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- Title
Dynamic Contributions of Stratified Groundwater to Streams Controls Seasonal Variations of Streamwater Transit Times.
- Authors
Marçais, Jean; Derry, Louis A.; Guillaumot, Luca; Aquilina, Luc; de Dreuzy, Jean‐Raynald
- Abstract
Streamwater transit time distributions display a variable proportion of old waters (≥1 year). We hypothesize that the corresponding long transit times result from groundwater contributions to the stream and that seasonal streamwater transit time variations result from (a) the variable contributions of different flowpaths (overland flow, seepage flow and baseflow) and (b) the stratification of groundwater residence times. We develop a parsimonious model to capture the groundwater contribution to the stream discharge and its effect on transient transit times. Infiltration is partitioned according to the aquifer saturation between Boussinesq groundwater flow and overland flow. Time‐variable transit time distributions are obtained with a new 2D particle tracking algorithm. Hydraulic conductivity, total and drainable porosities are calibrated by using discharge and CFC tracer data on a crystalline catchment located in Brittany (France). The calibrated models succeed in reproducing CFCs concentrations and discharge dynamics. The groundwater flow contribution to the stream is controlled by the aquifer hydraulic conductivity, while its age is controlled by the drainable and total porosities. Old groundwater (≥1 year) is the source for approximately 75% of the streamflow with strong seasonal variations (between 40% and 95%). Mean transit times are approximately 13 years, varying between 6 and 20 years, proportional to the groundwater contribution. These seasonal variations are driven by the groundwater versus overland flow partitioning. The stratification of groundwater residence times in the aquifer plays a minor role in the streamwater transit times but is key for the transit time dynamics of the groundwater contribution to the stream. Plain Language Summary: Water entering a catchment as precipitation can take multiple paths with different transit times to the stream. While a significant fraction of water has short transit times (i.e., is "young") when it reaches the stream, there is also an important contribution of "old" water with long residence times in the subsurface. The age distribution of this old component is important for understanding the resilience of watersheds to climate change, the behavior of persistent pollutants and chemical weathering processes. We developed a model, informed with discharge time series and atmospheric age tracer (CFCs), to constrain the age distribution in both groundwater and streamwater and how they vary seasonally. In our temperate test catchment with crystalline bedrock (Brittany, France), we find that the mean age of streamwater exiting the catchment is approximately 13 years, but varies seasonally from 6 to 20 years as the relative contributions of older groundwater and younger runoff change. Groundwater stratification further influences the mean age of the groundwater contribution arriving at the stream. Key Points: Hydraulic conductivity controls groundwater flow contributions to streams while porosity directly scales the streamwater mean transit timesSeasonal groundwater contributions to streams modulate the mean transit times dynamics from 6 years at high flows to 20 years at low flowsStratified groundwater ages lead to a significant fraction (>75%) of old water to streamflow (>1 year)
- Subjects
GROUNDWATER; PERSISTENT pollutants; HYDRAULIC conductivity; GROUNDWATER flow; STREAMFLOW; HYDROGEOLOGY; AQUIFERS
- Publication
Water Resources Research, 2022, Vol 58, Issue 3, p1
- ISSN
0043-1397
- Publication type
Article
- DOI
10.1029/2021WR029659