The use of KI as tracer in groundwater flow measurements
Keywords:hydrogeology, tracers, groundwater flow, coastal aquifer
The district of Casalabate (Apulia, Southern Italy) is located on the banks of the Adriatic Sea and has been affected by significant land erosion with some houses collapsing and many others becoming unusable. In order to investigate the causes, a detailed campaign of multidisciplinary investigations was performed. It consisted of an integrated geological survey by drilling 23 boreholes and georadar prospecting as well as chemical-physical analysis, thermo salinometric logs and groundwater flow measurements. The groundwater discharge to the sea, exclusively horizontal, mainly affects only some calcarenitic levels of fossil dune with a velocity of approximately 250 cm/day. The investigations regarding the measurement of groundwater flow were performed with the method of point dilution in a single well, by adopting and comparing three different surveying systems: 1) salinometric pole; 2) conductometric probe; 3) multiparameter probe with a specific sensor for the Iodine ion. The three different measurement systems has been carried out on a well with constant hydrodynamic and hydrochemical features at least for the time strictly necessary for the tests and have produced very similar results. The differences between the measurements made with different methods are very small: the percentage ratios of groundwater flow estimated with concentration changes of KI tracer and that with other two methods vary from 1% to 3%. The results of test well have been confirmed from a survey of 556 measurements, performed in 11 well of the same area by multiparametric probe and KI tracer. All the greater differences (>10%) have beeen recorded in the fresh-saline groundwater with salinity exceeding 4g/l, presumably due to the variations of basic salinity induced by tidal oscillations. Therefore, elaboration of the measurements performed in interface zone calls for the distinction of the effects of tracer dilution from those of the variations caused by the tide. By the double determination (with the iodine sensor and with the conducting one), a better analysis of data has been performed. Finally. the study highlights that the system based on the Iodine sensor is reliable and can be compared, showing itself to be valid in coastal aquifers.
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