Hydrogeochemical Changes Research Articles

Hydrogeochemical Evolution and Control Mechanism of Underground Multiaquifer System in Coal Mine Area

Mining activities interfere into the natural groundwater chemical environment, which may lead to hydrogeochemical changes of aquifers and mine water inrush disasters. The study of hydrogeochemical evolution processes of underground aquifers is helpful to the prevention and control of mine water inrush. The results show that the study area is mainly impacted by four hydrogeochemical processes: dissolution, cation exchange, desulfurization and reduction, and pyrite oxidation. The Cenozoic aquifers are dominated by carbonate dissolution and desulfurization. The Permian aquifers are impacted mainly by cation exchange and sulfate dissolution, followed by pyrite oxidation. The Carboniferous aquifers are mainly impacted by dissolving sulfate, followed by pyrite oxidation and cation exchange. The hydrogeochemical evolution of the aquifers was controlled by mining activities and tectonic changes, and a certain regularity in space. For the Cenozoic aquifers, sulfate dissolution and cation exchange increase from west to east, and desulfurization weakens. For the Permian aquifers, cation exchange and sulfate dissolution are stronger near synclines and faults, pyrite oxidation is enhanced, and desulfurization decreases from the middle to the east of the mining area. For the Carboniferous aquifers, there is a higher dissolution of rock salt, pyrite oxidation, and cation exchange from west to east, and the desulfurization effect weakens.

Biogeochemical characterization of a Mediterranean shallow lake using stable isotopes: Laguna del Cristo (NW Iberian Peninsula)

The present multi-isotopic study (δ18O–δDwater, δ34S–δ18Odissolved-sulphate, δ13Cdissolved-inorganic-carbon, δ13C–δ18Oshells-modern-gastropods, δ13Cplants, and δ13Csedimentary-organic-matter) is aimed at assessing the hydrogeochemical changes and biogeochemical dynamics in a Mediterranean shallow lake fed by a Quaternary–Tertiary aquifer, the “Laguna del Cristo” (NW Iberian Peninsula), a system sensitive to climate fluctuations, between 2010 and 2011. Lake water is of the bicarbonate type, and there are no major pollutants. δ18O-δDwater values plot on a local evaporation line (δD=5.29δ18O–12.29) indicating that evaporative enrichment had a significant impact on lake water isotopic features. Periods of high water levels are characterized by lower δ34S–δ18Odissolved-sulphate and δ13Cdissolved-inorganic-carbon values and suggest sulphate derived from weathering of sulphides in the catchment area, delivered to the lake by surface run-off or via groundwater, and in situ decay of organic matter. During lower water levels, sulphate reduction and enhanced primary productivity lead to higher δ34Sdissolved-sulphate and δ13Cdissolved-inorganic-carbon values. Evaporation induced enrichment in 18Osulphate, 13Cdissolved-inorganic-carbon and 13C–18Oshells-Galba-Gyraulus. δ13Cplant confirms the C3 photosynthetic pathway. Enrichment in 13C submerged aquatic plants indicates that HCO3− is the main carbon source, except for 13C-depleted Potamogeton. The TOC, δ13Corg values, and TC/TN ratios in sediments all confirm the autochthonous character of organic matter contribution. This study provides a baseline for isotopic research into shallow, flow-through lakes fed by siliciclastic aquifers, and stresses the importance of evaporation and refilling (direct precipitation and groundwater discharge) in controlling the solute chemistry and stable isotopic composition in temperate regions with contrasting seasonal climates. The results also provide a snapshot of modern lake isotope variability that can be applied to paleoenvironmental reconstructions.

Environmental geochemistry of rare earth elements in Cu-porphyry mine tailings in the semiarid climate conditions of Sarcheshmeh mine in southeastern Iran

