CSIRO SENSEI™ in Situ System for Groundwater Monitoring
Authors: Vepsalainen Mikko , Caruso Daniella , Levingstone Kathryn , Calley Rohan , Chen Miao , Macedo David , Molenaar David , Bendavid Avi , Acharya Durga , McGregor Kathie , Kilpatrick Tony , Marley Dylan 
Published: May 2020

Currently, standard electroanalytical multi-sensor systems used in-situ to monitor the chemistry of aqueous systems are based on conventional glass ion-selective electrodes. Issues with these electrodes are common and range from their fragile construction, to fouling of the junction and often reference electrode poisoning. Remote operations, such as mining sites, require new technologies to overcome these challenges, encouraging the development of low-maintenance options for monitoring mineral processes and environmental pollution in-situ.
CSIRO is developing a robust and cost-effective solid-state sensor and analytics platform, termed SENSEI™, which includes solid-state electrodes and system electronics embedded directly into the measurement unit. The monitoring system includes CSIRO’s patented pH and reference electrodes together with ORP and conductivity sensors. Third party sensors can be included as part of the unit via MODBUS connection. The all-solid construction of the unit enables operation at elevated pressures and in the presence of external forces, and direct contact of the electrodes with the electronics removes the need for analogue cables, reducing electronic interference.
A key feature of the SENSEI™ system is the patented solid-state reference electrode. Due to the issues mentioned earlier, conventional reference electrodes were not suitable in an all-solid system and this led to CSIRO developing a new solid salt-polymer composite reference electrode. The SENSEI™ reference electrode includes an Ag/AgCl reference element embedded in a composite material of crosslinked polyvinyl acetate and KCl. After the initial stabilisation time the potential of the reference electrodes stabilises at -45±5 mV vs. SCE. According to laboratory and pilot-scale experiments, the electrode functions maintenance-free during continuous online monitoring for 9-12 months in aqueous systems, including groundwater and acidic mineral leaching systems.
The first large-scale application for the system is in environmental groundwater monitoring of an in-situ recovery operation at Heathgate Resources Pty Ltd. In in-situ recovery mining systems the leaching solution is pumped underground, where it dissolves the metals of the ore deposit, and afterwards pumped back to the surface for post-processing, so proper process control is crucial for a safe and environmentally friendly operation to ensure mining fluids are contained within the mining region. Continuous monitoring of the lateral, overlying and underlying aquifers which ring the mining operation enables quick corrective actions to be taken in the event of possible contamination from a leaching solution. The field trial began in 2019 and will continue until the end of 2020.
Figure. A schematic drawing of the SENSEI™ sensor unit. The sensor pack includes solid-state (1) ORP electrode, (2) reference electrode, (3) pH electrode and (4) conductivity/temperature sensor.

Figure 1