Factors affecting the groundwater chemistry in a highly urbanized coastal area in Hong Kong: An example from the Mid-Levels area

Abstract

Coastal areas of Hong Kong Island are one of the most extensively urbanized areas in the world. Groundwater samples in natural slopes and developed spaces in the regions centered by the Mid-Levels area, Hong Kong Island, were collected and analyzed to investigate the natural and anthropogenic processes affecting the groundwater chemistry. The results presented may be of value to other coastal areas in the world for the identification of possible groundwater contamination sources. Groundwater samples in the natural slopes were in low total dissolved solid (TDS) (<100 mg/l), indicating that the waters were in the early evolutionary stage. Using chloride as a normalizing factor, the "non-marine" components of different major ions in the samples were calculated. The correlation analysis indicated the occurrence of weathering of plagioclase feldspars in the natural slopes. However, the breakdown of biotite and K-feldspar seems to be limited by short groundwater residence time and high resistance to weathering. The high variety in hydrochemical facies may suggest the presence of extremely heterogeneous subsurface geological conditions. In the developed spaces, groundwater samples exhibited a high range of TDS (∼100-5300 mg/l) and were mainly dominated by Na-Cl and Na-Ca-Cl water types. Besides water-rock interactions, the groundwater chemistry was significantly affected by leakage from service pipes and the dissolution of concrete materials. Some chemicals were used as signatures to identify the leakage from various service pipes. The area generally suffered from widespread, but small amount of leakages, and no obvious leakage was discovered. The strong correlations among major cations and chloride suggested that even a small amount of leakage from salty flushing water pipes can significantly affect the groundwater chemistry. Groundwater is found to be highly aggressive toward concrete as supported by three commonly used aggressiveness indices. Additional Ca 2+ may be released to groundwater by corrosion of subsurface concrete materials such as building foundations and basements. The strength of those subsurface engineering structures may be weakened. Besides, excess Ca 2+ may deposit in the dewatering systems in the area, which may affect their performance in lowering high water tables. The findings regarding leakage from service pipes will be useful for various government organizations such as the Water Supplies Department and Drainage Services Department. Discussion of the behavior of Ca 2+ is instructional to foundation and slope dewatering designs in the area. © Springer-Verlag 2005.