Occurrence of salt water above fresh water in dynamic equilibrium in a coastal groundwater flow system near De Panne, Belgium

A salt water lens is found above fresh water under the shore between Dunkerque (France) and Nieuwpoort (Belgium). This inverse density distribution is in a dynamic equilibrium. It develops due to the infiltration of salt water on the back shore during high tide. Under this salt water lens, water infiltrated in the adjacent dune area flows towards the sea and discharges at the seabed. This water quality distribution differs from the classic salt water wedge under fresh water described in the literature. Here, the evolution to this water quality distribution is simulated with a density dependent numerical model. A large tidal range, shore morphology and a permeable groundwater reservoir are the main conditions for the observed water quality distribution. By altering these conditions, intermediate water quality distributions between the classic salt water wedge and the one discussed here develop. Based on these simulations, it is expected that similar kinds of inverse density distribution could be present in a number of coastal areas, which have tides, a gently sloping shore and a permeable substratum.     

Improvement of groundwater quality due to fresh water ingress in Potharlanka Island, Krishna delta, India

Hydrochemical study had been carried out on the groundwater resources of Potharlanka Island, Krishna delta, India. Groundwater samples were collected and analyzed at 42 sites in December 2001 and October 2006. A comparative study of hydrochemical data indicates: groundwater is mildly alkaline with a pH of 7.0–8.2; electrical conductivity (EC) varies from 605 to 5,770 μS/cm in December 2001, and 652–5,310 μS/cm in October 2006. More than 62% of the groundwater samples in 2006 have TDS value <2,000 mg/l, which is within permissible limit of potable water, but 57% of the samples in 2001, are higher than the maximum permissible limit. Extremely low HCO₃/Cl and variable high Mg/Ca (molar ratios) had been indicated the transformation of the fresh groundwater aquifer systems to saline in 2001. Groundwater of this Island is mainly classified as Na–Cl and mixed types. A high percentage of Na–Cl type of these waters indicates the possibility of seawater ingression/intrusion process during 2001 and comparatively mixed water type indicates the dilution activities of groundwater. Excessive withdrawal of groundwater has caused the increase of saline water intrusion. Improvement of groundwater quality in this Island due to artificial recharge structures made by NGRI under RGNDWM project and affects of the flood due to heavy rainfall of the months of September–October 2005 are discussed in this paper.