Crystalline Bedrock Aquifers Research Articles

Long‐term trends in mountain groundwater levels across Canada and the United States

AbstractMountains have a critical role in freshwater supply for downstream populations. As the climate changes, groundwater stored in mountains may help buffer the impacts to declining water resources caused by decreased snowpack and glacier recession. However, given the scarcity of groundwater observation wells in mountain regions, it remains unclear how mountain groundwater is being impacted by climate change across ecoregions. This study quantifies temporal trends in mountain groundwater levels and explores how various climatic, physiographic and anthropogenic factors affect these trends. We compiled data from 171 public groundwater observation wells within mountain regions across Canada and the United States, for which at least 20 years of monthly data is available. The Mann‐Kendall test for monotonic trend revealed that 54% of these wells have statistically significant temporal trends (p < 0.05) over the period of record, of which 69% were negative and therefore indicating overall declining groundwater storage. Wells in the western mountain ranges showed stronger trends (both positive and negative) than the eastern mountain ranges, and higher elevation wells showed fewer negative trends than the low elevation (<400 m asl) wells (p < 0.05). Correlation, Kruskal‐Wallis tests, stepwise multiple linear regression and random forest regression were used to identify factors controlling groundwater trends. Statistical analysis revealed that lower‐elevation mountain regions with higher average annual temperatures and lower average annual precipitation have the greatest declines in groundwater storage under climate change. Trends in temperature and precipitation, and ecoregion were also important predictors on groundwater level trends, highlighting geographic differences in how mountain wells are responding to climate change. Furthermore, sedimentary bedrock aquifers showed markedly more negative trends than crystalline bedrock aquifers. The findings demonstrate that the impact of climate change on mountain water resources extends to the subsurface, with important implications for global water resources.

Integration of ground geophysical methods to characterize near‐surface aquifer zones within an active mine

AbstractUnderstanding near‐surface groundwater storage, flow patterns, surface and groundwater interactions in mining areas can assist in making mining more efficient and profitable. This is especially important in opencast mines affected by water inflows that may negatively affect production and increase mining costs. We map and characterize the near‐surface aquifer zones at the opencast site of Tharisa Minerals, located in the southwestern region of the Bushveld Complex (South Africa). The main goal is to infer pit water inflow at the mine site and determine how it may be better controlled. The Bushveld Complex hosts partially connected and unconfined alluvial, shallow‐weathered and crystalline bedrock aquifers, which are often connected by small‐scale permeable zones. Seismic refraction tomography, multichannel analysis of surface waves, electrical resistivity tomography and borehole data are used to map and understand the different aquifer zones in the vicinity of the mine, as well as infer their relation to water inflow in the mine pits. The geophysical surveys map the overburden, weathered bedrock aquifer zone, and the top of the crystalline aquifer rock zone reasonably well. They reveal extensive and deep weathering, and possible high hydraulic conductivity in the vicinity of the mine. The results provide a better understanding of the mine's near‐surface environment, which could be used to implement effective and targeted dewatering techniques, thus enabling better pit inflow water control to improve mine working conditions and production.

Delineating the mechanisms controlling groundwater salinization using chemo-isotopic data and meta-heuristic clustering algorithms (case study: Saguenay-Lac-Saint-Jean region in the Canadian Shield, Quebec, Canada).

