δ18O, δD and 3H measurements constrain groundwater recharge patterns in an upland fractured bedrock aquifer, Vermont, USA

Stable isotope ratios, measured in groundwater samples, reflect the effects of evapotranspiration on bedrock recharge and flow patterns in a 10.5 km2 upland mountainous watershed in northwestern Vermont. Precipitation and groundwater samples (n=619) were collected weekly over a 1.5 year period. Precipitation δ18O ranges yearly over ∼25 per mil (‰), and decreases 2.5‰ for every 1000 m of elevation gain, reflecting seasonal and altitudinal temperature changes. δD values are well correlated with δ18O and plot close to the Meteoric Water Line, indicating no evaporation effects. In the colder months (late November to early April), groundwater δ18O composition varies by as much as 4.3‰ in response to precipitation events, indicating rapid local recharge to the bedrock. In the warmer months (late April to early November), variation in groundwater δ18O is smaller (within 0.4‰) at the lower elevations in the watershed, reflecting a reduction of infiltration to the bedrock. However, at higher elevations (above 800 m asl), groundwater δ18O continues to respond to precipitation events due to the sparse vegetative cover and colder temperatures that result in lower evapotranspiration rates. Tritium concentrations in groundwater range from 6.7 to 26.7 TU, indicating that groundwater residence times may vary from less than 1 year to in excess of 30 years.