Latvian Environment, Geology and Meteorology Centre (LVGMC) supports a database, where several tens of thousands of groundwater composition analyses from Latvia are compiled including data from aquifers that have bee sampled only during geological mapping and exploration campaigns, and no open wells are preserved. Thus, only available knowledge about groundwater composition of the significant portion of the sedimentary rocks of Latvia is the historical data compiled in the database. However, the quality of this data is not known. It is suggested to evaluate it by analysing time series of groundwater composition in monitoring posts acquired by identical methods. This is justifiable because outside the zone of active influence of human and meteorological factors, the groundwater is characterised by constant, time-independent composition, and time series can be considered subsamples from a single large sample. To verify the data, concentrations of major components of the groundwater – cations Ca2+, Mg2+, Na+, K+ and anions Cl-, HCO3-, SO42- – are systematically analysed in time series starting in early 1961. All the results that significantly deviate from average values are considered to be improper. Although only Ca2+ and all mentioned anions have been determined directly until the mid-90s, a common validation criterion (CVC) has been proposed as the ratio between equivalent concentration of Ca2+ and sum of all the anions. The CVC helps to spot serious errors in cases, when ion balance is correct and average deviation of individual ion concentrations does not indicate an error. The concentrations of all the ions separately using CVC are evaluated for 29 different monitoring wells for a total of 560 groundwater samples. It has been found out that 4% to 6% of all analyses included in the database shall be considered serious errors. It is suggested that this proportion of serious errors can be attributed to the whole dataset found in the database.