Morphology and growth of aragonite crystals in hot-spring travertines at Lake Bogoria, Kenya Rift Valley

Pseudohexagonal aragonite crystals are common components in some hot-spring travertines at Chemurkeu on the western shore of Lake Bogoria, Kenya. Beds, lenses and pods of aragonite crystals are intercalated with beds of white non-crystallographic calcite dendrites. The pseudohexagonal aragonite crystals, which are up to 4 cm long and 4 mm wide, are formed of nested skeletal crystals. Each skeletal crystal is formed of cyclical twinned crystals that are constructed of stacked subcrystals. The latter are inclined at a consistent angle of 40° to the long axis of the pseudohexagonal aragonite crystal. Intense competition for space during growth modified the crystal morphology with the result that many of the pseudohexagonal crystals are distorted. Intercrystalline and intracrystalline pores are filled or partly filled by epitaxial aragonite overgrowths and/or reticulate microbial coatings that have a high concentration of Si and Mg. In places, this extracellular mucus induced etching of the underlying aragonite crystal. Today the hot (T>95 °C) Na-HCO₃-Cl spring waters at Chemurkeu have a salinity of 5–6 g L^?1 TDS, a pH of 8·1–9·1, Ca²⁺ concentrations of <2 mg L^?1 and Mg²⁺ concentrations of <0·7 mg L^?1, The springs of the Lake Bogoria Geothermal Field are fed by a shallow aquifer (T～100 °C) and a deeper aquifer (T～170 °C). Springs at Chemurkeu derive from meteoric groundwater, lake water and condensed steam, and are fed mainly from the shallow thermal aquifer. Much of the aragonite may have formed when the spring waters contained more dissolved Ca²⁺ than today, possibly under more humid conditions during the Holocene.     

Radiocarbon age distribution of groundwater in the Konya Closed Basin, central Anatolia, Turkey

Annual abstraction of 2.6?×?10⁹ m³ of groundwater in the 53,000 km² Konya Closed Basin of central Turkey has caused a head decline of 1 m/year over the last few decades. Therefore, understanding the hydrogeology of this large endorheic basin, in a semi-arid climate, is important to sustainable resource management. For this purpose, the groundwater’s radiocarbon age distribution has been investigated along a 150-km transect parallel to regional flow. Results show that the groundwater ranges in age from Recent at the main recharge area of the Taurus Mountains in the south, to about 40,000 years around the terminal Salt Lake located in the north. In this predominantly confined flow system, radiocarbon ages increase linearly by distance from the main recharge area and are in agreement with the hydraulic ages. The mean velocity of regional groundwater flow (3 m/year) is determined by the rate of regional groundwater discharge into the Salt Lake. Calcite dissolution, dedolomitization and geogenic carbon dioxide influx appear to be the dominant geochemical processes that determine the carbon isotope composition along the regional flow path. The groundwater’s oxygen-18 content indicates more humid and cooler paleorecharge. A maximum drop of 5°C is inferred for the past recharge temperature.     

Identification of He sources and estimation of He ages in groundwater of the North China Plain

