Impact Identification, Prediction and Significance

Introduction

Environmental impact assessment (EIA) is not merely a process for listing environmental effects. Its central analytical task is to determine how a proposed construction project may change environmental and social conditions, how important those changes may be, and whether they can be avoided, reduced, repaired or compensated. Impact identification, prediction and significance evaluation therefore connect project design with environmental decision-making.

Construction activities commonly involve land clearance, excavation, blasting, material transport, concrete production, water abstraction, dewatering, waste generation and the occupation of land. These activities can produce direct effects, such as dust from earthworks; indirect effects, such as habitat disturbance caused by access roads; induced effects, such as settlement growth around a new highway; and cumulative effects, such as the combined loss of wetlands from several projects.

The analyst must examine not only whether an impact occurs, but also its magnitude, duration, spatial extent, reversibility, likelihood, sensitivity of affected receptors and compliance implications. A technically sound EIA uses more than one method because no single tool captures every form of cause-and-effect relationship.

Learning Objectives

By the end of this chapter, students should be able to:

1. Distinguish direct, indirect, induced, cumulative, short-term and long-term impacts.
2. Select and apply checklists, interaction matrices, networks, overlays and risk assessment methods.
3. Use suitable qualitative and quantitative models to predict environmental change.
4. Evaluate impact significance using transparent, evidence-based criteria.
5. Recognize uncertainty, assumptions, limitations and ethical issues in impact prediction.
6. Recommend practical mitigation, monitoring and adaptive-management measures.

1. The Logic of Impact Analysis

An environmental impact is a change in an environmental or social receptor caused wholly or partly by a project, policy or activity. A useful conceptual chain is:

Project action → pressure or stressor → pathway → receptor → change in condition → consequence

For example:

Piling → underwater noise → transmission through water → fish and aquatic mammals → behavioural disturbance → reduced habitat use.

The chain prevents a common error: confusing an activity with an impact. “Road construction” is an activity; “increased sediment concentration in a stream” is an environmental change; “reduced fish spawning success” is a consequence.

1.1 Environmental components and receptors

An environmental component is a broad part of the receiving environment, such as air, water, soil, biodiversity, landscape, cultural heritage or the human environment. A receptor is a specific entity that may be affected: a school, wetland, groundwater aquifer, nesting colony, hospital, historic building or local livelihood.

Impact analysis begins with a baseline. The baseline should describe existing conditions, natural variability, current pressures and trends. A project effect is not necessarily the difference between a future project condition and today’s condition; it is more appropriately compared with the condition expected without the project.

The basic comparison is:

Here, ΔI(t) is the project-attributable change at time t, I_with(t) is the predicted indicator value with the project, and I_without(t) is the predicted value under the no-project or future baseline scenario. The two values must use the same indicator, units, location and time period. This expression is conceptual and does not by itself establish causation.

1.2 Types of impacts

Direct impacts result from a clear and immediate interaction between an activity and a receptor. Examples include land take, vegetation removal, construction noise and increased traffic at a site entrance.

Indirect impacts occur through intermediate pathways. A bridge project may increase sediment in a river; sediment may reduce aquatic plant growth; reduced plant growth may affect fish habitat. The latter effects are indirect relative to the bridge construction.

Induced impacts arise because a project changes behaviour, development patterns or economic activity. A new transport corridor may encourage industrial development, population growth and increased demand for water outside the original construction footprint.

Cumulative impacts result from the combined effect of the project with past, present and reasonably foreseeable activities. They may be additive, synergistic or compensatory. Several projects can collectively exceed an ecological threshold even when each project appears acceptable in isolation.

Short-term impacts occur over a limited period, such as dust during excavation. Long-term impacts persist for many years or become effectively permanent, such as loss of a mature wetland or altered river morphology.

Other useful classifications include positive or negative, reversible or irreversible, local or regional, regular or accidental, and certain or uncertain.

2. Scoping and Impact Identification

Scoping determines which issues require detailed assessment. It should combine professional judgment, legal requirements, baseline evidence, stakeholder knowledge and screening thresholds. A long list of every conceivable effect is not a substitute for a defensible scope.

