What is an environmental impact assessment (EIA)?
An environmental impact is any change a project makes to air, water, soil, plants, animals or people. An EIA is a study done before the project starts. It asks: what will change, how bad will it be, and what can we do about it?
- Screening: does this project need an EIA?
- Scoping: which effects must be studied?
- Baseline study: measure how things are now (clean air, river quality, trees).
- Predict the impacts and plan measures against them.
- Public hearing: local people can ask questions and give views.
- Decision: approve, approve with conditions, or refuse.
- Monitoring: check after building that the measures work.
Preventive measures: stop harm before it starts
Preventive measures (also called avoidance or mitigation at source) stop the harm from happening at all. They are the best choice because nothing needs to be repaired later, and they are usually cheaper.
- Choose the right site: keep away from rivers, wetlands, forests and homes.
- Change the design: make the project smaller, or use cleaner machines.
- Add controls: smoke filters, a waste-water tank, noise walls, dust covers.
- Plan the timing: do noisy work outside the nesting season of birds.
Corrective measures: repair harm that has happened
Corrective measures (also called restoration) repair the damage after it has happened, or reduce it once it is there.
- Clean a river after a spill.
- Fill a mine pit and cover it with soil.
- Replant trees on the cleared site once work is done.
- Treat polluted soil or water.
They work, but slowly. A replanted forest needs years to grow, and some harm, like a lost species, cannot be undone.
Compensatory measures: make up for harm that cannot be repaired
Sometimes harm is left even after prevention and correction. Compensatory measures (also called offsets) give back something of equal value somewhere else.
- Plant a new grove on a bare hill for a forest that was cut.
- Protect a wetland elsewhere for a wetland that was filled.
- Pay for a community to get clean water if its well was spoilt.
Compensation is the last step. A new grove is not the same as an old forest: the animals and the soil are different. So we only use it for what is left after prevention and correction.
The right order: avoid, reduce, repair, compensate
Experts call this the mitigation hierarchy. Work down the list and stop when no harm is left: 1. Prevent, 2. Reduce what cannot be prevented, 3. Correct what has happened, 4. Compensate for what is left. After this, monitor the result: measure air, water and trees at regular times and fix the plan if the numbers drop.
Key formulas and definitions
- Order of action: prevent โ reduce โ correct โ compensate โ monitor
- Preventive = before harm; corrective = after harm, on the same site; compensatory = elsewhere, for harm that is left
- EIA steps: screening โ scoping โ baseline โ prediction and measures โ public hearing โ decision โ monitoring
Worked examples
1. A cement factory adds a dust filter to its chimney. Which kind of measure is this, and why?
Preventive. The filter stops most dust from reaching the air in the first place, so little harm happens.
2. After a mine closes, the company fills the pit and plants trees on the same land. Which measure is this?
Corrective (restoration). The harm already happened on this site, and the company repairs it.
3. A road must cut through 40 hectares of forest and cannot avoid it. The builder protects 80 hectares of forest in another district. Which measure is this and why is it last?
Compensatory. It gives back something elsewhere for the forest that cannot be saved. It is last because the new forest is not the same as the old one, so we first try to prevent and correct.
4. List the EIA steps in order for a new river-side factory.
Screening, scoping, baseline study of the river and air, prediction of impacts with measures, public hearing, decision, monitoring after building.
5. In the 3D, the air bar is 100. A factory without a filter drops it by 55. A filter changes the drop to 12. What is the air bar in each case?
Without a filter: 100 โ 55 = 45. With a filter: 100 โ 12 = 88. The filter keeps 43 more points.
Common mistakes
- Doing the EIA after the factory is built. It is done before, so the plan can still change.
- Mixing up corrective and compensatory. Corrective repairs the same place; compensatory gives back somewhere else.
- Using compensation to excuse harm that could have been prevented. Prevention always comes first.
- Forgetting monitoring. A good plan on paper must be checked in real life.