Vadose-zone monitoring system for real-time characterization of contaminants leaching to groundwater in Israel

Groundwater contamination from anthropogenic origin is the result of the activities developed in the land-surface and top-soil. Contaminants are transported through the vadose zone to the groundwater below, imposing severe threats to the quality of all related water resources such as rivers and lakes, sometimes supporting groundwater dependent ecosystems. A lag time of years to decades between processes occurring in the root zone and their final imprint on groundwater quality prevents proper decision-making on land use and groundwater resource management. A key-solution is monitoring the vadose zone (or unsaturated zone).

This study implemented an innovative system, the Sensoil’s Vadose-zone Monitoring System™, which enables continuous monitoring and water sample collection directly from the vadose zone, providing real-time, continuous tracking of water percolation and contaminant transport across the vadose zone, from land surface to groundwater. Once installed, the VMS forms an accessible monitoring station (or network of stations) of the vadose zone.

Responsible entity

Sensoil Innovations Ltd. (Fig. 1) is a company developing and implementing smart soil sensing technologies and solutions, for the protection of groundwater and the environment.

Founded in 2013 to Commercialize sub-surface technologies developed at the Ben-Gurion University of the Negev Israel, Sensoil’s patented Vadose-zone Monitoring™ (VMS) technology.

The two main entities connected to this study are (Fig. 2) the Ben Gurion University of the Negev (Sde Boker Campus, Negev 84990, Israel) and The Volcani Center, Agricultural Research Organization (P.O. Box 6, Bet Dagan 50250, Israel).

Detailed explanation

The protection of groundwater resources from contamination is vital for the protection and sustainable use of this important water source.

In most situations, groundwater quality is assessed and sampled from wells and therefore the concentration of contaminants might already be at levels that will lead to disqualification of the aquifer as a source of drinking water, base flow for rivers or ecosystem support (cf. Fig. 4).

Fig. 4 – Illustration of contamination leaching through the vadose zone with until reaching groundwater (https://sensoils.com/technology/)

The transfer time of contaminants within the deep vadose zone has been estimated to take weeks to decades, depending on the water regime, thickness of the vadose zone and lithological characteristics of the subsurface (Spalding et al., 2001; Scanlon et al., 2010). Contaminants’ fate and transport below the root zone depends on issues such as recharge, soil spatial variability, soil chemistry and biology. Therefore, estimates of fluxes in the vadose zone have shown significant differences in the timing and concentrations. Furthermore, the cumulative effect of contaminants leaching from the root zone through the unsaturated zone on contaminants levels in the groundwater is blurred by mixing and dilution in the aquifer water.

The knowledge of the time lag between initiation of a contamination process in the vadose zone and its final effect on aquifer quality are fundamental to provide decision-makers the opportunity to implement measures and more time to plan possible alternative plans for water supply.

The Sensoil vadose-zone monitoring system is a recently developed tool that enables continuous monitoring of the hydrological and chemical properties of percolating water in the deep vadose zone under agriculture settings (Turkeltaub et al., 2014, 2015) and other hydrological settings (e.g., Dahan et al., 2009; Baram et al., 2013) (Fig. 5).

Fig. 5 – Schematic illustration of the vadose zone monitoring system, not to scale. (a) Side view and zoom in of the monitoring units: vadose zone pore water sampling ports (VSP) and flexible time domain reflectometers (FTDR). (b) All water content data are recorded and water samples are collected through a single control panel. (c) Top view, eight monitored plots. The plots are numbered and coloured according to the treatments schemes referred in Weissman et al., 2019 (https://doi.org/10.1002/vzj2.20041)

Data collected by the system comprises direct measurements of the water-percolation fluxes and the chemical evolution of the percolating water across the entire vadose zone.

The Vadose-zone Monitoring System (VMS) (Fig. 6) is composed of flexible sleeves (1) implemented in uncased, slanted boreholes. The flexible sleeves host multiple monitoring units, distributed along its length. Each monitoring unit is comprised of: water content sensors, vadose zone sampling ports and gas sampling ports for frequent sampling of the vadose zone. VMS data collection and water sampling is managed through Sensoil’s unique VMS control panel, installed on-site (2). Full access to real-time data, is available from anywhere, via the cloud (3). Data access is dedicated and customised to each customer (4).

The VMS sleeve is made of chemically resistant thin flexible liner, with integrally embedded monitoring units along its length. All monitoring units are connected to a control panel which is installed on the land surface. It is adapted for installation in 6″ diameter borehole drilled at 35º (to vertical), 55º (to horizon). Flexible Time Domain Reflectometry (FTDR) probes for monitoring of the sediment enable continuous measurement of water content and temperature of the sediment. Each and every FTDR sensor is tested for quality and performance and pre-calibrated for permittivity measurement in test medium of known dielectric properties.

Fig. 6 – Sensoil’s Vadose-zone Monitoring System (VMS) (https://sensoils.com/technology/)

Sampling of the vadose-zone pore water by the VSP (Vadose Sampling Port) is similar to standard tensiometers and suction cups, the hydraulic continuity is achieved via fine flexible porous medium. The VSP operates through a set of small diameter access pipes and control valves. The VSP is installed on the VMS sleeve together with the FTDR. The gas sampling probe (GSP) enables collection of the gas phase from the vicinity of the VSP. As such, comparison of the contaminant’s content in the water and gas phase may be achieved across the entire vadose zone.

The control panel is encased in a standard weather protected closet. A set of pressure manifolds valves and pressure gauges in the control panel allow direct access to each of the monitoring units for frequent sampling and regular maintenance.

A data-logger is used to collect data on water content, temperature and pressure measurements, providing real-time continuous data and advance warnings for decision support solutions. The software is able to receiving, storing, data-logging, processing, producing graphic charts, sending and accessing the raw and processed data. Access is dedicated and customised per customer. All data is protected by high-end security standards and under required privacy policy.

Fig. 7 shows an example of the results that can be obtained using Sensoil system. The objective of that study was to demonstrate the water flow and nitrate transport through the deep vadose zone underlying the crop field, with respect to rain patterns as well as the agricultural and fertilization setup.

It is possible to see the nitrate migration deeper into the vadose zone for different depths and the effect of rainy winters (e.g. 2012/13), with substantial nitrate breakthroughs noticeable throughout most of the vadose-zone cross section (marked with arrows in Fig. 7).

Fig. 7 – Time series of observed in nitrate (NO3-) concentrations in the vadose zone and daily rainfall for six consecutive years. (https://sensoils.com/technology/)

The nitrate concentration time series, which included variations of nitrate in time and at multiple depths, revealed, in real time, a major pulse of nitrate mass propagating down through the vadose zone toward the water table. These results indicate that nitrate fluxes in the unsaturated zone underlying agriculture land uses were associated with high nitrogen application rates and coarse-textured soils. Furthermore, pollution events that originated from agriculture land uses can be monitored in their early stages, long before pollution accumulates in the aquifer water.