Mass balance models for porewater chemistry and soil mineral chemistry have been completed for planar transects in the Shale Hills catchment. Mass balance is typically used to understand Mg concentrations because Mg is released to solution during weathering of illite and chlorite in the soils. In Figure A, a conceptual model of fluid flow in the soil is shown: high permeabilities at the A/B and B/C horizon interfaces allow fast fluid flow (large dark arrows). Within the B horizon, transport is dominantly by diffusion (white arrows). Water flow is fast in the vertical direction only through macropores (grey zones with vertical arrows). Fluid flow through vertical macropores allows water to enter the high-flux A/B and B/C interface zones quickly after rain events. Mg concentrations at these interfaces remain therefore generally low. In contrast, Mg concentrations reach high levels in the B horizon where flow is relatively insignificant. In (B), average concentrations are shown for south planar ridge top (SPRT), midslope (SPMS), and valley floor (SPVF) sites in the high-flow zones at the A/B and B/C interfaces (values in boxes). The average concentrations in the low-flow B horizons for SPMS and SPVF are also shown (values in ovals). Using the intensive observation network at Shale Hills catchment the conceptual model elucidates hydrogeochemical connections from rainfall, through soil horizons and groundwater, to stream chemistry.