Push-Pull test – Reactive transport modelling: A new approach to study water quality changes

“Groundwater technologies, such as Managed Aquifer Recharge, Aquifer Thermal Energy Storage and Subsurface Iron Removal, often result in desired or unwanted groundwater quality changes. For the first time, field Push-Pull Tests (PPTs) were combined with multi-component geochemical reactive transport modelling (RTM) to assess aquifer reactivity. This method, an alternative to column studies or field pilots, was applied to 2 groundwater wells at an agricultural Aquifer Storage and Recovery (ASR) site (Breezand, the Netherlands). The injected water at this ASR system has relatively high concentrations of nutrients (average: NO3 ~ 20 mg/L; PO4 ~ 15 mg/L). The fate processes, recycling of nutrients and other reactions were studied. The water quality changes observed during these PPTs were modelled with RTM. Equilibrium processes were used to simulate cation exchange, precipitation of Fe-(hydr)oxides and surface complexation on fresh Fe-(hydr)oxides and initial goethite minerals. The oxidation of pyrite and soil organic matter with O2 and NO3, and dissolved ferrous iron with O2, were simulated as kinetic rate expressions. This approach provided not only information about the rates, but also about reaction networks and the factors that control these rates. Oxygen and nitrate were consumed by ferrous iron, organic matter and pyrite oxidation in both PPTs and were depleted in maximum 2 and 7 days, respectively. In the shallow well (24 m-b.g.l.), organic matter was the largest consumer of oxygen and nitrate, whereas in the deeper well (35 m-b.g.l) this was pyrite. In both wells the precipitation of Fe- hydroxyphosphates seems the main process related to the decreased phosphate concentration in the abstracted water. Transformation and precipitation showed to be the most important processes concerning nutrient fate. The PPT-RTM method results in a better fundamental understanding of governing processes. It could be used as a relatively cheap and simple tool in exploratory studies for groundwater technologies. Therefore, the method will be used in further research on several multi-level wells at different locations, to get insights in the reactivity of shallow aquifers in the Northern part of the Netherlands. This data will be used to determine the feasibility of ASR systems in this region, related to water quality.”

(Citaat: Kruisdijk, E., Stuyfzand, P.J., van Breukelen, B.M. – Push-Pull test – Reactive transport modelling: A new approach to study water quality changes – ISMAR 10, Madrid, Spain, 20-24 May 2019, p,120)