Sarcheshmeh mine is one of the largest Oligo-Miocene porphyry Cu deposits in the world. It is located in the Kerman Cenozoic magmatic arc in southeastern Iran. This paper discusses the geo-environmental behavior of rare earth elements (REEs) in the Sarcheshmeh dammed tailing pond, situated in semiarid climate. For this purpose, solid and water samples were collected to consider: (1) weathering/oxidation reactions and upward migration of soluble elements; (2) super-saturation and formation/dissolution of the secondary evaporative phases; (3) hydro-geochemical changes of the surface and groundwater resources; (4) flotation of the Cu-sulfide ore and tailing management strategies. The mean values of the ∑REEs for fresh and weathered tailings and the evaporative phases were 113.0, 103.7 and 74.6mg/kg, respectively. When compared with the fresh and weathered tailings, the evaporative phases showed significant enrichment of HREEs and a well-depleted pattern of LREEs (except for that of Eu). This geochemical pattern suggests a higher solubility and mobility potential of HREEs and Eu compared with LREEs, resulting in good fractionation and enrichment of these elements in the surface evaporative layer formed on top of the weathered tailings. The water samples showed a range of hydro-geochemical features under acidic (pH of 2.5 to 4.7), near neutral (pH of 6.4 to 7.9), alkaline (pH of 8 to 10) and highly alkaline (pH>10) conditions. The maximum concentration of REEs was observed in the acidic waters produced by the weathered tailings after rainfall (∑REEs of 489 to 1903μg/L). Although ∑REEs decreased as the pH value increased, the general trend was compatible with the highest concentration of HREEs rather than LREEs. The speciation calculations indicate that LnSO4+, Ln(SO4)2− and Ln3+ are the primary forms of dissolved REEs at acidic and near neutral pH and the dominant species was LnSO4+. LnCO3+ (40.78%), Ln(CO3)2− (31.68%) and LnSO4+ (31.07%) were the dominant species under alkaline conditions. At high alkaline pH, the speciation pattern changed to LnOH2+ (53.15%), Ln(CO3)2− (44.88%) and LnCO3+ (2.61%), where Ln denotes lanthanide. Based on the results and considering previous works, it appears that ionic potential plays an important role in the geo-environmental behavior of REEs of adsorption tendency, mobility potential and ionic substitution.

Hydrogeochemical changes before and during the 2016 Amatrice-Norcia seismic sequence (central Italy)

Seismic precursors are an as yet unattained frontier in earthquake studies. With the aim of making a step towards this frontier, we present a hydrogeochemical dataset associated with the 2016 Amatrice-Norcia seismic sequence (central Apennines, Italy), developed from August 24th, with an Mw 6.0 event, and culminating on October 30th, with an Mw 6.5 mainshock. The seismic sequence occurred during a seasonal depletion of hydrostructures, and the four strongest earthquakes (Mw ≥ 5.5) generated an abrupt uplift of the water level, recorded up to 100 km away from the mainshock area. Monitoring a set of selected springs in the central Apennines, a few hydrogeochemical anomalies were observed months before the onset of the seismic swarm, including a variation of pH values and an increase of As, V, and Fe concentrations. Cr concentrations increased immediately after the onset of the seismic sequence. On November 2016, these elements recovered to their usual low concentrations. We interpret these geochemical anomalies as reliable seismic precursors for a dilational tectonic setting.

Hydrogeochemistry and water-rock interactions in the urban area of Puebla Valley aquifer (Mexico)

The urban area of Puebla Valley aquifer is seated in Puebla City and neighbor municipalities. Puebla is the fourth largest city in Mexico, where there are significant industrial zones and a large population. Water needs are almost exclusively met by groundwater, which has brought intense exploitation of groundwater resources and water quality degradation. The present study investigates the hydrogeochemical changes in groundwater, particularly focuses on the chemical changes produced by upwelling mineralized water. These concentrations may represent potential risks to the health of the population.The groundwater presented five types of families Ca-HCO3, Mg-HCO3, Mix-HCO3, HCO3-Ca-SO4 and Ca-SO4. The high concentrations of sulphates, calcium and magnesium are reflected in high TDS and Total Hardness. The samples collected showed detectable concentrations of F−, Fe, Mn, Ba, Sr, Cu, Zn, B and Li. The limitations for its use as drinking water are given by the high values of TDS, sulphates, total hardness and Mn.Geochemical modeling using Geochemist's Workbench (GWB) and PHREEQC software enabled the computation of the saturation index of mineral phases with ions in solution and speciation ions. Groundwater was initially in equilibrium with calcite; however, due to the changes in hydrogeological conditions, gypsum and dolomite are being dissolved until new equilibrium conditions are met. The additional calcium and carbonate in the water causes calcite to become oversaturated and to precipitate. Evidence of dedolomitization reactions and common ion effect is illustrated by concurrent increases in calcium and magnesium concentrations in the groundwater.

