Recharge into Southern High Plains aquifer—possible mechanisms,unresolved questions

The High Plains aquifer in the Southern High Plains (Texas and New Mexico), consisting of Tertiary, Cretaceous, and Triassic formations, has traditionally been considered to be recharged by its uppermost water-bearing unit, the Tertiary Ogallala aquifer. This article provides hydrologic, chemical, and isotopic evidence that in the Southern High Plains: (1) Cretaceous rocks actually contain independent recharge sources; (2) Triassic rocks cannot currently be recharged by the Ogallala aquifer in significant quantities; and (3) in places, both Cretaceous and Triassic aquifers recharge the overlying Ogallala aquifer. On the basis of chemical and isotopic data, playa lakes seem to act as the predominant recharge source of the Ogallala aquifer, suggesting recharge rates greater than 30 mm/yr, as opposed to the much lower rates reported by others. The Cretaceous aquifers are being recharged by cross-formational flow from the Ogallala aquifer but also from overlying Quaternary sands and the underlying Triassic aquifer in eastern New Mexico. Current recharge into the Triassic aquifer may be insignificant.     

Groundwater recharge and sustainability in the High Plains aquifer in Kansas, USA

Sustainable use of groundwater must ensure not only that the future resource is not threatened by overuse, but also that natural environments that depend on the resource, such as stream baseflows, riparian vegetation, aquatic ecosystems, and wetlands are protected. To properly manage groundwater resources, accurate information about the inputs (recharge) and outputs (pumpage and natural discharge) within each groundwater basin is needed so that the long-term behavior of the aquifer and its sustainable yield can be estimated or reassessed. As a first step towards this effort, this work highlights some key groundwater recharge studies in the Kansas High Plains at different scales, such as regional soil-water budget and groundwater modeling studies, county-scale groundwater recharge studies, as well as field-experimental local studies, including some original new findings, with an emphasis on assumptions and limitations as well as on environmental factors affecting recharge processes. The general impact of irrigation and cultivation on recharge is to appreciably increase the amount of recharge, and in many cases to exceed precipitation as the predominant source of recharge. The imbalance between the water input (recharge) to the High Plains aquifer and the output (pumpage and stream baseflows primarily) is shown to be severe, and responses to stabilize the system by reducing water use, increasing irrigation efficiency, adopting water-saving land-use practices, and other measures are outlined. Finally, the basic steps necessary to move towards sustainable use of groundwater in the High Plains are delineated, such as improving the knowledge base, reporting and providing access to information, furthering public education, as well as promoting better understanding of the publics attitudinal motivations; adopting the ecosystem and adaptive management approaches to managing groundwater; further improving water efficiency; exploiting the full potential of dryland and biosaline agriculture; and adopting a goal of long-term sustainable use.

