Klaus Hinsby

A mini slug test method for determination of a local hydraulic conductivity of an unconfined sandy aquifer

Abstract A new and efficient mini slug test method for the determination of local hydraulic conductivities in unconfined sandy aquifers is developed. The slug test is performed in a small-diameter (1 inch) driven well with a 0.25 m screen just above the drive point. The screened drive point can be driven from level to level and thereby establish vertical profiles of the hydraulic conductivity. The head data from the test well are recorded with a 10 mm pressure transducer, and the initial head difference required is established by a small vacuum pump. The method described has provided 274 spatially distributed measurements of a local hydraulic conductivity at a tracer test site at Vejen, Denmark. The mini slug test results calculated by a modified Dax slug test analysing method, applying the elastic storativity in the Dax equations instead of the specific yield, are in good accordance with the results from two natural gradient tracer experiments performed at the test site. The original Dax, the Bouwer and Rice, and the Chirlin analysing methods all led to an underestimation of the effective hydraulic conductivity by a factor of more than 2, when compared with the tracer tests. In contrast the spherical flow model of Karasaki et al. overestimated the results of the tracer tests by approximately a factor 1.4. The Dax and the Cooper et al. methods, assuming only radial flow to the partially screened well, yielded a better approximation of the horizontal hydraulic conductivity, than the Chirlin method, which also considers axial flow. This fact is suggested to be a result of aquifer anisotropy, as a significant higher horizontal than vertical hydraulic conductivity may suppress the significance of the axial flow component.

Spatial variability of hydraulic conductivity of an unconfined sandy aquifer determined by a mini slug test

Abstract The spatial variability of the hydraulic conductivity in a sandy aquifer has been determined by a mini slug test method. The hydraulic conductivity ( K ) of the aquifer has a geometric mean of 5.05 × 10 −4 m s −1 , and an overall variance of 1n K equal to 0.37 which corresponds quite well to the results obtained by two large scale tracer experiments performed in the aquifer. A geological model of the aquifer based on 31 sediment cores, proposed three hydrogeological layers in the aquifer concurrent with the vertical variations observed with respect to hydraulic conductivity. The horizontal correlation length of the hydraulic conductivity has been determined for each of the three hydrogeological layers and is found to be small (1–2.5 m). The asymptotic longitudinal dispersivity of the aquifer has been estimated from the variance in hydraulic conductivity and the horizontal correlation length, to be in the range of 0.3–0.5 m compared with a value of 0.42 m obtained in one of the tracer tests performed.

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).

The modern water interface: recognition, protection and development — advance of modern waters in European aquifer systems

Abstract Modern groundwater that has recharged aquifers within the past 50 a shows the influence of humans globally, either by the presence of small concentrations of environmental tracers or in some cases by severe pollution. This study describes important environmental tracers (e.g. 3H, 85Kr, chlorofluorocarbons, SF6) and contaminants (e.g. NO3−}, pesticides, chlorinated solvents) for modern groundwater dating and recognition of human impacts. Some applications of the described tracers in aquifers investigated in the PALAEAUX study are presented in order to illustrate the advance of modern waters in European aquifer systems. The study shows that the location of the modern water interface varies within a range of between c. 10 and c. 100 m in the investigated aquifers due to variations in hydrogeological setting, climate and exploitation of the groundwater resource. The subsurface distribution of the modern water indicators and contaminants demonstrate that the advance of modern groundwaters and the fate of harmful substances in them have important implications for protection and development of the water resources. Contaminants that do not degrade or degrade only very slowly will advance further into the aquifers and may eventually contaminate even deep groundwater systems.