This study employed meta-heuristic clustering algorithms to determine the source and mechanism of groundwater salinization in Quebec's Saguenay-Lac-Saint-Jean (SLSJ) region, utilizing hydrogeochemical (38 inorganic constituents, including minor, major, and trace elements) and isotopic data (δ18O and δ2H). A total of 382 groundwater and precipitation samples were examined. Among the meta-heuristic algorithms, Artificial Bee Colony K-Means (ABCKM), Differential Evolution K-Means (DEKM), Harmony Search K-Means (HSKM), Particle Swarm Optimization K-Means (PSOKM), and Genetic K-Means (GKM) were used and investigated, and finally, PSOKM displayed superior performance and was chosen for further investigation. Analysis of diverse plots and hydrogeochemical modeling unveiled the impact of the Laflamme Sea invasion on groundwater chemistry. PSOKM1, PSOKM4, and PSOKM5 exhibited notable carbonate and silicate dissolution, with PSOKM4 demonstrating predominant carbonate dissolution. Cation exchange was identified through binary plots and Chloro Alkaline Index (CAI), with reverse cation exchange predominantly observed in most PSOKM4 samples, while positive values suggested direct cation exchange in other clusters. Spatial dynamics analysis using HFE-D indicated that salinization occurs as groundwater flows through crystalline bedrock aquifers, resulting in a transition from HCO3- dominance in PSOKM4 to Cl- dominance in the remaining clusters. Interaction between groundwater and rock along this path facilitated a transformation towards a Na-Cl end-member. The closely aligned stable isotopes with the Global Meteoric Water Line (GMWL) indicated a blend of meteoric water and seawater as the groundwater source.

Hydrochemical and isotopic comparison of crystalline bedrock aquifers in two geological disposal research sites in South Korea using samples collected during and after borehole drilling

Hydrochemical and isotopic characteristics of crystalline bedrock aquifers were investigated down to 500 m below ground level at two geological disposal research sites. Groundwater samples were collected every 50 m during drilling including water from several fracture zones (n = 24) and at water-conducting fracture zones after the drilling was completed (n = 27) to understand the impact of sampling times, i.e., during versus after drilling, on the characterization result. Both aquifers were highly weathered and showed transport in fractured rocks, which made it difficult to determine the effect of sampling times. Despite the similar ranges of hydraulic conductivities, the BH (gneiss) groundwater showed higher total dissolved solids including Na, F, and HCO3 and carbon-13 isotopes of dissolved inorganic carbon, older carbon-14 ages, but lower dissolved oxygen, oxygen and hydrogen isotopes of water, and sulfur isotopes of sulfate than the AH (granite) groundwater, indicating the extensive water rock interaction through long pathways in BH. Sulfate reduction was also observed in deep anoxic environment in BH, whereas the AH groundwater seemed to be affected by recent recharge through permeable fractures regardless of depth. Redox potential (Eh) increased in samples collected after drilling, in particular in AH, and Fe and negative Eh were not detected in groundwater collected after drilling in both areas, which implied oxidization after drilling in particular in aquifers with recent recharge. The study result suggests isotopic investigations combined with hydrochemical characterization to assess anoxic conditions for geological disposal, if possible, during drilling before grouting to accurately assess the redox condition in crystalline bedrock aquifers.

Assessing the role of groundwater recharge from tanks in crystalline bedrock aquifers in Karnataka, India, using hydrochemical tracers

The majority of India’s rural drinking water supply is sourced from groundwater, which also plays a critical role in irrigated agriculture, supporting the livelihoods of millions of users. However, recent high abstractions are threatening the sustainable use of groundwater, and action is needed to ensure continued supply. Increased managed aquifer recharge (MAR) using the > 200,000 existing tanks (artificially created surface water bodies) is one of the Indian government’s key initiatives to combat declining groundwater levels. However, few studies have directly examined the effectiveness of tank recharge, particularly in the complex fractured hydrogeology of Peninsular India. To address this gap, this study examined the impact of tanks in three crystalline bedrock catchments in Karnataka, southern India, by analysing the isotopic and hydrochemical composition of surface waters and groundwaters, combined with groundwater level observations. The results indicate that tanks have limited impact on regional groundwater recharge and quality in rural areas, where recharge from precipitation and groundwater recycling from irrigation dominate the recharge signal. In the urban setting (Bengaluru), impermeable surfaces increased the relative effect of recharge from point sources such as tanks and rivers, but where present, pipe leakage from public-water-supply accounted for the majority of recharge. Shallow groundwater levels in the inner parts of the city may lead to groundwater discharge to tanks, particularly in the dry season. We conclude that the importance of aquifer recharge from tanks is limited compared to other recharge sources and highly dependent on the specific setting. Additional studies to quantify tank recharge and revisions to the current guidelines for national groundwater recharge estimations, using a less generalised approach, are recommended to avoid over-estimating the role tanks play in groundwater recharge.

