Recharge estimates for various land uses in the Guarani Aquifer System outcrop area

ABSTRACT

The Guarani Aquifer System (GAS) is a subsurface reservoir that contains the largest volume of fresh groundwater in South America. Despite the relevance of the GAS, a lack of attention has been paid to land use effects on its recharge. We present the most detailed long-term (2004–2011) results of land-use effects on recharge in an outcrop area of the GAS. Water table fluctuations (WTFs) were measured at 11 monitoring wells, which are distributed between different land uses (i.e. eucalyptus, sugarcane, citrus and grassland). Recharge was estimated using a point-scale method (WTF) for each monitored well. The annual recharge estimates for different land uses are eucalyptus forest (135 mm year^-1), sugarcane (248 mm year^-1), citrus areas (296 mm year^-1) and grassland (401 mm year^-1). The results indicate that the evapotranspiration seems to be a key parameter in the assessment of recharge in the study area.     

Estimating groundwater recharge using the chloride mass-balance method in the West Bank,Palestine

Abstract

The quantification of natural recharge rate is a prerequisite for efficient and sustainable groundwater resources management. Since groundwater is the only source of water supply in the West Bank, it is of utmost importance to estimate the rate of replenishment of the aquifers. The chloride mass-balance method was used to estimate recharge rates at different sites representing the three groundwater basins of the Mountain Aquifer in the West Bank. The recharge rate for the Eastern Basin was calculated as between 130.8 and 269.7 mm/year, with a total average replenishment volume of 290.3 × 10⁶ m³/year. For the Northeastern Basin, the calculated recharge rate ranged between 95.2 and 269.7 mm/year, with a total average recharge volume of 138.5 × 10⁶ m³/year. Finally, the recharge rate for the Western Basin was between 122.6 and 323.6 mm/year, with a total average recharge volume of 324.9 × 10⁶ m³/year. The data reveal a replenishment potential within the estimated replenishment volumes of previous studies for the same area. Also, the range was between 15 and 50% of total rainfall, which is still within the range of previous studies. The geological structure and the climate conditions of the western slope were clearly play an important role in the increment of total volume. In some cases, such as the geological formations in the Northeastern Basin, the interaction between Eocene and Senonian chalk formations result in minimum recharge rates.

Citation Marei, A., Khayat, S., Weise, S., Ghannam, S., Sbaih, M. & Geyer, S. (2010) Estimating groundwater recharge using the chloride mass-balance method in the West Bank, Palestine. Hydrol. Sci. J. 55(5), 780–791.     

Semiannual velocity variations around the 2008 M w 7.9 Wenchuan Earthquake fault zone revealed by ambient noise and ACROSS active source data

We continuously monitor the long-term seismic velocity variation of one of the major ruptured faults of the devastating 2008 M _w 7.9 Wenchuan earthquake in China from July 2009 to January 2012, jointly using accurately controlled routinely operated signal system active source and seismic noise-based monitoring technique. Our measurements show that the temporal velocity change is not homogeneous and highly localized in the damaged fault zone and the adjacent areas. Velocity variations from the active and passive methods are quite consistent, which both are characterized by ±0.2 % seasonal variation, with peak and trough at winter and summer, respectively. The periodic velocity variation within fault zone exhibits remarkably positive correlation with barometric pressure with stress sensitivity in the order of 10^?6 Pa^?1, suggesting that the plausible mechanism might be the crack density variation of the shallow subsurface medium of the damaged fault zone in response to the cyclic barometric pressure loading.     

Groundwater potential of the Egbe-Mopa basement area,central Nigeria

Abstract

An investigation on the groundwater potentials of the Egbe-Mopa area in central Nigeria, underlain by the Basement Complex, is presented. The investigation involved mapping of the subsurface by use of vertical electrical soundings; measurement of depth to groundwater; and evaluation of hydraulic conductivity, transmissivity and yield by means of pumping test interpretation. The results indicate subsurface units that range from three to five resistivity layers; depth to groundwater of 0–10 m; overburden thickness of 3–16 m; hydraulic conductivity of 6.2?×?10^?6 to 3.4?×?10^?4 m/s; transmissivity of 4.3?×?10^?7 to 2?×?10^?3 m²/s; and groundwater yield of 0.2–2.5 L/s. The hydraulic head assessments revealed a general northward groundwater flow direction. The study identified three aquifer potential types, of high, medium and low productivity, respectively. Based on the longitudinal conductance of the overburden units, four distinct Aquifer Protective Capacity zones were delineated, namely, poor, weak, moderate and good.

