Regional Groundwater Flow System Research Articles

Potential impact of climate change on groundwater resources in the Central Huai Luang Basin, Northeast Thailand

The Central Huai Luang Basin is one of the important rice producing areas of Udon Thani Province in Northeastern Thailand. The basin is underlain by the rock salt layers of the Maha Sarakham Formation and is the source of saline groundwater and soil salinity. The regional and local groundwater flow systems are the major mechanisms responsible for spreading saline groundwater and saline soils in this basin. Climate change may have an impact on groundwater recharge, on water table depth and the consequences of waterlogging, and on the distribution of soil salinity in this basin. Six future climate conditions from the SEACAM and CanESM2 models were downscaled to investigate the potential impact of future climate conditions on groundwater quantity and quality in this basin. The potential impact was investigated by using a set of numerical models, namely HELP3 and SEAWAT, to estimate the groundwater recharge and flow and the salt transport of groundwater simulation, respectively. The results revealed that within next 30years (2045), the future average annual temperature is projected to increase by 3.1°C and 2.2°C under SEACAM and CanESM2 models, respectively, while the future precipitation is projected to decrease by 20.85% under SEACAM and increase by 18.35% under the CanESM2. Groundwater recharge is projected to increase under the CanESM2 model and to slightly decrease under the SEACAM model. Moreover, for all future climate conditions, the depths of the groundwater water table are projected to continuously increase. The results showed the impact of climate change on salinity distribution for both the deep and shallow groundwater systems. The salinity distribution areas are projected to increase by about 8.08% and 56.92% in the deep and shallow groundwater systems, respectively. The waterlogging areas are also projected to expand by about 63.65% from the baseline period.

Modification of the SWAT model to simulate regional groundwater flow using a multicell aquifer

AbstractThe soil and water assessment tool (SWAT) has been widely used and thoroughly tested in many places in the world. The application of the SWAT model has pointed out that 2 of the major weaknesses of SWAT are related to the nonspatial reference of the hydrologic response unit concept and to the simplified groundwater concept, which contribute to its low performance in baseflow simulation and its inability to simulate regional groundwater flow. This study modified the groundwater module of SWAT to overcome the above limitations. The modified groundwater module has 2 aquifers. The local aquifer, which is the shallow aquifer in the original SWAT, represents a local groundwater flow system. The regional aquifer, which replaces the deep aquifer of the original SWAT, represents intermediate and regional groundwater flow systems. Groundwater recharge is partitioned into local and regional aquifer recharges. The regional aquifer is represented by a multicell aquifer (MCA) model. The regional aquifer is discretized into cells using the Thiessen polygon method, where centres of the cells are locations of groundwater observation wells. Groundwater flow between cells is modelled using Darcy's law. Return flow from cell to stream is conceptualized using a non‐linear storage–discharge relationship. The SWAT model with the modified aquifer module, the so‐called SWAT‐MCA, was tested in 2 basins (Wipperau and Neetze) with porous aquifers in a lowland area in Lower Saxony, Germany. Results from the Wipperau basin show that the SWAT‐MCA model is able (a) to simulate baseflow in a lowland area (where baseflow is a dominant source of streamflow) better than the original model and (b) to simulate regional groundwater flow, shown by the simulated groundwater levels in cells, quite well.

English

To enable sustainable management of groundwater resources, knowledge of dominant hydrogeological processes is fundamental. In this study, stable isotopes of water [δ18O  and δD] and major inorganic ions were used to investigate recharge and groundwater flow processes in a catchment underlain by fractured and faulted hornblende-biotite-gneiss. Spatial and temporal geochemical distributions consistently showed Mg-Ca-HCO3 dominated water facies. Evaporation was established to be the main process affecting isotopic enrichment in the study area. Stable isotopic and geochemical data revealed that a combination of thin overburden soil of up to 30 mm thickness and presence of fractures seem to enable localized rapid preferential recharge processes of isotopically enriched rainwater in shallow groundwater around the ridge section. However, the thicker overburden soil (up to 3 m) along the valley seems to allow only isotopically depleted large rain events to recharge deeper groundwater. The isotopically enriched small rain events seem to be allowed to evaporate before recharging groundwater in areas with thicker overburden soils. It was further established that the valley section also receives regional lateral groundwater recharge from high altitude areas. Regional groundwater flow system in the northwest-southeast (NW-SE) direction was thus established with local flows confirmed around the ridge section. It was also revealed that Ntcheu Fault acts as a conduit of regional groundwater flow in the NW-SE direction. Inter-aquifer connectivity and surface water and groundwater interaction were construed around the ridge section and around B12 and R8, respectively. The rapid recharge and flow phenomena in this type of geological media make the resource susceptible to pollution and inter-annual climatic variabilities. It is prudent therefore to consider such information when implementing other developmental plans in the catchment.   Key words: Groundwater recharge, groundwater flow, fractures, geological fault, stable isotopes, geochemistry, hornblende-biotite-gneiss, Malawi.

