Go to Learn - Step 2 Go to Learn - Step 4

In this section you will learn how to assess the impacts of the planned project (and its alternatives) on environmental and socioeconomic conditions, and then identify mitigation measures to reduce those impacts as well as potentially make positive contributions.

What is impact assessment and mitigation?

This step is the core part of an EIA. Impact assessment refers to the detailed evaluation of the environmental and social impacts of the planned project and identified alternatives, compared to the baseline conditions. Mitigation refers to minimizing or avoiding the described impacts.

This step is the core part of an EIA.

Impact assessment refers to the detailed evaluation of the environmental and social impacts of the planned project and identified alternatives, compared to the baseline conditions. This includes qualitative descriptions such as measuring high, medium and low impacts, and quantitative descriptions, such as indicating the cubic metres of water withdrawn, sewage produced, and pollutants released. This is done for the planned project as well as the identified alternatives, allowing for comparisons. Once the detailed assessment is complete, mitigation measures to reduce or avoid impacts are identified.

Mitigation refers to minimizing or avoiding the described impacts.Overall, mitigation measures are a response to the findings of impact assessment; they need to cover all the areas identified. The key focus of mitigation actions should be on:

  • Preventive measures that avoid the occurrence of impacts and thus avoid harm or even produce positive outcomes.
  • Measures that focus on limiting the severity and the duration of the impacts.
  • Compensation mechanisms for those impacts that are unavoidable and cannot be reduced further.

Key impacts and potential mitigation actions often relate to land. Almost all development proposals involve disturbance of the land surface. This is usually extensive for major linear projects (roads, pipelines), dams and reservoirs, and large-scale mining, agriculture, forestry and housing schemes. Environmental impacts of particular concern can include drainage of wetlands, conversion of natural areas, or expansion into areas that are vulnerable to natural hazards.

Why conduct impact assessment and mitigation?

The purpose of conducting an EIA is to clearly identify and understand (assess), and then prevent or minimize (mitigate) the adverse impacts of the planned project on the environment and people.

The purpose of conducting an EIA is to clearly identify and understand (assess), and then prevent or minimize (mitigate) the adverse impacts of the planned project on the environment and people. Because the impacts of the project can change over time, the impacts need to be assessed and measured over the lifetime of the project – from its construction through to operations and after closing.

Impact assessment is indispensable in order to provide systematic and detailed descriptions of the probable impacts in comparison to the identified project alternatives. Mitigation measures are a critical part of the EIA process, as these actions aim to prevent adverse impacts from the planned project on the environment and people, ensuring that unavoidable impacts are maintained within acceptable levels.

The key contributions of impact assessment and mitigation to a good EIA include:

  • It provides a clear and itemized list of relevant impacts on the environment and people, including cumulative effects, social impacts, and health risks.
  • Based on the results of the impact assessment, a detailed list of mitigation actions is identified.

What approaches exist?

Development BanksCentral AmericaExamples

Impacts and related mitigation actions are evaluated and identified according to the key environmental, social and cultural characteristics of the area where the project will be implemented.

Impacts and related mitigation actions are evaluated and identified according to the key environmental, social and cultural characteristics of the area where the project will be implemented. Characteristics are often used to describe impacts include: magnitude and frequency; duration; spatial distribution; reversibility; positive/negative effects; likelihood of occurrence; direct/indirect effects; and cumulative effects.

At the global level, international agencies and banks often provide a list of categories/subcategories that the EIA needs to include, such as environment/air, water, land. They also specify the types of impacts that need to be assessed, such as direct, indirect and cumulative (see table below).