River-spring connectivity and hydrogeochemical interactions in a shallow fractured rock formation. The case study of Fuensanta river valley (Southern Spain)

In upland catchments, the hydrology and hydrochemistry of streams are largely influenced by groundwater inflows, at both regional and local scale. However, reverse conditions (groundwater dynamics conditioned by surface water interferences), although less described, may also occur. In this research, the local river-spring connectivity and induced hydrogeochemical interactions in intensely folded, fractured and layered Cretaceous marls and marly-limestones (Fuensanta river valley, S Spain) are discussed based on field observations, tracer tests and hydrodynamic and hydrochemical data. The differential flow measurements and tracing experiments performed in the Fuensanta river permitted us to quantify the surface water losses and to verify its direct hydraulic connection with the Fuensanta spring. The numerical simulations of tracer breakthrough curves suggest the existence of a groundwater flow system through well-connected master and tributary fractures, with fast and multi-source flow components. Furthermore, the multivariate statistical analysis conducted using chemical data from the sampled waters, the geochemical study of water-rock interactions and the proposed water mixing approach allowed the spatial characterization of the chemistry of the springs and river/stream waters draining low permeable Cretaceous formations. Results corroborated that the mixing of surface waters, as well as calcite dissolution and CO2 dissolution/exsolution, are the main geochemical processes constraining Fuensanta spring hydrochemistry. The estimated contribution of the tributary surface waters to the spring flow during the research period was approximately 26–53% (Fuensanta river) and 47–74% (Convento stream), being predominant the first component during high flow and the second one during the dry season. The identification of secondary geochemical processes (dolomite and gypsum dissolution and dedolomitization) in Fuensanta spring waters evidences the induced hydrogeochemical changes resulting from the allogenic recharge. This research highlights the usefulness of an integrated approach based on river and spring flow examination, dye tracing interpretation and regression and multivariate statistical analysis using hydrochemical data for surface water-groundwater interaction assessment in fractured complex environments worldwide, whose implementation becomes critical for an appropriate groundwater policy.

Hydrogeochemical Changes in Thermal Waters of the Western Pannonian Basin

Due to the continuous thermal water abstractions of the last 50 years, in some sub-regions of the western part of the Pannonian Basin groundwater heads significantly decreased. New aquifer layers started to contribute to thermal wells, which in some cases leads to changes in the hydrogeochemical composition. Mixtures of different chemical types of thermal water are the results of these changes. To characterize the changing process, study of the hydrogeochemical composition of thermal waters was performed, and end-members of the mixing processes were identified. Finally, hydrogeochemical models were developed applying PHREEQC to simulate the processes and follow the ongoing changes.

Hydrogeochemical analysis and evaluation of groundwater in the reclaimed small basin of Abu Mina, Egypt

Agricultural reclamation activities during the last few decades in the Western Nile Delta have led to great changes in the groundwater levels and quality. In Egypt, changing the desert land into agricultural land has been done using transferred Nile water (through irrigation canal systems) or/and groundwater. This research investigates the hydrogeochemical changes accompanying the reclamation processes in the small basin of Abu Mina, which is part of the Western Nile Delta region. In summer 2008, 23 groundwater samples were collected and groundwater levels were measured in 40 observation wells. Comparing the groundwater data of the pre-reclamation (1974) and the post-reclamation (2008) periods, groundwater seems to have been subjected to many changes: rise in water level, modification of the flow system, improvement of water quality, and addition of new salts through dissolution processes. Generally, Abu Mina basin is subdivided into two areas, recharge and discharge. The dissolution and mixing were recognized in the recharge areas, while the groundwater of the discharge region carries the signature of the diluted pre-reclamation groundwater. The salts of soil and aquifer deposits play an important role in the salt content of the post and pre-reclamation groundwater. NaCl was the predominant water type in the pre-reclamation groundwater, while CaSO4, NaCl and MgSO4 are the common chemical facies in the post-reclamation groundwater. The post-reclamation groundwater mostly indicates mixing between the pre-reclamation groundwater and the infiltrated freshwater with addition of some ions due to interaction with soil and sediments.