Hydrogeochemical Characteristics and Geothermometry Applications of Thermal Waters in Coastal Xinzhou and Shenzao Geothermal Fields, Guangdong, China

Two separate groups of geothermal waters have been identified in the coastal region of Guangdong, China. One is Xinzhou thermal water of regional groundwater flow system in a granite batholith and the other is thermal water derived from shallow coastal aquifers in Shenzao geothermal field, characterized by high salinity. The hydrochemical characteristics of the thermal waters were examined and characterized as Na-Cl and Ca-Na-Cl types, which are very similar to that of seawater. The hydrochemical evolution is revealed by analyzing the correlations of components versus Cl and their relative changes for different water samples, reflecting different extents of water-rock interactions and clear mixing trends with seawaters. Nevertheless, isotopic data indicate that thermal waters are all of the meteoric origins. Isotopic data also allowed determination of different recharge elevations and presentation of different mixing proportions of seawater with thermal waters. The reservoir temperatures were estimated by chemical geothermometries and validated by fluid-mineral equilibrium calculations. The most reliable estimates of reservoir temperature lie in the range of 148–162°C for Xinzhou and the range of 135–144°C for Shenzao thermal waters, based on the retrograde and prograde solubilities of anhydrite and chalcedony. Finally, a schematic cross-sectional fault-hydrology conceptual model was proposed.

Investigating regional groundwater flow systems in baja california central desert region

In Northern Baja California environmental conservation may not be fully accomplished in the absence of a clear understanding of key factors controlling and protecting biodiversity Groundwater is an environmental agent triggering the presence of a wide range of arid ecosystems therefore any sustainable development in the region has to maintain a balance between environmental social and economical benefits The present study has placed a great emphasis on the understanding and distribution of groundwater flow systems and their environmental interactions in the protected area of the Valle de Los C iacute rios using physical chemical and isotope methods Results show the presence of three groundwater groups portraying contrasting flow systems conditions Analyzed samples have been classified as fresh brackish and thermal alkaline groundwater as suggested by pH temperature and TDS The Br Cl ratio in most samples was close to the ocean molar value x indicating the importance of marine aerosol fraction in the recharged waters however Mg Cl Na Cl Ca Cl SO Cl and Cl HCO Cl ratios were significantly different to those of seawater mixing conditions Dominant ions Na Ca and Cl chemical age proxies Li and O suggest waters with long residence time with the possibility of been recharge during the late glacial period Thermal ground water shows low minor and trace element content and exhibits the dominance of Na plagioclase hydrolysis their low levels of Sr and Ba suggest negligible contact with calcareous rocks and their estimated temperature at depth to deg C and TDS suggest groundwater has travelled through fractured media This introduction study suggests that desertic species in Valle de Los Cirios are highly sustained by groundwater of local and intermediate flows

The case of the Biscayne Bay and aquifer near Miami, Florida: density-driven flow of seawater or gravitationally driven discharge of deep saline groundwater?

Coastal groundwater flow investigations at the Cutler site of the Biscayne Bay south of Miami, Florida, gave rise to the dominating concept of density-driven flow of seawater into coastal aquifers indicated as a saltwater wedge. Within that wedge, convection-type return flow of seawater and a dispersion zone were concluded to be the cause of the Biscayne aquifer ‘seawater wedge.’ This conclusion was merely based on the chloride distribution within the aquifer and on an analytical model concept assuming convection flow within a confined aquifer without taking non-chemical field data into consideration. This concept was later labeled the ‘Henry problem,’ which any numerical variable-density flow program has to be able to simulate to be considered acceptable. Revisiting the summarizing publication with its record of piezometric field data (heads) showed that the so-called seawater wedge was actually caused by discharging deep saline groundwater driven by regional gravitational groundwater flow systems. Density-driven flow of seawater into the aquifer was not found reflected in the head measurements for low and high tide which had been taken contemporaneously with the chloride measurements. These head measurements had not been included in the assumption of a seawater wedge and associated dispersion zone and convection cell. The Biscayne situation emphasizes the need for any chemical interpretation of flow pattern to be backed up by head data as energy indicators of flow fields. At the Biscayne site density-driven flow of seawater did not and does not exist. This conclusion was confirmed by five independent methods. The hydrostatic use of vertical buoyancy forces needs, under hydrodynamic boundary conditions, to be replaced with buoyancy forces along the direction of the pressure potential forces [(grad p)/density] which, in the subsurface, can be pointed in any direction in space.

