Troels Laier

Transport and time lag of chlorofluorocarbon gases in the unsaturated zone, Rabis Creek, Denmark

et al., 1995, 1996; Szabo et al., 1996), flow and groundwater quality (Johnston et al., 1998; Böhlke and Denver, Transport of chlorofluorocarbon (CFC) gases through the unsatu1995), and groundwater–surface water interactions (Katz rated zone to the water table is affected by gas diffusion, air–water et al., 1995). A critical component in such analyses is the exchange (solubility), sorption to the soil matrix, advective–dispersive transport in the water phase, and, in some cases, anaerobic degradaconsideration of the transport and fate of the CFC tracers tion. In deep unsaturated zones, this may lead to a time lag between in the overlying unsaturated zone. For example, what entry of gases at the land surface and recharge to groundwater. Data are the CFC concentrations in recharge water and how from a Danish field site were used to investigate how time lag is aflong were the CFC gases in the unsaturated zone before fected by variations in water content and to explore the use of simple reaching the groundwater system? The residence time analytical solutions to calculate time lag. Numerical simulations demof a CFC tracer in the unsaturated zone is also called onstrate that either degradation or sorption of CFC-11 takes place, the time lag (Cook and Solomon, 1995). An accurate whereas CFC-12 and CFC-113 are nonreactive. Water flow did not estimate of the age of a groundwater sample also relies appreciably affect transport. An analytical solution for the period with on an accurate estimate of the time lag. a linear increase in atmospheric CFC concentrations (approximately For very shallow unsaturated zones of only a few early 1970s to early 1990s) was used to calculate CFC profiles and time lags. We compared the analytical results with numerical simulations. meters thickness, diffusion and barometric pumping sufThe time lags in the 15-m-deep unsaturated zone increase from 4.2 to ficiently mix the gases in the unsaturated zone so soilbetween 5.2 and 6.1 yr and from 3.4 to 3.9 yr for CFC-11 and CFC-12, gas CFC concentrations are similar to those in the trorespectively, when simulations change from use of an exponential to posphere. The input to the groundwater system is thus a linear increase in atmospheric concentrations. The CFC concentravery close to the atmospheric changes in CFC concentions at the water table before the early 1990s can be estimated by trations. This simple approach often has been used for displacing the atmospheric input function by these fixed time lags. A dating groundwater, where the concentrations measured sensitivity study demonstrates conditions under which a time lag in in groundwater are used directly together with the atmothe unsaturated zone becomes important. The most critical parameter spheric concentrations to estimate the time of recharge, is the tortuosity coefficient. The analytical approach is valid for the thus neglecting any time lag of the CFCs in the unsatulow range of tortuosity coefficients ( 0.1–0.4) and unsaturated zones greater than approximately 20 m in thickness. In these cases rated zone. the CFC distribution may still be from either the exponential or linear In deeper unsaturated zones the time lag can be imphase. In other cases, the use of numerical models, as described in portant, and various processes affecting CFC transport our work and elsewhere, is an option. become important. Gas diffusion and air–water exchange (solubility) are especially important in controlling the migration and attenuation rate. Both processes are a C hlorofluorocarbons are volatile organic comfunction of the water content and, thus, seasonal and pounds used, for example, as aerosol propellants year-to-year changes in infiltration and depth to the and refrigerants since the 1930s (Plummer and Busenwater table. Cook and Solomon (1995) investigated nuberg, 1999) and now found in the subsurface because of merically the conditions under which the time lag of the release to the atmosphere. There are numerous exCFCs in the unsaturated zone is of importance by examamples of the use of CFCs as tracers in groundwater ining the relative effects of various soil parameters on studies, including studies of dating recharge water (Ekthe distribution of gases in the unsaturated zone. Buwurzel et al., 1994; Plummer et al., 2000), dating young senberg and Plummer (2000) used the same model as groundwater ( 50 yr) (Busenberg and Plummer, 1992; Cook and Solomon (1995) to study lag times of SF6 in Oster et al., 1996; Plummer et al., 2001), groundwater unsaturated zones and found that the lag times of SF6 flow and transport processes (Reilly et al., 1994; Cook are smaller than for CFCs because of its low solubility. Oster et al. (1996) measured 57 profiles of both CFC-11 and CFC-12 in a 4.5-m-thick unsaturated zone in a forest P. Engesgaard and K.H. Jensen, Geological Institute, Univ. of Copensoil in Germany. A clear damping of the annual changes hagen, Øster Voldgade 10, 1350 Copenhagen K, Denmark; A.L. in CFC atmospheric concentrations were found with a Højberg, K. Hinsby, and T. Laier, Geological Survey of Denmark and Greenland, Øster Voldgade 10, 1350 Copenhagen K, Denmark; relaxation time (time lag) of 30 d for 4 m. Weeks et al. F. Larsen, Environment and Resources, Technical Univ. of Denmark, (1982) used analytical and numerical transport models Building 204, 2800 Lyngby, Denmark; E. Busenberg and L.N. Plumto simulate the observed distribution of CFC-11 and mer, USGS, 12201 Sunrise Valley Drive, Reston, VA 20192, USA. CFC-12 in a 50-m-deep unsaturated zone. The analytiReceived 4 July 2003. Original Research Paper. *Corresponding author (pe@geol.ku.dk). cal model was a pure diffusion model, whereas the nuPublished in Vadose Zone Journal 3:1249–1261 (2004).

