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close this bookConducting Environmental Impact Assessment in Developing Countries (United Nations University, 1999, 375 p.)
close this folder4. EIA methods
close this folder4.3 Matrix
View the document(introduction...)
View the document4.3.1 Descriptive matrix
View the document4.3.2 Symbolized matrix
Open this folder and view contents4.3.3 Numeric and scaled matrices
View the document4.3.4 The component interaction matrix
View the document4.3.5 Advantages of the matrix approach
View the document4.3.6 Limitations of the matrix approach

4.3.2 Symbolized matrix

A most noteworthy matrix presentation essentially improves the communication between the impact analysts, decision makers, and the public. In the symbolized matrices, symbols are used to capture the understanding of the impacts.

Environmental impacts may be described by words such as "important'' or "significant''. These subjective, qualitative words are difficult to deal with because their interpretation depend on cultural values and specific circumstances. Even when quantitative data are available, they must be gauged against some standard and often there is none or at least none widely accepted. There are, however, some useful guides for ranking impacts or assessing impact assessment.

There are several factors that must be taken into account when assessing the significance of an environmental impact arising from a project. The factors are interrelated and must not be considered in isolation. For a particular impact some factors may carry more weight than others, but it is the combination of all the factors that determines significance.

An example is to use abbreviations and scales, e.g., S for short term and L for long term or 10 to denote a very high order of the impact and 1 to denote almost negligible impact, etc. In this way, a symbolized matrix becomes a combination of descriptive and numeric scales. There are, however, some useful guides for the grading or classification of impacts. These are listed below:

Table 4.1 Environmental matrix for a quarry


Development action





Consents, district plan, EIS timetable

Law, regulation, public participation, employment, land values, alternative sites, justification, risks and anxieties, cultural/historical

Location of access road

Water table effects on adjacent land

Engineering design

Design of quarrying plan, resolve environmental factors, evaluate options

Landscape effects

Design of quarry, restoration plan

Design of drainage system, including sediment traps to protect water quality in the river


Access road, drainage system, site crushing plant, energy supply, traffic discharge, stormwaters, silt, sewage, site staff facilities

Cultural/historical, safety, noise, vibrations, effect on farm animals

Disposal of stripping, stability, nuisance, landform noise, blasting, drilling

Sedimentation, surface water pollution


Stripping overburden, drilling, blasting, excavation, crushing, loading, traffic, review and adjust environmental measures

Landscape, farm animals, noise/vibration/dust, emissions, safety/risk, staff facilities, working environment

Landscape effect

Progressive restoration plan, sedimentation, surface water pollution


Remove plant, check stability, replace topsoil, plant ground cover, maintenance

Safety, landscape restoration

Maintain and monitor sediment traps

• Sign of the impact
Positive or negative. This is unfortunately not that simple.

• Magnitude
This is defined as the probable severity of each potential impact. Will the impact be irreversible? If reversible, what will be the rate of recovery or adaptability of an impact area? Will the activity preclude the use of the impacted area for other purposes? The answer to these questions may be difficult and may have to be speculated on a subjective basis. The size often depends on the source release, mitigation measures adopted, if any, and the assimilative capacity of the receiving environment, etc.

• Type of change: reversible or irreversible
Irreversibilities always command attention because they signal a loss of future options. Species extinction, severe soil erosion, and habitat destruction are examples of irreversible changes. Pollution of groundwater is often irreversible because of its slow movement. Urbanization of agricultural land is virtually impossible to undo once the land use trend has begun.

• Prevalence
This is defined as the likely eventual extent of the impact as, for example, the cumulative effect(s) of a number of stream crossings. Each one taken separately might represent a localized impact of small importance and magnitude but a number of such crossings could result in a widespread effect. Coupled with the determination of cumulative effects is the remoteness of an effect from the activity causing it. The deterioration of fish production resulting from access roads could affect subsistence fishing in an area many miles away, and for months or years after the project activity has ceased.

• Duration and frequency
The significance of duration and frequency is reflected in the following questions. Will the activity be long term, short term, or sporadic? If the activity is intermittent, will it allow for recovery during inactive periods?

• Risk
This is defined as the probability of serious environmental effects. To accurately assess the risk, both the project activity and the area of the environment affected must be well known and understood.

• Importance
This is defined as the value that is attached to an environmental component in its present state. For example, a local community may value a short stretch of river for bathing or a small swamp for hunting. Alternatively, the impacted component may be of regional, provincial or even national importance.

• Mitigatability
Are solutions to problems available? Existing technology may provide a solution to a siting problem expected during construction of an access road, or to bank erosion resulting from a new stream configuration.

• Understanding
For example, if an access road is to cross a stream and the assessor does not know the extent of use of that stream (for fish spawning, fish migration, subsistence fishing, river transport, etc.), then the impact would be classed as unknown. Similarly, the nature of the river crossing (ford, bridge, ferry, or causeway) may not yet have been planned and so the significance of the environmental impact of that crossing is therefore unknown.

The assessment of significance is best done by holding at least two group discussion meetings involving interdisciplinary expertise.

The most frequently used presentation of a comparison of alternatives is a matrix, in which +, 0 and - show how each alternative affects the different environmental aspects. This can be a useful way to provide a quick overview of the differences between the alternatives.

• the impacts of each alternative are evaluated against a reference (usually the existing situation); or
• for each alternative, it is shown how it contributes to the environmental objectives; or
• the impacts of each alternative are compared with the preferred alternative.

+ + and - - give extra possibilities for differentiation.

In each case it is important that the significance of the symbols is properly defined. If necessary, a reference should guide the reader to more ample information (Table 4.2).

Table 4.2 Example of plus/minus matrix on the theme "Why does the proposed activity improve the soil condition?"

Existing situation

Proposed activity

Process alternative

Environmentally most friendly









Surface water


































Legend: - deterioration compared to the existing situation; + improvement; 0 no difference; (-) insignificant deterioration.

Table 4.3 Numeric matrix lending a quick but factual overview

Lead concentration in air (μg/m3)

NOx emission (μg/m3)

Noise level at periphery area (dB(A))

Hazardous waste produced tons/year

Alternative 1





Alternative 2





Background level existing situation





Predicted background level 2005





Environmental standard