High-resolution stable carbon isotope monitoring indicates variable flow dynamic patterns in a deep saline aquifer at the Ketzin pilot site (Germany)

Stable isotopes of injected CO₂ act as useful tracers in carbon capture and storage (CCS) because the CO₂ itself is the carrier of the tracer signal and remains unaffected by sorption or partitioning effects. At the Ketzin pilot site (Germany), carbon stable isotope composition (δ¹³C) of injected CO₂ at the injection well was analyzed over a time period of 4 months. Occurring isotope variances resulted from the injection of CO₂ from two different sources (an oil refinery and a natural gas-reservoir). The two gases differed in their carbon isotope composition by more than 27‰. In order to find identifiable patterns of these variances in the reservoir, more than 250 CO₂-samples were collected and analyzed for their carbon isotope ratios at an observation well 100 m distant from the injection well. An isotope ratio mass spectrometer connected to a modified Thermo Gasbench system allowed quick and cost effective isotope analyses of a high number of CO₂ gas specimens. CO₂ gas from the oil refinery (δ¹³C = −30.9‰, source A) was most frequently injected and dominated the reservoir δ¹³C values at the injection site. Sporadic injection of the CO₂ from the natural gas-reservoir (δ¹³C = −3.5‰, source B) caused isotope shifts of up to +5‰ at the injection well. These variances provided a potential ideal tracer for CO₂ migration behavior. Based on these findings, tracer input signals that were injected during the last 2 years of injection could be reconstructed with the aid of an isotope mixing model and CO₂ delivery schedules. However, in contrast to the injection well, δ¹³C values at the observation well showed no variances and a constant value of −28.5‰ was measured at 600 m depth. This is in disagreement with signals that would be expected if the input signals from the injection would arrive at the observation well. The lack of isotope signals at the observation well suggests that parts of the reservoir are filled with CO₂ that is immobilized.     

Use of sinkhole and specific capacity distributions to assess vertical gradients in a karst aquifer

The carbonate-rock aquifer in the Great Valley, West Virginia, USA, was evaluated using a database of 687 sinkholes and 350 specific capacity tests to assess structural, lithologic, and topographic influences on the groundwater flow system. The enhanced permeability of the aquifer is characterized in part by the many sinkholes, springs, and solutionally enlarged fractures throughout the valley. Yet, vertical components of subsurface flow in this highly heterogeneous aquifer are currently not well understood. To address this problem, this study examines the apparent relation between geologic features of the aquifer and two spatial indices of enhanced permeability attributed to aquifer karstification: (1) the distribution of sinkholes and (2) the occurrence of wells with relatively high specific capacity. Statistical results indicate that sinkholes (funnel and collapse) occur primarily along cleavage and bedding planes parallel to subparallel to strike where lateral or downward vertical gradients are highest. Conversely, high specific capacity values are common along prominent joints perpendicular or oblique to strike. The similarity of the latter distribution to that of springs suggests these fractures are areas of upward-convergent flow. These differences between sinkhole and high specific capacity distributions suggest vertical flow components are primarily controlled by the orientation of geologic structure and associated subsurface fracturing.     

Capture-zone design in an aquifer influenced by cyclic fluctuations in hydraulic gradients

Design of a groundwater pumping and treatment system for a wood-treatment facility adjacent to the tidally influenced Fraser River estuary required the development of methodologies to account for cyclic variations in hydraulic gradients. Design of such systems must consider the effects of these cyclic fluctuations on the capture of dissolved-phase contaminants. When the period of the cyclic fluctuation is much less than the travel time of the dissolved contaminant from the source to the discharge point, the hydraulic-gradient variations resulting from these cycles can be ignored. Capture zones are then designed based on the average hydraulic gradient determined using filter techniques on continuous groundwater-level measurements. When the period of cyclic fluctuation in hydraulic gradient is near to or greater than the contaminant travel time, the resulting hydraulic-gradient variations cannot be ignored. In these instances, procedures are developed to account for these fluctuations in the capture-zone design. These include proper characterization of the groundwater regime, assessment of the average travel time and period of the cyclic fluctuations, and numerical techniques which allow accounting for the cyclic fluctuations in the design of the capture zone. Electronic Publication     

High-resolution monitoring of nitrate variations in a typical subterranean karst stream, Chongqing, China

