Kristine Walraevens

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.

Isotopic methods and their hydrogeochemical context in the investigation of palaeowaters

Abstract Isotope and geochemical techniques are the primary way in which the residence time, recharge conditions and subsequent evolution of palaeowaters can be determined. Isotopic species and noble gas concentrations are used as residence time and palaeoclimate indicators. Among the former, 14C is pre-eminent in late Quaternary studies because of an age range which covers the Pleistocene-Holocene transition. However, its use is constrained by frequent difficulties in determining the dilution of dissolved 14C due to water-rock interaction. A combination of 14C data with 226Ra and 4He results may be useful for Holocene waters but they can also be used to validate the carbon systematics assumed for 14C dating. For waters beyond the range of 14C dating, 81Kr, 36Cl, 4He and chemical tracers can be applied. Stable isotope ratios and noble gas concentrations primarily reflect climatic conditions at the time of recharge. While the noble gases provide absolute values for recharge temperatures, stable isotopes are only relative indicators that vary regionally. The PALAEAUX programme has examined these aspects in some detail by looking at the δ18O shift between Pleistocene and Holocene waters on the European scale, and by calculating δ18O/ΔT ratios from δ18O v. recharge temperature plots for aquifers at different distances from the Atlantic Ocean. Indications are that the more positive δ18O value of ocean water during the Pleistocene dominates in the more westerly European countries over the negative δ18O shift during cooler conditions. There are also indications that air-mass circulation during the Pleistocene was similar to the present day. The evolution of a palaeowater can best be studied by measuring chemical tracers; this is possible in freshwater aquifers, where a clear trend of geochemical reactions is observed, and in freshening marine aquifers. Chemical and isotopic tracers can also be used to study the movement of the front between palaeowater and younger components that must be identified in coastal aquifers to guarantee a sustainable water use.

Inverse chemical modeling and radiocarbon dating of palaeogroundwaters: the tertiary Ledo-Paniselian aquifer in Flanders, Belgium.

Groundwater samples from the Ledo-Paniselian aquifer have been interpreted for chemical reaction patterns, 14C age, and recharge conditions. This confined Tertiary aquifer dips NNE from its outcrop in Belgium toward the North Sea over a length of ∼50 km. Conventional 14C ages of the water samples range from 3 to over 40 ka. Inverse chemical modeling was done to correct the 14C ages for the chemical reactions in the aquifer, while accounting for changes in the recharge water quality during the Holocene and late Pleistocene. The aquifer shows a zonal pattern with (going upstream) Na-, K-, NH4-, Mg-, and Ca-HCO3 water types. The pattern is a result of freshening: Ca displaces the saline cations Na, K, NH4, and Mg from the aquifers cation exchange complex in a Chromatographic sequence. The loss of Ca2+ from solution by cation exchange is by far the most important reaction for dissolution of calcite, which increases the apparent 14C age of the water samples. The 14C age furthermore depends on open/closed conditions of calcite dissolution and CO2 gas exchange and CO2 pressure in the recharge area. It is shown that δ13C and CO2 pressure in a soil are interrelated and that the changes in CO2 pressure can be included in an inverse model which considers variations in infiltration water quality. The overall correction for 14C age is obtained by inverse modeling of water quality and δ13C, with optimization on CO2 pressure in recharge water using PHREEQC [Parkhurst, 1995]. The optimized CO2 pressure for the recharge area varies with age and is generally lower in the water samples with an age above 13 ka. The lower CO2 pressure is corroborated by lower δ18O values of the water.

Hydrochemistry and source of high fluoride in groundwater of the Nairobi area, Kenya

Abstract This study aims to identify the hydrogeochemical processes influencing the high fluoride concentrations in groundwater of the Nairobi area, Kenya. For this purpose 16 groundwater samples were collected and analysed. Fluoride concentrations above the WHO standard are found in the downstream areas. The high F− concentrations are correlated with high sodium and pH and low Ca2+ concentrations. Weathering of sodium-rich alkaline igneous rocks causes a pH increase resulting in an increase in HCO3 − and CO3 2- by dissolution of CO2. Groundwater becomes oversaturated compared to calcite and calcite precipitation occurs, leading to a decrease in Ca2+. This causes a sub-saturation with respect to fluorite and dissolution of fluorite increases the F− concentration. These reactions were modelled using the PHREEQC model and the results showed a good agreement with the measured groundwater quality, indicating that the proposed reactions are plausible for explaining the observed concentrations in groundwater.