Using water stable isotopes for tracing surface and groundwater flow systems in the Barlow-Ojibway Clay Belt, Quebec, Canada

This study aims to improve the understanding of surface and groundwater flow systems based on water stable isotope data in a 19,549 km2 region of the Barlow-Ojibway Clay Belt, in western Quebec, Canada. The available geochemical database contains 645 samples including precipitation, snow cores, surface waters, groundwater and springs. All samples were analyzed for water stable isotopes (δ2H-δ18O) and complementary tritium analyses were conducted on 98 groundwater and spring samples. Precipitations depict a clear temperature-dependent seasonal pattern and define a local meteoric water line (LMWL) without a latitudinal trend in δ2H-δ18O. Samples collected from the snowpack plot on the LMWL, suggesting that the bulk snowpack preserves the isotopic composition of precipitation throughout the frozen period, prior to the spring snowmelt. Surface water samples define a local evaporation line (LEL), and evaporation over inflow (E/I) ratios range between 0 and 36%. Groundwater and spring samples are evenly distributed around the LMWL, suggesting that evaporation processes are limited prior to infiltration and that surface waters do not significantly contribute to groundwater recharge. Shallow unconfined aquifers present a greater variability in δ2H-δ18O compared to confined aquifers located farther down gradient, suggesting the mixing of varied recharge waters along the regional groundwater flow system. A three-component mixing model based on isotopic and specific electrical conductivity data allows the quantification of such mixing processes. The interpretation of isotopic data constrains a regional-scale conceptual model of groundwater flow systems and describes processes related to the timing of recharge, evaporation, mixing and discharge.

How does the groundwater influence the water balance of a lowland lake? A field study from Lake Stechlin, north-eastern Germany

The subsurface catchment of Lake Stechlin is quite complex. Water level measurements in groundwater wells with different screen depths show that groundwater in the lake’s catchment may partly discharge to the lake and partly leak to deeper aquifers. Simple steady-state 2D vertical modelling of eight different scenarios (different leakage areas, different hydraulic conductivities) was conducted in order to explore the local and regional groundwater flow system around Lake Stechlin and the origin and possible magnitude of lacustrine groundwater discharge. A comparison of the different scenarios with in situ measurements of lacustrine groundwater discharge by temperature sticks and seepage meters revealed that only a part of the groundwater recharge in the catchment discharges to the lake while the rest leaks into deeper aquifers. This implies a differentiation of the subsurface catchment into a local and a regional flow system and impacts on travel times and chemical reaction time scales.

Hydraulic connection affects uranium distribution in the Gas Hure salt lake, Qaidam Basin, China.

The widespread hydraulic connection is necessary for the formation of a salt lake. However, only limited studies have ever been carried out to investigate the influence of the hydraulic connection on the distribution of elements around certain salt lake. In this study, a total of 66 water samples (including river water, stream water, spring water, brine, intercrystalline brine, well water, and drilling brine) were collected around the Gas Hure salt lake (GSKLH) to investigate the relationship between hydraulic connection and uranium (U) distribution via hydrochemistry and isotope (234U/238U, δ11B) techniques. The results suggested that the GSKLH was recharged by water from the Kulamulekesay and Atetikan rivers, groundwater (borehole brine and some intercrystalline brine), and deep fluid (some intercrystalline brine), with each contributing 44.03%, 14.95%, and 41.02% of total recharge, respectively. The U-bearing rock was dominated mainly by silicates, carbonates, and evaporites in the high mountain area (region 1), overflow area (region 2), and plain area (region 3) of the GSKLH, respectively. In the GSKLH, the U distribution was strongly correlated with hydraulic connection and the U concentration was influenced by both groundwater flow system and flow velocity (represented by the γCl-/γCa2+ ratio). Thus, U was enriched under the conditions of regional groundwater flow system and slow velocity in the GSKLH.

