 |
|||||||
| About UNEP | UNEP offices | News centre | Publications | Calendar | Awards | Milestones | UNEP Store |
|
|||||||
|
 |
|
|
|
||
| UNEP>DTIE>IETC | |
|
|
<Proceedings of the International Symposium on Efficient Water Use in Urban Areas - Innovative Ways of Finding Water for Cities -> The high cost of land in some urban areas has provided the motivation for the development of vadose zone injection wells. As land prices escalate to greater than $100,000 per ha, the land costs associated with large recharge projects can become prohibitive. While still an unproven technology, vadose zone wells are rapidly becoming part of water resources planning in urban areas. The other key advantage to vadose zone injection wells is they can often be located near a water supply that eliminates the need for expensive conveyance systems. Pre-treatment requirements usually include removal of solids and disinfection with chlorine to prevent clogging. Vadose zone injection wells have been used for recharging filtered surface waters and for recharging reclaimed waters treated by secondary biological treatment followed by filtration and disinfection. Improvements in water quality as water percolates through the vadose zone and enters the saturated zone are expected but have not been well documented as compared to recharge basins. The life cycle of vadose zone injection wells is very uncertain since they are an emerging technology. Once a vadose zone injection well is clogged, they are very difficult to redevelop since there is no technique to backwash the well or to rapidly dry the well. Systems are being designed to allow for alternating flow patterns such that wells located adjacent to one another are never operated simultaneously (Small and Vernon, 1999). This operating scheme will allow wells to dry and to possibly be backwashed by horizontal flows from adjacent wells. Even when systems are designed with a life cycle of only five years, they are more economical than alternative recharge basins or direct injection wells when land is expensive. Recharge basins are still the most common method of recharge and provide excellent versatility for water resources planning. In the Montebello Forebay of Los Angeles County, California, USA, recharge basins are located adjacent to major water conveyance channels. Depending on the available water source, the basins are used to recharge stormwater, surface water from either the Colorado River or the State Water Project, or reclaimed water. Pre-treatment requirements for the stormwater and surface waters are essentially non-existent. Stormwater or surface waters are passed through stilling basins to reduce the sediment loading on the recharge basins. Reclaimed water is treated by secondary biological treatment followed by filtration and chlorination. Filtration and chlorination are primarily for the control of pathogens and are not required for many recharge projects applying reclaimed water on recharge basins. The versatile infrastructure available to deliver different waters to the recharge basins combined with effective water resource planning allows for efficient water use and maintenance of groundwater levels in the region. The Montebello Forebay also provides an example of indirect potable reuse since reclaimed water becomes part of the drinking water supply. Epidemiological studies have found no negative impact from over thirty years of indirect potable reuse at the Montebello Forebay (Nellor, 1984). A combination of low technologies can be used to accomplish groundwater recharge with reclaimed water or other poor quality water sources. For example, a sequence of lagoons followed by constructed wetlands is used as pre-treatment for groundwater recharge basins located in Kingman, Arizona, USA. Periodic high solids loadings from the wetlands do not have a permanent negative impact on infiltration rates since drying cycles dessicate the solids that are primarily organics. In Morroco, a sequence of lagoons followed by intermittent sand filtration was used prior to groundwater recharge using trenches. The intermittent sand filters clogged rapidly and were consequently labor intensive, however, an abundant supply of labor made the intermittent sand filters a feasible technology. The intermittent sand filters effectively prevented clogging of the groundwater recharge trenches. Regulations and laws have been developed to stimulate aquifer recharge in many urban areas. Laws that require assured water supplies, encourage groundwater recharge through water banking, and require irrigation with reclaimed water effectively promote water resource planning with aquifer recharge as an important component. Laws that require assured water supplies, as is the case in Arizona, USA, promote decentralization to reduce the costs associated with fully utilizing reclaimed water as a water resource. Water reclamation plants are located near sites where water is reused and where water is recharged into the ground. In Arizona, a typical water reclamation plant will provide reclaimed water for irrigation of parks and golf courses throughout the year. In the winter when irrigation water demand is less than the supply of reclaimed water, the reclaimed water is used to recharge groundwater. The net impact is an increase in groundwater levels providing that the primary drinking water source is a surface water. Groundwater is stored for future use and will be available during droughts and other periods of low surface water supply. The recharged groundwater is presently used primarily for non-potable purposes; however, many underground storage and recovery