Hydrogeochemical effects of a bulkhead in the Dinero mine tunnel, Sugar Loaf mining district, near Leadville, Colorado

The Dinero mine drainage tunnel is an abandoned, draining mine adit near Leadville, Colorado, that has an adverse effect on downstream water quality and aquatic life. In 2009, a bulkhead was constructed (creating a mine pool and increasing water-table elevations behind the tunnel) to limit drainage from the tunnel and improve downstream water quality. The goal of this study was to document changes to hydrology and water quality resulting from bulkhead emplacement, and to understand post-bulkhead changes in source water and geochemical processes that control mine-tunnel discharge and water quality. Comparison of pre-and post-bulkhead hydrology and water quality indicated that tunnel discharge and zinc and manganese loads decreased by up to 97 percent at the portal of Dinero tunnel and at two downstream sites (LF-537 and LF-580). However, some water-quality problems persisted at LF-537 and LF-580 during high-flow events and years, indicating the effects of the remaining mine waste in the area. In contrast, post-bulkhead water quality degraded at three upstream stream sites and a draining mine tunnel (Nelson tunnel). Water-quality degradation in the streams likely occurred from increased contributions of mine-pool groundwater to the streams. In contrast, water-quality degradation in the Nelson tunnel was likely from flow of mine-pool water along a vein that connects the Nelson tunnel to mine workings behind the Dinero tunnel bulkhead. Principal components analysis, mixing analysis, and inverse geochemical modeling using PHREEQC indicated that mixing and geochemical reactions (carbonate dissolution during acid weathering, precipitation of goethite and birnessite, and sorption of zinc) between three end-member water types generally explain the pre-and post-bulkhead water composition at the Dinero and Nelson tunnels. The three end members were (1) a relatively dilute groundwater having low sulfate and trace element concentrations; (2) mine pool water, and (3) water that flowed from a structure in front of the bulkhead after bulkhead emplacement. Both (2) and (3) had high sulfate and trace element concentrations. These results indicate how analysis of monitoring information can be used to understand hydrogeochemical changes resulting from bulkhead emplacement. This understanding, in turn, can help inform future decisions on the disposition of the remaining mine waste and water-quality problems in the area.

Hydrologic dynamics and geochemical responses within a floodplain aquifer and hyporheic zone during Hurricane Sandy

AbstractStorms dominate solute export budgets from catchments and drive hydrogeochemical changes in the near‐stream environment. We captured near‐stream hydrogeochemical dynamics during an intense storm (Hurricane Sandy, October 2012), by instrumenting a riparian‐hyporheic zone transect of White Clay Creek in the Christina River Basin Critical Zone Observatory with pressure transducers, redox probes, and pore water samplers. In the floodplain aquifer, preferential vertical flow paths such as macropores facilitated rapid infiltration early in the storm. Water table rose quickly and promoted continuous groundwater discharge to the stream. Floodplain‐hillslope topography controlled poststorm aquifer drainage rates, as the broad, western floodplain aquifer drained more slowly than the narrow, eastern floodplain aquifer adjacent to a steep hillslope. These changes in groundwater flow drove heterogeneous geochemical responses in the floodplain aquifer and hyporheic zone. Vertical infiltration in the floodplain and hyporheic exchange in the streambed increased DOC and oxygen delivery to microbially active sediments, which may have enhanced respiration. Resulting geochemical perturbations persisted from days to weeks after the storm. Our observations suggest that groundwater‐borne solute delivery to streams during storms depends on unique interactions of vertical infiltration along preferential pathways, perturbations to groundwater geochemistry, and topographically controlled drainage rates.

Metal(loid) Attenuation Processes in an Extremely Acidic River: The Rio Tinto (SW Spain)

This study deals with the hydrogeochemical changes and metal(loid) attenuation processes along the extremely acidic Rio Tinto (SW Spain). The geochemistry of Tinto headwaters is determined by the variability of mining discharges due to different geological, geochemical and hydrological controls. Downstream of the mining area, a decrease in most dissolved element concentrations is recorded. However, not all elements decreased its concentration to the same extent, and even some did not decrease (e.g., Ba and Pb). A group of elements formed by Al, Cd, Co, Cr, Cu, Li, Mg, Mn, Ni and Zn behaved quasi-conservatively; mainly affected by dilution, except at the lower part of the catchment where seem to be affected by sorption/coprecipitation (e.g., Cd, Cu, and Zn) or mineral dissolution processes (e.g., Al, Mg). Iron and As exhibited a non-conservative behaviour due to ochre precipitation and sorption processes, respectively. A group of elements formed by Ca, Na, Sr and Li did not behave conservatively; waters were enriched in these elements by dissolutive reactions of carbonates and aluminosilicates from bedrocks. The behaviour of Pb in the Rio Tinto is complex; values fluctuate along the river course and its solubility may be related to the nature of Fe precipitates.

