Data acquisition

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

Fig. 1: Borehole cores from the Akacievej field site.

Geology and hydrogeology 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 contaminant data. Information about the geology and hydrogeology at a site is primarily obtained from boreholes, either, if available, from existing ones or from boreholes that are drilled to provide access to the subsurface. 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 A common method to obtain knowledge about the local geology is to drill boreholes and to take borehole cores. An example is shown in Figure 1. 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 strongly vary. 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 because soft material was flushed out by the water required to cool the drill. 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 described by their aperture, spacing, distribution, length and height and their main orientation. The flow and transport in fractures is also controlled by the fracture connectivity. When open boreholes (without well screen) are available, optical and acoustical televiewers can be used to take images of the borehole walls. An example from the Akacievej site is shown in Figure 2. With vertical boreholes, mainly horizontal fractures can be identified by this method. For the determination of vertical fractures, diagonal boreholes can be beneficial, because they increases the likelihood that the borehole actually hits a vertical fracture. Flow logs with a propeller sonde, lowered into a borehole and pumping shows high conductive zones. Flow logging in a borehole can help to identify highly conductive zones in a borehole. Therefore, a propeller probe is lowered in the borehole and the speed of the propeller rotation is recorded while pumping the borehole and while slowly moving the propeller probe up- or downwards. High-flow zones will lead to a change of the propeller rotation speed and high-flow zones can be identified. 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. When the propeller probe is used without pumping, some information about vertical flows in the boreholes can be obtained. 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, exhibit often 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 Moench solution REFERENCE) 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. 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 analyze separated sections of the aquifer. 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 File:Slugtest.png Fig.3: Example of a vacuum slug test. 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 Water works can be a cheap and simple way of getting additional pumping test data. Water works are operating one or several wells, where usually 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 a usual pumping test. 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 water works in Fløng 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. Automated head measurements from a remediation system can also be useful to obtain hydraulic parameters. 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. Geophysical methods TODO: Flow logs with a propeller probe, lowered into a borehole and pumping shows high conductive zones. Electrical conductivity measurements to determine limestone type, caliper for borehole diameters, temperature measurements etc.

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