The use of artificial recharge to store surplus surface water underground can be expected to increase as growing populations demand more water, and as the number of good dam sites still available for construction becomes fewer. For example, artificial recharge may be used to store treated sewage effluent and excess stormwater runoff for later use. Groundwater recharge may also be used to mitigate or control saltwater intrusion into coastal aquifers. However, in order to accomplish the uses without deleterious environmental consequences, the optimum combination of treatment methodologies before recharge and after recovery from the aquifer must be identified. It will also be necessary to consider the sustainability of soil-aquifer treatment and health effects of water reuse when using treated wastewater as the recharge medium.
The main purpose of artificial aquifer recharge technology is to store excess water for later use, while improving water quality (decreasing the salinity level) by recharging the aquifer with better water. There are several artificial recharge techniques in use in Latin America and the Caribbean, including infiltration basins and canals, water traps, cutwaters, surface runoff drainage wells, septic-tank-effluent disposal wells, and diversion of excess flows from irrigation canals into sinkholes.
· Infiltration Basins and Canals
This technology has been used extensively in the San Juan River basin of Argentina, where two artificial recharge experiments have been conducted. The first experiment consisted of the construction of infiltration basins, 200 m by 90 m and 1.2m deep. These basins were combined with 9.30 ha of infiltration canals in the second experiment. This system was used to recharge the 10 hm3 aquifer in the Valley of Tulum. The system of canals was found to be more efficient than the infiltration basins because the high circulation velocities in the canals precluded the settling of fine material and resulted in higher infiltration rates.
· Water Traps
Water traps are used to increase infiltration in streambeds. The traps are earthen dams of variable height, usually 1 m to 3 m, that are constructed of locally available materials. They are normally perpendicular to river banks, depending on the characteristics of the stream system. Water traps are designed to operate during rainfalls of up to a l-in-50-year frequency. They are typically constructed along a 1 km stretch of river, at intervals of 70 m to 100 m. Their storage capacities fluctuate between 250 and 400 m3. They have an estimated life span of 20 to 25 years, given proper maintenance.
This technology can be used in areas where there are no rivers and creeks, such as in the Paraguayan Chaco. Cutwaters are excavations of variable dimensions, used as reservoirs, built in low-lying areas. Their primary objective is the harvesting of surface waters. Those to be used for artificial recharge are built on top of permeable strata; those for surface water storage are built on impermeable substrates.
· Drainage Wells
The limestone and coral rock formations that comprise the principal aquifer in Barbados consist of very pure calcium carbonate. Drainage wells, or "suckwells", are used to dispose of drainage waters (see Figure 13). The depth of the drainage wells is determined by the well digger and is based on reaching an adequate fissure or "suck" in the rock. They range in area from 16 ft2 to 36 ft2, and are either square or circular in shape. They are provided with guard walls of concrete or coral stone above the ground surface and drainage ports or underground pipes or culverts to conduct runoff into the wells.
Figure 13: Suckwell Construction.
Source: Government of Barbados, Stanley Associates Eng. Ltd., and Consulting Engineers Partnership Ltd. Barbados Water Resources Study, Vol. 3: Water Resources and Geohydrology, 1978.
· Septic Tanks and Effluent Disposal Wells
Another source of artificial groundwater recharge is effluents from septic tanks, using soakaways. The Barbados Water Resources Study of 1978 estimated that about half of the 128 million I/day water used for domestic consumption, or approximately 64 million I/day, is returned to the groundwater as septic tank effluent. The soakaways used for this purpose are very similar to suckwells in design and construction, except that they are used in conjunction with septic tanks and are always covered.
· Sinkhole Injection of Excess Surface Flows
In Jamaica, excess surface runoff is treated and discharged into sinkholes in karstic limestone aquifers. These aquifers are commonly associated with seawater intrusion and are highly saline. The recharged water is monitored through a series of monitoring and production wells. Monitoring is carried out to measure changes in groundwater levels and water quality (salinity levels).
Extent of Use
Artificial recharge has been widely used in several Latin American countries and the Caribbean. It may be expected to be utilized more frequently as demand for water increases and as surface water resources are fully committed.
In Argentina, a system of canals and infiltration basins has been used in the provinces of San Juan, Mendoza, and Santa Fe with relative success. Water traps have also been used in Mendoza. This is an effective technology for use in arid and semi-arid regions.
