Biomedical Engineering Reference
In-Depth Information
are more than 50 times those of the atmosphere. If groundwater is pumped to the surface and
allowed to equilibrate with the atmosphere, the concentration of dissolved CO
2
and carbonic
acid species (i.e., H
2
CO
3
*) becomes fixed at 10
4.8
M due to equilibrium with atmospheric CO
2
.
If no CaCO
3
precipitation occurs, alkalinity remains unchanged, and the pH increases from
7.25 (line 1 in Figure
9.10
) to 8.95 (line 2). At pH 8.95, however, the water is severely over-
saturated: the initial groundwater Ca
2+
concentration of 10
2.6
M greatly exceeds the equilib-
rium value of 10
4.4
M at pH 8.9. Precipitation is therefore expected. As it occurs, the alkalinity
of the water falls to 10
2.4
M and pH decreases to 8.7 (line 3), and the equilibrium Ca
concentration falls from the initial value of 10
2.6
M to a final value of 10
3.9
M. If the
precipitate resulting from this step is removed, strong acid could be added, for example as HCl,
to achieve a pH of 8, per the reaction H
+
+ HCO
3
-
>
H
2
CO
3
. Carbon dioxide would be
removed as bubbles to the atmosphere. Such a protocol would entail significant CO
2
out-
gassing. Graphically, the amount of HCl to be added for this case is given by the change in
HCO
3
(line A): 10
2.4
-10
3.1
10
3
M HCl. On the other hand, if precipitated CaCO
3
solids are not removed, even more acid must be added (7
¼
3
10
3
M HCl, line B) to convert the
carbonate in the precipitate into CO
2
.
In tests of the hydrodynamic factors affecting CaCO
3
precipitation, Hammer et al. (
2008
)
reported: “Calcite was
...
observed to grow on the gas-water interface of gas bubbles produced
from degassing of N
2
or CO
2
. Such bubbles increase the area of the gas-water interface and
thereby the total precipitation rate.” Based on this information and observations of well screen
clogging at the Schoolcraft site when an outgassing strategy was attempted, base addition to a
closed system is recommended. Moreover, since interfaces provide nucleation sites for precipi-
tation, the importance of careful well development before pH adjustment steps are implemen-
ted cannot be overemphasized.
9.8 FIELD EXPERIENCE: PILOT- AND
DEMONSTRATION-SCALE TESTING
9.8.1 Design and Site Characterization
Field tests of bioaugmentation with
P stutzeri
KC were performed at pilot-scale (Dybas
et al.,
1998
) and demonstration-scale (Dybas et al.,
2002
) in a CT- and nitrate-impacted region
of the St. Joseph aquifer located in southwest Michigan. Schoolcraft Plume A is a region of CT
contamination approximately 1.6 kilometers (km) long and 160 m wide (Figure
9.11
). Solute
transport simulations, site geology and a cost analysis are described in detail elsewhere (Hynd-
man et al.,
2000
). The aquifer consists of approximately 27 m of glacial outwash sediments,
with a water table approximately 5 m below ground surface (bgs). Average groundwater
velocity is 15 centimeters per day (cm/day). The formation is underlain by a nearly impermeable
clay unit, which appears to be lacustrine in origin. The aquifer sediments can be roughly
classified into three different zones (Table
9.3
). In addition to these three broad zones, a
narrow (5-10 cm) gravel lens was located just above the confining clay unit.
9.8.2 pH Adjustment, Inoculation and Biocurtain Colonization
In the initial pilot-scale experiment, chemicals (base and substrates) and
P. stutzeri
KC cells
were mixed, and then introduced directly into the aquifer as slug injections (Dybas et al.,
1998
).
Two key findings were obtained: (1) injection without recirculation resulted in non-uniform
delivery of chemicals and organisms, limited CT removal (60-65%), and failure to reliably
achieve groundwater CT levels below the MCL of 5 ppb; and (2)
P. stutzeri
KC could be
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