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Resumen El uso sostenible de aguas subterráneas debe garantizar tanto que el recurso futuro no esté amenazado por sobreutilización como que los ambientes naturales dependientes del recurso sean protegidos (ie el flujo base de los arroyos, la vegetación ripariana, los ecosistemas acuáticos y los pantanos). El manejo adecuado de los recursos de aguas subterráneas requiere información precisa con respecto a los influjos (recarga) y descargas (bombeo y descarga natural) en cada cuenca de aguas subterráneas de tal manera que se pueda estimar o reevaluar el comportamiento de largo plazo del acuífero y su tasa de sotenibilidad. En un primer paso hacia esta meta, este trabajo destaca algunos estudios claves de recarga de aguas subterráneas en las llanuras altas de Kansas. Dichos estudios se concentran en diferentes escalas: estudios regionales del presupuesto para aguas del suelo y modelos de aguas subterráneas, estudios de recarga de aguas subterráneas a nivel provincial y estudios locales experimentales de terrenos que incluyen algunos interesantes descubrimientos nuevos. Estas investigaciones comparten el énfasis en los presupuestos de partida y las limitaciones así como en los factores ambientales que afectan los procesos de recarga. El impacto general de las irrigaciones y cultivos sobre la recarga es un obvio incremento en el monto de recarga y en muchos casos excede a la precipitación como la fuente principal de recarga. Se puede observar que el desequilibrio entre el influjo de agua (recarga) del acuífero de las llanuras altas y la descarga (bombeo y flujos base de los arroyos principalmente) es severo. Asimismo, se describen las respuestas para estabilizar el sistema a través de la reducción del uso de agua por medio del incremento de la eficiencia de las irrigaciones y de la adopción de las prácticas de ahorro de agua y del uso de tierras así como otras medidas adicionales. Finalmente se describen los pasos básicos necesarios para evolucionar hacia el uso sostenible de las aguas subterráneas en las llanuras altas de Kansas. Estos pasos están constituidos por una mejora del conocimiento base, comunicar y proporcionar fácil acceso a la información, mejorar el conocimiento público general así como promover un mejor entendimiento de las motivaciones para las actitudes de la comunidad, adoptar los enfoques administrativos de ecosistemas y administración adaptable en el manejo de las aguas subterráneas, continuar las mejoras del uso eficiente del agua, explotar el potencial de la agricultura de terrenos áridos y biosalina y adoptar como meta el uso sostenible a largo plazo.

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Résumé Lutilisation durable de leau souterraine doit permettre non seulement que la pérennité de leau ne soit pas menacée, mais aussi que les environnements naturels qui dépendent de cette ressource, tels que la végétation riveraine, les écosystemes aquatiques et les milieux humides, soient protégés. Afin dassurer une gestion appropriée des ressources en eau souterraine, une information précise concernant les entrées (recharge) et les sorties (décharge naturelle et pompage) deau dans chacun des bassins est nécessaire afin que le comportement à long terme de laquifère et le taux de pompage durable puissent être estimés. En guise dinitiative, ce travail illustre certaines études clé concernant la recharge de leau souterraine à plusieurs échelles dans les hautes plaines du Kansas. Ces études comprennent le bilan sol-eau et la modélisation numérique à léchelle régionale, létude de la recharge des nappes souterraines à léchelle du comté, des études expérimentales à petite échelle et certaines découvertes originales. Lors de la présentation de ces études, lemphase est portée sur les hypothèses et limitations ainsi que sur les facteurs environmentaux qui affectent les processus de recharge. En général, les effets de lirrigation et de lagriculture sur la recharge sont daugmenter considérablement le taux de recharge, et dans plusieurs cas, dexcéder les précipitations comme principale source de recharge. Le déséquilibre entre les intrants deau (recharge) et entrants (pompage et écoulement de base dans les rivières) dans laquifère des hautes plaines est très important. Les résultats des efforts de stabilisation du système aquifère en diminuant lutilisation deau, en améliorant lefficacité des techniques dirrigation, en adoptant des pratiques dutilisation du territoire qui réduisent lutilisation deau et en adoptant certaines autres mesures sont présentés. Enfin, les étapes de base nécessaires afin datteindre une utilisation durable de leau souterraine dans les hautes plaines sontexposées. Elles comprennent lamélioration des connaissance de base, lapromotion dune meilleure compréhension de la motivation et de lattitudedu public, ladoption dune approche de gestion de leau souterraineadaptable et basée sur les écosystèmes, lamélioration de lefficacitédutilisation de leau, lexploitation du plein potentiel de lagriculturebiosaline et en milieu aride, et ladoption dun objectif pourlutilisation durable à long terme.