Globally, it is possible that up to 25% of the world’s population depends on karst water supplies. In karst areas, a high degree of groundwater and surface water linkage often results in the direct recharge of groundwater with polluted run-off following rainfall. In order to reveal the hydrochemical variations after rainfall, especially real-time variation of pollutants, high-resolution auto-monitoring techniques were used at the outlet of Qingmuguan subterranean stream (QSS), which is influenced by agricultural activities. In addition to rainfall, high-resolution measurements of pH, water level, electrical conductivity (Ec) and NO₃ ⁻ concentration were recorded in the monsoon season and fertilizer application period using a data logger with time intervals of 15 min. In the six observed rainfall events, the pH value was mainly controlled by acidic rainfall inputs. The pH showed sharp decline after the rainfall event, and then increased. The Ec was impacted by the rainfall chemistry, dilution effect of rainfall and agricultural wastewater. NO₃ ⁻ derived from agricultural activities was less impacted by rain chemistry; and its variations were mainly affected by the dilution effect of rainfall and agricultural wastewater. Under the influences of the R1 rainfall, the rapid changes of Ec and NO₃ ⁻ were opposite in direction. As the rain continued, both the Ec and NO₃ ⁻ rapidly changed in synchronization within the shortest period of 5 h and the longest of 27 h because of the impact of the agricultural wastewater. The groundwater quality changed due to the influx of agricultural wastewater over the entire monitoring period. According to the National Groundwater Quality Standard, People’s Republic of China (GB/T14848-9), the groundwater quality of the QSS moved through the following grades during the monitoring period: Grade III → Grade IV → Grade V → Grade IV → Grade V → Grade IV → Grade V → Grade IV → Grade III. Traditional sampling methods did not reveal accurate hydrochemistry changes of the QSS, and even generated misleading results. Consequently, the high-resolution auto-monitoring technique is necessary for the future protection and sustainable use of karst aquifer in Southwest China.     

Geochemical and flow modelling as tools in monitoring managed aquifer recharge

Due to a growing world population and the effects of anthropogenic climate change, access to clean water is a growing global concern. Managed aquifer recharge (MAR) is a method that can help society's response to this increasing demand for pure water. In MAR, the groundwater resources are replenished and the quality of the recharged surface water is improved through effects such as the removal of organic matter. This removal occurs through mechanisms such as microbial decomposition, which can be monitored by studying the isotopic composition of dissolved inorganic carbon (DIC). Nevertheless, the monitoring can be difficult when there are other factors, like dissolving calcite, affecting the isotopic composition of DIC.The aims of this study were to establish a method for monitoring the decomposition of organic matter (dissolved organic carbon – DOC) in cases where calcite dissolution adds another component to the DIC pool, and to use this method to monitor the beginning and amount of DOC decomposition on a MAR site at Virttaankangas, southwestern Finland. To achieve this, we calculated the mean residence times of infiltrated water in the aquifer and the fractions of this water reaching observation wells. We conducted geochemical modelling, using PHREEQC, to estimate the amount of DOC decomposition and the mineral reactions affecting the quality of the water.     

Flow and solute transport monitoring in the karst aquifer in SW Slovenia

The role of the unsaturated zone in the karst aquifer hydraulic behaviour was brought into focus in these studies of the catchment of the Hubelj spring (SW Slovenia). The variations of natural tracers in precipitation and in groundwater during a summer storm event made it possible to trace local flow and solute transport in the observed aquifer. The results produced data on the aquifer recharge, storage and discharge processes, as well as on mechanisms that affected them, which reflects a karst groundwater dynamics also at a regional scale. They point out the significance of effects of the fast preferential flow—epiflow that is the main factor controlling solute/contaminant transport towards the aquifer saturated zone. Numerous arguments indicate that the karst aquifer flow and solute transport mechanisms depend on the hydraulic behaviour of the epikarst zone.     

A multidimensional fuzzy least-squares regression approach for estimating hydraulic gradients in unconfined aquifer formations and its application to the Gulf Coast aquifer in Goliad County,Texas