A Practical, Robust Methodology for Acquiring New Observation Data Using Computationally Expensive Groundwater Models

AbstractRegional groundwater flow models play an important role in decision making regarding water resources; however, the uncertainty embedded in model parameters and model assumptions can significantly hinder the reliability of model predictions. One way to reduce this uncertainty is to collect new observation data from the field. However, determining where and when to obtain such data is not straightforward. There exist a number of data‐worth and experimental design strategies developed for this purpose. However, these studies often ignore issues related to real‐world groundwater models such as computational expense, existing observation data, high‐parameter dimension, etc. In this study, we propose a methodology, based on existing methods and software, to efficiently conduct such analyses for large‐scale, complex regional groundwater flow systems for which there is a wealth of available observation data. The method utilizes the well‐established d‐optimality criterion, and the minimax criterion for robust sampling strategies. The so‐called Null‐Space Monte Carlo method is used to reduce the computational burden associated with uncertainty quantification. And, a heuristic methodology, based on the concept of the greedy algorithm, is proposed for developing robust designs with subsets of the posterior parameter samples. The proposed methodology is tested on a synthetic regional groundwater model, and subsequently applied to an existing, complex, regional groundwater system in the Perth region of Western Australia. The results indicate that robust designs can be obtained efficiently, within reasonable computational resources, for making regional decisions regarding groundwater level sampling.

Lead and sulfur isotope composition of trace occurrences of Mississippi Valley-type mineralization in the U.S. midcontinent

Mississippi Valley-type (MVT) Zn-Pb mineralization in the U.S. mid-continent has formed in numerous widely dispersed trace occurrences and in a few large ore deposits that have been mined. Based on similarly high fluid inclusion salinities and high homogenization temperatures relative to inferred burial depth, the trace MVT occurrences and MVT ore deposits may be genetically related by having precipitated from the same regional groundwater flow system mobilized by Pennsylvanian-Permian tectonism. The purpose of the present study was to test the robustness of this inferred genetic relationship by characterizing the Pb and S isotope composition of trace MVT occurrences in light of the much more thoroughly studied MVT ore deposits.Lead and sulfur isotope compositions have been obtained for trace MVT occurrences from three broad regions of the U.S. mid-continent: the Forest City basin (FCB), the Illinois basin (ILB), and the Greater Upper Mississippi Valley (GUMV) region. Lead isotope compositions of trace MVT occurrences from each of these three areas define discrete linear trends that are similarly oriented but not strictly collinear with one another or with the Pb isotope trends of MVT ore deposits, suggesting that trace MVT occurrences in each of these three regions derived their Pb from separate pairs of end-member Pb sources. An exception is the GUMV trace MVT occurrences and the Upper Mississippi (UMV) district ores, which appear to have collinear Pb isotope trends and thus may be the most closely related genetically with respect to sources of Pb. All of the trace MVT occurrences have radiogenic Pb suggesting derivation from Precambrian igneous basement rocks and not from the formations that host trace MVT mineralization. Trace MVT occurrences tend to have less radiogenic Pb than the MVT ore deposits, although trace MVT occurrences from the GUMV region are the most heterogeneous and can have Pb that is as radiogenic as the MVT ore deposits. Regionally, Pb isotope compositions of trace MVT occurrences tend to become less radiogenic southwestward and southward, but do not correlate with host rock age, and by proxy, distance from the Precambrian basement.The sulfur isotope compositions of sphalerite and galena from the trace MVT occurrences are light compared to the MVT ore deposits and likely reflect incomplete reduction of marine sulfate reservoirs or derivation from organic matter. The Pb and S isotope compositions of the trace MVT occurrences do not correlate well, suggesting that Pb and S were not transported together in the same fluid.

The geochemistry of Don Juan Pond: Evidence for a deep groundwater flow system in Wright Valley, Antarctica