     

Reactions and reaction rates in the regional aquifer beneath the Pajarito Plateau,north-central New Mexico,USA

Reactions and reaction rates within aquifers are fundamental components of critical hydrological processes. However, reactions simulated in laboratory experiments typically demonstrate rates that are much faster than those observed in the field. Therefore, it is necessary to conduct more reaction rate analyses in natural settings. This study of geochemical reactions in the regional aquifer in the Pajarito Plateau near Los Alamos, New Mexico combines modeling with petrographic assessment to further knowledge and understanding of complex natural hydrologic systems. Groundwater geochemistry shows marked evolution along assumed flow paths. The flow path chosen for this study was evaluated using inverse mass balance modeling to calculate the mass transfer. X-ray diffraction and field emission gun scanning electron microscopy were used to identify possible reactants and products. Considering the mineralogy of the aquifer and saturation indices for the regional water refined initial interpretations. Calculations yielded dissolution rates for plagioclase on the order of 10⁻¹⁵ mol s⁻¹ m⁻² and for K-feldspar on the order of 10⁻¹⁷ mol s⁻¹ m⁻², orders of magnitude slower than laboratory rates. While these rates agree with other aquifer studies, they must be considered in the light of the uncertainty associated with geometric surface area estimates, ¹⁴C ages, and aquifer properties.     

Controls on the regional-scale salinization of the Ogallala aquifer,Southern High Plains,Texas, USA

An extensive saline plume (>250 km²) within the regionally important unconfined aquifer in the Neogene Ogallala Formation overlies the Panhandle oil and gas field in the Southern High Plains, Texas, USA. Relative to upgradient Ogallala water, the plume waters have δ¹⁸O (−6.7 to −8.8‰) and δD (−42 to −88‰) values that tend to be depleted and have higher Cl (>150 mg/l) and SO₄ (>75 mg/l) concentrations. Various end-member-mixing models suggest that the plume composition reflects the presence of paleowaters recharged during Middle to Late Wisconsinan time rather than salinization associated with petroleum production. Paleowaters probably mixed with salt-dissolution zone waters from the underlying Upper Permian formations before discharging upward into the Ogallala Formation. Cross-formational discharge is controlled primarily by the geometry of the underlying units, as influenced by the Amarillo uplift, pinch-out of the laterally adjoining confined aquifer in the Triassic Dockum Group, variations in the saturated thickness of the Ogallala aquifer and the presence of potential pathways related to salt dissolution.     

Elevated naturally occurring arsenic in a semiarid oxidizing system, Southern High Plains aquifer, Texas, USA

High groundwater As concentrations in oxidizing systems are generally associated with As adsorption onto hydrous metal (Al, Fe or Mn) oxides and mobilization with increased pH. The objective of this study was to evaluate the distribution, sources and mobilization mechanisms of As in the Southern High Plains (SHP) aquifer, Texas, relative to those in other semiarid, oxidizing systems. Elevated groundwater As levels are widespread in the southern part of the SHP (SHP-S) aquifer, with 47% of wells exceeding the current EPA maximum contaminant level (MCL) of 10 μg/L (range 0.3–164 μg/L), whereas As levels are much lower in the north (SHP-N: 9%  As MCL of 10 μg/L; range 0.2–43 μg/L). The sharp contrast in As levels between the north and south coincides with a change in total dissolved solids (TDS) from 395 mg/L (median north) to 885 mg/L (median south). Arsenic is present as arsenate (As V) in this oxidizing system and is correlated with groundwater TDS (Spearman’s ρ = 0.57). The most likely current source of As is sorbed As onto hydrous metal oxides based on correlations between As and other oxyanion-forming elements (V, ρ = 0.88; Se, ρ = 0.54; B, ρ = 0.51 and Mo, ρ = 0.46). This source is similar to that in other oxidizing systems and constitutes a secondary source; the most likely primary source being volcanic ashes in the SHP aquifer or original source rocks in the Rockies, based on co-occurrence of As and F (ρ = 0.56), oxyanion-forming elements and SiO₂ (ρ = 0.41), which are found in volcanic ashes. High groundwater As concentrations in some semiarid oxidizing systems are related to high evaporation. Although correlation of As with TDS in the SHP aquifer may suggest evaporative concentration, unenriched stable isotopes (δ²H: −65 to −27; δ¹⁸O: −9.1 to −4.2) in the SHP aquifer do not support evaporation. High TDS in the SHP aquifer is most likely related to upward movement of saline water from the underlying Triassic Dockum aquifer. Mobilization of As in other semiarid oxidizing systems is caused by increased pH; however, pH in the SHP aquifer is near neutral (10–90 percentiles, 7.0–7.6). Although many processes, such as competitive desorption with SiO₂, VO₄, or PO₄, could be responsible for local mobilization of As in the SHP aquifer, the most plausible explanation for the regional As distribution and correlation with TDS is the counterion effect caused by a change from Ca- to Na-rich, water as shown by the high correlation between As and Na/(Ca)^0.5 ratios (ρ = 0.57). This change in chemistry is related to mixing with saline water that moves upward from the underlying Dockum aquifer. This counterion effect may mobilize other anions and oxyanion-forming elements that are correlated with As (F, V, Se, B, Mo and SiO₂). Competition among the oxyanions for sorption sites may enhance As mobilization. The SHP case study has similar As sources to those of other semiarid, oxidizing systems (original volcanic ash source followed by sorption onto hydrous metal oxides) but contrasts with these systems by showing lack of evaporative concentration and pH mobilization of As but counterion mobilization of As instead in the SHP-S aquifer.     

