Case Study: Recharge Pits

  As surface waters become more polluted, a greater fraction of the world’s water supply is becoming dependent on groundwater. Simple pump wells are cheap and simple solutions to supply groundwater and are increasingly common among developing regions. The groundwater supply is limited however, and, as urbanization begins to concentrate population, water demand often is greater than the recharge rates of groundwater sources. This creates an unsustainable practice of groundwater dependence that could eventually result in the depletion of aquifers, causing a myriad of consequences including water scarcity in regions that need it most.

  Recharge pits are a simple technology that contributes to the effort of increasing recharge rates and extending the lifespan of groundwater supplies. These pits, and similar recharge techniques, are especially valuable in urbanized areas where development further limits infiltration. They can also be viewed as a type of water treatment technique as this infiltration works much like filtration, screening solids and other contaminates. Groundwater table maintenance is necessary to provide continued water sources as well as to protect existing infrastructure from damage due to soil matrix collapse. 

Technology

 

  Recharge pits and trenches are a rainwater harvesting technology that combines with groundwater recharge, collecting precipitation and runoff and storing it in the groundwater table for future use. Rainwater is collected from roofs of houses and community structures in order to minimize the recharge pit footprint while maximizing collection area. This water is transferred directly to the recharge pit or trench. The pit is generally 1 to 2 m wide and 2 to 3 m deep, an unlined excavation filled with boulders, gavel, and coarse sand from the bottom up in order to provide filtration while minimizing evaporation loss. Recharge trenches are 0.5 to 1 m wide and 1 to 1.5 m deep, spanning between 10 to 20 m in length depending on hydraulic load. Ground water recharge supplements shallow aquifers, increasing ground water supply beyond the capacity of mismanaged rainfall. The water collected from catchments built on impervious surfaces is routed to these highly transmissive recharge pits where it is stored, progressively infiltrating into the soil beyond the duration of the rainfall. This water is later extracted from the groundwater table through existing method according to demand.

  Cost is minimal and consists of excavation and filling material cost as well as catchment construction. In somewhat developed regions with a ready supply of manual labor, such as India, cost can be estimated at less than 100 USD and 100-200 USD for recharge trenches. As location changes labor cost and material rates will undoubtedly vary, soil types can change costs dramatically as well. As excavation increases in difficulty a higher volume of manual labor will be necessary as well as the number of pits due to the limited infiltration rates associated with low permeability soils. Maintenance costs are also minimal. These duties include the replacement of the coarse sand layer, typically after each wet season, in order to allow for effective transport through the sand layer which becomes clogged as it acts to filter receiving water.

  Recharge pits and trenches are best built in areas with water runoff present and shallow, unconfined aquifers. The pit should be placed in close proximity to its associated catchment or catchments as the catchment system should covey collected water directly into the pit. Areas near wells are also helpful in increasing infiltration rates due to the draw-down that results from pumping. These solutions are most effective when surface waters are somehow compromised or inadequate. In areas prone to flood events recharge pits can reduce related hazard by increasing runoff capacity through storage. The increased head associated with groundwater recharge can prevent sea water intrusion into fresh water aquifers. Groundwater is also improved due to the fact that rainwater is free of organics and bacteriologically safe.

 

   Recharge pits are typically either circular or rectangular and 1 to 2 m in width and 2 to 3 m deep. For a single square pit with a width of 1.5 m and depth of 2.5 m with 60% boulder, 20% gravel, 15% sand and 15% unfilled space yields about 2.6 m³ of storage when calculated using typical void ratios and the equation: Vs*e=Vv. Evaporation is negligible due to the filled pit design; therefore the total of the stored water will eventually reach the groundwater table. Infiltration rates vary greatly with soil characteristics and hydrological conditions and it is therefore difficult to estimate common values for the rate of infiltration. For type D clay soils, those typical to North Carolina, infiltration rates are typically between >0.1 m/d for sandy soils these values can be as high as 10 m/d. The unlined sides of the recharge pit work to encourage infiltration, allowing a greater surface area for water influx. The volume, associated catchment area and number of pits must be tailored to their location. Effective designs must take the groundwater table into account when designing these recharge pits. Seasonal highs and lows must be known as shallow groundwater tables are ideal for this technology, but not if they rise above the bottom of the pit. Soil profiles are also highly beneficial, though local knowledge will usually suffice. Regardless the pits should be sized and positioned in such a way as to encourage infiltration, which soil properties largely determine. Soil properties also effect excavation stability which is especially important regarding worker safety during construction. When designed to handle an areas capacity this technology can effectively capture and store available rainwater through largely natural processes in a highly sustainable way.

