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Artificial Recharge of Groundwater: Regulatory and Practical Aspects. SHARP PROJECT WORKSHOP VALLETTA 18th FEBRUARY 2011. Manuel Sapiano Directorate for Water Resources Regulation MALTA RESOURCES AUTHORITY. INTRODUCTION. ARTIFICIAL RECHARGE OF GROUNDWATER
SHARP PROJECT WORKSHOP
18th FEBRUARY 2011
Directorate for Water Resources Regulation
MALTA RESOURCES AUTHORITY
ARTIFICIAL RECHARGE OF GROUNDWATER
Artificial recharge to groundwater is a process by which the groundwater reservoir is augmented at a rate exceeding that obtained under natural conditions of replenishment. Any man-made scheme or facility that adds water to an aquifer may be considered to be an artificial recharge system.
The Water Framework Directive (2000/60/EC)considers ARTIFICIAL RECHARGE as one of the management tools which can be utilised by Member States for the achievement of good groundwater status.
The provisions of the Directive in this regard, are directed to ensure that the necessary controls are in place to eliminate the possibility of any degradation in the qualitative status of the receiving body of groundwater.
In fact, whilst listing ARTIFICIAL RECHARGE as one of the basic measures to be considered by Member States in their River Basin Management Plans, Article 11 of the WFD requires the establishment of:
“controls, including a requirement for prior authorisation of artificial recharge or augmentation of groundwater bodies.”
This in order to ensure that such practice does not
“compromise the achievement of the environmental objectives established for the source or the recharged or augmented body of groundwater”.
The protection of the qualitative status of the receiving bodies of groundwater is further considered in the Groundwater Directive (2006/118/EC).
By introducing the concept of preventing ‘inputs’ instead of ‘discharges’, this Directive widens the scope of the pollution-preventive actions to cover all pollutants that enter groundwater.
Under the old Groundwater Directive, 80/68/EEC such preventive actions were solely restricted to deliberate disposals.
The Groundwater Directivethus requires the enactment of all measures deemed necessary and reasonable to avoid the entry of hazardous substances into groundwater and to avoid any significant increase in concentration of such substances in groundwater, even at a local scale.
Therefore whilst attenuation processes in the unsaturated zone can be considered as part of the required pollution abatement measures, with the introduction of the ‘input prevention’ condition the Directive effectively eliminates the consideration of the abatement impacts of similar processes within the saturated zone.
Stricter controls are thus required in relation to direct discharges to groundwater.
When assessing whether new activities that may result in inputs are acceptable, i.e. whether they meet the requirements of the WFD, several questions need to be answered:
The GWD defines the ‘input’ of pollutant to groundwater as “the direct or indirect introduction of pollutants into groundwater as a result of human activity”.
Direct inputs can be identifed by one of the following properties:
- they bypass the unsaturated zone;
- the pollution source is in the saturated zone (or
discharges directly in the saturated zone);
- seasonal fluctuations in the water table mean that the
pollution source will be in direct contact with groundwater
from time to time
Indirect inputs are characterised by the discharge into groundwater after percolation through the soil or subsoil.
According to the GWD, substances to be PREVENTED from entering groundwater are substances indentified by Member States as being hazardous.
Substances to be LIMITED in groundwater such that pollution does not occur are all other pollutants.
However, since it is not technically feasible to stop all inputs of hazardous substances, and some small inputs are environmentally insignificant and thus do not present a risk to groundwater, the GWD introduces a series of exemptions.
Artificial recharge is considered under these exemptions.
Therefore to PREVENT an input into groundwater means:
Taking all measures deemed necessary and reasonable to avoid the entry of hazardous substances into groundwater and to avoid any significant increase in concentration in the groundwater, even at a local scale.
‘Reasonable’ means technically feasible without involving disproportionate costs.
When considering the need for measures to prevent an INDIRECT input of a hazardous substance into groundwater, one can take into account the attenuation (fixation, degradation) of the substance in the unsaturated zone. To this end, all the geological, hydro-geochemical and biological processes can be considered.