systems include plans for potable reuse. Another potential benefit of a decentralized system is reduced environmental impacts on receiving surface waters since discharge of reclaimed water into surface waters is eliminated. Laws that establish groundwater banks provide genuine economic motivation for groundwater recharge. Entities that recharge groundwater receive credit for the amount of groundwater that is recharged. These entities can then sell the groundwater to groundwater users in the future and the recharged groundwater can be viewed as money in the bank. A system of buyers and sellers can be established to effectively maintain or increase groundwater levels in an economically sound fashion. Banking of groundwater can also be integrated with the allocation of surface water supplies to allow for trading of groundwater with surface waters. Advantages of this strategy include the utilization of poor quality surface waters or reclaimed waters for irrigation while high quality groundwater is dedicated for drinking water purposes. The Groundwater Management Act (GMA) established a goal of zero groundwater mining by the Year 2025 in the State of Arizona, USA. The GMA established a groundwater banking systems combined with a system of penalties and incentives. Groundwater users are penalized on an escalating scale for withdrawing groundwater without recharging groundwater. The penalties force the groundwater user to either recharge groundwater or purchase recharged groundwater from a groundwater supplier. Increased groundwater reserves can also be used to protect baseline stream flows for environmental purposes. Laws that establish groundwater banks and provide incentives for groundwater recharge do not guarantee that water will be recharged adjacent to areas where groundwater overdraft is most severe. For example, the Groundwater Management Act in Arizona, USA, established Active Management Areas (AMA) and each AMA is viewed as one large aquifer. Therefore, groundwater can be recharged 40 km from an area where groundwater overdraft is severe. Groundwater users can continue to extract groundwater from an area where groundwater overdraft is severe by purchasing recharged groundwater regardless of the location of the recharge project. While decentralization tends to evenly distribute the recharge of reclaimed water as discussed above, large recharge projects that use surface water supplies are typically located adjacent to canals or rivers for economic reasons. The location of large recharge projects can causes increasing groundwater levels locally while doing very little to alleviate groundwater overdraft in other areas. Rising groundwater levels can also be a problem for adjacent industries and municipalities. LAND SUBSIDENCE: Land subsidence due to groundwater overdraft is an important consequence of groundwater overdraft. Subsidence is caused by compression of underground materials when water tables decline (Bouwer, 1977). Also, lateral flow of groundwater can cause lateral compression of the aquifer and, consequently, lateral movement of land surface. Movements from groundwater flow or overdraft are normally small, however, they become significant where underground materials are compressible and groundwater levels decline. Observed land subsidences vary from several centimeters to greater than 10 m (Poland, 1969) and the amount of subsidence depends on the thickness and compressibility of underground formations. Nonuniform subsidence results when different rates of groundwater declines exist or from differences in compressibility of formations and can lead to the production of cracks and fissures at the land surface. Land subsidence from groundwater overdraft is essentially irreversible. Land subsidence can be prevented or minimized by eliminating groundwater overdraft. Since subsidence is a slow process, groundwater replenishment can prevent residual compression of clay layers. However, increases in the land surface are normally insignificant, even when groundwater levels are returned to levels prior to land subsidence. As stated above, groundwater recharge to prevent continued damage from land subsidence must occur in the area of the overdraft. A groundwater depression created by overdraft will provide a gradient to enhance groundwater flow into the depression. Nevertheless, replenishing groundwater levels when overdraft is severe is a process that can take years or decades. Land subsidence can also occur in recharge basins as a consequence of infiltration of large water volumes. There are several possible reasons for this phenomena but regardless of the mechanism, the effect is local and is not related to land subsidence from overdraft. One mechanism is associated with collapsible soils where calcium carbonate or other cementing agents dissolve resulting in compaction of the soil. Hydrocompaction can occur in the vadose zone as the intergranular pressure increases the compaction of compressible layers below. Presettling soils can be done to prevent hydrocompaction during the operation of recharge basins. Finally, clays can shrink and swell if changes in the sodium and calcium concentrations occur. This can result in either compaction of fines migration. In most cases, land subsidence in a recharge basin will result in a permanent but small reduction in infiltration rates. If fines migrate to less permeable layer, a larger reduction in infiltration rates might occur requiring maintenance to remove the impeding layer.
|
|
|
|||
 |
© United
Nations Environment Programme | privacy
policy | terms
and conditions |contacts|support
UNEP | UNEP
sitemap
|