These values define the deterministic behavior of the screening tool. Fixed and locked.
Scope: Clastic sediment profiles only (soil, fill, alluvial/colluvial clay–sand–gravel sequences). Fractured rock and bedrock are outside model scope. Where bedrock is encountered, the clastic column above defines the assessed profile.
Multi-contaminant logic: Each selected contaminant group is evaluated independently. Overall risk = highest risk class across all groups. Driver = contaminant with highest risk class; ties broken by highest mobility class, then highest source strength score. This rule is deterministic and contaminant-neutral. Significant disagreement (≥2 risk levels apart) is flagged in results.
Surface Cover → Infiltration Score
| Cover | Score |
|---|
| Concrete | −7 |
| Asphalt | −3 |
| Compacted soil | −2 |
| Open soil | 0 |
| Unknown | −2 |
Soil Texture → Infiltration Score
| Texture | Score |
|---|
| Sand | +1 |
| Sandy loam / Clayey sand | 0 |
| Clay | −1 |
| Unknown | 0 |
Slope → Modifier
| Slope | Modifier |
|---|
| <5° | 0 |
| 5–15° | −1 |
| >15° | −2 |
| Unknown | −1 |
Horizontal Distance to Receptor → Connectivity
| Distance | Connectivity | Notes |
|---|
| <10 m | HIGH | Near-field — direct exposure plausible |
| 10–30 m | MEDIUM | Realistic impact under most GW conditions |
| 30–75 m | LOW | Attenuation significant — impact conditional on site factors |
| 75–250 m | VERY_LOW | Impact only under high-velocity, high-source conditions |
| >250 m | VERY_LOW | Same class as 75–250 m — bin retained for site representation accuracy |
| Unknown | MEDIUM | Conservative default |
Note: 75–250 m and >250 m both map to VERY_LOW. The distinction is preserved so users can accurately represent their site geometry — it does not affect model output.
Source Type
| Type | Strength | Definition |
|---|
| Point | HIGH | Discrete, mappable release |
| Area | MEDIUM | Distributed over a delineable zone |
| Unknown | MEDIUM | Default conservative |
Contaminant Group → Mobility
| Group | Mobility | Notes |
|---|
| VOC | HIGH | VI flag triggered for shallow sources + shallow foundation |
| PFAS | HIGH | Soil barrier penalised; mobility scored. Persistence and regulatory sensitivity (ng/L) are not scored — flagged as professional interpretation factors for Phase II scoping. |
| SVOC / Petroleum | MEDIUM | Petroleum treated as generalized TPH / LNAPL behavior (near-source non-aqueous phase + dissolved GW). DNAPL-phase behavior is outside this tool's scope and is not modelled. |
| Metals | LOW | Sorption-limited |
| Mixed / Unknown | MEDIUM | Precautionary default |
Source Magnitude / Confidence
| Option | Maps to | Use when |
|---|
| High (confirmed) | HIGH | Major release, significant exceedances |
| Moderate (confirmed) | MEDIUM | Confirmed release, moderate concentrations |
| Trace / incidental | LOW | Minor confirmed release |
| Suspected (land use only) | LOW | Land use implies risk, no confirmed release. More informative than Unknown. |
| Unknown | MEDIUM | No data — precautionary data-gap default |
Release Age → Source Strength Modifier
| Age | Modifier | Rationale |
|---|
| Recent (<10 yr) | +1 | High mass flux, limited attenuation time |
| Established (10–30 yr) | 0 | Neutral |
| Legacy (>30 yr) | −1 | Potential partial attenuation |
| Unknown | 0 | No adjustment; counted in uncertainty |
VI Flag Trigger
VOC or Mixed contaminant group AND source depth = Surface or Shallow AND foundation = Shallow. Does not affect baseline risk. Indicates a separate VI assessment (soil vapour / indoor air) should be scoped in Phase II.
