Groundwater is an important global resource, providing water for irrigation, industry, geothermal uses, and potable water all over the world. Moreover, groundwater contains the world’s largest terrestrial freshwater biome. Groundwater faunal communities undertake important ecosystem services including the provision of clean water. Despite this, investigations on the spatial and temporal variations and the influence of environmental parameters on these organisms, are still rare. The aim of this study is to provide a global overview on groundwater fauna (stygofauna) research, including the historical evolution of research topics and development of sampling methods. To achieve this, an extensive review of accessible groundwater fauna data was conducted. Over time, there has been an exponential increase in the number of studies together with changing paradigms in the research focus, particularly as sampling methods have developed and molecular analyses become common. Studies on groundwater fauna are spatially uneven and are dominated by studies in Europe and Australia, with few studies in Africa, Asia and the Americas. This has resulted in a potential geographic and climatically biased global view of stygofauna and groundwater ecology. In the future, a more evenly distributed sampling effort in underrepresented areas is necessary to enable global studies, thus allowing a more comprehensive perspective on stygofauna biodiversity, roles, and functional significances. This is increasingly important with the accumulating knowledge of the sensitivities of these ecosystems to anthropogenic activities, including climate change, and is fundamental to effective management of these ecosystems.

Harmonic Earth tide components in well water levels have been used to estimate hydraulic and geomechanical subsurface properties. However, the validity of various methods based on analytical solutions has not been established. First, we review the theory and examine the latest analytical solution used to relate well water levels to Earth tides. Second, we develop and verify a novel numerical model coupling hydraulics and geomechanics to Earth tide strains. Third, we assess subsurface conditions over depth for a range of realistic properties. Fourth, we simulate the well water level response to Earth tide strains within a 2D poroelastic layered aquifer system confined by a 100 m thick aquitard. We find that the analytical solution matches two observations (amplitudes and phases) to multiple unknown parameters leading to non-unique results. We reveal that undrained and confined conditions are necessary for the validity of the analytical solution. This occurs for the dominant M_2 frequency at depths >50 m and requires specific storage at constant strain of Sε ≥ 10-6 m-1, in combination with hydraulic conductivity of the aquitard kl ≥ 5*10-5 ms-1 and aquifer ka ≥ 10-4 ms-1. We further illustrate that the analytical solution is valid in unconsolidated systems, whereas consolidated systems require additional consideration of the Biot modulus. Overall, a priori knowledge of the subsurface system supports interpretation of the groundwater response. Our results improve understanding of the effect of Earth tides on groundwater systems and its interpretation for subsurface properties.