Dissolved helium concentrations and ³He/⁴He ratios were measured for 18 groundwater samples collected from the Quaternary confined aquifers in the North China Plain (NCP). The dissolved helium concentrations ranged from 1 × 10⁻⁷ to 1 × 10⁻⁶ cm³STP·g⁻¹ in the 14 samples from the central plain, but was approximately two orders of magnitude higher, between 6 × 10⁻⁶ and 9 × 10⁻⁵ cm³STP·g⁻¹, in 4 samples from the coastal plain. Based on these concentrations and the corresponding ³He/⁴He ratios varying from 0.09 to 0.55 R_a (where R_a is the ³He/⁴He ratio of air), the dissolved helium in groundwater in the central plain was identified to be primarily a mixture of atmospheric helium with radiogenic helium and a representative radiogenic helium ratio was estimated to be 0.035 R_a. Despite the high fraction of terrigenic ⁴He in the samples from the coastal plain, their ³He/⁴He ratios were found to be significantly above this radiogenic value, ranging between 0.20 and 0.37 R_a, indicating the presence of a mantle-derived He component in this area. About 2–4% mantle helium was estimated to be present in the groundwater of the coastal plain, which probably is associated with the regional Cangdong fault and tectonic activities. Based on the radiogenic He component, ⁴He ages of the groundwater in the central plain were calculated by assuming either pure in situ production or an external helium flux J₀ of 4.7 × 10⁻⁸ cm³STPcm⁻²a⁻¹. The estimated ⁴He ages fall between 9.5 and 51.4 ka and are comparable to the ¹⁴C ages, suggesting that the results of ⁴He dating are reasonable and can be an effective tool to estimate groundwater residence times under suitable conditions.     

Determining the groundwater basin and surface watershed boundary of Dalinuoer Lake in the middle of Inner Mongolian Plateau,China and its impacts on the ecological environment

The surface watershed and groundwater basin have fixed recharge scale, which are not only the basic unit for hydrologic cycle research but also control the water resources formation and evolution and its corresponding eco-geological environment pattern. To accurately identify the boundary of the surface watershed and groundwater basin is the basis for properly understanding hydrologic cycle and conducting the water balance analysis at watershed scale in complicated geologic structure area, especially when the boundary are inconsistent. In this study, the Dalinuoer Lake located in the middle of the Inner Mongolian Plateau which has complicated geologic structure was selected as the representative case. Based on the multidisciplinary comprehensive analysis of topography, tectonics, hydrogeology, groundwater dynamics and stable isotopes, the results suggest the following: (1) The surface watershed ridge and groundwater basin divide of Dalinuoer Lake are inconsistent. The surface watershed was divided into two separate groundwater systems almost having no groundwater exchange by the SW-NE Haoluku Anticlinorium Fault which has obvious water-blocking effect. The surface drainage area of Dalinuoer Lake is 6139 km². The northern regional A is the Dalinuoer Lake groundwater system with an area of 4838 km², and the southern regional B is the Xilamulun Riverhead groundwater system with an area of 1301 km². (2) The groundwater in the southern of regional A and the spring-feeding river are the important recharge sources for the Dalinuoer Lake, and it has greater recharge effects than the northern Gonggeer River system. (3) It is speculated that the trend of Haoluku Anticlinorium Fault is the boundary of the westerlies and the East Asian summer Monsoon (EASM) climate systems, which further pinpoints the predecessor’s understanding of this boundary line. At present, the Dalinuoer Lake watershed is proved to have gone through a prominent warming-drying trend periods, which leads to the precipitation reduction, temperature rise, human activities water usage increasement. So the hydrological cycle and lake eco-environment at watershed scale will still bound to be change, which may pose the potential deterioration risk on the suitability of fish habitat. The results can provide basic support for better understanding water balance evolution and lake area shrinkage cause as well as the ecological protection and restoration implementation of Dalinuoer Lake watershed.© 2021 China Geology Editorial Office.     

Study on the dynamic characteristics of groundwater in the valley plain of Lhasa City

The valley plain of Lhasa City is located on the Qinghai-Tibetan Plateau, which is one of the most developed and densely populated areas in Tibet. Groundwater is an important water supply source and plays an irreplaceable role in the social and economic development of Lhasa City. This study has investigated the dynamic characteristics of groundwater in the valley plain of Lhasa City through the methods of mathematical statistics and hydrochemical analysis. The results showed that local topography, climate, and urbanization substantially influenced the groundwater dynamics. Under the combined influences from urbanization and climate, the groundwater level decreased over three time periods, but the groundwater-level configuration has not shown significant changes in over 15 years. From 1997 to 2015, the hydrochemical type of groundwater has changed from HCO₃–Ca to HCO₃·SO₄–Ca·Mg and HCO₃·SO₄–Ca. The concentrations of Cl^?, Mg²⁺, and SO₄^2? in groundwater increased, but the concentrations of other ions were relatively stable. Water–rock interaction was the main mechanism controlling the groundwater chemistry in the study area, and it was mainly associated with the dissolution of silicate, carbonate, and halite.     

