Gerhard Strauch

An active subcontinental mantle volatile system in the western Eger rift, Central Europe: gas flux, isotopic (He, C, and N) and compositional fingerprints

The composition and flux of gas emanations, and the isotopic ratios of CO2, He and N2 of 74 mineral springs and dry gas vents (mofettes) in the western Eger rift (Czech Republic) have been analyzed. Four geochemically similar, but tectonically separate, gas escape centers are distinguishable, out of which 3 show a free gas flux >85000 dm3 h−1. All gases from the centers are CO2-rich (>99 vol.%) and have δ13C values ranging from −1.8 to −4.0‰. 3He/4He ratios are as high as R/Ra = 5, and are among the highest measured in Europe. The discharge of the gas mixture decreases with distance from the emanation centers with both decreasing fractions of CO2 and δ13C values, whereas the fractions of N2 and trace gases increase. These changes in chemical and isotopic composition are associated by a decrease in R/Ra ratios from about 5 in the centers to <2 in the peripheries. The changes of the contents and isotopic composition of CO2 can be explained by physico-chemical fractionations of CO2 between gaseous and aqueous phases. Towards the periphery, the contents of free CO2 and its δ13C are reduced by dissolution of CO2 in groundwater, whereby the content of N2 increases. 3He/4He ratios give evidence for mixing of He from both a deep-seated magmatic and a crustal source. The gas emanation centers, with their strongly magmatic δ13C value of about −2.7‰, seem to outline the intersections of the Eger rift and the Marianske Lazně fault, which are considered to represent a deep-reaching fracture system that enables the ascent of gases from a magmatic body in the European subcontinental mantle (SCM). Therefore, the European SCM is suspected to be the main source of CO2. The most mantle-like He (and probably N2) occurs in the centers of gas release. The total regional gas flux in the western Eger rift is determined to be 3.6 × 108 mol a−1. When related to the investigated area of 1500 km2, flux densities greater than 0.24 × 106, 52, and 0.65 mol km−2 a−1 for CO2, N2 and He respectively are calculated.

Micropollutant Loads in the Urban Water Cycle

The assessment of micropollutants in the urban aquatic environment is a challenging task since both the water balance and the contaminant concentrations are characterized by a pronounced variability in time and space. In this study the water balance of a central European urban drainage catchment is quantified for a period of one year. On the basis of a concentration monitoring of several micropollutants, a contaminant mass balance for the study areas wastewater, surface water, and groundwater is derived. The release of micropollutants from the catchment was mainly driven by the discharge of the wastewater treatment plant. However, combined sewer overflows (CSO) released significant loads of caffeine, bisphenol A, and technical 4-nonylphenol. Since an estimated fraction of 9.9-13.0% of the wastewaters dry weather flow was lost as sewer leakages to the groundwater, considerable loads of bisphenol A and technical 4-nonylphenol were also released by the groundwater pathway. The different temporal dynamics of release loads by CSO as an intermittent source and groundwater as well as treated wastewater as continuous pathways may induce acute as well as chronic effects on the receiving aquatic ecosystem. This study points out the importance of the pollution pathway CSO and groundwater for the contamination assessments of urban water resources.

Earthquake swarms in non‐volcanic regions: What fluids have to say

The detailed processes generating earthquake swarms are complex and not fully understood. Most earthquake swarms occur in volcanic regions and mid-ocean rifts. Here, we report new 3 He/ 4 He data of free gases monitored at CO 2 -rich degassing locations close to the Nový Kostel focal zone (NKFZ) located in the western Eger rift. The NKFZ is known for the recurrence of earthquake swarms at which the focal zone ranges between 6 and 12 km depth. At degassing locations neighboring to the NKFZ a progressive increase of mantle-derived helium has been observed during the last 15 years - actually the highest 3 He/ 4 He ratios (>6 Ra) in Central Europe. The 3 He/ 4 He anomalies indicate hidden magmatic activity. We assume that the latest strong earthquake swarm in October 2008 was initiated by a hidden magma intrusion process from the upper mantle into the lower crust that has been indicated by a three month lasting increase of the 3 He/ 4 He ratios in spring 2006 at all degassing locations near the NKFZ.

In-situ biodegradation of tetrachloroethene and trichloroethene in contaminated aquifers monitored by stable isotope fractionation.

