Managed Aquifer Recharge (MAR)
Managed aquifer recharge (MAR) involves building infrastructure and/or modifying the landscape to intentionally enhance groundwater recharge.
MAR is among the most significant adaptation opportunities for developing countries seeking to reduce vulnerability to climate change and hydrological variability. It has several potential benefits, including: storing water for future use, stabilizing or recovering groundwater levels in over-exploited aquifers, reducing evaporative losses, managing saline intrusion or land subsidence, and enabling reuse of waste or storm water.
Implementation of MAR requires suitable groundwater storage opportunities. Falling water levels or pressures in aquifers in many regions throughout the world are creating such opportunities, either as unsaturated conditions in unconfined aquifers or as a pressure reduction in confined aquifers. However, MAR is not a remedy for water scarcity in all areas. Aquifer conditions must be appropriate and suitable water sources (e.g. excess wet season surface water flows or treated waste water) are also required. MAR potential should be determined in any particular country or region before activities commence.
Detailed planning and assessment are required to determine whether MAR is a viable adaptation option. This may be carried out a national and watershed scale. Three fundamental planning steps should be considered:
- Water availability – assess the availability and quality of excess wet season surface water flows or other potential sources. The frequency and volume of availability of suitable water must be assessed for each planning region, as must the influence of natural climate variability and projected human-induced change.
- Evaluate the hydrogeological suitability of the MAR site or region – which largely depends on ease of injecting and recovering the water, the aquifer storage capacity and the aquifer’s resistance to clogging.
- Feasibility - the costs, benefit and feasibility of constructing and operating a MAR scheme, including those associated with transporting the recovered MAR water to demand centers needs to be determined.
Construction, operations and maintenance
MAR methods may be grouped into the following broad approaches:
- Spreading methods – such as infiltration ponds, soil-aquifer treatment, in which overland flows are dispersed to encourage groundwater recharge;
- In-channel modifications – such as percolation ponds, sand dam, subsurface harvesting systems, leaky dams and recharge releases, in which direct river channel modifications are made to increase recharge;
- Wells, shafts, and boreholes recharge – in which infrastructure are developed to pump water to an aquifer to recharge it and then either withdraw it at the same or a nearby location (e.g. aquifer storage and recovery, ASR);
- Induced bank infiltration – in which groundwater is withdrawn at one location to create or enhance a hydraulic gradient that will lead to increased recharge (e.g. bank filtration, dune filtration)
- Rainwater harvesting – in which rainfall onto hard surfaces (e.g. building roofs, paved car parks) is captured in above or below ground tanks and then allowed to slowly infiltrate into soil.
There are several common operational issues experienced by MAR schemes. These include: clogging of wells, stability of infrastructure under operating conditions, protection of groundwater quality, operation and management of the scheme, ownership of the stored water, monitoring, loss of infiltrated/injected water, policy and cultural acceptability and related stakeholder communications. Successful operation requires appropriate training for operators, access to successful demonstrations of the technologies being deployed and sound and integrated management of water resources.
The diverse range of managed aquifer recharge (MAR) schemes illustrates how the economics of different adaptation options can vary considerably. Low technology schemes such as controlled flooding / spreading basins and sand dams are less expensive (about US$10 to US$50 per ML, ignoring pipeline costs) than, for example, borehole injection methods (in the order of US$100 to US$1,000 per ML). Consequently borehole injection methods are often less viable, particularly for agricultural purposes, although in some areas may be suitable for urban and domestic water use. This provides an example where the economic feasibility is driven not only by cost, but also other considerations such as the scale of the scheme and the end-user of the water resource.
MAR example: sand dams in Kenya
Sand dams are made by constructing a wall across a riverbed, which slows flash floods/ephemeral flow and allows coarser sediment to settle out and accumulate behind the dam wall. The sedimentation creates a shallow artificial aquifer which is recharged both laterally and vertically by stream flow.
Since 1995, over 400 sand dams have been constructed in the Kitui District of Kenya, supported by the SASOL Foundation. Each of these dams provides at least 2,000 m3 of storage and has been constructed by local communities using locally available material. The benefits identified through this program include: water supplies more readily available in the dry season, enhanced food security during drought periods, and less travel time to obtain water supply.
Sand dams are not appropriate for all locations. They require unweathered and relatively impermeable bedrock at shallow depth; the dominant rock formation in the area should weather to coarse, sandy sediments; sufficient overflow is required for fine sediments to be washed away; and risk of buildup of soil and groundwater salinity needs to be low. Cooperative effort, ownership and ongoing maintenance by the local community are also necessary for the success of these schemes.
- Water and Climate Change: Impacts on groundwater resources and adaptation options. Water Working Notes, Note No. 25, June 2010. World Bank, Water Partnership Program.