The age of deep aquifers in Milan Province: Development of a new i.e.b.- tritium calibration curve
|Location||International Geological Congress,oslo 2008|
|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.