Stable carbon isotope analysis of tetrachloroethene (PCE) and trichloroethene (TCE) was applied to evaluate natural attenuation processes in the upper Quaternary and lower Tertiary aquifer in the area of a former dry-cleaning plant located in Leipzig, Germany. Groundwater samples were taken during one monitoring campaign in 2001. The 13C enrichment in contaminants along the water flow path suggested that both, PCE and TCE were degraded in the Quaternary aquifer. The enrichment of 13C in the residual PCE fraction and an isotope fractionation factor from laboratory experiments were used to calculate the extent of biodegradation in the Quaternary aquifer. These calculations indicated that a major portion of PCE was biodegraded in the course of the plume. In the Tertiary aquifer the carbon isotope ratios of PCE and TCE indicated that the decreasing concentrations of these contaminants were probably not caused by microbial processes.

Natural laboratory NW Bohemia: Comprehensive fluid studies between 1992 and 2005 used to trace geodynamic processes

Comprehensive studies of CO2-rich fluids close to the swarm earthquake region Nový Kostel at the Czech-German border have been started 15 years ago and have in particular included two extended chemical and isotope monitoring studies lasting for several years each. The regional surface distribution patterns of the fluid signatures including the identification of the origin of fluid components are the focus of the detailed studies. Three degassing centers (Cheb basin, Marianske Lazně, and Karlovy Vary) with high CO2 flux and the same level of δ13C values, but different levels of 3He/4He ratios, have been identified. The studies have located the CO2 source and have investigated seismically induced changes in fluid characteristics on the basis of unique weekly sampling campaigns at selected locations. A seismically triggered release of crustal helium was confirmed by both monitoring campaigns. Finally, indications for a presently active magmatic process beneath the Cheb basin have been found. In contrast to volcanically active regions, magma accumulation in the study area takes place at the crust-mantle boundary and is not yet accompanied by heat transfer to the surface. Likewise, reactive magma-derived components are absent in the degassing fluids. The area of investigation has the potential to be a natural laboratory for fundamental studies of active geodynamic processes. The results of our fluid monitoring, including the stunning observation of mantle-derived free fluids marked by 3He/4He ratios within the subcontinental mantle range, are supported by geophysical findings from seismic studies and geologic indications.

Contributions of the different water sources to the Elqui river runoff (northern Chile) evaluated by H/O isotopes

We present the results of an isotope (2H and18O) and hydrogeochemical study in order to constrain the origin, recharge, and evolution of the surface and groundwater in the arid Andean realm of the Elqui watershed. The results of2H and18O analyses of water samples obtained during our summer and winter campaigns indicate a generally meteoric origin of the river and spring waters of the watershed. The isotope signature of water of the Elqui river and its tributaries as well as that of groundwater in the coastal region fits the2H‒18O relation of δ2H=7.61δ18O+6.1. A relatively fast discharge and a quasi-closed catchment area can be asserted for water along the river flow path. The tributaries from the more arid coastal area, north of the Elqui river, differ in their isotopic signature due to evaporation and hydrochemically due to interactions with the strongly altered and fractured volcanic rocks of the basement. In the Andean zone, the18O-enriched hydrothermal spring of Baños del Toro exhibits the influence of water–rock interaction processes. The chemistry of the river water changes from sulphate- to chloride-rich along the river course from the high Andean mountains to the coast. The sulphate-rich character of these Andean waters reflects their passage through sulphide-rich rock massifs that were subjected to strong oxidation processes in the near superficial environment. This sulphate signature is enforced by past and present mining of precious metal epithermal deposits (e.g. those of El Indio-Tambo Au–Cu–As district), in which mineralised zones were developed during a series of Miocene magmatic-hydrothermal episodes in the Andean realm. Owing to the proximity of the lower Elqui river waters and its tributaries to the Pacific coast, the chloride character may be induced by agricultural and marine (sea spray, fog) sources. Generally, the main source of the Elqui river water is mainly attributed to surface runoff and less to contributions from the basement fractured aquifer.

Investigation of sewer exfiltration using integral pumping tests and wastewater indicators.

Leaky sewers affect urban groundwater by the exfiltration of untreated wastewater. However, the impact of sewer exfiltration on the groundwater is poorly understood. Most studies on sewer exfiltration focus on water exfiltration, but not on the impact on groundwater quality. In this paper we present a new monitoring approach to estimate mass flow rates M(ex) of different wastewater indicators (WWIs) from leaky sewers by applying integral pumping tests (IPTs). The problem of detecting and assessing heterogeneous concentrations in the vicinity of leaky sewers can be overcome with the IPT approach by the investigation of large groundwater volumes up- and downstream of leaky sewers. The increase in concentrations downstream of a leaky sewer section can be used to calculate M(ex) with a numerical groundwater model. The new monitoring approach was first applied using four IPT wells in Leipzig (Germany). Over a pumping period of five days we sampled five inorganic WWIs: B , Cl(-), K+, NO3(-), NH4+ and three xenobiotics: bisphenol-a, caffeine and tonalide. The resulting concentration-time series indicated an influence of wastewater at one IPT well downstream of the leaky sewer. We defined ranges of M(ex) by implementing the uncertainty of chemical analyses. The results showed a M(ex) of 0-10.9 g m(-1) d(-1). The combination of M(ex) with wastewater concentrations from the target sewer yielded an exfiltration rate Q(ex) of 28.0-63.9 Lm(-1)d(-1) for the conservative ion Cl(-). Most non-conservative WWIs showed reduced mass flow rates in the groundwater downstream of the leaky sewer that indicate a mass depletion during their passage from the sewer to the pumping well. Application of the IPT methodology at other field sites is possible. The IPT monitoring approach provides reliable M(ex) values that can help to assess the impact of leaky sewers on groundwater.

