Groundwater Age and Nonpoint Source Pollutant Mixing in Alluvial Aquifer Wells: Comparing the Role of Diffusion, Dispersion, Aquifer Heterogeneity, and Well Screen Length

Abstract Understanding the mixing of groundwater age and of nonpoint source (NPS) pollutants in water samples is crucial for interpreting age tracer and NPS pollutant data from production wells. Traditionally, diffusion and mechanical dispersion have been key mixing processes embedded in physical models for simulating and interpreting age tracer and NPS pollutant data. Also, a large body of literature highlights mixing due to aquifer heterogeneity across scales. Importantly, the collection of water samples through wells introduces additional mixing across the internal volume of the well screen. Here, we investigate and quantify the magnitude of mixing due to these four processes—diffusion, mechanical dispersion, aquifer heterogeneity, and in‐well mixing—through a Monte Carlo‐based modeling framework and sensitivity study. We consider wells in a typical unconsolidated alluvial aquifer system. We find that in‐well mixing and aquifer heterogeneity dominate the mixing process in larger production wells. In small production wells and in monitoring wells, diffusion and (sub‐grid scale) mechanical dispersion add significantly to age/pollutant mixing. Across an ensemble of larger production wells (e.g., in regional planning), the range of age (or pollutant) mixing observed is dominated by in‐well mixing, with aquifer heterogeneity not significantly changing the age mixing distribution. The depth of the well screen has some impact on age mixing only in small (monitoring) wells. Our work suggests that ensemble age (or mixed NPS pollutant concentration) distributions across large sets of production wells can be satisfactorily estimated from well construction information and by considering advective transport in equivalent homogeneous media.