Gschwend%20thesis.pdf

-152- ,
(figure 4-3). Propyl benzene showed a strong negative deviation (-80%)
from its mean ratio to o-xylene in August. Ethyl benzene also showed this
effect, although less pronounced (-50%). Grob and Grob (1974) also saw
strong relative removal of these compounds in Lake Zurich when water temper-
o
atures were above 20 C. They suggested that biological degradation was
occurring by a microorganism suited to these warm temperatures and capable
of metabolizing the elongated sidechains. A warm-weather metabolic degrada-
tion of these compounds may also occur in the coastal zone near CD.
Mesitylene (1,3, 5-trimethyl benzene) showed a 3-fold relative maximum
in September at CD. This compound was also found in relative abundance in
marsh samples (Schwarzenbach et al., 1978). Possibly an unknbwn biological
source of this compound exists.
O-xylene in Seawater from Other Vineyarò Sound Stations.
Four investigations of the concentrations of the C2- and C3-alkylated
benzenes in surface water from Vineyard - Nantucket Sound were conducted.
Samples were obtained within 3 km of CD. Salinity data are available from
3 of these studies (figure 4-4) and, along with knowledge of the tide at
the time of sampling, have been used to draw the lines of flow shown.
Salinity always increased offshore from CD. It appears that the shoals,
about 2 km off CD, do not provide a major barrier to mixing of surface
waters.
The concentrations of o-xylene in seawater from these stations are
shown in figure 4-5. No trends were consistently observed. Al though there
were always significant concentration differences, e.g. between samples from
stations at CD and 0.8 km. offshore, sometimes offshore levels were lower