 |
|||||||
| About UNEP | UNEP offices | News centre | Publications | Calendar | Awards | Milestones | UNEP Store |
|
|||||||
|
 |
|
|
|
||
| UNEP>DTIE>IETC | |
|
|
Newsletter and Technical Publications 1.3 Artificial Groundwater Recharge Artificial groundwater recharge is an important technology in water resources management, particularly for utilizing excess surface water. This technology can increase the water reserve of underground aquifers and utilize surface water that would otherwise be lost or contaminated. It also allows for the use of the water at later times when it is more needed. Although initial use of this technology dates back to the 19th Century, it has not been widely used, except after the Second World War in Europe and the United States of America. It became utilized in the Arabian region in 1958. Morocco was the first Arab country to apply the technology, including three projects in the Sharaf Al- Equab, Nafeis valley and Sous valley areas, primarily to ensure a dependable water supply for Tanga city. It was later introduced to most of the West Asia countries, and has been used in the United Arab Emirates, Bahrain, Saudi Arabia, Qatar, Kuwait, Oman, Syria, Lebanon and Jordan. The current groundwater recharge projects in these countries range from large productive projects contributing to the water reserve of groundwater aquifers used for drinking water supply and irrigation, to experimental projects for local testing and assessment of the technology as a means of determining whether to approve or disapprove its use in specific situations. In Saudi Arabia in the late-1950’s and early 1960’s,and in the United Arab Emiraates and Sultanh of Oman in the early 1980’s, dams were constructed in river valleys to retain water for the purpose of artificial recharge of groundwater aquifers by facilitating infiltration of the water into the soil, rather than having it flow to the sea or be evaporated in desert regions. It is estimated, for example, that about 120 million m3 of water is lost to the sea or is evaporated in Oman, which is a very significant water loss in the dry environmental conditions existing in the country. Large projects have been implemented in Qatar, involving the injection of stored surface water to recharge the underlying aquifers. Several experiments to test the efficiency and feasibility of the groundwater recharge technology for the prevailing environmental conditions are underway in Syria and Lebanon. The water shortage in West Asia countries results from the prevailing arid and semi-arid conditions of the region, and it is therefore very desirable to utilize every drop of this scarce resource. Artificial recharge technology for groundwater can obviously contribute to utilizing the surface water that is available in the region. It will allow for a general increase in the freshwater supply, as well as maximize the utilization of the available quantities of surface freshwater resources. Technology Description A number of artificial recharge concepts currently exist, and they also intermix with other concepts related to groundwater storage or to drainage of liquid wastes into different aquifers. Accordingly, artificial recharge of groundwater refers here to the process of feeding surface water into a groundwater aquifer, utilizing different artificial methods to increase the safe freshwater yield from the aquifer. When an aquifer is supplied with new quantities of freshwater that are not possible with natural recharge, the result is an increase in the quantity of water available to be extracted for beneficial human uses. With this perspective, the goal of artificial recharge is to increase the volume of freshwater reaching an aquifer, with the smallest environmental impact. Thus, artificial recharge can achieve one or more of the following objectives:
Some researchers have summarized the previous multiple objectives into two major objectives, as follows:
It is obviously important to determine the objectives of artificial recharge before initiation of relevant projects, as a means of avoiding failure in its use. Artificial recharge technologies have undergone major changes throughout the beginning of the 19th Century until the present time. There are currently ten different types of this technology available, and they can be classified into two major categories, as follows:
The variation of technologies within these two categories is attributed to variations in the types of groundwater aquifers, the different surrounding hydrogeological and geological conditions, and the general purpose for recharging the aquifers. The use of artificial recharge dams, Hafayer, and filtration basins comprise the infiltration technologies implemented in the West Asia region, while the use of wells represents the water injection technology. Dams for Artificial Recharge of Groundwater Aquifers Dams are widely used in Saudi Arabia, United Arab Emirates and Sultanate of Oman and, to a lesser extent, also in Syria and Jordan. The dams are built across valleys with recurrent surface water runoff to recharge aquifers spread on the valley beds. Large quantities of the surface water would otherwise have ended up flowing into the sea or into the desert areas or impermeable plains, all of which would promote water loss because of the small water infiltration capacity in these areas, relative to the volume of flowing surface water. This phenomenon is observed in tens of valleys in which large volumes of water accumulate and flow through the seasonal flood periods toward the plains, naturally recharging aquifers during their overland flow. Nevertheless, a great part of this water does not infiltrate to the underlying