Dynamics in leachate chemistry of Cu-Au tailings in response to biochar and woodchip amendments: a column leaching study

Phytostabilization has been advocated as a promising approach to reduce mine tailings' adverse effects to surrounding environment. With many years of efforts in both laboratory and field trials, organic amendments are found to be essential in tailing revegetation. Yet, the associate geochemical dynamics caused by different amendments has rarely been examined. As reactive minerals are usually rich in tailings, geochemical changes induced by amendments would influence seepage management and revegetation strategies. The present study aimed to investigate geochemical dynamics in Cu-Au tailing leachate, in response to amendments with biochar produced from hardwood timber at high charring temperature or woodchips of mixed native tree species in a column leaching experiment under laboratory conditions. Results showed that the Cu-Au tailings tested in this study were relatively stable after natural weathering, with little resilience of peak salinity, stable pH and low levels of metals in leachate against the six cycles of leaching over 20 weeks. In comparison with the control without any amendments, biochar treatment did not cause any substantial changes in most examined properties of leachate, except for the reduction in dissolved organic C and NO2-. In contrast, woodchip treatment had a reduced leachate pH and a strong resilience in leachate salinity (and thus major saline ions). The geochemical changes in the woodchip treatment may be related to the active decomposition of woodchips, as indicated by the sharp increases in microbial biomass and activity as well as labile organic C at the end of leaching. The present results suggest that dynamic hydrogeochemical changes may be induced by amendment of fresh biomass like woodchips, which can increase the load of salts and metals in tailing pore water. This may affect seepage water quality at least in the short-term and thus plant survival if tailings are immediately revegetated after amendment. In contrast, biochar with highly stable carbon may help alleviate geochemical environment in the tailings.

An Evaluation of Water Quality in Private Drinking Water Wells Near Natural Gas Extraction Sites in the Barnett Shale Formation

Natural gas has become a leading source of alternative energy with the advent of techniques to economically extract gas reserves from deep shale formations. Here, we present an assessment of private well water quality in aquifers overlying the Barnett Shale formation of North Texas. We evaluated samples from 100 private drinking water wells using analytical chemistry techniques. Analyses revealed that arsenic, selenium, strontium and total dissolved solids (TDS) exceeded the Environmental Protection Agency's Drinking Water Maximum Contaminant Limit (MCL) in some samples from private water wells located within 3 km of active natural gas wells. Lower levels of arsenic, selenium, strontium, and barium were detected at reference sites outside the Barnett Shale region as well as sites within the Barnett Shale region located more than 3 km from active natural gas wells. Methanol and ethanol were also detected in 29% of samples. Samples exceeding MCL levels were randomly distributed within areas of active natural gas extraction, and the spatial patterns in our data suggest that elevated constituent levels could be due to a variety of factors including mobilization of natural constituents, hydrogeochemical changes from lowering of the water table, or industrial accidents such as faulty gas well casings.

Integrated Assessment of Groundwater Influenced by a Confluence River System: Concurrence with Remote Sensing and Geochemical Modelling

