Physical, geochemical and microbial processes induced during the aquifer recharge using treated wastewaters: laboratory and pilot experiments and numerical simulations

M. Azarouala*, D. Thiérya, N. Amraouia, M. Pettenatia, N. Croiseta, J. Casanovaa, K. Besnardb, N. Rampnouxb

aBRGM, Water Division, Orléans, France
bVEOLIA Environnement, Rueil-Malmaison, Paris, France

In order to transform wastewater into a new water resources with specific and appropriate uses (irrigation, stop saltwater intrusion, etc..) several concepts and scenarios for artificial recharge of deep aquifers have been proposed. Even if this technology can help supply water to several parts of the world water shortage it is necessary to control the physical, physico-chemical and bacterial conditioning the fate of any recalcitrant organic and inorganic pollutants treated wastewater and / or surface water of poor quality. This knowledge will help ensure compatibility of the technology to implement with the features and quality of water recharge, geochemical and microbiological reactivity of different compartments of the soil, subsoil and aquifer as well as receiver under specific climatic conditions. A joint research project is developed by BRGM and VEOLIA aiming to develop a general and transferable technology based on the control of the key physical, chemical and microbial processes easy to integrate in the numerical predictive and quantitative tools.

Indeed, the joint research aims to develop/adapt numerical tools to estimate the purification capacity of natural soils vis-à-vis the wastewater and thus better understand the process of interaction and feedbacks between recharge water and the vadose zone (USZ). A detailed study of mechanisms of transfer reagents recharge water through the USZ is a prerequisite for the development of a proposed wastewater reuse. To this end a pilot consisting of a column of soil was build to analyze the scale of displacement reactive fronts, their amplification or mitigation. This is a cell of 3 m in diameter and 4 m in height, fully insulated from the ground to comply with regulatory constraints, and filled with a soil characteristic of the region. The top of the pilot is open to local climatic conditions to simulate a real operational site in terms of temperature, rainfall, etc.. In order to monitor the flow and changes in water content, the pilot is equipped with a set of 5 tensiometers, 5 water content probes, and 5 thermometers with measurements every hour. The atmospheric parameters (rainfall and temperature) are monitored on a daily basis. In a first phase a model of flow, mass and energy transfer through the vadose zone has been developed capturing the hydrodynamics of the pilot, which is a prerequisite before a coupled reactive transport model which is scheduled when geochemical and biochemical data are available for a long period of time.

The availability of pilot results over a period of eighteen months will be presented coupled with an interpretative modelling approach incorporating the mechanisms of transport and reactions of dissolution - precipitation of minerals, degassing and geobiochemical processes. Current results (at this stage of the progress of the experiment) show a marked decrease of nitrate concentrations without significant evolution of that of sulphates. The expected results of the pursuing experience with changes in temperature and perhaps of the feed water chemistry should provide important results for the development of an integrated approach. It is also planned to analyze the mineralogical stratification to identify areas of high reactive (precipitation of minerals, clogging zones, biofilm development, etc.).

Acknowledgement
This work is undertaken in the framework of joint project between BRGM and Veolia Environment.