Ref. No. [UMCES] CBL 2013-020
ACT VS12-03
The time series response of the CDOM, Crude Oil and Refined Fuel sensors to the
stepwise addition of crude oil and dispersant are plotted in figures 8, 9, and 10, respectively.
Each lettered panel represents a day-long test of specific source oil and dispersant ratio at seven
different concentrations, ranging from background to approximately 12 ppm oil (see figure
legends). The highest concentration was not tested on day 1, but this challenge experiment was
repeated on day 3 during which the highest concentration level was included. The background
fluorescence of the incoming seawater varied with each daily trial averaging between 100 – 300
RFU for the CDOM sensor, 100 – 450 RFU for the Crude Oil sensor, and 10 – 20 RFU for the
Refined Fuel sensor. The elevated ambient background fluorescence is also evident in the non-
zero EEM
QSE
values at the start of each trial series (Figs. 14, 15, & 16, panel B).
Representative EEM maps from reference samples collected after the fourth oil addition
(mass added ca. 85 grams; concentration ca. 3 ppm) are presented in figures 11, 12, and 13, with
the optical windows used for estimating the integrated fluorescent intensities of the CDOM,
Crude Oil, and Refined Fuel sensors overlaid on the maps, respectively. The degree of overlap
of the optical windows to the region of maximum fluorescence of the oil mixtures varies for each
of the sensors on the C3. Overall, the Crude Oil sensor had the largest potential EMM
fluorescence intensity based on both the bandwidth size and location and produced the greatest
instrument response.
Cross plots of instrument response versus oil concentration and estimated EEM
QSE
intensity clearly reveal differences in the detection capabilities of the three individual sensors
within the C3 unit (Figs. 14-16). In all cases, the background reading in seawater was not
subtracted from the instrument response during oil additions in order to reveal response
characteristics above ambient background.
The response of the CDOM sensor to the oil
additions was minimal and positively linear only in trials conducted with the chemical dispersant
Corexit 9500 (Fig. 14, panel A).
The response of the Crude Oil sensor was significantly greater
than that of the CDOM sensor but again only responded positively linear when chemical
dispersant was added in addition to physical wave dispersion (Fig. 15). The Refined Fuel sensor
had an overall lower response signal, reflective of the smaller range magnitude and range in
EMM Intensity within the optical window (Fig. 16). In addition, this sensor showed a positive
response to all of the oil addition treatments, even without chemical dispersion. However, when
dispersant was not added, the instrument response plateaued for concentrations greater than
approximately 2-3 ppm oil. It is not clear why the response of all three sensors differed between
day 1 and day 3 experiments using the same source oil with dispersant, beyond the small
differences that occurred from the initial ambient background signal.
Figure 17 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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