Don Juan Pond (DJP), Antarctica, is one of the most unusual surface waters on Earth because of its CaCl2-rich composition. To investigate the evolution of pond waters during closed-basin evaporation and to understand the source of brines responsible for the chemistry of DJP, we apply a newly developed low-temperature aqueous model in the Na–K–Ca–Mg–Cl system to DJP. By modeling the closed-basin evaporation of DJP and comparing ionic ratios between DJP surface water, deep groundwater, shallow groundwater, and other surface chemistries in Wright Valley, we find that DJP is best explained by upwelling deep groundwater, as opposed to recent hypotheses proposing shallow groundwater sources. The early closed-basin evolution of brines in our model accurately predicts observed chemistries in DJP; however, late-stage closed-basin evaporation produces Mg–K-rich brines and salts that do not match the CaCl2-rich brine in DJP. Based on groundwater inflow rates to DJP, we estimate that even the most concentrated brines in DJP have undergone closed-basin evaporation for less than a year. To explain the observed lack of Mg2+ and K+ accumulation in DJP over time, and the surprisingly young age for the brines, we deduce that DJP is a localized upwelling from a regional groundwater flow-through system in which evaporated DJP brines are recycled back into the subsurface over yearly timescales. The existence of a regional groundwater flow system beneath DJP has implications for water and solute budgets in cold desert ecosystems, and may provide clues for the formation of groundwater and aqueous flows on Mars.

Tracking changes in the specific storage of overburden rock during longwall coal mining

The hydraulic properties of geologic formations, such as specific storage and hydraulic conductivity, are known to vary spatially due to formation heterogeneity; however, they may also change temporally if the formation undergoes mechanical disturbance such as occurs during mining. Characterising changes in hydraulic properties is important for groundwater systems that are disturbed by mining. Conventional hydrogeological investigations rely on literature derived values or limited programs of field testing (e.g. pumping test) to define specific storage; but these approaches do not consider the change in compressibility that may occur due to mechanical disturbance. This paper presents, for the first time, direct in situ measurements of the changes in compressibility, and consequently specific storage, as a result of mechanical disturbance within geologic formations overlying underground mining operations. The in situ measurements of compressibility are derived in this study from the pore-pressure response to barometric loading or strains induced by earth tides. Even prior to disturbance, the compressibility obtained from in situ measurements was found to be an order of magnitude lower than that measured on core samples by unconfined strength test in the laboratory. The differences in compressibility for the intact formation are likely due to sample disturbance and the differences in strain level imposed by the field and laboratory methods. The increase in compressibility of the overburden rock varied with the location of the monitoring site relative to longwall extraction and the level of strain the formation sustained. The study found that quantifying the changes in compressibility, and consequently specific storage, as a result of mine-induced disturbance is critical to our understanding of subsidence, the extent of depressurization, and the impact that mining may have on regional groundwater flow systems.

Groundwater age determination using 85Kr and multiple age tracers (SF6, CFCs, and 3H) to elucidate regional groundwater flow systems

Study regionThe Kumamoto area (945km2) in the south of Japan, where almost 100% of the drinking water is dependent on groundwater. Study focusSimultaneous measurement of groundwater dating tracers (85Kr, chlorofluorocarbons [CFCs], sulphur hexafluoride [SF6], and 3H) was performed in the Kumamoto area, to elucidate the regional groundwater flow system and obtain improved estimates of groundwater ages. The groundwater ages were estimated from the 85Kr concentrations in nine locations from three areas: along two major groundwater flow lines (A–A’ and B–B’); and a high-nitrate-input recharge area (C area). New hydrological insights for the regionThe groundwater ages could not be estimated using CFCs or SF6, particularly in the urban areas because of artificial additions to the concentration over almost the entire study area. However, even in these regional circumstances, apparent ages of approximately 16, 36, and not less than 55 years were obtained for three locations on the A–A’ line (recharge area, discharge area, and stagnant zone of groundwater, respectively) from 85Kr measurements. This trend was also supported by lumped parameter model analysis using a time series of 3H observations. In contrast, along the B–B’ line, the groundwater age of not less than 55 years at three locations, including the recharge to discharge area, where CFCs and SF6 were not detected, implies old groundwater: this is also the area in which denitrification occurs. In the C area, very young groundwater was obtained from shallow water and older groundwater was detected at greater depths, as supported by the long-term fluctuations of the NO3−–N concentration in the groundwater. The results of this study can be effectively used as a “time axis” for sustainable groundwater use and protection of groundwater quality in the study area, where groundwater accounts for almost 100% of the drinking water resources.

