Aquifer Hydrogeochemistry Research Articles

Hydrogeochemistry of aquifers in the northern portion of the Lagoa Santa Karst Environmental Protection Area and surroundings, state of Minas Gerais, Brazil

The studied region, located in the central part of Minas Gerais state, Brazil, encompasses the northern portion of the Lagoa Santa Karst Environmental Protection Area (APA) and adjacent areas to the west. It is notable for harboring important karstic aquifers and archaeological and paleontological sites of global significance. The research aims to define the key parameters that characterize the waters of the aquifers, quantifying their most frequent maximum and minimum concentrations. For this purpose, water sampling and hydrochemical analyses were conducted at 51 water points. Hierarchical clustering analysis facilitated the association of samples with aquifer units. Utilizing Piper and Stiff diagrams, boxplot graphs, and proportional symbol maps enabled the classification of water and the determination of its hydrochemical signatures. All sampled waters are bicarbonated, with 87% being calcic. Waters associated with metacalcareous rocks exhibit higher concentrations of calcium (Ca2⁺), bicarbonate (HCO₃⁻), electrical conductivity (EC), total dissolved solids (TDS), and hydrogenionic potential (pH) compared to waters associated with non-carbonate rocks. Waters from the Lagoa Santa Member differ from those of the Pedro Leopoldo Member by higher concentrations of Ca2⁺ and HCO₃⁻ and slightly lower concentrations of sodium (Na⁺), magnesium (Mg2⁺), and silica (SiO₄2⁻). Non-carbonate waters related to granite-gneisses differ from those associated with metapelites by slightly higher concentrations of potassium (K⁺), sodium (Na⁺), silica (SiO₄2⁻), and barium (Ba2⁺). Additionally, hydrochemical variations have been linked to the regional groundwater flow (from west to east). The results obtained have enabled advancements in the hydrogeochemical understanding of the study area and can be valuable for comparative studies in other karst regions, contributing to more effective water resource management aimed at environmental conservation and water sustainability, a growing global concern.

Hydrogeochemical Assessment of Groundwater Quality and Suitability for Drinking and Agricultural Use. The Case Study of Fars Province, Iran

This study aims to evaluate the hydrogeochemistry of aquifers in Fars province, Iran, from 2007 to 2017 and assess the groundwater’s suitability for drinking and agricultural uses. A total of 35,000 samples were collected from wells and qanats across the province. Piper, Gibbs, and Durov diagrams were used to assess the hydrochemical facies and processes. Cross plots of different ions were investigated to assess ion exchange and determine the effects of anthropogenic activities, as well as the weathering and dissolution of different rocks and minerals in the aquifers. Groundwater quality and suitability for agricultural and drinking purposes were also assessed using physicochemical parameters including pH, Electrical Conductivity (EC), Total Dissolved Solids (TDS), Total Hardness (TH), and calcium, magnesium, sodium, potassium, bicarbonate, sulfate, and chloride concentrations. Suitability for domestic purposes was assessed by comparing these values with the WHO standards. Sodium and alkalinity hazards, including Sodium Adsorption Ratio (SAR), sodium percentage (Na%), Permeability Index (PI), Magnesium Hazard (MH), and Residual Sodium Carbonate (RSC) were used to assess irrigation suitability, along with plotting Wilcox and USSL diagrams.

Hydrogeochemistry of the Umm er Radhuma Unconfined Aquifer, Western Desert Iraq- Saudi border

The Umm Er Radhuma unconfined aquifer Hydrogeochemistry in the Saudi – Iraq border desert was studied to identify the main hydrogeochemical processes and rock-water interaction. The measurements were done using standard APHA procedures. The results indicated that Na+ and Cl- are the dominant ions in the groundwater. The average contribution of cations in the aquifer is Na+ + K+ ( 26 %), Ca2+ (14 %), and Mg2+ (10%) ; whereas anions contribution is Cl- (24 %) , SO42- (20 %), and HCO3- (6 %). The results reflect that the examined water is hard water of neutral to slightly alkaline, slightly brackish, with mineralized pH conditions. The main conclusion of chemical reactions indicated carbonates, evaporate dissolution, and clay minerals water interaction through ionic exchanging.