Geochemistry,radiocarbon ages,and paleorecharge conditions along a transect in the central High Plains aquifer,southwestern Kansas,USA

Water samples from short-screen monitoring wells installed along a 90-km transect in southwestern Kansas were analyzed for major ions, trace elements, isotopes (H, B, C, N, O, S, Sr), and dissolved gases (He, Ne, N₂, Ar, O₂, CH₄) to evaluate the geochemistry, radiocarbon ages, and paleorecharge conditions in the unconfined central High Plains aquifer. The primary reactions controlling water chemistry were dedolomitization, cation exchange, feldspar weathering, and O₂ reduction and denitrification. Radiocarbon ages adjusted for C mass transfers ranged from <2.6 ka (¹⁴C) B.P. near the water table to 12.8 ± 0.9 ka (¹⁴C) B.P. at the base of the aquifer, indicating the unconfined central High Plains aquifer contained a stratified sequence of ground water spanning Holocene time. A cross-sectional model of steady-state ground-water flow, calibrated using radiocarbon ages, is consistent with recharge rates ranging from 0.8 mm/a in areas overlain by loess to 8 mm/a in areas overlain by dune sand. Paleorecharge temperatures ranged from an average of 15.2 ± 0.7 °C for the most recently recharged waters to 11.6 ± 0.4 °C for the oldest waters. The temperature difference between Early and Late Holocene recharge was estimated to be 2.4 ± 0.7 °C, after taking into account variable recharge elevations. Nitrogen isotope data indicate NO₃ in paleorecharge (average concentration=193 μM) was derived from a relatively uniform source such as soil N, whereas NO₃ in recent recharge (average concentration=885 μM) contained N from varying proportions of fertilizer, manure, and soil N. Deep water samples contained components of N₂ derived from atmospheric, denitrification, and deep natural gas sources. Denitrification rates in the aquifer were slow (5 ± 2× 10⁻³ μmol N L⁻¹ a⁻¹), indicating this process would require >10 ka to reduce the average NO₃ concentration in recent recharge to the Holocene background concentration.     

Single-grain OSL dating of sediments from the Southern High Plains,USA

Single-grain OSL dating is applied to sediments from different depositional settings on the Southern High Plains of western Texas and eastern New Mexico. Criteria of acceptance are used to screen equivalent doses from individual grains and resulting distributions are evaluated in terms of normality. Wide variation is found in proportion of acceptable grains and in the distributions. While some of the latter are normal, many show broadening that may be the result of mixing of different-aged grains and skewness that may result from variant depositional and post-depositional modes. Geological modeling will be required to understand better these distributions, although for most samples means produced ages that agree with independent evidence. The resolution possible with single grains is necessary for best estimates of equivalent dose for other samples. A few samples do not agree with independent evidence, even with a normal distribution of equivalent dose. Sand dunes seem the most difficult to date accurately, probably because of mixing.     