広域地下水流動系を解明するための環境トレーサーと地下水位変動の有効性―十勝平野を例として―

広域地下水流動系を解明するための環境トレーサーと地下水位変動の有効性―十勝平野を例として―

Erratum to: Hydrogeochemical and isotopic tracers for identification of seasonal and long-term over-exploitation of the Pleistocene thermal waters

The aim of the study was to develop and test an optimal and cost-effective regional quality monitoring system in depleted transboundary low-temperature Neogene geothermal aquifers in the west Pannonian basin. Potential tracers for identification of seasonal and long-term quality changes of the Pleistocene thermal waters were investigated at four multiple-screened wells some 720 to 1570 m deep in Slovenia. These thermal waters are of great balneological value owing to their curative effects and were sampled monthly between February 2014 and January 2015. Linear correlation and regression analyses, ANOVA and Kolmogorov–Smirnov two-sample test for two independent samples were used to determine their seasonal and long-term differences. Temperature, pH, electrical conductivity, redox potential and dissolved oxygen did not identify varying inflow conditions; however, they provided sufficient information to distinguish between the four end-members. Characteristic (sodium) and conservative (chloride) tracers outlined long-term trends in changes in quality but could not differentiate between the seasons. Stable isotopes of δ 18O and δ 2H were used to identify sequential monthly and long-term trends, and origin and mixing of waters, but failed to distinguish the difference between the seasons. A new local paleo-meteoric water line (δ 2H = 9.2*δ 18O + 26.3) was outlined for the active regional groundwater flow system in the Pannonian to Pliocene loose sandstone and gravel. A new regression line (δ 2H = 2.3*δ 18O–45.2) was calculated for thermomineral water from the more isolated Badenian to Lower Pannonian turbiditic sandstone, indicating dilution of formation water. Water composition was generally stable over the 1-year period, but long-term trends indicate that changes in quality occur, implying deterioration of the aquifers status.

Hydrogeochemical and isotopic tracers for identification of seasonal and long-term over-exploitation of the Pleistocene thermal waters.

The aim of the study was to develop and test an optimal and cost-effective regional quality monitoring system in depleted transboundary low-temperature Neogene geothermal aquifers in the west Pannonian basin. Potential tracers for identification of seasonal and long-term quality changes of the Pleistocene thermal waters were investigated at four multiple-screened wells some 720 to 1570m deep in Slovenia. These thermal waters are of great balneological value owing to their curative effects and were sampled monthly between February 2014 and January 2015. Linear correlation and regression analyses, ANOVA and Kolmogorov-Smirnov two-sample test for two independent samples were used to determine their seasonal and long-term differences. Temperature, pH, electrical conductivity, redox potential and dissolved oxygen did not identify varying inflow conditions; however, they provided sufficient information to distinguish between the four end-members. Characteristic (sodium) and conservative (chloride) tracers outlined long-term trends in changes in quality but could not differentiate between the seasons. Stable isotopes of δ (18)O and δ (2)H were used to identify sequential monthly and long-term trends, and origin and mixing of waters, but failed to distinguish the difference between the seasons. A new local paleo-meteoric water line (δ (2)H = 9.2*δ (18)O + 26.3) was outlined for the active regional groundwater flow system in the Pannonian to Pliocene loose sandstone and gravel. A new regression line (δ (2)H = 2.3*δ (18)O-45.2) was calculated for thermomineral water from the more isolated Badenian to Lower Pannonian turbiditic sandstone, indicating dilution of formation water. Water composition was generally stable over the 1-year period, but long-term trends indicate that changes in quality occur, implying deterioration of the aquifers status.

Interpreting Variations in Groundwater Flows from Repeated Distributed Thermal Perturbation Tests.

To better understand the groundwater resources of southern Nye County, Nevada, a multipart distributed thermal perturbation sensing (DTPS) test was performed on a complex of three wells. These wells penetrate an alluvial aquifer that drains the Nevada National Security Site, and characterizing the hydraulic properties and flow paths of the regional groundwater flow system has proven very difficult. The well complex comprised one pumping well and two observation wells, both located 18 m from the pumping well. Using fiber-optic cables and line heaters, DTPS tests were performed under both stressed and unstressed conditions. Each test injects heat into the water column over a period of one to two days, and observes the rising temperature during heat injection and falling temperatures after heating ceases. Aquifer thermal properties are inferred from temperature patterns in the cased section of the wells, and fluxes through the 30-m screened section are estimated based on a model that incorporates conductive and advective heat fluxes. Vertical variations in flux are examined on a scale of tens of cm. The actively flowing zones of the aquifer change between the stressed and unstressed test, and anisotropy in the aquifer permeability is apparent from the changing fluxes between tests. The fluxes inferred from the DTPS tests are compared to solute tracer tests previously performed on the same site. The DTPS-based fluxes are consistent with the fastest solute transport observed in the tracer test, but appear to overestimate the mean flux through the system.

