Field sample collection
Water samples were collected at specified depths for each reservoir
using a Van Dorn water sampler and were filtered with a 0.7 µm glass
fiber filter (Thomas Scientific GF/F) before being stored in
acid-washed 125 mL polypropylene bottles. Sample timestamps for
samples collected between approximately 9 a.m. and 3 p.m. were
standardized to noon in Eastern Standard Time (EST). In FCR, water
samples were collected at specified depths including 0.1 m, 5.0 m, and
9.0 m and immediately placed on ice. In BVR, water samples were
collected at specified depths including 0.1 m, 6.0 m, and 11.0 m and
immediately placed on ice. Samples for ferrozine were analyzed in the
field within 1 hour after collection. The samples for sulfate analysis
were frozen within 6 hours of collection and analyzed within four
months of their collection date.
Chemical analysis and equipment
Fe (II)
Reagents and standards:
0.5 M HCl; prepared by diluting 43 milliliters of trace-grade Fisher
Scientific 36% HCl 11.6M into one liter of reagent grade deionized
water.
2.0 M HCl; prepared by diluting 17.2 milliliters of trace-grade Fisher
Scientific 36% HCl 11.6M into 100 mL of reagent grade deionized water.
10 mM Ferrozine; prepared by weighing 2.46 grams of ferrozine and 3.85
grams of ammonium acetate and dissolving in 500 mL reagent grade
deionized water.
10 mM Fe (II) standard solution; prepared by weighing 0.392 grams of
ferrous ammonium sulfate hexahydrate and dissolving into 100
millilters of the 0.5 M HCl.
Using the Fe (II) standard solution, make serial dilutions of 2.0,
1.5, 1.0, 0.5, 0.25, 0.1, 0.05, 0.025, 0.01, 0.005 mM and a blank
using 0.5 M HCl as the solvent.
Analytical procedure:
We added 100 mL of each serial dilution to 2 mL of volatile free water
and 500 mL of Ferrozine into a cuvette. We then mixed the solution by
inverting the cuvette five times. The standards were read on a
GENESYS-6 UV-Vis Spectrophotometer set to 562 nm. Using each of the
standards, we generated a standard curve that was used for the
filtered reservoir water samples.
For the filtered water samples collected in FCR and BVR, we added 100
mL of each sample to 2 mL of volatile free water and 500 mL of
Ferrozine into a cuvette. We then mixed the solution by inverting the
cuvette five times and then immediately read them on the GENESYS-6
UV-Vis Spectrophotometer set to 562 nm. The filtered water samples
were always within the range of the standard curve in FCR and BVR.
Sulfate
The frozen samples were set out to thaw and reach room temperature 24
hours prior to analysis. The sulfate samples were measured using ion
chromatography on a Dionex DX-120 following APHA Standard Method
3125-B, with a method detection limit of 100 ppb (APHA 1992).
References:
American Public Health Association. (1992). Standard methods for the
examination of water and wastewater. American Public Health
Association, American Water Works Association, Water Environment
Federation, Washington DC.
Gibbs, M. M. (1979). A simple method for the rapid determination of
iron in natural waters. Water Research, 13(3), 295-297.
Stookey, L. L. (1970). Ferrozine---a new spectrophotometric reagent
for iron. Analytical chemistry, 42(7), 779-781.
Viollier, E., Inglett, P. W., Hunter, K., Roychoudhury, A. N., &
Van Cappellen, P. (2000). The ferrozine method revisited: Fe (II)/Fe
(III) determination in natural waters. Applied geochemistry, 15(6),
785-790.