Attenuation processes in the saturated zone are not relevant for assessing DIRECT inputs of hazardous substances, since these substances should be prevented from entering the saturated zone.
Artificial recharge aims to increase the sustainable yield from aquifers by directing surface water into pits, trenches , boreholes and infiltration basins or storages.
Potential sources of surface water could be:
- naturally occuring sources (streams, springs or lakes);
- stormwater runoff from impervious surfaces;
- treated wastewater; and
- leaking pipelines.
The main advantages of artificial recharge schemes are:
Aquifer storage, transfer and recovery (ASTR) – injection of water into an aquifer for storage and recovery from a different well.
Infiltration ponds – ponds constructed usually off-stream where surface water is diverted and allowed to infiltrate (generally through an unsatured zone) to the underlying unconfined aquifer.
Rainwater harvesting – roof runoff is deverted into a well filled with sand or gravel and allowed to percolate to the water-table.
Percolation tanks – harvesting of rainwater runoff behind dams built across dry valleys where water is detained and infiltrates through the base to enhance storage in unconfined aquifers.
Control of sea-water intrusion in coastal areas: a line of injection wells parallel to the coast inject water into the aquifer. The result is a ridge in the potentiometric surface. Water levels behind the barrier could be drawn down below sea level with no fear of salt-water encroachment.
Managed Artificial Recharge is not a new concept to Malta.
To retain storm water discharge around 75 small dams have been built across the main valley lines. These dams had a double-purpose: storing water for agricultural irrigation and enahcing the recharge to the underlying aquifers.
The total design capacity of these dams amounts to around 150,000m3.
T.O. Morris - The Water Supply Resources of Malta (1950)
“... a large part of the replenishment of the Main Sea Level Table takes place through the small inliers of Lower Coralline Limestone which outcrop in the central region of the Island. ....... It follows therefore, that every effort should be made to conserve these outcrops, and all the channels that drain into them, in as clean and effective a condition as possible, Their importance to the Island’s water supply regime is completely out of proportion to the relatively small superficial area which they occupy. .....”
Rainwater runoff is only available during the wet-season whilst Treated Sewage Effluent is available throughout the whole year in constant volumes.
Urban rainwater runoff is polluted
Chemical Analysis for rainwater runoff in the Msida-Birkirkara Catchment (2008) indicate a high Total Organic Carbon (TOC) which is considered as an indicator of contamination by hydrocarbons.
Rural rainwater runoff is generally high in nutrients
Chemical analysis on surface water collected behind the Chadwick dams indicate a nitrate content exceeding the 50mg/l parametric value.
Treated Sewage Effluent
Treated sewage effluent has to undergo further polishing before being considered for artificial recharge.
Such further polishing includes purification processes such as ultra-filtration and reverse osmosis.
Construction of new dams has to fully consider the full environmental impacts of such development.
This since valleys are home to important and even endemic ecosystems.
A Pilot Artificial Recharge project is currently being implemented by the MRA and the WSC at Bulebel, Zejtun.
This pilot project will test the qualitative and quantitative impact of a direct artificial recharge exercise on the mean sea level aquifer system.
The project involves the following:
- Setting up of a polishing plant for the further treatment of effluent from the Sant’Antin Wastewater Treatment Plant.
- Detailed analytical testing of the polished effluent for indicators of emerging pollutants.
- Continuous monitoring in the immediate area of the recharge well for groundwater level and conductivity.
Expected results include:
1. A detailed characterisation of highly polished effluent with regards to the presence of unconventional pollutants;
2. The reaction time of the aquifer system to direct artificial recharge; and
3. The potential qualitative and quantitative benefits of recharging the aquifers with high quality water.
This pilot-project will be implemented under the MEDIWAT Project – Mediterranean Islands Network for the Implementation of Innovative Best Practices for Managing Water Scarcity and is part-financed by the European Union under the MED-Programme.