Deep Bedrock Groundwater Research Articles

Naturally occurring volatile organic compounds in deep bedrock groundwater

In deep continental bedrock groundwater, volatile organic compounds pose intriguing questions about their occurrence. Although their natural sources remain poorly characterized, they potentially have profound effects on global elemental cycles and safety assessment of underground facilities. Our investigation focused on non-methane volatile organic compounds in five groundwater-filled Precambrian bedrock fracture zones at 500 m to 2300 m depth in the Outokumpu Deep Drill Hole, Finland. The system revealed a range of indigenous volatile organic compounds, including alcohols, ketones, aldehydes, alkanes, aromatics, and sulfur-compounds. Metagenomic sequencing demonstrated spatially widely distributed genetic potential for microbial volatile organic compound metabolism. Integrating these findings with local geology and hydrogeochemistry, our results suggest that volatile organic compounds are not only formed in or released to groundwater but also degraded in the continental bedrock through interconnected abiotic and biotic processes.

Using multiple isotopes to determine groundwater source, age, and renewal rate in the Beishan preselected area for geological disposal of high-level radioactive waste in China

Understanding hydrogeological conditions is crucial for selecting and assessing the long-term safety performance of a high-level radioactive waste (HLW) disposal repository. Utilizing environmental isotopes as effective markers for analysing groundwater movement, this study investigates groundwater recharge sources, age, and renewal rates using multiple isotopes in China's potential HLW repository site, the Beishan area. The results indicated deep bedrock groundwater primarily derives from ancient precipitation infiltration under cold climatic conditions. A noteworthy distinction is that loose sedimentary groundwater exhibits higher tritium content (>10 TU) compared to bedrock groundwater (<3.2 TU). Groundwater within the recharge area, especially within gullies and piedmont slope deposits, is relatively youthful, with an age of less than 30 years and an annual renewal rate exceeding 5 %. In contrast, the shallow groundwater age in the intermountain basins and depressions of the discharge area generally exceeds 50 years, with an annual renewal rate often falling below 0.5 %. At the Beishan underground research laboratory site, deep groundwater at the disposal repository depth displays a corrected 14C age exceeding 8,000 years, indicating an extremely slow movement and alteration rate. As a result, the hydrogeological conditions in the Beishan area are expected to be relatively beneficial for ensuring the safety of HLW repository.

Lateral flow in bedrock dominates baseflow and flood flow in a catchment underlain with permeable sedimentary rock in the high‐relief Chichibu Mountains

AbstractDetermining the mechanisms of runoff generation is essential to land and water resource conservation, but lateral flow processes in high‐relief mountain areas remain unclear due to the difficulty to observe and measure these processes. This study demonstrates that lateral flow in bedrock dominates both high and low flows in a catchment underlain with permeable sedimentary bedrock in the high‐relief Chichibu Mountains. Through measurement of precipitation, discharge, soil, and stream temperatures and tracers (SiO2 and δ18O) in the first‐order stream Bakemonozawa (0.51 km2) and comparison of its water balance with the nested Kawamata catchment (93.58 km2) i, we showed that 831–1012 mm/year or 47%–53% of annual precipitation leaked from Bakemonozawa. The streamflow was discharged from an average depth of 2.3 m or deeper, which were deeper than the measured deepest soil near the gauge. Furthermore, stream temperature decreased significantly during a large summer storm, demonstrating that the relative contribution of the deep bedrock flowpath increased. Large isotopic variation in precipitation was dampened in the stream, suggesting that most baseflow and some higher flows arose from groundwater storage with a constant isotopic ratio due to dispersion and mixing. The discharge–SiO2 relationship suggested at least two groundwater sources, namely, shallow and deep bedrock groundwater, with different runoff responses. Discharge increased dramatically when the catchment became wet, coinciding with the deep bedrock groundwater contribution. Further research is needed to clarify the hydrological characteristics of bedrock.