A Statistical Analysis of Well Failures in Baltimore County

A statistical evaluation of the Baltimore County water well database is performed to gain insight on the sustainability of domestic supply wells in crystalline bedrock aquifers over the last 15 years. Variables potentially related to well yield that are considered included well construction, geol ogy, well depth, and static water level. A variety of statistical methods are utilized to assess correlation and significance from a database of approxi mately 8,500 wells, and a logistic regression model is developed to predict the probability of well failure by geology type. Results of a two-way analysis of variance technique indicate that the average well depth and yield are sta tistically different among the established geology groups, and between failed and non-failed wells. The static water level is shown to be statistically dif ferent among the geology groups but not among failed and non-failed wells. A logistic regression model results that well yield is the most influential vari able for predicting well failure. Static water level and well depth was not found to be significant in predicting well failure.

Neonicotinoids in groundwater: presence and fate in two distinct hydrogeologic settings in Ontario, Canada

Neonicotinoids are a group of insecticides that are commonly used in agriculture throughout the world. Despite their widespread use, there are significant knowledge gaps related to the presence and fate of neonicotinoids in groundwater. This research explores the environmental parameters governing the transport of the most commonly found neonicotinoids from surface application to deeper groundwater in two distinct hydrogeologic settings. Both of these research sites are located in Ontario, Canada: one has an unconfined, sandy Quaternary aquifer while the other has a fractured, crystalline bedrock aquifer (Canadian Shield) under a thin layer of soil and till. Groundwater sampling was conducted using 18–26 monitoring intervals at each research site during each sampling round (April 2016, July 2016, August 2016, November 2016, and April 2017). Analysis of six neonicotinoids, two fungicides, two herbicides, and one ryanoid insecticide were conducted using liquid chromatography–mass spectrometry (positive electrospray ionization)–tandem mass spectrometry. During the groundwater sampling period, soil sampling was conducted, and a crop survey completed. Results from groundwater sampling found the neonicotinoids clothianidin, imidacloprid, and thiamethoxam with maximum concentrations and detection frequencies above the level of quantitation of 2.09, 0.7, 0.46 μg/L, and 2.2, 0.9, 1.3%, respectively. Numerical analysis revealed that under similar environmental conditions, clothianidin and thiamethoxam are released in a similar pattern that resembles a pulse. Future research into this subject should include a finer sampling timeframe to confirm theories about the pulsing nature of neonicotinoids in groundwater.

Using specific capacity for assessing of the factors controlling borehole productivity in crystalline bedrock aquifers of N’Zi, Iffou and Moronou regions in the eastern area of côte d’Ivoire

Hard rock aquifers are characterized by high heterogeneity which leads to difficulties in boreholes implementation in the eastern region of Côte d’Ivoire. This is due to the combination of many factors including poor knowledge of hydrogeological environment. The thrust of this study is to highlight the influence of borehole depth, lithotypes, weathering thickness and electrical resistivity of the geological structures on borehole productivity that exploit the crystalline aquifer systems in N’Zi, Iffou and Moronou regions. Bivariate analysis was used to determine the relationships between these factors and specific capacity values for measuring borehole productivity. The values ranged from 0.02 to 4.09 m3 h−1.m−1. The analysis shows that there is no correlation between productivity and weathering thickness. However, weathering depths between 10 and 69 m provide the highest specific capacity values. Schist is more productive than granite. For hydrogeological discontinuities interest, boreholes located in H and KH anomaly curve types were the most productive. As productivity diminishes with depth, a deeper borehole can be more productive if it reaches a geological structure that is favorable for groundwater flow. Those geological and hydrogeological parameters are extremely important in borehole productivity in the above-mentioned regions.

