Page 19 - Performance Verification Statement for the Chelsea UV Aquatracka Fluorometer

Ref. No. [UMCES]CBL 2013-014
ACT VS12-01
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 3000 mV and was 75 – 85 % of the maximum fluorescence
signal in the presence of added oil. 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 6. The instruments’
optical window used for estimating the integrated fluorescent intensities closely mapped the
region of maximum fluorescence intensity of the oil mixtures.
Cross plots of instrument response versus oil concentration and estimated EEMQSE
intensity are shown in figure 7. Overall, instrument response was logarithmic but showed a
strong response to even the lowest oil addition with a resultant concentration of approximately
0.3 ppm of oil. Again, response was greater when Corexit 9500 dispersant was added, and
response was greater for ANS oil compared to ALC oil. 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 8 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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