Hydrogeochemical evaluation and mechanisms controlling groundwater in different geologic environments, Western Sokoto Basin, Northwestern Nigeria

The hydrogeochemistry of aquifers in the western Sokoto basin was assessed. The objective of this study is to identify the impact of geological variability on groundwater hydrochemistry and the mechanisms controlling the hydrochemistry of aquifers. Univariate statistics (several samples ANOVA), Pearson’s (r), and multivariate statistics-hierarchical cluster analysis (HCA) and Factor analysis (FA) were used to interpret the hydrochemistry of aquifers. Subsequently, geochemical modeling was applied to assess the saturation index (SI) of rock minerals. Forty groundwater samples were collected from Gwandu (n = 20) and Illo (n = 20) formations. ANOVA results indicated that geological variability exerted a considerable impact on groundwater in Gwandu and Illo aquifers. It is characterized by a substantial amount of Ca2+, Mg2+, SO42−, HCO3−, Na+, and K+. The hydrogeochemical facies indicated mixing conditions. FA and correlations analysis further revealed that groundwater received the noticeable influence of silicate weathering and dissolution of carbonates. There were significant differences in SI values for chrysotile, goethite, gypsum, H2(g), H2O(g), H2S(g), illite, and sepiolite minerals between the two geologic environments. Positive SI values for gibbsite were noticed in eleven sampling locations, indicative of potential recharge zones. Likewise, all the sampling locations have positive values for K-feldspar and are supersaturated with mica, suggesting both discharge and transition zones. The HCA classified aquifers into three clusters based on their hydrogeochemical characteristics. Geochemical modeling, univariate, and multivariate analyses are user-friendly tools for hydrochemical analysis.

Hydrogeochemistry of a strategic alluvial aquifer system in a semi-arid setting and its implications for potable urban water supply: The Lodwar Alluvial Aquifer System (LAAS)

Lodwar Municipality is one of the fastest-growing urban areas of Sub-Saharan Africa that depends mainly on groundwater for its municipal water supply. Most of the groundwater sources are located within the riparian zones of the Turkwel River. With limited understanding of its aquifers, the groundwater of Lodwar may be at risk of natural processes and anthropogenic activities. Statistical techniques and geochemical methods were applied to determine the aquifer hydrogeochemistry. Three distinct aquifers, which we collectively refer to as the Lodwar Alluvial Aquifer System, underlie Lodwar and its environs, the shallow alluvial, intermediate, and deep aquifers which are the main source of fresh water. A fourth, the shallow aquifer of the Turkana grit, is highly saline and with fluoride contamination. Just as the Turkwel River, the shallow alluvial aquifer (SAA) was dominated by Ca–HCO3 water type, while the TGSA was Na–Cl water type and became Na–HCO3 near the Holocene sediments. The intermediate aquifer (IA) was Na–HCO3water type. Pockets of Mg–HCO3 water occurred in the shallow alluvial and intermediate aquifers. The natural processes in the SAA include rock-water interaction, recharge by surface water, and oxidation reactions, while evaporation and dissolution are the major factors controlling the chemistry of the TGSA. Ion exchange, dilution, and dissolution are the major processes in the IA. Elevated levels of NO3− and SO42− during the wet season within the SAA and the IA reflects their vulnerability to pollution. Saline intrusion into the shallow and intermediate aquifers from the Turkana grit aquifers is likely to occur.

