Managing Coal Combustion Residues in Mines (2006) / Chapter Skim
Currently Skimming:
3 Behavior of Coal Combustion Residues in the Environment
3 Behavior of Coal Combustion Residues in the Environment
Pages 59-80
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 59... ...
. In a mine setting, subsurface water flow will normally be the primary mechanism for transporting CCR-derived contaminants from the disposal area to potential receptors (e.g., aquatic life in streams supported by groundwater flow, local residents relying on groundwater as a drinking water source)
|
From page 60... ...
however, in most minefill scenarios, CCRs are covered by several feet of soil or coal spoils, lessening the potential for overland transport of contaminants. Water Flow in the Saturated Zone Groundwater flow at CCR mine placement sites is controlled by the local hydrogeology, which may be significantly altered by mining activities.
|
From page 61... ...
. The strata in coal-bearing areas may be flat-lying, moderately undulating, or highly folded, leading to widely variable patterns of groundwater flow.
|
From page 62... ...
In some surface mine settings, large volumes of rock are removed to gain access to the coal, and during reclamation these materials are redeposited in the mine pit and surrounding area. Water flow through coal spoils and similar materials can occur both through discrete conduits or macropores that form between large pieces of spoil material (pseudokarstic flow)
|
From page 63... ...
. Mine reclamation activities aim to restore surface water flow paths and recreate similar recharge conditions, but, no effort is made to restore the specific subsurface water flow paths (NRC, 1990)
|
From page 64... ...
Water Flow in the Unsaturated Zone Above the water table, water flow occurs in response to gravitational and capillary forces and is therefore relatively complex. In homogeneous porous media (e.g., well-sorted sand)
|
From page 65... ...
Hydraulic conductivity in the unsaturated zone is a function of moisture content. In homogeneous porous media, the highest hydraulic conductivity in the unsaturated zone occurs within the capillary fringe, where all pores are saturated with water (Figure 3.2)
|
From page 66... ...
. As a result, large-volume CCR disposal can substantially alter groundwater flow paths.
|
From page 67... ...
Nevertheless, some observations of the potential impacts of CCRs on unsaturated flow at mine sites are provided here based on relevant studies of unsaturated flow through layered fine- and coarse-grained materials and through waste rock piles at coal and metal mine sites. The impacts of CCRs on unsaturated flow will depend on a number of
|
From page 68... ...
Oxidation-reduction conditions at a mine site are generally oxic, but suboxic conditions may occur at depth. The groundwater pH may be near neutral at some coal mine sites, particularly western mines, and highly acidic at others due to sulfide mineral oxidation reactions that cause acid mine drainage (AMD)
|
From page 69... ...
Acid mine drainage contains elevated concentrations of acid, iron, manganese, alumi num, and associated trace elements, such as zinc, nickel, and arsenic, which can be transported to surrounding waters (Williamson and Rimstidt, 1994; Blowes et al., 2003a)
|
From page 70... ...
(2003) conducted a series of 30- to 90-day column leaching experiments to evaluate the leaching of 32 fly ash samples by several different leaching fluids, including deionized water, simulated acid mine drainage (pH 1.2)
|
From page 71... ...
This section discusses research on the interactions between acid-generating coal spoils and alkaline CCRs, highlighting the implications for CCR placement design in the mine reclamation process.
|
From page 72... ...
Data were collected from fly ash, bottom ash, and flue gas desulfurization ash placed in landfills and surface impoundments. Boxes represent the range of data within the 25th and 75th percentiles (or the inter-quartile range, IQR)
|
From page 73... ...
These findings suggest that the addition of fly ash to coal spoils in a sufficient quantity can prevent AMD formation. However, less is known about the ability of CCRs to prevent AMD over extended time frames.
|
From page 74... ...
Only limited information is available on attenuation reactions influencing the fate of CCR elements of concern at coal mine sites where large-volume CCR disposal has occurred. However, insights can be gained through other studies on the transport of metals and metallic compounds under geochemical conditions that develop in mine settings or other types of sites, since many of the constituents of interest are the same as those found at CCR disposal sites (Table 3.3)
|
From page 75... ...
, zinc, nickel, arsenic, selenium, and boron often migrate + readily, especially in the suboxic conditions that exist in many coal spoils. In contrast, the concentrations and mobility of some other constituents, such as
|
From page 76... ...
However, at many sites this process is inefficient, and trace elements can migrate long distances from the mine in surface water, at unacceptable concentrations. POTENTIAL FOR CONTAMINANT TRANSPORT FROM COAL COMBUSTION RESIDUES IN COAL MINES Contaminants entering groundwater can be transported away from the CCR source area potentially resulting in the degradation of drinking water supplies or of surface-water quality.
|
From page 77... ...
77 al.
|
From page 78... ...
studies that encompass a range of climatic and geologic settings are needed to accurately characterize CCR behavior in mine sites so that the types of mine settings, CCRs, and placement techniques most protective of human and ecological health can be identified. Additional research is also necessary to determine whether placement of CCR in mines can ameliorate the adverse effects of AMD in surface waters, particularly over protracted time scales.
|
From page 79... ...
Thus, the committee concludes that acid neutralization will not reduce the mobility of all contaminants of concern from the CCR. Impacts on downgradient water quality from CCR disposal at mine sites will depend on the concentration and volume of contaminated water entering the flow system and the ability of the aquifer or receiving water body to dilute or attenuate the contamination.
|
From page 80... ...
This research should include studies to determine under which conditions CCRs can effectively ameliorate the adverse effects of AMD in surface waters, particularly over protracted time scales.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.