Regional Groundwater Circulation Research Articles

A conceptual and numerical model of fluid flow and heat transport in the Topusko hydrothermal system

Comprehensive understanding of hydrothermal systems is often obtained through the integration of conceptual and numerical modelling. This integrated approach provides a structured framework for the reconstruction and quantification of fluid dynamics in the reservoir, thereby facilitating informed decision-making for sustainable utilisation and environmental protection of the hydrothermal system. In this study, an updated conceptual model of the Topusko hydrothermal system (THS; central Croatia) is proposed based on structural, geochemical, and hydrogeological analyses. The stratigraphic sequence and the structural framework of the THS were defined based on geological maps and field investigations. As depicted by hydrochemical and isotope analyses, thermal waters in Topusko (temperatures of up to 65 °C) are of meteoric origin and circulate in a carbonate aquifer. The THS receives diffuse recharge approximately 13 km S from Topusko, where Triassic carbonates crop out. Gravity-driven regional groundwater circulation is favoured by regional thrusts that tectonically uplifted Palaeozoic low permeable rocks. These structures confine the fluid flow in the permeable, fractured and karstified, Triassic carbonates favouring the northward circulation of the water. A regional anticline lifts the aquifer closer to the surface in Topusko. Open fractures in the anticline hinge zone increase the fracturing and permeability field of the aquifer promoting the quick upwelling of the thermal water resulting in the Topusko thermal springs. Numerical simulations of fluid flow and heat transport corroborate the proposed conceptual model. In particular, a thermal anomaly was modelled in the Topusko subsurface with temperature values of 31.3 °C and 59.5 °C at the surface and at the base of the thermal aquifer, respectively, approaching the field observations. These findings showed that the circulation of Topusko thermal water is influenced by regional and local geological structures suggesting that the enhanced permeability field in the discharge area enables the formation of the natural thermal springs.

Effect of highway greenbelt constrution on groundwater flow in a semi-arid region.

The highway greenbelt, vigorously promoted in arid and semi-arid areas, has obvious impacts on beautifying the environment, absorbing dust, reducing noise, and maintaining soil and water. Moreover, it affects the characteristics of how water resources are distributed and the regional groundwater cycle. However, the impact of highway greenbelt construction on groundwater flow in semi-arid areas is unknown. The Hubao Highway greenbelt in the north part of the Tumochuan Plain was studied as an example. The paper combines field investigation, remote sensing and mathematical modeling to quantify the impact of highway green space construction on regional groundwater circulation. The results showed that: Trees, shrubs and grasses were the dominant vegetation types in the landscaped area, accounting for 42.17% of the studied area. The total evapotranspiration water consumption of the green belt during the growing season was 471.35 × 104m3. The groundwater recharge in the study area was mainly derived from the lateral recharge in front of the mountain, and the main discharge was the evapotranspiration water consumption of the green belt. This evapotranspiration accounts for 3.31% of the total groundwater recharge. Under the condition that the recharge in front of the mountain remains constant, the evapotranspiration water consumption of the green belt will still have an increasing trend in the future. Appropriate planting of poplar and other high water-consuming trees may be the best way to mitigate the adverse effects of greenbelt evapotranspiration on groundwater resources. The results of this study provide valuable insights for environmental protection and infrastructure development in similar areas.

Groundwater Circulation Mechanism of the Upstream Area of Beiniuchuan River Using Isotope–Hydrochemical Tracer

