By Kes Murray
Understanding Groundwater recharging
Managed Aquifer Recharge (MAR), sometimes referred to as Artificial Recharge, is the process by which a water source is intentionally injected or infiltrated into an aquifer. There are a wide variety of reasons for doing this, as well as a range of methods and technologies that are used to accomplish the desired outcome of increasing groundwater recharge.
What are the reasons for the Artificial Recharge of Aquifers?
- Maximise storage
- Manage water quality
- Replenish over-abstracted or depleted aquifers
- Hydraulic gradient control of natural or anthropogenic pollution
- Maintain ecological systems dependent on aquifer springs, or groundwater base-flows to rivers.
How does an aquifer recharge?
Typically, the way Managed Aquifer Recharge works can be broadly categorized into infiltration or injection.
Infiltration is done by directing the source water to areas that have high infiltration capacities (coarse sands and gravels) directly to the aquifer, while injection involves pumping the water down boreholes directly into the aquifer. In both cases, different technological approaches are taken, dependent on the objective of the MAR scheme. Some schematic examples of these are shown below (Dillon, 2005).
When it comes to storage, international groundwater abstraction was estimated to be in the order of 145 km3/year between 2001 – 2008 and in more recent years was estimated to be closer to 1 000 km3/year. In response to the increasing groundwater abstraction globally, international focus on MAR schemes has been increasing in the past few decades. It is estimated that worldwide, 10 km3/year of water is now being added to aquifers through MAR schemes, with an increase of ~5% /year. However, many of the international cases use MAR as a water treatment technology, rather than an aquifer storage management technology.
How to know what your groundwater recharge options are?
In order to determine what MAR options may be available for a given area there are several key, large scale questions to ask that will guide the objectives of an MAR study.
- Is there a potential source of water available? This can be seasonal overflows of surface water, excess run-offs in storm events, or sometimes even wastewater.
- What is the expected quality of the source water? This is critical in maintaining an MAR scheme.
- Is the aquifer a primary or secondary aquifer (porous sands and gravels, or fractured rock)? In most cases this will determine whether water quality management through MAR is worth considering.
- If primary, is the aquifer confined or unconfined? This will determine the aquifers connectivity to the surface, which is important for MAR infiltration schemes, but not MAR injection schemes.
- What is the groundwater quality? The legalization process of an MAR scheme addresses this.
Several MAR examples are shown in a bit more detail below, which illustrates how knowing the source water availability and quality, as well as the aquifer type and quality, are the controlling factors on what can be done.
Construction of the Kharkams Sand Bed Filter and Resultant Improvements (Murray, 2004)
The South African national guidelines for Aquifer Recharge
There are very detailed national guidelines on what is required for a Managed Aquifer Scheme. This is to ensure that the environment and aquifers are protected, but also to limit unnecessary expenditures on poorly designed schemes. An MAR scheme can vary hugely in size and application, and so can the costs.
Examples of water recharge in South Africa
Some schemes, such as the one along the N7 in Kharkams, are simple small-scale injection schemes. A sand bed filter of a few meters in length was installed along the nearby ephemeral river (pictured below). This sand bed filters and diverts small percentages of the excess fresh river water into the local supply borehole, which would otherwise be lost to evaporation. The opportunistic groundwater recharge decreased the salinity of the fractured bedrock aquifer to less than half its original slightly brackish state, and recharged the aquifer in less than half a year by more than two years of its natural recharge rate (Murray, 2004).
Groundwater Level Monitoring of Aquifer Storage
Groundwater Sampling for Chemical Analyses
The costs of Managed Aquifer Recharge
As can be expected, the financial costs of MAR schemes (setting up, implementing, monitoring and managing) can vary widely depending on the objectives of the scheme. MAR schemes for treatment purposes are usually focused on infiltration rates in primary aquifers, using the natural sands to filter out turbidity and suspended particles, but also can use reed beds and various other ecological systems to improve water quality by removing metals and microbiological contents of the source water. This way storm water and treated waste water from urban environments can be safely recharged to the local aquifers, where the residence time through these natural ecosystems and filters is carefully managed, to allow for re-use of the water once retrieved from the aquifer. Such systems require management costs that will maintain the infiltration areas, to ensure that silting up, or precipitation of chemicals does not clog the sands, reducing the effectiveness of the recharge. Systems that use ecological components must also be maintained during periods when there is less source water available to ensure these systems do not die out which can lead to clogged infiltration paths.
These require different set up and management costs to MAR schemes that directly inject excess clean water down boreholes into an aquifer for storage purposes. Aquifer storage and recovery allows for a greater availability during the times when surface water storage is insufficient. However, clogging within the aquifer itself can still be a concern in these schemes and must be managed through source water quality control.
How to make sure MAR is successful?
In order to ensure that the implementation of an MAR scheme is successful, both economically and objectively, South Africa’s Artificial Recharge Strategy (DWAF, 2007) lays out a clear structure for the steps, studies and tests required. At the Pre-Feasibility stage, the questions raised in the beginning of this document are discussed in order to determine what the potential MAR methodologies might be, as well as the objectives of such a scheme. Discussions are also required around what the costs of a Feasibility study might be.
If these seem viable, the Feasibility stage is used to perform recharge tests that will inform the preliminary infrastructure designs of the scheme, to identify the phases of the schemes for implementation, as well as to provide cost estimates on the scheme. The cost estimates address the implementation and management of the scheme, as well as comparing these costs to similar alternative options which are typically dams, water transfer schemes or the applicable water treatment components. A detailed project implementation plan, including the relevant licensing requirements, is provided at the end of this stage.
The Implementation stage of an MAR scheme involves drilling of any required boreholes (abstraction and/or injection) or the construction of infiltration basins, setting up of the necessary monitoring systems, development and completion of the infrastructure design and appointment of contractors to construct this infrastructure. During this stage the necessary licensing requirements are addressed, both in terms of water use and potential environmental authorisations where applicable.
The final stage is the Operation and Maintenance of the scheme. Unfortunately, this is where many schemes can fall apart, either due to a lack of economic planning for this stage, or simple negligence. The long-term operation of an MAR scheme is where the investment in such a project truly pays off, however this does require continuous monitoring and management to prevent clogging of the recharge mechanisms, whether infiltration or injection. Successful MAR is intended to meet water requirement objectives over decades of use; and will do so if properly maintained.
The intensity of these stages is dependent on the scale and complexity of the MAR scheme. The implementation and operation of the schemes are also best done by starting small and scaling up over time, which reduces the early financial demands and helps to optimize the final scheme successfully. For further information on potential MAR schemes and how to get started, please contact our offices.
- Department of Water Affairs and Forestry (2007) Artificial Recharge Strategy: Section C – Implementation and Authorisation.
- Dillon P (2005) Future management of aquifer recharge. Hydrogeology Journal. 13: 313 – 316.
- Dillon P, Pavelic P, Page D, Beringen H,Ward J (2009) Managed aquifer recharge: an introduction. Waterlines Report No. 13: 3
- Murray EC (2004) Artificial Groundwater Recharge: Wise Water Management for Towns and Cities. Water Research Commission Report No TT 219/03.