Multi-electrode Profiling Research Articles

The controlling of paleo-riverbed migration on Arsenic mobilization in groundwater in the Red River Delta, Vietnam

In the Red River Delta, the concentrations of Arsenic in groundwater of alluvial dominated systems are very high, exceeding the WHO’s permissible. The correlation between the Arsenic concentrations in groundwater and the age of Holocene sediment as a key controlling groundwater Arsenic concentration in the Red River delta has been investigated. The evolution of sediments in the Holocene is closely related to paleo-riverbed migration in the past. A combination of methods is implemented including remote sensing, multi-electrode profiling (MEP), gamma-logging, drilling, soil sample and groundwater modeling. The resul has identified the shape, sediment compositions and location of the six paleo-riverbed periods. The age of the paleo-riverbed is determined by drilling, soil sampling and optically stimulated luminescence (OSL) in the laboratory. The oldest sediments is 5.9±0.4 ka BP in Phung Thuong near the mountain, the youngest one is from 0.4÷0.6 ka BP in H-transect near the Red River and the rest of the other is around 3.5 ka BP. The modeling results by using MODFLOW and MT3D show that the dynamics of paleo-riverbeds controlling Arsenic mobilization in groundwater in the Red River Delta. When the river moved to another position, the current river position at that time was filled with younger sediments and became paleo-riverbed formation with reducing conditions, Arsenic content which was adsorbed in the previous stage then released into groundwater. Therefore, Arsenic concentration in groundwater of young Holocene sediments is higher than in older ones which elucidates that paleo-riverbed migration controls on Arsenic mobilization in groundwater in the study area.

The controlling of paleo-riverbed migration on Arsenic mobilization in groundwater in the Red River Delta, Vietnam

In the Red River Delta, the concentrations of Arsenic in groundwater of alluvial dominated systems are very high, exceeding the WHO’s permissible. The correlation between the Arsenic concentrations in groundwater and the age of Holocene sediment as a key controlling groundwater Arsenic concentration in the Red River delta has been investigated. The evolution of sediments in the Holocene is closely related to paleo-riverbed migration in the past. A combination of methods is implemented including remote sensing, multi-electrode profiling (MEP), gamma-logging, drilling, soil sample and groundwater modeling. The resul has identified the shape, sediment compositions and location of the six paleo-riverbed periods. The age of the paleo-riverbed is determined by drilling, soil sampling and optically stimulated luminescence (OSL) in the laboratory. The oldest sediments is 5.9±0.4 ka BP in Phung Thuong near the mountain, the youngest one is from 0.4÷0.6 ka BP in H-transect near the Red River and the rest of the other is around 3.5 ka BP. The modeling results by using MODFLOW and MT3D show that the dynamics of paleo-riverbeds controlling Arsenic mobilization in groundwater in the Red River Delta. When the river moved to another position, the current river position at that time was filled with younger sediments and became paleo-riverbed formation with reducing conditions, Arsenic content which was adsorbed in the previous stage then released into groundwater. Therefore, Arsenic concentration in groundwater of young Holocene sediments is higher than in older ones which elucidates that paleo-riverbed migration controls on Arsenic mobilization in groundwater in the study area.

Focused groundwater discharge of phosphorus to a eutrophic seepage lake (Lake Væng, Denmark): implications for lake ecological state and restoration

A study on Lake Vaeng in Denmark demonstrates a high potential for loading of phosphorous via groundwater to seepage lakes. Groundwater discharges are displayed as an important source of phosphorous to a lake due to: (1) high concentrations in the aquifer just below the lake, and (2) the main flow paths through the aquifer–lakebed interface either being overland flow through a seepage face, or focused in zones with very high discharge rates. In-lake springs have measured discharge of up to 7.45 m3 per m2 of lakebed per day. These findings were based on seepage meter measurements at 18 locations, stable isotope (δ18O) analyses, temperature profiles and mapping of ice cover distribution. Groundwater–lake interaction was modelled with a 2D conceptual flow model (MODFLOW) with hydrogeology interpreted from catchment multi electrode profiling, on-lake ground-penetrating radar, well logging and borehole data. Discharge was found to be much focused and opposite to expected increase away from the shoreline. The average total phosphorus concentration in discharging groundwater sampled just beneath the lakebed was 0.162 mg TP/l and thereby well over freshwater ecological thresholds (0.043–0.612, median = 0.117 mg TP/l). The study illustrates a direct link between groundwater and lake chemistry.

Evaluating the salinity distribution of a shallow coastal aquifer by vertical multielectrode profiling (Denmark)

A monitoring system, including five groups of piezometers and five vertical multielectrode profiling probes (VMEP), has been installed in an aquifer beneath a coastal dune in Denmark. In order to assess the salinity distribution within the aquifer, geoelectrical data were gathered in March, June and September 2008, by measuring a dipole-dipole and gradient array using multielectrode profiling. Interpretation of the processed resistivity data was performed by regularized inversion using a one-dimensional, horizontally layered model of formation resistivity. The standard deviation on estimated layer log-resistivity was 0.01–0.03. By estimating two parameters of a power function, observed fluid conductivities derived from samples of porewater were related to corresponding estimated formation resistivities. The conductivity profiles correlate with a winter situation in March with high sea level, active recharge and significant wave activity, causing increased hydraulic heads, a thicker freshwater lens and salt water overlying freshwater close to the sea. In June, the thickness of the freshwater lens is reduced due to less recharge and prevailing offshore winds, imposing density-stable conditions and a sharper transition between fresh and brackish water. During the autumn, aquifer recharge is enhanced and hydraulic heads increase, resulting in a thicker freshwater lens.

Discharge of nitrate-containing groundwater into a coastal marine environment

The discharge of nitrate-containing fresh groundwater from a sandy coastal aquifer and into the adjacent shallow marine environment was investigated near Esbjerg at the northern end of the Wadden Sea in Denmark. The geological structure of the coastal site was determined from drilling data and geoelectrical methods such as multi-electrode profiling (MEP). The distribution of shoreface sediment containing freshwater was mapped using MEP, underwater multi-electrode profiling (UMEP) and the electrical conductivity measured on pore waters. Freshwater discharge fluxes were measured using seepage meters and estimated from observed head gradients and measured hydraulic conductivities in the aquifer sediments. The nitrate distribution in the coastal sediments was obtained from water samples. A groundwater flow model was established to quantify the groundwater flow and travel times, and the distribution of freshwater discharge along the coastline. Results show that on the watershed scale the fresh groundwater discharge is controlled by sand filled buried channels that connect the aquifer with the shoreface sediments. The freshwater discharge along the coastline is at this scale probably best estimated by the groundwater flow model. However, at a more detailed scale the distribution of freshwater discharge in the intertidal zone is controlled by small scale geological heterogeneity and models are unable to predict what happens at a small scale on the beach and offshore. For that purpose UMEP, seepage meters, pore water sampling and local hydraulic gradients are more useful. These measurements indicate that the freshwater discharge occurs in distinct zones and that the highest discharge is near the high tide line, decreasing rapidly in a seaward direction. Nitrate is abundant in the shallow groundwater of the coastal aquifer and is also present in the discharging fresh groundwater at certain patches along the coast. However, on average very little nitrate is observed in the freshwater discharging at the coast. The maximum travel time of groundwater through the aquifer until it discharges at the coast is 100 years and since nitrate leaching from soils has only taken place during the last 40–50 years, part of the fresh groundwater discharging at the coast must be free of nitrate.