Epistemic uncertainties arise during the estimation of hydraulic gradients in unconfined aquifers due to planar approximation of the water table as well as data gaps arising from factors such as instrument failures and site inaccessibility. A multidimensional fuzzy least-squares regression approach is proposed here to estimate hydraulic gradients in situations where epistemic uncertainty is present in the observed water table measurements. The hydraulic head at a well is treated as a normal (Gaussian) fuzzy variable characterized by a most likely value and a spread. This treatment results in hydraulic gradients being characterized as normal fuzzy numbers as well. The multidimensional fuzzy least-squares regression has an exact analytical form and as such can be implemented easily using matrix algebra methods. However, the method was noted to be sensitive to round-off and truncation errors when the epistemic uncertainties are small. A closeness index based on the cardinality of a fuzzy number is used to evaluate how well the regression model fits the fuzzy hydraulic head observations. A fuzzy Euclidian distance measure is used to compare two fuzzy numbers and to evaluate how fuzziness in the observed hydraulic heads affects the fuzziness in the estimated hydraulic gradients. The Euclidian distance measure is also used to ascertain the influence of each well on the fuzzy hydraulic gradient estimation. The fuzzy regression framework is illustrated by applying it to evaluate hydraulic gradients in the unconfined portion of the Gulf Coast aquifer in Goliad County, TX. The results from the case-study indicate that there is greater uncertainty associated with the estimation of the hydraulic gradients in the vertical (Z-axis) direction. The epistemic uncertainties in the hydraulic head data at the wells have a significant impact on the gradient estimates when they are of the same order of magnitude as the most likely values of the observed heads. The influence analysis indicated that 5 of the 13 wells in the network had a critical influence on at least one of the hydraulic gradients. Three wells along the northeastern section of the study area and bordering the Victoria County were noted to have the least influence on the regression estimates. The fuzzy regression framework along with the associated goodness-of-fit and influence measures provides a useful set of tools to characterize the uncertainties in the hydraulic heads and gradients arising from data gaps and planar water table approximation.     

Radar remote sensing for monitoring of dynamic ecosystem processes related to biogeochemical exchanges in tropical peatlands

Peatlands are an important multipurpose ecosystem, supporting huge quantities of biomass and peat soil carbon. A time series of Japanese Earth Resource Satellite-1 (JERS-1) L-band Synthetic Aperture Radar (SAR) data was employed to monitor two dynamic ecosystem processes; deforestation and inundation patterns. Using a change detection analysis for three images acquired during dry seasons of 1994, 1997 and 1998, we detected the deforestation that has occurred in the region due to the anthropogenic and natural causes. At a threshold of ±2 dB change in backscattering response, an area of about 98 km² of these forests was found to have been cleared during 1994–1997 for conversion to cultivatable lands. However, the agricultural crops miserably failed to grow on these cleared lands because of the adverse water and soil chemistry conditions. The deliberate draining of these lands, by laying and extension of a huge network of canals, created congenial ecological conditions for the spread of forest fires, particularly during the 1997 El Niño period. An area of 250 km² of forests was thus detected to have been destroyed by these fires between September 1997 and January 1998. These deforested lands are rapidly regenerating since their abandonment and the regenerating carbon stocks were simulated using the CENTURY ecosystem dynamics model. Furthermore, the L-band SAR was able to detect the pixel-wise seasonal and spatial inundation information for particular forest types where the transmissivity of the L-band SAR signals was quite significant. These forest types corresponded to comparatively low biomass areas. The SAR derived information about these two important dynamic processes would be useful for improving the accuracy of modelling the spatial and temporal distribution of the carbon and other trace gases in these ecosystems.Edited by D. Boyd     

High-resolution coral records of rare earth elements in coastal seawater: biogeochemical cycling and a new environmental proxy

In this study we have used laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), to produce a high resolution coral record of rare earth elements (REE), Mn and Ba from coastal Porites corals from the Great Barrier Reef of Australia. Validation of the LA-ICP-MS technique indicated that the method provides accurate and reproducible (RSD = 13-18%) analysis of low concentration REE in corals (∼1 to 100 ppb). The REE composition in coral samples was found to closely reflect that of the surrounding seawater and distribution coefficients of ∼1-2 indicated minimal fractionation of the series during incorporation into coral carbonate. To explore the idea that coral records of REE can be used to investigate dissolved seawater composition, we analyzed two coastal corals representing a total of ∼30 yr of growth, including a 10-yr overlapping period. Comparable results were obtained from the two samples, particularly in terms of elemental ratios (Nd/Yb) and the Ce anomaly. Based on this evidence and results from the determination of distribution coefficients, we suggest that useful records of seawater REE composition can be obtained from coral carbonates. When compared to the REE composition of a mid shelf coral, coastal corals showed a significant terrestrial influence, characterized by higher REE concentrations (greater than 10 times) and light REE enrichment. The REE composition of coastal seawater inferred from the coral record was dependent on seasonal factors and the influence of flood waters. REE fractionation displayed a strong seasonal cycle that correlated closely with Mn concentration. We suggest that higher Nd/Yb ratios and higher Mn concentrations in summer result from scavenging of heavy REE by particulate organic ligands and Mn reductive dissolution respectively, both processes displaying higher rates during periods of high primary productivity. The Ce anomaly also displayed a strong seasonal cycle showing an enhanced anomaly during summer and during flood events. This is consistent with the Ce anomaly being primarily controlled by the abundance of Ce oxidizing bacteria. Based on these arguments, we suggest that the coral record of dissolved REE and Mn may be regarded as a useful proxy for biological activity in coastal seawater.     