In order to achieve the rational development and utilization of underground water resources in the Dongsheng mining area under coal mining conditions, we selected the upstream area of Beiniuchuan River as a typical region. Through field investigations, sampling tests, and the application of hydrochemical and isotope techniques, we traced the groundwater circulation mechanism in the Dongsheng mining area. The results indicate that the majority of the Quaternary alluvial and Salawusu Formation groundwater is of the HCO3-Ca type, with a TDS content below 300 mg/L. However, in some areas, the hydrochemical type becomes complex due to anthropogenic contamination. The shallow-buried Yan’an Formation groundwater is either of the HCO3-Ca·Mg type or the HCO3·SO4-Ca·Mg type, with TDS content ranging from 200 to 750 mg/L. The Yan’an Formation at depths greater than 40 m exhibits complex water chemistry, with a TDS content higher than 500 mg/L, and it belongs to the Cl-Na type, with TDS around 700 mg/L. The hydrogen and oxygen isotope results indicate that the local groundwater is primarily recharged via atmospheric precipitation. The 3H and 14C results show that the Quaternary alluvial and shallow-buried Yan’an Formation groundwater has a fast turnover rate, while the deep-buried Yan’an Formation and Yan’chang Formation groundwater have a slower turnover rate. The regional groundwater circulation can be generalized into three flow systems: shallow, intermediate, and deep. Under the influence of coal mining activities, the water circulation conditions in the study area have undergone significant changes. The sealing integrity of the Yan’an Formation has been compromised, and precipitation and shallow groundwater have enhanced the vertical infiltration capacity of the formation, increasing the proportion of groundwater participating in the intermediate flow system. As a result, the river runoff mainly dependent on the discharge from the shallow flow system has drastically decreased.

Groundwater Recharge and Circulation in Dolomitic Aquifer Located in Semi-Arid Region: Evidence from the δ18O and δ2H Record, South Africa

Dolomitic aquifers are regarded as important groundwater storage sites in South Africa. Since these aquifers occur in a semi-arid climatic setting with low rainfall, often characterized by a torrential downpour and high potential evapotranspiration, the occurrence of active recharge is very limited (<5% of mean annual rainfall) as compared with the rainfall amount. The Malmani dolomites that have undergone greenschist metamorphism contain widespread caves and open karst structures at shallow levels, which facilitate groundwater recharge, circulation, storage and spring occurrence. However, the open karst structures receive recharge that passes through fractures in the vadose zone, which regulates the recharge through retardation and mixing processes. The integrated approach involving major ions and stable isotopes of water was applied to understand the recharge mechanism. The cave drip water samples were represented by the δ18O values of −3.95‰ to 3.32‰ and the δ2H values ranging from −11.0‰ to 27.7‰. On the other hand, the rainfall isotope results for δ18O fall between −16.11‰ and 5.38‰, while the δ2H values fall between −105.7‰ and 35.6‰. The most depleted Malapa springs contain δ18O of −5.64‰ and δ2H of −32.4‰. Based on the results, the mixing of water in the vadose zone could be considered as an indicator of the dominance of a slow-diffusive flow process in the aquifer as a result of poor fracture permeability. However, regional groundwater circulation through faults and dykes besides interconnected karst structures helps in generating highly productive karst springs in the region characterized by low rainfall.

Understanding the Origin and Mixing of Deep Fluids in Shallow Aquifers and Possible Implications for Crustal Deformation Studies: San Vittorino Plain, Central Apennines

Expanding knowledge about the origin and mixing of deep fluids and the water–rock–gas interactions in aquifer systems can represent an improvement in the comprehension of crustal deformation processes. An analysis of the deep and meteoric fluid contributions to a regional groundwater circulation model in an active seismic area has been carried out. We performed two hydrogeochemical screenings of 15 springs in the San Vittorino Plain (central Italy). Furthermore, we updated the San Vittorino Plain structural setting with a new geological map and cross-sections, highlighting how and where the aquifers are intersected by faults. The application of Na-Li geothermometers, coupled with trace element and gas analyses, agrees in attributing the highest temperatures (>150 °C), the greatest enrichments in Li (124.3 ppb) and Cs (>5 ppb), and traces of mantle-derived He (1–2%) to springs located in correspondence with high-angle faults (i.e., S5, S11, S13, and S15). This evidence points out the role of faults acting as vehicles for deep fluids into regional carbonate aquifers. These results highlight the criteria for identifying the most suitable sites for monitoring variations in groundwater geochemistry due to the uprising of deep fluids modulated by fault activity to be further correlated with crustal deformation and possibly with seismicity.