A geospatial investigation of interlinkage between basement fault architecture and coastal aquifer hydrogeochemistry

The Mahanadi delta, deposited on a series of horst and graben basement structures, is considered an extension of the East Lambert Rift of Antarctica. Current study is based on the hydrogeochemical assessment of this deltaic aquifer system and geospatial analysis thereof, to appreciate the basement structure influence on groundwater chemistry. Major ion chemistry of subsurface waters portrays a distinct saline contamination across the terrain and varied regimes of water types, specifically with respect to southern and northern parts of this aquifer system. Findings of the study indicate a general near surface saline horizon and significant fragmentation of the hydrostatic units. This, in turn, implies noteworthy influence of formational water to salinity regimes and basin structural changes for the escape of these waters to surroundings. A plot of recent low intensity earthquakes displays proximity of epicenters to the faults as well as striking similarity to the trend of terrestrial faults indicating multiple reactivations of the faults. To further corroborate the above findings, spatial pattern analysis of individual hydrochemical variables is carried out which reveals specific clusters of sources (groundwater mixing) and sinks (groundwater dispersion) in proximity to basement fault dispositions. While the faults can be disregarded as conduits or barriers owing to their great depth, the overlying sedimentary mass, particularly, the horizons with significant clayey content have been distorted due to post rift subsidence and fault reactivations. A proximity analysis of ionic clusters points towards a greater influence of longitudinal faults to that of the transverse ones on groundwater mixing or dispersion.

Hydrogeochemical assessment of shallow aquifer in Chakdah, West Bengal for safe water supply

ABSTRACT The shallow aquifer hydrogeochemistry and arsenic exposure from drinking water in rural Bengal have been vividly investigated in the present study. It has been observed that shallow aquifers (<50 m) are often contaminated with arsenic, whereas deeper aquifers (>100 m) are mostly free from arsenic contamination. In some pockets, deeper aquifers are contaminated which is surrounded by safer aquifers. Mostly, shallow aquifers (<50 m) are contaminated with arsenic surrounded by safe and/or deep aquifers (>100 m) with low arsenic concentrations and are scattered throughout the study area with more heterogeneity. Furthermore, sedimentological (color of the sediment and its successions) and/or geomorphological features (land-use pattern and local recharge) are playing a key role to control the arsenic distribution pattern in groundwater from the study area. They control local hydrogeochemical factors thereby mobilizing arsenic into groundwater. It has also been found that yellow sand aquifers are mostly free from arsenic contamination. The deeper aquifers mainly consist of coarse to medium sand (mostly yellow sand) which incorporates the relatively lower concentration of arsenic in groundwater. For the long-term mitigation options, the findings from this investigation are very useful.

Temperature influence on mobilisation and (re)fixation of trace elements and heavy metals in column tests with aquifer sediments from 10 to 70 °C

The operation of seasonal underground thermal energy storages (UTES) as part of renewed heat supply systems can cause amplified temperature variations in the urban subsurface. Therefore, long-term environmental consequences for water extractions by trace elements and heavy metals (TEHMs) are a key point of concern regarding temperature effects on aquifer hydrogeochemistry. To address this issue, we report the results of flow-through and circular-flow column tests conducted with 4 anoxic northern German aquifer sediments, tempered to 10, 25, 40 and 70 °C and analysed for 20 TEHMs. Increased temperatures in column tests caused increasing Li, As, Mo, Sb and Ba concentrations and decreasing Ni concentrations in all of the sediments with a sediment-specific extent, whereas effects on V, Mn, Co, Tl and U concentrations varied sediment-specifically. Apart from Ba, all these components were released as a pulse in the initial heating phase, indicating a temperature dependent, finite, elutable pool. Re-cooling of the previously heated circular-flow column tests to 10 °C caused reversals of concentration changes by 30–95%. This indicates a return to initial hydrochemical conditions after termination of heat storage operation and downstream from heat storages during the operations. The latter was approximated for As with a simplified analytical 1-dimensional approach, presuming transferability from a laboratory to a field scale. This reversal in concentration changes enables active cooling as a countermeasure in cases of unexpected, adverse TEHM progression. From the perspective of our findings, TEHM concentration changes appear to be temporally and spatially limited.

Groundwater nitrate reduction versus dissolved gas production: A tale of two catchments.

