Use of groundwater level measurements to support hydrogeological studies in the Maputo aquifer, Mozambique

High-frequency dataloggers for groundwater level monitoring were used in combination with other tools to analyze tidal effects on groundwater levels (GWLs) in the Maputo aquifer, Mozambique. Power spectral analysis was used to ascertain the dominant periodic components in the tide and GWLs, and cross-spectral analysis was used to determine the lag time between them. Wavelet analysis was applied to investigate changes in periodic components over the measured period in the time-frequency domain. The estimated amplitudes and lag times were then used to estimate aquifer diffusivity and the water-table fluctuation (WTF) method was used to compute groundwater recharge. The results identified a 12.42 h dominant periodic component both in the tide and GWLs in the coastal area. However, GWLs lag behind the tide by 2-4 h, depending on the distance of the observation wells to the coastline. The wavelet analysis results showed no changes in the dominant periodic components over time. The estimated specific storage values for four piezometers were estimated to be 3.19 x 10(-5), 5.04 x 10(-5) and 1.02 x 10(-4) 1/m, respectively. Annual groundwater recharge for the young sand dune aquifer was estimated for one piezometer with a specific yield of 0.15-0.25 was within the range of 123-205, 185-309, 504-840 and 244-407 mm, for four hydrological years from 2018 to 2021. Estimated specific storage values and recharge rates are essential inputs to support the construction of transient groundwater models for the Maputo aquifer.