Hydrogeological characterization of crystalline aquifer in the Hout River Catchment, Limpopo province, South Africa

This study attempted to conceptualize the hydrogeological setting of the Hout River Catchment, located in the Limpopo River Basin, using multiple methods that include groundwater flow patterns, structural analysis, stable (18O, 2H and 13C) and radiogenic (14C) isotopes of water and Water Table Fluctuation methods. The hydrogeological system of the catchment is represented by fractured crystalline basement aquifer as the main host for groundwater and is overlain by weathered rocks that act as a vadose zone and shallow aquifer in various places. Groundwater from the fractured basement rocks is the main source of water for large-scale irrigation and domestic use. Potential aquifers in the area are evident within the Hout River granitic gneiss and the Goudplaats granitic gneiss besides the younger granites as a result of fracturing and weathering. Groundwater flow map shows a flow pattern from the southern part of the catchment towards the north-eastern part of the catchment dictated by dolerite dykes and tectonic lineaments that trend in the ENE and E direction (088° and 075°) with the dip angle of 50° to 55°. The deeper aquifer in the southern and central part of the catchment contain old groundwater with high salinity due to long residence time. The stable isotopes further confirmed the limited possibility of local recharge, with rather dominance of regional groundwater circulation into the catchment. The northern part of the catchment seems to be receiving recent recharge with the groundwater of high 14C content derived from the mountains that border the catchment.

Spatial-temporal evolutions of groundwater environment in prairie opencast coal mine area: a case study of Yimin Coal Mine, China.

The interactions between groundwater and its environment was investigated in prairie mining area in this study, through the groundwater system evolutions in mining area before and after the mining actions (from 1973 to 2016) of Yimin coal mine. The results showed that (1) the mining activities of the open-pit coal changed the original reduction environment into the oxidizing environment in the mining area. The pyrite and sulfur-bearing coal in the stratum oxidized, produced acid and triggered a series of subsequent reactions, resulting in the decrease in the pH value of the groundwater in the mining area. The concentration of SO42-, Fe2+, Fe3+, Ca2+ and Mg2+ and the total hardness increased. The regional hydrochemical type evolved from HCO3-Na·Ca·Mg type before mining to the type of HCO3·SO4-Na·Ca·Mg after mining. (2) Coal mining strongly draining underground water accelerated the regional groundwater circulation, and then made the groundwater desalination. The concentrations of TDS, COD and Na++K+ in the mining area all showed a decreasing trend. (3) The coal mining activities made the calcite and dolomite in saturated state under the natural condition of underground water to be unsaturated again. The hydro-geochemical action evolves from double control (water-rock interaction and evaporation-concentration) to water-rock interaction control.

Fragmentation of the Hyporheic Zone Due to Regional Groundwater Circulation

AbstractBy use of numerical modeling and field observations, this work quantified the effects of catchment‐scale upwelling groundwater on the hyporheic (below stream) fluxes over a wide range of spatial scales. A groundwater flow model was developed that specifically accounted for the hydrostatic and dynamic head fluctuations induced by the streambed topography. Although the magnitudes and relative importance of these streambed‐induced fluxes were found to be highly sensitive to site‐specific hydromorphological properties, we showed that streambed topographic structures exert a predominant control on the magnitude of hyporheic exchange fluxes in a Swedish boreal catchment. The magnitude of the exchange intensity evaluated at the streambed interface was found to be dominated by the streambed‐induced hydraulic head across stream order. However, the catchment‐scale groundwater flow field substantially affected the distribution of groundwater discharge points and thus decreased the fragmentation of the hyporheic zone, specifically by shifting the cumulative density function toward larger areas of coherent upwelling at the streambed interface. This work highlights the spectrum of spatial scales affecting the surface water‐groundwater exchange patterns and resolves the roles of key mechanisms in controlling the fragmentation of the hyporheic zone.