At the catchment scale, a complex mosaic of environmental, hydrogeological and physicochemical characteristics combine to regulate the distribution of groundwater and stream nitrate (NO3-). The efficiency of NO3- removal (via denitrification) versus the ratio of accumulated reaction products, dinitrogen (excess N2) & nitrous oxide (N2O), remains poorly understood. Groundwater was investigated in two well drained agricultural catchments (10km2) in Ireland with contrasting subsurface lithologies (sandstone vs. slate) and landuse. Denitrification capacity was assessed by measuring concentration and distribution patterns of nitrogen (N) species, aquifer hydrogeochemistry, stable isotope signatures and aquifer hydraulic properties. A hierarchy of scale whereby physical factors including agronomy, water table elevation and permeability determined the hydrogeochemical signature of the aquifers was observed. This hydrogeochemical signature acted as the dominant control on denitrification reaction progress. High permeability, aerobic conditions and a lack of bacterial energy sources in the slate catchment resulted in low denitrification reaction progress (0-32%), high NO3- and comparatively low N2O emission factors (EF5g1). In the sandstone catchment denitrification progress ranged from 4 to 94% and was highly dependent on permeability, water table elevation, dissolved oxygen concentration solid phase bacterial energy sources. Denitrification of NO3- to N2 occurred in anaerobic conditions, while at intermediate dissolved oxygen; N2O was the dominant reaction product. EF5g1 (mean: 0.0018) in the denitrifying sandstone catchment was 32% less than the IPCC default. The denitrification observations across catchments were supported by stable isotope signatures. Stream NO3- occurrence was 32% lower in the sandstone catchment even though N loading was substantially higher than the slate catchment.

The contribution of cluster and discriminant analysis to the classification of complex aquifer systems.

This paper presents an innovated method for the discrimination of groundwater samples in common groups representing the hydrogeological units from where they have been pumped. This method proved very efficient even in areas with complex hydrogeological regimes. The proposed method requires chemical analyses of water samples only for major ions, meaning that it is applicable to most of cases worldwide. Another benefit of the method is that it gives a further insight of the aquifer hydrogeochemistry as it provides the ions that are responsible for the discrimination of the group. The procedure begins with cluster analysis of the dataset in order to classify the samples in the corresponding hydrogeological unit. The feasibility of the method is proven from the fact that the samples of volcanic origin were separated into two different clusters, namely the lava units and the pyroclastic-ignimbritic aquifer. The second step is the discriminant analysis of the data which provides the functions that distinguish the groups from each other and the most significant variables that define the hydrochemical composition of the aquifer. The whole procedure was highly successful as the 94.7% of the samples were classified to the correct aquifer system. Finally, the resulted functions can be safely used to categorize samples of either unknown or doubtful origin improving thus the quality and the size of existing hydrochemical databases.

Identification of the hydrogeochemical processes and assessment of groundwater quality using classic integrated geochemical methods in the Southeastern part of Ordos basin, China

Insufficient understanding of the hydrogeochemistry of aquifers makes it necessary to conduct a preliminary water quality assessment in the southern region of Ordos Basin, an arid area in the world. In this paper, the major ions of groundwater have been studied aiming at evaluating the hydrogeochemical processes that probably affect the groundwater quality using 150 samples collected in 2015. The two prevalent hydrochemical facies, HCO3Mg·Na·Ca and HCO3Mg·Ca·Na type water, have been identified based on the hydrochemical analysis from Piper trilinear diagram. Compositional relations have been used to assess the origin of solutes and confirm the predominant hydrogeochemical processes responsible for the various ions in the groundwater. The results show that the ions are derived from leaching effect, evaporation and condensation, cation exchange, mixing effect and human activities. Finally groundwater quality was assessed with single factor and set pair methods, the results indicate that groundwater quality in the study region is generally poor in terms of standard of national groundwater quality. The results obtained in this study will be useful to understand the groundwater quality status for effective management and utilization of the groundwater resource.

