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Session 4: Augmentation of Groundwater Resources through Aquifer Recharge

ADVANTAGES OF AQUIFER RECHARGE FOR A
SUSTAINABLE WATER SUPPLY

By Peter Fox
PO Box 875306
Department of Civil and Environmental Engineering
Arizona State University, Tempe, Arizona, 85287-55306, USA

INTRODUCTION: Aquifer recharge is becoming an integral part of water resources planning as urban areas recognize the need for developing sustainable water supplies. Traditional approaches to meet growing water demand in urban areas require exploitation of surface and subsurface resources to the maximum extent possible. Future sustainable development of urban areas is dependent on solutions that provide sustainable water supplies without associated negative environmental impacts. Negative environmental impacts include damage to fisheries and ecosystems from dams, increased salinity from evaporation during surface water storage and land subsidence from decreasing groundwater levels. Aquifer recharge has many advantages as compared to conventional surface water storage. Groundwater recharge is preferred because there are negligible evaporation losses, the water is not vulnerable to secondary contamination by animals or humans, and there are no algae blooms resulting in decreasing surface water quality (Crook, 1998). Other benefits include prevention and minimization of land subsidence and reduced groundwater pumping costs. Additionally, passage of water through the subsurface provides soil-aquifer treatment (SAT) and the aquifers may offer seasonal or longer-term storage (Bouwer, 1985). Aquifer storage and recovery systems can allow for the storage of excess surface water during periods of high surface water flow combined with the recovery of stored surface water during periods of low surface water flows or drought (Pyne, 1994). In many cases, aquifers provide large amounts of storage capacity that can be made available through aquifer recharge. Public acceptance of groundwater recharge for indirect potable reuse of reclaimed waters has been favorable as compared to other forms of proposed potable reuse. As with the development of any water resource, the major costs are often associated with distributing the water and these costs can be exacerbated in urban areas that were developed to primarily utilize surface waters. Water resources planning including aquifer recharge provides many options to develop a sustainable water supply while reducing the costs associated with expensive distribution systems.

METHODOLOGY AND ECONOMICS: The most common and widely accepted method for aquifer recharge is the use of percolation basins. Another method for groundwater recharge includes direct injection into the saturated zone. An emerging method for groundwater recharge is the used of vadose zone injection wells which are analogous to trenches (Close et al., 1997). The three technologies are illustrated in Figure 1. The major characteristics of the three technologies are summarized in Table 1. Both recharge basins and vadose zone injection wells require the presence of an

Figure 1. The three commonly used methods for aquifer recharge. Recharge basins are the most common low technology method that require large amounts of land. Direct injection wells allow for injection into confined aquifers and require high technology pre-treatment. Vadose zone injection wells are an emerging technology that provides some of the advantages of both recharge basins and direct injection wells

unconfined aquifer with sufficient storage capacity. Direct injection wells can inject water directly into unconfined aquifers or confined aquifers. When unconfined aquifers are unavailable, direct inject wells are the only alternative for groundwater recharge and are capable of simultaneously injecting water into several aquifers. However, direct injection wells are expensive, require advanced pre-treatment technology and advanced technology for maintenance. Therefore, direct injection is not a viable option when low technology solutions are desired. The major costs associated with recharge basins are the required land and the conveyance system to deliver water to the recharge basins. Therefore, it is often desirable to locate recharge basins near water conveyance systems where land that is located in floodplains might be available.

Table 1. Major Characteristics of Aquifer Recharge Methodologies

The estimated costs associated with the use of recharge basins are highly variable since they depend on both infiltration rates and land values. Infiltration rates are a function of the soil hydraulic conductivity and the development of mounding on the groundwater table. Average infiltration rates must consider the cyclic operation of the recharge basins that includes both wetting and drying periods. Average infiltration rates can vary from 8 to 150 cm/d depending on the soil type and development of clogging layers. When groundwater levels are shallow, mounding below the recharge basins can increase groundwater levels to near the bottom of the infiltration basins thus decreasing infiltration rates. Under such conditions, extended drying periods are required to allow for dissipation of the mounds. Estimated land requirements for a recharge project using recharge basins depends on the volumetric rate of recharge and the average infiltration rate.

Land Required = Flowrate (m³/d) ¸ Infiltration Rate (m³/ha-d)

And the estimated costs are dependent on the land values.

Costs for Land = Land Required (ha) x Land Cost ($/ha)

For example, a 20,000 m³/d project with an average infiltration rate of 5,000 m³/ha-d will require 4 ha of recharge basins. If the land costs $20,000 per ha the total cost for the land will be $80,000. These costs do not consider the costs for a conveyance system to supply the water to the recharge basin. The City of Phoenix, Arizona, USA investigated a plan to recharge groundwater in recharge basins using reclaimed water from the 91^st Avenue Wastewater Treatment Plant. A system with a capacity to deliver 400,000 m³/d for a distance of 11.5 km was prohibitively expensive ($50,000,000). Therefore, a smaller project was designed to reduce the costs of the conveyance system and the remaining reclaimed water will be used to restore the ecosystem of receiving waters. An alternative surface water source was found to keep the capacity of the recharge project at greater than 400,000 m³/d of which less than 50% will be reclaimed water.

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• International Year of Forests
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