Demonstrating a Natural Origin of Chloroform in Groundwater Using Stable Carbon Isotopes

Chloroform has been for a long time considered only as an anthropogenic contaminant. The presence of chloroform in forest soil and groundwater has been widely demonstrated. The frequent detection of chloroform in groundwater in absence of other contaminants suggests that chloroform is likely produced naturally. Compound-specific isotope analysis of chloroform was performed on soil-gas and groundwater samples to elucidate whether its source is natural or anthropogenic. The δ(13)C values of chloroform (-22.8 to -26.2‰) present in soil gas collected in a forested area are within the same range as the soil organic matter (-22.6 to -28.2‰) but are more enriched in (13)C compared to industrial chloroform (-43.2 to -63.6‰). The δ(13)C values of chloroform at the water table (-22.0‰) corresponded well to the δ(13)C of soil gas chloroform, demonstrating that the isotope signature of chloroform is maintained during transport through the unsaturated zone. Generally, the isotope signature of chloroform is conserved also during longer range transport in the aquifer. These δ(13)C data support the hypothesis that chloroform is naturally formed in some forest soils. These results may be particularly relevant for authorities regulation of chloroform which in the case of Denmark was very strict for groundwater (<1 μg/L).

New geological aspects for freshwater seepage and formation in Eckernförde Bay, western Baltic

Abstract The subsurface geology relevant to the submarine freshwater seepage in Eckernforde Bay has been investigated using shallow seismic instruments and vibrocoring. Detailed surveying revealed that the pockmarks are aligned like pearls on a string or densely clustered in furrow-like structures depending on the glacial and postglacial setting of the underlying strata. Two possible aquifers have been verified: The older Miocene sand aquifer is partly sealed by a till unit forming the central part of the Mittelgrund. The younger aquifer consists of a mixture of glacial till and meltwater sediments partly sealed by till and partly by lateglacial galciolacustrine silt and clay sediments. The investigations imply that connections exist between the aquifers and that groundwater leakage takes place in the marginal zones of the bay due to thinning and coarsening of the sediment composition of the lateglacial seal. Within the seepage areas, the pockmarks are restricted to areas covered by unconsolidated Holocene mud of low thickness’ that are easy to penetrate by artesian groundwater. Macrofossil studies and AMS 14 C dating of the lateglacial and Holocene units reveal that the Mittelgrund shoal of glacial origin has been modified by coastal processes and formation of cuspate foreland deposits during the subsequent palaeo-lake phases of 15–20xa0m below the present sea level (b.s.l.). The lake phases correlate in time with the regional Baltic Ice Lake highstand about 10,000 14 C years BP and the Ancylus Lake highstand about 9200 14 C years BP. This means that local contemporary lakes existed or the western margin of the regional lakes can be moved considerably further west than expected hitherto. In the earliest phase of the Littorina Sea transgression, the Mittelgrund shoal was exposed to coastal erosion once more before the final drowning and the initiation of mud sedimentation in the surrounding basins took place.

