Determination of baseline groundwater levels for tree conservation in urban historical botanical gardens using applied geophysics

• Historical botanical gardens are part of cultural heritage. • Root and groundwater interactions are crucial for conserving tree collections. • Their spatial layouts require tailored methodologies for subsurface assessment. • Geophysical techniques digitally map subsurface conditions non-invasively. • 3D visualization of 2D electrical resistivity tomography models. Historical botanical gardens hold a significant place in cultural heritage. They serve as interpretive repositories of past botanical knowledge and practices, showcase plant collections cultivated over centuries, provide space for the emergence of new ecologies, offer numerous human well-being benefits, and supply vital regulating ecosystem services, which are especially important in urban areas. Nowadays, however, plants within urban historical botanical gardens can be at risk due to urban development. Therefore, it is crucial to achieve a comprehensive understanding of these spaces to help implement protective measures and support proper urban planning of the surrounding areas. This study investigates the subsurface of the Botanical Garden of Lisbon (JBL), which is subject to nearby construction works that may alter groundwater flow and depth. We employed a methodology designed for minimal on-site disturbance and high adaptability to the spatial constraints typical of these spaces. Two non-invasive applied geophysical techniques were used: ground-penetrating radar (GPR) and electrical resistivity tomography (ERT). Our main objectives were: (1) to assess groundwater levels in the construction area and establish the piezometric surface, and (2) to determine if the tree-root zones reach the saturated zone, establish a groundwater baseline, and suggest protective measures. The establishment of the piezometric surface and the delimitation of the tree-root zone, extending up to 3.0 m in depth, revealed that tree-root zones can reach groundwater levels. This finding underscores the critical need for vigilant monitoring and management of groundwater levels during excavation activities, as decreased lateral groundwater contributions from the potential excavation site can adversely affect groundwater levels in the plant beds. These findings and methodology can be applied to urban botanical gardens worldwide, as many of these gardens face similar challenges due to urbanization and environmental changes.