Lessons learned PART III: Interpreting flux results to reduce uncertainty and improve conceptual site model

Take into account the “alfa factor”: 

Flow paths bend when water flows from the soil to the filter pack, to the monitoring well and then into the cartridge, because of differences in hydraulic conductivity. Based on the diameter of the borehole and the monitoring well, the permeabilities of soil, filter gravel and the cartridge, the flux measurement is corrected with the "alpha factor". This calculation is the key of our expertise and fully described in this paper:

Paper: Groundwater flow field distortion by monitoring wells and passive flux meters - Verreydt et al. (2014) (pdf)

Do not directly compare fluxes with point concentrations: 

Flux average concentrations (= mass flux/water flux) are not directly comparable with measured concentrations by traditional sampling. With traditional sampling, you measure a concentration in the water present in the monitoring well and immediate surroundings. The concentration in the water from the most permeable layer (law of least resistance) will contribute the most. With passive flux measurements, we measure the amount of water and product that naturally flows through the cartridge in a certain period of time. This results in a time-averaged concentration in a well-defined soil layer. 

Quality over quantity: 

More samples give more certainty about your model but by using known data and performing targeted measurements the number of measurements can be limited. But beware, with a limited number of measurements, conclusions are limited and less nuance is possible. In order to support our customers, we have now drawn up a questionnaire from which the CSM as well as the objective of the flux measurements must emerge more clearly.  We go over these questions with our customers so that we are optimally informed at the start of the project. If we find any gaps or inconsistencies, we provide feedback to the customer. We are always prepared, before and after performing the measurements, to discuss the data.

Mass flux cartridges provide relevant information on their own: 

While in the past we often insisted on determining water and mass fluxes simultaneously. We now are confident to measure mass fluxes without adding water fluxes. When sufficient information is available about the soil and hydrogeology, mass fluxes alone can provide sufficient information. 

Continuous measurements to quantify variations in time and space: 

Groundwater flow (velocity and direction) can vary widely spatially and over time, for example due to tidal effects or abstraction wells. Depending on the objective of the measurement, it may be necessary to carry out several rounds of measurements. In addition, variations during the measurements can have an impact that should not be underestimated. To better understand these dynamics, the water flux (speed and direction) can now also be measured continuously and real-time with the real-time iFLUX sensors.

Combining 3 factors at once : 

Mass Flux and Mass Discharge combines  3 factors 1) contaminant concentration in water 2) groundwater flow rate and 3) size of the source zone, mass discharge is a relevant value to evaluate the contamination and its behaviour and risks. 

A summary about the use of mass flux and mass discharge can be found here.