CHEMICAL SPECIATION OF HEAVY METALS IN
POWER PLANT ASH POND LEACHATE
Thomas L. Theis, Assistant Professor
Richard 0. Richter, Graduate Student
Department of Civil Engineering
University of Notre Dame
Notre Dame, Indiana 46556
INTRODUCTION
The mobilization of undesirable materials into the environment through activities related to coal combustion has recently been given considerable attention [1,2,3,4].
These materials include heavy metals, trace organic compounds (such as polynuclear
aromatic hydrocarbons), large amounts of dissolved solids, and waste streams of extreme acidity or alkalinity.   A pathway of some interest is leaching from fly ash and/or
S02 scrubber sludge disposal sites.   It has been shown that leachates from these areas
can continue significant levels of heavy metals, sulfates, calcium, etc. [5].
Once a leachate containing various substances enters the soil environment, it is desirable to know the types and extent of interaction which occur between leachate and
soil particles, that is the "natural attenuation capacity" of a specific soil component
for a given leachate constituent.    In the case of dissolved ions, these interactions will
take the form of precipitation of a discrete solid phase or adsorption onto particle surfaces.   Soluble complexes, if they are formed to an appreciable degree, will also affect
these mechanisms.
In this study, data gathered from an active fly ash disposal pond will be presented
and analyzed with the aid of a chemical equilibrium computer model so as to provide
a realistic assessment of the subsurface reactions which various heavy metals have undergone since the initial construction of the facilities.   Although the interactions among
several soil and leachate components may be chemically rather complex, and are, of
course, specific to the site studied, they are considered typical of the reactions which
occur in a subsurface leaching zone.   The site reported on   here is viewed as representative of systems of generally wide environmental interest.
SITE DESCRIPTION
The site from which data were gathered is a 735 MW coal-fired power station located
in a dunal area adjacent to Lake Michigan.   There are two primary fly ash disposal
ponds which are operated on an alternating basis, i.e., while one pond is being actively
loaded the other is allowed to dry and the ash is removed and deposited elsewhere.   A
schematic diagram of the site is shown in Figure 1.   Numbered circles refer to borings
made as part of this study.   This layout was constructed and put into operation in
early 1974.    Each pond has a capacity of approximately 62,000 m3 (16.5 mg).   Under
normal operating conditions, the station produces between 310-360 metric tons/day
(340-400 tons/day) of fly ash.   This is sluiced to one of the ponds where it is allowed
to settle.   The normal hydraulic loading is 0.125 m3/sec (2.85 mgd).   The supernatant
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