A practical scoping sequence is:

1. Describe project phases: planning, construction, operation, maintenance, closure and post-closure.
2. List activities and emissions for each phase.
3. Identify environmental and social receptors.
4. Trace pathways between activities and receptors.
5. Consider credible accidents, abnormal events and climate-related hazards.
6. Include associated facilities, supply chains and induced development.
7. Screen impacts for significance and data requirements.
8. Record issues excluded from detailed assessment and justify the exclusions.

A project action–receptor register is useful. It should identify the activity, receptor, pathway, predicted indicator, spatial and temporal boundary, method, mitigation assumption and responsible specialist.

2.1 Checklists

Checklists are structured prompts used to ensure that common impact categories are considered. A simple checklist may contain air, water, soil, ecology, noise, traffic, landscape, heritage, health, safety, livelihoods and climate. More advanced checklists ask for magnitude, duration, reversibility and mitigation.

Their main advantages are speed, transparency and suitability for early screening. They are useful where project types are repetitive, such as housing developments or small bridges. Their limitations are equally important: they can encourage mechanical thinking, overlook interactions and fail to distinguish a serious impact from a minor one. Checklists should therefore initiate expert discussion rather than replace analysis.

2.2 Interaction matrices

An interaction matrix places project actions on one axis and environmental components on the other. A mark or score indicates a potential interaction. The Leopold matrix is a well-known example, although project-specific matrices are often more manageable.

A matrix may use a two-part score: magnitude and importance. For example, magnitude may be rated from −5 to +5, while importance is rated from 1 to 5. The two scores must not be multiplied automatically unless the scoring system has been justified; multiplication can create a false impression of precision. A matrix is primarily a screening and communication device.

Matrices are particularly useful for comparing construction phases. For example, excavation may interact strongly with groundwater, soil stability, dust and traffic, while operation may interact more strongly with energy use, noise and induced land development. Matrices show breadth, but usually do not explain pathways, feedbacks or secondary effects.

2.3 Networks and impact pathways

Impact networks represent chains and branching relationships. They are valuable for identifying indirect, induced and cumulative effects. A road project network might be represented as:

Road widening → increased accessibility → land-use change → vegetation clearance → habitat fragmentation → reduced ecological connectivity.

Networks help analysts ask whether an apparent “secondary” effect is actually the most important consequence. They also identify points at which mitigation can interrupt a pathway. For instance, controlling access and enforcing land-use zoning may reduce induced development more effectively than merely treating construction dust.

Networks become difficult to manage when they contain too many nodes or poorly supported assumptions. Each important link should be supported by evidence, expert knowledge or an explicit uncertainty statement.

2.4 Geographic information system overlays

Overlay analysis combines spatial layers such as protected areas, flood zones, groundwater vulnerability, population density, cultural sites and project footprints. It is particularly effective for route selection and site comparison.

A basic overlay may identify locations where several constraints coincide. A weighted suitability model can be expressed as:

Here, S is the suitability score, w_i is the weight assigned to criterion i, and x_i is the normalized score for that criterion. Weights should be established transparently with stakeholders and specialists. Scores must be placed on a comparable scale, and the model should be tested through sensitivity analysis. A high suitability score does not override legal prohibitions or the need for site-specific investigation.

Overlays can also reveal cumulative spatial patterns, such as several developments affecting the same watershed. Their weakness is that spatial coincidence does not prove impact, and map resolution may conceal local conditions.

3. Prediction of Environmental Change

Prediction estimates the likely magnitude, extent and timing of an impact. It may be qualitative, semi-quantitative or quantitative. The choice depends on the decision, data quality, receptor sensitivity, regulatory requirements and consequences of error.

3.1 Baseline and reference conditions

Prediction requires a defensible reference condition. Baseline surveys should account for seasonal variation, episodic events and existing trends. For example, a single dry-season water sample cannot characterize a river affected by strong seasonal flows. Where possible, baseline monitoring should include control locations or reference sites.

The future baseline may be affected by population growth, climate change, other approved developments or changing regulations. If these are ignored, the project’s incremental effect may be underestimated or overestimated.

3.2 Air-quality and dispersion models

For a simple steady-state screening estimate of pollutant concentration from a continuous point source, a Gaussian plume model may be used under restrictive assumptions:

C(x,y,z) is concentration, Q is emission rate, u is mean wind speed, σ_y and σ_z are horizontal and vertical dispersion parameters, and H is effective release height. The coordinates x, y and z describe downwind, crosswind and vertical position. Consistent units are required, such as kilograms per second, metres and seconds, producing concentration in kilograms per cubic metre.

