Groundwater storage as a major control of seasonal stream water quality dynamics during both base and storm flows

Stream water quality exhibits dynamics at various time scales that are controlled by the spatiotemporal dynamicsof hydrobiogeochemical compartments, particularly by the dynamics of their connectivity to the stream. Basedon long-term catchment observations from the AgrHyS Observatory (France), we established relationshipsbetween stream water quality and groundwater storage dynamics on various time scales. Those observationscombine stream water concentrations of Dissolved Organic Carbon (DOC), Nitrates (NO3) and Phosphorus (SRP)concentrations, and a network of piezometers along 2 transects to characterize groundwater storage and quality inspace and time.At the annual scale, the rise of groundwater table is delayed between the riparian area (early autumn) andthe upslope area (winter). This time lag along the hillslope is identified as the dominant control of the seasonalsignal in stream base flow nutrient concentrations.-DOC and SRP are mobilized from organic upper soils layers when reached by the groundwater table. Firstly,groundwater table rises in the riparian area, and connects the C and P sources in the organic rich riparian soilsleading to high stream DOC and SRP concentrations. Then, while groundwater connectivity remains high inthe riparian area, these sources are depleted leading to a progressive decrease in stream base flow DOC andSRP concentration. In the recession and low flow periods, shallow groundwater contribution decreases and otherprocesses can influence stream concentrations.-NO3 are mobilized from upslope shallow groundwater and therefore nitrate annual dynamics follows the annualfluctuations of catchment water storage. At the start of the water year, the water table rise in the denitrified riparianarea leads to low stream nitrate concentrations. Then, the water table rise in the upslope area leads to suddenincrease in stream nitrate concentration. In the recession period, the nitrate concentration decreases variablydepending on the dynamics of the groundwater recession, itself controlled by climate and vegetation demand.Groundwater dynamics appear to control concentration dynamics not only during base flow period but alsoduring storm events. Most of the storms occur during winter when groundwater is high and sub-surface connectivitymaximal. Conversely, the events that occur during dry periods can depict very different stream concentrationresponse because of reduced subsurface connectivity due to deeper groundwater table, leading to more variablestorm flowpaths.The two studied hillslopes show contrasting groundwater nitrate concentrations, soil organic matter andsoil phosphorus content. These differences are mainly attributed to the difference in soils-groundwater interactionsand in past and current farming practices.This conceptual model of the effect of spatio-temporal variations of groundwater table depth on soil andstream water quality highlights the importance of shallow groundwater in controlling the non-stationarity and thespatial variability of water quality response in catchments. This offer new constrains for modelling water qualityat different time scales.