A numerical groundwater flow model of Chienti River valley (central Italy): results and boundary problems

Abstract

Since ’90s the lower valley of Chienti river has been interested by a diffused contamination by chlorinated solvents (mainly PcE) used by local shoes companies. In order to analyze the feasible paths and travel times of a pollulant in the aquifer and so the possible problems that these contaminants can cause to the well field of Civitanova Marche, a detailed groundwater flow numerical model related to the drinking well field area has been developed, in steady and transient conditions, using Feflow 6.0 from Wasy inc (finite elements code).The model has four layers and reproduces a multilayer semiconfined aquifer characterized by a shallow and a deep levels. in the first aquifer the hydraulic conductivity (k) is ranging from 1*10-3 m/s to 5*10-3 m/s (storativity 0.20); in the second aquifer k is ranging from 1*10-3 to 5*10-4 m/s (storativity 1.3*10-3); the intermediate local aquitard has k 10-5 m/s and storativity 10-2. the recharge applied in steady model is 95 mm/y according with climatic data. in the first slice, along the chienti river, a cauchy boundary condition has been inserted. constant head conditions have been applied along western (22 m a.s.l) and eastern (0 m a.s.l.) limits of all slices and in correspondence with the river location, in the slices deeper than shallow one. along the northern limit a no-flow boundary condition inhibits flow entering or exiting from the hydrogeological basin.The model simulates the real W-E trend of groundwater flow, obtaining a good correlation between simulated and measured piezometric values (rMS 0.76 in the steady state simulation; rMS 0.69-0.9 in the transient one). the whole flow budget shows a comparable rate between entering and exiting flow from the model, but a critical situation in the SW area, at the contact between western and southern boundaries, is observed. this contact, combined with a signi cant hydraulic gradient (8‰) and with a hydraulic conductivity of about 10-3 m/s, generates an unexpected and perhaps unrealistic interchange of groundwater in that area.Detailed analysis of this local situation reveals modeling inaccuracy where different boundary conditions were applied in a boundary area characterized by complex hydrogeological setting.