Groundwater recharge and chemical evolution in the southern High Plains of Texas, USA

The unconfined High Plains (Ogallala) aquifer is the largest aquifer in the USA and the primary water supply for the semiarid southern High Plains of Texas and New Mexico. Analyses of water and soils northeast of Amarillo, Texas, together with data from other regional studies, indicate that processes during recharge control the composition of unconfined groundwater in the northern half of the southern High Plains. Solute and isotopic data are consistent with a sequence of episodic precipitation, concentration of solutes in upland soils by evapotranspiration, runoff, and infiltration beneath playas and ditches (modified locally by return flow of wastewater and irrigation tailwater). Plausible reactions during recharge include oxidation of organic matter, dissolution and exsolution of CO₂, dissolution of CaCO₃, silicate weathering, and cation exchange. Si and ¹⁴C data suggest leakage from perched aquifers to the High Plains aquifer. Plausible mass-balance models for the High Plains aquifer include scenarios of flow with leakage but not reactions, flow with reactions but not leakage, and flow with neither reactions nor leakage. Mechanisms of recharge and chemical evolution delineated in this study agree with those noted for other aquifers in the south-central and southwestern USA. Electronic Publication     

Lubbock Lake: Late Quaternary cultural and environmental change on the Southern High Plains,USA

Lubbock Lake (Southern High Plains of Texas) contains a cultural, faunal, and floral record within a virtually complete geological record spanning the past 11 100+ years. More than 88 archaeological occurrences have been excavated from five major stratigraphic units. The Paleoindian record (11 500–6500yr BP) begins with Clovis-age occupation (ca. 11 100yr BP) found within fluvial deposits (stratum 1). Subsequent Paleoindian occupations are found in lake and marsh sediments (stratum 2). Archaic occupations (8500-2000yr BP) are contained within aeolian and marsh deposits (strata 3 and 4). Ceramic occupations (2000-500yr BP) are found on a soil developed in stratum 4, in marsh sediments (strata 4 and 5), and in slopewash and aeolian sediments (stratum 5). The Protohistoric (500-300yr BP) and Historic (300-100yr BP) remains are in slopewash, aeolian, and marsh sediments (stratum 5) and associated soils. The Southern High Plains remained a grasslands throughout the last 11 500 years and neither man nor bison abandoned the region. The successive local faunas reflect changing ecosystems under pluvial to arid to more mesic to semiarid conditions. The occupation of Lubbock Lake through time appears to have been by small groups of people for both economic and short-term residential uses. These hunter-gatherer peoples underwent adaptive change brought about by climatic stress and alterations to food resources.     

Strontium isotope geochemistry of groundwater in the central part of the Dakota (Great Plains) aquifer,USA

The Dakota aquifer of the central and eastern Great Plains of the United States is an important source of water for municipal supplies, irrigation and industrial use. Although the regional flow system can be characterized generally as east to northeasterly from the Rocky Mountains towards the Missouri River, locally the flow systems are hydrologically complex. This study uses Sr isotopic data from groundwater and leached aquifer samples to document the complex subsystems within the Dakota aquifer in Nebraska and Kansas. The interaction of groundwater with the geologic material through which it flows has created spatial patterns in the isotopic measurements that are related to: long-term water–rock interaction, during which varying degrees of isotopic equilibrium between water and rock has been achieved; and the alteration of NaCl fluids by water-rock interaction. Specifically, Sr isotopic data distinguish brines from Kansas and western Nebraska from those in eastern Nebraska: the former are interpreted to reflect interaction with Permian rocks, whereas the latter record interaction with Pennsylvanian rocks. The Sr isotopic composition of groundwater from other parts of Nebraska and Kansas are a function of the dynamic interaction between groundwater and unlithified sediments (e.g., glacial till and loess), followed by interaction with oxidized and unoxidized sediments within the Dakota Formation. This study illustrates the power of combining Sr chemistry with more conventional geochemical data to obtain a more complete understanding of groundwater flow systems within regional aquifer systems where extensive monitoring networks do not exist.     