  Soil clogging is the primary problem with recharge pits and many artificial groundwater recharge systems and designs must take this into account. The top sand layer of a recharge pit acts much like a filter bed, removing suspended solids. Because of this upper sand layer clogging of pit boundaries is avoided and does not need maintenance. When operating effectively, this naturally treats rainwater and runoff and works to improve groundwater quality. If not maintained, these sand layers can become clogged by sediment and algae decreasing effectiveness of the recharge pit by preventing runoff inflow and causing the pit to backup. Clogging is caused by physical processes such as the accumulation of solids present in the received water and the downward movement of fine sand particles causing an uneven distribution of particle size. These processes form thin layers of low permeability that limit infiltration. Biological processes than can cause clogging are the accumulation of algae on the infiltration surface and micro-organism growth within the sand bed due to incoming biomass. These process block or reduce pore size, improving effluent water quality while requiring more head to drive the water through. Chemical processes such as chemical precipitation, vapor barriers caused by the trapping of gasses produced by bacteria and air binding can reduce infiltration rates as well. Since it is not possible to backwash the filtering sand layer the sand layer must be replaced periodically in order to maintain proper inflow rates into the recharge pit. When infiltration rate though the filtering layer becomes greater than infiltration rate into the soil it becomes the limiting factor in the recharge rate of the pit. This cause the pit to lose effectiveness over time and can cause problems due to concentrating runoff around the pit location or creating areas of standing, stagnate water over the pit.

  Water table depth is also an important factor in recharge pit design. Shallow aquifers are preferred as the infiltration rate is much more responsive to water table depths in these cases and residence time within the water table is reduced, allowing for the more immediate use of the captured water. Small differences in water level of the recharge pit and water table depth also cause groundwater flow away from the pit to be lateral, making use of the large surface area of the unlined sides of the pit rather than the pit bottom alone. Shallow aquifers also avoid “perched mounding” where the infiltrated water reaches a layer of reduced permeability and head must build on the layer before the water can penetrated it and reach the well aquifer. It is also important to insure that the wet season water table does not rise above the pit bottom which can, with the additional recharge created by the pit, cause the soil to become waterlogged. This limits infiltration in the surrounding area prolonging the period where the top layer of soil is saturated and water cannot infiltrate. This prolonged period of topsoil saturation increases risk of flooding and erosion, causing additional runoff and standing water.

Artificial Recharge Movement in India

  Artificial groundwater recharge has been historically common in India and continues today. Many communities in arid regions have managed recharge pits or like structures for many years. Without government intervention or scientific knowledge base residents used knowledge of the land and local climate to store water in locations necessary. As problems with over pumping and the subsequent effects became clearer, government and scientific bodies soon began an effort to promote healthy groundwater management. National and non-governmental bodies began to put I place pilot programs to and studies to assess groundwater conditions and solutions to existing problems. This involved a number of techniques, including recharge pits and trenches. The study of artificial recharge and application to development projects was a main strategy for India in managing its water supply. These resulted in the development and release of technical guidelines in the area which caused the practice to become more common.

   Now that water scarcity is an undeniable issue in the country India and other nations have placed a renewed importance on artificial recharge. The synthesis of local, regional and national effort to further develop and implement simple and easily manageable recharge systems and practices has largely benefited dry areas. In some cases nation orders have mandated rainwater harvesting and artificial recharge. This has created a transition from transporting water from remote reservoirs to the local and individual management of immediately available water sources giving the artificial recharge movement momentum to continue to grow and propagate through nearby locations.

   The Mazhapolima Participatory Well Recharge Programme is a well suited example of this phenomenon. Local government, the Thrissur District Administration, along with NGOs, private sector sponsors and individual households has developed a network of water providers and users to coordinate in an effort to maintain suitable groundwater conditions and improved access to drinking water. To achieve this, the program pans to recharge groundwater and improve water availability and service level in order to reduce drought impact, improve public health and increase agricultural productivity.

   By creating a community based, participatory program the MPWRP relies on decentralized solutions, such as recharge pits and withdrawal wells, to solve existing problems. The water demand motivates individuals and groups to make small investments in order to take advantage of the meaningful benefits of clean, reliable water. This approach is seen as the most cost effective as many of the maintenance and initial cost is not supplied by the program, but the community members themselves. The program promotes and assists in determining feasible solutions and circulating information on the subject. Eventually the program hopes to be able to recharge 4.5 million open wells in the affected area.

Conclusion

   Recharge pits are an extremely cost effective approach with a design based on the reliance of naturally occurring processed. This make solutions like recharge pits and trenches suitable for developing regions with limited capital and infrastructure. The low footprint and unobtrusive nature of these technologies make it suitable for more developed, urban regions as well. By improving the groundwater health of a location artificial recharge has a domino effect on the health of the environment and community. This simple technology that has been practiced for many years is now more applicable than ever in solving many of our water problems.

References
 

Bouwer, Herman (02/28/2002). "Artificial recharge of groundwater: hydrogeology and engineering". Hydrogeology journal (1431-2174), 10 (1), p. 121.

Ruffino, L. Water Conservation Technical Briefs. SAI Platform, 2009. Web. 17 Nov. 2011. <http://www.saiplatform.org/uploads/Library/>.

Sakthivadivel, Ramaswamy. The Groundwater Recharge Movement in India. Sri Lanka: IWMI, Digital file.

Thrissur District Government of Kerala. "Participatory Well Recharge Program." Mazhapolima Project Report. June 2008. Web. 17 Nov. 2011. <http://www.indiawaterportal.org/sites/indiawaterportal.org/>.