Hydrochemical Approach to the Alterations of the Recharge of a Karst Aquifer Consecutive to a Long Pumping Period: Example Taken from Pinchinade Graben (Mouans‐Sartoux, French Riviera)

Abstract

AbstractThe carbonate aquifer of the Pinchinade graben, which has a well‐delimited geometry, involves water with well‐defined chemical types from calcium sulfate to magnesium bicarbonate. Each ground water type corresponds to a well‐delimited area of the aquifer: the magnesium bicarbonate for the Liassic and Rhetian limestone water (with 10 to 30 mg L‐1 of sulfate) and calcium sulfate for the water of the relatively impermeable layer of the underlying Keuper (with 300 to 1500 mg L−1 of sulfate). A four‐year pumping test with a bimonthly to daily monitoring of water chemistry has allowed evaluation of the renewal of the exploitable water reserve. During the four‐year period, the survey shows that the average discharge is balanced by natural recharge (2.8 to 105 m3 y−1). A change in the chemical character of the water was observed from Rhetian to Keuper type (from 35 to 167 mg L−1 of sulfate). Such a change indicates a progressive exhaustion of the Rhetian reserves, which are the greater part of the exploitable reserve in the area. The same phenomenon is observed daily depending on the pumped discharge and the season. For pumping rates below 26 m3 h−1, the borehole drains the Rehetian inflows to a degree depending on high or low water levels. For pumping rates above 26 m3 h−1, whatever the period, the permeable Keuper layers are pumped and sulfate peaks ensue.