Data acquisition

Highlights
Methods to describe geology and hydrogeology Fracture characterization Aquifer tests to obtain parameters

Geology and hydrogeology Fig. 1: Borehole cores from the Akacievej field site. Knowledge about geology and hydrogeology of a contaminated site is usually limited and not easily obtainable. This page gives a description of some useful methods to determine relevant aquifer parameters and to characterize the fracture system. Information about the geology and hydrogeology at a site is primarily obtained from locations that provide access to the subsurface, such as outcrops, excavations, and boreholes, and from geophysical measurements. This can be combined with information from geologic maps and interpretations, as, for example, provided by the Geological Survey of Denmark and Greenland. Borehole cores Fig. 2: Borehole core with substantial core loss. A common method to obtain knowledge about the local geology is to drill boreholes and to take borehole cores. Some examples from a borehole cores of are shown in Figure 1. However, in limestone geologies with flint layers and the hardness varying with depth, core losses may be significant. The cores can be analyzed and the depth-discrete stratigraphy at the location of the borehole can be determined. With a microfossile analysis, the age and the type of limestone can be further characterized. However, one has to bear in mind that cores can only give information about the geology at the location of the borehole and that the geology can have a strong spatial variability. At the Akacievej site, several cores were taken and a microfossile analysis was conducted. The limestone had a strongly varying hardness and losses of crushed material were considerable, despite using elaborate drilling methods (dry drilling / tørboring, symmetrix, DTH drilling). Especially when drilling through flint inclusions, substantial core losses were observed (see for example Fig. 2), because soft material was flushed out by the water required to cool the drill. TODO: Conductivity measurements, caliper, temperature measurements etc. Aquifer tests Pumping tests Pumping tests are very useful to characterize the local hydrogeology at a contaminated site. Usually, a well is pumped and the drawdown behavior (head changes) in the pumping well and (if available) in neighboring observation wells is measured. This can be done with manual head measurements, if the hydraulic head changes relatively slow. Fractured aquifers have, however, often exhibit a high hydraulic conductivity and the drawdown happens quickly, which makes it hard to manually measure the drawdown caused by the pumping. In this case, the head changes can be monitored with programmable pressure transducers (divers), which measure the hydraulic heads at a high measurement frequency. Two measurements per seconds or even more are recommended for fracture-flow dominated aquifers or aquifers with a high hydraulic conductivity. The drawdown curves have to be interpreted with a suitable tool (e.g. Aqtesolv), which allows the determination of aquifer parameters like the hydraulic conductivity. In fractured aquifers, long-term pumping tests with a high pumping rate can potentially give information about the hydraulic conditions in the fractures and the matrix. The drawdown curves show different stages - first, pumped water comes mainly from the fractures, then there is a fracture-matrix interflow, and finally the water comes from the matrix. For the interpretation of such drawdown curves, specialized dual-continuum solution schemes (e.g. the dual-porosity solution presented in Moench, 1984 ) can be employed. However, it has to be kept in mind that the obtained values only describe the area affected by the pumping test and an extrapolation has to be done with care, especially if the aquifer is very heterogeneous. A higher pumping rate can potentially increase the affected area. Further, in screened wells, the (vertical) location of the screen is important, since water is mainly pumped from the part of the aquifer at the depth of the screen (or in case of an open borehole, of the entire borehole). Packers can be installed to separate sections of the aquifer and to determine hydraulic conductivities (transmissivities) for these segments. Long-term pumping tests can give additional information in fractured limestone aquifers. The drawdown curve may exhibit different stages (as described in Nielsen, 2007). In the first stage, water is mainly withdrawn from the fractures, followed by a stage with interflow between matrix and fractures. In the last stage, the water is mainly abstracted from the matrix. The drawdown curves can then be interpreted using specialized solution schemes. At the Akacievej site, the full drawdown had developed within a time period of 10 days. Fig.1: Photos from the pumping test and the tracer tests at Akacievej (spring 2016). The following report contains a detailed description of a long-term pumping test that was conducted at the Akacievej site in combination with six tracer tests and PCE measurements: Report about the pumping and tracer test at Akacievej (PDF) Slug tests Fig.3: Example of a vacuum slug test at the Akacievej site. Slug tests are relatively cheap and easy single-borehole aquifer tests, where the water table in a borehole is abruptly changed, for example, by releasing a slug of water into the borehole while monitoring the pressure response, when the water table changes back to its original state again. They can be used to obtain approximate values of the hydraulic conductivities at the location of a borehole. Therefore, the hydraulic head has to be measured with a high frequency. In low conductivity aquifers, this can be done with manual measurements using common dip-meters. In highly conductive aquifers, the water table responds quickly, and automated measurements with pressure transducers should be chosen. These pressure transducers should be set to a sufficiently small time interval (ideally several measurements per second). The interpretation of slug tests with standard aquifer test software like Aqtesolv allows for obtaining information about the local hydraulic conductivity. Therefore, a