In the past, the assessment and management of groundwater contamination has been driven by contaminant concentrations. However, concentration data alone are sometimes not sufficient to fully understand the behaviour or effect of a plume over time. Mass flux and mass discharge estimates are important tools to help practitioners and regulators characterise and remediate groundwater contamination. Their inclusion within remedial design and optimisation, when relevant, may ultimately result in time- and cost-efficient groundwater remediation programs.
In 2014, CRC CARE completed an initial review of available technical information on flux-based criteria for management of groundwater.1 This review served as the background for developing guidance on flux-based assessment and management of groundwater contamination.
The CRC CARE guidance illustrates how flux concepts, tools and measurements can be used to assess and manage groundwater contamination.2 Further to the guidance, numerical site-specific metrics for mass flux and mass discharge may need to be developed on a site-specific basis. Mass flux and mass discharge estimates are typically used to complement concentration-based assessments, rather than to replace them.
Using mass flux-based techniques to support regulations
Australian regulations generally emphasise a pragmatic, risk-based approach to the management of groundwater contamination. Mass flux-based techniques are a valuable tool in supporting this approach, in some circumstances. Acceptance of mass flux-based tools and techniques in the management of site contamination will depend on their technical basis and how they are relevant to achieving the overall objectives for a site. For example, by demonstrably reducing risk to an acceptable level.
Mass flux and mass discharge concepts can help fill the gap in understanding, and have been applied successfully both in Australia and internationally to:
- enhance the conceptual site model
- complement concentration criteria
- assist with remedy selection
- optimise remedial design
- assess remedial performance
- demonstrate risk reduction
- evaluate compliance/long-term monitoring.
Environmental effects on mass flux
At a location along a groundwater contaminant plume, the mass flux represents the integrated effects of transport, storage and degradation along the flow path. By definition, mass flux estimates are impacted by factors that affect groundwater flow, such as the hydraulic conductivity and the hydraulic gradient. Therefore, estimates of mass flux are affected by such things as:
- changes in groundwater extraction rates
- groundwater elevation changes
- seasonal variations in velocity or flow directions.
Mass flux estimates are also affected by variations in contaminant concentrations. Factors that may cause contaminant concentrations to vary include redox changes as a result of the infiltration of rainwater, and variations in dissolved phase concentrations due to sorption and the precipitation/dissolution of contaminants.
Heterogeneity in the lithology can significantly affect mass flux. Thus, heterogeneity should be understood before implementing a program to measure and use mass flux at a site. Rather than being homogeneous across the full extent of an aquifer, groundwater flow tends to be concentrated in zones of high hydraulic conductivity that often occupy a relatively small proportion of the aquifer cross-section. This heterogeneity results in a range of mass fluxes across the aquifer.
Estimating mass flux and mass discharge for dissolved phase contaminants in water
In many circumstances, mass flux and mass discharge can provide useful information in addition to concentration data, to help define the contamination plume structure and its evolution over time. Typical monitoring focuses on delineating plume boundaries and concentration trends. However, both concentrations and groundwater flux can vary greatly across a plume, and by focusing only on the plume boundaries, areas of significant contaminant mass flux may be missed.
The CRC CARE guidance identifies 5 key methods to derive mass flux and mass discharge estimates for dissolved phase contaminants in groundwater:
- transect methods
- passive flux meters
- well capture or pump test methods
- transects based on iso-contours
- solute transport models.
As with other site investigation approaches, it is necessary to determine the acceptable level of uncertainty for the intended application of the mass flux and/or discharge information, and how that level of uncertainty can be achieved, managed and assessed.
About the guidance
The guideline provides practical steps on communicating results, along with information on mass flux within the Australian regulatory context for the assessment and remediation of contaminated land.
The guidance on flux has been endorsed for release by stakeholders engaged in the project; see Technical Report 37 on the CRC CARE website.
- CRC CARE. Flux-based criteria for management of groundwater, CRC CARE Technical Report 31, CRC CARE, Adelaide, 2014.
- CRC CARE. Flux-based assessment and management of groundwater contamination, CRC CARE Technical Report 37, CRC CARE, Newcastle, 2016.
Photo: Dennis Cowals