Pumping test interpretation by combination of Latin hypercube parameter sampling and analytical models

Pumping tests in groundwater reservoirs are a much used and recommended method to derive hydraulic properties of aquifers and aquitards at the field scale. Interpretation of pumping tests can be done by fitting analytical or numerical models to the obtained field data using optimisation procedures. In this paper, an interpretation methodology is presented and implemented in the form of a computer code that combines analytical solutions for confined, semi-confined and unconfined single and multiple layer schematisations, with random parameter generator routines to find best fitting parameter sets to measured pumping test data. As this requires a large number of Monte Carlo (MC) runs, the number of required simulations is restricted by using a stratified instead of a pure random sampling technique. Latin hypercube sampling (LHS) is used as the stratified random sampler. The analytical solutions are defined in the Laplace domain and inverted numerically using the well-known Stehfest algorithm. The program uses a number of iteration cycles during which parameters sets are generated within predefined limits using the LHS technique. Parameters sets which satisfy a chosen maximum value for a selected objective function (root mean square (RMSE) or mean relative deviation (MRD)) are retained and used to update parameter limits for the next cycle. The objective function criterion is decreased during subsequent cycles while the limits of the sampling intervals are adapted. The number of determinable parameters is dependent on the aquifer schematisation and conceptual model that is chosen. After each cycle, statistics of the parameter values of the realisations which satisfy the objective function criterion are calculated. The method is demonstrated with examples including synthetic datasets and field data. The synthetic data examples show that the program is able to retrieve the parameter values used for generating drawdown sets very well. Typical runtimes on a PC are no more than a few minutes. The program can easily be extended with additional analytical solutions for other schematisations.

Geological and geotechnical constraints for urban planning and natural environment protection: a case study from Mekelle City, Northern Ethiopia

The study was conducted in the rapidly expanding city of Mekelle located in the northern part of Ethiopia, East Africa. An integrated approach including geomorphological, geological and engineering geological, geotechnical and hydrogeological methods were used to characterize the natural urban environment to be used as a baseline for protection and rational planning of development. Conventional field observation, test pits and drilled boreholes were used to collect soil, rock and water samples and standard laboratory procedures were applied. The results revealed that the main geologic constraints to sustainable development of the city are active erosion, instability of slopes, flood hazard, cyclic/alternating hard and weak rock layers, high swelling–shrinkage soils, shallow and unconfined groundwater with variable composition and high susceptibility to corrosion and pollution. It is recommended to avoid development and expansion of the city in areas of steep land forms, at the foot of steep slopes and in areas covered by black alluvial soil deposits. Development of a systematic multidisciplinary database aided by GIS, with continuous monitoring and updating is also highly recommended and will be useful for further refined geotechnical microzonation. It is the authors’ belief that this study highlights the basic constraints and hazards the city is facing, and provides baseline information and awareness to the city planners, decision makers, geo-environmentalists, engineers and the community for future expansion and rehabilitation plans and to tackle the existing hazards.