Cutwaters have been used in the Paraguayan Chaco, where rainwater is the main source of aquifer recharge. This technology is normally used for recharge of surficial aquifers, and its application is limited by the hydrogeologic conditions. In Barbados, suckwells are extensively used for recharge, except in areas on the east coast which lack the necessary coral formations and where the exposed oceanic soil (consisting of a mixture of clay, marl, silt, and sand) has a low permeability. There are probably more than 10 000 suckwells, mostly on private lands or estates. They are at elevations of 20 ft above sea level or higher, and usually are well maintained.
The technology in Jamaica of using sinkholes as injection points is applicable where karstification of a limestone aquifer has taken place. Artificial recharge is suitable for areas upgradient of an aquifer where there is significant water for recharge purposes and land area available for treatment of the runoff before recharge. Treatment consists primarily of settling suspended solids. It is best used in areas where pumping is not needed to move the water to the sinkholes.
Operation and Maintenance
Infiltration basins and canals require minimal maintenance, consisting mostly of avoiding excessive sedimentation in the basins and canals and preventing erosion of canal banks. A bulldozer is often used in the infiltration basins to remove accumulated sediments and to rehabilitate the system.
Water traps require maintenance during the first few years of operation, until the natural vegetation grows again in the area. Intense rainfalls may damage or destroy the traps, and they will have to be rebuilt.
Maintenance of cutwaters is similar to that required in infiltration basins. Runoff from areas with unpaved streets can carry large loads of sediment, which may be deposited in cutwaters and will need to be removed during dry periods.
Road drainage is also a source of water for suckwells in Barbados. These roadside wells are built and maintained by the government. Other suckwells, on residential and plantation lands, are maintained by the landowners. Maintenance is labor-intensive and generally involves the removal of silt, which accumulates at die bottom of the well and may plug the "suck", rendering it useless. Repairs to the guard walls, covers, and iron grilles are also needed. Unfortunately, owing to increased labor costs and declines in profitability at most sites, many of these wells have fallen into a state of disrepair and have been either plugged, stuffed up, or overgrown with trees. Some of these wells have been contaminated by garbage dumped into them.
In sinkhole injection, operations are simple. The canal attendant, who normally resides nearby, visits the site twice a day to read the Parshall flumes, collect water samples, and open or close sluice gates. The earth canals need to be kept clear to ensure maximum delivery of water. The settling basin has to be cleaned of accumulated sediment and vegetative growths once every four to five months. Vandalism, resulting in damage to sluice gates, sinkholes, and monitor wells, is also a problem in the maintenance of the system.
Level of Involvement
In Argentina, most of the experimental use of this technology has been done by the government in both the provinces of San Juan and Mendoza.
In Paraguay, the government, in conjunction with international organizations, has been conducting experiments to quantify the recharge provided by different recharge systems. In general, the implementation and maintenance of these technologies in urban areas have been carried out by municipal governments, but in rural areas by (he private sector.
Both the private sector and the Government of Barbados have been involved in the successful implementation of artificial recharge schemes. The private sector, primarily represented by the sugar industry, has encouraged the development of this technology and provided land, manpower, and water. The government, represented by the Water Authority, has provided technical expertise and financing.
In Jamaica, there is a cadre of well-diggers who can be contracted by the government, plantation owners, and other landowners to both dig and maintain drainage wells. An ongoing educational program informs landowners of the need to maintain wells on a regular basis, the potential for groundwater recharge from the wells, and the need to monitor contamination of the groundwater.
The estimated cost of infiltration of surface water in Argentina, using basins and canals, is $0.20/m3. The basins and canals used in the 1977 experiment in the San Juan River basin incurred a capital cost of $31 300. The comparable cost of water traps in Argentina has been estimated at between $133 and $167. The capital cost of a 5 700 m3 cutwater, equipped with a 14 m extraction well, is estimated at $6 325. The operation and maintenance cost is estimated at $248 per year. The production costs are estimated to be about $0.30/m3 for the first five years of operation, $0.17/m3 for the next five years (five to ten years of operation), and $0.15/m3 for the following five years (ten to fifteen years of operation).
In Jamaica, the initial capital cost of the sinkhole injection system is estimated at less than $15 000. This cost is primarily related to the construction of the inflow settling basin and channels conveying the runoff water to the sinkholes. Maintenance costs are low, less than $5 000 for the 18-month project (or under $3 500/year).
