A simple graphical method to visualize the end-member fractions in a three-component groundwater mixing: the example of Ferrarelle system (Riardo, southern Italy)

The Ferrarelle groundwater system (FGS) is located in a highly fractured area in the Riardo Plainsouthern Italy) at the base of the northern slope of Mt. Maggiore, covered by a sequence of volcanoclastic deposits. It is the product of three endmembers mixing: deep mineralized, shallow volcanic and lateral carbonate groundwater inflows. The three-component mixing has been calculated by using V and K which act as reliable conservative elements. The mixing fractions calculated through mass balance were plot in a element-element triangle plot. This graphic method provides similar results obtained by numerical computation obtained by mass balance. FGS is mainly governed by the mixing of two endmembers: i) a deep CO2 super-saturated and mineralized (CO2 ~ 0.01 mol/kg; Electric Conductivity ~ 3500 ìS/cm) groundwater hosted in the carbonate basal aquifer and ii) a low mineralized (CO2 ~ 0.004 mol/kg; Electrical Conductivity ~ 400 ìS/cm) aquifer hosted in the shallow volcanic deposits. Relative fractions of each endmember in mixed groundwater can be calculated by mass balance through an element-element binary plot considering conservative elements in solution. A lateral groundwater input from the Mt Maggiore carbonate aquifer introduces a third endmember in the FGS. The final scenario shows a threecomponent mixing in a binary diagram where the endmember (components) are located in the three angles of a triangle. IC coupled with ICPMS analytical techniques provide a large number of variables in the dataset, both major (HCO3, Cl, NO3, SO4, Na, K, Mg, Ca) and minor/trace elements (F, Li, B, V, Mn, Fe, As, Sr, Rb, Ba, Cs), useful to be used for a validation in the mass balance processes. More than 120 groundwater samples were monthly collected from eight wells tapping the FGS since October 2017 in order to test this methodology. In the FGS the detected end-members are as follows: i) the deep and mineralized aquifer (V 0.2 ppb; K 87 ppm), ii) the shallow, volcanic and low mineralized aquifer (V 15 ppb; K 28 ppm), and iii) the lateral inflow from the Mt. Maggiore carbonate aquifer (V 2 ppb; K 3 ppm). The three numerical end-members define a triangle plot. We used three equation mixing (for the 3 end-members represented by each side of the triangle. The final calculated fractions for each sample for the three different end-members, allowed to define the relevance of each fraction on the final groundwater composition in different monitoring well in the FGS area. V and K seem be the most functional variables to evaluate groundwater mixing in volcanic vs. carbonate water reservoirs in the FGS. Triangulation of mixing functions in the element-element plot allows a fast visualization of fractions of the three components calculated by the numerical mass balances