Recharge Areas and Geochemical Evolution of Groundwater in an Alluvial Aquifer System in the Sultanate of Oman
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REPORT Recharge areas and geochemical evolution of groundwater in an alluvial aquifer system in the Sultanate of Oman Juerg M. Matter · H. N. Waber · S. Loew · A. Matter Abstract A regional hydrogeochemical model was de- Keywords Arid regions . Groundwater flow . veloped to evaluate the geochemical evolution of differ- Groundwater recharge . Hydrochemical modeling . Stable ent groundwaters in an alluvial aquifer system in the In- isotopes terior of Oman. In combination with environmental iso- topes the model is able to extract qualitative and quanti- tative information about recharge, groundwater flow paths Resum´ eOnad´ evelopp´ eunmod´ ele` regional´ and hydraulic connections between different aquifers. The hydrogeochimique´ pour evaluer´ l’evolution´ geochimique´ main source of water to the alluvial aquifer along the flow des differentes´ eaux souterraines dans le systeme` aquifere` paths of Wadi Abyadh and Wadi M’uaydin in the piedmont de l’interieur´ d’Oman. Avec la contribution des isotopes is groundwater from the high-altitude areas of the Jabal d’environnement, le modele` est capable de fournir des Akhdar and local infiltration along the wadi channels. In informations qualitatives et quantitatives sur la recharge, contrast, the piedmont alluvial aquifer along Wadi Halfayn les directions d’ecoulement´ et sur les connections hy- is primarily replenished by lateral recharge from the ophio- drauliques entre les differents´ aquiferes` du systeme.` La lite foothills to the east besides smaller contributions from source principale pour l’aquifere` situe´ au long de la di- the Jabal Akhdar and local infiltration. Further down gra- rection d’ecoulement´ des oueds de Abdyah et de M’uaydin dient in the Southern Alluvial Plain aquifer a significant dans le piemont´ est l’eau souterraine provenant des zones de source of recharge is direct infiltration of rain and surface haute altitude de Jabal Akhbar ainsi que l’infiltration locale runoff, originating from a moisture source that approaches au long des canaux de l’oued. Par contre, a` part d’une petite Oman from the south. The model shows that the main contribution provenant des infiltrations locaux et de Jabal geochemical evolution of the alluvial groundwaters occurs Akhbar, l’ aquifere` alluvial de Wadi Halfayan est en prin- along the flow path from the piedmont to the Southern cipal alimente´ par la recharge laterale´ provenant des ophio- Alluvial Plain, where dedolomitization is responsible for lites situees´ dans la partie est. Les plus importantes sources the observed changes in the chemical and carbon isotope de recharge dans l’aval de l’aquifere` de la Plaine Alluviale composition in these waters. sont l’infiltration directe et le ruissellement provenant d’une source d’humidite´ qui s’approche d’Oman par le sud. Le modele` montre que la principale evolution´ geochimique´ se produise au long de la direction d’ecoulement´ qui part de piemont´ de la Plaine Alluviale du Sud, ou` le proces` Received: 30 September 2004 / Accepted: 9 December 2004 de dedolomisation´ est responsable pour les changements Published online: 8 September 2005 observes´ dans la composition geochimique´ et en carbone des eaux. C Springer-Verlag 2005 Resumen Un modelo hidrogeoqu´ımico regional fue de- sarrollado, para evaluar la evolucion´ geoqu´ımica de difer- J. M. Matter () · S. Loew Geological Institute ETH Zuerich, Engineering Geology, ETH entes aguas subterraneas´ en un sistema acu´ıfero aluvial ubi- Hoenggerberg, cado en el interior de Oman.´ El modelo, en combinacion´ 8093 Zuerich, Switzerland con isotopos´ ambientales, es capaz de obtener informacion´ e-mail: [email protected] cualitativa y cuantitativa sobre recarga, direcciones de flujo Tel.: +1-845-365-8543 de agua subterranea´ y sobre las conexiones hidraulicas´ en- Fax: +1-845-365-8155 tre diferentes acu´ıferos. La fuente principal de agua para J. M. Matter el acu´ıfero aluvial en el piedemonte son las aguas sub- Present address: Lamont-Doherty Earth Observatory of terraneas,´ a lo largo de las direcciones de flujo del Wadi Columbia University, Geoscience 105B, 61 Route 9W, Palisades, NY, 10964 USA Abyadh y del Wadi M’uaydin, desde las areas´ de gran altura de Jabal Akhdar y como infiltracion´ local a lo largo de los H. N. Waber · A. Matter canales del wadi. En contraste, el acu´ıfero aluvial del piede- Institute of Geological Sciences, University of Bern, Baltzerstrasse 1, monte a lo largo del Wadi Halfayn, esta alimentado prin- 3012 Bern, Switzerland cipalmente por recarga lateral desde las partes bajas de las Hydrogeology Journal (2005) 14: 203–224 DOI 10.1007/s10040-004-0425-2 204 montanas˜ de ofiolitas hacia el Este, ademas´ por pequenas˜ ble. Different recharge areas and moisture sources are pro- contribuciones desde el Jabal Akhdar e infiltracion´ local. posed based on environmental isotope data and tested for Siguiendo gradiente abajo en el