Kristine Martens

Saltwater intrusion and nitrate pollution in the coastal aquifer of Dar es Salaam, Tanzania

Dar es Salaam Quaternary coastal aquifer is a major source of water supply in Dar es Salaam City used for domestic, agricultural, and industrial uses. However, groundwater overdraft and contamination are the major problems affecting the aquifer system. This study aims to define the principal hydrogeochemical processes controlling groundwater quality in the coastal strip of Dar es Salaam and to investigate whether the threats of seawater intrusion and pollution are influencing groundwater quality. Major cations and anions analysed in 134 groundwater samples reveal that groundwater is mainly affected by four factors: dissolution of calcite and dolomite, weathering of silicate minerals, seawater intrusion due to aquifer overexploitation, and nitrate pollution mainly caused by the use of pit latrines and septic tanks. High enrichment of Na+ and Cl− near the coast gives an indication of seawater intrusion into the aquifer as also supported from the Na–Cl signature on the Piper diagram. The boreholes close to the coast have much higher Na/Cl molar ratios than the boreholes located further inland. The dissolution of calcite and dolomite in recharge areas results in Ca–HCO3 and Ca–Mg–HCO3 groundwater types. Further along flow paths, Ca2+ and Na+ ion exchange causes groundwater evolution to Na–HCO3 type. From the PHREEQC simulation model, it appears that groundwater is undersaturated to slightly oversaturated with respect to the calcite and dolomite minerals. The results of this study provide important information required for the protection of the aquifer system.

Groundwater recharge and flow in a small mountain catchment in northern Ethiopia

Abstract The hydrodynamic behaviour of a sloped phreatic aquifer in the Tigray Highlands in northern Ethiopia is described. The aquifer is situated in the soils of a plateau on top of a basalt sequence and lies on steep slopes; the latter lead to hydraulic gradients that can cause high discharge fluxes. Distinct wet and dry seasons characterize the climate of the Tigray Highlands and recharge is absent during the dry season. Because of the fertile vertisols that have developed, the plateau is heavily cultivated and thus has great local economic, and hence social, importance. Water for land irrigation is almost exclusively delivered by rainfall, which is largely restricted to the period June—September. During the dry season, the water table drops dramatically and the aquifer drains nearly completely, under the strong gravity-driven, sustained discharges. This study strives to give insights into recharge and discharge mechanisms of the aquifer, in order to improve the effectiveness of the implemented water conservation measures.

Effects of multi-annual climate variability on the hydrodynamic evolution (1833 to present) in a shallow aquifer system in northern Belgium

Abstract Using a groundwater flow model and long historical meteorological time series data, the evolution of the groundwater flow regime in a multi-layered groundwater flow basin in northern Belgium during the last one and a half centuries (since 1833) is reconstructed. Model output parameters such as piezometric levels, depth to water table, seepage fluxes in the valleys and calculated baseflow to the river system are presented and inter-annual and decadal variations are evaluated against seasonal fluctuations. The main time-varying boundary condition in the model is the aquifer recharge which was estimated using the method of Thornthwaite and Mather based on precipitation and temperature data. The model does not take into account changes in boundary conditions due to changes in land use (deforestation, drainage of cultivated land) or groundwater exploitation. Variations in model output parameters are therefore only due to climatological forcing. Only the natural non-exploited state of the aquifer is considered. Although few historical piezometric measurements are available to verify model output, the results give an indication of the natural hydrodynamic variations on a time scale of decades. Citation Van Camp, M., Coetsiers, M., Martens, K. & Walraevens, K. (2010) Effects of multi-annual climate variability on the hydrodynamic evolution (1833 to present) in a shallow aquifer system in northern Belgium. Hydrol. Sci. J. 55(5), 763–779.

An integrated approach for detection and delineation of leakage path from Micro-Dam Reservoir (MDR): a case study from Arato MDR, Northern Ethiopia

Water is at the center of all efforts to address food security, poverty reduction, economic growth, energy production and human health. In arid and semi-arid regions, groundwater and water harvesting structures (such as Micro-Dam Reservoirs, MDRs) play a significant role for irrigation and maintaining the sustainability of water resources. Fighting poverty and drought using construction of MDRs is becoming a common practice in Tigray (northern Ethiopia), but their implementation is not without challenges. Reservoir leakage, inter alia, is one of the main problems facing MDRs in the region. This paper presents geological, geophysical, hydrogeological and hydrochemical study results in MDR to better understand and delineate leakage zones and mechanisms. Conventional geological mapping, geo-electrical methods (VES and ERP), hydrogeological and geochemical methods were used to understand the geological and geo-hydrological situation of the MDR. Integrated interpretation and synthesis of the data enable to delineate the leakage zone and path. The limestone-shale-marl intercalation unit that makes up the foundation and reservoir area is found to be the leaky unit due to syngenetic and epigenetic discontinuities, while the dolerite unit is relatively impervious. Installation of a cut-off wall at the upstream toe of the dam could be a possible solution to minimize the leakage problem underneath the dam foundation and left abutment/flank. As an alternative, proper utilization of the water from big diameter shallow hand dug wells from the leakage zone downstream of the MDR may be also conceived as a target without incurring additional costs of maintenance.