TableCategories and subcategories of impact assessment

Categories Subcategories
Environment Land use and soil: soil removal and pollution Land-cover change: wetlands, floodplains, mangroves, other subsoil (composition, depth, etc.); surface (types and distribution, characteristics, uses, etc.); Topography (altitude, gradients, relief variations, orientation, etc.). Sewage release (treated, untreated) Groundwater (location, description of aquifers, recharge areas, level of use, etc.); Surface water (location and description of areas that could be affected by the project; identification of use of surface water; description of drainage areas and channels; discussion on potential for floods, sedimentation, erosion, and eutrophication of water sources). Air: emissions, noise, odour, dust; Climate (rainfall, temperature, radiation, fog, wind, etc.); Air quality (description of existing air quality levels); identification of sources of pollutants; fragile receptors in the area; description of supervision programs. Biodiversity: impacts on endangered species; trees and canopy, migratory birds, indicator species and different groups of species and introduction of invasive species; Vegetation and terrestrial and aquatic flora (identification of types and characteristics of local species; ); Land and aquatic fauna (identification and discussion on wildlife species and their characteristics); Fragile areas (identification and characteristics).
Social issues Characteristics of the population (parameters, growth projections, etc.); Sociocultural features (ethnic minorities, habits, population of special interest). Present land use and zoning; Land-use plans (utilization or master plans, including the project area and surroundings; discussion of future trends or development pressures, etc.); Resettlements. Community impacts: impact on poor, low income people; impacts on culture, heritage; social impacts (access to jobs, food, energy), aesthetics. Visual resources (physical description of the community); natural areas of significant scenic value; identification of structures with significant architectural design; etc.); Historic and archaeological resources (listed historic areas or structures, according to national or community designation; sites of potentially significant archaeological value).
Other Disaster management Adaptation and increasing vulnerability to climate change

Development BanksDifferent types of impact assessment & mitigation actions of selected development banks

Details on impact assessment procedures
Inter-American Development Bank (IDB) An EIA needs to account for indirect and cumulative impacts and include associated facilities. It needs to include mitigation measures to address adverse impacts. Bank-financed operations require mitigation measures; and for impacts that cannot be fully mitigated, compensation or offsets should be implemented. Here, the second consultation should preferably occur when the impact assessment is being reviewed, in order to inform, gather comments, and adjust the assessment and the corresponding environmental and social management plan. The process must have the necessary budget, time, and human resources to ensure good technical quality. When the project team considers it necessary, the Bank may complement the sociocultural evaluation process carried out by the project proponent with its own studies and evaluations.
World Bank Impact assessment takes into account the natural environment (air, water, and land); human health and safety; social aspects (involuntary resettlement, indigenous peoples, and physical cultural resources) and transboundary and global environmental aspects. It considers natural and social aspects in an integrated way. It also takes into account the variations in project and country conditions; the findings of country environmental studies; national environmental action plans; the country’s overall policy framework, national legislation, and institutional capabilities related to the environment and social aspects; and obligations of the country, pertaining to project activities, under relevant international environmental treaties and agreements.
European Union Evaluation of the environmental impacts in the baseline scenario and then comparing the impacts of the outlined alternatives in scoping. It also includes an assessment of environmental and wider socioeconomic impacts in a qualitative and quantitative manner. Alternatives that have the same impacts can be group together and then separately the differences in impacts between the alternative need to be highlighted.

Source: IDB, 2011; World Bank, 1999; European Union, 2001

In Central America, national legislation specifies the type of environmental resources which need to be assessed.

Central AmericaApproaches to impact assessment and mitigation in selected Central American countries

Country Impact assessment and mitigation requirements
Belize Must include direct, indirect, cumulative and short- and long-term impacts on humans, flora, fauna, soil, water, air, climatic factors, material goods, cultural heritage, landscape, natural resources, and ecological balance.
Costa Rica Identification of positive and negative, direct and indirect, cumulative, and synergistic impacts on physical, chemical, biological, social, and cultural conditions and environments. EIA must predict and evaluate potential impact; and proposed prevention, mitigation, or compensation measures.
Dominican republic Identification, prediction, and control of environmental impacts. “Impact” is defined as any significant alteration, positive or negative, of environmental categories and natural resources, caused by human activity or natural events.
Ecuador Identification and evaluation of impacts on people and biodiversity; economic valuation of impacts, EMP, and mechanisms for citizen participation.
El Salvador Must consider positive and negative impacts on physical, chemical, biological, socioeconomic, and cultural conditions at the site and in the site’s area of influence. EIA must include a EMP.
Guatemala Identification, prediction, and evaluation of environmental impacts of a project, work, industry, or activity.
Honduras Description of project’s components and project site; alternatives considered; results of consultations; evaluation of potential impacts on the physical and social environment; and plans for mitigation, contingency, and risk management.
Mexico Environmental forecasts, and identification, description, and evaluation of the following impacts: environmental, cumulative, synergistic, significant or relevant, and residual.
Nicaragua Identification, prediction, and oversight of environmental impacts of projects and of their alternatives.
Panama Identification, analysis, evaluation, and ranking of all negative and positive impacts and induced risks.

