Ref. No. [UMCES]CBL 2013-021
ACT VS12-05
The time series response of the WETLabs FLCDRTD fluorometer 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 trials and
was not subtracted from the instrument response during oil additions. The background
fluorescence of the seawater ranged between 180 to 240 counts and was 75 – 85 % of the
maximum fluorescence signal in the presence of added oil. The FLCDRTD only responded to
the crude oil when Corexit 9500 was added at the typical dispersant-to-oil application ratio
(DOR) of 1:25 (Fig. 4, panel A, B, and C). The instrument response actually diminished at
higher oil concentrations when crude oil was added without dispersant. As indicated by the
physical presence of oil on the instrument at the completion of the test, this result may have
indicated a fouling of the sensor window when the oil was not effectively dispersed into small
droplets or dissolved within the seawater. The instrument response was quite similar between
the two types of crude oil, the Alaskan North Slope oil compared to the Arabian Light Crude.
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 was significantly shifted
from the region of maximum fluorescence intensity of the oil mixtures.
Cross plots of instrument response versus oil concentration and estimated EEM
QSE
intensity are shown in figure 6. Overall, instrument response to increasing oil concentration was
linear for the three treatments with dispersant, however, a noticeable non-linear increase
occurred with the highest concentration of ANS (trial 2). With the exception of that highest
reading, the response slopes were similar for these first three trials and two sources of oil, with
some offset from different background fluorescence. Again, instrument response decreased with
added oil concentrations above 1.5 ppm when dispersant was not added, despite a relatively high
EEM
QSE
intensity noted in the reference samples (Fig. 6). This behavior reflects changes in oil
droplet particle size and solubility at higher concentrations as well as potential contamination of
the optical window at higher oil concentrations in the absence of chemical dispersant. As noted
previously, a greater presence of oil was observed on the optical window for the non-dispersed
trials upon recovery of the instrument. The instrument was able to clearly detect the lowest oil
addition level at a concentration of approximately 0.3 ppm when dispersant was added.
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. The levels
of chlorophyll, CDOM and turbidity varied at the start of each day, and the relatively high
CDOM present for trial 3 (ALC, DOR 1:25) likely accounts for the higher initial background
fluorescence and higher overall readings exhibited for this test. Changes in chlorophyll and
CDOM concentrations during the step-up oil additions for a given days’ trial 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.
