Leaching is a process by which soluble constituents are dissolved form a solid material (solid or waste) into a contact water phase. The extent to which they are dissolved depends on the site, material specific conditions (chemical, physical and biological) and the length of time involved. The process of leaching includes the partitioning of contaminants between a solid and liquid phase and the mass transport of aqueous or dissolved constituents. When fill material comes in contact with a liquid like rainwater, groundwater, or surface water constituents in the solid dissolve forming a leachate. This leachate can then have an impact on local water quality.
Several laboratory methods are utilized to evaluate the leaching potential of a soil sample including the following:
EPA Method 1311 – toxicity characteristic leaching procedure (TCLP) was first used by the USEPA in 1990 to replace EPTOX as the regulatory method for classifying wastes as hazardous based on toxicity. This method was essentially a pass or fail where if the TCLP extract contained one of the TC constituents equal to or exceeding the concentrations specified then the material would be considered hazardous. This method was later criticized for being too broad. In 1999 the USEPA SAB recommended developing multiple leaching testes that could be more flexible, cases specific and have tier testing or a suit of related tests incorporation the most important parameters affecting leaching.
No one leaching test can provide accurate assessment for all potential soil impacts, so many different methods are used. New leaching tests take into account the following aspects of leachability:
- broad range of conditions;
- Influence of pH on equilibrium; and
- Influence of L/S on equilibrium and influence of mass transfer rates.
Some of these new methods are as follows:
EPA Method 1311 – Toxicity Characteristic Leaching Procedure (TCLP): Designed to simulate leaching a waste will undergo if co-disposed with municipal solid waste. Used for hazardous waste determination.
EPA Method 1312 – Synthetic Precipitation Leaching Procedure (SPLP): Designed to assess the leaching potential of soils and waste disposed in a monofill when exposed to rainfall. There are no federal regulatory requirements for the use of SPLP.
EPA 1313 – Liquid/solid partitioning as a function of extract pH using parallel batch extraction procedure. This method had nine parallel extractions and the contact time was determined by particle size.
EPA 1314 – Liquid/solid partitioning as a function of liquid solid ratio for constituents in solid materials using an up-flow percolation column procedure. This tried to minimize the air entrapment and flow channeling so the entire effluent volume is collected.
EPA 1316 – Liquid/solid partitioning as a function of liquid/solid ratio for constituents in solid materials using a parallel batch extraction procedure. This method had five parallel extractions where the contact time was determined by the particle size.
EPA 1315 – Mass transfer rates of constituents in monolith or compacted granular material using a semi-dynamic tank leaching procedure. This was a flux based leaching method for monolith/compacted material where the sample is immersed in reagent water at specific liquid/solid surface area providing the mass transfer rate of COPC under diffusion control leaching conditions as a function of leaching time. This method states that it is for non-volatile organics only. There is a modified 1315 method for PAHs and VOAs.
ANSI/ANS – 16.1 – 2003 – The measurement of the leachability of solidified low-level radioactive wastes by a short-term test procedure. This method was developed for low-level radioactive waste and can be used to measure the leach resistance of any waste solidified into a well-defined geometric shape.
Tiered Testing was developed so each tier can provide leaching data which is more specific to the material being tested and leaching conditions.
Thank you to Test America for the original presentation on this material.