安徽马鞍山市当涂地区地下水水化学特征及演化机制

【研究目的】 了解长江中下游平原地区地下水流系统并深入分析其地下水水化学特征及其演化机制。【研究方法】 综合马鞍山市当涂地区的水文地质条件、水动力场等,基于研究区水化学基本特征,运用多元统计分析、水化学图件、离子比值和反向水文地球化学模拟等方法对该地区浅层地下水水化学演化进行分析。【研究结果】 结果表明：（1）研究区地下水主要为低矿化度偏碱性水,地下水组分中阳离子以Ca²⁺和Mg²⁺为主,阴离子以HCO₃^-和SO₄^2-为主。（2）研究区地下水水化学类型主要可分为7类,其中松散岩类孔隙含水岩组和碎屑岩类孔隙裂隙含水岩组的水化学类型主要为HCO₃^-Ca型、HCO₃^-Ca·Na型、HCO₃·Cl-Ca·Na型以及HCO₃^-Ca·Mg型;基岩类裂隙含水岩组的化学类型主要为HCO₃·SO₄-Ca·Mg型和SO₄·HCO₃^-Ca·Mg型。（3）研究区浅层地下水水样超标率为46%,总体水质较差,超标率较高的组分依次为Mn、高锰酸盐指数（COD_Mn）、硝酸盐（以N计）、Fe、As、氨氮（以N计）等。（4）研究区地下水的化学组分主要受到岩石风化作用的控制;此外,还存在Na-Ca的正向阳离子交替吸附作用。反向水文地球化学模拟结果进一步定量论证了水岩相互作用对本区浅层地下水组分的形成和演化起着主导作用。【结论】 研究区地下水主要为低矿化度偏碱性,主要可分为松散岩类孔隙水、碎屑岩类孔隙裂隙水和基岩类裂隙水。主要离子比例和反向水文地球化学模拟揭示了本区浅层地下水化学组分主要是地下水溶滤方解石、白云石等碳酸盐矿物、石英、长石等硅酸盐矿物,高岭土等黏土矿物以及岩盐、石膏等达到过饱和之后形成的。     

Hydrogeochemistry and groundwater origin of the Basses-Laurentides sedimentary rock aquifer system, St. Lawrence Lowlands, Québec, Canada

A comprehensive hydrogeochemical study was carried out in the Paleozoic Basses-Laurentides sedimentary rock aquifer system in Québec over a 1500 km² study area. Groundwater samples were collected at 153 sites, characterizing all geological and hydrogeological units to a maximum depth of 140 m. Groundwater was analyzed for major, minor and trace inorganic constituents, stable isotopes δ ²H, δ ¹⁸O, and δ ¹³C of dissolved inorganic carbon (DIC), and some samples were analyzed for ³H, and ¹⁴C of DIC. The regional distribution of groundwater types shows that the hydrogeological conditions exert a dominant control on the major ions chemistry of groundwater. Preferential recharge areas are characterized by tritiated Ca-Mg-HCO₃ groundwater, and confined conditions by submodern Na-HCO₃ and Na-Cl groundwater types. Two groundwater end-members are identified in the aquifer system, modern meteoric water and Pleistocene Champlain Sea water. The region displays significant variations of groundwater geochemistry and quality controlled by glaciation, Champlain Sea invasion, lithological rock diversity, and flow system scales. This situation leads to varied groundwater types and origins within a restricted area.     

