasing application rates resulting from intensifying agricultural demands may advance the nitrate-N front to 16–23 m, within the zone of groundwater pumping.     

Flow of river water into a Karstic limestone aquifer. 1. Tracing the young fraction in groundwater mixtures in the Upper Floridan Aquifer near Valdosta, Georgia

The quality of water in the Upper Floridan aquifer near Valdosta, Georgia is affected locally by discharge of Withlacoochee River water through sinkholes in the river bed. Data on transient tracers and other dissolved substances, including Cl⁻, ³H, tritiogenic helium-3 (³He), chlorofluorocarbons (CFC-11, CFC-12, CFC-113), organic C (DOC), O₂ (DO), H₂S, CH₄, δ¹⁸O, δD, and ¹⁴C were investigated as tracers of Withlacoochee River water in the Upper Floridan aquifer. The concentrations of all tracers were affected by dilution and mixing. Dissolved Cl⁻, δ¹⁸O, δD, CFC-12, and the quantity (³H+³He) are stable in water from the Upper Floridan aquifer, whereas DOC, DO, H₂S, CH₄, ¹⁴C, CFC-11, and CFC-113 are affected by microbial degradation and other geochemical processes occurring within the aquifer. Groundwater mixing fractions were determined by using dissolved Cl⁻ and δ¹⁸O data, recognizing 3 end-member water types in the groundwater mixtures: (1) Withlacoochee River water (δ¹⁸O=−2.5±0.3‰, Cl⁻=12.2±2 mg/l), (2) regional infiltration water (δ¹⁸O=−4.2±0.1‰, Cl⁻=2.3±0.1 mg/l), and (3) regional paleowater resident in the Upper Floridan aquifer (δ¹⁸O=−3.4±0.1‰, Cl⁻=2.6±0.1 mg/l) (uncertainties are ±1σ). Error simulation procedures were used to define uncertainties in mixing fractions. Fractions of river water in groundwater range from 0 to 72% and average 10%. The influence of river-water discharge on the quality of water in the Upper Floridan aquifer was traced from the sinkhole area on the Withlacoochee River 25 km SE in the direction of regional groundwater flow. Infiltration of water is most significant to the N and NW of Valdosta, but becomes negligible to the S and SE in the direction of general thickening of post-Eocene confining beds overlying the Upper Floridan aquifer.     

Using multiple chemical indicators to characterize and determine the age of groundwater from selected vents of the Silver Springs Group, central Florida, USA

The Silver Springs Group, Florida (USA), forms the headwaters of the Silver River and supports a diverse ecosystem. The 30 headwater springs divide into five subgroups based on chemistry. Five selected spring vents were sampled in 2007 to better understand the contaminant sources and groundwater flow system. Elevated nitrate-N concentrations (>0.8 mg/L) in the five spring vents likely originate from inorganic (fertilizers) and organic sources, based on nitrogen and oxygen isotope ratios of nitrate. Evidence for denitrification in the Lost River Boil spring includes enriched δ¹⁵N and δ¹⁸O, excess N₂ gas, and low dissolved O₂ concentrations (<0.5 mg/L). Multiple age-tracer data (SF₆, ³H, tritiogenic ³He) for the two uppermost springs (Mammoth East and Mammoth West) indicate a binary mixture dominated by recent recharge water (mean age 6–7 years, and 87–97% young water). Tracer data for the three downstream spring vents (Lost River Boil, Catfish Hotel-1, and Catfish Conventional Hall-1) indicate exponential mixtures with mean ages of 26–35 years. Contamination from non-atmospheric sources of CFCs and SF₅CF₃ precluded their use as age tracers here. Variations in chemistry were consistent with mean groundwater age, as nitrate-N and dissolved O₂ concentrations were higher in younger waters, and the Ca/Mg ratio decreased with increasing mean age.     

