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UNESCO-IHE Institute for Water Education, Delft (2012)

Comparative assessment of hydrological impacts of a large dam and cascading small dams using SWAT model : case study of the White Volta basin (Ghana)

Afosah-Anim E.K.

Titre : Comparative assessment of hydrological impacts of a large dam and cascading small dams using SWAT model : case study of the White Volta basin (Ghana)

Auteur : Afosah-Anim E.K.

Université de soutenance : UNESCO-IHE Institute for Water Education, Delft

Grade : Master of Science 2012

Résumé
Tourism is the main economic base in Sharm el Sheikh and is booming. Howerver, nofresh water resources are available. Currently groundwater, which is salt in the entirecoastal area of this part of the Sinai, is desalinated to produce drinking water.Ridgewood is the major actor in desalination in this area currently running four plantsin Sharm el Sheikh. Due to increasing demand the capacity is extended regularly.Desalination is done by RO (reversed osmosis). This process splits the extracted waterinto about 30% drinking water and 70% so-called brine, which contains all the salts.This very salt water is discharged back into the aquifer by means of a number ofreturn wells. Discharge into the sea is not permitted because of the danger feared forthe coral reef ecosystem, which is the basis of the tourist economy.However, three dangers are involved with the method of using the aquifer in this way.First there is a small risk of deep saline water to upcone (be attracted from below) intothe wells. Second, there is the very real risk of short circuiting, in which the injectedbrine flowing through the aquifer reaches the extraction wells in a limited time. Thirdproblem is that increasing capacity is limited by increasing drawdowns, due to whichwells and pumps have to be lowered and more wells have to be installed. Last there isthe risk of pollution of the groundwater by other sources, for instance, loss ofwastewater form sewerage pipes, which may cause the extracted water quality todeteriorate over time, for instance causing increase iron concentrations. The plantstudied in this thesis suffers from most of these problems. This thesis analysis theproblems for one of the Ridgewood sites, El Hadab, and suggests some solutions.To analyze the situation, data were gathered on site and obtained form Ridgewood.These data were combined with other data and analyzed for the geology and lithologyof the aquifer. The pumping tests that have been carried out in the past have also beenanalyzed and used to obtain the necessary hydraulic data of the aquifer.In order to analyze the situation systematically, an extended groundwater model wasbuilt, that includes flow modeling, solute transport and density of the groundwater.The analyzed data served to construct the model. The model was based on the wellknowncodes of MODFLOW, MT3DMS and Seawat2000, all public domain productsof the USGS. The modeling was done using a graphical user interface (GUI) ofGroundwater Vistas, which is a commercial product. In order to be accurate enoughthe model cell size ranged from 25 to 50 m, while 14 layers were chosen to allowsufficiently accurate modeling of the influence of density on the groundwater flow.Several pumpage-injection scenarios have been analyze for El Hadab to examinegroundwater flow and solute transport system characteristic and to address the effectof anticipated future demands on the system.The modeling provided the following results:The drawdown reaches a steady state in about one year. This implies that increasingdrawdowns can only be due to increasing extractions and possibly due to clogging ofwells.The conductivity of the aquifer (about 3 m/d) is low. Therefore the extraction at anylocation is limited due to drawdown.The salinity of the extracted water in El Hadab show a tendency of increase. So theirsalinity has not increased far yet. However, the model shows that the brine is currentlyflowing towards the wells and will reach the extraction wells within a couple of years with high probability. Once this happens, salinity will increase to almost that of theinjected brine within about two decades. This will increase cost and probably increasethe salinity and amount of brine in the future, thus aggravating the problem.Based on the flow computations, a scenario was run in which the injection wells havebeen moved to the very south of the modeled peninsula. This delays short-circuitingfor possibly about a century (depending on the increase of the desalination capacity).However, the brine will also reach the wells in this optimal case on the long run. Bythat time, most of the groundwater will have been replaced by the brine. Therefore,research in recommended in long-term sustainable solutions, which will probablyinvolve larger desalination plants and injection of the brine into the sea in anenvironmentally sustainable way. A suggestion of this is presented in the report.One problem encountered in El Hadab was high iron concentration in the extractedwater, fouling the RO-membranes. The data show that only well 19 has a high Feconcentration, which is probably due to some source of pollution with organicmaterial (wastewater) upstream of this well. The model path lines show where to lookfor a possible source. It is recommended that well 19 is not shut off, because thenanother well will get the iron. Ridgewood may investigate treatment of this wellseparately.When analyzing the data, it was revealed that the manganese concentration aboutdoubled between 2005 and 2007. This is a new problem, which should be addressedby Ridgewood in the near future.

Mots clés : dams ; streamflow ; river basins ; case studies ; Ghana

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Page publiée le 26 février 2015, mise à jour le 11 octobre 2018