Impact of lake‐level changes on the formation of thermogene travertine in continental rifts: Evidence from Lake Bogoria,Kenya Rift Valley

Travertine is present at 20% of the ca 60 hot springs that discharge on Loburu delta plain on the western margin of saline, alkaline Lake Bogoria in the Kenya Rift. Much of the travertine, which forms mounds, low terraces and pool‐rim dams, is sub‐fossil (relict) and undergoing erosion, but calcite‐encrusted artefacts show that carbonate is actively precipitating at several springs. Most of the springs discharge alkaline (pH: 8·3 to 8·9), Na‐HCO₃ waters containing little Ca (<2 mg l^?1) at temperatures of 94 to 97·5°C. These travertines are unusual because most probably precipitated at temperatures of >80°C. The travertines are composed mainly of dendritic and platy calcite, with minor Mg‐silicates, aragonite, fluorite and opaline silica. Calcite precipitation is attributed mainly to rapid CO₂ degassing, which led to high‐disequilibrium crystal morphologies. Stratigraphic evidence shows that the travertine formed during several stages separated by intervals of non‐deposition. Radiometric ages imply that the main phase of travertine formation occurred during the late Pleistocene (ca 32 to 35 ka). Periods of precipitation were influenced strongly by fluctuations in lake level, mostly under climate control, and by related changes in the depth of boiling. During relatively arid phases, meteoric recharge of ground water declines, the lake is low and becomes hypersaline, and the reduced hydrostatic pressure lowers the level of boiling in the plumbing system of the hot springs. Any carbonate precipitation then occurs below the land surface. During humid phases, the dilute meteoric recharge increases, enhancing geothermal circulation, but the rising lake waters, which become relatively dilute, flood most spring vents. Much of the aqueous Ca²⁺ then precipitates as lacustrine stromatolites on shallow firm substrates, including submerged older travertines. Optimal conditions for subaerial travertine precipitation at Loburu occur when the lake is at intermediate levels, and may be favoured during transitions from humid to drier conditions.     

Linking groundwater quality to residence times and regional geology in the St. Lawrence Lowlands,southern Quebec,Canada

The assessment of groundwater quality in shallow aquifers is of high societal relevance given that large populations depend directly on these water resources. The purpose of this study was to establish links between groundwater quality, groundwater residence times, and regional geology in the St. Lawrence Lowlands fractured bedrock aquifer. The study focuses on a 4500 km² watershed located in the St. Lawrence Lowlands of the province of Quebec in eastern Canada. A total of 150 wells were sampled for major, minor, and trace ions. Tritium (³H) and its daughter element, ³He, as well as radiocarbon activity (A¹⁴C) were measured in a subset of wells to estimate groundwater residence times. Results show that groundwater evolves from a Ca–HCO₃ water type in recharge zones (i.e., the Appalachian piedmont) to a Na–HCO₃ water type downgradient, toward the St. Lawrence River. Locally, barium (Ba), fluoride (F), iron (Fe), and manganese (Mn) concentrations reach 90, 2, 18, and 5.9 mg/L respectively, all exceeding their respective Canadian drinking water limits of 1, 1.5, 0.3, and 0.05 mg/L. Release of these elements into groundwater is mainly controlled by the groundwater redox state and pH conditions, as well as by the geology and the duration of rock–water interactions. This evolution is accompanied by increasing ³H/³He ages, from 4.78 ± 0.44 years upgradient to more than 60 years downgradient. Discrepancies between calculated ³H/³He and ¹⁴C water ages (the latter ranging from 280 ± 56 to 17,050 ± 3410 years) suggest mixing between modern water and paleo-groundwater infiltrated through subglacial recharge when the Laurentide Ice Sheet covered the study area, and during the following deglaciation period. A linear relationship between ³H activity and corrected ¹⁴C versus Mg/Ca and Ba support a direct link between water residence time and the chemical evolution of these waters. The Ba, F, Fe, and Mn concentrations in groundwater originate from Paleozoic rocks from both the St. Lawrence Platform and the Appalachian Mountains. These elements have been brought to the surface by rising hydrothermal fluids along regional faults, and trapped in sediment during their deposition and diagenesis due to reactions with highly sulfurous and organic matter-rich water. Large-scale flow of meltwater during subglacial recharge and during the subsequent retreat of the Laurentide Ice Sheet might have contributed to the leaching of these deposits and their enrichment in the present aquifers. This study brings a new and original understanding of the St. Lawrence Lowlands groundwater system within the context of its geological evolution.     

