Research on groundwater mixing between aquifers by isotope technique: A case study in Ca Mau, Nam Bo Plain

Abstract

Ca Mau is a province in the Mekong Delta with a complex hydrogeological structure, many faults, hydrogeological windows, and non-uniform distribution of aquitards, including many interspersed salt/fresh groundwater lenses, directly or indirectly affecting the recharge and movement of water in aquifers. In addition, the exploitation of groundwater at high density, focusing on only 3 out of 7 existing aquifers, has seriously reduced the groundwater level, strengthening hydraulic relationships and mixing of water sources between aquifers. To clarify the hydraulic relationships as well as the mixing process between aquifers in Ca Mau province, this study investigated and analysed the mixing of water sources between aquifers using isotopic techniques. We analysed 32 groundwater samples for 14C activity, δ18 O, and δ2 H. Results show that Ca Mau's groundwater is exclusively "Old Groundwater" with no modern recharge. The correlation between 14C activity and sampling depth is weak (r2 = 0.11 for fresh and 0.26 for saline groundwater), indicating complex mixing processes. Two main factors affecting groundwater mixing were identified: the hydrogeological structure and local mixing processes, including mixing with saline water of marine origin and mixing with other freshwater sources due to the strengthening of the hydraulic relationship between aquifers. The spatial distribution map of 14C activity and δ18 O in Ca Mau Province indicates areas of intense mixing. This study provides crucial insights for sustainable groundwater management in Ca Mau Province.