Environmental Engineering Reference
In-Depth Information
radiation into the deeper layers, whilst penetration of radiation at longer depth of
the water column is usually observed in water bodies with low DOC (Morris et al.
1995
). UV-B radiation penetrates at a depth of 0.1-5 m, while UV-A penetrates
at 0.2-15 m (Farmer et al.
1993
). Therefore, UV-A plays a more important role
into the photoinduced degradation of DOM in deeper layers compared to UV-B
(Piazena and Häder
1994
; Blough et al.
1993
). It can be concluded that the pho-
toinduced degradation of DOM at any depth of the water column in freshwater
systems and in oceans is greatly dependent on the penetration of light intensity.
3.9 Physical Mixing in the Surface Mixing Zone
Physical or turbulent mixing in the surface mixing zone of the water column might be
an important factor to enhance the photoinduced degradation of DOM in waters. The
mixing process allows the reactants of a chemical reaction to come more frequently in
contact, thus accelerating the reaction rate. It has been shown that the production rates
of H
2
O
2
are higher for both Suwannee River Fulvic Acid (445 nM h
−
1
) and seawa-
ter (86 nM h
−
1
) in stirred samples compared to unstirred ones (211 and 51 nM h
−
1
,
respectively) (Mostofa K, Sakugawa H unpublished data). These photoexperiments
were conducted using a solar simulator . Simultaneously, the fluorescence intensity of
fulvic acid is decreased to a higher extent in stirred samples compared to non-stirred
ones. Therefore, physical mixing is an essential factor to increase the reaction rate or
promote the photoinduced processes in natural surface waters. Moreover, production
of H
2
O
2
is merely observed in the surface mixing zone, where H
2
O
2
is derived from
the photoinduced degradation of DOM (Moore et al.
1993
; Sikorski and Zika
1993a
,
b
;
Sakugawa et al.
2000
; Johnson et al.
1989
). Furthermore, the fluorescence intensity of
fulvic acids is much lower in the surface mixing zone in lake or seawaters due to solar
effects (Mostofa et al.
2005
; Hayase and Shinozuka
1995
). Therefore, physical mixing
in the surface mixing zone is an important factor for promoting the photoinduced degra-
dation of DOM in waters. Mixing processes are typically dependent on physical factors
such as strong or weak wind, presence of artificial or natural dams, power-dam outfalls,
stream riffles, waterfall, and finally water temperature which affects the stratification-
stagnant regime of natural waters.
3.10 Increasing UV Radiation During Ozone Hole Events
The ozone hole due to stratospheric ozone depletion because of anthropogenic activi-
ties is a well-known phenomenon in the Antarctic (Qian et al.
2001
; Jones and Shanklin
1995
) and Arctic oceanic regions (Rex et al.
1997
; Randall et al.
2005
). Moreover,
the incident UV-B radiation is increased at a rate of 10-20 % per decade at temper-
ate latitudes (Kerr and McElroy
1993
; Madronich
1992
). An increase in UV-B radia-
tion may greatly enhance the production of HO
•
by inducing higher rates of photolysis