Geochemistry of carbonate precipitation from the groundwater in a coastal region

Groundwater samples were collected from a coastal region of Andhra Pradesh to assess the possible conditions of the formation of carbonates. The area experiences a semi-arid climate and is underlain by khondalites, over which the Quaternary sediments occur. The study of the geochemistry of groundwater indicates that groundwater is mostly of fresh, with alkaline nature. The study further suggest that the breakdown of feldspars as kaolinite during rock-water interaction, releases Ca²⁺. Soils/weathered products contribute high CO₂ under the open system. The Ca²⁺ and CO₂ are added to the groundwater through the infiltrating recharge water. They subsequently precipitate as fine-grained carbonates in the weathering profile due to evapotranspiration under a freshwater environment.     

Application of hydrochemical signatures to delineating portable groundwater resources in Ordos Basin, China

The Ordos Basin of China encompasses Shaanxi, Gansu, and Shanxi provinces, Ningxia and Inner Mongolia autonomous regions. It lacks significant surface water resources. Among the water-bearing formations, the Luohe formation, with an area of 1.32×10⁵ km², is the most prospective aquifer. Groundwater quality data collected at 211 boreholes drilled into the Luhe formation indicate a complex distribution of groundwater chemistry. The hydrochemical properties were used to study the recharge, runoff, and discharge conditions of the groundwater in Ordos Basin and to evaluate sustainable groundwater resources. In the northern part of the basin, the hydrochemistry types and the total dissolved solids (TDS) show a clear lateral transition from SEE to NWW, indicating that the groundwater gets recharge in the northwest region and discharges in the southeast region. In the southern part of the basin, maximum TDS occurs at the center of the Malian River valley, from which the TDS decreases radially. Therefore, the groundwater in the southern basin gets recharge from the southeast and southwest regions, and the Malian River valley is the discharge zone. As a result of this research, the areas with portable groundwater were delineated. They include most of the southeast region of the Sishili Ridge, east of the Ziwu Mountain, and the southwest corner of the basin. The TDS of the groundwater in these regions is less than 1 g/l, and the hydrochemistry type is either HCO₃ or HCO₃·SO₄.     

Use of hydrogeochemistry and environmental isotopes for evaluation of groundwater in Qingshuihe Basin,northwestern China

Hydrogeochemistry and environmental isotope data were utilized to understand origin, geochemical evolution, hydraulic interconnection, and renewability of groundwater in Qingshuihe Basin, northwestern China. There are four types of groundwater: (1) shallow groundwater in the mountain front pluvial fans, originating from recent recharge by precipitation, (2) deep paleo-groundwater of the lower alluvial plains, which was formed long ago, (3) shallow groundwater in the lower alluvial plains, which has undergone evaporation during the recharge process, and (4) mixed groundwater (shallow and deep groundwater in the plain). The main water types are Na–HCO₃, which dominates type (1), and Na–SO₄, which dominates types (2) and (3). Geochemical evolution in the upper pluvial fans is mainly the result of CO₂ gas dissolution, silicates weathering and cation exchange; in the lower alluvial plains, it is related to mineral dissolution. The evaporative enrichment only produces significant salinity increases in the shallow groundwater of the lower alluvial plains. Shallow groundwater age in the upper plain is 10 years or so, showing a strong renewability. Deep groundwater ages in the lower plain are more than 200 years, showing poor renewability. In the exploitation areas, the renewability of groundwater evidently increases and the circulation period is 70–100 years.     

