Column Experiments Investigating Wetting and Drying of Soil and Consumption of Organic Contaminants for Managed Aquifer Recharge

Abstract:

The EU FP7 project MARSOL seeks to address water scarcity challenges in arid regions. Within this framework, we conduct a series of experiments to evaluate the potential for water quality improvement and changes in hydraulic conductivity when managed aquifer recharge (MAR) is performed by infiltrating treated wastewater in soils that do not have high potential for sorption. For example, in the Attica (Athens and vicinity) region of Greece, the bedrock is mostly marble, resulting in calcite-rich soils that present little potential for sorption of contaminants to mineral surfaces. This leaves consumption of organic contaminants by microbes as the critical mechanism for water quality improvement, when treated wastewater is infiltrated through such soils.

In order to enhance the potential for contaminant consumption by aerobic bacteria in a way that would be realistic to later perform in an infiltration basin, we conduct experiments using a series of wetting and drying cycles. The experimental setup consists of 90-cm long soil columns, fitted with oxygen sensors, time-domain reflectometry sensors (to measure moisture content), sampling ports, oxidation-reduction probes, and head observation tubes. We use the data collected from these sensors and features of the experimental setup to answer the following questions:

1. Does hydraulic conductivity change, from formation of a biofilm or dissolution of calcite (or both)?

2. Are organic contaminants consumed?

3. What effect do wetting and drying cycles have on consumption of organic contaminants?

4. How long can infiltration of treated wastewater last, before oxygen is consumed and conditions become reducing?

These questions are investigated by observing the hydraulic head and outflow, performing tracer tests, taking samples from the sampling ports and outflow for chemical analyses, and measuring moisture content and oxygen concentration, in the course of performing multiple wetting and drying cycles.

These column experiments will be used to evaluate the potential for new MAR applications in areas facing water scarcity challenges. In the future the experiments will be expanded to test multiple soils and optimize both the soil type and infiltration patterns in order to best obtain water quality improvements through MAR.