The model assumes steady emissions and meteorological conditions, a relatively simple terrain, and a source that can be represented as a point. It is not automatically appropriate for complex terrain, street canyons, changing weather or highly intermittent construction emissions. More advanced regulatory models may be required.

3.3 Noise prediction

Sound levels are logarithmic. For independent sound sources, the combined level is:

L_i is the sound pressure or sound power level of source i, expressed in decibels on the same weighting and reference basis. Decibels cannot be added arithmetically. Two identical sources are approximately 3 dB higher than one source because 10 log₁₀(2) ≈ 3.0. Distance, barriers, ground absorption, building reflection and operating schedules must also be considered.

3.4 Water-quality mass balance

A completely mixed receiving-water estimate can be represented by:

Q_r and Q_e are receiving-water and effluent flow rates, while C_r and C_e are their respective concentrations. All flow units must match, and concentrations must use the same units. The equation assumes complete and instantaneous mixing and does not account for decay, settling, reaction or stratification. It is therefore a screening calculation rather than a universal prediction.

3.5 Traffic and greenhouse-gas estimation

Construction vehicle emissions may be estimated from activity data:

E is emissions, A is activity such as vehicle-kilometres or tonnes of fuel, EF is the emission factor, and η is the controlled fraction, expressed between 0 and 1. Units must be compatible. For example, vehicle-kilometres multiplied by grams per vehicle-kilometre produces grams. Emission factors should be appropriate to vehicle type, fuel, age, operating conditions and jurisdiction.

3.6 Risk assessment

Risk assessment is appropriate when an event may be uncertain but its consequences could be serious. A basic qualitative or semi-quantitative representation is:

R is risk score, P is the likelihood of an event and C is consequence severity. This is a decision aid, not a probability law. Multiplication is meaningful only if the scales and categories are defined consistently. A low-probability catastrophic failure may require stronger controls than a high-probability minor nuisance even if both receive similar numerical scores.

A more formal annualized expected loss may be written:

where p_j is the probability of scenario j and L_j is its loss or consequence. Probabilities must refer to a specified time period and mutually exclusive scenarios. Rare-event estimates may have large uncertainty and should not be presented as exact facts.

Risk analysis should address construction accidents, fuel spills, slope failure, flooding, fire, hazardous-material release, dam or excavation failure and climate-related events. It should consider existing vulnerability and emergency response capacity.

4. Evaluating Impact Significance

Significance is a reasoned judgment about the importance of an impact for decision-making. It is not identical to magnitude. A small change in a highly sensitive protected habitat may be more significant than a larger change in a resilient, already disturbed area.

Common significance criteria include:

• magnitude or intensity;
• spatial extent;
• duration and frequency;
• reversibility and recoverability;
• likelihood and uncertainty;
• sensitivity, rarity or legal protection of the receptor;
• ecological, health, cultural or livelihood consequences;
• exceedance of standards, thresholds or carrying capacity;
• equity and distribution of benefits and burdens;
• effectiveness and reliability of mitigation.

A transparent assessment may use a significance matrix. For example, magnitude can be classified as negligible, low, moderate or high, and receptor sensitivity can be classified on the same four-level scale. The resulting significance should then be reviewed by professional judgment rather than treated as an automatic mathematical result.

4.1 Significance before and after mitigation

Impact significance should be described both before mitigation and as residual significance after mitigation. Mitigation must be specific and enforceable. “Good environmental management” is not a mitigation measure. A suitable measure identifies the action, performance requirement, timing, responsible party and monitoring indicator.

The mitigation hierarchy is:

1. Avoid the impact through site, route or design selection.
2. Minimize the impact through engineering and management controls.
3. Restore or rehabilitate affected environments.
4. Offset or compensate for residual impacts only where appropriate and permitted.

Mitigation assumptions must be realistic. If a significance conclusion depends on a sediment pond operating at a particular efficiency, that efficiency must be supported by design, maintenance and monitoring arrangements.

4.2 Thresholds and standards

Legal standards provide important decision boundaries for air, noise, water, waste, occupational health, biodiversity and heritage. However, compliance does not automatically mean that an impact is insignificant. A standard may protect an average population, whereas a vulnerable receptor may require additional protection. Conversely, an exceedance may be temporary and manageable, but it must be disclosed and addressed.