Tracing Soil and Groundwater Pollution with Electromagnetic Profiling and Geo-Electrical Investigations

Geophysical investigation is used to differentiate lithological units. In addition to this, geophysics is also useful for the detection. definition and monitoring of pollution in the ground on the condition that the pollution. due to Spills. leakage or illegal discharges causes a significant difference in conductivity/resistivity. The methodology of geophysical investigation in environmental issues will be illustrated. The first presented method is the electromagnetic profiling method. which measures the lateral variation in ground conductivity using a transmitter and a receiver. At the transmitter coil, a time-varying electromagnetic field is induced by an alternating current. This field interacts with the ground, proportional to ground conductivity. The resulting field is measured and recorded by the receiver. The background conductivity needs to be defined along a profile in a non-polluted zone. Subsequently, all the collected data can be used to plot the lateral variation in conductivity. Areas with higher conductivity reflect in most cases pollution from which the Source can be traced. A second method is geo-electrical tomography, which is a combination of resistivity profiling and sounding where a large number of electrodes arc placed at a constant distance along a line. During each measurement, the electrical potential caused by a current sent into the soil by two current electrodes is measured between the two potential electrodes. By automatically addressing a combination of four electrodes, and increasing the distance between the electrodes, the depth of penetration increases. Considering the resistivity of the corresponding lithology, pollution along the profile can be delimited vertically and horizontally. Finally, the investigation of the conductivity (resistivity) carried out with borehole loggings delivers information on the vertical distribution of the conductivity in the groundwater reservoir, resulting in the vertical delimitation of pollution close to the borehole. A great advantage for these methods applied to soil pollution consists in the avoidance of direct contact with the Pollution, resulting, in a reduction of health risks. These methods are non-destructive and fairly fast investigations are possible. They will be illustrated by case studies where the results are validated based on the analyses of soil and groundwater samples.

Quantification of water table dynamics as a reference for impact assessment of ecohydrological enhancement measures in a dune area in Belgium

Quantification of water table dynamics is an essential part of eco-hydrological characterisation as they are a controlling factor for vegetation and groundwater-dependent ecosystems. This is often done using the concept of groundwater depth classes, which are defined based on mean high winter and mean low summer water levels. However, this concept does not consider inter-yearly variability of groundwater levels. Also, groundwater depth classes can depend strongly on local topography, which is specially the case in dune belts in coastal regions. In this paper, the relation between groundwater depth classes and the size of seasonal and inter-yearly variations is investigated in a dune region along the Belgian coast, which has the special hydrological and ecological feature of including a dune river. The spatial pattern of the seasonal fluctuations and inter-yearly variability here is much simpler than the heterogeneous distribution of groundwater depth classes. This may provide additional useful information in relation to the occurrence of vegetation and can be used in the context of nature development programmes.

Lateral variation in ground conductivity in a nature reserve

The apparent ground conductivity has been investigated in a deforested area by means of electromagnetical profiling. The variation in apparent ground conductivity is compared with groundwater samples. The structure of the groundwater reservoir, the groundwater and surface water quality, and the groundwater flow are taken into account for the interpretation of the electromagnetical investigation.

Migration of VOCl in the groundwater reservoir modelled by RBCA Tier 2 Analyzer

At the study area, for more than 50 years, an open type of PCE dry cleaning machine was used. This causes a pollution of the groundwater reservoir by PCE and its degradation products. The degradation of the pollution is modeled by means of RBCA Tier 2 analyzer. The results will help to predict the mobilization of the pollution. It is a first step in the estimation of taking precaution measures. One of the degradation products is VC which is very carcinogen.

Geophysical Investigation (Geo-electrical & Electromagnetical Profiling) is Still Indispensable in Environmental Issues

Geophysical investigations have been used since long time in environmental studies to delineate groundwater and soil pollution. This method is very effective in combination with the traditional method of investigation of soil and groundwater pollution. The application of both investigation methods can be continued in monitoring remediation, which is an innovative approach and is illustrated in two case studies.

Electromagnetical investigation of a dune valley in Oostduinkerke (Belgium)

Along several profiles, an electromagnetic prospection (horizontal dipole mode) has been performed in a draining dune river valley. Different factors are at the basis of the variability of the apparent ground conductivity: groundwater flow, lithology, topography and sea spray.