The Ribe Formation in western Denmark- Holocene and Pleistocene groundwaters in a coastal Miocene sand aquifer

Abstract The Ribe Formation is a regionally extensive Miocene sand aquifer that is present in western Denmark at depths ranging from 100 to 300 m below ground surface. Groundwater chemistry and isotope data collected from more than 40 wells show that the Ribe Formation mainly contains high quality Cabi-carbonate type groundwater of Holocene age (100–10 000 a bp). Pleistocene age groundwaters, identified on the basis of stable isotopes, noble gases and corrected 14C values, are present below the island of Rømø, in discharge areas near the coast, and in hydraulically isolated inland areas. The groundwater age distribution in the Ribe Formation was successfully simulated with a numerical groundwater flow model and particle tracking only when the 14C content in groundwater was corrected for both geochemical reactions and diffusion. The results indicate that geochemical and physical processes significantly influence the 14C content of groundwater and that the correction factors required for the two processes are of the same magnitude. Flow modelling results indicate that Pleistocene groundwaters were emplaced at depth within the Ribe Fromation under low base-level conditions that prevailed throughout the late Pleistocene — near the coast these waters are essentially isolated from the present flow system, and Pleistocene freshwater may be present offshore. Seismic surveys show that conditions offshore are favourable for the presence of Pleistocene freshwater within the Ribe Formation and other aquifers.

Evolution of groundwater systems at the European coastline

Abstract An overview is given of the status and origin of fresh and saline groundwaters in the sedimentary aquifers at or near the present European coastline. Results are presented as six regional maps summarizing, as far as possible, the conditions likely to have existed at the end of the Pleistocene, after the impact of glaciation, when groundwaters might be expected to have reached their maximum offshore evolution prior to the encroachment of sea water during the Holocene marine transgression. In the eastern Baltic, the groundwater evolution was different to other European regions in that freshwater heads were higher than the present day, promoting recharge during much of the Late Pleistocene. Near the North Sea coasts, there is generally evidence of freshwater movement to depths of 100–150 m but the absence of deeper freshwater (palaeowater) storage may relate to the low hydraulic gradients in the North Sea Basin. In the southeastern part of the North Sea brackish palaeowater is found between Tertiary marine sediments c. 300 m below the island of Rømø, 10 km off the Danish west coast. Freshwater of Pleistocene and Holocene ages is found in aquifers at the English Channel coast in several areas, to depths in excess of −300 m; offshore flow in the Late Pleistocene took place towards the central palaeovalley and some of this groundwater may be preserved off the present coastline. In the two Atlantic coastal areas of France and Portugal-Spain a contrast exists due to the proximity of the continental margin and different hydraulic gradients. In Portugal, freshwaters are found at the coastline, and probably offshore, that contain evidence of recharge during the lowered sea levels. In most of the southwestern Atlantic coast of Spain, fresh recent groundwater discharges along and beyond the coastline, while in the old estuary of the Guadalquivir River, saline Holocene water still encroaches the sediments. On the Mediterranean coast of France and Spain some salinity encroachment took place during sea-level rise. In most of the Spanish aquifers fresh recent groundwater has substituted for palaeowater, except for the low-lying areas (Ebro Delta, Inca-Sa Pobla Plain) where brackish Holocene water is still present.

Assessment of climate change impacts on the quantity and quality of a coastal catchment using a coupled groundwater–surface water model

The hydrology of coastal catchments is influenced by both sea level and climate. Hence, a comprehensive assessment of the impact of climate change on coastal catchments is a challenging task. In the present study, a coupled groundwater–surface water model is forced by dynamically downscaled results from a general circulation model. The effects on water quantity and quality of a relatively large lake used for water supply are analyzed. Although stream inflow to the lake is predicted to decrease during summer, the storage capacity of the lake is found to provide a sufficient buffer to support sustainable water abstraction in the future. On the other hand, seawater intrusion into the stream is found to be a significant threat to the water quality of the lake, possibly limiting its use for water supply and impacting the aquatic environment. Additionally, the results indicate that the nutrient load to the lake and adjacent coastal waters is likely to increase significantly, which will increase eutrophication and have negative effects on the surface water ecology. The hydrological impact assessment is based on only one climate change projection; nevertheless, the range of changes generated by other climate models indicates that the predicted results are a plausible realization of climate change impacts. The problems identified here are expected to be relevant for many coastal regimes, where the hydrology is determined by the interaction between saline and fresh groundwater and surface water systems.