Late Quaternary sedimentology and geochronology of small playas on the Southern High Plains, Texas and New Mexico, U.S.A.

Playas are small, circular basins forming a ubiquitous component of the southern High Plains landscape. They are filled with carbonaceous mud deposited since the terminal Pleistocene. The stratigraphy and geochronology of 30 playas was investigated to better understand the paleoenvironmental record of basin filling. At the base of the fill in some playas is a well sorted eolian sand dated between ~ 13,000 and ~ 11,000 ¹⁴C yr BP. The beginning of mud deposition, representing aggradation of eolian dust on a moist, vegetated playa floor was largely between ~ 12,000 and ~ 10,500 ¹⁴C yr BP. Playa filling slowed ~ 9000 to ~ 4000 ¹⁴C yr BP, probably due to dry conditions, increased ~ 4000 to ~ 2000 ¹⁴C yr BP, then slowed again. Eolian sand and loam, likely representing regional aridity, accumulated in some basins episodically just prior to ~ 10,700 ¹⁴C yr BP, between ~ 8600 and ~ 4700 ¹⁴C yr BP, and at ~ 1300 ¹⁴C yr BP. Stable C isotopes from one basin indicate that the playa was inundated only seasonally throughout the record beginning ~ 11,500 ¹⁴C yr BP. The phytolith record in that basin indicates an abrupt shift toward cooling ~ 11,400 to ~ 11,200 ¹⁴C yr BP and then increasing importance of xeric-adapted C₄ grasses through the Holocene.     

Non-linear connections between dune activity and climate in the High Plains, Kansas and Oklahoma, USA

Discrete dune fields are found throughout much of the Great Plains of North America, and the timing of past dune activity is often used as a proxy for paleoclimate because of the intuitive link between dune activity and a more arid climate. This research suggests that feedbacks in the soil-geomorphic system create a relationship between dune activity and climate that varies both spatially and temporally. Older eolian landforms are more resistant to activation because of the long-term accumulation of finer soil particles in a Bt horizon which retain moisture and anchor the deposit even during more arid times. Conversely, younger deposits lack these fines and are more easily reactivated. This spatially variable relationship is supported by soil stratigraphy, particle size analysis, and optical age control. Additionally, the water retention of the Bt horizons is quantifiably greater than that of the soils found in the younger dunes of the area. This complication in the relationship between eolian activity and climate is important because it suggests that caution is needed when using past dune activity as the lone proxy for paleoclimate.     

Lithium and strontium isotopic systematics in playas in Nevada,USA: constraints on the origin of lithium

Lithium-rich brine in playas is a major raw material for lithium production. Recently, lithium isotopic ratios (δ⁷Li) have been identified as a tool for investigating water–rock interactions. Thus, to constrain the origin of lithium in playas by the use of its isotopes, we conducted leaching experiments on various lacustrine sediment and evaporite deposit samples collected from playas in Nevada, USA. We determined lithium and strontium isotopic ratios and contents and trace element contents of the leachate, estimated the initial δ⁷Li values in the water flowing into the playas, and examined the origin of lithium in playas by comparison with δ⁷Li values of the possible sources. In samples from the playas, δ⁷Li values show some variation, reflecting differences both in isotopic fractionation during mineral formation and in initial δ⁷Li value in water flowing into each playa. However, all δ⁷Li values in this study are much lower than those in river water and groundwater samples from around the world, but they are close to those of volcanic rocks. Considering the temperature dependence of lithium isotopic fractionation between solid and fluid, these results indicate that the lithium concentrated in playas in Nevada was supplied mainly through high-temperature water–rock interaction associated with local hydrothermal activity and not directly by low-temperature weathering of surface materials. This study, which is the first to report lithium isotopic compositions in playas, demonstrates that δ⁷Li may be a useful tracer for determining the origin of lithium and evaluating its accumulation processes in playas.     

