Tamiru Abiye

The interference of a deep thermal system with a shallow aquifer: the case of Sodere and Gergedi thermal springs, Main Ethiopian Rift, Ethiopia

An integrated survey program involving geological, hydrogeological and geophysical techniques has been employed to characterize the aquifer geometry, recharge and circulation dynamics of thermal springs within a shallow aquifer system in Ethiopia. The selected springs for the case study are Sodere and Gergedi, which are situated within the tectonically active Main Ethiopian Rift (MER). Geologically, the studied springs are located on Plio-Quaternary volcanic rocks. The geophysical results indicate the presence of subsurface weak zones represented by extensional tectonics and weathering zones which are responsible for thermal water circulation and facilitate recharge from the adjacent surface-water bodies. The structures inferred by the resistivity survey, both sounding and electrical tomography, present contrasts in rock resistivity response. The anomalous zones in the magnetic data are in good agreement with the zones that are revealed by geological mapping and surface manifestation of the thermal water discharge zones. The shallow aquifer of the central MER is under the influence of thermal water, which increases the groundwater temperature and mineral content.RésuméUn programme d’investigation intégrée comprenant géologie, hydrogéologie et géophysique a été mis en œuvre afin de caractériser la géométrie de l’aquifère, la recharge et les circulations thermales au sein d’un aquifère de surface en Ethiopie. Les sources retenues pour ce cas d’étude sont les sources de Sodere et de Gergedi, qui sont localisées au sein du rift éthiopien tectoniquement actif (RE). Du point de vue géologique, les sources étudiées émergent au niveau des formations volcaniques du Plio-Quaternaire. Les résultats géophysiques indiquent la présence de zones souterraines « faibles » correspondant aux zones d’extension tectonique et d’altération, responsables de la circulation des eaux thermales et facilitant la recharge des eaux de surface. Les structures mises en évidence à partir des sondages et des tomographies électriques, sont associées à des réponses contrastées en termes de résistivité. Les zones d’anomalies magnétiques sont cohérentes avec les zones identifiées lors de la cartographie géologique et par les zones de décharge des eaux thermales. L’aquifère de surface de la partie centrale du RE est influencé par les circulations des eaux thermales, ayant pour conséquence une augmentation de la température des eaux souterraines et de la minéralisation.ResumenSe empleó un programa integrado de relevamiento que involucra técnicas geológicas, hidrogeológicas y geofísicas para caracterizar la geometría del acuífero, la recarga y la dinámica de circulación de los manantiales termales dentro de un sistema acuífero somero en Etiopía. Los manantiales seleccionados para el caso de estudio son Sodere y Gergedi, los cuales están situados dentro del Rift Principal Etíope (MER) tectónicamente activo. Geológicamente los manantiales están localizados en rocas volcánicas Plio-Cuaternarias. Los resultados geofísicos indican la presencia de zonas de debilidad subsuperficial representadas por una tectónica extensional y zonas meteorizadas las cuales son responsables de la circulación de agua termal y facilitan la recarga desde los cuerpos de aguas superficiales adyacentes. Las estructuras inferidas por los relevamientos de resistividad, tanto sondeos como tomografía eléctrica, presentan contrastes en la respuesta de la resistividad de la roca. Las zonas anómalas en los datos magnéticos están en buen acuerdo con las zonas que son revelados por el mapeo geológico y las manifestaciones en superficie de las zonas de descarga de aguas termales. El acuífero somero del MER central está bajo la influencia de agua termal, la cual incrementa la temperatura y el contenido mineral del agua subterránea.摘要为刻画埃塞俄比亚境内一个浅部含水层系统的含水层形态特征以及温泉补给与循环机理,开展了一个包括地质、水文地质和地球物理技术的综合性调查。本文以位于构造活跃的Main Ethiopian Rift (MER)上的Sodere和Gergedi温泉为例。地质上,本文所研究的温泉位于上新世-第四纪火山岩上。地球物理探测结果表明存在有以伸展构造和风化带为代表的地下薄弱地带,是热水循环的原因,并促进了附近地表水体的补给。根据电阻率测量结果(包括测深与电子成像)推断的构造,在岩性电阻率信号上形成对照。基于磁场数据确定的异常区与基于地质填图和排泄区地表热显示所圈定的范围一致。MER中央的浅部含水层受热水影响,提高了地下水温度和矿物质含量。RiassuntoAl fine di definire le dinamiche di ricarica e di circolazione di sorgenti termali profonde e di delimitare la geometria dell’acquifero superficiale alimentante è stata realizzata un’indagine integrata idrogeologica e geofisica. Le sorgenti Sodere e Gergedi, ubicate all’interno del Main Ethiopian Rift (MER) (Etiopia centrale), sono state selezionate come rappresentative. Nell’area in studio sono presenti rocce vulcaniche riferibili al Plio-Quaternario. Le interpretazioni dei dati geofisici indicano la presenza nel sottosuolo di zone fratturate legate a tettonica estensionale e alterazione, che sono responsabili della circolazione dei fluidi termali e che facilitano la ricarica dai limitrofi corpi idrici superficiali. Le strutture dedotte dalle indagini di tomografia elettrica ed acustica mostrano un contrasto di resistività nelle rocce. Le zone ad anomalia magnetica sono in accordo con quelle evidenziate dalla geologia di terreno e dalle manifestazioni superficiali delle emergenze delle acque termali. L’acquifero superficiale del MER centrale è influenzato dalle acque termali profonde, che fanno aumentare la temperatura e la mineralizzazione delle acque sotterranee.ResumoFoi desenvolvido um programa integrado de pesquisa que envolveu técnicas geológicas, hidrogeológicas e geofísicas, para caraterizar a geometria do aquífero e a recarga e a dinâmica da circulação de nascentes termais dentro de um sistema aquífero superficial na Etiópia. As nascentes selecionadas para o estudo de caso foram as de Sodere e Gergedi, situadas dentro do Rift Principal Etíope (Main Ethiopian Rift – MER), que se encontra tectonicamente ativo. Geologicamente, as nascentes estudadas situam-se em rochas vulcânicas de idade Plio-Quaternária. Os resultados geofísicos indicam a presença de zonas de fraqueza na subsuperfície, representadas por tectónica extensional, e zonas de meteorização, as quais são responsáveis pela circulação de águas termais e facilitam a recarga a partir de massas de água superficiais adjacentes. As estruturas inferidas pelos ensaios de resistividade, tanto em sondagem como em tomografia elétrica, apresentam contrastes na resposta da resistividade da rocha. As zonas anómalas nos dados magnéticos estão de acordo com as zonas delimitadas por mapeamento geológico e com as zonas de manifestação superficial da descarga de águas termais. O aquífero superficial do MER encontra-se sob influência de águas termais, aumentando a temperatura e o conteúdo mineral da água subterrânea.