Application of geophysical borehole logging techniques to examine coastal aquifer palaeohydrogeology

Abstract Geophysical logging is shown to be a useful technique to support investigations of coastal aquifer hydrogeology. Formation logging can identify the geological layering and fluid logging can characterize the salinity distribution. The measurements also reveal wellbore flow to be common in coastal boreholes, which can mask the salinity stratification in the aquifer matrix. Geophysical logging can be used to guide water sampling and to provide information on the palaeohydrogeology. In combination with water sampling and age determinations, it has shown modern groundwaters overlying Holocene age groundwaters, in turn overlying groundwaters of Pleistocene age, within 150 m of the surface in some of the aquifers studied. Sea-level change in response to Pleistocene glaciations and deglaciations is recognized as a major control on the salinity of groundwaters and on the development of permeable flow routes in coastal aquifers. The permeable routes that developed by groundwater circulation to older and deeper base levels are now partly or wholly occupied by groundwaters of modern flow systems, and can be the focus for saline intrusion. The effects of Pleistocene sea-level change on aquifer development appear to be worldwide and are being increasingly recognized. Examples are described to illustrate the development of flow horizons in relation to rock layering, structure and base levels.

Aquifer Vulnerability Assessment Based on Sequence Stratigraphic and 39Ar Transport Modeling

A large-scale groundwater flow and transport model is developed for a deep-seated (100 to 300 m below ground surface) sedimentary aquifer system. The model is based on a three-dimensional (3D) hydrostratigraphic model, building on a sequence stratigraphic approach. The flow model is calibrated against observations of hydraulic head and stream discharge while the credibility of the transport model is evaluated against measurements of (39)Ar from deep wells using alternative parameterizations of dispersivity and effective porosity. The directly simulated 3D mean age distributions and vertical fluxes are used to visualize the two-dimensional (2D)/3D age and flux distribution along transects and at the top plane of individual aquifers. The simulation results are used to assess the vulnerability of the aquifer system that generally has been assumed to be protected by thick overlaying clayey units and therefore proposed as future reservoirs for drinking water supply. The results indicate that on a regional scale these deep-seated aquifers are not as protected from modern surface water contamination as expected because significant leakage to the deeper aquifers occurs. The complex distribution of local and intermediate groundwater flow systems controlled by the distribution of the river network as well as the topographical variation (Tóth 1963) provides the possibility for modern water to be found in even the deepest aquifers.

The Baltic Sea as a time machine for the future coastal ocean

Science-based, multinational management of the Baltic Sea offers lessons on amelioration of highly disturbed marine ecosystems. Coastal global oceans are expected to undergo drastic changes driven by climate change and increasing anthropogenic pressures in coming decades. Predicting specific future conditions and assessing the best management strategies to maintain ecosystem integrity and sustainable resource use are difficult, because of multiple interacting pressures, uncertain projections, and a lack of test cases for management. We argue that the Baltic Sea can serve as a time machine to study consequences and mitigation of future coastal perturbations, due to its unique combination of an early history of multistressor disturbance and ecosystem deterioration and early implementation of cross-border environmental management to address these problems. The Baltic Sea also stands out in providing a strong scientific foundation and accessibility to long-term data series that provide a unique opportunity to assess the efficacy of management actions to address the breakdown of ecosystem functions. Trend reversals such as the return of top predators, recovering fish stocks, and reduced input of nutrient and harmful substances could be achieved only by implementing an international, cooperative governance structure transcending its complex multistate policy setting, with integrated management of watershed and sea. The Baltic Sea also demonstrates how rapidly progressing global pressures, particularly warming of Baltic waters and the surrounding catchment area, can offset the efficacy of current management approaches. This situation calls for management that is (i) conservative to provide a buffer against regionally unmanageable global perturbations, (ii) adaptive to react to new management challenges, and, ultimately, (iii) multisectorial and integrative to address conflicts associated with economic trade-offs.