Arsenic variability and groundwater age in three water supply wells in southeast New Hampshire

Three wells in New Hampshire were sampled bimonthly over three years to evaluate the temporal variability of arsenic concentrations and groundwater age. All samples had measurable concentrations of arsenic throughout the entire sampling period and concentrations in individual wells had a mean variation of more than 7 μg/L. The time series data from this sampling effort showed that arsenic concentrations ranged from a median of 4 μg/L in a glacial aquifer well (SGW-65) to medians of 19 μg/L and 37 μg/L in wells (SGW-93 and KFW-87) screened in the bedrock aquifer, respectively. These high arsenic concentrations were associated with the consistently high pH (median ≥ 8) and low dissolved oxygen (median <0.1 mg/L) in the bedrock aquifer wells, which is typical of fractured crystalline bedrock aquifers in New Hampshire. Groundwater from the glacial aquifer often has high dissolved oxygen, but in this case was consistently low. The pH also is generally acidic in the glacial aquifer but in this case was slightly alkaline (median = 7.5). Also, sorption sites may be more abundant in glacial aquifer deposits than in fractured bedrock which may contribute to lower arsenic concentrations.Mean groundwater ages were less than 50 years old in all three wells and correlated with conservative tracer concentrations, such as chloride; however, mean age was not directly correlated with arsenic concentrations. Arsenic concentrations at KFW-87 did correlate with water levels, in addition, there was a seasonal pattern, which suggests that either the timing of or multiple sampling efforts may be important to define the full range of arsenic concentrations in domestic bedrock wells.Since geochemically reduced conditions and alkaline pHs are common to both bedrock and glacial aquifer wells in this study, groundwater age correlates less strongly with arsenic concentrations than geochemical conditions. There also is evidence of direct hydraulic connection between the glacial and bedrock aquifers, which can influence arsenic concentrations. Correlations between arsenic concentrations and the age of the old fraction of water in SGW-65 and the age of the young fraction of water in SGW-93 suggest that water in the two aquifers may be mixing or at least some of the deeper, older water captured by the glacial aquifer well may be from a similar source as the shallow young groundwater from the bedrock aquifer. The contrast in arsenic concentrations in the two aquifers may be because of increased adsorption capacity of glacio-fluvial sediments, which can limit contaminants more than fractured rock. In addition, this study illustrates that long residence times are not necessary to achieve more geochemically evolved conditions such as high pH and reduced conditions as is typically found with older water in other regions.

Borehole Productivity Controlling Factors in Crystalline Bedrock Aquifer of Gk&amp;amp;#234;k&amp;amp;#234; Region, Center of C&amp;amp;#244;te d’Ivoire

The presence of both weathered rocks and fractured crystalline bedrock aquifers makes Hydrogeology in Gbeke region of Cote d’Ivoire. Access to water in this region is not easy. This study focuses on the influence of borehole depth, weathering thickness and electrical resistivity of the geological structures on borehole productivity that exploit the crystalline aquifer system. Bivariate analysis was used to determine the relationships between these factors and specific capacity for measuring borehole productivity. The values ranged from 0.0088 to 2.20 m3∙h−1∙m−1. The analysis shows that there is no correlation between productivity and weathering thickness. However, weathering depths between 15 and 35 m provide the highest specific capacity values (Qs ≥ 1 m3∙h−1∙m−1). For hydrogeological discontinuities interest, boreholes located in KH, QH and H anomaly curve types were the most productive. As productivity diminishes with depth, a deeper borehole can be more productive if it reaches a geological structure that is favorable for groundwater flow. Those hydrogeological parameters are extremely important in borehole productivity in Gbeke region.

