PATHOGENIC VIRUSES


The problem

The presence of virus in the environment is ubiquitous in all three media (liquid, solid and gas). Our research is focused on the groundwater transport of those viruses, but a main question arises when dealing with this issue: which are the main mechanisms involved in virus groundwater transport? When studying virus transport as a whole process, four phases have to be considered:

  1. 1) Source

    It is very important to know the type of contamination source and its distance to the collection area. The first part is useful in order to know the type, quantity and periodicity of the virus intrusion into the system. The second part is needed in order to know precisely the distance/time that the viruses will be being transported through unsaturated and saturated porous media.

  2. 2) Transport through unsaturated zone

    The soil properties and the micro/microorganisms present in the soil define the hydraulic conductivity of the soil, the adsorption rate and the decay rate of viruses, amongst other parameters that are key aspects of virus transport. It should be also stated that the unsaturated zone is where most part of decay and reactions occur during this general virus transport process, due to the interaction between stagnant water, oxygen and macro/microorganisms present.

  3. 3) Transport through saturated zone

    Once the viruses travel through the unsaturated zone, they arrive to the saturated zone. This section has the highest residence time of the transported particles will take place (which can vary from a few days in very transmissive unconfined aquifers to many years in some confined aquifers). The transport through this area is where our research is mainly focused. Because it is the one containing the water that will be extracted and as some studies stated (Macías et al., 2017; Schijven and Hassanizadeh, 2002) the travel time between the virus source and the water extraction are relevant in order to ensure a virus free water. As well as the unsaturated zone, the processes affecting this saturated zone are: sorption, decay or deactivation of viruses and hydraulic parameters derived from geology. However, the fact that the zone is fully saturated in water, implies that there are other processes that are key for the virus transport: aquifer geology (rock surface properties, heterogeneity, geochemistry, matrix diffusion, etc.), infiltrated water chemical properties, residence time, etc.

  4. 4) Water extraction at the end point

    Well design is one of the most important aspect of safe and sustainable water extraction. Ensuring an adequate depth of the borehole and a complete insulation of the well’s water (so no debris or runoff water from the surface may go into the well). More details about this kind of design and operation can be found at the USEPA website (Handbook of Suggested Practices for the Design and Installation of Ground-Water Monitoring Wells).



The challenge

The transport of viruses through aquifers is prone to having significant challenges in its research. The main one being the fact that an aquifer is an underground structure, which implies that studying it directly is very complicated. Observatory wells, geophysical studies, mathematical models, geological models amongst others are the most typical ways of studying an aquifer. However, there is no way to “cut” an aquifer and watch directly how it works. Thus relying on indirect measurements and models to know the processes involved. Mainly, the study of an aquifer can be divided into two groups: theoretical and experimental. For the experimental part, the main challenge is to reproduce exactly or very similarly the characteristics and processes that occur in an aquifer in order to have reliable data in the experiments undertaken (or in the field, to know where and how to take samples and data). On the other hand, the theoretical part or modeling aspect, has the challenge of defining and quantifying in an adequate way the processes involved in the transport so that the model developed can represent similarly the processes observed in the field or in the laboratory.

How experiments can help

Virus experimentation is subject to some complications, especially related to the use of real virus. Whenever the study of viruses is undertaken, the safety measures have to be very strict in order to ensure the safety of the researchers involved. Moreover, the avoidance of virus outbreaks outside the laboratory or in the field is needed for environmental and health reasons. Therefore, bacteriophages are often use for having less security requirements or virus like particles (nanoparticles). In addition, the counting of viruses or virus-like particles is nontrivial because the transport experiments may inactivate the viruses and the methods chosen may need to distinguish between active and non-active viruses. However, both column and field experiments are viable options in virus experimentation when taking the proper safety measures.

How modeling can help

The modeling of viruses has been preliminarily done in GHS by considering a Colloid Facilitated Transport of viruses. The viruses appeared either free, attached to colloids or attached to the geological matrix.

This transport modelled is affected in our preliminary models by: attachment (to colloids and matrix), diffusion into the matrix, free transport (Advection dispersion of viruses and colloids) and retardation (mainly due to electrostatic interactions). Some preliminary modelling results are presented below and were also exposed in EGU-2018 where the effect of pH and ionic strength (IS) was tested in the cotransport of viruses and colloids.

  1. 1- Virus concentration breakthrough curves
  2. 2- Retardation factor variation
  3. 3- pH and IS variations