Prøvetagning af porevand i umættet zone

English summary

Background

Included in "Guidance for Remediation of Contaminated Sites" (The Danish EPA, 1998a) is a method for performing a risk-based evaluation for contamination of a groundwater resource. The level of contamination concentrations in the water infiltrating the groundwater, is an essential element in the evaluation of a given soil contamination to determine if the contamination is posing a risk for the groundwater. In some cases, the concentrations may be found by analysis of a water sample collected in the bottom of a tank bed or from a utility trench. Alternative approaches include the fugacity method, where contamination concentrations in the water, located in the unsaturated zone are estimated, or to use the solubility of each contamination compound. These alternative methods normally lead to significant overestimation of the risks of groundwater contamination. Furthermore, calculations based on the fugacity method are not valid for product mixtures or metals.

Purpose of study

The Danish EPA has initiated this study to identify the methods described in literature and papers for sampling of pore liquids from the unsaturated zone. The study generated a comprehensive list of the identified sampling methods, and included a technical evaluation of each methods pros and cons under typical Danish conditions. Furthermore, the study provides examples of methods that may be developed into sampling methods that may be provided as normal environmental consulting services.

Compounds

The study has been narrowed to the compounds included in Danish EPA’s Groundwater Standards

Content of the report

...transport of water and compounds

After the Introduction in Chapter 1, Chapter 2 includes a brief description of adhesive forces that restrain the pore water, a brief description of retention curves is also provided. Chapter 2 gives a short review of the transportation mechanisms for water and compounds in the unsaturated zone, including the most important equations for describing the transport. Also, this chapter contains a brief explanation of the parameterization of water and compound transport for use in the MACRO model.

Chapter 3 presents a general discussion of hydrogeological and chemical changes in the vicinity of, and inside the samplers along with an assessment of the required sample volume. After this discussion, as a major part of this report, Chapter 3 presents a comprehensive review of the identified sampling methods along with the methods pros and cons. The types of samplers mentioned in this study, are divided into three groups: Zero-tension samplers, tension samplers, and other samplers.

...tension samplers

The group of vacuum samplers mainly include suction cells, which is the most commonly described sampling method. A general description of the different vacuum systems is provided along with a discussion of the pore size, design, chemical structure and the cells area of efficiency.

There are only few examples of utilizing suction cells for sampling of organic compounds. Highly volatile compounds would presumably evaporate from the suction cells. Sampling in suction cells of compounds with strong sorption to the soil particles combined with only limited solubility in water, as e.g. PAH’s, will probably also prove less useful. However, different types of metals, which generally also have a strong sorption to soil particles and are undissolvable in water, are proven successfully sampled in suction cells. Heavy petroleum compounds, and compounds that only have a limited sorption to the soil particles combined with a solubility in water, as e.g. MTBE and phenoles, would probably be successfully sampled in suction cells. Also, suction cells can be utilized for sampling a wide range of pesticides.

In addition to suction cells, Chapter 3 describes the filter tip sampling system, sampling with hollow fibers, membrane filtering and the vacuum plate sample system. Compared with the suction cell sampling system, these systems generally seem less effective or inadequate for sampling at contaminated sites.

...zero-tension samplers

Zero-tension sampling systems are divided into three groups: Lysimeters, wick sampling systems and other passive sampling systems.

Installation of lysimeters, (along with the wick sampling systems) requires excavation, which will disturb the structure of both soil and contamination in the investigation area. These sampling methods are, therefore, useless for investigation of magnitude and extent of known contaminations. Lysimeters, however, are unsurpassed for assessment of the magnitude of infiltration or spills over an preinstalled lysimeter. The lysimeter method provides unequaled "reference values" for the infiltration.

Zero-tension sample systems may be designed as e.g. vertical screens, pipes or compartments to be installed by various drilling methods. The pipe and compartment type systems most often include flaps that may be opened and closed for sampling. These systems may be inserted into the ground with minor disturbing of soil or groundwater.

A disadvantage to these types of sampling systems is that the sampling only occurs when the pore water pressure is above the field capacity. To minimize the loss of volatile compounds, sampling should only take place immediately after an infiltration event.

...other sampling methods

Other sampling methods described in this study include water extraction of soil samples and centrifugation of pore water from soil samples. Both methods are based on collection of soil samples, and both sampling methods seem to be potentially adequate sampling methods. These two methods, however, require further development and documentation.

Conclusion

This literature study has found only few papers concerning the compounds which are included in the Danish Groundwater Standards. At this time, none of the identified sampling systems are documented to be adequate for conducting sampling of the pore water for the specified compounds. The following sampling methods, however, seems to be potential adequate and may be developed into suitable "standard sampling methods":

• Suction cell systems
• Passive sampling systems (e.g., screens)
• Water extraction of soil samples
• Centrifugation of pore water from soil samples
• Calculations based on the fugacity method

Testing of the methods may be completed in three phases, where the methods are initially tested in a controlled laboratory environment. Based on the laboratory results, one or two methods are selected for tests in outdoor located lysimeters. The third test phase should include tests of the selected methods on contaminated sites.

For test and evaluation of the sampling methods, two organic compounds with different leaching/sorption potentials may be selected for the tests along with one or two metals. The tests should be conducted in two soil types, till and outwash sand as these two soil types are representative for the soil types in the eastern and western part of Denmark respectively. During installation of the lysimeters disturbing of the soil should be minimized. Also, it should be determined how naturally occurring zones of cracks and fractures of the soil may be preserved. If possible, old lysimeters which was packed several years ago, and has now achieved a natural soil structure should be utilized for the tests.