aquifers, which gave rise to trying to devise a means of retaining the water for a sufficiently long period of time for it to infiltrate into the aquifers. Different types of dams are used to retain the water, including earth dams, rockfill dams and, to a lesser extend, concrete dams. The dams are usually provided with a spillway to release the retained water to infiltration basins immediately downstream of the dams. An example is the Al-Kabeer valley dam in the Al-Zahera region of Oman, with the goal of improving and increasing the the Aflaj and well discharge between the two towns of Al-Iraqi and Al-Abry, by recharging the aquifer in the region with floodwater that previously was lost to the desert. The water filtration basin was built alongside and immediately downstream of the dam to a width of 22 m and a depth of about 0.5 m. It was covered with gabion boxes with thickness of at least one meter. Water is released from the dam to the infiltration basin through one opening, and then to the natural channels of the valley, for the purpose of recharging the groundwater aquifer and for increasing the wells and Aflaj discharges, which are widespread in the region’s farms. There are some other dams that were designed on the basis that the infiltration will take place from the dam’s lake itself, or from the terraces spread along the side of the dam. An example is the Ghalfa valley dam in the United Arab Emirates, where the infiltrating water volume across the rocky terraces surrounding the dam reach up to 2,330 m3/day. Across the dam lake bottom, the infiltrating water volume reaches about 225 m3/day. A striking example of the successful use of infiltration dams is the Rabiat Al-Sheikh dam of the Al-Asharnah in Syria. It was built on a Karst geological formation, with a water storage capacity of 16 million m3. In 1991, the dam recharged the aquifer with about 25 million m3, resulting in a rise in the aquifer water table to more than 15 m. Similar results were achieved with the Al-Kafat Dam of the Al-Salmeyah region in Syria. An earth dam was also constructed on the Al-Sawaqah valley in Jordan to recharge the karstic aquifer in the region. There is a third category of dams, based on spreading the water after its storage. With this technology, the water is spread on vast areas downstream of the dam to ensure its infiltration to recharge the aquifer. An example of this type of dam is the Al-Bih valley dam, which store about 7 million m3/year, most of which would otherwise have been lost to the sea. The dam obstructed the waterflow, redistributing it into special basins attached to the dam, thereby ensuring its ts infiltration, storage and later reuse in the wells. The dam, located east of Ras Al-Kheimah, is about 14 m high and has a length of about 160 m. In addition to increasing groundwater storage, most of the ground recharge dams have also achieved other objectives as well, such as protection against floods. Rainstorms in these arid and semi-arid regions can readily cause flash floods. The dams also help reduce seawater intrusion, improve groundwater quality, and can be used directly to irrigate agricultural lands. Examples are the Al-Geezy valley and Al-Khoud valley dams in Oman. The Al-Geezy dam stores about 3.6 million m3 of water, which is spread to recharge groundwater and improve its quality, to reduce seawater intrusion in this region, and to protect against floods. The same objectives were achieved with the Al-Khoud dam in the Al-Sayeb region. In Jordan, the Al-Soltanah Dam was built in the Al-Mowgeb valley to ensure artificial recharge for Al-Mowgbasin, as well as to provide irrigation water for the neighboring agricultural lands. Its storage capacity reached 1.1 million m3. The Al-Qaryat valley Dam was constructed in Bahlaa State of Oman. It has a water storage capacity of 0.5 million m3, which is distributed on an area of 180,000 m2, and is also used to cultivate wheat. The dam also resulted in a raising of about 2 meters of the aquifer water table in nearby wells. The artificial recharge dams constructed in the above-mentioned countries vary with respect to their designs and volumes, based on the prevailing hydrogeological and geological conditions in the valleys where they were constructed. Two main types of dam design are approved for use in the Gulf countries. The first type is where the dam location is near the valley outfall (i.e., where the main valley channel ends and branches to several secondary channels at the valley delta). In this type, the dam is equipped with outlets to regulate the release and distribution of water through secondary channels to cover the land between them and spread the water over the designated spreading areas, thereby accelerating the leakage process. The dam openings are designed in such a way that the water flow velocity is dissipated within a short distance. Thus, the valley delta will be covered with still water that will infiltrate to the aquifer in a matter of days. Among the dams based on this type of design are the Al-Khoud, Halty Salahy valley and Al-Geezy valley dams in Oman. The second type of design is used in valleys with permeable beds, where dams of about 2-5 m height are built along the main course of the valley. They are flooded with the floodwaters and form upstream lakes, thereby facilitating natural leakage of the water through the valley bed. Trenches and basins are sometimes built to facilitate the infiltration process. This type of design slows the water flow velocity in the valley to ensure sufficient time for the water to infiltrate. Examples of this type of design include the Al-Goul valley, Tanouf valley and Al-Qaryat valley dams in Oman. Since