Citation Okogbue, C.O. and Omonona, O.V., 2013. Groundwater potential of Egbe-Mopa basement area, central Nigeria. Hydrological Sciences Journal, 58 (4), 826–840.     

Early detection of brine and CO2 leakage through abandoned wells using pressure and surface-deformation monitoring data: Concept and demonstration

In this paper, we develop a methodology for early detection of potential CO₂ leakage from geological storage formations using pressure and surface-deformation anomalies. The basic idea is based on the fact that leakage-induced pressure signals travel much faster than the migrating CO₂; thus such anomalies may be detected early enough for risk management measures taking effect in avoiding substantial CO₂ leaks. The early detection methodology involves automatic inversion of anomalous brine leakage signals with efficient forward pressure and surface-deformation modeling tools to estimate the location and permeability of leaky features in the caprock. We conduct a global sensitivity analysis to better understand under which conditions pressure anomalies can be clearly identified as leakage signals, and evaluate signal detectability for a broad parameter range considering different detection limits and levels of data noise. The inverse methodology is then applied to two synthetic examples of idealized two-aquifer-and-one aquitard storage systems, with an injection well and a leaky well, for different monitoring scenarios. In Example 1, only pressure data at the monitoring and injection wells are used for leakage detection. Our results show that the accuracy of leakage detection greatly depends on the level of pressure data noise. In Example 2, joint inversion of pressure and surface-deformation measurements significantly improves the speed of convergence toward the true solution of the leakage parameters and enables early leakage detection. In both examples, successful detection is achieved when two monitoring wells are appropriately placed within up to 4 km from the leaky well.     

Use of Geophysical and Remote Sensing Data for Assessment of Aquifer Depletion and Related Land Deformation

An integrated approach [field, Interferometric Synthetic Aperture Radar (InSAR), hydrogeology, geodesy, and spatial analysis] was adopted to identify the nature, intensity, and spatial distribution of deformational features (sinkholes, fissures, differential settling) reported over fossil aquifers in arid lands, their controlling factors, and possible remedies. The Lower Mega Aquifer System (area 2 × 10⁶ km²) in central and northern Arabia was used as a test site. Findings suggest that excessive groundwater extraction from the fossil aquifer is the main cause of deformation: (1) deformational features correlated spatially and/or temporally with increased agricultural development and groundwater extraction, and with a decline in water levels and groundwater storage (? 3.7 ± 0.6 km³/year); (2) earthquake events (years 1985–2016; magnitude 1–5) are largely (65% of reported earthquakes) shallow (1–5 km) and increased from 1 event/year in the early 1980s (extraction 1 km³/year), up to 13 events/year in the 1990s (average annual extraction > 6.4 km³). Results indicate that faults played a role in localizing deformation given that deformational sites and InSAR-based high subsidence rates (? 4 to ? 15 mm/year) were largely found within, but not outside of, NW–SE-trending grabens bound by the Kahf fault system. Findings from the analysis of Gravity Recovery and Climate Experiment solutions indicate that sustainable extraction could be attained if groundwater extraction was reduced by 3.5–4 km³/year. This study provides replicable and cost-effective methodologies for optimum utilization of fossil aquifers and for minimizing deformation associated with their use.     

Temporal dynamics of soil water balance components in a karst range in southeastern Spain: estimation of potential recharge

Abstract

This paper analyses the temporal dynamics of soil water balance components in a representative recharge area of the Sierra de Gádor (Almeria, southeastern Spain) in two hydrological years. Two approaches are used to estimate daily potential recharge (PR): Approach 1 based on deriving PR from the water balance as the difference between measurements of rainfall (P) and actual evapotranspiration (E) obtained by eddy covariance; and Approach 2 with PR obtained from the dynamic pattern of the soil moisture (θ) recorded at two depths in the site's thin soil (average 0.35 m thickess). For the hydrological year 2003/04, which was slightly drier than the 30-year average, E accounted for 64% of rainfall and occurred mainly in late spring and early summer. The PR estimated by Approach 1 was 181 ± 18 mm year^-1 (36% of rainfall), suggesting an effective groundwater recharge in the study area. In the unusually dry hydrological year 2004/05, E was about 215 mm year^-1, close to the annual rainfall input, and allowing very little (8 ± 12 mm year^-1) PR according to Approach 1. Estimation of PR based on Approach 2 resulted in PR rates lower than those found by Approach 1, because Approach 2 does not take into account the recharge that occurs through preferential flow pathways (cracks, joints and fissures) which were not monitored with the θ probes. Moreover, using Approach 2, the PR estimates differed widely depending on the time scale considered: with daily mean θ data, PR estimation was lower, especially in late spring, while θ data at 30 min resolution yielded a more reliable prediction of the fraction of total PR resulting from the downward movement of soil water by gravity.

