Ecotoxicology
Transport of contaminants
The transport of contaminants in the atmosphere takes place:
Globally
Large-scale:
> 1000 km
Meso-scale:
10 - 1000 km
Micro-scale:
< 10 km
The dispersal of chemicals is a function
of their atmospheric lifetime
Boundary layer → tropopause:
> 5 days
Whole tropospheric hemisphere: > 1 month
Global troposphere:
> 2 years
Troposphere → Stratosphere:
> 10 years
Examples
NO2 ( ~ 1 day)
- only small exchange to the free tropopause
Aerosol from Chernobyl ( ~ 1 month)
- spreading to the whole Northern hemisphere
Metan ( ~ 4 years)
- mixing between both hemispheres
Chlorfluorocarbon (CFC) (~ 100 years)
- transport to the stratosphere
The global transport
Hadley
circulation
South
Equator
North
Typical forms and sizes of trace metals in aquatic systems
Metal
species
Free
metal
ions
Inorganic ion
pairs
High molecular
org. complexes
Inorg. complexes
Metals
adsorbed
onto inorganic
colloids
Metals adsorbed
Into living cells
Co-MnO2
Cu-clays
PbCO3(5)
Low mol. Wt. organic complexes
Ex.
Mn2+
NiCl+
Cd2+
HgCl4 2-
Soluble
Pb-humates
Pb-Fe(OH)3
Colloidal
Metals adsorbed
Onto or incorporated into mineral
soils and precipitates
Particulate
Size---------------- 1 nm ------------------10 nm ----------------------100 nm ----------------1000 nm----------
Concentration of chromium in the River Thames as a function of flow rate
Concentration of Cr
µg / l
15
Dissolved
Suspended
12
9
6
3
50
100
150
200
250
Average flow (m3 s-1)
Nitrate
Phosphor
Wind + Coriolis force
Thermic boundary layer
Atmosphere-land surface exchange
Dry deposition:
Wet deposition:
particles and gases
Rainfall and fog
Flux to surface = Vg • atmospheric concentration at 1 m
Vg = deposition velocity
Vg =
Flux to surface
Atmospheric concentration at 1 m
Resistance to deposition = R =
1
= ra + rb + rc
Vg
- ra : aerodynamic resistance to within 1-2 mm of the surface
- rb : the resistance of the individual roughness elements in
the boundary layer (ca 1 mm)
- rc : the resistance of the surface itself to take-up of the gas
Deposition velocity of particles depends on their size
0.001-0.1 µm
0.01-1.0 µm
1.0-100 µm
: Vg high (high diffusivity – like gasses)
: Vg small (atmospheric lifetime: 7-30 days)
: Vg high (heavy)
Washout from the atmosphere
Washout factor W
=
Concentration in rain (mg/kg)
Concentration in in air (mg/kg)
W(drops) = 200 -1000
Air:
Rain:
microgram/m3 (1 m3 air weighs 1,2 kg)
mg/l
The flux between air and sea
H = solubility of gas
K = constant
α = the gas’ coefficient of absorption
P = partial pressure of the gas
Henry’s law: H=K·α·P
Cair · H-1 = Cwater
Cair · H-1 > Cwater
Cair · H-1 < Cwater
: equilibrium (H-1 Henry’s law constant)
: net transport air  water
: net transport water  air
If:
ΔC = Cair · H-1 – Cwater
The transport of material is:
F = K(T)W · ΔC (K(T)W : ”transfer velocity”)
1
K (T )W
1
1
=
+
akW Hka
where kw and ka are the individual transfer velocities for gasses in water and air
Air – sea transfer velocity for CO2
Points:
Experimental data
Line:
Calculated
Water – sediment exchange
Dissolved
Suspended
Sediment
Water - suspended
Distribution coefficient
Concentration in solid phase material (mg/kg)
K =
d Concentration in dissolved phase (mg/kg)
Water – sediment
-less efficient due to limited extent of contact
Suspended – sediment (sedimentation)
most important for the removal of contaminants from the water
Sedimentation rates are expressed as the rate of bottom increase
-oceans
: mm / 1000 years
-lakes, estuaries : cm / year
Vand – sediment fluxe
Sedimentation
Exchange of
pore water
Net flux Fz =0 = P ( - D
P
D
C
CS
U
US
z
Physical mixing
of sediment
Upward flow of pore
water
Hydrostatic pressure
dC
+ UC + U S C S )
dz
: Porosity (volume fraction occupied by water)
: Diffusion coefficient
: Concentration in solution
: concentration in solids
: rate of pore water advection
: Sedimentation rate
: Vertical distance
Redox-active compounds
Example: Ion and Manganese
Fe2O3
MnO2
Bound to
sediment
Dissolved in
pore water
Oxic
zone
Oxic
Fe+++  Fe++
Mn++++  Mn++
Anoxic
Anoxic
zone
Atmospheric CO2-concentration
measured in Hawaii (1950-1988)
Lead concentration in
ice core from Greenland