Hydrogeological and thermal characterization of shallow aquifers in the plain sector of Piemonte region (NW Italy): implications for groundwater heat pumps diffusion

The low annual and seasonal variability of the shallow groundwater temperature in the alluvial plain aquifers of the Piemonte region (NW Italy) confirmed the potentiality of the low-enthalpy open-loop groundwater heat pumps (GWHP) diffusion to contribute to the reduction of regional greenhouse gas emissions. The distribution of mean groundwater temperatures ranged from a minimum of 10.3°C to a maximum of 17.9°C with a mean of 14.0°C. Differences among diverse areas were slight according with the modest variations in the general climatic condition. Like the air, temperature distribution of the shallow groundwater temperatures is generally similar to topographic elevations in reverse manner. Higher temperature values recorded were typical of summer months (June, July). On the opposite lower values were measured in January and February. No significant difference phase (time) difference between air and groundwater temperature appeared in the data analysis. Besides air-temperature influence (seasonal variability) seemed strictly connected to the depth to groundwater in the measure point and it was negligible when the value was over 9.5 m. For the application of the open-loop systems, extensive examinations of the hydrogeological local conditions should be conducted at site scale and groundwater heat transport modelling should be developed.     

The application of isotope and chemical analyses in managing transboundary groundwater resources

Managing transboundary groundwater resources requires accurate and detailed knowledge of aquifers and groundwater bodies. The Pannonian Basin is the largest intracontinental basin in Europe with a continuous succession of more than 7 km of Miocene to Quaternary sediments and with an average geothermal gradient of about 5 °C/100 m. Geographically the Pannonian basin overlaps eight countries (Hungary, Romania, Serbia, Croatia, Slovenia, Austria, Slovakia and Ukraine), so the issue of transboundary cold and thermal water resources is regionally very important. The T-JAM bilateral Hungarian–Slovenian (HU–SLO) project is the first to apply modern isotopic and chemical analyses in the characterization and correlation of a number of shared groundwater resources in the Mura-Zala Sub-basin of the Pannonian. The aims of this work were the identification of groundwater flow paths, the delineation of transboundary aquifers based on thermal and cold groundwater geochemical and isotope properties in the Mura-Zala Basin, and providing input to calibrate a hydraulic numerical model. Following a common groundwater sampling campaign, 24 cold and thermal groundwater samples from seven aquifers were collected for chemical, isotope, gas and noble gas analyses. Chemical analyses, and D, O and C isotopes were used to correlate cross border aquifers. A regional groundwater flow is hydrogeologically possible in some aquifers in the Mura-Zala Basin, and has been confirmed by hydrogeochemistry. The Újfalu (HU) and Mura (SLO) Formations are a part of the active regional thermal groundwater flow system, probably hydraulically separated from the shallower flow system of the Ptuj-Grad (SLO), Zagyva and Somló-Tihany (HU) Formations. The thermal water is of meteoric origin, reductive and alkaline. The predominant water type in the Quaternary and Pliocene aquifers is Ca–Mg–HCO₃, changing to Na–HCO₃ in the main Pannonian geothermal aquifer, and Na–Cl brine in deeper and older Miocene aquifers. Total dissolved solids and Na content generally increase with depth. Deuterium is in the range −87‰ to −75‰, ¹⁸O from −11.9‰ to −10.4‰, while ¹⁴C values are less than 6.1 pmC in the samples of the active regional thermal groundwater flow system. These and results of noble gas analyses indicate recharge during the Pleistocene interglacial period with temperatures around 6–7 °C. Regional thermal water resources are limited and environmental isotopes can be used as an early warning in the management of thermal water.     