Accumulation and seepages of biogenic gas in northern Denmark

Abstract Gas from two shallow submarine seeps in northern Kattegat were found to consist of methane (94.8–98.6%), carbon dioxide (0.3–2.1%) and nitrogen (up to 3%). The stable isotopic ratios of methane, δ13C: −65.3 to −68.4‰ and δ2 H: −168 to −191‰, clearly show it to be biogenic. The gas most likely originates from the Late Pleistocene marine deposits in which gas with similar chemical and isotopic composition has been found by the authors in several on-land shallow (100–120 m) wells. These marine deposits were found to contain 0.6–0.9% total organic matter of mainly terrestrial origin (based on Rock-Eval data), deposited at high sedimentation rates. The processes of generation and accumulation of biogenic gas during the Late Pleistocene is found to parallel those known to be acting today in a major depocentre in northern Kattegat. Recent tectonic activity in the area has created fractures through which the gas escapes. With regard to sediment type, seeps in the seabed from the area are more likely to occur in coarser sediments rather than fine grained sediments, since consumption of methane by sulphate reduction is probably more important in the latter.

A conceptual model linking functional gene expression and reductive dechlorination rates of chlorinated ethenes in clay rich groundwater sediment.

We used current knowledge of cellular processes involved in reductive dechlorination to develop a conceptual model to describe the regulatory system of dechlorination at the cell level; the model links bacterial growth and substrate consumption to the abundance of messenger RNA of functional genes involved in the dechlorination process. The applicability of the model was tested on a treatability study of biostimulated and bioaugmented microcosms. Using quantitative real time PCR, high-resolution expression profiles of the functional reductive dehalogenase genes bvcA and vcrA were obtained during two consecutive dechlorination events of trichlorethene, cis-dichlorethene and vinyl chloride. Up-regulation of the bvcA (for the biostimulated microcosms) and vcrA (for the bioaugmented microcosms) gene expression fitted well with high rates of dechlorination of vinyl chloride, while no known transcripts could be measured during trichloroethene and cis-dichlorethene dechlorination. Maximum concentrations of 2.1 and 1.7 transcripts per gene of the bvcA and vcrA genes, respectively, were measured at the same time points as maximum dechlorination rates were observed. The developed model compared well with the experimental data for both biostimulated and bioaugmented microcosms under non-steady state conditions and was supported by results from a recently published study under steady state conditions.

Fluid flow and methane occurrences in the Disko Bugt area offshore West Greenland: indications for gas hydrates?

The present study is the first to directly address the issue of gas hydrates offshore West Greenland, where numerous occurrences of shallow hydrocarbons have been documented in the vicinity of Disko Bugt (Bay). Furthermore, decomposing gas hydrate has been implied to explain seabed features in this climate-sensitive area. The study is based on archive data and new (2011, 2012) shallow seismic and sediment core data. Archive seismic records crossing an elongated depression (20×35 km large, 575 m deep) on the inner shelf west of Disko Bugt (Bay) show a bottom simulating reflector (BSR) within faulted Mesozoic strata, consistent with the occurrence of gas hydrates. Moreover, the more recently acquired shallow seismic data reveal gas/fluid-related features in the overlying sediments, and geochemical data point to methane migration from a deeper-lying petroleum system. By contrast, hydrocarbon signatures within faulted Mesozoic strata below the strait known as the Vaigat can be inferred on archive seismics, but no BSR was visible. New seismic data provide evidence of various gas/fluid-releasing features in the overlying sediments. Flares were detected by the echo-sounder in July 2012, and cores contained ikaite and showed gas-releasing cracks and bubbles, all pointing to ongoing methane seepage in the strait. Observed seabed mounds also sustain gas seepages. For areas where crystalline bedrock is covered only by Pleistocene–Holocene deposits, methane was found only in the Egedesminde Dyb (Trough). There was a strong increase in methane concentration with depth, but no free gas. This is likely due to the formation of gas hydrate and the limited thickness of the sediment infill. Seabed depressions off Ilulissat Isfjord (Icefjord) previously inferred to express ongoing gas release from decomposing gas hydrate show no evidence of gas seepage, and are more likely a result of neo-tectonism.