Characteristics of high-intensity groundwater abstractions from weathered crystalline bedrock aquifers in East Africa

Weathered crystalline bedrock aquifers sustain water supplies across the tropics, including East Africa. Although well yields are commonly <1 L s−1, more intensive abstraction occurs and provides vital urban and agricultural water supplies. The hydrogeological conditions that sustain such high abstraction from crystalline bedrock aquifers remain, however, poorly characterised. Five sites of intensive groundwater abstraction (multiple boreholes yielding several L s−1 or more) were investigated in Uganda and Tanzania. Analysis of aquifer properties data indicates that the sites have transmissivities of 10–1,000 m2 day−1, which is higher than generally observed in deeply weathered crystalline bedrock aquifers. At four of the five sites, weathered bedrock (saprolite) is overlain by younger superficial sediments, which provide additional storage and raise the water table within the underlying aquifer. Residence-time indicators suggest that: (1) abstracted water derives, in part, from modern recharge (within the last 10–60 years); and (2) intensive abstraction is sustained by recharge occurring over several decades. This range of encountered residence times indicates a degree of resilience to contemporary climate variability (e.g. short-term droughts), although the long-term sustainability of intensive abstractions remains uncertain. Evidence from one site in Tanzania (Makutapora) highlights the value of multi-decadal groundwater-level records in establishing the long-term viability of intensive groundwater abstraction, and demonstrates the influence of intra-decadal climate variability in determining the magnitude and frequency of recharge.

Hydrogeochemical and multi-tracer investigations of arsenic-affected aquifers in semi-arid West Africa

The semi-arid Sahel regions of West Africa rely heavily on groundwater from shallow to moderately deep (<100 m b.g.l.) crystalline bedrock aquifers for drinking water production. Groundwater quality may be affected by high geogenic arsenic (As) concentrations (>10 μg/L) stemming from the oxidation of sulphide minerals (pyrite, arsenopyrite) in mineralised zones. These aquifers are still little investigated, especially concerning groundwater residence times and the influence of the annual monsoon season on groundwater chemistry. To gain insights on the temporal aspects of As contamination, we have used isotope tracers (noble gases, 3H, stable water isotopes (2H, 18O)) and performed hydrochemical analyses on groundwater abstracted from tube wells and dug wells in a small study area in southwestern Burkina Faso. Results revealed a great variability in groundwater properties (e.g. redox conditions, As concentrations, water level, residence time) over spatial scales of only a few hundred metres, characteristic of the highly heterogeneous fractured underground. Elevated As levels are found in oxic groundwater of circum-neutral pH and show little relation with any of the measured parameters. Arsenic concentrations are relatively stable over the course of the year, with little effect seen by the monsoon. Groundwater residence time does not seem to have an influence on As concentrations, as elevated As can be found both in groundwater with short (<50 a) and long (>103 a) residence times as indicated by 3He/4He ratios spanning three orders of magnitude. These results support the hypothesis that the proximity to mineralised zones is the most crucial factor controlling As concentrations in the observed redox/pH conditions. The existence of very old water portions with residence times >103 years already at depths of <50 m b.g.l. is a new finding for the shallow fractured bedrock aquifers of Burkina Faso, suggesting that overexploitation of these relatively low-yielding aquifers may be an issue in the future.

Occurrence of geogenic contaminants in private wells from a crystalline bedrock aquifer in western Quebec, Canada: Geochemical sources and health risks

