Q+A - Managed Aquifer Recharge

The conservation and management of water may not have been a concern 10 years ago, but it is increasingly becoming an issue in our country.

As global temperatures continue to rise, bringing more frequent droughts, and the activities of our primary sectors increasingly affect pollution levels in national water bodies, it is essential that strategies and procedures are put in place to ensure our most valuable resource is maintained and preserved.

One of these strategies is managed aquifer recharge (MAR).

WHAT IS MANAGED AQUIFER RECHARGE?

MAR is the recharge and storage of water into aquifers for subsequent recovery or environmental benefits.  The water is obtained and stored through mechanisms such as injection wells, infiltration basins and galleries for rainwater, stormwater, reclaimed water, mains water and water from other aquifers that is subsequently recovered for all types of uses.

WHAT IS AN AQUIFER?

An aquifer is a body of permeable rock through which water can easily move. The rock types most commonly identified as aquifers include sandstone, fractured limestone, sand, gravel, and fractured volcanic rocks. Rocks like granite and schist are usually poor aquifers (unless heavily fractured) as they have a low porosity. 

The water stored in an aquifer is known as groundwater. Groundwater tends to be cheaper and less vulnerable to pollution than surface water (water above ground, in lakes, streams etc). Therefore, it is commonly used for public water supplies. 

WHAT IS THE PURPOSE OF MAR?

Along with the public water use mentioned above, managed aquifer recharge also commonly acts as a source of water for industry, irrigation, and for sustaining groundwater dependent ecosystems. It is also implemented for the purposes of improving water quality.

HOW IS MAR USED IN THIS COUNTRY?

A key focus of managed aquifer recharge in New Zealand is improving the quality and flow rate of national water bodies. 

A prime example of this is the Hinds/Hekeao managed aquifer recharge project in Mid Canterbury, where water quality has been deteriorating since the early 2000s.  

Launched in May of 2016, this scheme involves the diversion of un-utilised stock water from the Rangitata River through the Valetta Irrigation Scheme and into two infiltration basins at the MAR site (where the aquifer water levels have declined sharply due to a range of environmental and industrial factors). The first of these two basins is designed to help retain natural sediments in the source water to manage potential clogging at the site. The water passes through this first basion into the second (main) basin which helps filter it into the underlying shallow aquifer. 

By supplementing aquifers of a high nitrate concentration with clean water from the Rangitata, the Hinds/Hekeao scheme dilutes nitrate levels, providing better reliability for groundwater takes and allowing natural ecosystems to regenerate.

The first report on the scheme, delivered in August 2017, showed significantly increased groundwater levels and a decrease in nitrate levels from an average of fourteen grams per cubic meter to less than three. 

MAR OVERSEAS

Managed aquifer recharge has been employed successfully in various countries, for example:

- In Australia, MAR has been in use since the 1960s. Currently, more than twenty separate systems are in operation across the nation, with types ranging from aquifer storage & recovery, to soil aquifer treatment and pond infiltration. One of the most successful of these projects is at Langhorne Creek, where severe drought conditions in the Murray-Darling Basin reduced water levels and increased salinity in Lake Alexandria, which supplied water to vineyards at Langhorne Creek. A managed aquifer recharge project was undertaken in 1987, which not only saved the vines, but also drastically increased recharge, allowing 30 irrigation wells to be established, with up to 2.4GL being recharged per year.

- In Kenya, MAR has been invaluable through the use of sand dams. Sand dams are made by constructing steel reinforced concrete wall across a seasonal sandy riverbed. Rivers formed during the rainy season carry sand and silt downstream, where they eventually meet the wall. Lighter silt material flows over the dam wall, whilst  the heavier sand accumulates behind it. Eventually, the dam completely fills with sand, which, being highly permeable, stores large volumes of water, creating a shallow aquifer. Water can then by abstracted from the dam-aquifer through scoop holes or an infiltration gallery. Since 1996, over 400 sand dams have been constructed across Kenya, providing readily available water supplies during the dry season and providing a boost to agriculture that enhances food security during droughts. Charity organisations around the world recognise sand dams as a vital asset to combating food and water shortages in the third world. 

- North America has a long history of employing managed aquifer recharge to keep it's largest cities supplied with sufficient water all year round. One example is the Water Replenishment District (WRD) of Southern California, established in 1959. The WRD protects the central and west coast groundwater basins from salt water intrusion and provides half of the four million residents of Southern LA County with water through the management of aquifers. 

LOOKING TO THE FUTURE

Given that much of New Zealand's agricultural land is situated on alluvial deposits like silt and clay, we are a nation well suited to managed aquifer recharge. The continued success of the Hinds/Hekeao project is likely to see other regions adopt MAR in an attempt to reduce nutrient levels in waterways and store water to meet the demands of agriculture. 

 

 

 

Information sourced from:

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