Ecological thresholds are often uncertain. A precautionary approach is justified where potential harm is serious or irreversible and scientific uncertainty is substantial. Precaution does not mean refusing all development; it means using proportionate safeguards, alternatives analysis and monitoring.

4.3 Distributional and cumulative significance

An impact can be significant because of who bears it. Construction traffic may be acceptable at a regional scale but impose disproportionate noise and safety burdens on one low-income neighbourhood. Significance evaluation should therefore include environmental justice, access to information, meaningful consultation and respect for affected communities and Indigenous peoples.

Cumulative assessment should identify valued environmental components, define a geographic and temporal boundary, list other stressors, and assess the combined condition. The project’s contribution may be small, but still unacceptable if it pushes a system beyond a critical threshold.

5. Worked Example: Illustrative Urban Bypass Project

Background

The following is a fictional illustrative case. A municipality proposes a 6-kilometre bypass road around a growing town. Construction will require earthworks, a river bridge, two borrow areas and approximately 18 months of heavy-vehicle activity. The preferred alignment passes near a wetland, three villages and agricultural land.

Problem

Decision-makers must compare the preferred alignment with an alternative route. Key concerns are dust, traffic safety, wetland habitat, flood risk, noise, land acquisition and induced development.

Analysis

A checklist identified the principal receptors. A matrix showed strong interactions between excavation and soil erosion, bridge works and aquatic habitat, and traffic diversion and community safety. A network identified a longer-term pathway:

Improved road access → roadside commercial development → land conversion → wetland fragmentation.

GIS overlay analysis showed that the preferred route intersected a flood-prone wetland buffer, while the alternative affected more agricultural land but avoided the wetland. Field surveys confirmed that the wetland supported seasonal breeding habitat for amphibians.

For a screening water-quality estimate, assume receiving-water flow Q_r = 0.80 m³/s, receiving concentration C_r = 20 mg/L, construction discharge flow Q_e = 0.05 m³/s, and discharge concentration C_e = 180 mg/L. Then:

This estimate assumes complete mixing and no treatment or reaction. It indicates that an uncontrolled discharge could substantially increase concentration and therefore supports the need for sediment control, discharge limits and monitoring. It does not prove that the river will reach exactly 29.4 mg/L in the field.

Approach and solution

The project team selected the alternative route after considering the wetland’s high sensitivity and the preferred route’s induced-development risk. The design included a bridge span that avoided in-channel piers, stabilized exposed soil, covered haulage trucks, restricted working hours near villages, installed pedestrian crossings and prepared a wetland buffer-management plan. A construction environmental management plan assigned responsibilities and trigger levels for corrective action.

Residual impacts were judged moderate for agricultural land acquisition, low to moderate for construction noise, and low for wetland habitat after avoidance and design changes. The bypass still required cumulative monitoring because other developments were proposed along both routes.

Outcome and learning points

The case demonstrates that significance is not determined by a single score. The alternative performed better because it avoided a sensitive receptor, reduced long-term induced effects and provided more reliable mitigation. It also shows why matrices should be combined with networks, spatial analysis, quantitative screening and stakeholder knowledge.

6. Uncertainty, Quality Assurance and Adaptive Management

All predictions contain uncertainty. Sources include incomplete baseline data, natural variability, model assumptions, uncertain future development, changing climate, uncertain mitigation performance and imperfect knowledge of ecological thresholds.

A good EIA should distinguish:

• aleatory variability, which arises from natural randomness or variation; and
• epistemic uncertainty, which arises from limited knowledge or data.

Uncertainty can be addressed through sensitivity analysis, conservative assumptions, alternative scenarios, confidence intervals where appropriate, independent review and monitoring. The analyst should report what is known, what is assumed and what would change the conclusion.

Monitoring should be linked to predictions and decisions. A useful adaptive-management arrangement specifies baseline conditions, indicators, locations, frequency, trigger levels, responsible organizations and corrective actions. Monitoring is not a substitute for good design; it is a means of testing predictions and managing residual uncertainty.

Quality assurance includes calibrated instruments, documented methods, chain of custody for samples, validated models, reproducible GIS procedures, version control and review by competent specialists. Data should not be selectively reported to support a preferred project alternative.