Citation Cantón, Y., Villagarcía, L., Moro, M. J., Serrano-Ortíz, P., Were, A., Alcalá, F. J., Kowalski, A. S., Solé-Benet, A., Lázaro, R. & Domingo, F. (2010) Temporal dynamics of soil water balance components in a karst range in southeastern Spain: estimation of potential recharge. Hydrol. Sci. J. 55(5), 737–753.     

Natural 222Rn as a tracer of mixing and volatilization in a shallow aquifer during a CO2 injection experiment

This study aims to evaluate the application of ²²²Rn in groundwater as a tracer for monitoring CO₂ plume migration in a shallow groundwater system, which is important to detect potential CO₂ leakage in the carbon capture and storage (CCS) project. For this research, an artificial CO₂-infused water injection experiment was performed in a shallow aquifer by monitoring hydrogeochemical parameters, including ²²²Rn. Radon in groundwater can be a useful tracer because of its sensitivity to sudden changes in subsurface environment. To monitor the CO₂ plume migration, the data were analysed based on (a) the influence of mixing processes on the distribution of ²²²Rn induced by the artificial injection experiment and (b) the influence of a carrier gas role by CO₂ on the variation of ²²²Rn. The spatio-temporal distributions of radon concentrations were successfully explained in association with horizontal and vertical mixing processes by the CO₂-infused water injection. Additionally, the mixing ratios of each monitoring well were calculated, quantitatively confirming the influence of these mixing processes on the distribution of radon concentrations. Moreover, one monitoring well showed a high positive relationship between ²²²Rn and Total dissolved inorganic carbon (TIC) by the carrier gas effect of CO₂ through volatilization from the CO₂ plume. It indicated the applicability of ²²²Rn as a sensitive tracer to directly monitor CO₂ leakage. When with a little effect of carrier gas, natural ²²²Rn in groundwater can be used to compute mixing ratio of CO₂-infused water indicative of CO₂ migration pathways. CO₂ carrier gas effect can possibly increase ²²²Rn concentration in groundwater and, if fully verified with more field tests, will pose a great potential to be used as a natural tracer for CO₂.     

Temporal Gravity Variations near Shrinking Vatnajökull Ice Cap, Iceland

Repeated gravity measurements were carried out from 1991 until 1999 at sites SE of Vatnajökull, Iceland, to estimate the mass flow and deformation accompanying the shrinking of the ice cap. Published GPS data show an uplift of about 13 ± 5 mm/a near the ice margin. A gravity decrease of –2 ± 1 μGal/a relative to the Höfn base station, was observed for the same sites. Control measurements at the Höfn station showed a gravity decrease of –2 ± 0.5 µGal/a relative to the station RVIK 5473 at Reykjavík (about 250 km from Höfn). This is compatible, as a Bouguer effect, with a 10 ± 3 mm/a uplift rate of the IGS point at Höfn and an uplift rate of ~20 mm/a near the ice margin. Although the derived gravity change rates at individual sites have large uncertainties, the ensemble of the rates varies systematically and significantly with distance from the ice. The relationship between gravity and elevation changes and the shrinking ice mass is modelled as response to the loading history. The GPS data can be explained by 1-D modelling (i.e., an earth model with a 15-km thick elastic lithosphere and a 7·10¹⁷ Pa·s asthenosphere viscosity), but not the gravity data. Based on 2-D modelling, the gravity data favour a low-viscosity plume in the form of a cylinder of 80 km radius and 10¹⁷ to 10¹⁸ Pa·s viscosity below a 6 km-thick elastic lid, embedded in a layered PREM-type earth, although the elevation data are less well explained by this model. Strain-porosity-hydrology effects are likely to enhance the magnitude of the gravity changes, but need verification by drilling. More accurate data may resolve the discrepancies or suggest improved models.     