Seismically triggered anomalies in the isotope signatures of mantle‐derived gases detected at degassing sites along two neighboring faults in NW Bohemia, central Europe

The Vogtland and NW Bohemia region is known for its earthquake swarms; the most intensive swarm since 1985/86 occurred in October 2008. To find further indications for the interaction of ascending mantle-derived fluids and the occurrence of earthquake swarms, detailed fortnightly studies of gas compositions (CO2, N2, Ar, He, H2, and CH4) and isotope ratios (δ13C, δ15N, and 3He/4He) were carried out between October 2008 and April 2011 at four locations close to the Nový Kostel focal zone and at the Wettinquelle spring (Bad Brambach). From the start of the 2008 earthquake swarm seismically induced isotope-geochemical anomalies were recorded at locations along the Pocatky-Plesna fault zone (PPZ) and were, for the first time, also found at degassing locations on the Marianske Laznĕ fault zone (MLF). Variations were observed in both the temporal and spatial distributions of the anomalies as well in anomaly strengths, probably due to the positions of these fault zones relative to the focal zone, and to differences in fluid migration pathways. Prior to both the 2000 and 2008 swarms, 3He/4He ratios > 6 Ra were recorded at the Bublak mofette. These anomalous pre-seismic 3He/4He ratios suggest that both the 2000 and 2008 swarms may have been associated with the supply of fresh magma from a less degassed reservoir in the lithospheric mantle. The temporal δ13CCO2 pattern from detailed studies at Bublak between 2005 and 2011 indicates progressive magma degassing, as well as seismically induced variations in the δ13C, providing additional support to the interpretation derived from the 3He/4He ratios.

Assessment of the Flow Dynamic of a Mining Lake by Stable Isotope Investigations

For water samples from mining lake ML 111 of the Lusatian Lignite District and surrounding aquifers δ34S(SO4-b), δ15N(NH4+), δD, and δ18O (H2O) values have been determined to asess the flow dynamic of the in- and outflow scenario. The high mineralization and acidification (pH > 2.6) of the lake water results from an intensive interaction with the surrounding dump sites and the inflow of extremely sulfate rich dump water (up to 4,100mg/1 SO4-). δ34S values range from -7‰ to + 7°CDT in the inflow aquifers. The lake water is considered to be a mixture of different water bodies. The evaporation signature in the outflow aquifer suggests that the groundwater consists of 60% lake water. The nitrogen transfer is explained by 15N signature of ammonium. Sulfate reduction, perceptible in outflow and dump aquifer, cannot be proved within the mining lake by 34S and hydrochemistry.

Evaluation of groundwater dynamics and quality in the Najd aquifers located in the Sultanate of Oman

The Najd, Oman, is located in one of the most arid environments in the world. The groundwater in this region is occurring in four different aquifers A to D of the Hadhramaut Group consisting mainly of different types of limestone and dolomite. The quality of the groundwater is dominated by the major ions sodium, calcium, magnesium, sulphate, and chloride, but the hydrochemical character is varying among the four aquifers. Mineralization within the separate aquifers increases along the groundwater flow direction from south to north-northeast up to high saline sodium-chloride water in aquifer D in the northeast area of the Najd. Environmental isotope analyses of hydrogen and oxygen were conducted to monitor the groundwater dynamics and to evaluate the recharge conditions of groundwater into the Najd aquifers. Results suggest an earlier recharge into these aquifers as well as ongoing recharge takes place in the region down to present day. Mixing of modern and submodern waters was detected by water isotopes in aquifer D in the mountain chain (Jabal) area and along the northern side of the mountain range. In addition, δ2H and δ18O variations suggest that aquifers A, B, and C are assumed to be connected by faults and fractures, and interaction between the aquifers may occur. Low tritium concentrations support the mixing assumption in the recharge area. The knowledge about the groundwater development is an important factor for the sustainable use of water resources in the Dhofar region.