The age of deep aquifers in Milan Province: Development of a new i.e.b.- tritium calibration curve

Category Hydrogeology
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Gorla, Maurizio
Holding Date 17 September 2008

Deep aquifer systems will play an increasingly strategic role in supplying drinking fresh water during the next future. At province scale, two main deep aquifer systems can be recognized: the so called Continental and Marine aquifers. Both of them are confined aquifers, with transmissivity values mainly ranging from 1-2•10-3 m2/s to 8-9•10-3 m2/s. They are mostly wedge-shaped, with thicknesses ranging from 5-6 m, due south, up to > 20 m, within the highland zone (northern part of the province).
Flow direction is from north to south, with a hydraulic gradient varying from 5-6 ‰ upstream to 1-2 ‰ downstream. Despite we reached a quite good knowledge of deep aquifers’ hydrogeological setting, we know just a bit about their age. A better understanding of deep groundwater paths, their time of residence or in a word how old are them, represents a fundamental managing data to achieve a good level of knowledge and to carry out a sustainable withdrawal of groundwater resources. How can we obtain this kind of information? Tritium is a natural instable isotope used to determine the age of groundwater or better to estimate if groundwater has been recharged before or after 1953. In SI units, 1 T.U. is about 0,118 Bq/l or approximately 3,19 pCi/l. Scientists can also use the ratio of tritium to its decay product Helium-3 (3He) to date groundwater. If all the Helium-3 was derived from tritium decay and from air, a sample’s age can be calculated from this formula:
t =T1/2/ln 2 • ln(1 + 3Hetrit/3H) where: T1/2 = half-life of tritium, 12,43 years; 3Hetrit = amount of tritiogenic Helium-3 in T.U.; 3H = sample’s tritium concentration in T.U.. I.E.B., called "alkali-chloride disequilibrium index", is a ionic ratio (concentrations in meq/l): I.E.B. = [Cl - (Na + K)]/Cl.
This ionic ratio is in direct proportion with "oldness" of groundwater. A new way to reach this goal is here and now proposed. Using all the available data pairs I.E.B. (meq/l) - 3H (T.U.), a new calibration curve was developed. The numerical terms of the curve are: 3H (T.U.) = 4,43e^[0,2 • I.E.B. (meq/l)] with a regression coefficient R^2 equal to 0,70.
The curve I.E.B./time of residence is:
time (years) = - 0,712 • I.E.B.^2 - 12,484 • I.E.B. + 20 or log time (years) = - 0,725 • I.E.B.^2 - 12,705 • I.E.B. + 19,292 with a regression coefficient R^2 equal to 0,81.
These curves highlight respectively an exponential and a logarithmic trend, with a direct proportion between these two variables. The ages of these deep groundwater encompass a time span of about fifty-sixty years, in perfect agreement with the flow velocity values hydrogeologically computated. These experimental curves can be considered a new practical, operative hydrogeologic tool, useful for the sustainable management of deep fresh groundwater. Nevertheless, just after collecting a huge number of "field" data, we could completely vindicate its goodness.