Site-specific risk assessment and management of point and diffuse sources - Environmental Deterioration and Contamination: Problems and their Management - page 348

i) Mapping and characterizing the point and diffuse sources in the watershed.

ii) Delineation of the primary/significantly emitting point and diffuse sources on

the map.

iii) Watershed scale modeling of the water flow using GIS-tools.

iv) Quantification of the water fluxes according to the water balance of the

watershed.

v) Determination of the emission or runoff flux of the delineated contaminant

sources.

vi) Validation of the flow accumulation model using the water flux/flow of the

creek.

vii) Estimation of the contaminant input from point and diffuse sources.

viii) Calculation of the risk reduction and contaminant retention capacity of the

watershed/ (a black box from this point of view) comparing the quantity and
quality of the input contaminant to the black box (entrance into the black
box) with the existing contaminant concentration in the creek (exit of the
black box).

ix) Having the delineated sources and the flow accumulation - and assuming an

average and homogenous retention capacity - one can model the effect of the
elimination/removal of any source (area) on the metal content of the creek.

x) Targeting no-risk concentration in the creek, the model may be used in inverse

mode for planning the risk reduction measures (removal, isolation or other ways
of contaminant transport mitigation.

Calculations

- i and ii) mapping and delineation of subareas on digital maps:

◦ The total surface area of the pollution sources: 192 000 m 2

Watershed area of the pollution sources: 822 800 m 2

◦

Diffusely contaminated watershed: 1 km 2

◦

Total watershed area: 15 km 2 ◦

- iii, iv, v, vi) based on meteorological data and the GIS-based flow accumulation

model:
◦

Input water to the watershed area: annual rainfall (20-year average) = 756 mm/

year

◦ =

Distribution of the rain: runoff 326 mm/year

◦ infiltrated, evaporated and transpired waters = 430 mm/year.

◦ The output flow of the watershed is the average flow rate of the creek =

2450 m 3 /day.

◦ Total runoff volume from the diffusely polluted area and its watershed =

333 825 m 3 /year.

- vii) Estimation of the metal emission from diffusely contaminated area the

contaminant input to the black box. Based on microcosm and lysimeter models:
◦

=

As: 99 kg/year in 333 825 m 3 /year = 296 µ g/L

Cd: 186 kg/year in 333 825 m 3 /year ◦ = 557 µ g/L

Pb: 455 kg/year in 333 825 m 3 /year

◦ = 1366 µ g/L

Zn: 24 306 kg/year in 333 825 m 3 /year

◦ = 72 772 µ g/L