Site-specific risk assessment and management of point and diffuse sources - Environmental Deterioration and Contamination: Problems and their Management - page 347

Table 10.4 Average and maximum metal contents in the creek

Average Maximum Target concentration Reduction (x fold)

As ( µ g/L) 50 120 10 5-12

Cd ( µ g/L) 2 8 1 2-4

Pb ( µ g/L) 30 80 10 3-8

Zn ( µ g/L) 800 1200 100 8-12

the relevant risk reducing fate and transport processes in the watershed from the source
to the compliance point. These processes include volatilization, diffusion, dispersion,
dilution, sorption, plant uptake, physico-chemical transformation, biotransformation
and biodegradation.

Case study 3

Diffuse pollution—from abandoned primary and diffuse secondary sources in a zinc
and lead mining area—poses threat to a creek, the main surface water flow of the
watershed. The environmental risk has to be assessed and the necessary risk reduction
rate should be planned. Also, the risk reducing/mitigation efficiency of the selected
measures should be demonstrated. The management and the tiered risk assessment
consist of three steps, applicable both in forward and inverse modes.

Tier 1
Concept

Calculation of the quantitative risk based on contaminant concentrations from long-
term monitoring data of the creek water. Comparison of the actual risk to the desired
RCR=1 value and determination of the necessary extent of risk reduction. In this tier
the watershed is handled as a black box, assuming an unknown metal input/inflow
into it from diffuse sources, resulting in the dissolved metal concentration in the (Toka)
creek water, as the output/ouflow of the black box.

Calculations

Average and maximum metal contents of the Toka creek water and the site-specific
target is shown in Table 10.4.
According to these data, most of the mine waste (92%) (both from point and
diffuse sources) should be eliminated to reach the target metal concentrations in the
Toka creek water. This approach is definitely not feasible at watershed scale.

Tier 2
Concept

Combination of the GIS flow accumulation model with the black box approach for
contaminant transport and fate modeling.