***A geometrical method for quantifying endmembers' fractions in three-component groundwater mixing*** Authors: E. Cuoco(1,6), S. Viaroli(2), T. H. Darrah(3,4), V. Paolucci(5), R. Mazza(2), and D. Tedesco(1,6) (1)Istituto Nazionale di Geofisica e Vulcanologia - Osservatorio Vesuviano (INGV-OV), Via Diocleziano 328, Napoli Italy. (2)Dipartimento di Scienze, Università degli Studi Roma Tre, L.go S. Leonardo Murialdo 1, 00145 Rome, Italy. (3)School of Earth Sciences, The Ohio State University, Columbus, OH 43210 USA. (4)Global Water Institute, The Ohio State University, Columbus, OH 43210 USA. (5)Ferrarelle S.p.A., Contrada Ferrarelle, 81053 Riardo, Italy. (6)Dipartimento di Scienze e Tecnologie Ambientali, Biologiche e Farmaceutiche, Università della Campania "Luigi Vanvitelli", Via Vivaldi 43, Caserta (Italy). Corresponding author: Emilio Cuoco Contact Information: emilio.cuoco@ingv.it ***General Introduction*** This dataset contains data collected during monitoring and sampling activities were performed monthly between October 2017 to January 2019 on eight monitoring wells placed within Ferrarelle™ company’s water concession in the Riardo Plain. The data elaborations consist in correlations computing among the analyzed chemical variables, and fractions estimation of the investigated groundwater three component mixing. This research project was made possible by Ferrarelle™ company collaboration and founding in the research projects on Riardo Plain hydrogeologic system. ***Analytical equipment*** Temperature, pH and Electric Conductivity (EC, with automatic compensation to 20°C) were measured in each surveyed point using portable probes (WTW pH/EC 340i). Major elements were analysed by ion chromatography on unacidified (F, Cl, NO3 and SO4) and acidified (Na, K, Mg and Ca) samples. The total alkalinity (as HCO3) was determined in the field (Kartell™ technotrate digital burette) by titration using standardized 0.1 M HCl (Merck™) against methyl orange indicator. Minor and trace metal analysis were carried out using Agilent 7500ce-ORS ICP-MS with collision cell technology. ***Purpose of monitoring campaign*** The hydrogeochemical investigation has allowed the detailed description of groundwater hydrodynamics in the investigated aquifers system. The chemical composition of analyzed groundwater is due the mixing of three different aquifers (endmembers). It was possible to compute the fractions of each endmember in each sample over an entire hydrogeologic year. The outcomes have allowed a conceptualization and the testing of a new geometrical method capable of quantifying and illustrating the outcomes of a three-component mixing dynamics. The hydrogeochemical investigation has allowed the detailed description of groundwater hydrodynamics in the investigated aquifers system. The chemical composition of analyzed groundwater is due the mixing of three different aquifers (endmembers). It was possible to compute the fractions of each endmember in each sample over an entire hydrogeologic year. The outcomes have allowed a conceptualization and the testing of a new geometrical method capable of quantifying and illustrating the outcomes of a three-component mixing dynamics. ***Description of the data in this data set*** Row data are presented in mg/L (milligrams per liter or ppm) for the following ions: HCO3, F, Cl, NO3, SO4, Na, K, Mg, Ca, and in ug/l (micrograms per liter or ppb). The correlations among all the analyzed variables are presented as a correlation matrix. The mixing fractions of each endmember are present as fD, fN and fF, where the f stand for "fraction" and is reported in italic. The uncertainties are reported with (delta) symbol, thus are presented as plus/minus (delta)f. These notations are the same for both mixing computations methods, algebraic and EMMA. ***Recommendation*** Please, cite also the related paper publisched in Hyrological Processes Journal, doi:10.1002/hyp.14409 .