Local climate change induced by groundwater overexploitation in a high Andean arid watershed,Laguna Lagunillas basin,northern Chile

The Laguna Lagunillas basin in the arid Andes of northern Chile exhibits a shallow aquifer and is exposed to extreme air temperature variations from 20 to ?25 °C. Between 1991 and 2012, groundwater levels in the Pampa Lagunillas aquifer fell from near-surface to ~15 m below ground level (bgl) due to severe overexploitation. In the same period, local mean monthly minimum temperatures started a declining trend, dropping by 3–8 °C relative to a nearby reference station. Meanwhile, mean monthly maximum summer temperatures shifted abruptly upwards by 2.7 °C on average in around 1996. The observed air temperature downturns and upturns are in accordance with detected anomalies in land-surface temperature imagery. Two major factors may be causing the local climate change. One is related to a water-table decline below the evaporative energy potential extinction depth of ~2 m bgl, which causes an up-heating of the bare soil surface and, in turn, influences the lower atmosphere. At the same time, the removal of near-surface groundwater reduces the thermal conductivity of the upper sedimentary layer, which consequently diminishes the heat exchange between the aquifer (constant heat source of ~10 °C) and the lower atmosphere during nights, leading to a severe dropping of minimum air temperatures. The observed critical water-level drawdown was 2–3 m bgl. Future and existing water-production projects in arid high Andean basins with shallow groundwater should avoid a decline of near-surface groundwater below 2 m bgl and take groundwater-climate interactions into account when identifying and monitoring potential environmental impacts.     

A study of the thermal behaviour of exposed karst water systems in a mountainous area of Zigui County,Hubei Province,Central China

Temperature, discharge, and stable isotope ratios of five karst springs in a mountainous area of Zigui County, Hubei Province, Central China, were analyzed. The purpose was to illustrate the heat exchanges linked to circulation depth in the exposed karst water systems through the development of a method for estimating heat input and heat flux during a rainstorm. Meteorological water in the study area conformed to a local meteoric water line (δD?=?8.37 δ¹⁸O?+?12.99) with a mean δ¹⁸O elevation gradient of ?4.0‰ km^?1, which was used to estimate mean circulation depths of 209–686 m. The mean spring temperatures defined a vertical gradient of ?5.4 °C km^?1, which resembled that of the stable atmosphere of the Earth, indicating that the thermal response patterns are mainly controlled by surface air temperature. Thermal convection after rainfall events dominated heat exchange between baseflow and recharge water, leading to a warmer and colder recharge during summer and winter, respectively, whereas thermal conduction dominated the heat exchange only between groundwater, surrounding geology, and the interface air under a condition of no rainfall, resulting in only small temperature variations of the baseflow. Successful application of the method for estimating heat exchange showed that the characteristics of shallow circulation, strong karstification, and well-developed epikarst readily allowed disruption of the thermal balance of the Yuquandong system, resulting in a poor heat regulation capacity, a larger variation of heat input, a lower mean heat flux, and lower baseflow temperatures compared to those of the Dayuquan system.

     

Surface air temperature changes over the Tibetan Plateau: Historical evaluation and future projection based on CMIP6 models

With its amplification simultaneously emerging in cryospheric regions, especially in the Tibetan Plateau, global warming is undoubtedly occurring. In this study, we utilized 28 global climate models to assess model performance regarding surface air temperature over the Tibetan Plateau from 1961 to 2014, reported spatiotemporal variability in surface air temperature in the future under four scenarios (SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5), and further quantified the timing of warming levels (1.5, 2, and 3 °C) in the region. The results show that the multimodel ensemble means depicted the spatiotemporal patterns of surface air temperature for the past decades well, although with differences across individual models. The projected surface air temperature, by 2099, would warm by 1.9, 3.2, 5.2, and 6.3 °C relative to the reference period (1981–2010), with increasing rates of 0.11, 0.31, 0.53, and 0.70 °C/decade under the SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5 scenarios for the period 2015–2099, respectively. Compared with the preindustrial periods (1850–1900), the mean annual surface air temperature over the Tibetan Plateau has hit the 1.5 °C threshold and will break 2 °C in the next decade, but there is still a chance to limit the temperature below 3 °C in this century. Our study provides a new understanding of climate warming in high mountain areas and implies the urgent need to achieve carbon neutrality.     