A methodology for identifying ecologically significant groundwater recharge areas

A methodology has been developed to delineate recharge areas that support groundwater-dependent ecological features such as wetlands and coldwater streams. Numerical groundwater models are employed together with particle tracking techniques to link recharge areas to specific ecological features. Kernel density estimation methods are used to identify areas that contain the maximum number of particle endpoints per unit area. These high-density endpoint clusters are subsequently mapped as ecologically significant groundwater recharge areas (ESGRAs). The technique can be utilized with either finite-element or finite-difference groundwater models which are loosely coupled or fully integrated with a distributed hydrologic model. A good representation of both the surface water and the shallow groundwater system is required and the models should be based on a rigorous understanding of the regional, watershed and local-scale geologic, hydrogeologic and hydrologic setting. This manuscript describes how the methodology was employed to delineate ESGRAs for the Oro North, Oro South and Hawkestone Creeks subwatersheds, located in the northwest portion of the Lake Simcoe watershed in southern Ontario, Canada. The three study subwatersheds are connected to the Oro Moraine complex, a high-recharge feature that supports multiple watersheds and the regional groundwater flow system. A transient, integrated surface water/groundwater model was developed for the watersheds flanking the moraine and was calibrated to daily streamflow and groundwater-level observations. Particle tracking was then used to define the groundwater flow system between all streams and wetlands and the areas of recharge, and provided insight into the hydrologic function of the Oro Moraine. After optimization of the cluster analysis procedure, 24% of the 125-km² study area was mapped as having recharge areas that support significant ecological features. As required by the Lake Simcoe Protection Plan, the ESGRAs will be protected from future development to ensure the maintenance of the groundwater-fed ecosystems they support. The methodology described in this paper provides a consistent, objective and technically sound means of identifying and delineating ESGRAs, and has recently been applied to other watersheds in southern Ontario.

3H/3He, 14C and (U–Th)/He groundwater ages in the St. Lawrence Lowlands, Quebec, Eastern Canada

This study attempts to place constraints on groundwater residence times using helium isotopes and 14C of a regional groundwater flow system – the Becancour River watershed – located midway between Montreal and Quebec City (Quebec, Canada). This densely populated region is one of the main targets for shale gas exploitation in Eastern Canada. For this reason, this watershed has been the focus of a detailed aquifer study to gain a better understanding of groundwater resources, both in terms of availability and quality. In the current study, noble gases were sampled and analyzed in twenty-eight wells from a Quaternary granular aquifer and a regional bedrock aquifer of Ordovician age. Tritium (3H) and radiocarbon (A14C) activities were measured on selected wells. Helium isotopic ratios 3He/4He (R) normalized to that of the atmosphere (Ra=1.386×10−6) range from R/Ra=0.039±0.003 to 3.109±0.065. The helium isotopic signatures point to the presence of three water bodies: 1) modern infiltration water with nearly atmospheric helium isotopic ratio and little post-bomb tritium recharging the shallower granular aquifer; 2) mid-50s tritium-rich water slightly mineralized; and 3) an older water component rich in terrigenic helium flowing in from the bedrock fractured aquifer. Uncorrected 14C ages range from 15ka to modern. 14C is affected by several dead carbon reservoirs related to carbonate dissolution, cations exchange, oxidation of organic matter and methanogenesis. Adjusted 14C ages calculated with NETPATH range from 7ka to modern. Older 14C ages correspond to the end of the regional re-organization of the hydrological system following deglaciation and isostatic rebound. Noble gas recharge paleotemperatures are 4–9°C colder than the present temperatures, although no clear relation with ages has been found. The relationship between the helium isotopic ratios and 14C ages suggests that the regional bedrock aquifer is affected by mixing between these three water sources. Calculated (U–Th)/4He ages can be partially explained by in situ production of 4He in the aquifer rock and the addition of a radiogenic helium source external to the aquifer. Calculated minimum helium fluxes of 1.0×10−8 to 1.8×10−7cm3STPcm−2yr−1 are tens to hundreds of times lower than the average continental crust flux of 3.3×10−6cm3STPcm−2yr−1, suggesting local sources, possibly from production of radiogenic helium in the shale gas formations underlying the studied aquifers. The occurrence of old groundwater in this aquifer system clearly limits the renewable resource and increases the risk of overexploitation in the case of increased use or in the case of pollution from different sources.