The textbook "Geochemistry, Chemical Speciation, and Environmental Impacts of Trace Elements and Rare Earths" provides a comprehensive analysis of hydrogeochemical processes and the chemical speciation of rare earth elements. The study is based on water samples collected from various sources in the Oban Massif and the Mamfe Embayment in southeastern Nigeria. The author, Gregory Sikakwe, details the methods for measuring physical parameters such as pH, redox potential, and electrical conductivity, as well as laboratory analyses of anions and cations. Special attention is given to the analysis of trace elements and rare earth elements, conducted using modern techniques such as inductively coupled plasma mass spectrometry. The results of chemical speciation and modeling with VISUAL-MINTEQ 3.1 provide valuable insights into the environmental impacts of these elements and their significance in geochemistry. This book is aimed at professionals and students in the fields of geosciences, environmental sciences, and chemistry.