Anomalies in the ionic and gas composition of groundwater before earthquakes (hydrogeochemical precursors, hereinafter HGCP) are considered according to data of systematic observations in ten selfflowing wells and springs in seismically active regions of Russia (Kamchatka Peninsula), Uzbekistan, Japan,and Iceland. Using an extended set of earthquake parameters, it was shown that HGCP were manifested for 1–9 months in the near and intermediate zones of the future earthquake sources with Mw = 5.3–7.8. Such properties of hydrogeochemical precursors allow their use for predicting the magnitude and time of expected seismic events, as well as their impact in the observation area. A model is presented for a registered precursor in the form of the ionic composition of groundwater in self-flowing well based on observational data on concentrations of anions and cations and on a mathematical model for the mixing of two waters of contrastingly different composition in a zone of increased conductivity. Calculations for one of the wells are presented as an illustrative example of the chemical composition of two mixing waters in the aquifer and in the wellbore. This model and observational data are used to estimate parameters of the disturbed state of the well–aquifer system during the preparation of an earthquake: the relaxation time of the water pressure pulses (t0) and the movement time of mixed water (τ0). The morphology and duration of HGCP are controlled by relations between t0 and τ0. The further development of HGCP studies and their application in predicting earthquakes require a technical modernization of the system of hydrogeochemical observations and the development of HGCP models for individual wells.