Field scale investigation and lysimeters
Teemu Karlsson, Geological Survey of Finland, P.O. BOX 1237, FI-70211 Kuopio, FINLAND, e-mail: teemu.karlsson(at)gtk.fi
Leaching tests play an important role in the characterisation and classification of mine waste material in relation to assessing their actual or potential environmental impacts. These days widely used short-term leaching tests are generally performed in a laboratory under controlled conditions, which are not typical of natural conditions. (Hansen et al. 2000)
Lysimeter experiments are outdoor leaching or percolation tests which are carried out under more natural field-resembling conditions on a relatively large scale and over longer periods of time. Therefore they simulate better the actual field conditions, and are suitable for verification of results obtained in the laboratory leaching tests and for demonstrating similarities and differences between field and laboratory experiments. (Hansen et al. 2000)
Description of the method
Lysimeter tests resemble large-scale column leaching tests where the test material is placed in a container with an open surface. In field lysimeters and laboratory column leaching tests, the solid granular waste material is stationary and a leachant flows through the test material and carries away dissolved constituents and fine particles. The rate at which constituents are carried away is influenced by a number of chemical, physical and biological factors, including e.g. pH in the solution, redox conditions and sorption processes in the system, presence of complexing ligands in solution, colloids, and micro-organisms. (Hansen et al. 2000)
The usually rectangular or circular container is supplied with a drainage system at the bottom, to conduct the leachate to a collection vessel for sampling. The lysimeter setup may include equipment for in situmonitoring of chemical and physical properties at different depths below the test material surface. Lysimeter experiments are usually performed outdoors and the leachant is natural rain water. The duration of a lysimeter test is typically from one to several years. (Hansen et al. 2000)
The size of the lysimeter is dependent on the representativeness of the material investigated; if the material is very heterogeneous or if the particle size is large the lysimeter must be large enough to contain a representative sample of the material. A “rule of thumb” is that the maximum diameter of the test material particles should be under 1/10 of the lysimeter diameter. The material used for the construction of the lysimeter should not contaminate the leachate or interfere with the parameters to be tested. Other aspects to be taken into account are e.g. testing the material, filling-in the lysimeter container, edge effects or preventing preferential flow along the side walls of the lysimeter. (Hansen et al. 2000)
For more detailed guidelines for selection, design and operation of lysimeters, see “Use of lysimeters for characterization of leaching from soil and mainly inorganic waste materials” (Hansen et al. 2000)
Lysimeter studies are useful for scientific studies of the fate and movement of water, pesticides, salts/nutrients, tracers, trace elements, heavy metals and other contaminants. They have also been used in studies concentrating on water percolation through and evaporation from soil and waste and to evaluate solute transport models (Hansen et al. 2000). Lysimeter studies are suitable for investigations related to easily leachable substances, e.g. explosives originated nitrogen, as the undetonated explosives leach out relatively fast. For ARD prediction and metal leaching assessments this kind of tests should be implemented at early stages of the mining operations, since they require a number of years before meaningful results and conclusions can be made (Urrutia et al. 2011). As waste rock material is usually relatively heterogeneous, various lysimeters should be installed to obtain more precise data. Also the size of the lysimeter is essential; the larger the lysimeter, more accurate information will be obtained.
- Simulates well the actual field conditions
- Produces reliable data
- No standards, therefore each lysimeter test has to be designed and adjusted individually
- Is laborious to construct
- Requires long time periods
Lysimeters have been used for nearly three centuries to study water percolation through soil, but lysimeter tests are not standardised; lysimeters are usually designed and adjusted to suit individual research requirements (Hansen et al. 2000).
Hansen, J.B., Holm, P.E., Hansen, E.A. & Hjelmar O 2000. Use of lysimeters for characterization of leaching from soil and mainly inorganic waste materials, Nordtest Technical Report 473.
Urrutia, P., Wilson, W., Aranda, C., Peterson, H., Blackmore, S., Sifuentes, F. & Sanchez, M. 2011. Design and Construction of Field-Scale Lysimeters for the Evaluation of Cover Systems at the Antamina Mine, Peru. Proceedings Tailings and Mine Waste 2011, Vancouver, BC, November 6 to 9, 2011.