GEOPHYSICAL INDICATORS OF AUFEIS IN THE ANMANGYNDA RIVER (MAGADAN REGION)

Giant aufeis fields are the indicators of water exchange processes in the permafrost zone. The study of aufeis dynamics is relevant to assessing the state of the cryosphere in a changing climate. The Anmangynda aufeis which forms upstream of the Kolyma River basin, was as large as 6.8 km2in the last century and is considered representative of the mountainous territories in the northeast Russia. In recent decades, there have been significant changes in the aufeis formation regime that require updating the understanding of the cryosphere processes. The historical observational data obtained in 1962-1990 for the Anmangynda aufeis allows using it as the present-day object of research.The main goal of geophysical research in 2021-2022 was to study the structure of the Anmangynda aufeis and aufeis glade to determine its genesis and development processes. The main tasks were to identify the boundary between frozen and unfrozen rocks, to reveal groundwater discharge channels, and to assess the aufeis thickness distribution. There were carried out georadar survey using an antenna unit with central frequency of 250 MHz and ground-based capacitively coupled electrical resistivity tomography. It was found that by April 5, 2021, the maximum ice thickness was 4.35 m with an average value of 1.35 m, and the volume of aufeis was 3.56 million m3. By means of georadar, there were performed identification and drilling verification of the groundwater discharge channels from alluvium to aufeis surface, located in the stream beds. There is geophysical evidence of deep bedrock groundwater sources. Based on the electrical resistivity tomography data, there were identified large and locally freezing river taliks, from which the water is squeezed onto the ice surface. It is recommended to conduct further interdisciplinary research to clarify the geophysical results obtained.

Groundwater Dynamics near the Saltwater–Freshwater Interface in an Island of Seto Inland Sea

Groundwater dynamics near the saltwater–freshwater interface were investigated in an island of the Seto Inland Sea, using multiple tracers (δD, δ18O, Cl−, SF6, and 14C) at two coastal groundwater monitoring wells at depths of 10–40 m. The groundwater recharge area and age were also estimated using these tracers. Additionally, bedrock groundwater at a depth of 40 m at the 2.7 m altitude was brackish and considered to be near the saltwater–freshwater interface, and a mixture of seawater (2–3.5%) and fresh groundwater (97–98%) was estimated by the Cl− concentration. Based on the δ18O of fresh groundwater estimated from the seawater mixing ratio, the recharge area was estimated to range from near to above the summit; however, this region is unlikely to be the actual recharge area, as the groundwater may be old freshwater that was recharged during a previously colder period. Groundwater dating using SF6 and 14C suggests that the fresh groundwater originated during the last glacial period (assumed 20,000 years ago) and that the 40 m deep bedrock groundwater is a mixture of old water (0–28%), 30 m deep groundwater (76–100%), and stagnant seawater (1–3%).

Fluctuation in deep groundwater chemistry and microbial community and their impact on corrosion of stainless-steels

Low and intermediate level radioactive waste produced during the operation and decommissioning of nuclear power plants is disposed of in an underground geological repository. The majority of metallic waste is made of various stainless-steels and carbon steel. Microbial communities and groundwater composition in deep bedrock at repository sites were believed to stay stable over time, allowing the prediction of evolution of the repository environment. However, a two-year survey of chemical components and microbial community composition within deep bedrock revealed changes in both. An in situ corrosion monitoring system was developed to monitor real-time corrosion rates of two stainless-steel grades (AISI 304 and 316) to study the evolution of corrosion, and correlation between environmental changes and corrosion rate. Surprisingly, higher corrosion rates of steel coupons were detected in the higher alloyed stainless-steel grade 316 compared to the lower alloyed grade 304. Pitting was the main corrosion form. Sulphate reducing bacteria and methanogenic archaea were enriched on surfaces of both types of steel coupons. These microbes likely have a role in the corrosion of stainless-steel in this environment. The changes in groundwater conditions and microbial communities within deep bedrock groundwater at this repository site may have implications for the nuclide release and transport of radioactive material and the long-term evolution and safety of this repository and continental repositories in general and thus needs to be thoroughly understood.

Microbially Induced Corrosion in Deep Bedrock

This paper covers an overview of recent research of microbially induced corrosion in Fennoscandian terrestrial deep bedrock groundwater environment. The assessment of microbially induced corrosion of metals in the deep bedrock environment has become important in evaluating the long-term safety of disposal of radioactive waste.

Influence of bedrock groundwater on streamflow characteristics in a volcanic catchment

