Geophysical Investigation of the Fresh-Saline Water Interface in the Coastal Area of Abergwyngregyn ()
1. Introduction
In order to delineate the problems caused by saline intrusions, caused by both natural and anthropogenic activities, different methods have been employed. The application of borehole surveying is intrusive, cumbersome, quite expensive, and involves a large time frame. This project investigated the effectiveness of the use of nonintrusive surface geophysical techniques to map and delineate the fresh/saline-water interface in the Abergwygregyn area of North Wales, United Kingdom. Abstraction of freshwater resources is a major cause of saline intrusion. Over-withdrawal leads to head loss causing saline-water to be drawn into a freshwater body. Many countries the world over rely on groundwater for a large proportion of their high quality water supply. Consequently the study of saline intrusion is crucial in order to avoid extracting saline-water for consumption and general usage. There is therefore a need to undertake a monitoring exercise in order to identify and assess saline intrusion.
Combining electrical resistivity and electromagnetic
(EM) techniques partially compensates for their individual shortcomings. The electrical resistivity technique is one of the geophysical methods, which enables the determination of sub-surface resistivity by introducing artificially produced electric current into the ground through a set of two electrodes, and measuring the potential field generated by the current by the aid of another set of two electrodes. Electromagnetic induction profiling is a surface geophysical technique, which is used to measure terrain conductivity, that is, the bulk electrical conductivity (EC) of sub-surface materials. It employs a changing primary electromagnetic field created around a currentcarrying transmitter coil to induce a current flow within the ground, which in turn creates a secondary electromagnetic field sensed by a receiver coil. The strength of the secondary field created is a measure of the ground conductivity.
The reliability of resistivity mapping in delineating coastal zones invaded by saline-water was shown in the work of Soomro [1] when he found that the salinity of groundwater could be directly detected by determining its electrical conductivity and hydro-chemistry. Rosalomanana [2] also described electrical and EM methods as being well suited for coastal aquifer studies because of the large contrast in resistivity between freshwater-bearing and saltwater-bearing formations. The high electrical conductivity of saline-water made the electromagnetic and electrical resistivity techniques particularly appropriate for this study.
2. Study Area: Geography and Geology
The project site was a beach area along the North Wales coastline adjacent to the A55, North Wales Coast Highway at Abergwygregyn, situated between Bangor and Llanfairfechan along the banks of the Menai Strait. Figures 1 and 2 show the locations of North Wales and the study area, respectively. The site can be accessed through a nature reserve car park.
The area comprises essentially flat sandy and muddy deposits. Gravels and boulders bound the southern end, and this area marks the boundary between the marine environment and the farmland (Plate 1). As the sea is approached, the dominant tall grassland at the southern end gives way to shrub-like grasses (salt marsh), which begin to break up into disseminated parts, as the sea is approached. About 30 m from a fence that demarcates the higher elevated land from the inter-tidal area, the grassland gives way to the Traeth Lafan (the Lavan Sands)
Figure 1. Map of north Wales showing the location of the study area.
Figure 2. Location map of the study area.
and as the sea is approached mudflats dominate. A continuously flowing thin layer of surface water characterizes the sands and the mudflats (Plate 2).
3. Methodology
A geophysical survey consisting of 12, Frequency Domain Electromagnetic (FDEM) profiles and 5,2-D Resistivity Imaging (RI) profiles was conducted on the site. The position of the transect lines were determined using a hand-held GPS system. Soil and water samples were collected from specific points along some of the survey lines in order to carry out geochemical analyses to ground truth the geophysical data.
3.1. Frequency Domain Electromagnetic (FDEM) Survey
Electromagnetic mapping was carried out along premarked transects. Every 5-m along each transect, an apparent conductivity reading was taken using a Geonics EM-31 conductivity meter carried on the shoulder. Readings were continued along the transects for at least 400 m or until the conductivity values indicated that the sediments were fully saturated with seawater. A total of 12 survey lines were completed (Figure 3).
Plate 1. Study area showing southern bounded area with gravels, boulders and grasses.
Plate 2. Study area showing thin layer of water flowing on the sands and mudflats.
Figure 3. EM31 conductivity contour plot with pseudo-section lines; sample lines and borehole points superimposed.
3.2. 2-D Resistivity Imaging (RI)
Resistivity profiles were carried out almost perpendicular to the shore. They were positioned in a way that would allow easy correlation with the EM31 surveys (Figure 3). Twenty-five electrodes were deployed with 5 m spacing along survey lines 1 and 2, while fifty electrodes with 5 m spacing were deployed along survey lines 3, 4 and 5. Good contacts were achieved and measurements were completed automatically using a laptop computer interfaced to a Campus Geopulse resistivity meter. 2-D resistivity models were obtained by inverting the apparent resistivity pseudo-section data using Geotomo RES2- DINV software.
3.3. Geochemical Analysis
Soil water and soil samples were collected along RI and EM survey lines, from which the salinity and electrical conductivity (EC) were measured. Particular attention was paid to sites that showed anomalous conductivity values in the EM31 survey.
4. Results and Analysis
The 2-D resistivity imaging, the EM profiling and soil/ water samples were all carried out perpendicular to the direction of the advancing sea. For each method, data acquisition was superimposed in order to generate results that would be correlative.
A contour map of the EM31 conductivity values was produced (Figure 3).
It can be observed that the conductivity varies indicating an uneven distribution of saline-water within the beach sediments. A slightly high value of conductivity (150 - 200 mS/m) occurs in the southeast corner, indicating a slightly more saline-water medium. In a previous study carried out to the east of this study area [3] a conductivity value of 200 - 300 mS/m was found adjacent to this area. This is probably due to the presence of salt marsh vegetation and soil retaining saline-water after being covered by spring tides. A freshwater plume, extending from points 64620 - 64660 E and 73120 - 73220 N can be seen. There is a central area with conductivity of about 300 mS/m but it is quite small in extent. Further offshore trending northwest is a saline-water region, which extends from the limits of the study site and heads down the beach with increasing conductivity as the main channel in the Menai Strait is approached. This area is predominantly mudflats.
Figure 3 also displays the positions of the 2-D electrical image lines (pseudo-sections) within the study site. The resistivity images have an advantage over the EM31 conductivity contour plot in that they are able to define both the lateral and vertical extent of the sediment layers, as can be observed in Figures 4-8.