Multiyear Increase in the Stable Isotopic Composition of Stream Water From Groundwater Recharge Due to Extreme Precipitation

The isotopic composition of surface and groundwater is impacted by many hydrologic processes. The long-term stable water isotopic response of systems to hydrologic change is critical for interpreting isotopic information for streamflow generation, stream-aquifer-coupling, and recharge processes. To evaluate the response of stream-aquifer systems to extreme precipitation events we use 743 surface and groundwater isotopes with drainage areas ranging from 0.1 to >800 km(2). Results show multiannual trends from high to low isotopic compositions associated with increases in the composition of shallow groundwater. The year 2011 was one of the wettest years and the months of August and September were the wettest consecutive 2-month period in the 123-year record. This increase in the isotopic composition has long-term impact on the isotopic composition of surface and groundwater highlighting the importance of groundwater sources of baseflow to streams and the transient storage and release mechanisms of groundwater at the catchment scale. Plain Language Summary We present and analyze surface and groundwater samples collected from late 2011-2016 across a large region of western Massachusetts. The data together show multiyear trends in the stable water isotope compositions, a tracer of water source and hydrological processes. These trends are not reflective of typical seasonal behavior and point to a change in the isotopic composition due to a unique hydrologic event. The year of 2011 was the wettest calendar year on record for western Massachusetts and the months of August and September of 2011 were the wettest consecutive 2-month period in the 123-year precipitation record. This wet period saturated the ground with excess water leading to large increases in the regional water table and high infiltration rates. The isotope data suggest that 4-5 years later we still observe this water draining from the groundwater system to surface streams. These results point to the important role of extreme precipitation events to surface and groundwater catchments, even in regions with high annual precipitation rates. The inclusion of groundwater processes (recharge, storage, and discharge) are important controls on surface water isotopic composition.