Water FAQs

Brush up on your water and ground water knowledge

What is Groundwater?

Underground water occurs in two zones, namely the unsaturated zone (where voids and pore spaces are partially filled with water) and the saturated zone (where all voids and pore spaces are filled with water). The saturated zone is referred to as groundwater.

Groundwater occurs in either porous media (such as sandstone or unconsolidated gravel deposits) in primary openings, or within fractured rock (such as fracture shale, granite or in dolomitic cave systems) in secondary openings, as illustrated in the figure below. The majority of South Africa’s groundwater is located within fractured rock systems.

Types of groundwater systems (after Heath, 1983)
How do I locate and utilise groundwater as a water source?

Groundwater is located using a variety of geophysical methods, which identify the most likely location of groundwater by quantifying the underlying geology’s natural properties. These properties include magnetism (e.g. magnetics, electromagnetics methods), resistivity (e.g. electrical resistivity methods) and gravity (e.g. the gravity method). The geophysical method is selected based on the site geology and groundwater locations are based on anomalies in the resultant data.

Once groundwater targets have been identified a hydrogeological borehole can be drilled, using air percussion drilling, and constructed using a mixture of solid and slotted casing (either steel or uPVC). The casing is essential to prevent soil and weathered material entering the borehole and potentially damaging any pump equipment installed at the borehole.

Once the borehole is installed it should undergo pump testing, where water is abstracted at a known rate for a fixed period of time and the water level response recorded, to allow for the design and specification of the most suitable and sustainable abstraction pump equipment to be installed at the borehole for use.

What is groundwater recharge and how does it affect me?

Groundwater recharge is the portion of precipitation that infiltrates the subsurface and reaches the saturate zone. Recharge occurs mainly during the rainy season, but may also occur as recharge from losing streams (i.e. where water from a river enters the groundwater system) or through lateral aquifer flow (i.e. water from a neighbouring aquifer system). The water cycle is illustrated in the figure below, where the various types of surface and subsurface flows in the environment can be seen.

Groundwater recharge ensures reliable water supply and should be considered during the design of a groundwater resource development project. Recharge can be determined using a number of techniques (e.g. chloride mass balance, cumulative rainfall departure, etc.) and should be considered during sustainable borehole yield determination.

The water cycle (after Colvin et al., 2007)
What is numerical groundwater modelling?

A model, in general, is a conceptual description of a physical system using mathematical equations, where real-world entities are described and simulated using mathematical equations. A numerical groundwater model is a model representing the hydrogeological environment at a site, using the governing groundwater flow equation. A number of numerical modelling solutions exits, however the most commonly applied solutions are the finite difference (FD) and finite element (FE) modelling methods. Groundwater models are used to simulate groundwater flow and/or contaminant transport at a specific site.

Groundwater models are generally based on a number of assumptions, which are consolidated and described in the conceptual hydrogeological model for the site, which generally include assumptions on the direction of flow, aquifer extent, contaminant transport mechanisms and aquifer properties. Fetter (2001) described modelling as a method “to understand why a system is behaving in a particular observed manner or to predict how it will behave in the future.” 

How will a numerical groundwater model benefit my operations?

Numerical groundwater modelling is a tool to be incorporated into the overall decision making process and is a valuable tool for long term planning at a site. A groundwater model can be either a flow model, which simulates groundwater flow, or a contaminant transport model, which simulates contaminant migration at a site. A model can assist with proactive management design and mitigation of risk at a site at a fraction of the cost should a reactive approach be taken.

A groundwater flow model can assist in the mine-design and operational processes in terms of dewatering management, groundwater abstraction regime design and management, as well as provide input to environmental risk assessments and management plans for the site.

Groundwater contaminant transport models can assist in the prediction of contaminant migration path simulation over time, which provides input into environmental impact assessments and management plan design. 

Is groundwater use regulated in South Africa?

Yes, the use of groundwater for water supply is regulated under the National Water Act (NWA) of 1998, along with all other water in South Africa. The NWA defines water use very broadly and includes:

  • Taking and storing of water;
  • Activities which may result in a reduction of streamflow;
  • Waste discharge and disposal; and
  • Activities which impact negatively on water resources and which can be declared controlled activities.

The NWA sets out rules to use water efficiently and sustainably and can be summarised to state that the larger the potential risk to the water resource, the more stringent the rules to be applied to that water use. The 

Hierarchy of water use authorisation (DWAF, 2004)
What are the causes of poor water quality?