Urbanization effects on shoreline phytobenthos: a multiscale approach at lake extent

To understand how littoral biota respond to anthropogenic disturbances, limnologists seek to detect the scale at which patterns and processes occur. We conducted an extensive study on the shoreline phytobenthos of Lake Garda (Italy) with the following main objectives: (i) to examine the importance of urbanization for species distribution within a set of hierarchical spatial scales (10¹–10⁴ m), and then (ii) to test the spatio-temporal interactions on a reduced set of scales (10¹–10² m, and 10¹–10² days). Results showed that most of the variation in most abundant species and habitat characteristics occurred at the spatial scale of 10¹–10² m. Species richness was positively related with microheterogeneity, but the relationship occurred only at low urbanization and not at highly-urbanized sites where artificial shores were less heterogeneous. The similarity of species assemblages was regulated by two interacting processes, one operating at a fine spatial scale (10² m), reflecting the physical-habitat requirements of the species, and the other one operating at a broader scale (10⁴ m) in relation to the N–S nitrogen gradient. Overall, time explained 73 % of the total variation of species assemblages, space 7 %, and 20 % was explained by the interaction between space and time (the patch scale, 10s of m, and area scale, 100s of m, interacted with the finest temporal scale, 10s of days). This interaction might be explained by the process of species recruitment operating at different rates at the two spatial scales. Since the largest variation in species assemblages was at the temporal scale (due to the seasonal succession of phytobenthos), we recommend collecting at least one sample per season when monitoring littoral habitats.     

Is net ecosystem production higher in natural relative to constructed wetlands?

Ecosystem metabolism is an important measure of wetland restoration efficiency, and serves to indicate if the system is capable of processing energetic resources. Despite its value, ecosystem metabolism has rarely been included in monitoring programs. In this study, we aimed to achieve the following objectives: (i) compare net ecosystem production (NEP) rates of constructed vs. natural wetlands; (ii) identify the highest NEP rate habitats; and (iii) define the main environmental factors regulating NEP in different wetland types. Pelagic and benthic NEP rates and physicochemical features were measured in three natural and five constructed wetlands in the middle Ebro River floodplain (NE Spain). Statistical analyses showed pelagic NEP rates peaked in natural wetlands, which produced up to 187.5 mg C m^?3 h^?1 compared to lower rates in constructed wetlands (up to 46.2 mg C m^?3 h^?1). Pelagic NEP responded positively to temperature, total dissolved solids, and nutrients. Benthic NEP rates were 3 to 30-fold greater than pelagic in natural (up to 994.9 mg C m^?3 h^?1) and constructed (up to 1,551.5 mg C m^?3 h^?1) wetlands, and were heavily influenced by habitat type, with NEP peaking in areas dominated by submerged vegetation and fine organic sediment. Rapid recovery in aquatic communities (i.e. macroinvertebrate diversity) has been previously reported for the studied wetlands; however, our study suggests a slower recovery of functional processes (i.e. pelagic NEP) in constructed habitats. We therefore strongly advocate the inclusion of ecosystem function in the design and evaluation of restoration projects to optimise long-term wetland ecosystem sustainability.     

Anisotropy in the Ironton and Galesville Sandstones Near a Thermal-Energy-Storage Well, St. Paul, Minnesota

The U.S. Geological Survey is studying the potential for storage of heated water in a sandstone aquifer in St. Paul, Minnesota. The efficiency of the aquifer to store thermal energy is related, in part, to the hydrogeologic characteristics of the aquifer. The movement of heat away from the injection well is directly related to the anisotropy. Aquifer tests were conducted to determine the anisotropy near the heated-water injection well. The maximum and minimum values of transmissivity along the principal directions of the hydraulic conductivity tensors of the Ironton and Galesville Sandstones in St. Paul, Minnesota are approximately 1,090 and 480 feet squared per day. The storage coefficient is 4.5 × 10⁻⁵. These values represent the average of four determinations of nonsteady flow to a well in an idealized infinite anisotropic aquifer. Analysis of the values of transmissivity and storage coefficient for hypothetical changes in location of two of the monitoring wells where depth-deviation surveys were not available indicates that computed transmissivities vary less than 5 percent and storage coefficients vary less than ±6 percent.     