Groundwater-flow-system characterization with hydrogeochemistry: a case in the lakes discharge area of the Ordos Plateau, China

Understanding the pattern of regional groundwater circulation is essential for sustainable management of groundwater resources and ecosystems protection. A large-scale basin may develop nested groundwater flow systems including local, intermediate and regional flow systems. Hydrogeochemical tracing may be an effective methodology to identify different groundwater flow systems, considering its routine application in field investigation. This study uses wavy-topography-driven regional groundwater flow in the groundwater-fed lakes area of the northern Ordos Plateau, China, as an example to test the effectiveness of a hydrogeochemical method for groundwater-flow-system characterization. Samples of groundwater from wells with different depths and lake water were collected and analyzed. Hierarchical cluster analysis was conducted using the pH, electrical conductivity, and major ions as the input, which leads to three clusters with distinct geochemical compositions. Considering the hydrochemical characteristics, wells depths, and sampling locations, different groundwater flow systems were identified. Geochemical evolution was affected by processes such as leaching, cation exchange, evaporation and human activities. The relationship between δD and δ18O indicates that the shallow and deep groundwater were recharged by atmospheric precipitation during the modern time and a past colder period, respectively. The groundwater geochemistry is closely related to groundwater circulation depth within different flow systems, indicated by comparison of geochemical processes among the three clusters. This work highlights a hydrochemical method that can identify nested groundwater flow systems in the lakes discharge area of this large-scale basin and provides a better understanding of the hydrogeochemical evolution from the processes involved in relation to groundwater flow systems.

ASSESSMENT OF THE INFLUENCE OF DISCREPANCY OF THE LOCATION OF GROUNDWATER AND SURFACE WATERSHEDS ON THE QUANTITY OF CALCULATED RENEWABLE RESOURCES, EXEMPLIFIED BY THE P-IX BALANCE REGION IN THE WARTA WATER REGION

The article presents the results of research on the renewal of a hydrogeological system that is characterized by a significant discrepancy of the location of groundwater and surface watersheds. The area of the hydrological and hydrogeological studies is part of the P-IX balance region (Warta River from the Prosna to the Mosiński Canal) located south of the Warta River. The evaluation of groundwater renewability has been made using a hydrological method and mathematical modelling. When using the hydrological method, which is a commonly used control method for water balance calculations, it is assumed that the surface and groundwater watersheds are identical. Comparison of the results of water balance calculations obtained with both methods required the assessment of the compatibility of the groundwater catchment and morphological boundaries. This was done by calculating the difference between the quantities of renewable resources determined by two variants of the hydrological method. In the first (standard) variant, renewable resources were determined based on data from water-gauge and hydrometric profiles closing the catchments of the Warta tributaries draining the analyzed water-management regions. In the second (extended) method, they were determined based on data from water gauges controlling the Warta sub-basin. Enlargement of the balanced unit as far as the boundaries of the Warta basin was consistent with the principle that the reliability of calculations of groundwater resources increases proportionally to the analyzed catchment area and the significance of its drainage zone in the regional groundwater circulation (Toth, 1963). The quantity of renewable resources determined by the standard and extended variants of the hydrological method amounted to 107,100 m3/day and 184,000 m3/day, respectively. The difference is 72% of the value determined using the standard method. The article also provides an analysis of the spatial and temporal variability of the nature of the hydraulic relationship between surface water and groundwater, and the influence of this variability on the reliability of water balance calculations, based on stationary observations of surface water flow.