Regional aquifer hydrogeochemistry in the confined aquifer system below the Boom Clay (NE Belgium): data analysis and modelling

Abstract Geochemical data from the deep aquifer system below the Boom Clay in NE Belgium have been collected and reinterpreted. The data were obtained between 1980 and 2014 within the framework of radioactive waste disposal studies. Currently, groundwater in the deep aquifer system mainly ranges between NaHCO 3 type and NaCl type. Because of the low groundwater velocity in this aquifer system, re-equilibration with the mineralogical composition of the host formations generally occurs. The main geochemical indicators point out that the current formation waters are a mixture between the original saline marine porewaters and freshwater recharge. SE–NW gradients of increasing ion concentrations are observed and can be explained in agreement with the pattern of natural groundwater flow. Calcite dissolution and cation exchange are still ongoing in this freshening aquifer system. A low sulphate content in the deep aquifer system indicates sulphate reduction associated with organic matter degradation. Inverse modelling along a flow path using PHREEQC generally confirms the reactions that have been derived in the data analysis.

Hydrogeochemical overview and natural arsenic occurrence in groundwater from alpine springs (upper Valtellina, Northern Italy)

Summary High arsenic (As) concentrations (up to 230 μg/L) have been historically observed (since 1999) in the upper Valtellina valley groundwater (UVV, central Italian Alps), and measured in samples collected during four campaigns of one full hydrological year (summer 2012–summer 2013). During these campaigns, water has been collected from both cold springs and thermal springs. The hydrogeochemistry of aquifers and superficial waters through the hydrologic year, and the long-term regional As distribution and time variability were analyzed. Although the studied springs belong to different catchments with different hydrochemical and lithological conditions, they present some typical characteristics: (1) the water types are dominated by Ca–Mg and SO4–HCO3 main ions, with seasonal variations for the second end members; (2) the Cl concentration is always very low, and poorly correlated with other ions; (3) the circulation time obtained from isotopic data ranges between 5 and 10 years for thermal springs and it is lower than 2 years for cold springs; (4) dominant oxidizing conditions have been observed for most of the cold and for the thermal springs; (5) anthropogenic contamination is absent, while natural contamination of arsenic affects most of the springs, with a natural background level for the entire UVV of 33 μg/L; (6) both As (V) and As (III) are present in all the springs analyzed, with a marked prevalence of As (V) among the cold ones. These conditions suggest that the latter belong to recent hydrochemical immature aquifers, where the presence of arsenic is mostly related to alkali desorption and sulfide oxidation, while the thermal springs derive from the rapid uprising of deep-circulation water, with a high concentration of geothermal arsenic.

Impacts of subsurface heat storage on aquifer hydrogeochemistry

The use of shallow aquifers for subsurface heat storage in terms of energy management and building climatisation can lead to a temperature rise in the aquifer to 70 °C and above. The influence of temperature changes on individual mineral and sorption equilibria, reaction kinetics and microbial activity is largely known. However, the impact of heating to temperatures as high as 70 °C on the aquifer overall system has not been quantified yet. Temperature-related changes in sediment ion exchange behaviour, dimension and rates of mineral dissolution and precipitation as well as microbially enhanced redox processes were studied in column experiments using aquifer sediment and tap water at 10, 25, 40, and 70 °C. At 70 °C, a change in sediment sorption behaviour for cations and organic acids was postulated based on temporal changes in pH, magnesium, and potassium concentration in the experimental solution. No clear changes of pH, TIC and major cations were found at 10–40 °C. Redox zoning shifted from oxic conditions towards nitrate and iron(III) reducing conditions at 25 and 40 °C and sulphate reducing conditions at 70 °C. This was attributed to (a) a temperature-related increase in microbial reduction activity, and (b) three times higher release of organic carbon from the sediment at 70 °C compared to the lower temperatures. The findings of this study predict that a temperature increase in the subsurface up to 25 °C and above can impair the usability of ground water as drinking and process water, by reducing metal oxides and thus possibly releasing heavy metals from the sediment. Generally, at 70 °C, where clear cation and organic carbon desorption processes were observed and sulphate reducing conditions could be achieved, a site-specific assessment of temperature effects is required, especially for long-term operations of subsurface heat storage facilities.