Results from the Big Spring basin water quality monitoring and demonstration projects,Iowa, USA

Agricultural practices, hydrology, and water quality of the 267-km² Big Spring groundwater drainage basin in Clayton County, Iowa, have been monitored since 1981. Land use is agricultural; nitrate-nitrogen (-N) and herbicides are the resulting contaminants in groundwater and surface water. Ordovician Galena Group carbonate rocks comprise the main aquifer in the basin. Recharge to this karstic aquifer is by infiltration, augmented by sinkhole-captured runoff. Groundwater is discharged at Big Spring, where quantity and quality of the discharge are monitored. Monitoring has shown a threefold increase in groundwater nitrate-N concentrations from the 1960s to the early 1980s. The nitrate-N discharged from the basin typically is equivalent to over one-third of the nitrogen fertilizer applied, with larger losses during wetter years. Atrazine is present in groundwater all year; however, contaminant concentrations in the groundwater respond directly to recharge events, and unique chemical signatures of infiltration versus runoff recharge are detectable in the discharge from Big Spring. Education and demonstration efforts have reduced nitrogen fertilizer application rates by one-third since 1981. Relating declines in nitrate and pesticide concentrations to inputs of nitrogen fertilizer and pesticides at Big Spring is problematic. Annual recharge has varied five-fold during monitoring, overshadowing any water-quality improvements resulting from incrementally decreased inputs. Electronic Publication     

Fraction of young water as an indicator of aquifer vulnerability along two regional flow paths in the Mississippi embayment aquifer system,southeastern USA

Wells along two regional flow paths were sampled to characterize changes in water quality and the vulnerability to contamination of the Memphis aquifer across a range of hydrologic and land-use conditions in the southeastern United States. The flow paths begin in the aquifer outcrop area and end at public supply wells in the confined parts of the aquifer at Memphis, Tennessee. Age-date tracer (e.g. SF₆, ³H, ¹⁴C) data indicate that a component of young water is present in the aquifer at most locations along both flow paths, which is consistent with previous studies at Memphis that documented leakage of shallow water into the Memphis aquifer locally where the overlying confining unit is thin or absent. Mixtures of young and old water were most prevalent where long-term pumping for public supply has lowered groundwater levels and induced downward movement of young water. The occurrence of nitrate, chloride and synthetic organic compounds was correlated to the fraction of young water along the flow paths. Oxic conditions persisted for 10 km or more down dip of the confining unit, and the presence of young water in confined parts of the aquifer suggest that contaminants such as nitrate-N have the potential for transport. Long-term monitoring data for one of the flow-path wells screened in the confined part of the aquifer suggest that the vulnerability of the aquifer as indicated by the fraction of young water is increasing over time.     

Climate controls on nitrate concentration variability in the Abbotsford-Sumas aquifer,British Columbia,Canada

Understanding the linkage between temporal climate variability and groundwater nitrate concentration variability in monitoring well records is key to interpreting the impacts of changes in land-use practices and assessing groundwater quality trends. This study explores the coupling of climate variability and groundwater nitrate concentration variability in the Abbotsford-Sumas aquifer. Over the period of 1992–2009, the average groundwater nitrate concentration in the aquifer remained fairly steady at approximately 15 mg/L nitrate-N. Normalized nitrate data for 19 individual monitoring wells were assessed for a range of intrinsic factors including precipitation, depth to water table, depth below water table, and apparent groundwater age. At a broad scale, there is a negative correlation between nitrate concentration and apparent groundwater age. Each dedicated monitoring well shows unique, non-uniform cyclical variability in nitrate concentrations that appears to correspond with seasonal (1 year) cycles in precipitation as well as longer-period cycles (~5 years), possibly due to ENSO (El Niño Southern Oscillation) or the Pacific North American (PNA) pattern. These precipitation cycles appear to influence nitrate concentrations by approximately ±30 % of the critical concentration (10 mg/L NO₃–N). Not all wells show direct correlation due to many complex local-scale factors that influence nitrate leaching including spatially and temporally variable nutrient management practices and soil/crop nitrogen dynamics (anthropogenic and agronomic factors).     