There is growing evidence of escalating pollution threats to groundwater owing to change in mineral phase chemistry arose due to high anthropogenic activities. This manuscript primarily deals with the results of physicochemical parameters in relation to the hydro-geochemistry of groundwater influenced by two confluence river systems i.e. Ganga and Yamuna at Allahabad, India. The groundwater samplings was performed in two seasons namely pre-monsoon and post-monsoon respectively to explore the link of seasonality and impact of land use change on mineral phase and quality of groundwater in the region. Computation of saturation index for both the seasons was performed independently by WATEQ4F geochemical model. The analysis indicates that Halite and Brucite solutions showed a very high degree of under-saturation in both seasons indicated that they were largely affected by the dilution. The chemical categorizations of groundwater samples represented through piper diagram suggested a common composition and origin. It has been found that sodium, magnesium and calcium were the most dominant cations present in the groundwater of the study area. Most of the stations, the water facies during the pre-monsoon season was showing a Mg-HCO3 type (70 %) water while in post-monsoon season its dominant nature was Na-HCO3 type (50 %). The investigation of salinity hazard, residual carbonate and magnesium hazard indicates that groundwater during the both pre and post-monsoon for some of the stations were even not fit for irrigation purpose. At some stations high fluoride content were also reported having elevated concentrations of fluoride in the water samples as compared to WHO/BIS standards. The analysis of land use estimated from Landsat TM imagery and seriation analysis based on iterative performances of permutation matrix suggests that the main driving factors for hydro-geochemical changes are governed by the human induced factors followed by the natural processes.

Variation in the uranium isotopic ratios234U/238U,238U/total-U and234U/total-U in Indian soil samples: Application to environmental monitoring

The uranium isotopes 238 U, 235 U and 234 U are found naturally in the environment. 238 U and 235 U are parent nuclides of two independent decay series of isotopes, while 234 U is a member of the 238 U decay chain. When decay series occur in a closed system the series tends to reach, with time, the state of secular equilibrium in which the activities of all series members are equal to the activity of its first nuclide. The activity ratio 234 U/ 238 U in natural uranium may vary as a consequence of decay chain disequilibrium due to alpha recoil and biogeochemical processes. A study based on measurement of uranium concentration and 234 U/ 238 U activity ratios in soil samples collected from Nalgonda district, Andhra Pradesh, India, a proposed mining site, was carried out to find the spatial distribution of uranium and the state of secular equilibrium of 234 U/ 238 U to examine the possibility of applying uranium concentration and uranium isotopic activity ratios to detect any hydrogeochemical changes in the environment during/post-operation. Soil samples were collected and analyzed for uranium concentration using the conventional UV fluorimetric method, showing a uranium concentration in the range of 0.7 ± 0.2 ppm to 7.9 ± 0.4 ppm with an average of 3.4 ppm, and 234 U/ 238 U activity ratios were estimated using the alpha spectrometry technique, showing an activity ratio in the range of 0.92 ± 0.11 to 1.02 ± 0.11. The 234 U/ 238 U activity ratio obtained indicated that these two uranium isotopes are in the state of secular radioactive equilibrium. The percent activity ratio of 238 U/total U and 234 U /total U is observed to vary from 47.94 ± 4.83 to 50.76 ± 4.87 and 45.80 ± 3.83 to 49.14 ± 3.99, respectively.

Development of thermodynamically-based models for simulation of hydrogeochemical processes coupled to channel flow processes in abandoned underground mines

Abstract Accurate modeling of changing geochemistry in mine water can be an important tool in post-mining site management. The Pollutant Sources and Sinks in Underground Mines (POSSUM) model and Pollutant Loadings Above Average Pyrite Influenced Geochemistry POSSUM (PLAYING POSSUM) model were developed using object-oriented programming techniques to simulate changing geochemistry in abandoned underground mines over time. The conceptual model was created to avoid significant simplifying assumptions that decrease the accuracy and defensibility of model solutions. POSSUM and PLAYING POSSUM solve for changes in flow rate and depth of flow using a finite difference hydrodynamics model then, subsequently, solve for geochemical changes at distinct points along the flow path. Geochemical changes are modeled based on a suite of 28 kinetically controlled mineral weathering reactions. Additional geochemical transformations due to reversible sorption, dissolution and precipitation of acid generating salts and mineral precipitation are also simulated using simplified expressions. Contaminant transport is simulated using a novel application of the Random-Walk method. By simulating hydrogeochemical changes with a physically and thermodynamically controlled model, the ‘state of the art’ in post-mining management can be advanced.

Flow Changes and Geochemical Anomalies in Warm and Cold Springs Associated with the 1992–1994 Seismic Sequence at Pollina, Central Sicily, Italy

During a three-year discontinuous geochemical monitoring of some warm springs and cold discharges located in central-northern Sicily, some hydro-geochemical changes were observed. Excluding a possible related to a moderate seismic activity were accidentally identified. The observed anomalies showed amplitudes that were modulated by the different geometries and volumes of the feeding aquifers. A poroelastic aquifer contraction, a shaking-induced dilatancy theory as well as seismogenetic-induced changes in the properties of the aquifers have been proposed as possible mechanisms for the water flow and hydro-geochemical changes. These preliminary results could be used to design a monitoring network aimed at surveilling the seismic activity of the studied area from a geochemical standpoint.