AbstractGroundwater movements in volcanic mountains and their effects on streamflow discharge and representative elementary area (REA) have remained largely unclear. We surveyed the discharge and chemical composition of spring and stream water in two catchments: the Hontani river (NR) catchment (6.6 km2) and the Hosotani river (SR) catchment (4.0 km2) at the southern part of Daisen volcano, Japan. Daisen volcano is a young volcano (17 × 103 years) at an early stage of erosion. Our study indicated that deep groundwater that moved through thick lava and pyroclastic flows and that could not be explained by shallow movements controlled by surface topography contributed dominantly to streamflow at larger catchment areas. At the NR catchment, the deep groundwater contribution clearly increased at a catchment boundary defined by an area of 3.0 km2 and an elevation of 800 m. At the SR catchment, the contribution deep groundwater to the stream also increased suddenly at a boundary threshold of 2.0 and 700 m. Beyond these thresholds, the contributions of deep bedrock groundwater remained constant, indicating that the REA is between 2 and 3 km2 at the observed area. These results indicate that the hydrological conditions of base flow were controlled mainly by the deep bedrock groundwater that moved through thick lava and pyroclastic flows in the undissected volcanic body of the upper part of the catchment. Our study demonstrates that deep and long groundwater movements via a deep bedrock layer including thick deposits of volcanic materials at the two catchments on Daisen volcano strongly determined streamflow discharge instead of the mixing of small‐scale hydrological conditions. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Revealing the unexplored fungal communities in deep groundwater of crystalline bedrock fracture zones in Olkiluoto, Finland.

The diversity and functional role of fungi, one of the ecologically most important groups of eukaryotic microorganisms, remains largely unknown in deep biosphere environments. In this study we investigated fungal communities in packer-isolated bedrock fractures in Olkiluoto, Finland at depths ranging from 296 to 798 m below surface level. DNA- and cDNA-based high-throughput amplicon sequencing analysis of the fungal internal transcribed spacer (ITS) gene markers was used to examine the total fungal diversity and to identify the active members in deep fracture zones at different depths. Results showed that fungi were present in fracture zones at all depths and fungal diversity was higher than expected. Most of the observed fungal sequences belonged to the phylum Ascomycota. Phyla Basidiomycota and Chytridiomycota were only represented as a minor part of the fungal community. Dominating fungal classes in the deep bedrock aquifers were Sordariomycetes, Eurotiomycetes, and Dothideomycetes from the Ascomycota phylum and classes Microbotryomycetes and Tremellomycetes from the Basidiomycota phylum, which are the most frequently detected fungal taxa reported also from deep sea environments. In addition some fungal sequences represented potentially novel fungal species. Active fungi were detected in most of the fracture zones, which proves that fungi are able to maintain cellular activity in these oligotrophic conditions. Possible roles of fungi and their origin in deep bedrock groundwater can only be speculated in the light of current knowledge but some species may be specifically adapted to deep subsurface environment and may play important roles in the utilization and recycling of nutrients and thus sustaining the deep subsurface microbial community.

Contributions of bedrock groundwater to the upscaling of storm‐runoff generation processes in weathered granitic headwater catchments

AbstractWe examined the contributions of bedrock groundwater to the upscaling of storm‐runoff generation processes in weathered granitic headwater catchments by conducting detailed hydrochemical observations in five catchments that ranged from zero to second order. End‐member mixing analysis (EMMA) was performed to identify the geographical sources of stream water. Throughfall, hillslope groundwater, shallow bedrock groundwater, and deep bedrock groundwater were identified as end members. The contribution of each end member to storm runoff differed among the catchments because of the differing quantities of riparian groundwater, which was recharged by the bedrock groundwater prior to rainfall events. Among the five catchments, the contribution of throughfall was highest during both baseflow and storm flow in a zero‐order catchment with little contribution from the bedrock groundwater to the riparian reservoir. In zero‐order catchments with some contribution from bedrock groundwater, stream water was dominated by shallow bedrock groundwater during baseflow, but it was significantly influenced by hillslope groundwater during storms. In the first‐order catchment, stream water was dominated by shallow bedrock groundwater during storms as well as baseflow periods. In the second‐order catchment, deeper bedrock groundwater than that found in the zero‐order and first‐order catchments contributed to stream water in all periods, except during large storm events. These results suggest that bedrock groundwater influences the upscaling of storm‐runoff generation processes by affecting the linkages of geomorphic units such as hillslopes, riparian zones, and stream channels. Our results highlight the need for a three‐dimensional approach that considers bedrock groundwater flow when studying the upscaling of storm‐runoff generation processes. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Effects of bedrock groundwater on spatial and temporal variations in soil mantle groundwater in a steep granitic headwater catchment

We investigated processes of soil mantle groundwater generation in a granitic headwater catchment in central Japan. Two types of groundwater were observed: ephemeral‐type groundwater (EG), which developed in response to rainfall events and disappeared rapidly after the events ceased, and semiperennial‐type groundwater (SPG), which remained formed for more than several months. The groundwater level, chemistry, and temperature within the soil and bedrock layers indicated that the source of EG was rain or soil water, whereas the source of SPG was deep bedrock groundwater. The generation processes of soil mantle groundwater varied both spatially and temporally under the influence of the underlying bedrock. Whereas only EG was generated in upslope areas, bedrock groundwater continuously seeped into the soil layers in downslope areas to generate SPG. In middle‐slope areas, an increase in the bedrock groundwater level generated SPG in soil layers, but the SPG disappeared when the bedrock groundwater level fell. Our results indicate that bedrock is important in controlling soil mantle groundwater generation and water flow processes in headwater catchments and that direct measurements of bedrock conditions are vital for clarifying the roles of bedrock in these processes.

