Mapping the Impact of Salinity Derived by Shrimp Culture Ponds Using the Frequency-Domain EM Induction Method

Geophysical characterization of saltwater intrusion in a coastal aquifer: The case of Martil-Alila plain (North Morocco)

Water saturated sand and a shallow bay: Combining coastal geophysics and underwater archaeology in the south bay of Tel Dor

As sea levels rose since the end of Last Glacial Maximum (LGM) ancient coastal communities were often forced to abandon their settlements and move inland. Today, many of these abandoned sites are covered by sand or lay in shallow water. Examining these can shed light on past coastal communities as well as settlement patterns in ancient times. Archaeological excavation along the coast is particularly tricky and often sporadic in nature. Thus, high‐resolution shallow geophysical methods, which have become a standard in archaeological studies since they provide a noninvasive way of imaging the subsurface before an excavation, would seem like a perfect solution. However, most methods are limited in their ability to work near the shoreline – the transitional zone between classical land-based methods and standard marine ones. Ground penetrating radar (GPR), for example, is greatly affected by moisture and salinity and is therefore limited in its ability to work in areas saturated with seawater. Seismic reflection is time consuming to overcome issues of poor vertical and spatial resolution and sensitive to urban noise, while magnetics would provide poor results for sand covered sandstone. Other techniques, such as electrical resistivity tomography (ERT) have been shown to work in coastal areas and in shallow water. However, this method can be slow, as it involves setting up complex arrays for each cross section measured. This study will present the frequency domain electromagnetic (FDEM) method, which has the potential to overcome these problems and can bridge the gap in knowledge by measuring in the nearshore environment. The ease of use and quick scanning capability means that large areas can be covered in a relatively short time. There are no electrodes or loops to set up. Since it measures swaths, results are obtained in map-view and not cross-section, with little interpolation. Different frequencies penetrate to different depths (lower frequencies corresponding to deeper penetration). Therefore, the result is a series of frequency maps corresponding to the integration of all subsurface data in a specific sampled volume (i.e. down to the frequency-related depths), providing important information on shallow subsurface properties. The use of multiple frequencies allows for the resolving of internal structures within the depth range. Overall, the FDEM method has proven to be a valuable tool for studying coastal archaeology, and it is likely to continue to play an important role in the field in the coming years. Its ability to detect buried objects and structures and to study the geomorphology of submerged landscapes makes it an essential tool for researchers working in this field.

The subsurface illuminating strength of integrating Electrical Resistivity (ER) and Frequency Domain Electromagnetic (FDEM) geophysical investigation techniques to evaluate the level of subsurface leachate contamination plume generated by indiscriminate dumping of cassava product processing effluents and wastes around a dumpsite near cassava processing mill at Ilero southwestern Nigeria, has been demonstrated in this study. The lateral spread as well as depth extent of the cassava waste effluent leachate plume was determined from the subsurface resistivity and conductivity models generated from combined ER and FDEM geophysical surveys carried out through eight (8) Electrical Resistivity Tomography (ERT), eight (8) FDEM and (2) Vertical Electrical Sounding (VES) measurement profiles and stations purposively established across the dumpsite.Quality checked, inverted and depth matched ER and FDEM data generated 1D, 2D, and 3D resistivity and conductivity distribution in the form of subsurface logs, sections, constant depth maps and earth model employed to characterize the study area. Electrical resistivity result presents resistivity distribution that range in value between 21 Ωm to 213 Ωm from the topsoil to depths beyond 7.5 m. The FDEM sections and depth maps show conductivity distribution range between 4 mS/m – 286 mS/m across the dump site and provided information up to a depth of 60 m. The VES results generally delineate three (3) georesistivity layers, which include topsoil, weathered and fractured basement layers, occurring at depth between 0.4 – 10.0 m with characteristic resistivity values from 33.5 Ωm – 37.9 Ωm. The 3D resistivity and conductivity models generated by integrating the various results indicate anomalously low resistivity and relatively high conductivity (21 – 30 Ωm; 11 mS/m – 40 mS/m) zone which defines contamination plume in the central part of the cassava effluents and waste dumpsite.This study has through integrated electrical resistivity and frequency domain electromagnetic geophysical methods of investigation highlighted the subsurface contaminating potential of indiscriminating dumping of agricultural processing effluent and wastes, especially of cassava effluent, which has toxic cyanogenicglucosides constituent that is deleterious to the environment.

