Environmental Engineering Reference
In-Depth Information
An increase in UV radiation reaching the surface due to for instance ozone
depletion will be noticed at all depths in the water. The effect of increased UV will
affect the doses measured at certain depths. The profiles given in Figure 1 will be
moved parallel to the right (higher irradiances). The Z(1%) will remain the same,
because attenuation depends on properties within the water column.
One of the most important factors for UV attenuation in water is the content of
CDOM (coloured dissolved organic matter). Bricaud et al.
6
have shown that the
absorption coefficient (a) for natural filtered waters in the UV-visible range varies
according to the relationship
a(O) = a
o
exp[-S(O-Oo)]
where S is a parameter describing the slope of the exponential curve and a
o
is the
absorption at a reference wavelength O
o
. Equation 4 explains spectral absorption
properties in the water, thus the value of S can be used as an indicator of different
absorption properties. In many freshwater systems S lies within the range 0.010 - 0.023
nm
-1
. The slope values have been used to explain spectral absorption properties within a
water system, but care should be taken because S depends on the investigated
wavelength interval.
Laurion et al.
7
hypothesised that the diffuse attenuation coefficient (k
d
)
,
including both absorption and scattering properties, would follow the same exponential
function of wavelength in the UV spectral range as in Equation 4
k
d
(O)=k
d440
exp[-S(O-440)]
where S would be a constant and where k
d440
, the attenuation coefficient at a reference
wavelength of 440 nm, would be a function of CDOM variables.
(4)
(5)
3. Results
In studies performed in Trondheim Fjord (Norway) downward irradiance was
measured at the same station (63° 29' N, 10° 18' E) throughout the year. The station
was chosen to be representative for the Fjord system with a high circulation rate and
directly dominated by input from rivers around (Figure 4, map).
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