Redox-driven changes in porewater chemistry in the unsaturated zone of the chalk aquifer beneath unlined cattle slurry lagoons

Farm waste stores such as cattle slurry lagoons are widespread in the UK and many overly important aquifers. Stores can be serious risks to water quality because they are important sources of N species, organic C and pathogenic microbes. At two sites on the Chalk aquifer of southern England, inclined boreholes were drilled and cored to obtain aquifer material from directly beneath unlined slurry stores. Vertical boreholes were also drilled adjacent to the slurry stores to determine any lateral movement of contaminants. Interstitial porewaters were analysed for major and minor ions and S isotopes. At the second site, unsaturated zone gases were sampled from the inclined hole. Infiltration of slurry into the unsaturated zone caused significantly elevated concentrations of metals such as Cu and Ni at both sites. Sulphate reduction was occurring at Site 1, as evidenced by SO₄ concentrations decreasing from 150 to 50 mg/l and enhanced ratios of δ³⁴S–SO₄ and δ¹⁸O–SO₄. Ammonium-N also leaches along with dissolved organic C which were found 17 m below ground surface at concentrations up to 400 and 260 mg/l, respectively. Contaminant concentrations were similar in the porewaters from both the inclined and vertical boreholes. At Site 2, higher contaminant concentrations were found in the inclined borehole compared with the vertical borehole. Organic C concentrations were considerably lower than at Site 1, ranging from 10 to 70 mg/l. Ammonium–N concentrations reached a maximum concentration of 25 mg/l, however NO₃-N concentrations were up to 500 mg/l and SO₄ concentrations were generally higher than Site 1. Data for N₂/Ar and δ¹⁵N–N₂ from the gas samplers show a peak of 102 and 2.2‰, respectively, at 14 m below ground level indicating denitrification was taking place. Evidence from δ³⁴S–SO₄ and δ¹⁸O–SO₄ suggest that some SO₄ reduction was taking place simultaneously. From CH₄ and NH₃ detected at depth it is suggested that slurry contamination, emanating from early use of the store, has passed through the top 18 m of the unsaturated zone at Site 2. The presence of high concentrations of NO₃ and lower concentrations of organic C suggests that this lagoon has formed a relatively impermeable seal at its base within the first few years of its lifetime. The anoxic conditions at both sites may have mobilised U from N–P–K fertilisers. Both sites are continuing to impact on the porewater chemistry and pose a risk of groundwater contamination.     

Fossil flat-slab subduction beneath the Illinois basin, USA

The Illinois basin is one of several well-studied intracratonic sedimentary basins within the North American craton whose formational mechanisms and subcrustal structure are not well understood. We study the S-velocity structure of the upper mantle beneath the Illinois basin and its surrounding area through seismic tomography. We utilize continental scale waveform data of seismic S and surface waves, enhanced by regional earthquakes located near the Illinois basin. Our 3D tomographic model, IL05, confirms the existence of a slow S-velocity structure in the uppermost mantle beneath the Illinois basin region. This anomalously slow region exists from the base of the crust to depths of  90 km, and is slower than the North American cratonic average by about 200 m/s. This anomalous uppermost mantle beneath the Illinois basin is underlain by a faster lithosphere, typical of the surrounding craton, to depths of  200 km. Excluding the formation of the Reelfoot Rift, this area of North American has been stable for over 1.0 Gy. Thus, we do not expect thermal anomalies from before that time to persist into present day S-velocity anomalies and we consider a delamination origin as an explanation of Illinois basin subsidence unlikely. We cannot rule out that the slow mid-lithosphere beneath the Illinois basin is caused by an uppermost mantle enriched by a deep, but weak plume. We attribute the slow mid-lithosphere to the presence of either oceanic, hydrous crust, or, a relatively cool mantle wedge with preserved hydrous minerals in the Illinois basin's uppermost mantle, related to a fossilized flat subduction zone.     