A multi-method study of regional groundwater circulation in the Ordos Plateau, NW China

The Ordos Basin is one of the most intensively studied groundwater basins in China. The Ordos Plateau, located in the north part of the Ordos Basin, is ideal to study the pattern of regional groundwater circulation induced by water-table undulations due to the wavy topography and the relatively simple aquifer systems with macroscopically homogeneous sandstone. In catchments located near the first-order divide, the water table is found to be a subdued replica of the topography, and the nonclosed water-table contours in topographic highs of a catchment are indicative of regional groundwater outflow to other catchments. In topographic lows, groundwater-fed lakes/rivers, topography-driven flowing wells, water-loving and/or salt-tolerant vegetation, and soap holes are all indicative of discharge areas. In discharge areas, although groundwater inflow from recharge areas is relatively stable, seasonal variations in groundwater recharge and evapotranspiration lead to significant seasonal fluctuations in the water table, which can be used to estimate groundwater inflow and evapotranspiration rates based on water balance at different stages of water-table change. In the lowest reaches of a complex basin, superposition of local flow systems on regional flow systems has been identified based on groundwater samples collected from wells with different depths and geophysical measurements of apparent resistivity, both of which can be used for characterizing groundwater flow systems. This study enhances understanding of the pattern of regional groundwater circulation in the Ordos Plateau, and also tests the effectiveness of methods for groundwater flow-system characterization.

Drastic hydrological changes caused by hydroelectrical development in karst: a case of the karst river Zrmanja (Croatia)

This paper has analysed changes in the hydrological regime along the karst river Zrmanja (the Dinaric karst of Croatia) caused by operation of the reversible or pumped-storage hydroelectric power plant Velebit (RHEPP Velebit), which started operations in 1985. The total river length from the spring to the mouth in the Adriatic Sea is 70 km. Discharges along the whole river course are monitored at nine hydrological gauging stations. For all nine analysed stations, the decrease of the mean annual discharges starting from 1985 has proven to be statistically significant. However, it should be stressed that the Zrmanja River natural regime is extremely complex due to a deep, well developed and generally undiscovered subsurface karst morphology and its influence on regional surface and groundwater circulation. The influence of RHEPP Velebit operation on the maximum annual discharges is not statistically significant. Statistically significant decrease on the minimum annual discharges is observed at only one gauging station located at the Zrmanja River downstream of the Razovac Reservoir. At the same time, climatological changes have been monitored in the analysed area. It has been established that there has been an annual increase in air temperature since 1988 and an annual decrease in rainfall since 1983. A general conclusion is that the decrease in the mean annual discharges along the Zrmanja River is mainly caused by the RHEPP Velebit operation and only in a smaller degree by climatological changes. On the basis of the available data it was not possible to separate precisely the influences of these two factors.

Influence of major fault zones on 3-D coupled fluid and heat transport for the Brandenburg region (NE German Basin)

<p><strong>Abstract.</strong> To quantify the influence of major fault zones on the groundwater and thermal field, 3-D finite-element simulations are carried out. Two fault zones – the Gardelegen and Lausitz escarpments – have been integrated into an existing 3-D structure of the Brandenburg region in northeastern Germany. Different geological scenarios in terms of modelled fault permeability have been considered, of which two end-member models are discussed in detail. In addition, results from these end-member simulations are compared to a reference case in which no faults are considered. <br><br> The study provides interesting results with respect to the interaction between faults and surrounding sediments and how it affects the regional groundwater circulation system and thermal field. <br><br> Impermeable fault zones seem to induce no remarkable effects on the temperature distribution; that is, the thermal field is similar to the no-fault model. In addition, tight faults have only a local impact on the fluid circulation within a domain of limited spatial extent centred on the fault zone. Fluid flow from the surrounding aquifers is deviated in close proximity of the fault zones acting as hydraulic barriers that prevent lateral fluid inflow into the fault zones. <br><br> Permeable fault zones induce a pronounced thermal signature with alternating up- and downward flow along the same structures. Fluid flow along the plane of the faults is principally driven by existing hydraulic head gradients, but may be further enhanced by buoyancy forces. Within recharge domains, fluid advection induces a strong cooling in the fault zones. Discharge domains at shallow depth levels (~<−450 m) are instead characterized by the presence of rising warm fluids, which results in a local increase of temperatures which are up to 15 °C higher than in the no-fault case. <br><br> This study is the first attempt to investigate the impact of major fault zones on a 3-D basin scale for the coupled fluid and heat transport in the Brandenburg region. The approach enables a quantification of mechanisms controlling fluid flow and temperature distribution both within surrounding sediments and fault zones as well as how they dynamically interact. Therefore, the results from the modelling provide useful indications for geothermal energy exploration.</p>

