The Effects of Diabase Dikes on the Spatial Distribution of Baseflow: Geology Trumps Topography

Storage and release of groundwater during baseflow are controlled by both topography and geology. In some geological settings, intrusive structures such as diabase dikes can affect groundwater movement by acting as either pathways or barriers to flow. While dikes can modify groundwater flow patterns, their influence on baseflow remains uncertain. Here, we evaluate how topography and diabase dikes influence the spatial distribution of specific baseflow and δ¹⁸O in a catchment with heterogeneous geology. We conducted synoptic campaigns to measure δ¹⁸O and discharge during baseflow across 26 nested subcatchments (0.02–5.33 km²) of Peri Lake Experimental Catchment (20 km²) in southern Brazil. All subcatchments are underlain by granite, with 12 intruded by diabase dikes. The spatial variability of δ¹⁸O and specific baseflow was analysed against subcatchment area, mean elevation, mean slope, and geology using Spearman correlation and multiple linear regression. The results show that in subcatchments without diabase dikes, the spatial pattern of baseflow and δ¹⁸O is related to topography and reflects the high storage capacity at high elevation. In contrast, in areas with diabase dikes, irregular hydraulic conductivity and fracture distribution disrupt topography-related spatial patterns. These findings highlight that combining synoptic sampling and process-based approaches enhances the understanding of spatial patterns of groundwater flow, particularly in small catchments in data-limited regions.