The effects of human activities on oasis climate and hydrologic environment in the Aksu River Basin,Xinjiang, China

Under the influence of human activities, the differences in climate changes emerged obviously in the Aksu Oasis. The summer air temperature in Aksu City increased at a rate of 0.20°C·(10 annum)⁻¹, while the air temperature in Aler in summer decreased at a rate of −0.27°C·(10 annum)⁻¹. Human intervention has become the dominant factor in the the changes of water resources in the Aksu Oasis. Compared with the Xidaqiao hydrological station, which is located on the upper reaches of the Aksu River, the runoff ratio at the Aler hydrological station increased in summer and decreased in spring. Though the natural water inflow has increased in the last 50 years, irrigation diversion and water consumption for human activities have increased unceasingly due to the increased reclamation of land and the development of extensive agriculture. The water quality at the Xidaqiao station is in Grade I at different periods, and the water quality at the Aler station of the Tarim River is good because the river did not suffer from human disturbance before the large-scale development of oasis agriculture; but the salinity of river water increased from 0.67 g L⁻¹ in 1960 to 5.27 g L⁻¹ in 2000 at the Aler station after farmland development, reclamation and salt washing in the oasis. The geographic distribution of surface water resources has changed due to human activities and in turn affected groundwater replenishment, bringing about changes to groundwater table and quality.     

Characterization of the subsurface urban heat island and its sources in the Milan city area,Italy

Urban areas are major contributors to the alteration of the local atmospheric and groundwater environment. The impact of such changes on the groundwater thermal regime is documented worldwide by elevated groundwater temperature in city centers with respect to the surrounding rural areas. This study investigates the subsurface urban heat island (SUHI) in the aquifers beneath the Milan city area in northern Italy, and assesses the natural and anthropogenic controls on groundwater temperatures within the urban area by analyzing groundwater head and temperature records acquired in the 2016–2020 period. This analysis demonstrates the occurrence of a SUHI with up to 3 °C intensity and reveals a correlation between the density of building/subsurface infrastructures and the mean annual groundwater temperature. Vertical heat fluxes to the aquifer are strongly related to the depth of the groundwater and the density of surface structures and infrastructures. The heat accumulation in the subsurface is reflected by a constant groundwater warming trend between +0.1 and?+?0.4 °C/year that leads to a gain of 25 MJ/m² of thermal energy per year in the shallow aquifer inside the SUHI area. Future monitoring of groundwater temperatures, combined with numerical modeling of coupled groundwater flow and heat transport, will be essential to reveal what this trend is controlled by and to make predictions on the lateral and vertical extent of the groundwater SUHI in the study area.

     

Evidence for deep groundwater flow and convective heat transport in mountainous terrain, Delta County, Colorado, USA

The Tongue Creek watershed lies on the south flank of Grand Mesa in western Colorado, USA and is a site with 1.5 km of topographic relief, heat flow of 100 mW/m², thermal conductivity of 3.3 W m^–1 °C^–1, hydraulic conductivity of 10^-8 m/s, a water table that closely follows surface topography, and groundwater temperatures 3–15°C above mean surface temperatures. These data suggest that convective heat transport by groundwater flow has modified the thermal regime of the site. Steady state three-dimensional numerical simulations of heat flow, groundwater flow, and convective transport were used to model these thermal and hydrological data. The simulations provided estimates for the scale of hydraulic conductivity and bedrock base flow discharge within the watershed. The numerical models show that (1) complex three-dimensional flow systems develop with a range of scales from tens of meters to tens of kilometers; (2) mapped springs are frequently found at locations where contours of hydraulic head indicate strong vertical flow at the water table, and; (3) the distribution of groundwater temperatures in water wells as a function of surface elevation is predicted by the model.     

Modeling spatial and temporal variations in temperature and salinity during stratification and overturn in Dexter Pit Lake,Tuscarora, Nevada,USA

