Spatial Variation in Transit Time Distributions of Groundwater Discharge to a Stream Overlying the Northern High Plains Aquifer, Nebraska, USA

Groundwater transit time distributions (TTDs) describe the spectrum of flow-weighted apparent ages of groundwater from aquifer recharge to discharge. Regional-scale TTDs in stream baseflow are often estimated from numerical models with limited calibration from groundwater sampling and suggest much younger groundwater discharge than has been observed by discrete age-dating techniques. We investigate both local and regional-scale groundwater TTDs in the Upper Middle Loup watershed (5,440 km²) overlying the High Plains Aquifer in the Nebraska Sand Hills, USA. We determined flow-weighted apparent ages of groundwater discharging through the streambed at 88 discrete points along a 99 km groundwater-dominated stream segment using ³H, noble gases, ¹⁴C, and groundwater flux measurements at the point-scale (<7.6 cm diameter). Points were organized in transects across the stream width (3–10 points per transect) and transects were clustered in five sampling areas (10–610 m in stream length) located at increasing distances along the stream. Groundwater apparent ages ranged from 0 to 8,200 years and the mean groundwater transit time along the 99 km stream is >3,000 years. TTDs from upstream sampling areas were best fit by distributions with a narrow range of apparent ages, but when older groundwater from downstream sampling areas is included, the regional TTD is scale dependent and the distribution is better described by a gamma model (α ≈ 0.4) which accommodates large fractions of millennial-aged groundwater. Observations indicate: (a) TTDs can exhibit spatial variability within a watershed and (b) watersheds can discharge larger fractions of old groundwater (>1,000 years) than commonly assumed.