Nineteen private wells were investigated in order to evaluate the groundwater quality and the issues associated with well water use in a fractured metasedimentary aquifer of the Canadian Shield, in western Quebec (Canada). Groundwater sampling and analysis reveal that the quality of well water is both a potential aesthetic and health concern for the residents. Aesthetic problems are mainly related to the high levels of hardness and dissolved iron and manganese. Potential health risks are associated with the occurrence of brackish groundwater, high manganese concentrations, and arsenic concentrations exceeding the Canadian guideline value of 10 µg/l. Brackish groundwater is suspected to be derived from the mixing of fresh groundwaters with deep calcium-sodium-chloride brines of the Canadian Shield. The occurrences of iron, manganese and arsenic, primarily derived from the natural weathering of bedrock, are highly dependent on the geochemical conditions in groundwater, particularly the redox potential. Arsenic occurs mainly as arsenite (As(III)) and is thought to be released by the dissolution of iron and manganese oxyhydroxides under reducing conditions. Information obtained from well owners indicates that most households use ion exchange water softeners to minimize aesthetic problems of excessive hardness and dissolved iron and manganese concentrations. Homeowners generally take protective measures to reduce their exposure to arsenic when they are aware of the contamination. The exposure to arsenic and manganese may pose health risks for residents that do not take protective measures. The quality of well water is of paramount importance for human health in rural areas. Information on the contaminant sources and individual mitigation measures is essential to assess the health risks associated with groundwater consumption and to ensure the protection of public health.

Hydrogeochemical mechanisms governing the mineralization and elevated fluoride $$\left( {\text{F}^{ - } } \right)$$ F - contents in Precambrian crystalline aquifer groundwater in central Benin, Western Africa

In the central part of Benin (Western Africa), high fluoride $$\left( {{\text{F}}^{ - } } \right)$$ contents have been reported in groundwater from Precambrian crystalline bedrock aquifer which is the main source of drinking water. The hydrogeochemical mechanisms leading to such elevated fluoride concentrations are usually not fully understood. In this context, the objective is to identify the hydrogeochemical processes governing groundwater mineralization and the origin of the high fluoride concentrations. A dataset of 162 groundwater samples was collected from the aquifer consisting of a thin altered bedrock layer (shallow aquifer) and a deep fractured crystalline bedrock (deep aquifer). Geochemical approaches and multivariate statistics have been used to explore the data. Fluoride concentrations vary between 0.00 and 7.19 mg/L in groundwater. Samples collected in the southern part of the investigated area, close to Dassa-Zoume, show the highest concentrations, with more than 75% greater than the guideline value of 1.5 mg/L. The deep fractured aquifer shows higher concentrations of fluoride than the shallow regolith reservoir. Results reveal that groundwater mineralization is derived mainly from the hydrolysis of silicate minerals, but it is also influenced by anthropogenic effects, particularly in the shallow reservoir. However, fluoride has a geogenic origin, primarily from the weathering of silicate minerals, primarily biotite. $${\text{Ca}}^{2 + }/{\text{Na}}^{ + }$$ cation exchanges and $${\text{F}}^{ - }/{\text{OH}}^{ - }$$ anion exchanges between groundwater and the rock matrix also occur as minor processes. Saturation of the water with respect to calcite and the precipitation of this mineral, which reduces calcium activity, also favor the release of fluoride from rocks.

Modelling of the dissolution and reprecipitation of uranium under oxidising conditions in the zone of shallow groundwater circulation

Generic hydrochemical modelling of a grantoid-groundwater system, using the Russian software “HydroGeo”, has been carried out with an emphasis on simulating the accumulation of uranium in the aqueous phase. The baseline model run simulates shallow granitoid aquifers (U content 5 ppm) under conditions broadly representative of southern Norway and southwestern Siberia: i.e. temperature 10 °C, equilibrated with a soil gas partial CO2 pressure (PCO2, open system) of 10−2.5 atm. and a mildly oxidising redox environment (Eh = +50 mV). Modelling indicates that aqueous uranium accumulates in parallel with total dissolved solids (or groundwater mineralisation M – regarded as an indicator of degree of hydrochemical evolution), accumulating most rapidly when M = 550–1000 mg L−1. Accumulation slows at the onset of saturation and precipitation of secondary uranium minerals at M = c. 1000 mg L−1 (which, under baseline modelling conditions, also corresponds approximately to calcite saturation and transition to Na-HCO3 hydrofacies). The secondary minerals are typically “black” uranium oxides of mixed oxidation state (e.g. U3O7 and U4O9). For rock U content of 5–50 ppm, it is possible to generate a wide variety of aqueous uranium concentrations, up to a maximum of just over 1 mg L−1, but with typical concentrations of up to 10 μg L−1 for modest degrees of hydrochemical maturity (as indicated by M). These observations correspond extremely well with real groundwater analyses from the Altai-Sayan region of Russia and Norwegian crystalline bedrock aquifers. The timing (with respect to M) and degree of aqueous uranium accumulation are also sensitive to Eh (greater mobilisation at higher Eh), uranium content of rocks (aqueous concentration increases as rock content increases) and PCO2 (low PCO2 favours higher pH, rapid accumulation of aqueous U and earlier saturation with respect to uranium minerals).

