Biomedical Engineering Reference
In-Depth Information
Ca
2+
concentration of 10
2.6
M (dashed line labeled “
Groundwater
[
Ca
2+
]”), this groundwater
can be considered saturated with respect to CaCO
3
at a temperature of 15
C. However, the
actual temperature of Schoolcraft groundwater is 13.1
C. At this temperature, the groundwater
will be slightly undersaturated.
In order to increase pH to 8, strong base can be added, for example as NaOH, according to
the reaction OH
+H
2
CO
3
HCO
3
. Graphically, the NaOH to be added can be determined
from the change in H
2
CO
3
levels as pH increases from 7.25 to 8: 10
3.1
-10
3.8
!
10
4
M
NaOH (line
A
). How this base is introduced into the aquifer becomes critical. It would be
unwise to add it directly into the wells as a high pH solution near the well screen would result
and scaling would ensue. Extraction of groundwater, with aboveground mixing of base to
achieve the target pH allows a gradual titration of subsurface solids. Such a strategy was
successfully implemented at Schoolcraft, Michigan (Dybas et al.,
2002
).
In theory, pH adjustment could be accomplished by allowing aboveground degassing of
carbon dioxide. Figure
9.10
illustrates the expected situation for Schoolcraft groundwater
pumped to the surface and allowed to equilibrate with the atmosphere.
When groundwater is allowed to equilibrate with the atmosphere, outgassing of CO
2
can be
expected because carbonic acid and dissolved levels of CO
2
far exceed the levels that would
be in equilibrium with atmospheric CO
2
. At the Schoolcraft site, CO
2
levels in the groundwater
¼
6
Figure 9.10. pC-pH diagram for Schoolcraft groundwater at 15
C in an open system. Line 1
represents initial pH. Line 2 indicates conditions after degassing of CO
2
, with calcite still present.
Line 3 indicates conditions after degassing of CO
2
and calcite precipitation. Line A represents acid
added to decrease pH to 8 if precipitate is removed from solution. Line B represents the acid added
to decrease pH to 8 if precipitate is not removed from solution.
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