Formation of an ‘a’ā lava delta: insights from time-lapse multibeam bathymetry and direct observations during the Stromboli 2007 eruption

Five consecutive multibeam bathymetries collected before, during, and after the 2007 Stromboli eruption, combined with visual inspections, allowed us to document the morphological evolution of an ‘a’ā lava-fed delta and to reconstruct the main processes acting during its submarine emplacement. The 2007 Stromboli delta extended down to 600-m water depth and covered an area of 420?×?10³ m², with a maximum thickness of 65 m and a total estimated volume of ≈7?×?10⁶ m³, i.e., three times larger than its subaerial counterpart. The lava delta grew mainly through the emplacement of discrete lobes about 50–150 m in size. Lobes were fed from point sources along the paleoshoreline, and their subaqueous pathways seem to be mainly controlled by the submarine morphology, with flows mostly filling in depressions left by previous lobes. The main controlling factors on the lobe morphology and thickness are the effusion rates and the pre-eruption morphology, i.e., the geometry and gradients of the basal surface. Data also shows that sudden slope failure of portions of the submarine delta may occur simultaneously with accretion, implying that a significant part of the delta material can be transported to greater depths by submarine gravity flows. The present study is relevant for future monitoring and hazard assessment during the growth of active lava-fed deltas as well as for a better interpretation of ancient volcaniclastic successions inland.     

Volumetric characteristics of lava flows from interferometric radar and multispectral satellite data: the 1995 Fernandina and 1998 Cerro Azul eruptions in the western Galápagos

We have used a suite of remotely sensed data, numerical lava flow modeling, and field observations to determine quantitative characteristics of the 1995 Fernandina and 1998 Cerro Azul eruptions in the western Galápagos Islands. Flank lava flow areas, volumes, instantaneous effusion rates, and average effusion rates were all determined for these two eruptions, for which only limited syn-eruptive field observations are available. Using data from SPOT, TOPSAR, ERS-1, and ERS-2, we determined that the 1995 Fernandina flow covers a subaerial area of 6.5×10⁶ m² and has a subaerial dense rock equivalent (DRE) volume of 42×10⁶ m³. Field observations, ATSR satellite data, and the FLOWGO numerical model allow us to determine that the effusion rate declined exponentially from a high of ~60–200 m³ s^-1 during the first few hours to <5 m³ s^-1 prior to ceasing after 73 days, with a mean effusion rate of 4–16 m³ s^-1. Integrating the ATSR-derived, exponentially declining effusion rate over the eruption duration produces a total (subaerial + submarine) DRE volume of between 27 and 100×10⁶ m³, the range in values being due to differing assumptions about heat loss characteristics; only values in the higher part of this range are consistent with the independently derived subaerial volume. Using SPOT, TOPSAR, ERS-1, and ERS-2 data, we determine that the 1998 Cerro Azul flow is 16 km long, covers 16 km², and has a DRE volume of 54×10⁶ m³. FLOWGO produces at-vent velocity and effusion rate values of 11 m s^-1 and ~600 m³ s^-1, respectively. The velocity value agrees well with the 12 m s^-1 estimated in the field. The mean effusion rate (total DRE volume/duration) was 7–47 m³ s^-1. Dike dimensions, fissure lengths, and pressure gradients along the conduit based on magma chamber depth estimates of 3–5 km produce mean effusion rates for the two eruptions that range over nearly four orders of magnitude, the range being due to uncertainty in the magma viscosity, dike dimensions, and pressure gradient between magma chamber and vent. Although somewhat consistent with mean effusion rates from other techniques, their wide range makes them less useful. The exponentially declining effusion rates during both eruptions are consistent with release of elastic strain being the driving mechanism of the eruptions. Our results provide independent input parameters for previously published theoretical relationships between magma chamber pressurization and eruption rates that constrain chamber volumes and increases in volume prior to eruption, as well as time constants of exponential decay during the eruption. The results and theoretical relationships combine to indicate that at both volcanoes probably 25–30% of the volumetric increase in the magma chamber erupted as lava onto the surface. In both eruptions the lava flow volumes are less than 1% of the magma chamber volume.     

