Groundwater Quality

Groundwater quality is determined by the solutes and gases dissolved in the water, as well as the matter suspended in the water.  Water quality is a consequence of the natural physical and chemical state of the water as well as any alterations that may have occurred as a consequence of human activity.  The usefulness of water for a particular purpose is determined by the water quality.  If human activity alters the natural water quality so that it is no longer fit for a use for which it was previously suited, the water is said to be polluted or contaminated.  It should be noted that in many areas water quality has been impacted by human activity but the water is still usable.

One basic measure of water quality is that of total dissolved solids (TDS), which is the total amount of solids, in milligrams per litre, that remain when a water sample is evaporated to dryness.  Electrical conductivity (EC) is also a measure of the mineral content of the water.

Water naturally contains different dissolved inorganic constituents.  The major cations are: calcium; magnesium; sodium; and potassium.  The major anions are chloride; sulphate; carbonate and bicarbonate.  Although not in ionic form, silica can also be a major constituent.  These major constituents constitute the bulk of the mineral matter contributing to total dissolved solids.  In addition there may be minor constituents present, including: iron; manganese; fluoride; nitrate; strontium and boron.  Trace elements such as arsenic; lead; cadmium and chromium may be present in amounts of only a few micrograms per litre, but they are very important from a groundwater quality standpoint.

Natural groundwater is never pure.  They always contain at least small amounts of dissolved gases (oxygen and carbon dioxide mainly plus nitrogen (however this is essentially inert)) and solids.  The chemical composition of groundwater is a function of a number of factors.  For example, initial composition of the water; partial pressure of the gas phase; the type of mineral matter the water contacts, and the pH and oxidation potential of the solution (Fetter, 2001).

The natural quality of groundwater varies substantially from place to place.  It can range from total dissolved solids content of 100 mg/L or less for fresh groundwater to more than 100 000 mg/L for some brines found in deep aquifers.  In South Africa of the many factors influencing groundwater quality, the amount of rainfall and the geological setting play an important role.  These two factors also influence groundwater recharge (which is a complex process also influenced by additional factors) and broadly speaking the higher the recharge rates the better the groundwater quality.

EC is sub-divided into a number of categories according to the drinking water quality standards (DWAF, 1998).  The latest national EC groundwater map of South Africa (Murray et al., 2012) is shown in Figure 1.  Using this map, an assessment of the groundwater quality based on aerial extent is shown in Table 1.

Figure 1: Map showing the interpolated distribution of EC in South Africa  (Murray et al., 2012)

From a broad overview of South Africa, the groundwater quality (in terms of EC and TDS) does have a spatial trend.  The groundwater quality is better in the eastern portion of South Africa, and the water quality is worse in the western portion of the country, especially in the north-western portion.  This broadly reflects the rainfall pattern, with annual rainfall being higher in the eastern portion and the becoming very low in the north western corner.  There are of course exceptions to this regional pattern, notably the good water quality in the Western Cape, associated with the very “pure” nature of the geological formations of the Table Mountain Group of rocks.  There is also poor quality water in the southern Cape and Kwa-Zulu Natal area and this is attributable to the geological nature of the rock formations.  The rock types are very fine grained and mineral rich, thus recharge rates are very low allowing a lot of contact time between the recharging rainfall and mineral rich rocks (typically siltstones / clays / shales etc) to interact, and thus for the EC and TDS of the groundwater to increase significantly.

Besides the regional trends related to EC and TDS there are many differing trends for other groundwater parameters.  One example is nitrate which can be elevated through naturally occurring processes or elevated through anthropogenic activities  (farming, landfills, sewage systems etc).  Another aspect of groundwater quality also to be taken into account is the microbiological content of the groundwater.  This aspect is highly variable and a very important consideration.

In summary groundwater quality is highly variable across South Africa with the main influencing factors being rainfall volumes, geological setting and anthropogenic activities.  It depends on the nature of the project however it is often best to assess an area in detail, as groundwater quality varies across an area (spatially), it varies with depth as well and it also varies with time.

References:

DWAF, 1998.  Quality of domestic water supplies, Volume 1:  Assessment guide.  Department of Water Affairs and Forestry, Department of Health, Water Research Commission, 1998.

Fetter, C. W., 2001.  Applied Hydrogeology, fourth edition. Prentice-Hall, Inc. Upper Saddle River, New Jersey 07458.

Murray R, Beker K, Ravenscroft P, Musekiwa, C AND Dennis, R. (2012). A Groundwater Planning Toolkit for the Main Karoo Basin: Identifying and quantifying groundwater development options incorporating the concept of wellfield yields and aquifer firm yields. WRC Report No. 1763/1/11, Pretoria, South Africa.