Geoscience Reference
In-Depth Information
Table 13-2.
Summary of i nal conceptual design
process units, primary targeted water-quality
parameters and design function. Note the role of
artii cial wetlands. Based on Nairn et al. (2009).
Targeted
parameter
Process unit
Function
1. Oxidation
pond
Fe
Oxidation, hydrolysis
and settling of iron
oxyhydroxide solids
and trace-metal
sorption
Figure 13-11.
Reclamation of mined land in progress at
Treece, Kansas. Photo by J.S. Aber.
2. Surface-fl ow
wetlands and
ponds
Fe
Solids settling
<
discharge has pH just
6, total alkalinity
∼
400 mg/L as CaCO
3
, l ow rates of 400-700 L/
minute, and elevated levels of Fe, Zn, Pb, and
Cd.
Given the nature of the target discharge, a
multi-process unit conceptual design was devel-
oped, which included six specii c treatment steps
(Table 13-2). In addition, an identical parallel
treatment train approach was deemed appropri-
ate for at least two reasons. First, the parallel
trains allow for simultaneous performance of
necessary maintenance and continued treatment.
Second, given the research focus of this site, the
parallel trains allow experimental manipulations
to be conducted. During construction, a third
mine-water discharge was discovered and incor-
porated into the design (Fig. 13-12).
The completed system includes ten distinct
process units with a single initial oxidation
pond (cell 1) followed by parallel surface-l ow
aerobic wetland-ponds (cells 2N and 2S),
vertical-l ow bioreactors (cells 3N and 3S), re-
aeration ponds (cells 4N and 4S), horizontal-
l ow limestone beds (cells 5N and 5S), and a
single polishing pond-wetland (cell 6). Cattails
are well established in cells 1, 2N and 2S; wind
and solar power provide the only necessary
energy for the re-aeration ponds (Fig. 13-13).
Mine water was diverted into the passive treat-
ment system late in 2008. Monitoring began in
January 2009, and preliminary results indicate
highly effective reduction or removal of metals
from the treated water (Table 13-3). This system
represents a state-of-the-art ecological engineer-
ing research site for passive-wetland treatment
of ferruginous lead-zinc mine waters.
3. Vertical-fl ow
bioreactors
Zn, Pb & Cd Retention of trace
metal sulfi des via
reducing mechanisms
4. Re-aeration
ponds
Oxygen
demand and
odor
Wind- and solar-
powered re-aeration.
Stripping oxygen
demand and H
2
S.
Adding O
2
5. Horizontal-fl ow
limestone beds
Zn, Mn &
hardness
Final polishing of Zn
as ZnCO
3
Final
polishing of Mn as
MnO
2
.
Adding hardness to
offset bioavailability
of any remaining
trace metals
6. Polishing pond
and wetland
Residual
solids
Solids settling.
Photosynthetic
oxygenation.
Ecological buffering
Table 13-3.
Comparison of system inl uent and
efl uent data for selected metals (mg/L) and calculated
percentage differences. SE is standard error; BDL refers
to data below detectable limits. Adapted from Aber et
al. (2010, Table 3).
Infl uent
Effl uent
Change
%
Metal
Mean
SE
Mean
SE
As
0.062
0.0005
BDL
BDL
100
Cd
0.017
0.003
BDL
BDL
100
Fe
172.51
5.54
1.055
0.439
99.4
Ni
0.893
0.012
0.049
0.024
94.5
Pb
0.063
0.009
BDL
BDL
100
Zn
8.093
0.092
0.232
0.062
97.1