The deep Cretaceous aquifer in the Aleppo and Steppe basins of Syria: assessment of the meteoric origin and geographic source of the groundwater

A drilling project was carried out in Syria to assess the potential of the deep groundwater resources of the Cretaceous aquifer, composed of Cenomanian-Turonian limestones and dolomites. In this context, isotope (¹⁴C, ³H, δ¹³C, δ¹⁸O, δ²H) and hydrochemical analyses were performed on wells in and around the Aleppo and Steppe basins. The interpretation includes complementary results from published and unpublished literature. The results provide evidence that many new wells pump mixed groundwater from the Cretaceous aquifer and the overlying Paleogene aquifer. Radiocarbon measurements confirmed dominating Pleistocene groundwater in the Cretaceous aquifer and mainly Holocene groundwater in the Paleogene aquifer. Most groundwater in the Cretaceous aquifer seems to be recharged in the western limestone ridges, stretching from Jebel az Zawiyah (south of Idlep) via Jebel Samane (south of Afrin and A’zaz) to the region north of Aleppo, and in the Northern Palmyrides mountain belt. Some recharge also occurs around the basalt plateau of the Jebel al Hass, south east of Aleppo. It is concluded that the Taurus Mountains and the Euphrates River do not recharge the Cretaceous aquifer. The sources of recharge seem to be occasionally occurring intensive winter storms that approach from Siberia.     

Assessing groundwater availability and the response of the groundwater system to intensive exploitation in the North China Plain by analysis of long-term isotopic tracer data

The use of isotope tracers as a tool for assessing aquifer responses to intensive exploitation is demonstrated and used to attain a better understanding of the sustainability of intensively exploited aquifers in the North China Plain. Eleven well sites were selected that have long-term (years 1985–2014) analysis data of isotopic tracers. The stable isotopes δ¹⁸O and δ²H and hydrochemistry were used to understand the hydrodynamic responses of the aquifer system, including unconfined and confined aquifers, to groundwater abstraction. The time series data of ¹⁴C activity were also used to assess groundwater age, thereby contributing to an understanding of groundwater sustainability and aquifer depletion. Enrichment of the heavy oxygen isotope (¹⁸O) and elevated concentrations of chloride, sulfate, and nitrate were found in groundwater abstracted from the unconfined aquifer, which suggests that intensive exploitation might induce the potential for aquifer contamination. The time series data of ¹⁴C activity showed an increase of groundwater age with exploitation of the confined parts of the aquifer system, which indicates that a larger fraction of old water has been exploited over time, and that the groundwater from the deep aquifer has been mined. The current water demand exceeds the sustainable production capabilities of the aquifer system in the North China Plain. Some measures must be taken to ensure major cuts in groundwater withdrawals from the aquifers after a long period of depletion.     

Factors affecting the movement and persistence of nitrate and pesticides in the surficial and upper Floridan aquifers in two agricultural areas in the southeastern United States

Differences in the degree of confinement, redox conditions, and dissolved organic carbon (DOC) are the main factors that control the persistence of nitrate and pesticides in the Upper Floridan aquifer (UFA) and overlying surficial aquifer beneath two agricultural areas in the southeastern US. Groundwater samples were collected multiple times from 66 wells during 1993–2007 in a study area in southwestern Georgia (ACFB) and from 48 wells in 1997–98 and 2007–08 in a study area in South Carolina (SANT) as part of the US Geological Survey National Water-Quality Assessment Program. In the ACFB study area, where karst features are prevalent, elevated nitrate-N concentrations in the oxic unconfined UFA (median 2.5 mg/L) were significantly (p = 0.03) higher than those in the overlying oxic surficial aquifer (median 1.5 mg/L). Concentrations of atrazine and deethylatrazine (DEA; the most frequently detected pesticide and degradate) were higher in more recent groundwater samples from the ACFB study area than in samples collected prior to 2000. Conversely, in the SANT study area, nitrate-N concentrations in the UFA were mostly <0.06 mg/L, resulting from anoxic conditions and elevated DOC concentrations that favored denitrification. Although most parts of the partially confined UFA in the SANT study area were anoxic or had mixed redox conditions, water from 28 % of the sampled wells was oxic and had low DOC concentrations. Based on the groundwater age information, nitrate concentrations reflect historic fertilizer N usage in both the study areas, but with a lag time of about 15–20 years. Simulated responses to future management scenarios of fertilizer N inputs indicated that elevated nitrate-N concentrations would likely persist in oxic parts of the surficial aquifer and UFA for decades even with substantial decreases in fertilizer N inputs over the next 40 years.     