7. Professional, Ethical and Regulatory Responsibilities

Impact identification and significance evaluation involve value judgments as well as technical analysis. Ethical practice requires competence, independence, disclosure of conflicts of interest, protection of confidential information where justified, respect for affected people and honest communication of uncertainty.

Regulatory systems commonly require screening, scoping, public participation, alternatives analysis, assessment of cumulative effects, mitigation planning, environmental management and monitoring. Requirements differ by jurisdiction, so practitioners must consult applicable statutes, permits, standards and institutional guidance.

Public participation should occur early enough to influence alternatives and project design. Consultation is not meaningful if communities receive technical conclusions only after major decisions have been made. Information should be accessible, culturally appropriate and available in relevant languages. Indigenous and local knowledge can identify seasonal, cultural and ecological relationships that short technical surveys may miss; it should be used with consent and proper recognition.

Artificial intelligence and digital tools can support impact analysis by classifying documents, detecting land-cover change, forecasting traffic or identifying spatial patterns. They do not remove the need for field verification, professional accountability or stakeholder engagement. AI outputs may reproduce biased training data, obscure assumptions or generate plausible but unsupported conclusions. Any AI-assisted result should be traceable, validated and clearly disclosed.

8. Best-Practice Workflow

A robust workflow for construction-project impact analysis is:

1. Define the project, alternatives, phases and associated facilities.
2. Establish the baseline and future no-project condition.
3. Identify activities, stressors, receptors and pathways.
4. Use checklists for completeness.
5. Use matrices to screen interactions.
6. Use networks to trace indirect, induced and cumulative effects.
7. Use overlays for spatial constraints and alternatives.
8. Apply quantitative models only when assumptions and data are adequate.
9. Assess accidents, climate hazards and vulnerable receptors.
10. Rate significance before and after mitigation.
11. Test conclusions through sensitivity and uncertainty analysis.
12. Consult affected stakeholders and independent reviewers.
13. Specify mitigation, monitoring, responsibilities and trigger actions.
14. Present conclusions with clear evidence, limitations and alternatives.

The objective is not to create a highly complicated assessment. It is to use the simplest defensible method that answers the decision question while revealing important interactions and uncertainties.

Chapter Summary

Impact identification, prediction and significance evaluation form the analytical core of EIA. The process begins by linking project activities to pressures, pathways, receptors and consequences. Direct impacts arise immediately, whereas indirect, induced and cumulative impacts require pathway and system-level analysis. Short-term and long-term effects must be considered across construction, operation, maintenance and closure.

Checklists support completeness, matrices display activity–receptor interactions, networks expose secondary and induced pathways, and GIS overlays support spatial comparison. Quantitative models can estimate air pollution, noise, water quality, emissions and risk, but only when their assumptions, units and data requirements are respected. Significance depends on magnitude, extent, duration, reversibility, likelihood, receptor sensitivity, thresholds, uncertainty, mitigation performance and distributional consequences.

A credible EIA compares with the future no-project condition, evaluates alternatives, discloses uncertainty, uses the mitigation hierarchy and connects predictions to monitoring and adaptive management. Professional integrity, public participation, environmental justice and regulatory compliance are essential to defensible conclusions.

Key Terms

Impact: A change in an environmental or social condition caused wholly or partly by an action.

Receptor: A person, ecosystem, resource, property or cultural feature that may be affected.

Direct impact: An immediate effect of a project activity on a receptor.

Indirect impact: An effect transmitted through an intermediate pathway or process.

Induced impact: An effect caused by changes in development, behaviour or economic activity resulting from a project.

Cumulative impact: The combined effect of a project with other past, present or foreseeable activities.

Baseline: Existing environmental and social conditions used as a reference for assessment.

Future baseline: The expected condition without the proposed project at a future time.

Checklist: A structured list used to identify potentially relevant impact categories.

Interaction matrix: A table showing relationships between project actions and environmental components.

Impact network: A diagram or analytical structure showing chains and branches of cause and effect.

Overlay analysis: Combination of spatial information layers to identify constraints, opportunities or cumulative patterns.

Receptor sensitivity: The vulnerability, rarity, value or legal importance of a receptor.

Magnitude: The intensity or scale of an environmental change.

Significance: The importance of an impact for environmental decision-making.

Residual impact: The effect remaining after mitigation has been applied.

Risk: A representation of uncertain likelihood and consequence.

Mitigation hierarchy: Avoidance, minimization, restoration and, where appropriate, compensation or offsetting.

