| Iron
and Manganese Analysis
Well
conditions:
A large production well with high manganese and iron concentrations
was analyzed under dynamic (pumping) conditions. After a spinner
analysis, water samples were taken from different depths and
immediately tested for iron and manganese. Split samples were
also collected and send to a laboratory.
Test
conditions:
Pumping was started about 7 AM (two hours before the first
sample was collected). Starting around 9 AM, samples at the
wellhead were regularly tested for iron and manganese
Test
result:
The iron concentration varied significantly throughout the
day. The concentration decreases from a high at 9:30 AM of
0.88 mg/l to a low at 3:30 PM of 0.46 mg/l; a decrease of
almost 50%. However, around 12 PM more stable iron concentration
were observed (see chart below). Manganese concentrations
were very stable all day. By testing for iron and manganese
immediately at the site, information was gained, which showed
that the well was not stable concerning iron concentrations.
This is most likely caused by the short period of pumping.
Mass
balance analysis:
The depth specific sampling was delayed by a few hours until
more stable iron concentrations were observed after 12 PM,
because the quality of the mass balance analysis relies heavily
on stable concentrations for the parameters tested. Therefore,
knowledge about the unstable and decreasing iron concentrations
improved the quality of the mass balance analysis significantly.
Comparison
of on-site data with laboratory results
Iron:
The observed concentrations from the on-site testing were
generally higher than the laboratory results. One explanation
for the difference is that iron precipitation might have formed
during the time delay from sampling to testing in the laboratory,
which was more than 30 days. The tendency to form solid iron
hydroxides is strongest in the samples with high dissolved
iron concentrations. In fact the largest differences between
on site and laboratory results are observed in the samples
from the deepest formation, which both contained the highest
iron concentrations.
Manganese:
The was little difference between the on-site and laboratory
data. The largest difference was observed in the two samples
from 1450 ft and 1495 ft, which had the highest iron concentrations.
In these two samples it is likely that manganese co-precipitated
with the iron hydroxide precipitation.
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