Groundwater recharge: an overview of processes and challenges

Since the mid-1980s, a relative explosion of groundwater-recharge studies has been reported in the literature. It is therefore relevant to assess what is now known and to offer further guidance to practitioners involved in water-resource development. The paper summarizes current understanding of recharge processes, identifies recurring recharge-evaluation problems, and reports on some recent advances in estimation techniques. Emphasis is accorded to (semi-)arid regions because the need for information is greatest in those areas – groundwater is often the only water source, is vulnerable to contamination, and is prone to depletion. Few studies deal explicitly with groundwater recharge in temperate and humid zones, because recharge is normally included in regional groundwater investigations as one component of the water balance. The resolution of regional water-balance studies in (semi-)arid areas is, in contrast, often too low to quantify the limited recharge component with sufficient precision. Despite the numerous studies, determination of recharge fluxes in (semi-)arid regions remains fraught with uncertainty. Multiple tracer approaches probably offer the best potential for reliable results in local studies that require 'at-point' information. However, many investigations indicate that these approaches are not straightforward, because in some cases preferential flow contributes as much as 90% of the estimated total recharge. Tracer results (e.g. Cl^–, ³H) must therefore be interpreted with care in areas with multi-modal flow in the vadose zone. Moreover, accurate estimation of total chloride deposition is essential, and tritium may be influenced by vapour transport at low flux rates. In addition, paleoclimatic and paleohydrological conditions may cause discrepancies between measured actual processes and calculated long-term averages. The frequently studied issues of localized recharge and spatial variability need not be a problem if concern is with regional estimates. The key for practitioners is the project objective, which dictates whether 'at-point' or area-/groundwater-based estimation methods are appropriate. Many indirect (wadi) recharge studies reported in the literature are site specific; the relationship between 'at point' hydraulic properties and channel-reach losses demands further investigation. Electronic Publication     

Hydrogeochemical characteristics of groundwater and pore-water and the paleoenvironmental evolution in the past 3.10 Ma in the Xiong’an New Area,North China

The groundwater level has been continuously decreasing due to climate change and long-time overexploitation in the Xiong’an New Area, North China, which caused the enhanced mixing of groundwater in different aquifers and significant changes in regional groundwater chemistry characteristics. In this study, groundwater and sediment pore-water in drilling cores obtained from a 600 m borehole were investigated to evaluate hydrogeochemical processes in shallow and deep aquifers and paleo-environmental evolution in the past ca. 3.10 Ma. Results showed that there was no obvious change overall in chemical composition along the direction of groundwater runoff, but different hydrochemical processes occurred in shallow and deep groundwater in the vertical direction. Shallow groundwater (< 150 m) in the Xiong’an New Area was characterized by high salinity (TDS > 1000 mg/L) and high concentrations of Mn and Fe, while deep groundwater had better water quality with lower salinity. The high TDS values mostly occurred in aquifers with depth < 70 m and >500 m below land surface. Water isotopes showed that aquifer pore-water mostly originated from meteoric water under the influence of evaporation, and aquitard pore-water belonged to Paleo meteoric water. In addition, the evolution of the paleoclimate since 3.10 Ma BP was reconstructed, and four climate periods were determined by the δ¹⁸O profiles of pore-water and sporopollen records from sediments at different depths. It can be inferred that the Quaternary Pleistocene (0.78–2.58 Ma BP) was dominated by the cold and dry climate of the glacial period, with three interglacial intervals of warm and humid climate. What’s more, this study demonstrates the possibilities of the applications of pore-water on the hydrogeochemical study and further supports the finding that pore-water could retain the feature of paleo-sedimentary water.© 2021 China Geology Editorial Office.     