Sampling of sub-seafloor aquifers by a temporary well for CFC age dating and natural tracer investigations

Abstract:Based on a sediment vibro corer, a tool for the sampling of sub-seafloor groundwater aquifershas been developed and successfully deployed in a coastal area of the western Baltic. Thedevice was designed to obtain pure groundwater samples from coarse sediments to be used fortracer investigations and CFC age dating.Operated from a medium size research vessel, a well pipe tipped with a filter segment isvibrated into the sediment down to the aquifer. Groundwater entering the filter is pumped tothe ship by a conventional submersible pump situated in the wells filter tip. Groundwater iscontinuously analysed on board for O2, salinity, pH, Eh and temperature, prior to sampling forCFC and radioisotope analysis. All parametres indicate that pure groundwater had beenobtained.CFC concentrations are very low suggesting that the groundwater of this shallow sub-seaflooraquifer recharged prior to 1950. This finding is in accordance with other hydrogeologicalevidence that this aquifer, located only 4-5 metres below the seafloor, is connected to fairlydeep confined sandy aquifers on land of Pleistocene or Miocene age.Applying the method described, it is possible to obtain sufficient sample volumes foranalyses of natural groundwater tracers such as radon-222 and CFCs which can beused to trace submarine groundwater discharge as well as the origin ofgroundwater in such environments.Keywords: submarine springs, groundwater, pockmark, submarine well, CFC, radon

The impact of bioaugmentation on dechlorination kinetics and on microbial dechlorinating communities in subsurface clay till

A molecular study on how the abundance of the dechlorinating culture KB-1 affects dechlorination rates in clay till is presented. DNA extracts showed changes in abundance of specific dechlorinators as well as their functional genes. Independently of the KB-1 added, the microbial dechlorinator abundance increased to the same level in all treatments. In the non-bioaugmented microcosms the reductive dehalogenase gene bvcA increased in abundance, but when KB-1 was added the related vcrA gene increased while bvcA genes did not increase. Modeling showed higher vinyl-chloride dechlorination rates and shorter time for complete dechlorination to ethene with higher initial concentration of KB-1 culture, while cis-dichloroethene dechlorination rates were not affected by KB-1 concentrations. This study provides high resolution abundance profiles of Dehalococcoides spp. (DHC) and functional genes, highlights the ecological behavior of KB-1 in clay till, and reinforces the importance of using multiple functional genes as biomarkers for reductive dechlorination.

Geothermal Energy in Denmark – Potential, Policy and Progress

A new assessment of the geothermal resources in Denmark published by GEUS concludes that the Danish subsurface contains huge geothermal resources (Mathiesen et al. 2009). To rationalise administration the Danish Energy Agency (DEA) has established a new simple application procedure with a standard license period and work program. These initiatives and rising prizes on fossil fuels have together with public concerns related to climatic changes and increasing emission of CO2 to the atmosphere triggered the awareness of the large potential of the geothermal resources, which may contribute to a safe, sustainable, price stable and reliable supply of energy. It is thus expected that geothermal energy may play an important role in the future energy strategy in Denmark (Fenham et al. 2010; Nielsen et al. 2011).