Effectiveness of the Technology
In Argentina, sites near the San Juan and Mendoza rivers recharged the underlying groundwater aquifer at the rate of 60 l/sec/ha during a three-month period.
Water traps have been successfully used for more than 25 years in Argentina. They have been very useful in reducing sedimentation and risk of flooding.
Cutwaters proved a significant source of water to communities during the droughts of 1993 and 1994 in the Paraguayan Chaco. Recovery of 75% of the infiltrated water has been reported in that region.
Even though groundwater recharge is not the principal intended use of drainage wells, it is a major indirect beneficiary. Infiltration rates in coral rock in Barbados have been estimated at between 6.0 and 6.5 cm/hr and are known to be highest where solution openings (or "sucks") occur.
In Jamaica, total recharge over 18 months amounted to 4 million m3. Two groundwater mounds were detected downgradient of sinkholes. One mound indicated an increase of 4.1 m in water levels, while at the other the increase was 6.7m. Divergent radial flows developed from both of these mounds. Once recharge ceased, the mounds gradually disappeared over a two-month period. Chloride concentrations in some wells in Jamaica have decreased from 2 300 mg/l to 1 700 mg/l and in others from 170 mg/l to 25 mg/l before reaching an equilibrium at 50 mg/l. In general, most wells influenced by artificial recharge have shown declines in salinity levels.
In areas where groundwater is an important component of the water supply, and rainfall variability does not allow for a sufficient level of aquifer recharge by natural means, these technologies provide for the artificial enhancement of the natural recharge. Storage of surface runoff in underground aquifers in arid and semi-arid areas has the advantage of minimizing evaporative losses. However, use of these technologies requires an appropriate geological structure. In areas underlain by igneous rock, the natural fracture lines can be expanded by injection of water under pressure and infusion of a sand slurry into the gaps thus created. Given the cost of this latter measure, however, use of natural limestone or sandstone formations, such as are common in the Caribbean islands, is preferred and most cost-effective.
· The technology is appropriate and generally well understood by both the technicians and the general population.
· Very few special tools are needed to dig drainage wells.
· Because of the structural integrity of the coral rock formations, few additional materials are required (concrete, softstone or coral rock blocks, metal rods) to construct the wells.
· Groundwater recharge stores water during the wet season for use in the dry season, when demand is highest.
· Aquifer water can be improved by recharging with high quality injected water.
· Recharge can significantly increase the sustainable yield of an aquifer.
· Recharge methods are environmentally attractive, particularly in arid regions.
· Most aquifer recharge systems are easy to operate.
· In many river basins, control of surface water runoff to provide aquifer recharge reduces sedimentation problems.
· Recharge with less-saline surface waters or treated effluents improves the quality of saline aquifers, facilitating the use of the water for agriculture and livestock.
· In the absence of financial incentives, laws, or other regulations to encourage landowners to maintain drainage wells adequately, the wells may fall into disrepair and ultimately become sources of groundwater contamination.
· There is a potential for contamination of the groundwater from injected surface water runoff, especially from agricultural fields and roads surfaces. In most cases, the surface water runoff is not pre-treated before injection.
· Recharge can degrade the aquifer unless quality control of the injected water is adequate.
· Unless significant volumes can be injected into an aquifer, groundwater recharge may not be economically feasible.
· The hydrogeology of an aquifer should be investigated and understood before any future full-scale recharge project is implemented. In karstic terrain, dye tracer studies can assist in acquiring this knowledge.
· During the construction of water traps, disturbances of soil and vegetation cover may cause environmental damage to the project area.
Artificial groundwater recharge is generally well accepted by communities in areas where it is used.
Further Development of the Technology
Potential improvements in artificial recharge technologies include:
· Improvements in the design of pre-injection silt chambers, grease traps, and oil interceptors to reduce the amount of contaminants entering drainage wells.
· Improvements in the design of injection wells to eliminate the use of "sucks".
· Evaluation of groundwater contamination potentials from various sources of artificial recharge, and the adoption of techniques to reduce the associated impacts or risks.
· Improvements in the design of water traps to increase groundwater recharge efficiency.
· A better understanding of the causes and consequences of bacterial and viral contamination of aquifer systems, and the means of minimizing and mitigating such risks.
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