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Examining the Elevated Iron
Concentrations at Landfill Sites
Background:
The elevation of iron
concentrations in groundwater monitoring wells at landfill sites have been
noted at a growing number of facilities in the state, both lined and
unlined. The hypothesis that has been proposed by many is that the source
of this iron is not the landfill leachate, but the native soils beneath the
landfill. It is speculated that the iron becomes mobilized as a result of
changing pH and/or redox conditions in the groundwater underneath the
landfill. In the case of unlined (C&D debris) disposal facilities, reducing
conditions can develop as a result of leachate migration into the
groundwater. In the case of lined landfills, the reducing conditions are
thought to develop as a result of changes to the natural hydrology that
result from the use of a liner and possibly from other site changes such as
the installation of storm water control ponds.
Iron is a naturally-occurring
element in many Florida soils. As a chemical species, iron can exist in the
environment in many different forms. Iron in soils typically occurs in the
more oxidized Fe+3 form as opposed to the Fe+2 form.
In the oxidized form, the iron is relatively immobile and stays as part of
the soil matrix. When reducing condition convert some of the oxidized iron
to a more reduced form ( ),
the iron may become liberated from the native soil matrix into groundwater
(Pedersen et al. 2005; Benner et al. 2002; Zachara et al. 2001).
In the environment (including the groundwater environment), a
number of biological reactions occur in which organic matter is consumed as
part of the life cycle of different bacteria. These bacteria utilize a
variety of different compounds as electron acceptors. Since some electron
acceptors are energetically more favorable than others, some bacteria out
compete other bacteria depending on which electron acceptors they use.
For example, iron reducing bacteria (IRB) are known to
out-compete sulfate reducing bacteria (SRB) (Lovley, 1991). This
concept is a basic component of most water chemistry texts, and has been
used to describe different zones that occur in groundwater underneath
landfills that have been contaminated with leachate.
When the
groundwater gets into reducing conditions with low concentration of oxygen
and nitrate, Fe(III) is slowly released from soil and rocks to groundwater,
resulting in increases in dissolved Fe(II) concentrations. Increasing Fe(II)
concentrations in groundwater has also been reported by the Minnesota
Pollution Control Agency (1999).
Recent Studies and Presentations
Assessment of groundwater monitoring data at Aucilla landfill
A trend
has been noted in several surficial groundwater monitoring wells at the
Aucilla Area Solid Waste Facility in Madison County, Florida: iron
concentrations have been increasing. Several surficial wells have
concentrations of iron above the secondary drinking water limit of 0.3
mg/L. The iron concentrations in these wells also exceed the 4.2 mg/L
health-based criterion for iron recently developed for FDEP. These wells
are located adjacent to an operating Class I lined landfill unit. The
possible cause of the elevated iron concentrations was investigated.
A study report titled "Assessment
of Groundwater Monitoring Data from the Aucilla Landfill and the Cause of
the Elevated Iron Concentrations" was prepared by the research
summarizing the findings of the study.
Presentations
Below are the links for several
presentations made at different meetings on this and related issues.
1. Proposal Presentation for
Hinkley Center Project
2. Examining
the Problem of Elevated Iron Concentrations
3. Solid
Waste Chemistry 101 (presented at FDEP Workshop)
(To use any material from these report and
presentation file, please request permission from
Dr. Tim
Townsend)
Project
Sponsored by:
Hinkley Center for Solid and Hazardous
Waste Management
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