Assessment of water quality and pollution of the Lake Seyfe basin, Kırşehir, Turkey

Lake Seyfe is located in a closed basin near K?r?ehir in the central Anatolian region, Turkey. The aim of this study is to evaluate the groundwater quality and effects of lithogenic contamination carried out in the Lake Seyfe basin, which is represented by various lithologies and groundwater types. Seyfe, Horla and Akp?nar springs are recharged through marbles at the western and southwestern of the basin are ultimately and discharged into the K?z?l?rmak Formation and Lake Seyfe. The waters of deep wells drilled into the marbles are of bicarbonate type (type I) in the Ca²⁺–Mg²⁺–HCO₃ ^? and Ca²⁺–HCO₃ ^? facies. Özlühüyük spring and waters from most of trenches and shallow wells, which are fed by the K?z?l?rmak Formation, have a mixed (type II) composition in the Ca²⁺–Mg²⁺–HCO₃ ^?–Cl^? facies. Groundwater in the alluvium and K?z?l?rmak Formation along Lake Seyfe has a salty (type III) character in the Na⁺–Cl^? facies. The main reasons of formation and change of the groundwater salinity and hydrochemical facies in the Seyfe basin are causing the various (a) lithogenic pollution and heterogeneity of the K?z?l?rmak Formation, (b) salinity of the upper soil zones, and (c) evaporation of the trench and channel waters open to the atmosphere. Considering parameters such as sodium hazard, specific conductivity, bicarbonate and carbonate hazards, waters in the study area are generally suitable for agricultural usage.     

Groundwater flow patterns in the San Luis Valley,Colorado, USA revisited: an evaluation of solute and isotopic data

Groundwater systems in the San Luis Valley, Colorado, USA have been re-evaluated by an analysis of solute and isotopic data. Existing stream, spring, and groundwater samples have been augmented with 154 solute and isotopic samples. Based on geochemical stratification, three groundwater regimes have been identified within 1,200 m of the surface: unconfined, upper active confined, and lower active confined with maximum TDS concentrations of 35,000, 3,500 and 600 mg/L, respectively. The elevated TDS of northern valley unconfined and upper active confined systems result from mineral dissolution, ion exchange and methanogenesis of organic and evaporate lake sediments deposited in an ancient lake, herein designated as Lake Sipapu. Chemical evolutions along flow paths were modeled with NETPATH. Groundwater ages, and δ¹³C, δ²H and δ¹⁸O compositions and distributions, suggest that mountain front recharge is the principle recharge mechanism for the upper and lower confined aquifers with travel times in the northern valley of more than 20,000 and 30,000 ¹⁴C years, respectively. Southern valley confined aquifer travel times are 5,000 ¹⁴C years or less. The unconfined aquifer contains appreciable modern recharge water and the contribution of confined aquifer water to the unconfined aquifer does not exceed 20%.     

Climatic and non-climatic effects on the δO and δC compositions of Lake Awassa, Ethiopia, during the last 6.5 ka

A comparison of a 6450 ¹⁴C yr δ¹⁸O and δ¹³C record of authigenic calcite from Lake Awassa, Ethiopia, with other proxy climate records in the area suggests that the lake records long-term regional climate changes. Co-varying and increasing δ¹⁸O and δ¹³C values from 4800 BP suggest an aridification of climate after the early Holocene insolation maximum. After 4000 BP, humid conditions return until after 2800 BP when δ¹⁸O increases again, reflecting more arid conditions recorded elsewhere in Ethiopia. In addition to these long-term changes, there are abrupt decreases in both δ¹⁸O_calcite and δ¹³C_calcite immediately after tephra layers. The likeliest explanation for these abrupt decreases in isotopes is the effect of tephra on the lake's catchment vegetation. δ¹⁸O, δ¹³C and lake-level measurements from Lake Awassa since the 1970s suggest that the lake is currently isotopically sensitive to short-term (annual–decadal) climate change. However, during this period, the catchment has undergone progressive deforestation that may have caused an increase in runoff. Caution is therefore required when reconstructing palaeoclimates as a contemporary lake may not always be a good analogue for lake hydrology in the past.     