Untangling complex shallow groundwater dynamics in the floodplain wetlands of a southeastern U.S. coastal river

Understanding the hydrological functioning of tidally influenced floodplain forests is essential for advancing ecosystem protection and restoration goals in impacted systems. However, finding direct relationships between basic hydrological inputs and floodplain hydrology is hindered by complex interactions between surface water, groundwater, and atmospheric fluxes in a variably saturated matrix with heterogeneous soils, vegetation, and topography. Thus, an explanatory method for identifying common trends and causal factors is required. Dynamic factor analysis (DFA), a time series dimension reduction technique, models temporal variation in observed data as linear combinations of common trends, which represent unidentified common factors, and explanatory variables. In this work, DFA was applied to model water table elevation (WTE) in the floodplain of the Loxahatchee River (Florida, USA), where altered watershed hydrology has led to changing hydroperiod and salinity regimes and undesired vegetative changes in the floodplain forest. The technique proved to be a powerful tool for the study of interactions among 29 long‐term, nonstationary hydrological time series (12 WTE series and 17 candidate explanatory variables). Regional groundwater circulation, surface water elevations, and spatially variable net local recharge (cumulative rainfall – cumulative evapotranspiration) were found to be the main factors explaining groundwater profiles. The relative importance of these factors was spatially related to floodplain elevation, distance from the river channel, and distance upstream from the river mouth. The resulting dynamic factor model (DFM) simulated the WTE time series well (overall coefficient of efficiency, Ceff = 0.91) and is useful for assessing management scenarios for ecosystem restoration and predicted sea level rise.

Isotopic and geochemical characterization of salinization in the shallow aquifers of a reclaimed subsiding zone: The southern Venice Lagoon coastland

The coastal plain bordering the southern Venice Lagoon is a reclaimed lowland characterized by high subsidence rate, and ground level and water-table depth below sea level. In this agricultural region, where the surface hydrologic network is entirely artificially controlled by irrigation/drainage canals, salinization problems have long been encountered in soils and groundwaters. Here we use isotopic and geochemical tracers to improve our understanding of the origin of salinization and mineralization of the semi-confined aquifer (0–40 m), and the freshwater inputs to this hydrological system. Water samples have been collected at different seasons in the coastal Adriatic Sea, lagoon, rivers and irrigation canals, as well as in the semi-confined aquifer at depths between 12 and 35 m (14 boreholes), and in the first confined aquifer (three boreholes drilled between 40 and 80 m depth). Stable isotopes (δ18O and δD) and conductivity profiles show that direct saline intrusion from the sea or the lagoon is observed only in a restricted coastal strip, while brackish groundwaters are found over the entire topographic and piezometric depression in the centre of the study area. Fresh groundwaters are found only in the most western zone. The sharp isotopic contrast between the western and central regions suggests disconnected hydrological circulations between these two parts of the shallow aquifer. The border between these two regions also corresponds to the limits of the most strongly subsiding zone. Our results can be interpreted in terms of a four end-member mixing scheme, involving (1) marine water from the lagoon or the open sea, (2) alpine and pre-alpine regional recharge waters carried either by the main rivers Adige, Bacchiglione and Brenta (irrigation waters) or by the regional groundwater circulation, (3) local precipitation, and (4) evaporated waters infiltrated from the surface. Infiltration from the surface is also revealed by the stratification of the electrical conductivity profiles, showing that the brackish groundwaters are overlain by a shallow layer of less saline water all over the central depression. In the first confined aquifer, the groundwaters have isotopic compositions similar to the deep groundwaters of the Venetian confined aquifers (40–400 m depth). The isotopic data and the Br/Cl ratio show that the origin of the salinization of the phreatic aquifer can be ascribed to seawater intrusion alone, with no indication of the involvement of deep brines (identified at 450 m depth) in the process. The chemical composition of the saline and brackish groundwaters is characterized by an excess of sodium and a deficit of calcium compared to conservative mixing between fresh groundwaters and seawater. This suggests that the phreatic aquifer is progressively freshening, as a consequence of the beneficial influence of the extensive irrigation/drainage network, including raised canals acting as a hydraulic barrier along the coast. This freshening tendency may have been lasting since the reclamation in the mid-twentieth century, and has probably been accelerated by the ban on groundwater abstraction since the 1970s.