Preliminary groundwater quality assessment in the central region of Ghana

Insufficient knowledge of the hydrogeochemistry of aquifers in the Central Region of Ghana has necessitated a preliminary water quality assessment in some parts of the region. Major and minor ions, and trace metal compositions of groundwater have been studied with the aim of evaluating hydrogeochemical processes that are likely to impair the quality of water in the study area. The results show that groundwater in the area is weakly acidic with mean acidity being 5.83 pH units. The dominant cation in the area is Na, followed by K, Ca, and Mg, and the dominant anion is Cl−, followed by HCO3− and SO42−. Two major hydrochemical facies have been identified as Na–Cl and Na–HCO3, water types. Multivariate statistical techniques such as cluster analysis (CA) and factor analysis/principal component analysis (PCA), in R mode, were employed to examine the chemical compositions of groundwater and to identify factors that influenced each. Q-mode CA analysis resulted in two distinct water types as established by the hydrochemical facies. Cluster 1 waters contain predominantly Na–Cl. Cluster 2 waters contain Na–HCO3 and Na–Cl. Cluster 2 waters are fresher and of good quality than cluster 1. Factor analysis yielded five significant factors, explaining 86.56% of the total variance. PC1 explains 41.95% of the variance and is contributed by temperature, electrical conductivity, TDS, turbidity, SO42−, Cl−, Na, K, Ca, Mg, and Mn and influenced by geochemical processes such as weathering, mineral dissolution, cation exchange, and oxidation–reduction reactions. PC2 explains 16.43% of the total variance and is characterized by high positive loadings of pH and HCO3−. This results from biogenic activities taking place to generate gaseous carbon dioxide that reacts with infiltrating water to generate HCO3−, which intend affect the pH. PC3 explains 11.17% of the total variance and is negatively loaded on PO43− and NO3− indicating anthropogenic influence. The R-mode PCA, supported by R-mode CA, have revealed hydrogeochemical processes as the major sources of ions in the groundwater. Factor score plot revealed a possible flow direction from the northern sections of the study area, marked by higher topography, to the south. Compositional relations confirmed the predominant geochemical process responsible for the various ions in the groundwater as mineral dissolution and thus agree with the multivariate analysis.

Modeling of limestone aquifer in the western part of the River Nile between Beni Suef and El Minia

A robust classification scheme for partitioning groundwater chemistry into homogeneous groups was an important tool for the characterization of Eocene limestone aquifer. The aquifer locally is composed of chalky limestone with thin clay intercalated (Samalut Fm.), the fissures, the joints, and the fractures are represented the conduits of the aquifer system. The flow patterns are conditioned by karstification processes which develop a conduit network and preserve low permeability microfractured blocks. The aquifer is mainly recharged by surrounding aquifers and agricultural wastewaters. The groundwater flows in the eastern part (due the Bahr Yossef and River Nile), which is a discharge area rather than a recharge. Twenty-eight groundwater samples was collected from the Eocene limestone aquifer and analyzed for isotopes, major, and trace elements. δD and δ18O concentrations ranged widely due to geology, infiltration of different surface waters, evaporation, and hydrogeology. The concentration of δD and δ18O isotopes is depleted in the northern zone of the northern part and western zone of the central and southern part of the study area. They are enriched due the eastern area of the central and southern part of the study area. δD vs. δ18O delineate the Pleistocene aquifer and has a strong influence than other waters on aquifer hydrogeochemistry. It is confirmed by the AquaChem outputs of the mixing proportions of different water types included in the aquifer system. Cl-δD and Cl-δ18O relationships indicate the role of evaporation especially due the eastern area of the central and southern part of the study area. This research tests the performance of the many available graphical and statistical methodologies used to classify water samples. R-mode clustering, correlation analysis, and principal component analysis were investigated. All the methods were discussed and compared as to their ability to cluster, ease of use, and ease of interpretation. Nearly most low-salinity waters are in equilibrium to supersaturate with respect to both carbonate minerals, while it is shifted to undersaturate with salinity. The inverse modeling findings clarify that the calcite, gypsum, and anhydrite dissolution increased due the northeastern area, middle zone, and southern corner of the northern, central, and southern part of the study area, respectively. The latter areas also were characterized by the lowest precipitation of the dolomite. Such areas are distinguished by much more enhancement for aquifer permeability and therefore transmissivity. The latter areas can be use as injection zone by fresh water. It can be a triple function; firstly, it recharges the saline Eocene limestone aquifer through the enhancement hydraulic conductivity and dilutes it. Secondly, it enhances much more the aquifer permeability and therefore the transmissivity. The Eocene limestone aquifer can be improved in quality and quantity by using such a model and exploits it as an alternative water resource with Quaternary aquifer and Nile water. Thirdly, it irrigates more areas to increase the income/capita. The dedolomitization represents the main hydrogeochemical process in the aquifer system. The geomedia (limestone, clay, marl, shale, and sand deposits) are in contact with water, therefore, the rock/water interaction, mixing, and ion exchange were estimated by the geochemical evolution of the groundwater systems.