acu´ıfero de la Llanura Alu- their consistency with the overall geochemical evolution of vial del Sur, all´ı tanto la infiltracion´ directa de lluvia, como the groundwaters. It is shown that a sound understanding la escorrent´ıa superficial constituyen fuentes significativas of groundwater evolution including quantification by geo- de recarga, las cuales se originan en una fuente de humedad chemical modeling refines the conclusions obtained from que llega a Oman´ proveniente del Sur. El modelo muestra, isotope studies alone. que la evolucion´ geoqu´ımica principal de las aguas sub- terraneas´ en el aluvion,´ sucede a lo largo de la direccion´ de flujo, que va desde el piedemonte hacia la Llanura Alu- Hydrogeological setting vial Sur, en donde el proceso de des-dolomitizacion´ es el responsable de los cambios observados en la composicion´ Study area qu´ımica y tambien´ del contenido de isotopos´ de carbono The Halfayn alluvial aquifer system is located in Oman en estas aguas. in the eastern part of the Dakhiliya Region, approximately 150 km southwest of the capital Muscat (Fig. 1). The area is characterized by a large alluvial plain, lining the flank of the Introduction Northern Oman Mountains (Fig. 1). To the north, the study area is bounded by the water divide of the Jabal Akhdar, a This study focuses on the geochemical evolution of ground- mountain chain rising up to 3,000 m. The southern limit is waters in the Halfayn alluvial aquifer system in the in- marked by the frontal ranges forming the Adam Mountains. terior (Dakhiliya region) of the Sultanate of Oman. The The area has an arid climate with a mean annual precipita- aquifer encompasses an area of approximately 4,000 km2 tion of about 300 mm on the Jabal Akhdar and 90 mm on and is the principal groundwater source for the whole re- the alluvial plain (MWR 1995). Rainfall is characterized by gion. Previous investigations in parts of the study area in- large variability both in time and space. Precipitation is re- clude Cansult/Gartner and Lee (1986) who described the lated to local summer convective storms and winter frontal origin and residence times of groundwater in the alluvial storms, approaching from the north and northwest and orig- and bedrock aquifers in the Manah area between Manah inating from the Mediterranean Sea (Stanger 1986; Clark and Adam based on environmental isotopes. Clark et al. 1987; Macumber et al. 1997; Weyhenmeyer et al. 2002). (1987) presented a detailed isotope study on modern and Summer convective storms occur locally and are of short fossil groundwater in Oman, including the data of Can- duration (min) compared to the frontal storms (h) during sult/Gartner and Lee (1986) for the shallow alluvial and the winter months, whereby most of the rainfall occurs in bedrock aquifers in the Halfayn system between Nizwa and the mountainous areas (Meteorological Reports, Ministry Adam. A review of all available data of the entire Dakhiliya of Communications, Sultanate of Oman). Tropical cyclonic region from previous studies is presented in MWR storms, originating in the southeastern Arabian Sea or the (1995). Bay of Bengal approach northern Oman occasionally and In arid regions, the assessment of available groundwa- may cause heavy rainfall for several days. The frequency of ter resources is key to the economic development and in- these storms is estimated as once every 5–10 years (Taylor creased prosperity. In order to evaluate the existing ground- et al. 1990). In addition, low-pressure systems originating water resources and to develop improved water manage- from the south (e.g. Horn of Africa) occasionally approach ment strategies, it is primarily necessary to identify the northern Oman and are the source for sporadic but heavy dominant infiltration areas, sources of recharge and major rainfall in this area (Macumber 2003). The mean annual groundwater flow paths as well as to estimate groundwater temperature is between 17◦C in the high mountain areas residence times. Many studies from semi-arid to arid ar- and 28◦C in the low-lying alluvial plain (MWR 1995). eas have focused on the evaluation of recharge, its sources Natural vegetation and soil development is almost absent and spatial variability as well as the estimation of ground- in the mountainous areas, where bedrock is exposed, and water residence times by using mainly environmental iso- sparse in the alluvial plain. topes and to a lesser extent hydrochemical data, e.g. chlo- ride concentrations (e.g. Edmunds and Walton 1980; Adar and Neuman 1988; Issar et al. 1993; Edmunds and Gaye Geology and hydrostratigraphy 1994; Simmers 1997; Weyhenmeyer et al. 2002; Macumber The alluvial plain of the Halfayn System lies in a syn- 2003). clinal basin between the Jabal Akhdar to the north and This report presents results from a hydrogeochemical the Adam Mountains to the south (Fig. 2). The geology investigation that details the sources of recharge, the geo- of northern Oman has been described in detail by Glen- chemical evolution along the flow paths, groundwater mix- nie et al.