Ref. No. [UMCES]CBL 2013-018
ACT VS12-04
The time series response of the Hach FP 360 sc to the series of oil and dispersant
additions is plotted in figure 4. Each lettered panel represents a day-long test of specific source
oil and dispersant ratio at seven different concentrations including ambient background (see
figure legend). The highest concentration was not tested on day 1, but this whole experiment
was repeated on day 3 during which the highest concentration level was included. Background
fluorescence of the source bay water was similar for all five days and was not subtracted from
the instrument response during oil additions. The background fluorescence of the seawater
averaged around 200 ppb Oil compared to the maximum fluorescence signal of 1800 ppb Oil in
the presence of added oil with dispersant and 900 ppb Oil in the presence of oil without
dispersant. Fluorescence response was slightly greater for the Alaskan North Slope oil
compared to the Arabian Light Crude, and the response to both oil sources was enhanced when
the dispersant Corexit 9500 was added at the typical dispersant-to-oil application ratio (DOR) of
1:25.
Representative EEM maps from reference samples collected after the fourth oil addition
(mass added ca. 85 grams; concentration ca. 3 ppm) are presented in figure 5. The instruments’
optical window used for estimating the integrated fluorescent intensities closely mapped the
region of maximum fluorescence intensity of the oil mixtures particularly that of Alaskan North
Slope oil.
Cross plots of instrument response versus oil concentration and estimated EEM
QSE
intensity are shown in figure 6. Overall, instrument response was linear with oil added up to 1.5
ppm and asymptotic above dependent upon the crude oil source and dispersant combination. This
behavior reflects changes in oil droplet particle size and solubility at higher concentrations as
well as reduction in sensitivity due to contamination of the optical window at higher oil
concentrations, particularly in absence of chemical dispersant. The instrument was able to clearly
detect the lowest oil addition level at a concentration of approximately 0.3 ppm. Again, the
instrument response was greater when Corexit 9500 dispersant was added (maximum of 1800
ppb versus 570 ppb), and response was greater for ANS oil compared to ALC oil (maximum of
1800 ppb versus 1200 ppb). Some of the difference between the two source oils may have
resulted from the ALC source oil being 7% weathered from previous handling (Paul Kepkay,
BIO, personal communication).
Figure 7 summarizes various water quality parameters over the course of the five tests.
Concentrations of chlorophyll, CDOM, and turbidity were conducted on discrete reference
samples, while particle concentration estimates were generated in situ with a LISST. Although
levels of chlorophyll, CDOM and turbidity varied at the start of each day, their effect on the
initial background fluorescence of the seawater was relatively small. Changes in chlorophyll and
CDOM concentrations during the step-up oil additions were relatively small. Turbidity
increased almost linearly when dispersant was present with the oil, but showed little change to
increasing oil concentrations above 1.5 ppm without dispersant. Similarly, the increase in mean
particle concentrations was much greater in the presence of dispersant than without, indicating a
physical repacking of the oil is also taking place, which would likely account for much of the
differences in fluorescent response of the test mixtures.
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