Nitrate distribution and potential attenuation mechanisms of a municipal water supply bedrock aquifer

The Silurian bedrock aquifer constitutes a major aquifer system for groundwater supply across the Ontario province in Canada. The application of natural and industrial fertilizers near urban centers has led to groundwater NO₃⁻-N concentrations that sometimes have exceeded the drinking water limit, posing a threat to the usage of groundwater for the human consumption. Therefore, there is a growing interest and concern about how nitrate is being leached, transported and potentially attenuated in bedrock aquifers. This study assesses the local distribution of groundwater NO₃⁻ in the up-gradient area of two historically impacted municipal wells, called Carter Wells, in the City of Guelph, Canada, in order to evaluate the potential nitrate attenuation mechanisms, using both groundwater geochemical and isotopic analysis (³H, δ¹⁵N-NO₃, δ¹⁸O-NO₃, δ¹⁸O-SO₄, δ³⁴S-SO₄) and a detailed vertical hydrogeological and geochemical bedrock characterization. The results indicate that probably the main source of nitrate to the Carter Wells is the up-gradient Arkell Research Station (ARS), an agricultural research facility where manure has been historically applied. The overburden and bedrock groundwater with high NO₃ concentrations at the ARS exhibits a manure-related δ¹⁵N and δ¹⁸O signature, isotopically similar to the high nitrate in the down-gradient groundwater from domestic wells and from the Carter Wells. The nitrate spatial distribution appears to be influenced and controlled by the geology, in which more permeable rock is found in the Guelph Formation which in turn is related to most of the high NO₃⁻ groundwater. The presence of an underlying low permeability Eramosa Formation favors the development of oxygen-depleted conditions, a key factor for the occurrence of denitrification. Groundwater with low NO₃⁻-N concentrations associated with more oxygen-limited conditions and coincident with high SO₄²⁻ concentrations are related to more enriched δ¹⁵N and δ¹⁸O values in NO₃⁻ and to more depleted δ³⁴S and δ¹⁸O values in SO₄²⁻, suggesting that denitrification coupled with pyrite oxidation is taking place. The presence of macro crystalized and disseminated pyrite especially in the Eramosa Formation, can support the occurrence of this attenuation process. Moreover, based on tritium analysis, some denitrification can occur in shallow bedrock and within relatively short residence times, associated with less permeable conditions in depth which facilitates oxygen consumption through sulfide oxidation. The role of denitrification mediated by organic carbon cannot be discarded at the study site. This study suggests that the geological configuration and particularly the presence of low permeability Eramosa Formation can play an important role on nitrate natural attenuation, which may serve as a decision factor on defining the bedrock water supply system for both domestic and municipal purposes.     

Stable isotope geochemistry of dissolved chloride in relation to hydrogeology of the strongly exploited Quaternary aquifers,North China Plain

Deep Quaternary groundwater is the main source for industrial, domestic, and agricultural water supply in the North China Plain (NCP). There is currently a regional decline of groundwater levels, deterioration of water quality and environmental geological problems induced by increasing exploitation of the NCP Quaternary aquifer system. To trace sources and transport processes of dissolved Cl⁻ in a regional aquifer system and to reveal hydrogeological characteristics of Quaternary complexes, δ³⁷Cl, δ¹⁸O and δD, and chemical compositions (including F⁻, Cl⁻, Br⁻) of the deep groundwater sampled from the northern flow system of the NCP were measured along the west–east groundwater flow paths. The measured δ³⁷Cl values decreased from 0.39‰ to −2.22‰ (SMOC) along the groundwater flow direction, with increasing Cl⁻ concentrations. Marine aerosol input via rainfall is the main source of Cl⁻ in the deep groundwater near the recharge areas, and subsequent evaporation/evapotranspiration appears to be responsible for Cl⁻ accumulation. Mixing of recharge water with water of high-Cl⁻ and low-δ³⁷Cl accounts for the pattern of δ³⁷Cl and Cl⁻ concentration observed in Aquifer-3 along the west–east transect. The water with high-Cl and low-δ³⁷Cl is likely from pore water released from compacted clays induced by over-exploitation of deep groundwater, suggesting that clay is a dominant subsurface source of Cl⁻ for groundwater where a regional depression cone is present in the Quaternary aquifers. The groundwater of Aquifer-4 in the Huang-Hua depression is potentially mixed with an upward flux of Cl⁻ from the Neogene aquifer through subvertical faults. Diffusion and ion filtration are two mechanisms invoked to explain the highly negative δ³⁷Cl data for groundwater of Aquifer-4 in the Yanshan–Haixing areas, which provides new insight into solute migration and the hydraulic relationship in the strongly exploited groundwater system. This study using the conservative solute Cl⁻ provides additional important information for further investigations of the geochemistry of a wide range of reactive solutes in the Quaternary aquifer system, so guiding water resource management.     