Anomalous behavior of soil mantle groundwater demonstrates the major effects of bedrock groundwater on surface hydrological processes

The formation of groundwater in the soil mantle has a dominant effect on rainwater discharge and shallow landslide occurrence in headwater catchments. Here, we report two completely different groundwater responses within a single well excavated into the soil mantle. One was an ephemeral‐type response that is well described by physical hydrology models based on a geographic information system (GIS). The other was a semi‐perennial‐type response, rarely reported in previous studies, which cannot be explained by the existing physical models. The semi‐perennial groundwater caused considerably high antecedent groundwater tables between storms, leading to an increased peak in the groundwater level during later heavy storm events and a likely increase in the risk of shallow landslides. Moreover, peaks in the semi‐perennial groundwater lagged considerably behind rainstorm events, which probably affected base flow discharge by forming a delayed peak. Geochemical and geothermal observations indicated that the source of the semi‐perennial groundwater was deep bedrock groundwater, demonstrating the considerable effects of bedrock groundwater on surface hydrological processes.

Fluorine geochemistry in bedrock groundwater of South Korea

High fluoride concentrations (median=4.4 mg/L) in deep bedrock groundwater of South Korea prevent the usage of it as a drinking water source. The hydrogeochemistry of deep thermal groundwaters (N=377) in diverse bedrocks has been studied in order to evaluate the geologic and geochemical controls on fluoride concentrations in groundwater. The groundwater samples were clustered geologically, and the average and median concentrations of fluoride were compared by the Mann-Whitney U test. The order of median fluoride concentration with respect to geology is as follows: metamorphic rocks> or =granitoids > or =complex rock>>volcanic rocks> or =sedimentary rocks. This result indicates that the geological source of fluoride in groundwater is related to the mineral composition of metamorphic rocks and granitoids. With respect to groundwater chemistry, the fluoride concentration was highest in Na-HCO3 type groundwater and lowest in Ca-HCO3 type groundwater. Ionic relationships also imply that the geochemical behavior of fluoride in groundwater is related to the geochemical process releasing Na and removing Ca ions. The thermodynamic relationship between the activities of Ca and F indicates that fluoride concentration is controlled by the equilibrium of fluorite (CaF2). In other words, the upper limits of fluoride concentration are determined by the Ca ion; i.e., Ca concentrations play a crucial role in fluoride behavior in deep thermal groundwater. The result of this study suggests that the high fluoride in groundwater originates from geological sources and fluoride can be removed by fluorite precipitation when high Ca concentration is maintained. This provides a basis for a proper management plan to develop the deep thermal groundwater and for treatment of high fluoride groundwater frequently found in South Korea.

Atmospheric, Geological, Marine and Anthropogenic Effects on Groundwater Quality in Finland

The atmospheric, geological, marine and anthropogenic factors affecting the chemical quality of groundwater are evaluated. Sulphates and nitrates derive mainly from the atmosphere as wet or dry deposition in weakly mineralized shallow groundwater in natural uncontaminated surficial aquifers. The texture, structure and lithologic composition of aquifer material have an impact on water chemistry although marine influence and anthropogenic pollution generally outweigh the geological influence. Relict seawater trapped in deeper parts of confined aquifers and in bedrock covered by clay deposits has a marked effect on the quality of groundwater drawn from wells drilled into bedrock in the broad coastal belt. Brackish or saline groundwater is encountered all over the country in holes drilled in crystalline bedrock to a depth of more than 500 to 1000 metres. The lithologic environment affects the quality of deep bedrock groundwater which tends to change from Na-Cl type towards Ca-Na-Cl type with increasing salinity and depth of occurrence. Anthropogenic pollution is reflected in elevated amounts of all major dissolved components, although the most sensitive indicators of contamination are nitrates, chlorides and potassium. Nation-wide hydrogeochemical mapping of rural water sources revealed that shallow wells dug into glacial till deposits had the highest median NO3 contents, and that spring water discharging from glaciofluvial sand deposits had the lowest. The quality of groundwater distributed by public waterworks is generally good.