Groundwater origins and recharge in a well field near Chien-Shih, Shinchu, Taiwan

Based on global hydrological data, Taiwan has been classified as an area of water shortage. In some regions, and particularly during dry periods, groundwater is the only reliable water resource in Taiwan. This is particularly true for mountain regions. To determine the potential yield, and also to support sustainable management of groundwater resources, groundwater recharge must be estimated. The results of this study showed that there was a close relationship between measurements of the water table and the accumulated monthly precipitation in a well field close to Chien-Shih, Shinchu, NW Taiwan, which suggests that precipitation is the major source of recharge in the study area. However, the difference in the water table between the wet and dry seasons indicated that, in addition to precipitation, there is recharge from another stable source. We determined the pathway from, and the geological setting of, the recharge source areas by comparing precipitation events and temporal transformations in the water types. Out of four potential water types, the water corresponded to the Ca/Mg–SO4 2− type (III) at different points throughout the study period, indicating geochemical controls, such as dissolution–precipitation, dilution, cation exchange, silicate weathering, and mineralization. Meteoric water stored in the mountain region was another source of recharge that contributed to hydrochemical variability during the dry season. When this source was added, the mean δ18O value dropped from −15.8 to 1.4‰ in precipitation, from −9.6 to −7.8‰ in river water, from −9.1 to −6.9‰ in gully 1, from −7.4 to −5.8‰ in gully 2, and from −9.8 to −6.6‰ in groundwater. The isotopic compositions of Well C and Well W1 became lighter with additional recharge from the river, while those of Well E, Well W, and Well W2 became heavier during the rainy season. In the dry season, the isotopic composition was lighter in Well W1 and heavier in the other wells. It is worth noting that the isotopic composition of Well W1 was similar to that of the river, suggesting high connectivity between the river and Well W1, possibly through fractures.