Source: Tiffer-Sotomayor et al., 2015


Methodological approaches to impact assessment combine qualitative and quantitative methods depending on the available information and data. These methods include multi-sectoral and sectoral modelling such as hydrological modelling, case studies to address specific challenges, and expert-based assessment of specific impacts where other more exact approaches are not available. Most commonly used methods are:

  • Expert judgment
  • Quantitative physical and mathematical models
  • Cumulative impact assessment
  • Matrices and interaction diagrams
  • Rapid Impact Assessment Matrix (RIAM)
  • Battelle Environmental Evaluation System

Read more about each of these methods here.

More recently, there has been an increasingly stronger focus within EIAs to assess the social impacts and consequences of the planned project and alternatives. This has been formulated in a standardized procedure called Social Impact Assessment (SIA).

Mitigation actions to address impacts: The development of alternatives to a proposed project is part of a comprehensive approach to mitigation. Specific mitigation actions are described in project alternatives to reduce or avoid the identified impacts. Examples of these include changes in the technological process to eliminate organic effluents in fish meal plants, replanting of vegetation on slopes after road construction, building additional protection or using trees to limit noise, and training people for new kinds of jobs.

There are four major guidelines for devising mitigation actions:

  • Identify the mitigation actions that most reduce the impacts, and that are practical and feasible to integrate into the planned alternatives.
  • Identify mitigation actions that reduce the severity of the impacts to the lowest possible level over the lifetime of the project, to avoid irreversibly and cumulative effects.
  • Include activities designed to monitor compliance with agreements established in the assessment, and provide specific and timely information on the environmental conditions and social variables in the area under study.
  • Identify potential risks of accidents, malfunctions and other emergencies that may occur during all the project phases.

The mitigation hierarchy is a widely accepted approach for environmental conservation. It is a set of prioritized steps to limit negative impacts as far as possible through avoidance, mitigation (or reduction), restoration, and offsetting (compensation).

mitigation hierarchy graph

Source: Adapted from BBOP, 2009

How to conduct impact assessment and mitigation?

Impact assessment and mitigation is done by a multidisciplinary team of experts who have the skills and qualifications to assess diverse environmental and social impacts.

Impact assessment and mitigation is done by a multidisciplinary team of experts who have the skills and qualifications to assess diverse environmental and social impacts.

The team begins by systematically assessing the impacts of the planned project and its alternatives, using one or more assessment methods, such as those described in the downloadable resource Impact Assessment Methods. Ideally, it would also carry out a Social Impact Assessment (SIA). The outcomes of these assessments are then summarized in the form of a matrix.

These are the key steps for conducting impact assessment and related mitigation:

  1. Perform a detailed assessment of impacts of all project phases on the environment, socioeconomic systems and other areas as requested by the designated agency’s guidelines and legislation, taking into consideration regional and international best practices.
  2. Based on the completed analyses, conduct an assessment of cumulative impacts.
  3. Compile similar impacts into groups in order to make the impact analyses easier to understand.
  4. Identify mitigation actions to eliminate and/or reduce the identified impacts.
  5. Identify specific mitigation measures to reduce cumulative impacts.

In Honduras, impact assessment for the reference areas must consider the total and net project areas. Identification of environmental impacts is conducted using a Leopold Matrix – a method based on the completion of table fields, which assists in conducting an inventory of the identified environmental impacts.


Read more about how the Leopold Matrix is done here.

Once the more significant environmental impacts have been identified and placed in order (according to either the construction or operational phase) and according to relevant environmental factors, the impacts are then evaluated according to Matrix of Importance of Environmental Impact (MIIA). This methodology scores each of the identified environmental impacts from 1 to 100, to produce a total score of environmental impacts generated by the activity or work.

The outcomes of the impacts assessment and mitigation include:

  1. A list of impacts and description of the severity of the impacts over the lifetime of the project in the context of the environmental, social, cultural and aesthetic resources and issues using the Leopold Matrix.
  2. A grouping of similar types of impacts using the MIIA methodology.
  3. A list of mitigation actions linked to the groups of impacts.
  4. The presentation of the groups of impacts and related mitigation measures in a table.