A zonal evaluation of intrinsic susceptibility in selected principal aquifers of the United States

A method was developed to evaluate intrinsic groundwater susceptibility in 11 study areas across the United States. Calibrated groundwater-flow models and a variable-advection particle-tracking scheme that accounts for uncertainty were used to derive ranges of conservative solute concentration and groundwater age within spatially defined zones from solute loading to the water table. Aquifers were partitioned into six zones; four relative depth zones and two zones to represent pumping wells and surface water. Five years after solute was introduced in simulated recharge and stream leakage, normalized zone concentrations were detected at values above 10^-4 in the shallowest aquifer zone, well zone, and surface-water zone for 10 of the 11 study areas. At the 125-year time scale, 9 out of the 11 study areas exhibited detectable concentrations in all zones and the majority of zones possess concentrations that are substantial relative to the source concentration (ClC_o > 10-1). Thresholds defined by the time representing the earliest 1% of groundwater-transit times were used to identify fast transport pathways within the groundwater. The 1% thresholds occurred in a period of days to years for the shallow zone, days to decades for the well and surface-water zones, and years to millennia for the deeper zones. Thresholds defined by the 99th percentile of groundwater travel times were used to reflect late-time response and ranged considerably between study area (~10² to ~10⁶ years), which highlights the potential for chemical constituents to persist in groundwater for long periods under a conservative state. The results of this investigation provide an instructive example of the intricate relations between climate and aquifer characteristics and their role on solute transport in groundwater. The proposed method accounts for dynamical processes in the aquifer and complements more traditional assessments of susceptibility using (apparent) mean water age.