The potential of collected rainwater as an alternative water resource for domestic purpose to mitigate groundwater overexploitation in terms of water quality and people’s acceptance

• This study assesses rainwater as alternative domestic water resource to reduce groundwater depletion. • Rainwater potential was evaluated through surveys and sampling in Ca Mau Province. • Most rainwater parameters met Vietnamese standards, but microbial treatment is required. • Public acceptance of rainwater as an alternative to groundwater is high. • Government communication and support strongly influence public acceptance. Ca Mau province, located in the south of the Mekong Delta, is strongly affected by land subsidence and seawater intrusion. Overexploitation of groundwater has been highlighted as a significant driving factor for these processes. As groundwater currently plays a crucial role in the local water supply, this study examines the potential of rainwater as an alternative water source for domestic use to reduce groundwater extraction and contribute to ensure the safe and sustainable water resource for public use (SDG 6). The present study follows a multi-disciplinary evaluation of the potential of rain water as an alternative water resource, covering an assessment of (i) social perception of rainwater usage, (ii) rainwater availability as well as water quality analyses of rainwater stored under various conditions. For this, a combined survey and sampling campaign throughout Ca Mau province was conducted in two main phases between 2019 and 2022. The survey includes a questionnaire which was completed by 473 participants together with in-depth interviews in several selected locations. Complementary, time series of precipitation were analyzed to determine the potentially available rainwater quantity. Stored rainwater samples were collected and analyzed according to their quality based on physical parameters, chemical parameters using ICP-MS and Ion Chromatography with 49 water samples and biological parameters using the IDEXX Colilert® system and MALDI-TOF-MS with 75 water samples. Based on the results, factors that could affect the quality of rainwater were examined. The results show that in some rainwater samples, threshold values of the standards from Vietnamese regulation were exceeded for some parameters, raising the issue of rainwater treatment before use. The questionnaire revealed that rainwater is considered as a potential water source which can replace groundwater for domestic purposes by participants. Despite concerns about rainwater storage during the dry season, public acceptance of using rainwater is generally high. However, the government’s interest in communicating information about current environmental issues as well as support in guiding rainwater storage and economic support during the transition process is an issue that people are concerned about. The results of this study provide new insights to further identify potential pilot sites in Ca Mau for rainwater harvesting as an alternative to groundwater extraction, as well as to design concepts for optimizing the use of rainwater.