This paper examines the seasonal cycling of temperature and salinity in Dexter pit lake in arid northern Nevada, and describes an approach for modeling the physical processes that operate in such systems. The pit lake contains about 596,200 m³ of dilute, near neutral (pHs 6.7–9) water. Profiles of temperature, conductivity, and selected element concentrations were measured almost monthly during 1999 and 2000. In winter (January–March), the pit lake was covered with ice and bottom water was warmer (5.3 °C) with higher total dissolved solids (0.298 g/L) than overlying water (3.96 °C and 0.241 g/L), suggesting inflow of warm (11.7 °C) groundwater with a higher conductivity than the lake (657 versus 126–383 μS/cm). Seasonal surface inflow due to spring snowmelt resulted in lower conductivity in the surface water (232–247 μS/cm) relative to deeper water (315–318 μS/cm). The pit lake was thermally stratified from late spring through early fall, and the water column turned over in late November (2000) or early December (1999). The pit lake is a mixture of inflowing surface water and groundwater that has subsequently been evapoconcentrated in the arid environment. Linear relationships between conductivity and major and some minor (B, Li, Sr, and U) ions indicate conservative mixing for these elements.Similar changes in the elevations of the pit lake surface and nearby groundwater wells during the year suggest that the pit lake is a flow-through system. This observation and geochemical information were used to configure an one-dimensional hydrodynamics model (Dynamic Reservoir Simulation Model or DYRESM) that predicts seasonal changes in temperature and salinity based on the interplay of physical processes, including heating and cooling (solar insolation, long and short wave radiation, latent, and sensible heat), hydrologic flow (inflow and outflow by surface and ground water, pumping, evaporation, and precipitation), and transfers of momentum (wind stirring, convective overturn, shear, and eddy diffusion). Inputs to the model include the size and shape of the lake, daily meteorological data (short wave radiation, long wave radiation or cloud cover, air temperature, vapor pressure, wind speed, and rainfall), rates for water inputs and outputs, the composition of inflowing water, and initial profiles of temperature and salinity. Predicted temperature profiles, which are influenced by seasonal changes in the magnitude of solar radiation, are in good agreement with observations and show the development of a strong thermocline in the summer, erosion of the thermocline during early fall, and turnover in late fall. Predicted salinity profiles are in reasonable agreement with observations and are affected by the hydrologic balance, particularly inflow of surface and groundwater and, to a lesser degree, evaporation. Defining the hydrodynamics model for Dexter pit lake is the first step in using a coupled physical – biogeochemical model (Dynamic Reservoir Simulation Model-Computational Aquatic Ecosystem Dynamics Model or DYRESM-CAEDYM) to predict the behavior of non-conservative elements (e.g., dissolved O₂, Mn, and Fe) and their effect on water quality in this system.     

Chloride imbalance in a catchment undergoing hydrological change: Upper Barwon River,southeast Australia

Documenting whether surface water catchments are in net chemical mass balance is important to understanding hydrological systems. Catchments that export significantly greater volumes of solutes than are delivered via rainfall are not in hydrologic equilibrium and indicate a changing hydrological system. Here an assessment is made of whether a saline catchment in southeast Australia is in chemical mass balance based on Cl. The upper reaches of the Barwon River, southeast Australia, has total dissolved solids, TDS, concentrations of up to 5860 mg/L and Cl concentrations of up to 3370 mg/L. The high river TDS concentrations are due to the influxes of groundwater with TDS concentrations of up to 68,000 mg/L. Between 1989 and 2011, the median annual Cl flux from the upper Barwon catchment was 17.8 × 10⁶ kg (∼140 kg/a/ha). This represents 340–2230% of the annual Cl input by rainfall to the catchment. Major ion and stable isotope geochemistry indicate that the dominant source of solutes in the catchment is evapotranspiration of rainfall, precluding mineral dissolution as a source of excess Cl. The upper Barwon catchment is not in chemical mass balance and is a net exporter of solutes. The chemical imbalance may reflect the transition within the last 100 ka from an endorheic lake system where solutes were recycled producing shallow groundwater with high TDS concentrations to a better drained catchment. Alternatively, a rise in the regional water table following land clearing may have increased the input of groundwater with high TDS concentrations to the river system.     

A numerical simulation of impact of groundwater seepage on temperature distribution in karst collapse pillar

This paper presents an investigation of the impacts of groundwater seepage on temperature distribution in the karst collapse pillar (KCP). A thermal mathematic model with groundwater seepage was proposed, and a numerical model was established to describe the impacts of groundwater seepage on the temperature distribution in the KCP. The results show that the groundwater seepage velocity improved with the increasing permeability of porous medium in the KCP. The increasing seepage velocity improves thermal convection and then improves thermal conduction in the porous medium. As a result, the heat in the deep Ordovician limestone groundwater gradually diffuses. The seepage velocity occurred differently in different locations of KCP, and the heat exchange was also different, resulting in temperature differences for different locations. This temperature divergence in the surrounding rocks can reach 5~9°C at distances less than 50 m to the KCP. These findings can be used to protect the mine from water inrush by monitoring temperature changes in the surrounding rocks of KCP.     