Uncertainty: Lack of complete knowledge about conditions, relationships, predictions or outcomes.

Adaptive management: A management approach that uses monitoring and learning to adjust actions.

Review Questions

Short Answer Questions

1. Distinguish between a project activity, an environmental impact and an environmental consequence.
2. What is the difference between a direct impact and an induced impact?
3. State two strengths and two limitations of interaction matrices.
4. Why must a future no-project condition be considered in impact prediction?
5. What is meant by residual impact?

Long Answer Questions

1. Explain how checklists, matrices, networks and GIS overlays can be combined in a construction-project EIA.
2. Discuss the criteria that should be used to determine impact significance.
3. Explain the assumptions and limitations of a simple receiving-water mixing calculation.
4. Describe how cumulative impacts differ from indirect impacts and explain why both may be missed in a narrow site assessment.
5. Discuss the role of uncertainty, monitoring and adaptive management in environmental impact prediction.

Analytical / Application Questions

1. A proposed highway crosses agricultural land, a wetland and a densely populated settlement. Design an impact-identification process using at least four EIA tools and justify the sequence.
2. A construction discharge has a high pollutant concentration but a very small flow, while the river has a low pollutant concentration and a large flow. Explain how you would predict the mixed concentration and what additional field conditions must be checked before relying on the result.
3. A project complies with an ambient noise standard but disproportionately affects a low-income neighbourhood. Evaluate whether the impact may still be significant and propose mitigation and stakeholder-engagement measures.

Multiple Choice Questions with Answers

Question 1

Which statement best defines an indirect impact?

A. An effect occurring immediately at the construction site

B. An effect transmitted through an intermediate pathway

C. An effect that is always positive

D. An effect caused only by natural processes

Correct Answer: B. An effect transmitted through an intermediate pathway

Question 2

What is the principal purpose of an interaction matrix?

A. To prove that an impact will occur with certainty

B. To replace all field surveys

C. To display potential relationships between project actions and environmental components

D. To calculate construction costs

Correct Answer: C. To display potential relationships between project actions and environmental components

Question 3

In the conceptual comparison ΔI(t) = I_with(t) − I_without(t), I_without(t) represents:

A. The construction contractor’s estimate

B. The future condition without the proposed project

C. The maximum legal concentration

D. The worst possible accident

Correct Answer: B. The future condition without the proposed project

Question 4

Which method is particularly useful for tracing chains of indirect and induced effects?

A. Impact network

B. Equipment inventory

C. Payroll schedule

D. Single-point sampling

Correct Answer: A. Impact network

Question 5

If two identical independent noise sources operate together, their combined level is approximately:

A. 1 dB higher than one source

B. 2 dB lower than one source

C. 3 dB higher than one source

D. Twice the numerical decibel value

Correct Answer: C. 3 dB higher than one source

Question 6

In the receiving-water mixing equation, complete and instantaneous mixing is an assumption concerning:

A. The legal status of the project

B. How the discharge combines with the receiving water

C. The project’s construction cost

D. The age of the receiving ecosystem

Correct Answer: B. How the discharge combines with the receiving water

Question 7

Which option follows the mitigation hierarchy most directly?

A. Offset first, then investigate alternatives

B. Avoid the impact before minimizing it

C. Monitor without changing the design

D. Ignore impacts below a preferred threshold

Correct Answer: B. Avoid the impact before minimizing it

Question 8

Why can a small change be highly significant?

A. Magnitude is the only significance criterion

B. The affected receptor may be highly sensitive or legally protected

C. Small impacts are always irreversible

D. Small changes automatically exceed standards

Correct Answer: B. The affected receptor may be highly sensitive or legally protected

Question 9

What is a major limitation of GIS overlay analysis?

A. It cannot display spatial information

B. Spatial coincidence alone does not prove causation

C. It can only be used for noise assessment

D. It eliminates the need for stakeholder consultation

Correct Answer: B. Spatial coincidence alone does not prove causation

Question 10

If Q_r = 0.80 m³/s, C_r = 20 mg/L, Q_e = 0.05 m³/s and C_e = 180 mg/L, the complete-mixing concentration is approximately:

A. 20.0 mg/L

B. 25.0 mg/L

C. 29.4 mg/L

D. 180.0 mg/L

Correct Answer: C. 29.4 mg/L