Diel biogeochemical cycling in a hyperventilating shallow estuarine environment

A diel biogeochemical study was performed to assess the influence that periods of elevated biological activity have on the biogeochemical cycling of macronutrients and redox-sensitive elements in a natural estuarine environment. High-resolution data (15 min sampling) illustrates periodic extreme variations in dissolved oxygen (DO) in the shallow waters of Azevedo Pond, Elkhom Slough, California. During periods of low tidal flushing, DO values can range from highly oxic (>560 μM O₂: >250% saturation) during sunny days to suboxic conditions (<5 μM) at night. Nutrient cycling and redox-sensitive trace element biogeochemistry were evaluated in response to the extreme daily DO fluctuations. A diel sampling study was conducted over a 26-h period, where O₂ concentrations ranged from 346 μM to sustained non-detectable levels in the night hours. In concert with the DO fluctuations, diel phosphate cycling was on the order of 4 μM in response to tidal flushing events and biological assimilation and regeneration. The IO₃ ⁻/I⁻ redox couple quickly responded to suboxic conditions in the water column by a marked increase in I⁻ concentrations and corresponding depletion of IO₃ ⁻. The extreme fluctuations of the p∈ in the water column resulted in diel dissolved Mn²⁺ variations of nearly 5 μM, with observed dissolved Mn removal rates on the order of 1 μM h⁻¹. The elevated biogeochemical cycling of oxygen, nitrogen, phosphorus, iodine, manganese, and iron found in this shallow estuarine environment suggest that tidal restrictions and anthropogenic nutrient enrichments can amplify diel variations and potentially hinder the functional and ecological stability of these systems. These data suggest that accurate chemical monitoring of the health of an estuarine ecosystem must account for the diel variability inherent in these highly productive environments.     

High-resolution monitoring of water temperature and oxygen concentration in Lake Murten (Switzerland)

Lake Murten is located in the Lake District in western Switzerland and so far, it has been poorly investigated. The Environmental Service of Fribourg (SEn) has monitored this lake once a month for several years by water profiles (water temperature, pH, dissolved oxygen, conductivity) at its deepest part. The SEn identified the stratification of water masses as one of the main causes for oxygen consumption in the hypolimnion. In the present study, a Lander System was deployed in Lake Murten from April to September 2015 at 20 m depth at the northwestern part of the lake to monitor water temperature and dissolved oxygen at higher resolution (30 s interval). These records were compared with time series of atmospheric parameters like atmospheric temperature, wind speed and precipitation. A clear correlation of lake temperatures with wind speed was observed during the Spring and Autumn. The water mass stratification evolved through the season and reached its peak during Summer, preventing surface turbulences to reach the deeper part (20 m) of the water column and to partially oxygenate the metalimnion.     

Hydrogeology of limited aquifer in a granitic terrain

 Detailed hydrogeological studies in a granitic micro-watershed have been carried out to determine the extent, behavior, and characteristics of the aquifer. The study includes analysis of lithologs, drill time log, pumping tests, and slug tests. Realistic field conditions have been taken into account for characterizing the aquifer system. Slug tests were carried out to estimate aquifer parameters at the wells which could not sustain pumping. Received: 20 November 1997 · Accepted: 23 February 1998     

Solute transport characteristics and influencing factors in a coastal unconfined aquifer under tidal action identified by image monitoring in a laboratory experimental setup

Hydrogeology Journal - Studies of solute transport in unconfined coastal aquifers typically neglect oceanic oscillations and assume a static seaward boundary condition defined by the mean annual...     