Initiation and Propagation of Compaction Bands in Dry and Wet Bentheim Sandstone

We investigated initiation and propagation of compaction bands (CB) in six wet and four dry Bentheim sandstone samples deformed in axial compression tests with strain rates ranging from 3.2 × 10^?8 s^?1 to 3.2 × 10^?4 s^?1. Circumferential notches with 0.8-mm width and 5-mm depth served to initiate CB at mid-sample length. Wet samples were saturated with distilled water and deformed at 195 MPa confining pressure and 10 MPa pore pressure. Dry samples were deformed at 185 MPa confining pressure. Twelve P-wave sensors, eight S-wave sensors and two pairs of orthogonally oriented strain-gages were glued to the sample surface to monitor acoustic emission (AE), velocities and local strain during the loading process. Nucleation of compaction bands is indicated by AE clusters close to the notch tips. With progressive loading, AE activity increased and AE hypocenters indicated propagation of a single CB normal to the sample axis. CB propagation from the sample periphery towards the centre was monitored. Microstructural analysis of deformed samples shows excellent agreement between location of AE clusters and CBs. In both dry and wet samples the lateral propagation of CBs was about 100 times faster than axial shortening rates. At the slowest displacement rate, AE activity during band propagation was reduced and CB nucleation in wet samples occurred at 20% lower stresses. This may indicate an increasing contribution of stress corrosion processes to the formation of the compaction bands. In dry and wet samples inelastic compaction energy per area ranged between 16 and 80 kJ m^?2. This is in good agreement with previous estimates from laboratory and field studies.     

Forthcoming papers

Abstract

The Hai River Basin (HRB) is a heavily irrigated region encompassing the North China Plain (NCP) in northeast China. In the last decades, continuous lowering of groundwater levels had been reported in the NCP. This study used data from the Gravity Recovery and Climate Experiment (GRACE) and in situ measurements to quantify recent changes in groundwater storage from 2003 to 2012. The signal from GRACE observations highlight a sharp decline in the deep subsurface water stores (deep unsaturated zone and groundwater systems) up to a rate of 17.0 ± 4.3 mm year^-1 between 2003 and 2012 over the HRB, equal to a volumetric loss of 5.5 ± 1.4 km³ year^-1. This result shows good consistency with in situ observations of groundwater hydraulic heads compiled from monitoring bores, and emphasizes GRACE’s ability to monitor large-scale groundwater storage variations. Results from GRACE also provide an independent assessment of the effectiveness of water saving programmes that have been implemented by the government so far. Our study indicates that groundwater overdrawal is still prevalent and the dominant factor for the persistent loss in groundwater storage over the HRB/NCP; the current groundwater consumption pattern is far beyond the natural recharge ability in groundwater system.

Editor D. Koutsoyiannis; Associate editor T. Wagener     

A New Automatic Subsurface Gas Monitoring System for Seismogeochemical Studies, Installed in Haruno Borehole, Shizuoka Prefecture, Central Japan

The results of subsurface gas monitoring by application of gas chromatography (GC) to the gas composition of bubbles associated with groundwater for seismogeochemical studies are reported. An automated gas monitoring system was used to determine gas compositions in a 500-m borehole at the Haruno Crustal Movement Observation Site (HOS), central Japan during period 1, from December 1999 to December 2000. The average ± two standard deviation (2SD) compositions of gases in this period were He = 82 ± 29 ppmV, H₂ = 170 ± 62 ppmV, Ar = 0.05 ± 0.07%, N₂ = 50 ± 8%, and CH₄ = 45 ± 6%. A new automated gas monitoring system equipped with a micro-GC was installed in the borehole at the HOS, and gas bubbles from the borehole were monitored during period 2, from December 2006 to March 2007. The average ± two standard deviation (2SD) compositions of gases in this period were He = 8 ± 7 ppmV, H₂ = 13 ± 15 ppmV, Ar = 0.6 ± 0.3%, N₂ = 66 ± 7%, and CH₄ = 14 ± 14%. The gas concentration ratios (He/Ar, H₂/Ar, N₂/Ar, and CH₄/Ar) fluctuated significantly over time and repeatedly showed abrupt spike-like increases during period 2. The gas compositions obtained in period 1 and 2 were markedly different. Over the period from 2006 to 2007, the gas bubbles were depleted in He, H₂, and CH₄ of deep origin, but enriched in Ar and N₂ of atmospheric origin. This difference can be interpreted as being due to an irreversible change of the aquifer/gas system. The present deep component in the HOS gas is estimated to have composition He = 63 ppmV, H₂ = 37 ppmV, Ar = 0.17%, N₂ = 63%, and CH₄ = 37%. The new monitoring system is able to analyze the gas composition using a smaller volume of sample gas and with greater precision than the previous system. During the 3-month monitoring period 2, the separation capacity of the capillary column of the micro-GC was sufficiently maintained to determine gas-chromatographic peak areas for the five gaseous species examined. This study confirms that the new monitoring system with micro-GC is promising for continuous subsurface gas monitoring for earthquake prediction studies.     