Mobility and speciation of geogenic arsenic in bedrock groundwater from the Canadian Shield in western Quebec, Canada

High arsenic concentrations occur in groundwater collected from a fractured crystalline bedrock aquifer in western Quebec (Canada). Sampling and analysis of water from 59 private wells reveal that more than half of the bedrock wells exceed the Canadian guideline value of 10μg/l for arsenic, whereas shallow wells in unconsolidated surficial deposits are not affected by the contamination. The weathering of arsenic-bearing sulfides present along the mineralized fault zone is considered to be the primary source of arsenic in groundwater. High-arsenic wells are generally characterized by mildly reducing conditions (Eh<250mV), weak alkaline conditions (pH>7.4), low Ca/Na ratios, elevated dissolved Fe and Mn concentrations and high proportions of As(III). Private bedrock wells are open boreholes that likely receive groundwater from multiple contributing fractures. Hence, it is proposed that dissolved arsenic is mainly derived from the contribution to the well discharge of reducing and alkaline geochemically evolved groundwater that contains arsenic as As(III). Geochemically evolved groundwater provides favorable conditions to release arsenic by reductive dissolution of iron and manganese oxyhydroxides and alkaline desorption from mineral surfaces. Thus, high-arsenic wells would contain a high proportion of geochemically evolved groundwater, while oxidizing low-pH recharge water causes dilution and sequestration of arsenic. In relation with the chemical evolution of groundwater along the flow path, most contaminated wells are located in confined areas whereas most of the wells located in unconfined recharge areas are not contaminated. The occurrence of boreholes with high dissolved arsenic as As(V) and oxidizing conditions is attributed to extensive sulfide oxidation and alkaline desorption. This work shows that the determination of arsenic speciation provides a valuable tool to investigate the behavior of arsenic in bedrock groundwater.

A comparison of fracture transmissivities in granite water wells before and after hydrofracturing

In many regions of the world, crystalline bedrock aquifers are the only choice for groundwater supply. This is the case in northern Wisconsin, located in the upper Midwest of the continental United States. Here, groundwater flow to wells occurs only through fractures in the granitic basement. Although hydrofracturing of these wells is common and generally increases well yield, the precise mechanism for the increased yields remained unknown. Stressed and ambient flow logs were obtained in two 305-m-deep granitic boreholes in northern Wisconsin prior to hydrofracturing. From those logs, it was determined that nearly all of the groundwater flow to the boreholes occurred in less than 10 fractures in the upper 80 m, with no measureable contribution below that depth. Following hydrofracturing of the boreholes, stressed and ambient flow logs were again obtained. The transmissivity of the two boreholes increased by factors of 8.6 and 63 times. It was found that (1) the fractures that had contributed flow to the boreholes increased in transmissivity, (2) although the applied pressures were large enough in some instances to create new fractures, those new fractures did not increase the borehole transmissivities significantly, and (3) fractures without measureable flow before hydrofracturing remained without measureable flow. Hydrofracturing increases yield in granitic boreholes; however, that increase seems to only occur in fractures where flow was pre-existing and in the upper 80 m of the boreholes. These observations suggest that efforts to enhance yield in granitic aquifers should be focused on the upper part of the borehole.

