Using Ternary Plots for Interpretation of Ground Gas Monitoring Results - 19th May 2021

Oct 26, 2026

Ternary plots are used to interpret ground gas monitoring results and reveal trends in gas composition that can indicate potential sources and help assess risk. The paper compares two plotting approaches for ternary graphs, clarifying how axis labelling and data normalization affect interpretation. Guidance is provided for characterising the ground gas regime in wells where only low concentrations of carbon dioxide and/or methane are recorded, especially when flow rates are not elevated.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages9

Reading Time23 min

AccessFree to download

Introduction

How to Read a Ternary Plot

Axis label placement approaches

Plotting Data on a Ternary Plot

Constant-sum requirement and gas selection

Two plotting methods and implications

Using ternary plots for interpretation of ground gas monitoring results Steve Wilson MSc BEng CEng MICE CEnv CSci CWEM MCIWEM FGS ROGEP ASoBRA Technical Director, The Environmental Protection Group Limited, Warrington, UK (corresponding author: stevewilson@epg[ltd.co.uk](ltd.co.uk)) Fiachra Collins PhD BE Chief Technology Officer, Ambisense Richard Lavery MSc BSc Business Development Manager, Ambisense Ternary plots are commonly used to allow interpretation of ground gas monitoring results. There are various ways of plotting data in ternary graphs. This paper explains two different approaches that can be used and the implications that need to be considered when interpreting the data. It also provides advice on using ternary plots to help characterise the ground gas regime and risk where only low concentrations of carbon dioxide and/or methane have been recorded in monitoring wells without elevated flow rates. Introduction Ternary plots are commonly used to plot ground gas monitoring results (Teasdale et al 2014 and 2015) . They are also used to assess the flammability of methane and air mixtures (Figure 1) and to assess geological and hydrogeological data. A ternary plot is a triangular plot of three variables which must sum to a constant value, e.g. 1.0 or 100%(Howarth, 2009) . In most cases the ratios of the three variables are plotted as points on an equilateral triangle (West, 1982) . There are two ways of plotting data in ternary graphs. This paper explains each of the approaches that are used and the implications that need to be considered when interpreting the data. Ternary plots allow trends in gas composition to be identified and this can help to identify the potential source of the ground gas. It should not be used in isolation of other data nor without a robust conceptual site model. BS8485: 2015 and previous assessment methods (Wilson and Card, 1999, Boyle and Witherington, 2007 and Wilson et al, 2007) require that where the carbon dioxide concentration exceeds 5% consideration is given to increasing a site classification from Characteristic Situation 1 up to Characteristic Situation 2. Carbon dioxide is widely present in natural soils at concentrations up to 21% and this often leads to uncertainty about whether to increase the classification. A ternary plot can help to assess gas monitoring data to identify if there isan elevated risk and whether the classification should be increased. Figure 1 Flammability diagram for methane How to read a ternary plot There are different ways of plotting the axis labels on ternary plots. These are: [1. place](1. place) the axis label at the apex of the triangle atthe point where the axis value is 100 (Figure 2a) . 2. place the label alongside the axis zero line (Figure 2b) . This can be confusing at first glance because the labels are not alongside the axis values. [3. place](3. place) the label alongside the axis values (Figure 2c) . Whichever approach is taken it is useful to colour the axis labels, values and the gridlines so it is clear which axis relates to which gridlines. This makes it much easier for users to read the data from the graph. An example of how to read the data from a graph is shown in Figure 3. (a) label at apex (b) label alongside zero line (c) Label alongside axis values Figure 2 Different methods of axes labelling for ternary plots Figure 3 Reading data from a ternary plot Plotting data on a ternary plot As discussed in the introduction the three variables being plotted on a ternary diagram must sum to a constant value. For ground or landfill gas assessment when considering the permanent gases methane, carbon dioxide, oxygen and nitrogen, this is normally 100% . The contribution from any trace gases at ppm level is ignored (e.g. hydrogen sulphide or carbon monoxide) . However, these trace components should be considered separately as they can give useful information regarding the source of the ma

FAQs

What is the purpose of using ternary plots in ground gas monitoring?

Ternary plots help interpret ground gas monitoring results by identifying trends in gas composition. They can support interpretation of potential sources and assist risk characterisation when used with other evidence and a robust conceptual site model.

What are the two main approaches for plotting data in ternary graphs described in the paper?

One approach plots methane, carbon dioxide, and oxygen plus nitrogen so the three variables sum to a constant. The second approach plots the relative proportion of methane, carbon dioxide, and oxygen compared to their sum, ignoring nitrogen.

Why is simply dividing each gas by nitrogen (normalising by N2) not consistent with ternary plot requirements?

Simply dividing by N2 does not ensure that the totals plotted across the three gases remain a constant for all result sets. The paper explains that the combined sum of the nitrogen-based ratios does not stay constant, affecting consistency of interpretation.

Oct 26, 2026

Upload by anakgang17