Plot ground gas monitoring data on a ternary diagram to visualise the relationship between CH₄, CO₂, and balance gas (effectively N₂). Identify gas sources, distinguish between landfill gas migration and natural processes, and support your ground gas conceptual site model.
The chart includes reference zones for typical landfill gas, methane oxidation, aerobic soil respiration, and thermogenic sources, plus the 5% CO₂ threshold line from BS 8485.
Add monitoring locations — enter a label (e.g. BH01), then CH₄ %, CO₂ % and O₂ % for each location. Balance gas is calculated automatically as the remainder. Use the + button to add more rows.Review the ternary plot — data points appear as dots on the chart. Hover over any point to see its label and full composition breakdown.Interpret using the key — compare your data against the colour-coded reference zones shown in the legend below the chart to characterise gas sources.Export or share — download the chart as an SVG for reports, or share the results summary.The ternary plot uses the approach described in CIRIA C665 and the EPG Ground Gas Information Sheet (GGIS No. 1). Balance gas is the residual left once the measured gases are subtracted (Balance = 100 − CH₄ − CO₂ − O₂), so it is nitrogen-dominated plus any trace gases — not N₂ and O₂ combined.
Oxygen is measured and excluded rather than folded into the balance, matching how field instruments report BAL. This is what keeps the plot diagnostic: an air-affected borehole and a genuinely anaerobic one would otherwise plot in the same position. Because a ternary diagram shows the ratio of its three components, CH₄, CO₂ and balance gas are then rescaled to sum to 100% before plotting. The tooltip shows both the values as measured and the plotted, oxygen-free position.
Readings with elevated O₂ are flagged in the input table. Air ingress dilutes the sample, so the plotted position reflects the ingress rather than the source gas.
Reference zones
Landfill gas — Phase IV methanogenic gas, typically CH₄ 45–60%, CO₂ 25–55%.Methane oxidation — where bacterial oxidation consumes CH₄ (CH₄ + 2O₂ → CO₂ + 2H₂O), shifting points away from the air dilution line toward higher CO₂:CH₄ ratios.Aerobic respiration — natural CO₂ production from soil organic matter with negligible CH₄. Key for demonstrating that CO₂ above 5% may pose low risk under BS 8485.Thermogenic — deep source gas from coal measures or natural gas seepage, characterised by high CH₄ (>80%) with very low CO₂.Reference lines
Air dilution line — theoretical mixing line from typical 60:40 landfill gas to atmospheric air. Points on this line suggest simple dilution without chemical change.5% CO₂ line — the BS 8485 threshold above which the characteristic situation may need to be increased from CS1 to CS2, unless the source can be shown to be aerobic respiration.Note on zone boundaries
The reference zones shown on the ternary plot are approximate and interpretive. No published standard provides exact coordinate boundaries for these regions — they are derived from the compositional ranges described in CIRIA C665, the EPG GGIS No. 1, and published gas chemistry literature. Real-world gas compositions can fall between or outside these zones. The zones are intended as a visual aid to support interpretation, not as definitive classification boundaries. Professional judgement and a site-specific conceptual model should always be used when characterising ground gas regimes.
Limitations Reference zones are indicative and based on typical UK landfill gas compositions. Actual zone boundaries vary by site geology, waste composition, and gas migration pathways.
The ternary chart shows bulk gas composition only. It does not account for gas flow rate, borehole response, barometric pressure effects, or temporal variation — all of which affect gas risk assessment.
Gas compositions should be interpreted alongside borehole construction details, monitoring frequency, and weather conditions per BS 8576 and CIRIA C665.
Revision history 27 October 2026: Content update and publish
30 August 2026: Corrected the balance gas definition. Oxygen is now entered and excluded from the balance (Balance = 100 − CH₄ − CO₂ − O₂) rather than counted within it, and the three plotted components are rescaled to sum to 100%.
Disclaimer This tool is provided for educational and general information purposes only. It is not a substitute for professional engineering advice, design or verification.
Diggy and its contributors are not licensed engineering consultants and no results generated by this tool should be used directly for construction, design or safety-critical decisions.
All values and outputs are based on published empirical correlations and should be independently checked and confirmed by a qualified geotechnical engineer before use.
By using this tool, you accept full responsibility for how you interpret and apply the information provided.
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