Impact of agricultural activity and geologic controls on groundwater quality of the alluvial aquifer of the Guadalquivir River (province of Jaén,Spain): a case study

The alluvial aquifer of the Alto Guadalquivir River is one of the most important shallow aquifers in Jaén, Spain. It is located in the central-eastern part of the province, and its groundwater resources are used mainly for crop irrigation in an agriculture-dominated area. Hydrochemical and water-quality data obtained through a 2-year sampling (2004–2006) and analysis program indicate that nitrate pollution is a serious problem affecting groundwater due to the use of nitrogen (N)-fertilizers in agriculture. During the study, 231 water samples were collected from wells and springs to determine water chemistry and the extent of nitrate pollution. The concentration of nitrate in groundwater ranged from 1.25 to 320.88 mg/l. Considerable seasonal fluctuations in groundwater quality were observed as a consequence of agricultural practices and other factors such as annual rainfall distribution and the Guadalquivir River flow regime. The chemical composition of the water is not only influenced by agricultural practices, but also by interaction with the alluvial sediments. The dissolution of evaporites accounts for part of the Na⁺, K⁺, Cl⁻, SO₄ ²⁻, Mg²⁺, and Ca²⁺, but other processes, such as calcite precipitation and dedolomitization, also contribute to groundwater chemistry.     

Distribution of nitrate in groundwater affected by the presence of an aquitard at an agricultural area in Chiba, Japan

Geological and geographical parameters including land use, stratigraphic structure, groundwater quality, and N- and O-isotopic compositions of nitrate in groundwater were investigated to elucidate the mechanism of groundwater pollution by NO₃ ^? in the agricultural area of Katori, Chiba, Japan. An aquitard distributed in the western part of the study area has produced two unconfined aquifers. The average concentrations of NO₃ ^? and dissolved oxygen (DO) were high in the aquifer above the aquitard (7.5 and 7.6 mg/L, respectively) and in the aquifer of the eastern part of the study area that was not influenced by the aquitard (11.9 and 7.8 mg/L, respectively); however, the levels in the aquifer under the aquitard were relatively low (2.2 and 3.7 mg/L, respectively). The δ¹⁵N and δ¹⁸O values of NO₃ ^? generally increased exponentially in the groundwater that flowed into the aquifer under the aquitard as the concentration of NO₃ ^? decreased, although this decrease in NO₃ ^? also occasionally occurred without isotopic changes. These results indicated that the aquitard prevented the penetration of NO₃ ^?, DO, and gaseous O₂. Under the aquitard, denitrification and dilution with unpolluted water that entered from natural forested areas reduced the NO₃ ^? concentrations in the groundwater. The major sources of NO₃ ^? in groundwater in the study area were estimated to be NH₄-chemical fertilizer, livestock waste, and manure.     