Basement VI Consideration Trigger
VOC or Mixed contaminant group AND source depth = Surface or Shallow AND foundation = Deep AND distance ≤30 m AND receptor present = Yes. Warning only — does not affect transport score, baseline risk, or overall risk. Intended to flag possible basement / sub-slab vapour pathway for separate assessment. Structurally exclusive with the VI Flag Trigger (which requires Shallow foundation).
Risk Baseline Logic (Priority Order)
Derived from transport class and source strength only. No proximity, regulatory, or screening heuristics at this layer.
| Rule | Condition | Outcome |
|---|
| Transport Gate | Both V & H ≤ LOW | LOW (MEDIUM if magnitude=High confirmed) |
| High Transport | Combined = HIGH | HIGH |
| High Source + High Pathway | Source=HIGH & (V or H=HIGH) | HIGH |
| High Source + Medium Transport | Source=HIGH & Combined=MEDIUM | MEDIUM |
| Low Envelope | One MEDIUM, other ≤LOW, source≠HIGH | LOW |
| Low Source | Source=LOW & Combined=MEDIUM | LOW |
| Default | All other combinations | MEDIUM |
Post-Baseline Escalations & Guardrails
Applied after baseline risk is determined. Each rule is input-driven and independent of the baseline output. These are deterministic screening decisions, not transport physics.
| Rule | Condition | Effect | Rationale |
|---|
| Proximity Escalation | Source=HIGH & Combined=MEDIUM & distance ≤30 m | Raise to HIGH | Confirmed large release with demonstrated transport within close proximity — HIGH concern at screening level regardless of individual pathway class |
| Near-field Guardrail | Distance <10 m & source Surface/Shallow & baseline=LOW | Raise to MEDIUM | Direct contact, vapour, and shallow lateral pathways plausible at this distance — not captured in GW transport scoring |
Note: Proximity Escalation takes priority over Near-field Guardrail. Both are shown explicitly in the audit trail and explanation output. The audit trail preserves "Risk Class (physics)" separately from "Overall Risk Class" to maintain transparency.
Urgency Level
| Condition | Urgency |
|---|
| GW Protection Zone = Yes OR Regulatory Trigger = Yes | HIGH |
| Sensitive Receptor = Yes | ELEVATED |
| None | STANDARD |
References / Scientific Basis
This tool applies a deterministic, heuristic screening approach informed by established contaminated land and vapour intrusion assessment frameworks. It does not implement numerical models or regulatory equations from these sources — the conceptual framework for source-pathway-receptor analysis, transport pathway classification, and vapour intrusion screening is aligned with the guidance listed below. Results require interpretation by a qualified professional with reference to applicable national or regional regulatory guidance.
| Reference | Relevance to this tool |
|---|
| US EPA (2015). Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway from Subsurface Vapor Sources to Indoor Air. | Informs VI flag trigger conditions, basement VI consideration, and vapour pathway screening logic |
| ITRC (2020). Vapor Intrusion Pathway: A Practical Guideline. | Aligned with VI screening distances, exposure pathway classification, and receptor proximity considerations |
| ITRC (2014). Petroleum Vapor Intrusion: Fundamentals of Screening, Investigation, and Management. | Informs petroleum vapour pathway considerations, including vadose zone attenuation |
| US EPA (2004). User's Guide for Evaluating Subsurface Vapor Intrusion into Buildings (Johnson & Ettinger model documentation). | Conceptual basis for vapour attenuation and sub-slab exposure pathway logic |
| UK Environment Agency (2020). Land Contamination Risk Management (LCRM). | Source-pathway-receptor conceptual framework underlying the tool's overall assessment architecture |
| Freeze, R.A. & Cherry, J.A. (1979). Groundwater. Prentice-Hall. | Foundational reference for subsurface transport concepts including advection, dispersion, and vadose zone behavior |
| Fetter, C.W. (2001). Applied Hydrogeology (4th ed.). Prentice-Hall. | Supporting reference for horizontal and vertical transport pathway classification and saturated zone transport |