The timescale and mechanisms of fault sealing and water‐rock interaction after an earthquake

AbstractHydrogeochemical monitoring of a basalt‐hosted aquifer, which contains Ice Age meteoric water and is situated at 1220 m below sea level in the Tjörnes Fracture Zone, northern Iceland, has been ongoing since July 2002. Based on hydrogeochemical changes following an earthquake of magnitude (Mw) 5.8 on 16 September 2002, we constrained the timescales of post‐seismic fault sealing and water–rock interaction. We interpret that the earthquake ruptured a hydrological barrier, permitting a rapid influx of chemically and isotopically distinct Ice Age meteoric water from a second aquifer. During the two subsequent years, we monitored a chemical and isotopic recovery towards pre‐earthquake aquifer compositions, which we interpret to have been mainly facilitated by fault‐sealing processes. This recovery was interrupted in November 2004 by a second rupturing event, which was probably induced by two minor earthquakes and which reopened the pathway to the second aquifer. We conclude that the timescale of fault sealing was approximately 2 years and that the approach to isotopic equilibrium (from global meteoric water line) was approximately 18% after >104 years.

Evaluation of groundwater contamination beneath an urban environment: The Besòs river basin (Barcelona, Spain)

The urban groundwater of the central Besòs river basin (the La Llagosta aquifer) has become contaminated due to the infiltration of wastewater from septic tanks and sewage networks, and by industrial activities located in urban areas. The groundwater hydrogeochemistry of the La Llagosta aquifer was characterized using isoconcentration maps, hydrogeochemical diagrams (Piper, Schoeller–Berkaloff) and by analyzing hydrogeochemical changes along a flow-path that crosses an urban and peripheral industrial area in the main alluvial aquifer (the La Llagosta unit). The evolution of cations, anions and heavy metals along the flow path and the use of the PHREEQC numerical code indicate a complex set of geochemical processes, which result from the interaction between the sources of pollution, the groundwater flow and the mineral composition of the aquifer materials. The contaminated groundwater below the urban areas shows high contents of NO 3 − (90–100 mg/L) and an increase in the concentrations of Ca 2+ and Mg 2+ which coincides with a decrease in pH. The Eh shows greater variation than the pH along the flow line studied, with values ranging between 56 mV in the industrial area and 370 mV in the urban area. The area with the lowest Eh value coincides with the highest concentrations of dissolved Fe (4.7 mg/L) and Mn (0.22 mg/L).

The impact of variably saturated conditions on hydrogeochemical changes during artificial recharge of groundwater

Artificial recharge of groundwater is often used to either purify partially treated wastewater or to enhance the quality of surface water by percolation through a variably saturated zone. In many cases, the most substantial purification process within the infiltration water is the redox-dependent biodegradation of organic substances. The present study was aimed at understanding the spatial and temporal distribution of the redox reactions that develop below an artificial recharge pond near Lake Tegel, Germany. At this site, like at many artificial recharge sites, the hydraulic regime immediately below the pond is characterised by cyclic changes between saturated and unsaturated conditions. These changes, which occur during each operational cycle, result from the repeated formation of a clogging layer at the pond bottom. Regular hydrogeochemical analyses of groundwater and seepage water in combination with continuous hydraulic measurements indicate that NO 3 - - and Mn-reducing conditions dominate beneath the pond as long as water-saturated conditions prevail. Manganese-, Fe- and SO 4 2 - -reducing conditions are confined to a narrow zone directly below the clogging layer and in zones of lower hydraulic conductivity. The formation of the clogging layer leads to a steady decrease of the infiltration rate, which ultimatively causes a shift to unsaturated conditions below the clogging layer. Atmospheric O 2 then starts to penetrate from the pond fringes into this region, leading to: (i) the re-oxidation of the previously formed sulphide minerals and (ii) the enhanced mineralisation of sedimentary particulate organic C. The mineralisation of sedimentary particulate organic C leads to an increased H 2CO 3 production and subsequent dissolution of calcite.