Seepage of groundwater nitrate from a riparian agroecosystem into the Wye River estuary

Intensive research in Chesapeake Bay has indicated that reductions in nitrogen inputs to the bay will be necessary to restore water quality to levels needed for resurgence of bay living resources. Fall-line water quality monitoring efferts have characterized diffuse-source nitrogen inputs from a large percentage of the bay drainage basin, but relatively little information exists regarding rates of nitrogen delivery to tidal waters from coastal plain regions. Extensive nitrate contamination of shallow groundwater due to agricultural activities, coupled with the dominant role of subsurface flow in discharge from Coastal Plain regions of the drainage basin, creates the potential for high rates of nitrogen delivery to tidal waters via groundwater seepage. This study utilized intensive hydrologic and water chemistry monitoring from April 1992 through September 1994 to determine the spatial characteristics of the groundwater-estuarine interface, as well as the rates of subsurface nitrogen transport from an agricultural field into nearshore waters of the Wye River, a subestuary of Chesapeake Bay. The hydrogeologic characteristics of the study site resulted in groundwater discharge to the Wye River occurring almost exclusively within 15 m of the shoreline. Calculated groundwater discharge rates were found to vary widely in the short term due to tidal fluctuations but in the long term were driven by seasonal changes in groundwater recharge rates. The zone of groundwater discharge contracted shoreward during summer months of low discharge, and expanded to a maximum width of approximately 15 m during high discharge periods in late winter. Average discharge rates were more than five times higher in winter versus summer months. Groundwater nitrate concentrations entering the discharge zone were relatively stable throughout the study period, with little evidence of denitrification or nitrate uptake by riparian vegetation. Consequently, nitrogen discharge patterns reflected the strong seasonality in groundwater discharge. Annual nitrate-N discharge was approximately 1.2 kg m^?1 of shoreline, indicating drainage basin rates of nitrogen delivery to tidal waters of approximately 60 kg ha^?1.     

Stable chlorine isotopic signatures and fractionation mechanism of groundwater in Anyang,China

The present work provides an online Bench II-IRMS technique for the measurement of stable chlorine isotope ratio, which is used to measure the δ³⁷Cl of 38 groundwater samples from the Karst and Quaternary aquifers in Anyang area. The regional distribution and signature of δ³⁷Cl value are characterized on the base of isotopic data. The results suggest that the δ³⁷Cl value of Quaternary groundwater decreases with increasing Cl^? concentration, and has no correlation with δ¹⁸O and δD values, but closely correlates with the depth to water table. The fractionation mechanism of the chlorine isotope is expounded according to the type of groundwater. The δ³⁷Cl value of karst water is generally positive, which is relevant to the dissolution of evaporite (gypsum mine), and may be caused by the mixing of groundwater and precipitation. The groundwater of Quaternary unconfined aquifer is mainly recharged by precipitation, and the δ³⁷Cl value of groundwater is generally negative. The δ³⁷Cl value of groundwater in Quaternary confined aquifer is more negative with increasing the depth to water level and elevated Cl^? concentration, which is possible to result from the isotope fractionation of ion filtration. The groundwater with inorganic pollutants in Quaternary unconfined aquifer has generally a positive δ³⁷Cl value.     

Using stable isotopes of nitrogen to study its source and transformation in a heavily farmed watershed

Twenty private wells and ten stream locations were sampled to assess the source and fate of dissolved nitrate in the Cedar River watershed of Iowa, USA. The average levels of nitrate in groundwater decreased from 39.5 mg/L in May, to 38 mg/L in July, to 30 mg/L in September. Although several surface water samples exceeded MCL in May, most values dropped to below 20 mg/L by July and September. The decreasing N levels were attributed to the gradual uptake of nitrate by growing crops as well as the cyanobacterial growth in the aquatic systems. The δ¹⁵N values of dissolved nitrate in groundwater ranged from +0.45 to +5.35‰, whereas those in surface water ranged from +1.48 to +5.16‰. The results suggested that commercial fertilizers and soil organic nitrogen were probably mixed up in their transport pathways. A fertilizer-only source would provide much lower delta values, whereas soil nitrogen would provide higher than observed delta values. Denitrification was considered unlikely because of the low δ¹⁵N values, high nitrate concentrations, and moderately high DO content in groundwater. Animal wastes were not found as a possible source of nitrate in the water. This is supported by the low chloride concentrations and lower than 10‰ delta values in the water samples. The study demonstrates that nitrogen isotope data in coordination with the dissolved nitrate levels and land use can be effectively used in nitrogen source identification and its transformation studies.     

