Groundwater Age Tracer and NPS Pollutant Mixing in Groundwater Samples: Comparing Effects of Diffusion, Mechanical Dispersion, Aquifer Heterogeneity, and Well Screen Mixing

The mixing of groundwater age in water samples - or, similarly, the mixing of NPS pollutants in water samples - and understanding the processes controlling such mixing are key to accurately interpreting age tracer and NPS pollutant sample data typically obtained from networks of production wells. Traditionally, diffusion and mechanical dispersion have been key mixing processes embedded in process models for simulating and interpreting age tracer and NPS pollutant data. Also, a large body of research literature has been developed to account for the additional, non-ideal mixing that occurs on account of aquifer heterogeneity across scales. The collection of water samples through wells introduces additional mixing across the internal volume of the well screen, a main reason for developing highly localized sample ports in research monitoring wells that characterize point source contaminant plumes. However, most age tracer and NPS pollutant campaigns occur on existing production well networks across aquifer systems. Significant mixing occurs within each well screen as water samples are obtained. Here, we investigate the magnitude of mixing due to these four processes - diffusion, mechanical dispersion, aquifer heterogeneity, and in-well mixing - through a Monte Carlo-based numerical simulation framework and sensitivity study. We consider wells in a typical unconsolidated alluvial aquifer system, common across many developed landscapes around the world. We find that in-well mixing and aquifer heterogeneity are dominating the mixing process in larger production wells. Diffusion and (sub-grid scale) mechanical dispersion add significantly to age/pollutant mixing only in small production wells and in monitoring wells. More-over, across an ensemble of larger production wells, the range of age (or pollutant) mixing observed is dominated by in-well mixing, with aquifer heterogeneity not significantly changing the ensemble distribution of the age mixing observed. The depth to the top of the well screen has some impact on age mixing only in very small (monitoring) wells. Our work suggests that ensemble age distributions across large sets of production wells can be obtained as a function of well construction information and by considering advective transport in equivalent homogeneous media.