62 Recent Advances in Hydrogeology

Sunday, 19 October 2014
1:00 PM - 5:00 PM
202/203

THE REACTION OF AN AQUIFER ON A DYNAMIC SALTWATER-FRESHWATER INTERFACE: A CASE STUDY IN A SEDIMENTARY COASTAL BASIN

Paper number: 62-11
Program: Technical Sessions
Time: 3:15 PM – 3:30 PM

Authors

Abstract

The groundwater system of Horonobe (island of Hokkaido, Japan) is a deep coastal basin, which is composed of heterogeneous porous sediments. Aim was to investigate the impact of a dynamic saltwater-freshwater interface on the aquifer. The area experienced significant sea level fluctuations in the past one million years (+5 m to -120 m to present day sea level). Future sea level fluctuations, which may caused either by man-made or natural climate change, have to be taken into account. The fluctuations will cause large scale movement of the local saltwater-freshwater interface. Accordingly, many hydrochemical and hydraulic reactions are expected between the aquifer and the intruding fluids. The estimation of potential changes of aquifer characteristics induced by intruding fluids is a crucial task for many applications, as aquifer storage and recovery systems, supply with drinking water in coastal areas, or usage of geothermal heat and energy.

As access to the aquifer of Horonobe is limited to a single-well setting, the application of “classic” experiments (e.g. multiple-well tracer tests) is not possible. Therefore, the “push-pull” method was used to simulate potential in-situ effects of a moving interface. The test solutions injected in the “push” phase differed in their chemical composition (artificial produced brackish water and deionized water). The gained conservative and reactive BTCs were used to describe the hydrochemical response of the aquifer to intruding fluids with different ion contents. The injection of brackish water leads to the expected conservative mixing with the origin groundwater. Mass balances reveal complete recovery of the dissolved species. Curve fitting of the conservative breakthrough curve lead to an effective porosity of ~5%. In contrast, injection of very low mineralized water caused complex reactions within the aquifer, which could be identified as ion exchange, dissolution of some species, redox, and sorption-desorption reactions. Curve fitting reveal in a very low effective porosity. However, while clay swelling may be affect the effective porosity during the experiment, other results show that clay dispersion can be excluded as a major process.