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

Abstract

AbstractGroundwater 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.