Regional recharge assessment in the crystalline bedrock aquifer of the Kenogami Uplands, Canada

Assessing recharge in unconfined fractured rock aquifers is a challenging task due to the discontinuous nature of this medium. In this study, a methodology is proposed based on an analytical interpretation of the regional hydraulic head profiles of an unconfined fractured rock aquifer, to determine its recharge. This calculation is conducted for a regional groundwater system based on a one-dimensional Dupuit-Forchheimer model in steady-state conditions that yields the value of the ratio of the aquifer recharge to the aquifer hydraulic conductivity. The approach is tested on a crystalline bedrock aquifer (Kenogami Uplands) of the Canadian Shield located in the Province of Quebec in Canada. The results show an average hydraulic conductivity of the aquifer of 4.3 × 10−7 m/s and a recharge equivalent to approximately 0.4% (3.5 mm/year) of precipitation. These results are in agreement with other studies conducted on similar crystalline aquifers in the Canadian Shield. Editor D. Koutsoyiannis Citation Chesnaux, R., 2013. Regional recharge assessment in the crystalline bedrock aquifer of the Kenogami Uplands, Canada. Hydrological Sciences Journal, 58 (2), 421–436.

Characterization of bacterial diversity to a depth of 1500 m in the Outokumpu deep borehole, Fennoscandian Shield

This paper demonstrates the first microbiological sampling of the Outokumpu deep borehole (2516 m deep) aiming at characterizing the bacterial community composition and diversity of sulphate-reducing bacteria (SRB) in Finnish crystalline bedrock aquifers. Sampling was performed using a 1500-m-long pressure-tight tube that provided 15 subsamples, each corresponding to a 100-m section down the borehole. Microbial density measurements, as well as community fingerprinting with 16S rRNA gene-based denaturing gradient gel electrophoresis, demonstrated that microbial communities in the borehole water varied as a function of sampling depth. In the upper part of the borehole, bacteria affiliated to the family Comamonadaceae dominated the bacterial community. Further down the borehole, bacteria affiliated to the class Firmicutes became more prominent and, according to 16S rRNA gene clone libraries, dominated the bacterial community at 1400-1500 m. In addition, the largest number of bacterial classes was observed at 1400-1500 m. The dsrB genes detected in the upper part of the borehole were more similar to the dsrB genes of cultured SRBs, such as the genus Desulfotomaculum, whereas in the deeper parts of the borehole, the dsrB genes were more closely related to the uncultured bacteria that have been detected earlier in deep earth crust aquifers.

Evaluation of geologic and geomorphologic influences on borehole productivity in crystalline bedrock aquifers of Limpopo Province, South Africa

Geologically complex crystalline aquifers underlie large parts of the semi-arid Limpopo Province where some of the greatest groundwater needs in South Africa occur. It is important to identify potentially high-yielding zones that can be targeted for water supply. The study covered four distinct geologic and morpho-structural domains within Limpopo Province, together covering about 23,500 km2. Results from over 2,500 pumping test analyses indicate that bedrock type (e.g. pegmatite), lithological setting (e.g. aureole of granitoids), proximity and orientation of dykes and lineaments, topographic setting (e.g. slopes or valleys) and proximity of surface-water drainages may exert an influence on borehole productivity. No correlation between borehole productivity and weathering depth was found. Lineaments and dykes striking perpendicular to the current maximum horizontal stress seem to be more favourable targets, which is inconsistent with the predicted regime. Due to the complex geologic history, it is difficult to link open discontinuities to a distinct recent or past tectonic event. Regional stress-field data, as in this case, may not account for local, possibly highly significant, stress-field variations. The hydrogeologic importance of several factors related to groundwater occurrence, here presented, can be used as a working reference for future groundwater development programmes.