Geophysical Investigations of Saltwater Intrusion into the Coastal Groundwater Aquifers of Songkhla City, Southern Thailand

Abstract

Saltwater intrusion into coastal aquifers is an emerging problem for many cities located along coastlines as they are dealing with an increase in the population numbers. The Bo Yang District of Songkhla Province, comprising the City of Songkhla, one of the main provincial capitals in Southern Thailand, is facing a similar problem, although it is already decades old. The city, abound 10 km² in area, is bounded to the east by the Gulf of Thailand and to the west by the seasonal saline Lower Songkhla Lake. Geophysical investigations were carried out in order to delineate the saltwater intrusion utilizing widely available equipment. Four seismic refraction and 6 seismic reflection survey lines were acquired, as well as 12 vertical electrical sounding surveys were done mainly in the northern and western part of the city. All methods require several 100 m long straight lines with ground access. In the densely populated city these were mainly found along streets, on football fields, at the beaches, and also on a military airfield. Additionally, data from seven drilling locations provided lithology data, mainly shallow boreholes. Further, for several wells screen depth intervals, chloride and total dissolved solids (TDS) concentrations were known. From all available results three cross sections were drawn, two in EW direction perpendicular to the beaches and one in approximately NS direction crossing the city. In each of the cross sections four main resistivity layers were outlined. A near surface higher resistivity layer can be related to top soil or beach sand. The second later shows medium resistivity values, comprising layers of sand and clay, partially saturated from rainwater infiltration. The related groundwater of this unconfined aquifer has relatively low TDS values. The third layer has resistivity values of about 2–20 Ωm with TDS values of the groundwater of about 2,300–8,200 mg/L. This resistivity layer also comprises different geological layers, sand, clay and gravel layers, likely with brackish to saline water. The fourth resistivity layer shows resistivity values in general around 140 Ωm. For this layer no TDS are available, but the higher resistivity values indicate sand and gravel sediments with minor clay layers and saturated with freshwater. However, no further information is available for this layer. The cross sections further show that the subsurface layers show some topography which is likely to be related to hard rock outcrops, in the south there are granite hills, whereas in the northern part hornfels hills separate the aquifers. For any further groundwater development deeper boreholes into the third resistivity layer might yield the desired freshwater. This process must ensure that during any exploration and exploitations efforts the deeper aquifer is not contaminated by the saltwater intrusion into the second aquifer. However, since some decades a pipeline from a reservoir further south in Songkhla Province is supplying the people in Bo Yang District with tap water.