Keywords

Tracer tests; vadose zone; tailing; underground storage

Location

Session D4: Water Resources Management and Planning - Modelling and Software for Improving Decisions and Engaging Stakeholders

Start Date

12-7-2016 8:50 AM

End Date

12-7-2016 9:10 AM

Description

The “Beauce” limestone aquifer is contaminated by nitrate and pesticides mostly coming from agricultural activities. In this region the vadose zone can extend to more than 20 m in depth, appears to be highly heterogeneous and plays a significant role in the retention and/or release of pollutants to aquifers and wells. In order to gain insight into contaminant transfer in the vadose zone, tracer experiments were conducted in a pit, 4 meters wide, 1.6 meters deep, using (1) 30 L of brilliant blue (BB) at a concentration of 6 g/L to map the preferential flow along the percolation profile and (2) 30 L of a Bromide solution at a concentration of 5 g/L to study the water dispersion. A lithological description of the pit was carried out. BB mapping with depth was used to map the preferential flows. The bromide breakthrough curves were monitored at 8 points, within the vadose zone profile using suction caps.

The lithological description evidenced a highly heterogeneous limestone. At the soil interface, tracers flowed through the different materials and delays were observed in the less permeable material. With depth, BB spreading showed that flows concentrate in the more permeable material, bypassing the poorly permeable ones. Thus after a few meters, only the highly permeable material drives the flows. The two dispersion paths described here can be modeled by considering that the entire vadose zone is represented by the high-permeability homogeneous material and that the initial boundary condition is not a pulse but a pulse followed by a slow decrease in the concentration power law. This solution made it possible to model the observed bromide tracer breakthrough curve and opens up interesting perspectives for modeling the residence time distribution of nitrate at the watershed scale.

 
Jul 12th, 8:50 AM Jul 12th, 9:10 AM

Evidence of contaminant storage at the soil/vadose zone interface and implication for nitrate residence time modeling in groundwaters

Session D4: Water Resources Management and Planning - Modelling and Software for Improving Decisions and Engaging Stakeholders

The “Beauce” limestone aquifer is contaminated by nitrate and pesticides mostly coming from agricultural activities. In this region the vadose zone can extend to more than 20 m in depth, appears to be highly heterogeneous and plays a significant role in the retention and/or release of pollutants to aquifers and wells. In order to gain insight into contaminant transfer in the vadose zone, tracer experiments were conducted in a pit, 4 meters wide, 1.6 meters deep, using (1) 30 L of brilliant blue (BB) at a concentration of 6 g/L to map the preferential flow along the percolation profile and (2) 30 L of a Bromide solution at a concentration of 5 g/L to study the water dispersion. A lithological description of the pit was carried out. BB mapping with depth was used to map the preferential flows. The bromide breakthrough curves were monitored at 8 points, within the vadose zone profile using suction caps.

The lithological description evidenced a highly heterogeneous limestone. At the soil interface, tracers flowed through the different materials and delays were observed in the less permeable material. With depth, BB spreading showed that flows concentrate in the more permeable material, bypassing the poorly permeable ones. Thus after a few meters, only the highly permeable material drives the flows. The two dispersion paths described here can be modeled by considering that the entire vadose zone is represented by the high-permeability homogeneous material and that the initial boundary condition is not a pulse but a pulse followed by a slow decrease in the concentration power law. This solution made it possible to model the observed bromide tracer breakthrough curve and opens up interesting perspectives for modeling the residence time distribution of nitrate at the watershed scale.