Modeling of limestone aquifer in the western part of the River Nile between Beni Suef and El Minia

A robust classification scheme for partitioning groundwater chemistry into homogeneous groups was an important tool for the characterization of Eocene limestone aquifer. The aquifer locally is composed of chalky limestone with thin clay intercalated (Samalut Fm.), the fissures, the joints, and the fractures are represented the conduits of the aquifer system. The flow patterns are conditioned by karstification processes which develop a conduit network and preserve low permeability microfractured blocks. The aquifer is mainly recharged by surrounding aquifers and agricultural wastewaters. The groundwater flows in the eastern part (due the Bahr Yossef and River Nile), which is a discharge area rather than a recharge. Twenty-eight groundwater samples was collected from the Eocene limestone aquifer and analyzed for isotopes, major, and trace elements. δD and δ¹⁸O concentrations ranged widely due to geology, infiltration of different surface waters, evaporation, and hydrogeology. The concentration of δD and δ¹⁸O isotopes is depleted in the northern zone of the northern part and western zone of the central and southern part of the study area. They are enriched due the eastern area of the central and southern part of the study area. δD vs. δ¹⁸O delineate the Pleistocene aquifer and has a strong influence than other waters on aquifer hydrogeochemistry. It is confirmed by the AquaChem outputs of the mixing proportions of different water types included in the aquifer system. Cl-δD and Cl-δ¹⁸O relationships indicate the role of evaporation especially due the eastern area of the central and southern part of the study area. This research tests the performance of the many available graphical and statistical methodologies used to classify water samples. R-mode clustering, correlation analysis, and principal component analysis were investigated. All the methods were discussed and compared as to their ability to cluster, ease of use, and ease of interpretation. Nearly most low-salinity waters are in equilibrium to supersaturate with respect to both carbonate minerals, while it is shifted to undersaturate with salinity. The inverse modeling findings clarify that the calcite, gypsum, and anhydrite dissolution increased due the northeastern area, middle zone, and southern corner of the northern, central, and southern part of the study area, respectively. The latter areas also were characterized by the lowest precipitation of the dolomite. Such areas are distinguished by much more enhancement for aquifer permeability and therefore transmissivity. The latter areas can be use as injection zone by fresh water. It can be a triple function; firstly, it recharges the saline Eocene limestone aquifer through the enhancement hydraulic conductivity and dilutes it. Secondly, it enhances much more the aquifer permeability and therefore the transmissivity. The Eocene limestone aquifer can be improved in quality and quantity by using such a model and exploits it as an alternative water resource with Quaternary aquifer and Nile water. Thirdly, it irrigates more areas to increase the income/capita. The dedolomitization represents the main hydrogeochemical process in the aquifer system. The geomedia (limestone, clay, marl, shale, and sand deposits) are in contact with water, therefore, the rock/water interaction, mixing, and ion exchange were estimated by the geochemical evolution of the groundwater systems.     

Environmental tracers as indicators of karst conduits in groundwater in South Dakota,USA

Environmental tracers sampled from the carbonate Madison aquifer on the eastern flank of the Black Hills, South Dakota, USA indicated the approximate locations of four major karst conduits. Contamination issues are a major concern because these conduits are characterized by direct connections to sinking streams, high groundwater velocities, and proximity to public water supplies. Objectives of the study were to estimate approximate conduit locations and assess possible anthropogenic influences associated with conduits. Anomalies of young groundwater based on chlorofluorocarbons (CFCs), tritium, and electrical conductivity (EC) indicated fast moving, focused flow and thus the likely presence of conduits. δ¹⁸O was useful for determining sources of recharge for each conduit, and nitrate was a useful tracer for assessing flow paths for anthropogenic influences. Two of the four conduits terminate at or near a large spring complex. CFC apparent ages ranged from 15 years near conduits to >50 years in other areas. Nitrate-N concentrations >0.4 mg/L in groundwater were associated with each of the four conduits compared with concentrations ranging from <0.1 to 0.4 mg/L in other areas. These higher nitrate-N concentrations probably do not result from sinking streams but rather from other areas of infiltration.