Regional groundwater flow modeling of the Geba basin, northern Ethiopia

The Geba basin is one of the most food-insecure areas of the Tigray regional state in northern Ethiopia due to recurrent drought resulting from erratic distribution of rainfall. Since the beginning of the 1990s, rain-fed agriculture has been supported through small-scale irrigation schemes mainly by surface-water harvesting, but success has been limited. Hence, use of groundwater for irrigation purposes has gained considerable attention. The main purpose of this study is to assess groundwater resources in the Geba basin by means of a MODFLOW modeling approach. The model is calibrated using observed groundwater levels, yielding a clear insight into the groundwater flow systems and reserves. Results show that none of the hydrogeological formations can be considered as aquifers that can be exploited for large-scale groundwater exploitation. However, aquitards can be identified that can support small-scale groundwater abstraction for irrigation needs in regions that are either designated as groundwater discharge areas or where groundwater levels are shallow and can be tapped by hand-dug wells or shallow boreholes.RésuméLe bassin de Geba est l’une des régions les plus touchées par l’insécurité alimentaire de l’état régional de Tigray dans le nord de l’Ethiopie en raison de sécheresse récurrente résultant d’une répartition erratique des précipitations. Depuis le début des années 1990, l’agriculture pluviale a été soutenue par des projets d’irrigation à petite échelle, principalement par la collecte des eaux de surface, mais le succès a été limité. Par conséquent, l’utilisation des eaux souterraines pour l’irrigation a attiré une attention considérable. Le but principal de cette étude est d’évaluer les ressources en eau souterraine dans le bassin de Geba au moyen d’une modélisation MODFLOW. Le modèle est calibré en utilisant les niveaux d’eau souterraine observés, conduisant à une idée précise concernant les systèmes d’écoulement des eaux souterraines et les réserves. Les résultats montrent qu’aucune des formations hydrogéologiques ne peut être considérées en tant qu’aquifères permettant une exploitation des eaux souterraines à grande échelle. Cependant, des aquitards capables de supporter des prélèvements d’eaux souterraines à petite échelle pour les besoins de l’irrigation peuvent être identifiées soit dans les régions qui sont désignées comme des zones de décharge des eaux souterraines ou des zones où les niveaux d’eau souterraine sont peu profonds et peuvent être captés par des puits creusés à la main ou des forages peu profonds.ResumenLa cuenca de Geba es una de las zonas con mayor inseguridad alimentaria del estado regional de Tigray en el norte de Etiopía debido a las recurrentes sequías resultantes de la distribución irregular de las lluvias. Desde comienzos de los años noventa, la agricultura de secano ha sido apoyada mediante regímenes de riego a pequeña escala, principalmente mediante la recolección de aguas superficiales, pero el éxito ha sido limitado. Por lo tanto, el uso de agua subterránea para fines de riego ha ganado considerable atención. El propósito principal de este estudio es evaluar los recursos de agua subterránea en la cuenca de Geba mediante un enfoque de modelado MODFLOW. El modelo se calibra usando los niveles observados de agua subterránea, lo que da una idea clara de los sistemas de flujo de agua subterránea y reservas. Los resultados muestran que ninguna de las formaciones hidrogeológicas puede ser considerada como acuíferos que pueden explotarse en una explotación de agua subterránea a gran escala. Sin embargo, se pueden identificar acuitardos que pueden apoyar la extracción de agua subterránea a pequeña escala para las necesidades de riego en regiones designadas como áreas de descarga de aguas subterráneas o donde los niveles de aguas subterráneas son superficiales y pueden ser aprovechados por pozos excavados a mano o pozos poco profundos.摘要由于降雨分布极度不均从而造成周期性的干旱,致使Geba流域是埃塞俄比亚北部Tigray州粮食最得不到保障的地区之一。自从20世纪90年代开始,旱作农业主要通过地表水收集而进行的小规模灌溉计划得到支持,但成就有限。因此,使用地下水用于灌溉受到了人们的极大关注。本研究的主要目的就是通过MODFLOW模拟方法评价Geba流域的地下水资源。采用观测的地下水位校正模型,深入了解了地下水流系统和地下水储量。结果显示,没有一个水文地质层可以作为含水层进行大规模地下水的开采。然而,发现一些弱透水层,这些弱透水层在地下水排泄区或地下水位浅的地方可以进行地下水开采以满足灌溉所需,可以通过手工挖掘井或者浅的机井抽取。ResumoA bacia de Geba é uma das áreas de maior insegurança alimentar no estado regional de Tigray, Norte da Etiópia, devido às secas recorrentes resultantes da distribuição errática da precipitação. Desde o inicio da década de 1990, a agricultura dependente da chuva vem sendo mantida por sistemas de irrigação de pequena escala, majoritariamente utilizando águas superficiais, mas o êxito tem sido limitado. Desse modo, a utilização de águas subterrâneas para irrigação ganhou uma atenção considerável. O principal propósito desse estudo é avaliar os recursos hídricos subterrâneos na bacia de Geba por meio de modelagem utilizando o MODFLOW. O modelo é calibrado utilizando níveis freáticos observados, fornecendo uma visão clara sobre o sistema de fluxo e reservas de água subterrânea. Os resultados demonstram que nenhuma das formações hidrogeológicas podem ser consideradas como aquíferos que permitem a extração de água subterrânea em larga escala. Entretanto, podem ser identificados aquitardos que podem apoiar a captação de águas subterrâneas em pequena escala para utilização na irrigação em regiões que são tanto designadas como áreas de descarga como onde os níveis freáticos são rasos e podem ser captados por poços escavados a mão ou poços rasos.

