microsensors & soil columns

Soil Columns and Capillary Fringe

 Dynamics in the capillary fringe are difficult to measure in the field. Scientists and engineers, therefore, use soil columns in the lab to understand the capillary fringe. Image source: USGS.

Continuous measurements of biogeochemical parameters in the field can be extremely difficult. In order to understand biogeochemical process and patterns, scientists and engineers often resort to simulating field conditions in the laboratory via the soil column methodology.

Soil columns in the laboratory are extremely useful for understanding increased areas of biogeochemical activities such as the capillary fringe. The capillary fringe is the transition zone between the soil and the groundwater. As water levels rise and fall, the capillary zone can be highly dynamic. There are steep physical-chemical processes occurring within the capillary fringe as the zone fluctuates between oxic and anoxic conditions. Biological activity, in particular, can be high as groundwater increases and decreases. Greenhouse gases (GHG), such as carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), can fluctuate widely within the capillary fringe.

pH soil column

The automated soil column from the experiment of Rezanezhad et al 2014. Image source: https://uwaterloo.ca/ecohydrology/biogeochemical-kinetics-laboratory

In soil column studies, scientists and engineers manually increased and decreased water table levels in order to simulate oxic and anoxic conditions in the capillary fringe. A recent experiment by Rezanezhad et al (2014) introduced an automated soil column system where the water levels fluctuated according to pre-set cycle. Rezanezhad et al demonstrated how the biogeochemistry of their fluctuating water level soil column differed to a stable water level soil column.

In order to demonstrate how the biogeochemistry differed between the systems, the scientists measured the reduction / oxidation potential (redox, Eh) at two depths within the soil column: 10cm and 30cm. The scientists deployed Unisense redox microsensors (microelectrodes) and pH microelectrodes connected to a Unisense mV meter. Measurements were continuously recorded every 60 seconds by connecting the mV meter to a computer.

redox sensor

The Unisense redox or pH microsensors are ideally suited for soil columns research.

Within the soil column, the scientists also installed pore water samplers at four depths. The pore water samplers were used to manually collected aqueous solutions for subsequent chemical analysis. In the experiment, the researchers used an Inductively Coupled Plasma Optical Emission Spectrometer to measure concentrations of the following elements: Fe, Mn, Si, K, Mg, Na, P and S. In order to measure ammonia, nitrate, potassium and phosphorus, a soil nutrient analyzer could have been used.

In the stable water column, the redox measurements were consistent over the entire course of the experiment. At the 10cm depth, in the oxic zone, the redox values were approximately +600mV, indicating oxidizing conditions. At a depth of 30cm, in the groundwater or anoxic zone, the redox values were approximately -200mV, indicating reducing conditions.

In contrast, the redox values in the fluctuating soil column varied depending on the level of the water table. As the water table increased, the redox values became negative at both 10 and 30cm depths. As the water table decreased, oxidizing conditions returned to the soil column and redox values became positive.

The study by Rezanezhad et al demonstrated how microsensors can be deployed in soil columns to continuously monitor environmental parameters. As the microsensors have a fine measurement tip, with tip diameters as small as 10μm, they can be deployed in soil columns with minimal disturbance to the soil profile. 

 

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