Interaction matrices were one of the earliest methods used. The simple matrix refers to a display of project actions or activities along one axis, with appropriate environmental factors listed along the other axis of the matrix. Many variations of the simple interaction matrix have been used in environmental impact studies, including stepped matrices (Economic and Social Commission for Asia and the Pacific 1990, Lohani and Halim 1990, and International Institute for Applied Systems Analysis 1979).

The matrix method developed by Leopold et al. 1971, is an example. The method involves the use of a matrix with one hundred specified actions and eighty-eight environmental items. Figure 2.3.1 illustrates the concept of the Leopold matrix; in its usage, each action and its potential for creating an impact on each environmental item is considered. Where an impact is anticipated, the matrix is marked with a diagonal line in the interaction box. The second step in using the Leopold matrix is to describe the interaction in terms of its magnitude and importance.

The magnitude of an interaction is the extensiveness or scale and is described by assigning a numerical value from one to ten, with ten representing a large magnitude and one a small magnitude. Values near five on the magnitude scale represent impacts of intermediate extensiveness. Assigning a numerical value for the magnitude of an interaction is based on an objective evaluation of facts.

Actions causing impact

Actions causing impact

The importance of an interaction is related to the significance, or assessment of the consequences, of the anticipated interaction. The scale of importance also ranges from one to ten, with ten representing an interaction of high importance and one an interaction of low importance. Assignment of an importance numerical value is based on the subjective judgment of the interdisciplinary team working on the environmental assessment study.

A simpler approach than the Leopold matrix can be used in an environmental impact study. Using the matrix entails considering the potential impacts, either beneficial or detrimental, of each project action relative to each environmental factor. Each interaction is delineated in terms of a predefined code denoting the characteristics of the impacts and whether certain undesirable features could be mitigated. Table 2.3.1 displays the concept of this type of an interaction matrix for a proposed wastewater collection, treatment, and disposal project in Barbados (Canter 1991). For this analysis, the following definitions are used for the codes:

SB = Significant beneficial impact (represents a highly desirable outcome in terms of either improving the existing quality of the environmental factor or enhancing that factor from an environmental perspective)

SA = Significant adverse impact (represents a highly undesirable outcome in terms of either degrading the existing quality of the environmental factor or disrupting that factor from an environmental perspective)

B = Beneficial impact (represents a positive outcome in terms of either improving the existing quality of the environmental factor or enhancing that factor from an environmental perspective) A = Adverse impact (represents a negative outcome in terms of either degrading the existing quality of the environmental factor or disrupting that factor from an environmental perspective) b = Small beneficial impact (represents a minor improvement in the existing quality of the environmental factor or a minor enhancement in that factor from an environmental perspective) a = Small adverse impact (represents a minor degradation in the existing quality of the environmental factor or a minor disruption in that factor from an environmental perspective)

TABLE 2.3.1 INTERACTION MATRIX FOR SOUTH COAST SEWERAGE PROJECT (CANTER 1991)

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