The Diavik Waste Rock Project: Measurement of the thermal regime of a waste-rock test pile in a permafrost environment

The interior thermal regime of a field-scale experimental waste rock pile in the Northwest Territories, Canada, was studied. Test pile construction was completed in the summer 2006, and temperature data was collected continuously since that time to February 2009. The temperature data indicates the test pile cooled over the study period, with an average heat energy release of −2.5 × 10⁴ and −2.6 × 10⁴ MJ in 2007 and 2008, respectively. The mean annual air temperature (MAAT) at the site was −8.9 °C during the period between 2006 and 2009, with a permafrost table at a depth of 4 m in bedrock away from the pile. Because of this cold environment, the upward movement rate of the 0 °C isotherm into the test pile at its base was approximately 1.5 m a⁻¹ during 2007 and 2008. Thermistor strings installed immediately below the base of the test pile showed the test-pile basal temperatures remained near and below 0 °C during the study period. Furthermore, due to low rates of sulfide mineral oxidation, elevated temperatures in the interior of the test pile were not observed. The average air velocity in the pore space in July 2007 and 2008 was about one third of that during January of each year based on temperature distributions. Therefore, due to higher air velocity during the winter, it is expected that heat transfer is greater during winter.     

A new method for analysis of osmium isotopes and concentrations in surface and subsurface water samples

We describe a new method for the determination of osmium (Os) content and ¹⁸⁷Os/¹⁸⁸Os isotopic ratio in water samples, particularly adapted to analysis of reducing waters such as groundwaters. The critical feature of the method is the improved oxidation step. We use a high pressure asher (HPA-S) to achieve oxidation of all Os species at high pressure (~ 130 bars) and high temperature (250 °C). A series of tests was performed on two groundwater samples in order to study the general behavior of Os during oxidation and to optimize the various parameters of the method. At high temperature (≥ 250 °C), using hydrogen peroxide as an oxidant, complete spike-sample equilibration and oxidation of Os species to the highest oxidation state (OsO₄) are obtained in less than 10 h. A strong contrast in oxidation behavior was observed between tests performed at high (250 °C) and low temperature (100 °C). The results for both samples indicate a difference in the oxidation kinetics between spike and sample. They also demonstrate that a non-negligible proportion of each sample is easily oxidized at low temperature whereas high temperatures are required to oxidize the rest of the sample. This last observation could provide evidence for the existence of several species of osmium in groundwater samples. Our results underscore the need for high temperatures to assure spike-sample equilibration and complete oxidation of all Os species potentially present in water. The method presented here, which is adapted to various matrices, opens the possibility of analyzing Os concentrations and isotopic compositions in groundwater and thus to better constrain the potential sources of osmium delivered to the ocean.     

Modelling the effects of climate change on methane emission from a northern ombrotrophic bog in Canada

Peatlands are a large potential source of methane (CH₄) to the atmosphere. In order to investigate the effects of climate change on CH₄ emission from northern ombrotrophic peatlands, a simulation model coupling water table dynamics with methane emission was developed for the Mer Bleue Bog in Ontario, Canada. The model was validated against reported values of CH₄ flux from field measurements and the model outputs exhibited high sensitivity to acrotelm thickness, leaf area index, transmissivity and slope of water table. With a 2–4°C temperature rise over the 4-year simulation period, the rate of CH₄ release dropped significantly to under 0.1 mg m⁻² day⁻¹. On the other hand, mean CH₄ emission increased by >26-fold when the increase in precipitation was >15%. When looking at the combined effects, the highest CH₄ release (13.3 mg m⁻² day⁻¹) was attained under the scenario of 2°C temperature rise and 25% precipitation increase. Results obtained in this study highlight the importance of avoiding more extreme climate change, which would otherwise lead to enhanced methane release from peatlands and further atmospheric warming through positive feedback.     