Shrimp farming has contributed a large share in Indonesia’s aquaculture portfolio for at least a decade, and a national plan to increase shrimp production by 250% has been recently laid out. However, boosting shrimp productions could lead to unintended consequences in environmental and socio-economic negative impacts. The rapid development of vannamei farming in Java has increased coastline land clearings and demands of fertilizers, feeds, and chemicals to sustain the farming activities. Such pressures will eventually lead to a reduced environmental capacity and the farming efficiency itself. This study aimed to study the environmental impacts and business performance of intensive shrimp farming in Indonesia. The study was conducted in Aquaculture Business Center (ABC) in Karawang for four months, from July to October 2020. In-situ and ex-situ measurements of water quality parameters were done at six sampling stations directly post-harvest water discharge. The measured parameters consisted of temperature, pH, dissolved oxygen, ammonia (NH3), nitrite (NO2), nitrate (NO3), phosphate (PO4), alkalinity, and salinity. Pollution Index (PI) was used as the primary method to determine the environmental impacts of the shrimp farming. The R/C Ratio was used to analyze the business performance of the company. The results showed that the water quality index in the ABC area was categorized as lightly polluted in station 1 (PIj 4.52) and station 5 (PIj 4.37), moderately polluted in station 2 (PIj 6.24), station 3 (PIj 6.72), and station 4 (PIj 6.13) and heavily polluted in station 6 (PIj 111.06). The determined R/C ratio was 1.10, meaning that the shrimp farming is classified as economically profitable. Although the shrimp farming’s economic performance value is very good, the water conditions affected by waste from the shrimp pond culture will reduce the R/C ratio in the future if not properly managed.

The increasing installation of shrimp farms in vulnerable coastal areas around the world generates an environmental impact and makes it urgent to develop methodologies and studies for assessing and scaling the potential risks and sustainability of these activities. One of the main hazards of these activities is that the prolonged inundation of excavated ponds for shrimp farming allows the percolation of saltwater in the surroundings, resulting in increasing groundwater salinity. Saltwater intrusion in coastal aquifers, accompanied by salinization of soils, causes a decrease in available freshwater resources, a decline in crop productivity and the deterioration of the natural ecosystem. The coastal aquifer of Rio Grande do Norte State (Brazil) where, for years, several shrimp farm factories have been operating, reported some issues related to aquifer and soil salinization. The present study aims to assess the origin of and delineate groundwater salinization in a sector of this coastal aquifer using a low-budget procedure. The integration of hydrogeological and hydrogeochemical characterization by drilling shallow piezometers, measuring the hydrostatic level and analyzing the major ion concentrations of the groundwater has made it possible to establish that the origin of groundwater pollution in the studied area is caused by saltwater percolation from shrimp farms. The joint use of both characterization techniques has been shown to have an efficient cost–benefit ratio and less-intrusive methodology, which can be applied in other areas with similar environmental concerns.

Geophysical methods are beneficial for mapping of possible fresh and salt water intrusion in coastal regions. As it well knows, possible seawater intrusion might have negative impacts on groundwater quality and agricultural soil. In this study, a geophysical survey were performed at Trabzon, Eastern Black Sea, Turkey in order to determine possible seawater intrusion and to reveal the shallow underground. These survey consist of frequency domain electromagnetic method (FDEM) and electric resistivity tomography (ERT). In this context, FDEM and 2D ERT data were collected along 7 profiles and 4 profiles at the selected area, respectively. 2D section were created from FDEM measurement profiles. In addition to, 2D sections and 3D image were obtained from ERT measurements. At Trabzon, geophysical survey revealed saline/fresh water zones and shallow underground in the investigated area.

The Frequency Domain Electromagnetic (FDEM) method is a cost-effective geophysical technique that simultaneously studies the electrical and magnetic properties of a medium, providing data as in-phase and out-of-phase components of the electromagnetic field. Although FDEM yields valuable insights, its results can be complex to interpret, and the two EM field components are normally only visually inspected to support findings from other techniques. This study aims to enhance FDEM data interpretation using an unsupervised learning technique. The proposed approach seeks to automate and expedite the interpretative phase. By applying the K-Means clustering algorithm, we divided the FDEM data into several clusters based on specific intervals of the in-phase and quadrature components, resulting in integrated maps of EM components. Combining these maps with geological and archaeological insights helped identifying areas of potential archaeological interest. This method was applied to the Torre Galli archaeological site in Calabria, Italy, known for its significance in Iron Age studies.Based on comparisons with the findings of earlier excavations and results from a magnetic survey, the proposed procedure shows promise in improving the efficiency and accuracy of the FDEM method in identifying areas of archaeological interest. This suggests that automating the interpretation process could lead to a better cost management and time optimization in geophysical and archaeological studies.

