Data acquisition

Knowledge about geology and hydrogeology of a contaminated site is usually limited and not easy to obtain. This page gives a description of some useful methods to determine relevant aquifer parameters and contaminant data.

Geology and hydrogeology

Borehole cores

A common method to obtain knowledge about the local geology is to drill vertical boreholes and to take borehole cores. 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 vary strongly.

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

Fractures are described by their aperture, spacing, distribution, length and height and their main orientation. Information about the fracture geometry and location is generally scarce and it is challenging to determine appropriate parameters.

Flow logging in a borehole can help to identify high-flow zones in a borehole. Therefore, a propeller probe is lowered in the borehole and the propeller rotations while pumping the borehole are recorded. High-flow zones will lead to a change of the propeller rotation speed and the 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 open boreholes (without well screen) are available, optical and acoustical televiewers can be used to analyze the borehole walls. An example is shown in Figure... (reference possible?). With vertical boreholes, mainly horizontal fractures can be identified by this method. For the determination of vertical fractures, diagonal boreholes can be beneficial (it increases the likelihood to hit a fracture with the borehole).

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 separate sections of the aquifer. In limestone aquifers, the horizontal hydraulic conductivity is usually much higher (typically factor 2-10) than the vertical one.

The following report describes a long-term pumping test combined with six tracer tests conducted at the Akacievej site:

• Report about the pumping and tracer test at Akacievej (PDF)

Slug tests

File:Slugtest.png

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 (e.g. by releasing a slug of water into the borehole) and the pressure response, when the water table changes back to its original state, is monitored. Therefore, the hydraulic head has to be measured. This can be done with manual measurements using common dip-meters or with automated measurements with pressure transducers. If the hydraulic conductivity close to the borehole is high (f.e. in a heavily fractured aquifer), the recovery happens very fast and it is necessary to have automated measurements.

The interpretation of slug tests allows for obtaining information about the local hydraulic conductivity. Note that slug tests usually affect only a small volume around a borehole and the determined parameters are valid for this volume. Furthermore, they 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 testing can yield some 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.

• 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. Water can be added in a slug into the borehole. A method which is particularly useful for aquifers with a high hydraulic conductivity (such as fractured limestone aquifers) is to apply a vacuum on the borehole in order to pull up the water table. The vacuum is released and the equilibration of the water table can be interpreted.

• Rising-head slug test

For a rising-head slug test, water is removed from the borehole and the recovery of the water level is recorded afterwards.

Additional information from water works data and remediation systems

Water works offer the opportunity of getting cheap pumping test data. Usually, they are operating one or several wells, where loggers monitor the water levels in the wells. We show an example, where the water works in Fløng operates an alternating pumping scheme in 4 drinking water wells. When the pumps are switched on and off, drawdown and recovery curves are obtained and can be analyzed with standard aquifer test software (e.g. Aqtesolv). 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.

Geophysical methods

Flow logs with a propeller sonde, lowered into a borehole and pumping shows high conductive zones. Conductivity measurements, caliper, temperature measurements etc. ADD SAMPLE PICTURES FROM OPTICAL TELEVIEWER AND BOREHOLE LOGS

Transport parameters and contaminant data

Advective transport happens mainly with the groundwater flow. However, to describe the transport of a substance in a fractured limestone aquifer properly, additional parameters are required. Important transport parameters that influence the migration of a substance are

• diffusion coefficient of the substance in the limestone (often estimated as molecular diffusion coefficient times the tortuosity or porosity of the limestone
• sorption coefficient
• limestone porosity

Transport parameters

Tracer tests are very useful to analyze the transport behavior in a limestone aquifer. Different types of tracer tests can be distinguished

• push-pull tracer tests, where a tracer is injected and monitored in the same borehole
• tracer tests with an injection well and one or several observation wells
• forced-gradient (with pumping) or natural gradient tracer tests

In the limestone project, a forced-gradient tracer test with several injection wells and a central pumping well for tracer monitoring was conducted. Details are described in the report LINK TO REPORT.

Furthermore, measurements from core material can be used to determine the porosity and hydraulic conductivity of a limestone sample (poroperm tests).

Contaminant data

Different sampling and monitoring techniques to determine the depth-discrete contaminant distribution in boreholes have been developed. Depth-discrete sampling is important, since due to a very heterogeneous nature of the aquifer, the concentration can vary strongly over depth. For the planning of a site remediation it is important to know the vertical extent of the contamination, so the remediation system can be planned targeted on the high concentration zones.

The following list gives an overview of some useful sampling methods in wells in limestone aquifers:

• Snap samplers
• Diffusion cells
• Bladder pump

The following ones require open (unscreened) boreholes:

• NAPL-FLUTe
• FACT-FLUTe
• Passive flux meters
• etc.

Report by Mette et al. gives a comparison of different methods. Link Contaminant report Sammenligning af...

• Comparison of depth-discrete sampling methods and contaminant measurements at Akacievej (PDF)

Integral pump tests can be used to quantify the contaminant mass discharge through a control plane.

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