Hydrogeology and geochemistry of the multilayered confined aquifer of the Pisa plain (Tuscany - central Italy)

The Pisa plain contains a multilayered confined aquifer made up of Pleistocene sands and gravels. The groundwater from the wells tapping these horizons are generally of poor quality: they exhibit significant TDS, relatively high Cl content and considerable hardness. During geothermal prospecting of the Pisa plain, about 80 wells ranging in depth from 20 to 250 m were sampled, and both chemical (major ions) and isotope analyses were conducted. The data collected show that TDS is strongly influenced by HCO3 and Cl, and that a 3-component mixing process affects the groundwater's chemical composition. The end members of this mixing process have been identified as: (a) diluted HCO3 meteoric water, which enters the plain mainly from the eastern and northern sides of the study area; (b) Cl-rich water, which largely characterizes the shallow sandy horizons of the multilayered aquifer system and has been attributed to the presence of seawater, as also suggested by δ18O data; and (c) SO4-rich groundwater, which is linked to the hot groundwater circulation within Mesozoic carbonate formations and, at first sight, seemed to affect only the gravelly aquifer. A SO4-rich water also contributes to the sandy aquifer; it probably enters the plain both laterally, from the margins of the Pisan Mountains and from depth, but promptly undergoes substantial SO4 reduction processes by bacteria. That such processes are at work is suggested both by the low SO4 and high HCO3 concentrations found in the well waters and by their C and S isotope compositions. The collected data have allowed zones with higher quality waters to be identified, which may someday be used for the local water supply.