Environmental impact and vulnerability of the surface and ground water system from municipal solid waste disposal site: Koshe, Addis Ababa

Geo-environmental assessment and geophysical investigations were carried out over the only functional municipal solid waste disposal site of the city of Addis Ababa, Ethiopia, known locally as Koshe. The accumulated wastes from Koshe have impact on the surrounding human and physical environment since the disposal site was not designed. The study deserves emphasis because the city of Addis Ababa currently obtains a considerable portion of its domestic water supply from a well field developed not much farther from and along a groundwater flow direction in relation to the waste disposal site. It was found out that the leachates from the site contain high concentration of biological oxygen demand, chemical oxygen demand, chloride and sulphate besides high concentration of cobalt, nickel and zinc in the surrounding soils. The geophysical results have mapped weak zones and near-vertical discontinuities that could potentially be conduits for the leachate from the wastes into the deep groundwater system. Further, a zone of potential leachate migration from the landfill was identified from the electrical models; the location of this zone is consistent with the predicted direction of groundwater flow across the site. The results further suggested that the open dump site tends to cause increasing amount of pollution on the surrounding soil, surface and ground waters. Furthermore, it was observed that the Koshe waste disposal site has grown beyond its capacity and the poor management of the open dump landfill has reduced the aesthetic value of the surrounding environments. The need to change/relocate the existing waste disposal site to a more suitable and technologically appropriate site is emphasized.

Hydrogeophysical investigation for groundwater at the Dayspring Children's Village, South Africa

Over the past 30 years, the water available in boreholes at the Dayspring Childrens Village has slowly diminished to the point where the schools viability is threatened. In this same time period, a large stand of eucalyptus and yellow wattle trees has become well established. These trees are known to consume large quantities of water and we are investigating the effect of these trees on the local groundwater hydrology. Many types of geophysical and hydrological data—including gravity, magnetic, electrical resistivity, electromagnetic (EM) and seismic—are being acquired to understand the regional geology, seasonal changes and ultimately the impact of the trees on the local hydrology. Electrical resistivity data collected at the end of the dry season and at the end of the rainy season are used to define seasonal changes of a near-surface aquifer. By quantifying the effect of these trees on the hydrology, predictive recommendations on the benefits of tree removal can be made to government.

Hydrogeophysical investigations at the Dayspring Children's Village: Quantifying the effect of invasive tree species

Invasive alien tree species are known to use significantly more water than local indigenous trees; however, quantifying the effect of these trees on groundwater resources is a challenging task. We are currently investigating aquifers and recharge at the Dayspring Children’s Village. There is a large stand of mature invasive alien trees which is proposed to be depleting a shallow groundwater aquifer. We have collected gravity, magnetic, resistivity, refraction seismic and TEM data to map out the subsurface and determine temporal variations in the groundwater. Structural controls, such as fractures, appear to host the groundwater, as the diabase sills and metamorphosed shales are not porous enough to host groundwater. Two aquifers have been identified at the school, an aquifer at ~50 m depth with a shallow circulation system age of ~49 years and a deeper aquifer at ~162 m with a closed circulation system hosting water older than 60 years. It is possible that the syenite dyke to the west of the school may compartmentalize the aquifer system. Preliminary resistivity and gravity data collected in 2009, 2010 and 2011 demonstrate significant variations between the end of the dry season and the end of the rainy season.