Research on the sudden changes and the controlling factors of deep coal mining conditions

It was illustrated that the mining conditions inducing disasters changed with depth both in regularity of gradual and sudden change. The sudden change depth for different disaster conditions are different and controlled by different factors. The high temperature and its change with depth are mainly controlled by strata structures and rock heat conductivity property, the high rock stress and dynamical engineering disasters and their change with depth are mainly controlled by tectonic conditions, roof strata rock property and deep rock mechanical property, coal mine water disasters and their change with depth are mainly controlled by rock mechanical property of coal seam floor and regional groundwater circulation conditions, gas disaster conditions and their change with depth are mainly controlled by buried conditions of coal seam and opening conditions of geological structures. It is mentioned that the key point for the control of deep coal mining disaster is to clearly understand the sudden change depth of different factors causing disasters.

Groundwater circulation patterns and isotope geochemistry in the Chalk of Northern Ireland

The Cretaceous Chalk or Ulster White Limestone Formation (UWLF) of Northern Ireland predominantly occurs as a fracture flow aquifer that exhibits both deep artesian conditions at subcrop, and shallow karstic behaviour at outcrop. The UWLF is underlain by impermeable Triassic/Jurassic mudstones and overlain by Tertiary basalt lavas. Aconceptual regional groundwater circulation model has been developed using limited hydraulic head information. This has been tested using isotope geochemistry. Springs draining the outcrop section of the aquifer produce ‘recent’ waters with abundant modern carbon. Stable isotopes and hydrochemistry have demonstrated that surface runoff is the predominant source of recharge to the outcrop region (as opposed to basalt leakage recharge). UWLF spring water chemistry is often similar to that of the potential recharge waters. Deep regional flow in the aquifer can occur by seepage from the outcrop areas, but limited ‘outlet’ potential means that circulation is slow and distinct hydrochemical evolution occurs. Radiocarbon ages for this groundwater have beendetermined at between 10 000 years and 20 000 years. Isotope geochemistry in the UWLF of Northern Ireland thus provides a powerful method for differentiating groundwaters in terms of their interaction with rock,their circulation route and their age. These have major implications for resource modelling and vulnerability.

