Site-specific risk assessment and management of point and diffuse sources - Environmental Deterioration and Contamination: Problems and their Management - page 371

the transport pathways. This objective can be achieved by preventive, restrictive or
remediation measures (EPA, 2000). Preventive or restrictive measures on the use of the
already existing diffusely contaminated, abandoned mining sites cannot be taken, but
the transport rate and the mobility of the contaminants in the contaminated area can
be reduced by water treatment and soil remediation technologies.
Traditional AMD and ARD treatment technologies aim to protect surface waters
downstream from the mine outflow treating it with lime followed by sedimentation of
the lime precipitate.
The same problem can be solved by passive systems such as reactive soil zones
or subsurface reactive barriers and ecological water treatment technologies in ponds,
requiring minimum energy and labor.
Passive treatment of acidic or alkaline mine waters can hinder both point and
diffuse contamination of surface and groundwater systems. Passive treatment of mine
waters uses chemical and biological processes to decrease metal concentrations and
neutralize acidity. The three most common types of passive technologies are aerobic
ponds and wetlands, anoxic limestone drains (ALDs) and reducing and alkalinity pro-
ducing systems (RAPS). Aerobic wetlands promote mixed oxidation and hydrolysis
reactions and are effective when the mine water is net alkaline. ALDs generate bicar-
bonate alkalinity and can be used to convert net acidic waters into net alkaline water
for treatment in aerobic ponds and wetlands. RAPS extend the concept of ALDs by
pretreating the water before it contacts the limestone to eliminate dissolved oxygen
and reduce dissolved ferric iron to ferrous iron. These systems can generally be used
to treat more acidic water than ALDs.
The uncontrolled disposal of metal sulfide or sulfur containing mine waste, sulfide
minerals containing radioactive elements cause severe problems all over the world.
Dispersion of the diffuse acidic leachate may be prevented by a life cycle approach in
mining. Ore transportation in the mine area, ore fallen off from trucks and mine cars
show their fingerprints even after 40-50 years.
Combined chemical and phytostabilization proved to be the most effective reme-
diation alternative for diffusely contaminated mining areas (Volume 5), resulting in
the decrease in the runoff flow and improved water quality (Feigl et al., 2007).
The watershed-scale approach enables modeling the emission reduction effect of the
selected remediation technology if the PEC at watershed scale is known and the effi-
ciency of the planned remediation technology has been forecasted based on microcosm
and field experiments (Gruiz et al., 2009).
Chemical stabilization is an in situ approach applied over a wide range in the
case of metal contamination of mining origin. Chemical stabilization of contaminants
in the soil using additives is based on the following physico-chemical and biological
processes:

- Positive metal ions form hydroxide or oxy-hydroxide precipitate with lime or other

reactive chemicals present in the soil or added to the soil, which can remain in this
form if no acidification is going on in the soil;

- Chemically or microbiologically induced low redox potential in the soil or in sur-

face water sediments is able to keep some of the metals in completely immobile
(stable) physico-chemical form, e.g., precipitates of CdS, ZnS, CuS, PbS;