Low signal-to-noise FDEM in-phase data: Practical potential for magnetic susceptibility modelling

The coastline of China is approximately 18,000 km long. In most coastal cities, seawater intrusion is a serious threat to groundwater resources. Nine shallow monitoring wells were constructed to study the dynamics of shallow groundwater level and salinity in the coastal plain region of Jiangsu province, China. Results showed that precipitation, evaporation, and river stage affected the groundwater level in our study area. Positive correlations were observed among the groundwater level, precipitation, and river stage; then negative correlation existed between the groundwater level and evaporation. The influencing factors on the groundwater level were in the order precipitation > river stage > evaporation. Sufficient precipitation during the wet season diluted the groundwater salinity. After the dilution, between two continuous precipitation events, the groundwater salinity increased as the groundwater level decreased. During the dry season, the groundwater salinity rapidly increased and reached its peak in December. The groundwater salinity in December was 23 times higher than that in July. The groundwater level and salinity in this study were generally associated with the season. Climate factors led to fluctuation of groundwater levels and salinity during the wet season, and seawater intrusion increased the groundwater salinity during the dry season.

Shrimp vs prawn-rice farming in Bangladesh: A comparative impacts study on local environments and livelihoods

Summary In the past decade drones have become available and affordable for civil applications, including monitoring with geophysical sensors. In 2017 and 2019 the feasibility of executing frequency domain electromagnetic (FDEM) method surveys using an off the shelf drone, was investigated at Deltares. This paper reports on the preparatory tests executed to determine the optimal configuration, processing and inversion scheme and on the field validation tests demonstrating the capability of the drone-borne electromagnetic survey. One demonstration example shows how the system can efficiently map shallow groundwater and surface water salinity in areas where seepage of saline water occurs. The other demonstration example shows how the system can map shallow lithology as well as buried metal cables and pipelines. The general findings are that the system provides for the flexibility to fly a combination of FDEM soundings, profiles or grid patterns and its ability and efficiently to fly over inaccessible areas and surface water. Compared to the HLEM surveys, the spatial resolution is much higher which allows for detailed 3D mapping of subsurface and the costs are, certainly for small study areas, relatively low, which also makes monitoring of changes by repeated drone-borne electromagnetic (DREM) surveys affordable.

The normal function of modern society requires a vast number of buried pipelines with differing specifications, which could become blocked to varying degrees during routine operation. Consequently, a major technical municipal engineering problem is how best to accurately, nondestructively, and efficiently locate blockages in underground pipelines. The frequency-domain electromagnetic (FDEM) method has been widely used in geophysical exploration because of its features of nondestructive examination and high efficiency. Considering the application of FDEM to the problem of locating blockages in underground pipelines, numerical simulations and physical experiments are conducted on different pipeline types with different blocked states, different blockage substances, and different working configurations. Results demonstrate that FDEM is most applicable for the detection of blockages in insulated pipelines, especially when the single-ended charging method is used or when the blockage has reasonable impermeability because the induction signal exhibits a particularly obvious abrupt drop at the blockage position. However, owing to the low resistivity of metals and the current propagating to the surrounding ground, which makes it easier to form more conductive paths than blockages, the FDEM displays no obvious change in signal characteristics at the position of a blockage in a metal pipeline, which means that this method has certain challenges for detecting such blockages.

A modified water quality index for intensive shrimp ponds of Litopenaeus vannamei

Traditional sheep and cattle grazing in natural semiarid Mediterranean, Asian and African regions is based on night corrals, where animal secretions accumulate. Lack of management and disregard for the long-term effects of using the same sites for corrals on underground soil characters may negatively affect soil values. This locally increases the content of organic matter and nutrients such as nitrogen, potassium, phosphorus and others that are stockpiled in the corrals. As these activities are long-lasting, they affect the soil parameters, leading to nutrient leakage and contamination of the upper and sub-soil surface. This alarming situation demands a technique to reveal and estimate sub-soil contamination in corrals by using the frequency domain electromagnetic method (FDEM) for measuring soil salinity. The aim of this study is to correlate electrical conductivity measurement with the FDEM to study the influence of sheep corrals on the changes within the sub-soils of corrals in the semiarid region of the northern Negev desert. The results show that a correlation was found between the laboratory soil analysis and the electromagnetic analysis in all sites. Plugot forest site results found to be anomalous indicated sub-surface conductivity resulting from the presence of the corral, with a higher conductivity value of about 230 mS/m, while no differences were found between the soil layers outside the active corral and the corral edge. High values were found in the center of the active corral: 960 mS/m by the laboratory analysis and 200 mS/m by the FDEM. The values obtained in the abandoned corral in the laboratory were about 10 times lower than those obtained from the active corral and six times lower that those found with the FDEM. At the Beit Nir site, high values were found in the center of the active corral: 300 mS/m by the laboratory analysis and 130 mS/m by the FDEM. With different sources of manure, cattle and sheep have shown similar patterns of electrical conductivity (EC) obtained in the sub-soil layers between active and abandoned corrals: high in the center and low at the edge and outside the corral and decreased with depth.