groundwater recharge dams are built along the water flood route, measures must be taken in all cases to retain rocks, stones, sand and all other solid materials, in order to conserve the lake’s water storage capacity. Protective measures also must be implemented to protect the body of the dam from scour and failure, usually by lining it with gabions, cement blocks and spillways. In regard to their volumes, there are some large dams with storage capacities of millions of cubic meters per year. An example is the Najran valley dam in Saudi Arabia, with a maximum height of 73 m, which retains about 83 million m3/year. The Al-Khoud valley dam in Oman has a length of 4.9 km and a height of 8 m, and retains a volume of 11.5 million m3/year. There also are small dams that retain water volumes not exceeding tens of thousands of cubic meters per year, such as the Galfa dam in the United Arab Emirates. In general, most dams built for artificial recharge have medium volumes, and retain a few million cubic meters of water per year. In designing the dams, and in selecting the dam locations and volumes, a water storage period of not more than a few weeks is optimal to reduce water losses to evaporation in the dry West Asia region. The types of aquifers located along the valley help achieve this goal, since they are formed of crushed rocks of high permeability. Other aqufiers have carbonated or cracked rocks. The design criteria for dam construction must consider flood frequency, peak flows, expected recharged volumes, and the permeability of the infiltration region. In selecting the dam site, the possibility of obtaining the construction material from the site should be considered, especially backfill and rocks used for the gabion boxes and other protective structures. Hafayer Hafayer are mechanically-excavated ponds of different shapes (square, rectangular, circular) and depths. They are usually excavated in low ground along floodwater courses and seasonal valleys in desert and semi-desert regions. This technology can serve one of two objectives. If the Hafir bottom is permeable, it can be used to ensure infiltration of the surface runoff flow retained in it, to recharge the groundwater in the Hafir region. In contrast, if the Hafir bottom is impermeable, it can be used as a storage tank for drinking water supply, irrigation or raising livestock. In both cases, Hafir play a role of surface runoff harvesting technology, similar to artificial groundwater recharge dams. It is presented here as an artificial recharge technology. Hafayer were used for artificial recharge purposes in Syria and Jordan in the Hammad basin. The proper selection of the Hafir sites was determined on the basis of soil permeability, gentle topography and probable surface runoff. The Hafir water-feeding system and its protection from high floods also was considered in the Hafir design. The Hafayer bottoms are sometimes covered with a layer of stones to facilitate water infiltration, and to limit the process of blockage of voids in the overlain layer. Infiltration Basins Between 1964-1977, Kuwait has conducted experiments on artificial infiltration with the use of recharge pit basins. It has engaged in water spreading, utilizing the excess desalinated water to recharge the Kuwait aquifer formation in the Rawdatyn well field and the Dammam aquifer formation in the Saleiba well field. These experiments included evaluation of the possibility of artificially recharging groundwater and improving its quality, as well as the possibility of preventing the clogging of voids in the soil layers due to lithological, bacteriological and organic factors. The technical feasibility of using this technology also was part of the experiments. Ten infiltration tests were conducted with two recharge pits, in which the surface soiil was removed and the rock surface in direct contact with the aquifer was exposed to facilitate the infiltration process. The exposed layer was covered with a layer of gravel to prevent clogging, and chlorine solution (4-8 mm/L) was applied to reduce bacterial growth. The results were encouraging, with the infiltration capacity reaching 10 m/day. Similar experiments were conducted in Al-Mowaquar valley in Jordan. Kuwait also has conducted similar experiments in the 1990’s in cooperation with the Islamic Bank, for the purpose of simultaneously facilitating artificial recharge and water quality improvement. Seven ponds were constructed (six ponds with an area of 7x15 m; one pond with an area of 18x15 m) above a saline aquifer of about 3-4 gm/L. The treated sanitary drainage water was poured into the ponds to recharge the groundwater aquifer. The results indicated the vertical recharge was weak. Ten small pits with 23 cm diameters and depths reaching to about one meter above the groundwater table were excavated in the largest pit, and were filled with gravel with mean diameters of about 3 mm. This resulted in an increase in the groundwater recharge to about 1.5 m/day (the fall value of the water in the pit). The experiment also indicated a reduction in the concentrations of total dissolved solids to 2 gm/L, of biochemical oxygen demand (BOD) from 20-25 mg/L to less than 10 mg/L, and of bacteria from 100-300 colonies/100 mL to less than 2 colonies/100 mL. Similar experiments are curentloy being conducted in Kuwait, using treated sanitary drainage water under changing hydraulic conditions, as a means of attempting to determine the optimal means of improving the efficiency and effectiveness of this technology.
|
|
|
|||
Table
of Contents
 |
© United
Nations Environment Programme | privacy
policy | terms
and conditions |contacts|support
UNEP | UNEP
sitemap
|