HYDRAULIC AND HYDROCHEMICAL CHARACTERIZATION OF DEEP COASTAL SEDIMENTARY BASINS BY SINGLE-WELL PUSH-PULL TESTS

Recently, investigations were conducted for geological and hydrogeological characterization of the sedimentary coastal basin of Horonobe, which is located on the northern Japanese main island of Hokkaido. The basin is composed of poorly compacted neozoic sand-, silt- and mudrocks. Coastal basins are typical geological settings in Japan, which are less tectonically active than the mountain ranges and good accessible by ship. For that reasons, coastal basins are considered as potential host rocks for the Japanese final repository for radioactive waste program.

Methods and procedures are required for hydrogeological deep basin characterization. The aim of the project is to test and enhance existing and new methods for this purpose.

As a first step, a Single-well push-pull test was conducted in the shallowest well (100 m depth) at the test site of Horonobe. This method was already used as partitioning push-pull test to quantify organic pollutants at contaminated sites in relatively shallow depths of about 10 to 20 metres. During the first push-pull test (“saltwater test”) a known amount of several solutes including a conservative tracer was injected into the aquifer ("push") and afterwards extracted ("pull"). After a certain time the test was repeated with distilled water (“freshwater test”).

The during the pull phase obtained tracer breakthrough curves (BTC) of uranine differed considerable between both tests. Tracer test characteristics, as recoveries, dispersions, and penetration depths of the tracer into the aquifer, seem to be dependent not only from the aquifer itself but also from the salinity/density of the fluid. Furthermore, separate single preferential flow paths could be derived from the shapes of the BTC.

The final goal of the interpretation of push-pull tests is the hydrochemical (sorption-desorption processes) and the hydraulic characterization (dispersions, transport velocities) of density dependent transport of groundwater within a coastal basin. The push-pull method may be suitable as a relatively cheap (because of single-well setting), fast (tracer test duration of only some days also in large depths possible), and easy-to-apply method for hydraulic and hydrochemical characterization of groundwater flow processes in deep aquifers.