Heat transport by groundwater flow during the Baikal rift evolution

A two-dimensional modelling study of sedimentation, fluid flow, and heat flow in the Baikal rift basin undergoing flank uplift and basin subsidence has been performed in order to understand the impact of these processes on the surface heat flow signal. Heat flow anomalies of different scales and magnitudes have been observed at the sediment surface of the lake Baikal basin, and the presence of a hydrothermal vent suggests that fluids play an important role in the regional distribution of heat flow. The BASIN-code applied for this study allows to simulate topographically and compaction-driven hydrodynamical fluid flow and coupled heat transfer. The flank uplift history provides the basis for a regional groundwater circulation towards the central basin area, with predicted Darcy velocities at present-day situation in the basement varying between 1 and 100 cm/year. Within the basin, the presence of aquifers and the pinch-out layering has a major control on the flow field, and compaction-driven flow velocities are strongly altered when combined with topography-driven flow. When velocities in the basement are larger than several centimeters per year, the regional fluid circulation is an effective mechanism of heat redistribution. Heat is brought from the flanks towards the basin area, with largest heat transported at a depth of 1–2 km at both sides. During the flank uplift, heat advection increases, with secondary variation related to the deposition of sedimentary layers. The heat flow is increased over the basin and reduced in the flanks, with a total heat output balance always positive. The extra heat output over the modelled transection is 2–10% of the initial heat output. The maximum computed heat fluxes are smaller than measured in the heat flow anomalies of the lake Baikal basin. Nevertheless, the model suggests that flow in the sedimentary basin combined with a topographically driven heat advection in the surrounding basement is a sufficient mechanism to account for the increased heat flow over the basin and the main features of the heat flow distribution.

Application of uranium-series disequilibrium to studies of groundwater mixing in the Harwell region, U.K.

Regional groundwater systems in a mixed sedimentary sequence beneath the Harwell site have been studied as part of a national research programme of investigation into the feasibility of disposal of low- and intermediate-level radio-active wastes into argillaceous rocks. The principal aim of the programme is to establish the groundwater flow patterns using hydrogeological and geochemical methods. Basic groundwater chemical patterns in association with variations in isotope content and uranium-series disequilibrium can provide data on the permeabilities of clay lithologies in a sedimentary sequence, which can be independent of hydraulic models, and thus lend support to them. Groundwater samples derived from the high-permeability, formations in the Harwell region have been analysed for U and Th content and 234U/ 238U 230Th/ 234U and 230Th/ 232Th activity ratios. The U isotopic signatures have been interpreted in terms of the regional groundwater circulation and mixing patterns. The most significant zones of groundwater mixing determined from hydraulic and hydrochemical data, and supported by the U isotopic data presented here are situated just beneath the edge of the confined strata. Such zones coincide with the locations of hydraulic lows particularly in the Corallian formation towards which the regional groundwaters move. The work reported here concludes that U isotopic signatures can be used to identify water masses and to evaluate mixing of groundwaters in a sedimentary sequence on a regional scale, thus effectively complementing hydrogeological, hydrochemical and isotopic methods.

Ground-Water Circulation in the Meade Thrust Allochthon Evaluated by Radiocarbon Techniques

The Meade thrust, in southeastern Idaho, is a major element of the Western Overthrust Belt. The allochthon is of geo-economic importance both as a potential hydrothermal area and as the principal mining area within the Western Phosphate Field. To assist in the development of these two resources, an understanding of the regional ground-water circulation was sought. Geologic and hydrologic data from boreholes in this area are virtually nonexistent. Waterwell development in the area has not occurred because of the abundance of springs and only a few hydrocarbon exploration boreholes have been drilled. Thus, the problem lends itself to evaluation by isotope hydrologic and geochemical methods. Ten springs from within the thrust block and around its periphery were sampled for major ions, 2H/18O, and 14C/13C analysis. Data from these analyses and from field geologic evidence have identified two distinct flow regimes within the Meade thrust allochthon. Shallow flow systems lie above the impermeable Phosphoria Formation, usually within a few hundred meters of the surface. Most of the spring waters from this system are recent and cool. In all cases, they have mean subsurface residence times of less than a few hundred years. The deeper flow systems which lie below the Phosphoria formation are hydraulically isolated from the shallow system. Warm waters from these springs have 14C contents suggesting mean ground-water residence times on the order of 15,000 years. Although these waters could have circulated to as deep as 1900m, 2H/18O results show that high temperatures were never reached. There is no evidence to suggest that water from beneath the Meade thrust has contributed to the circulation in the allochthon.