Shallow groundwater that develops on hillslopes is the main compartment in headwater catchments for flow and solute transport to rivers. Although spatial and temporal variations in its chemical composition are reported in the literature, there is no coherent description of the way these variations are organized, nor is there an accepted conceptual model for the mechanism involved. On the basis of the results from an experimental hillslope, we propose a description of the variation in spatial and temporal patterns of groundwater chemical composition and discuss the mechanisms involved. We instrumented a headwater catchment with a network of 42 nested piezometers along a 200 m hillslope and monitored major anion composition frequently over a 2-year period. The results demonstrated that (i) the anionic composition in water table fluctuation zone varied significantly compared to deeper portions of the aquifer on the hillslope, confirming that this layer constitutes a main compartment for the mixing of new recharge water and old groundwater, (ii) deeper parts of the aquifer exhibited seasonal variations with structured hysteretic patterns, suggesting that mixing processes also occurred at greater depths and (iii) these hysteretic patterns were dampered from upslope to downslope, indicating an increased influence of lateral flow downslope. We recommend that the generality of these observations be assessed in different environments, as they can constitute a new set of processes to include in hydrological models. Copyright © 2010 John Wiley & Sons, Ltd.