Trace element source terms for mineral dissolution

We present an approach for determining source terms for modeling trace element release from minerals, using arsenic (As) as an example. The source term function uses laboratory-measured mineral dissolution rates to predict the time rate of change of As concentrations (mol/L s) released to water by the dissolving mineral. Application of this function to As-bearing minerals (realgar, orpiment, arsenopyrite, scorodite, pyrite, and jarosite) in air saturated water at 25 °C shows that mineralogy, grain size and pH are important factors affecting the As source term while DO concentration and temperature are relatively unimportant for conditions found in typical aquifers. The derived function shows that the source term decreases as a function of (1 − t/t_L)², where t_L is the grain lifetime, due to the shrinkage of the mineral grains as they dissolve. For some models, either a constant or an instantaneous term might be used, provided that certain time constraints are met. The methods outlined in this paper are intended to help bridge the gap between laboratory measurements and field-based models. Although this paper uses As as an example, the methods are general and can be used to predict source terms for other mineral-derived trace elements to groundwater.     

Controls on permafrost thaw in a coupled groundwater-flow and heat-transport system: Iqaluit Airport,Nunavut, Canada

Numerical simulations of groundwater flow and heat transport are used to provide insight into the interaction between shallow groundwater flow and thermal dynamics related to permafrost thaw and thaw settlement at the Iqaluit Airport taxiway, Nunavut, Canada. A conceptual model is first developed for the site and a corresponding two-dimensional numerical model is calibrated to the observed ground temperatures. Future climate-warming impacts on the thermal regime and flow system are then simulated based on climate scenarios proposed by the Intergovernmental Panel on Climate Change (IPCC). Under climate warming, surface snow cover is identified as the leading factor affecting permafrost degradation, including its role in increasing the sensitivity of permafrost degradation to changes in various hydrogeological factors. In this case, advective heat transport plays a relatively minor, but non-negligible, role compared to conductive heat transport, due to the significant extent of low-permeability soil close to surface. Conductive heat transport, which is strongly affected by the surface snow layer, controls the release of unfrozen water and the depth of the active layer as well as the magnitude of thaw settlement and frost heave. Under the warmest climate-warming scenario with an average annual temperature increase of 3.23 °C for the period of 2011–2100, the simulations suggest that the maximum depth of the active layer will increase from 2 m in 2012 to 8.8 m in 2100 and, over the same time period, thaw settlement along the airport taxiway will increase from 0.11 m to at least 0.17 m.

     

The nature of MIS 3 stadial–interstadial transitions in Europe: New insights from model–data comparisons

15 abrupt warming transitions perturbed glacial climate in Greenland during Marine Isotope Stage 3 (MIS 3, 60–27 ka BP). One hypothesis states that the 8–16 °C warming between Greenland Stadials (GS) and Interstadials (GI) was caused by enhanced heat transport to the North Atlantic region after a resumption of the Atlantic Meridional Overturning Circulation (AMOC) from a weak or shutdown stadial mode. This hypothesis also predicts warming over Europe, a prediction poorly constrained by data due to the paucity of well-dated quantitative temperature records. We therefore use a new evidence from biotic proxies and a climate model simulation to study the characteristics of a GS–GI transition in continental Europe and the link to enhanced AMOC strength. We compare reconstructed climatic and vegetation changes between a stadial and subsequent interstadial – correlated to GS15 and GI14 (～55 ka BP) – with a simulated AMOC resumption using a three-dimensional earth system model setup with early-MIS 3 boundary conditions. Over western Europe (12°W–15°E), we simulate twice the annual precipitation, a 17 °C warmer coldest month, a 8 °C warmer warmest month, 1300 °C-day more growing degree days with baseline 5 °C (GDD5) and potential vegetation allowing tree cover after the transition. However, the combined effect of frequent killing frosts, <20 mm summer precipitation and too few GDD5 after the transition suggest a northern tree limit lying at ～50°N during GI14. With these 3 climatic limiting factors we provide a possible explanation for the absence of forests north of 48°N during MIS 3 interstadials with mild summers. Finally, apart from a large model bias in warmest month surface air temperatures, our simulation is in reasonable agreement with reconstructed climatic and vegetation changes in Europe, thus further supporting the hypothesis.     