solution scheme that is appropriate for fractured aquifers like the . In very conductive aquifers, an oscillating water table can occur. There are specialized solution schemes, which can be applied then. Note that slug tests are very local measurements, because they usually affect only a small volume around a borehole and an extrapolation of the determined parameters should be done with care. Furthermore, the slug test results can be influenced by the borehole filling (gravel or sand pack around the well screens). If several boreholes are close by or if there are boreholes with several well screens at different depth, slug tests can yield information about the local variability of the hydraulic conductivity. There are different types of slug tests. A main distinction can be done between rising-head and falling-head slug tests. Rising-head slug test To perform a rising-head slug test, water is removed from the borehole and the recovery of the hydraulic head in the borehole is recorded. This can be a good choice for aquifers with moderate flow velocities but can be problematic for highly conductive aquifers. Falling-head slug test For a falling-head slug test, the water level in the borehole is abruptly increased. This can be done on different ways. A slug of water can be added into the borehole and the head change monitored. For aquifers with a high hydraulic conductivity, a different method is suggested. A vacuum is applied on the borehole to pull the water table up at the borehole. The raised water table is then released and the equilibration of the water table measured with pressure transducers with a short measurement interval. Additional information from water works data and remediation systems Waterworks can be a cheap and simple way of getting additional pumping test data. Waterworks are operating one or several wells, where often automated loggers are installed, which monitor the hydraulic heads in the pumped wells. When the pumps are switched on and off, drawdown and recovery curves can be obtained, which can be analyzed with standard aquifer test software (e.g. Aqtesolv) like usual pumping tests. To obtain data useful for interpretation, a high measurement frequency should be set for the hydraulic head logging in the wells, particularly for aquifers with a high hydraulic conductivity. For highly conductive aquifers like the one at Akacievej, a good choice of the measurement interval is 1 second or even less. The Fløng waterworks close to the Akacievej site operates an alternating pumping scheme in 4 drinking water wells, where the individual wells are automatically switched on and off according to the water demand. This creates a sequence of pumping-test like events, which were evaluated to obtain the hydraulic conductivity at the wells of the water works. Note that the wells used by the water works have often long screens, and that the measurements represent average values over the screen length or borehole length (in case of an open borehole). Furthermore, head measurements from a remediation system can be interpreted to obtain hydraulic parameters at the location of the remediation system. At the Akacievej site, the remediation system was turned on and off several times, and the hydraulic heads in the remediation well and in observation wells next to it were measured. This information was used to calibrate a flow model to the observed drawdown. Fracture characterization Fig.2: Example of an optical televiewer measurement in Geo18 showing a fracture (dark) at 35.4 m bgs. Courtesy of Geo. In fractured limestone aquifers, the fractures are the primary travel pathways for substances, and their characterization is important for risk assessment and remedial planning. Information about the fracture geometry and location is, however, generally scarce and the determination of appropriate parameters may be challenging. Fractures are generally described by their aperture, spacing, distribution, length and height and their main orientation. The flow and transport in fractures is furthermore controlled by the fracture connectivity. The following measurements are useful to determine fracture parameters: slug tests and pumping tests flow logs in boreholes temperature logs in boreholes and induced temperature gradients tracer tests fracture mapping at analogue sites like outcrops and excavations borehole cores geophysical measurements open-borehole methods like flexible liners (FLUTe) or televiewers Flow logs in boreholes can be very useful to identify highly conductive zones. Therefore, a propeller probe is lowered in the borehole and the speed of the propeller rotation is recorded while pumping the borehole. The propeller probe is slowly moved up- or downwards. High-flow zones will lead to a change of the propeller rotation speed and high-flow zones can be identified from the logs. For this method, even borehole walls are beneficial. Holes or gaps in the borehole walls can lead to disturbances in the flow logs due to local turbulences and backflow of water. When the propeller probe is used without pumping, some information about vertical flows in the boreholes can be obtained. When open boreholes (without well screen) are available, FLUTe liners can be used to identify high-flow zones. Moreover, optical and acoustical televiewers can be used to take images of the borehole walls. With vertical boreholes, mainly horizontal fractures and flint layers can be identified by this method. For the determination of vertical fractures, diagonal boreholes can be beneficial, because they increases the likelihood that vertical fractures intersect with the borehole. A sample image from a vertical borehole at the Akacievej site (Geo18d) is shown in Figure 2. The dark area at a depth of 35.4 m below ground surface is a significant horizontal fracture. This is reflected in the flow log in the right part of Figure 2. At the same depth as the dark area, a strong change in the flow log can be observed. This is an indication for a high-flow zone. TODO: Electrical conductivity measurements, gamma logs, density logs to determine the stratigraphy, caliper for borehole diameters, temperature measurements etc. Return to Content