Pleistocene and Holocene groundwaters in the freshening Ledo-Paniselian aquifer in Flanders, Belgium

Abstract The Ledo-Paniselian aquifer presents a case study of evolution of fresh groundwater from sea water under the changing piezometric and climatic conditions of the Pleistocene and Holocene. Hydrogeochemical, isotopic, experimental and hydrodynamic results are used in the interpretation. The distribution of groundwater types in the Ledo-Paniselian aquifer is determined by two end members: fresh Ca-HCO3 recharge water and sea water-saturated sediments. Hydrogeochemical modelling supports the view that mixing of the end members and cation exchange are the main processes; calcite dissolution is also important. Cation exchange consists, in the first place, of desorption of the adsorbed marine cations (Na+, K+ and Mg2+) in exchange for the freshwater cation Ca2+. Groundwater δO is around the value of modern precipitation in the area (−6.5‰) for the samples with higher radiocarbon contents; it is <−7.0‰ for the groundwater containing the lowest radiocarbon levels. An overlapping transition zone exists between both groups. δ13C becomes heavier for the samples containing the lowest radiocarbon levels, indicating chemical dilution. Pore waters from the Bartonian clay show preferential flow paths. Faster flow paths are more strongly leached, leading to low total dissolved solids (TDS), low sulphate concentrations and low Mg2/Ca2 ratios; the slower pathways still contain gypsum, increasing the sulphate concentrations and TDS, and Mg2/Ca2 ratios are higher because they were less reduced by cation exchange resulting from freshening. Four methods for determining cation exchange capacity (CEC) and adsorbed cations are compared: the NH4OAc method, two BaCl2 methods (one in unbuffered and the other in buffered conditions) and a new NaCl/NH4Cl method. Reasonable CEC values are obtained with the NHOAc method. Comparing the measured equivalent fractions of the adsorbed cations with those calculated from the pore solutions, using the computer programme PHREEQC, it can be concluded that the NaCl/NH4Cl method produces the best results. The proton exchange capacity of decalcified sand from the Ledo-Paniselian aquifer was determined to be c. 1–1.5 meq/100 g in the pH range 5–8.5. A hydrodynamic model is developed to explain the evolution of groundwater and for evaluating the effects of pumping at both local and regional scales. Model calculations show that the observed freshwater-saltwater distribution is not the result of the present freshwater flow conditions but the result of different flow regimes during the ice ages when sea levels were much lower. Occurrence of a permafrost layer during cold periods could have had a dramatic impact on the groundwater flow system by, at least temporarily, decreasing the recharge of the aquifers. The existence of the Saalian ice sheet in The Netherlands could have influenced the flow in the deeper Eocene-Oligocene aquifers. The high pressures that existed under the ice sheet could have reversed the flow direction from north to south.

Drought impacts on long-term hydrodynamic behavior of groundwater in the tertiary–quaternary aquifer system of Shahrekord Plain, Iran

Shortage of water resources in arid and semi-arid areas causes water supply to be one of the most important subjects and major concerns within NGO and governments’ policies in recent years. The Shahrekord Plain aquifer system is located in a semi-arid area and acts as a key source of water supply. Groundwater management in this area is thus very important. Although change in the climatological factors is not possible, long-term fluctuation studies can help in managing the available water resources to overcome from drought or decrease its negative impact. The hydrodynamic study of the aquifer system coupled with the drought indices in each region can be useful in making decisions related to the hydro-ecosystem management of that region. In this article, hydrodynamics of the aquifer system of the Shahrekord Plain coupled with the ratio of P/PET as a drought index, are assessed on the long term. In Shahrekord Plain aquifer, there is a short-term seasonal fluctuation, which is increased by overexploitation during the dry season, when water is needed for irrigation. The hydrodynamic behavior of the plain aquifer on the long term is changing. This fluctuation at first is a function of time. Secondly, it is spatially dependent. Groundwater behavior is directly sensitive to the variation of drought index, both seasonally and on the long term.