Feasibility of geoelectrical monitoring and multiphase modeling for process understanding of gaseous CO2 injection into a shallow aquifer

Potential pathways in the subsurface may allow upwardly migrating gaseous CO₂ from deep geological storage formations to be released into near surface aquifers. Consequently, the availability of adequate methods for monitoring potential CO₂ releases in both deep geological formations and the shallow subsurface is a prerequisite for the deployment of Carbon Capture and Storage technology. Geoelectrical surveys are carried out for monitoring a small-scale and temporally limited CO₂ injection experiment in a pristine shallow aquifer system. Additionally, the feasibility of multiphase modeling was tested in order to describe both complex non-linear multiphase flow processes and the electrical behavior of partially saturated heterogeneous porous media. The suitability of geoelectrical methods for monitoring injected CO₂ and geochemically altered groundwater was proven. At the test site, geoelectrical measurements reveal significant variations in electrical conductivity in the order of 15?C30?%. However, site-specific conditions (e.g., geological settings, groundwater composition) significantly influence variations in subsurface electrical conductivity and consequently, the feasibility of geoelectrical monitoring. The monitoring results provided initial information concerning gaseous CO₂ migration and accumulation processes. Geoelectrical monitoring, in combination with multiphase modeling, was identified as a useful tool for understanding gas phase migration and mass transfer processes that occur due to CO₂ intrusions in shallow aquifer systems.     

Validity of a sharp-interface model in a confined coastal aquifer

 The problem of seawater intrusion is considered for the case of a confined coastal aquifer in which there is steady seaward flow of fresh water. Using the GWCH2O model, the problem is solved first for the case of no dispersion where a distinct interface exists separating the fresh water from the salt water. The problem is solved next by taking into account dispersion and diffusion of the salt-water component, along with the density effect. In this respect, a two-dimensional finite-element model, 2D-VDTRAN, is developed to simulate density-dependent solute transport. To investigate the limitation of the sharp-interface approach in coastal aquifers for conditions of both steady state and unsteady state, the problem is solved twice using the two models with different parameter values. These parameters are combined in dimensionless form, resulting in four named parameters: seepage factor (A); dispersion-to-advection ratio (B); geometry ratio (C); and time-scale factor (T). Using the density-dependent model, the dimensionless width of the transition zone (W/L) is determined for different values of A, B, C, and T. Steady-state simulations show that the sharp-interface approach is valid only when the system is dominated by advection, i.e., when 0<B≤5% for all values of A and C, or when A≥65% for all values of B and C. However, the unsteady-state analysis shows that the applicability of the sharp-interface approach is sufficiently accurate at early times. Received, October 1997 Revised, June 1998, October 1998 Accepted, November 1998     

Time-lapse crosswell seismic and VSP monitoring of injected CO<Subscript>2</Subscript> in a brine aquifer

Seismic surveys successfully imaged a small scale CO₂ injection (1,600 ton) conducted in a brine aquifer of the Frio Formation near Houston, Texas. These time-lapse borehole seismic surveys, crosswell and vertical seismic profile (VSP), were acquired to monitor the CO₂ distribution using two boreholes (the new injection well and a pre-existing well used for monitoring) which are 30 m apart at a depth of 1,500 m. The crosswell survey provided a high-resolution image of the CO₂ distribution between the wells via tomographic imaging of the P-wave velocity decrease (up to 500 m/s). The simultaneously acquired S-wave tomography showed little change in S-wave velocity, as expected for fluid substitution. A rock physics model was used to estimate CO₂ saturations of 10–20% from the P-wave velocity change. The VSP survey resolved a large (∼70%) change in reflection amplitude for the Frio horizon. This CO₂ induced reflection amplitude change allowed estimation of the CO₂ extent beyond the monitor well and on three azimuths. The VSP result is compared with numerical modeling of CO₂ saturations and is seismically modeled using the velocity change estimated in the crosswell survey.     

利用观测孔中地下水位潮汐效应计算天津滨海新区含水层参数

在沿海地区,尤其是围海造陆工程形成的陆域地区地下水水位受潮汐影响较大,使传统水文地质试验求取含水层参数存在较大误差。因此通过合理概化地下水在潮汐作用下运动规律,建立数学模型,推导解析公式求取沿海含水层参数具有重要意义。分析天津滨海新区两处观测孔地下水位及潮汐波动特征,在滞后时间不明显的情况下,利用观测孔水位变幅数据计算了含水层水头扩散系数,并根据承压含水层储水系数经验值进一步获得含水层渗透系数。通过两个观测孔分别计算,对比计算结果互相验证发现,该方法取得了令人满意的结果。利用地下水潮汐效应计算含水层参数可以广泛应用于沿海地区水文地质工作中。