Towards drought preparedness: A microfluidic lab study to the process of BioSealing

Abstract

The Provence of Yunnan (People’s Republic of China) is known for its severe droughts between November and April. The people living in this Provence have to deal with serious water scarcity problems as a consequence. Especially the subsurface is unfavourable for (rain) water storage. The subsurface is built up of fractured bedrock below a few meters of soil. When rain is falling water filtrates into the soil and flows towards the fractures in the bedrock leaving the soil. This causes the groundwater to be unreachable for groundwater extraction. The soil and groundwater are known to be enriched by many types of microorganisms. Especially the soil bacteria are potentially helpful to solve the water scarcity problem in the Provence of Yunnan. By feeding the ambient bacteria in the soil with a nutrient solution a process called BioSealing can be triggered that causes the fractures in the bedrock to be clogged. This process involves biological and physical clogging. The combination of these clogging mechanisms potentially leads to permanent clogging of the fractures. Rain water that seeps into the soil can now be stored in the soil and be pumped up to use for different purposes. This study assesses the rate and behaviour of the products of the BioSealing process on the pore scale. Besides, different experimental approaches are tried to identify the different components present in the groundwater. A spectrophotometer is used to measure the optical density at 600 nm (OD600) of several groundwater samples. These measurements give a first impression for the presence of bacteria and other organic material in the groundwater. The main focus of this study is to simulate the biological clogging part of the BioSealing process. For the simulation of this part an open-microscope set-up is used. This set-up consists of a PDMS micromodel containing a thin section of a porous medium that has two inlet channels and one outlet channel. The groundwater and nutrition (Nutrolase) are separately injected with a syringe pump through the micromodel. Images are taken with a digital photo camera of the porous medium while injecting the fluids. Finally a Confocal Laser Scanning Microscope (CLSM) has been used to acquire more detailed images of the groundwater, Nutrolase and a mixture of both fluids to get a better view of the particles inside the fluids. Fluoresbrite has been added to a few samples to observe the bacteria even better. The measurements of the optical density of the groundwater gave a minor proof of the presence of bacteria in the groundwater. The following open-microscope experiments showed that biological clogging needs at least several days to have significant impact on obstructing groundwater flow. The potential biomass had the tendency to occupy the air bubbles that unwillingly entered the micromodel. The biomass also became denser with time and eventually occupied the boundaries of the air bubbles. The confocal microscope images did show a clearer view on the content of the fluids, but did not clarify for the presence of bacteria.