呼图壁地下储气库地表盖层变形的GPS研究

利用新疆呼图壁地下储气库地表盖层由13个点位组成的形变监测网的前5期GPS观测资料,研究地下储气库注采过程中地表盖层的变形响应。通过获取地下储气库运行过程中地表盖层形变的三维时间序列,并结合井口压力数据,区分地下储气库在不同过程中的变形信号。研究结果表明,地表盖层在储气库注采过程中水平方向上存在明显的“呼吸效应”,储气库每MPa气井压力变化在注、采周期内对地表变形造成的影响在水平方向上分别达到1.02、1.24mm,垂直方向分别达到-1.11、0.86mm。     

Rapid entrenchment of stream profiles in the salt caves of Mount Sedom,Israel

Rock salt is approximately 1000 times more soluble than limestone and thus displays high rates of geomorphic evolution. Cave stream channel profiles and downcutting rates were studied in the Mount Sedom salt diapir, Dead Sea rift valley, Israel. Although the area is very arid (mean annual rainfall ≈ 50 mm), the diapir contains extensive karst systems of Holocene age. In the standard cave profile a vertical shaft at the upstream end diverts water from a surface channel in anhydrite or clastic cap rocks into the subsurface route in the salt. Mass balance calculations in a sample cave passage yielded downcutting rates of 0–2 mm s^?1 during peak flood conditions, or about eight orders of magnitude higher than reported rates in any limestone cave streams. However, in the arid climate of Mount Sedom floods have a low recurrence interval with the consequence that long-term mean downcutting rates are lower: an average rate of 8·8 mm a^?1 was measured for the period 1986–1991 in the same sample passage. Quite independently, long-term mean rates of 6·2mm a^?1 are deduced from ¹⁴C ages of driftwood found in upper levels of 12 cave passages. These are at least three orders of magnitude higher than rates established for limestone caves. Salt cave passages develop in two main stages: (1) an early stage characterized by high downcutting rates into the rock salt bed, and steep passage gradients; (2) a mature stage characterized by lower downcutting rates, with establishment of a subhorizontal stream bed armoured with alluvial detritus. In this mature stage downcutting rates are controlled by the uplift rate of the Mount Sedom diapir and changes of the level of the Dead Sea. Passages may also aggrade. These fast-developing salt stream channels may serve as full-scale models for slower developing systems such as limestone canyons.     

HCl emissions at Soufrière Hills Volcano,Montserrat, West Indies,during a second phase of dome building: November 1999 to October 2000

HCl:SO₂ mass ratios measured by open path Fourier transform spectroscopy (OP-FTIR) in the volcanic plume at Soufrière Hills Volcano, Montserrat, are presented for the second phase of dome building between November 1999 and November 2000. HCl:SO₂ mass ratios of greater than 1 and HCl emission rates of greater than 400 t day^–1 characterise periods of dome building for this volcano. The data suggest that chlorine partitions into a fluid phase as the magma decompresses and exsolves water during ascent. This is substantiated by a correlation between chlorine and water content in the melt (derived from the geochemical analysis of plagioclase melt inclusion and matrix glasses from phase I and II of dome growth). The matrix glass from the November 1999 and March 2000 domes indicate an open system degassing regime with a fluid-melt partition coefficient for chlorine of the order of 250–300. September 1997 glasses have higher chlorine contents and may indicate a switch to closed system degassing prior to explosive activity in September and October 1997. The OP-FTIR HCl time series suggests that HCl emission rate is strongly related to changes in eruption rate and we infer an emission rate of over 13.5 kt day^–1 HCl during a period of high extrusion rate in September 2000. A calculation of the HCl emission rate expected for varying extrusion rates from the open-system degassing model suggests a HCl emission rate of the order of 1–4 kt day^–1 is indicative of an extrusion rate of between 2 and 8 m³ s^–1. Monitoring of HCl at Soufrière Hills Volcano provide a proxy for extrusion rate, with changes in ratio between HCl and SO₂ occurring rapidly in the plume. Order of magnitude changes occur in HCl emission rates over the time-scale of hours to days, making these changes easy to detect during the day-to-day monitoring of the volcano. Mean water emission rates are calculated to range from 9–24 kt day^–1 during dome building activity, calculated from the predicted mass ratio of H₂O:HCl in the fluid at the surface and FTIR-derived HCl emission rates.