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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. Plain Language Summary: The transit time distribution (TTD) of an aquifer describes the complete spectrum of groundwater transit times from the water table to discharge at an outlet such as a stream. TTDs indicate how susceptible aquifers may be to contamination, drought, or pumping. Despite their importance, TTDs are difficult to quantify and many approaches either overestimate the relative fraction of young groundwater discharge or have been conducted in small systems (areas <1,000 km2 or stream lengths <10 km). We measured the flow‐weighted transit time of groundwater through the streambed at 88 points along a 99 km stream in the Upper Middle Loup watershed (5,440 km2) in Nebraska, USA to estimate TTDs of the underlying High Plains Aquifer at various spatial scales. Transit times ranged from 0 to 8,200 years and the mean transit time is >3,000 years along the entire study stream with substantially older groundwater discharge downstream. Upstream TTDs were well‐fit by distributions with larger fractions of young groundwater (<15 years), but TTDs for the entire study stream were better fit by distributions that accommodate large fractions of millennial‐aged groundwater. Our findings suggest that TTDs can vary within a watershed and ancient groundwater may be a significant component of modern streamflow. Key Points: Groundwater age and flux along a 99 km stream overlying the High Plains Aquifer, USA yield transit time distributions (TTD) and meanGroundwater ages ranged from 0 to 8,200 years with mean age >3,000 years and increasing apparent age of baseflow downstreamTTD evolution with stream distance suggests streams overlying regional systems may intersect large fractions of millennial‐aged groundwater [ABSTRACT FROM AUTHOR]

Gas exchange between streams and overlying air is an important physical‐chemical environmental process that is typically determined by injecting a tracer gas into a stream at a steady rate and sampling steady‐state tracer gas concentrations in the stream water. Previous modes of tracer gas injection allow gas‐water partitioning of the tracer gas, making the rate of gas injection and thus the measured gas transfer velocity potentially sensitive to temperature variation. Presented here is a modification to the tracer solution injection method in which a tracer gas solution was prepared in Tedlar® bags from which all headspace was removed before injecting the solution into the stream. Along with four other strategies to prevent a headspace from forming in the bags during tracer injection in the field, this zero‐headspace tracer solution method prevents gas‐water partitioning anywhere in the injection system, allowing a steady delivery of tracer gas to the stream even in the presence of variation in air and/or stream water temperature. A field test of the method in Nebraska yielded a gas transfer velocity of 4.1 m/day, within the range found in the literature for similarly‐sized streams. Plain Language Summary: This paper reports on the development and testing of a method that allows injection of tracer gases into streams and rivers at a constant rate even if the temperature of the air or water is changing over time. The new method allows accurate estimation of gas transfer velocities from injected tracer gases at times and places with significant temporal variation in temperature. Key Points: Steady tracer gas injections are regularly used to measure the gas exchange velocity (v) between streams and the overlying airTemperature variation affects tracer gas solubility, which can lead to unsteady gas injection rate and error in measured vA zero‐headspace tracer solution method was field tested and is proposed to measure v in the presence of large temperature variation [ABSTRACT FROM AUTHOR]

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