Chironomids as a tool for inferring Holocene climate: an assessment based on six sites in southern Scandinavia

Chironomid subfossil assemblages from six low-alpine and sub-alpine Holocene stratigraphies are presented and compared. They are from five lakes in mid-southern and western Norway and one in central Sweden. When comparing the chironomid-inferred July air temperatures, there are many time segments with a poor among-lake fit in inferred temperatures. Possible environmental variables influencing the fossil chironomid assemblages are discussed using a modern Norwegian calibration data set to indicate taxon–environment relationships. These analyses indicate that local changes in pH, water chemistry, and productivity at times may have overridden the regional temperature signal. In addition, other causes of poor among-site temperature fit are discussed, in particular those related to chronological uncertainties. Holocene temperature inferences from single cores based on chironomids may not always be able to provide a reliable regional temperature signal, but can act as a guide from which hypotheses about past environmental conditions can be tested with the aid of chironomid-inferred temperatures from several sites and from other environmental proxies. We have obtained a regional picture of Holocene summer temperature change by developing a consensus reconstruction based on the overall temperature signal from all six sites. This consensus is developed by fitting a smoother through all 330 site-specific temperature-deviations from the Holocene mean. The consensus temperature deviations vary from −0.8 °C at 8800 cal years BP to +0.8 °C at 6500 cal years BP.     

Ultra‐high temperature metamorphism recorded in Fe‐rich olivine‐bearing migmatite from the Khondalite Belt,North China Craton

We document the first occurrence of Fe‐rich olivine‐bearing migmatitic metapelite in the Khondalite Belt, North China Craton. Petrological analyses revealed two exotic assemblages of orthopyroxene+spinel+olivine and orthopyroxene+spinel+cordierite. Phase relation modelling suggests that these assemblages are diagnostic of ultra‐high temperature (UHT) metamorphism in the Fe‐rich system, with temperatures from 1,000 to 1,050°C at 0.6 GPa. U–Th–Pb SIMS analyses on zircon reveal a similar age of c. 1.92 Ga for the olivine‐bearing migmatite and an adjacent gabbronoritic intrusion that is therefore identified as the heat source for the UHT metamorphism. These results, coupled with additional analysis of the famous Tuguiwula sapphirine‐bearing granulite, lead to a re‐appraisal of the P–T path shape and heat source for the UHT metamorphism. We suggest that UHT metamorphism, dated between 1.92 and 1.88 Ga, across the whole Khondalite belt, proceeded from a clockwise P–T evolution with an initial near‐isobaric heating path at ~0.6–0.8 GPa, and a maximum temperature of 1,050°C followed by a cooling path with minor decompression to ~0.5 GPa. Considering our results and previous works, we propose that the orogenic crust underwent partial melting at temperature reaching 850°C and depth of ~20 to ~30 km during a period of c. 30 Ma, between 1.93 and 1.90 Ga. During this time span, the partially molten crust was continuously intruded by mafic magma pulses responsible for local greater heat supply and UHT metamorphism above 1,000°C. We propose that the UHT metamorphism in the Khondalite belt is not related to an extensional post‐collisional event, but is rather syn‐orogenic and associated with mafic magma supplies.     

Holistic assessment of groundwater resources and regional environmental problems in the North China Plain

Water balance components of the North China Plain (NCP) were analyzed, indicating the decrease both in precipitation and evaporation. The decreased precipitation and expansion of water use for agriculture, industrial and domestic purposes have caused a water crisis, which was managed until now by diverting water from the Yellow River and over exploitation of groundwater. The groundwater resource was assessed by estimating its recharge in both upper unconfined and lower confined layers, yielding a total value of 1.65 × 10¹⁰ m³/a. Total groundwater use was estimated and judged by the actual water table drawdown. Salt accumulation, water table decrease, fluoride and nitrate pollution were all found to be major regional environmental problems. Furthermore, heavy metals were found in high content in the soil and surface water in suburbs of large cities, posing a potential risk of pollution in the groundwater. It has been verified by isotropic data that dry conditions have occurred since 10 ka and are therefore part of the natural process. Possible solutions for water crises in the NCP are proposed.