Recharge and sustainability of a coastal aquifer in northern Albania

The River Mati in Albania has formed a coastal plain with Holocene and Pleistocene sediments. The outer portion of the plain is clay, with three underlying aquifers that are connected to an alluvial fan at the entry of the river into the plain. The aquifers supply water for 240,000 people. Close to the sea the aquifers are brackish. The brackish water is often artesian and found to be thousands of years old. Furthermore, the salinity, supported by δ¹⁸O results, does not seem to be due to mixing with old seawater but due to diffusion from intercalated clay layers. Heavy metals from mines in the upstream section of River Mati are not an immediate threat, as the pH buffering of the river water is good. Moreover, the heavy metals are predominantly found in suspended and colloidal phases. Two sulphur isotope signatures, one mirroring seawater sulphate in the brackish groundwater (δ³⁴S >21 ‰) and one showing the influence of sulphide in the river and the fresh groundwater (δ³⁴S <10 ‰), indicate that the groundwater in the largest well field is recharged from the river. The most serious threat is gravel extraction in the alluvial fan, decreasing the hydraulic head necessary for recharge and causing clogging of sediments.     

Recharge rate estimation in the Mountain karst aquifer system of Figeh spring, Syria

Figeh watershed spring is one of the important groundwater aquifer, which is considered a major source for drinking waters of Damascus city and countryside. The origin identification and recharge estimates of groundwater are significant components of sustainable groundwater development in this Mountain karst aquifer of Figeh spring. During the period 2001–2009, monthly groundwater and precipitation samples were taken and the isotopic compositions of δ¹⁸O, δ²H, and chloride contents were analyzed to identify groundwater origins and to estimate recharge rates. The δ¹⁸O, δ²H of the groundwater show that the groundwater recharge is of meteoric origin. The chloride mass balance (CMB) method was used to quantify recharge rates of groundwater in the Mountain karst aquifer of Figeh spring. The recharge rate varies from 192 to 826 mm/year, which corresponds to 43 and 67% of the total annual rainfall. Recharge rates estimated by CMB were compared with values obtained from other methods and were found to be in good agreement. This study can be used to develop effective programs for groundwater management and development.     

Nutrient chemistry and salinity mapping of the Delhi aquifer,India: source identification perspective

Present study is an effort to distinguish between the contributions of natural weathering and anthropogenic inputs towards high salinity and nutrient concentrations in the groundwater of National Capital Territory (NCT) Delhi, India. Apart from the source identification, the aquifer of entire territory has been characterized and mapped on the basis of salinity in space and water suitability with its depth. Major element chemistry, conventional graphical plots and specific ionic ratio of Na⁺/Cl⁻, SO₄ ²⁻/Cl⁻, Mg²⁺/Ca²⁺ and Ca²⁺/(HCO₃ ⁻ + SO₄ ²⁻) are conjointly used to distinguish different salinization sources. Results suggest that leaching from the various unlined landfill sites and drains is the prime cause of NO₃ ⁻ contamination while study area is highly affected with inland salinity which is geogenic in origin. The seasonal water level fluctuation and rising water level increases nutrients concentration in groundwater. Mixing with old saline sub-surface groundwater and dissolution of surface salts in the salt affected soil areas were identified as the principle processes controlling groundwater salinity through comparison of ionic ratio. Only minor increase of salinity is the result of evaporation effect and pollution inflows. The entire territory has characterized into four groups as fresh, freshening, near freshening and saline with respect to salinity in groundwater. The salinity mapping suggests that in general, for drinking needs, groundwater in the fresh, freshening and near freshening zone is suitable up to a depth of 45, 20 and 12 m, respectively, while the saline zones are unsuitable for any domestic use. In the consideration of increasing demand of drinking water in the area; present study is vital and recommends further isotopic investigations and highlights the need of immediate management action for landfill sites and unlined drains.