Geochemical impacts of CO2 storage in saline aquifers with various mineralogy—Results from laboratory experiments and reactive geochemical modelling

Abstract Investigations of reservoirs, cap-rocks and traps in the Norwegian-Danish Basin have indicated a large potential for geological storage of CO2 onshore Denmark. The possible reservoir rocks include a variety of sandstones with different mineralogical composition, ranging from the Bunter Sandstone Formation containing K-feldspar, clay minerals, calcite, and dolomite as primary reactive minerals to the Gassum Formation containing albite, clay minerals, siderite, and ferroan dolomite as the most reactive minerals. A laboratory and modelling study was carried out to investigate the geochemical response of five potential reservoir rocks to CO2 storage in order to constrain predictions of mineral- CO2 reactions prior to geological storage. The study includes hydrogeochemical experiments, petrographical and mineralogical analyses, and reactive geochemical modelling. During up to 13 months of exposure to CO2 at reservoir conditions (70 °C; 20 MPa), the five different rock samples show little mineral reactivity as compared to similar rock samples exposed to N2 for the same period of time. However, during the period covered by the experiments, dissolution of carbonates present in the host rock is observed both from petrographical analysis and geochemical analysis using speciation calculations. Thus, for the Bunter Sandstone Formation calcite dissolution is apparently taking place in the laboratory experiments while for the Gassum Formation samples ferroan dolomite and siderite dissolution are the dominant mineral dissolution reactions taking place. As a result of the speciation calculations, a procedure for back calculation of chemical analyses to true experimental conditions is suggested in order to reflect the correct saturation state of the pore water with respect to carbonates, primary silicates and aluminosilicates. It is suggested that performing such back calculations is essential to the understanding of the future evolution of the hydrogeochemistry of aquifers aimed at as CO2 storage reservoirs.

Simulation of reactive geochemical transport in groundwater using a semi-analytical screening model

A reactive geochemical transport model, based on a semi-analytical solution to the advective-dispersive transport equation in two dimensions, is developed as a screening tool for evaluating the impact of reactive contaminants on aquifer hydrogeochemistry. Because the model utilizes an analytical solution to the transport equation, it is less computationally intensive than models based on numerical transport schemes, is faster, and it is not subject to numerical dispersion effects. Although the assumptions used to construct the model preclude consideration of reactions between the aqueous and solid phases, thermodynamic mineral saturation indices are calculated to provide qualitative insight into such reactions. Test problems involving acid mine drainage and hydrocarbon biodegradation signatures illustrate the utility of the model in simulating essential hydrogeochemical phenomena.