Site-specific risk assessment and management of point and diffuse sources - Environmental Deterioration and Contamination: Problems and their Management - page 369

6.4 Modeling of the fate and transport of diffuse pollution
from mining
Nonpoint source pollution from mining affects large areas where the source is consid-
ered to be made up of an infinite number of points. The dominant transport pathway
is the runoff water delivering dissolved metals and suspended solids from eroded mate-
rial. Dissolved metals are transported into the watershed: solids are deposited in low
lying areas or in the stream sediment. The transport model predicts the watershed-
scale contaminant emission from the source based on which the PEC of the watershed
(or of any other point of the mapped area) is forecasted according to the site-specific
conditions in the watershed. To calculate the PEC, we need the following information:

- Water balance of the site quantifying the water inflow into and the outflow from the

watershed. It shows the proportion in which the incoming rainfall is distributed in
the watershed: infiltrated water (pore water, soil moisture, contaminated leachate,
subsurface water), surface and subsurface runoff, water in biomass, water by
evapo-transpiration;

- The size of the delineated diffusely polluted subareas: in the case of diffuse mine

waste, the polluted subareas are identified as bare surfaces without any vegetation;

- Estimated thickness of the mine waste layer in the bare areas, or an estimated mass

of the abandoned deposits;

- The average contaminant (toxic metal) content of the diffusely scattered mine

wastes;

- Partition of the contaminants (metals) between solid waste and runoff water;

- Digitized maps of the watershed and of the mining site and information on land

covers and land uses.

Delineating the diffusely polluted areas on the map, the accumulated water flow
is calculated from the GIS-based transport model, treating the delineated subareas as
individual units with water input (direct and flow-through rain) and output. Knowing
the average contaminant content of these individual subareas, we can calculate the
hazardous contaminant concentration and the amount transported by the output flow.
The solid phase-related contaminant emission can be predicted, for example,
by the revised universal soil loss equation (RUSLE) (Foster, 2005) using geographic
resources analysis support system (GRASS GIS) (Raghunath, 2002). This model results
in the average annual soil erosion rates (t/ha/year) using six factors: rainfall erosivity,
soil erodibility, topography, land cover, land use and soil protection. The model can
predict contaminant emissions from the eroded solid material at watershed and/or sub-
site levels using typical contaminant concentration ranges, rainfall and soil conditions
of the area (Gruiz et al , 2008).
A case study on the management of point and diffuse contamination from an
abandoned base metal sulfide mining area in Hungary is presented in Volume 5.

6.5 Measures to reduce diffuse pollution from mining
Nonpoint sources in general cannot be exactly delineated and removed, and this may
also be true for mining. In many cases, however, diffuse pollution from mining can be