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Local groundwater in coastal aquifers in Shenzhen has experienced heavy pumping since the 1980s when it began to expand very quickly from a fishing village to a modern city. Meanwhile, large-scale land reclamation was carried out to meet the needs of various urbanization projects. In this paper we analyzed the groundwater from a coastal aquifer system in Shenzhen and examined the evolution of groundwater chemistry over the last 20 years. The temporal changes of ionic ratios of rCa/(rHCO3 + rSO4) and the relationship between sodium and chloride in the coastal area over this period indicate that the aquifer experienced seawater intrusion in the 1980s but underwent gradual freshening in the 1990s. It is speculated that seawater intrusion was induced by excessive groundwater pumping and that the aquifer freshening was caused both by the recent reduction in groundwater pumping and by coastal reclamation which moved the interface between fresh groundwater and saline groundwater seaward.