The hydrogeology and hydrogeochemistry of the Barwon Downs Graben aquifer, southwestern Victoria, Australia

The Barwon Downs Graben lies on the northern flanks of the Otway Ranges and is situated approximately 70 km southwest of Geelong, Victoria, Australia. The major lower Tertiary Barwon Downs Graben aquifer comprises highly permeable sands and gravels interbedded with clays and silts of the hydraulically interconnected Pebble Point, Dilwyn and Mepunga Formations. Groundwater flows east into the Barwon Downs Graben from the Barongarook High, and yields ¹⁴C ages up to ～20 ka implying that recharge rates are low and, consequently, that the resource could be impacted by overabstraction. The presence of three different lithological units has led to the development of localized flow systems that has resulted in a lack of regular spatial variations in groundwater chemistry. Stable isotopic data suggests that groundwater was recharged under similar climatic conditions as of today. The major ion chemistry of the freshest groundwater is dominated by Na and HCO₃ while higher TDS groundwater, from the confining Narrawaturk Marl, is dominated by Na and Cl. Cl/Br ratios are close to rainfall suggesting that halite dissolution is not the principle source of salts. An excess of Na relative to Cl in fresher groundwater suggests that feldspar dissolution has occurred, however, water–rock interaction is limited. The concentrations of Ca, Mg, and SO₄ are controlled by silicate dissolution and ion-exchange reactions with clays.     

Hydrogeochemical and isotopic evidences of the groundwater regime in Datong Basin, Northern China

Datong Basin is one of the Cenozoic faulted basins in Northern China’s Shanxi province, where groundwater is the major source of water supply. The results of hydrochemical investigation show that along the groundwater flow path, from the margins to the lower-lying central parts of the basin, groundwater generally shows increases in concentrations of TDS, HCO₃ ^?, SO₄ ^2?, Cl^?, Na⁺ and Mg²⁺ (except for Ca²⁺ content). Along the basin margin, groundwater is dominantly of Ca–HCO₃ type; however, in the central parts of the basin it becomes more saline with Na–HCO₃-dominant or mixed-ion type. The medium-deep groundwater has chemical compositions similar to those of shallow groundwater, except for the local area affected by human activity. From the mountain front to the basin area, shallow groundwater concentrations of major ions increase and are commonly higher than those in medium-deep aquifers, due to intense evapotranspiration and anthropogenic contamination. Hydrolysis of aluminosilicate and silicate minerals, cation exchange and evaporation are prevailing geochemical processes occurring in the aquifers at Datong Basin. The isotopic compositions indicate that meteoric water is the main source of groundwater recharge. Evaporation is the major way of discharge of shallow groundwater. The groundwater in medium-deep aquifers may be related to regional recharges of rainwater by infiltrating along the mountain front faults, and of groundwater permeating laterally from bedrocks of the mountain range. However, in areas of groundwater depression cones, groundwater in the deep confined aquifers may be recharged by groundwater from the upper unconfined aquifer through aquitards.     

The impact of human activities in the Malia coastal area (Crete) on groundwater quality

The evaluation of the long-term effects of seawater intrusion into the aquifers due to negative water balance and nitrate pollution of drinking-water quality due to human activities requires detailed knowledge of both the transport of the chemical constituents and the geochemical processes within aquifers. Hydrogeological and hydrochemical studies in the unconfined aquifer of Malia have provided the necessary data to define the areas at increased risk from these phenomena. The solution of the second Fick's low under given boundary conditions gave an estimate of the propagation of groundwater pollution by NO₃ ^–. Additionally, in order to simulate the ion concentration changes during a period, for example a period of positive water balance or refreshening, groundwater transport and cation exchange reactions were modelled using the code PHREEQM. Received: 25 July 1997 · Accepted: 4 November 1997