Groundwater is generally of a good quality, depending on its vulnerability to surface contamination and the aquifer material. Poor quality water may be as a result of natural water-rock interactions (e.g. in Limpopo Province where naturally elevated nitrate and fluoride concentrations occur due to interactions between the host rock and groundwater) or due to anthropogenic activies such as agriculture (where nitrate may be released into water from fertilizers or animal waste), urbanisation (where contaminants may be released through urban infrastructure such as sewage plants, hydrocarbon storage areas or manufacturing plants) and mining activities. The typical types of water contamination associated with mining is shown in the table below.

Causes of Poor Water Quality
Impact Type Description Associated Contaminant
Acid Rock/Acid Mine Drainage (ARD/AMD) Sulphide-bearing rock is oxidised (i.e. exposed to air) and sulphuric acid is generated as a result.
  • Increased sulphate concentrations
  • Lowering of pH
  • Increased metal concentrations (due to changing redox conditions)
Metal Contamination & Leaching When extracted rock (generally waste rock) is exposed to water, heavy metals are leached out over time and carried downstream in the water. 
  • Heavy Metal contamination (e.g. arsenic, cobalt, copper, lead, zinc, etc.)
  • Radioactive contamination (e.g. gold mining where U is associated with the deposit)
  • Increased EC/TDS
Processing Chemical Contamination Chemicals used in the ore extraction and beneficiation processes may enter the water environment (surface and groundwater) through spills at the plant/storage area or through leaching at tailings disposal facilities over time
  • Cyanide (gold benificiation)
  • Acid used in ore beneficiation
  • Nitrates from explosives
Erosion & Sedimentation Disturbances of the natural surfaces result in exposed earth, which allows for increased erosion and sediment loads in surface water channels
  • Increased sediment (i.e. TDS/EC)
  • Could result in clogging of rivers/streams, resulting in loss of aquatic ecosystems

 

What water treatment solutions are available on the market?

There are a number of water treatment solutions available on the market today, each with its associated advantages and disadvantages. The most commonly applied water treatment solutions include reverse osmosis (which effectively treats a large variety of contaminants, but only recovers a percentage of feedwater which results in waste generation), ion exchange (which deals with ion contaminants effectively and generates minimal waste, but requires regular replenishment of ionic reagent), granular activated carbon (which can be used to treat a variety of contaminants with low waste generation, but requires regular replenishment of activated carbon) and ultrafiltration (which is a mechanical filter that removes selected pathogens, bacteria and physical material, but is limited in terms of removal of chemical contaminants).

There are a wide range of water treatment options available at a variety of scales, each customisable to meet the site budget, environmental and quality requirements. 

How do I select the best water treatment solution for my problem?

Each water quality issue is unique and has individual project constraints and limitations which must be considered when choosing a water treatment solution. These include the actual water quality (i.e. what is the parameter of concern), budget (how much can I afford), environmental considerations (is the environment going to benefit from the water treatment solution) and water demand (how much water do I need per day to be treated).

All of these factors need to be considered during the selection and design process. In terms of general treatment solutions for selected contaminants of concern, please refer to the figure below.

 

Treatment Technology

Contaminant of Concern

Arsenic

Copper

Lead

Other Heavy Metals

Fluoride

Nitrate

Activated Alumina

X

 

 

S

X

 

Granular Activated Carbon

 

 

 

S

 

 

Distillation

X

X

X

X

 

X

Anion Exchange

X

 

 

S

 

X

Cation Exchange

 

X

X

S

 

 

Ozonation

 

 

 

 

 

 

Reverse Osmosis

X

X

X

X

X

X

Ultrafiltration

 

 

 

 

 

 

Ultraviolet Light

 

 

 

 

 

 

Treatment Technology

Contaminant of Concern

Microbial Contaminants

Radium

Selenium

Uranium

Synthetic Organic Compounds (SOC's)

Activated Alumina

 

 

X

X

 

Granular Activated Carbon

 

X

X

X

X

Distillation

X

 

 

 

 

Anion Exchange

 

 

X

X

 

Cation Exchange

 

X

 

 

 

Ozonation

X

 

 

 

 

Reverse Osmosis

X

X

X

X

X

Ultrafiltration

X

 

 

 

 

Ultraviolet Light

X

 

 

 

 

 

What is Acid Mine Drainage (AMD)?

Acid mine drainage (AMD), also referred to as acid rock drainage (ARD), is caused when water flows over or through sulphide-bearing rocks and metal-rich, acidic water is formed via chemical reactions between the water and rock. AMD is formed when pyrite (FeS2) is oxidised and forms sulphuric acid, which lowers the water pH to a point where metals, such as iron, are mobilised.

The acidic water may result in other heavy metals being mobilised as well, such as copper, lead etc. which leads to a deterioration of the overall water quality within the region. The oxidation of pyrite, as summarised in Coetzee et al., 2007, is shown below.