Rietvlei Natural Springs EarthCache

Rietvlei Natural Springs

Rietvlei Nature Reserve belongs to the City of Tshwane Metropolitan Municipality and was established to supply the city with clean drinking water. The dam and a part of its catchment area were proclaimed as a nature reserve and are managed as such.

The supply of clean drinking water and the protection of the catchment area are the main reasons for the establishment and maintenance of the reserve. There are three sources of water on the reserve, namely the dam with its purification plant below the dam wall, five fountains and five boreholes. The reserve supplies about 15% of the city's drinking water, which equals about 40 million liters of water a day.

Geology of Rietvlei Nature Reserve
As far as geology is concerned the eastern portion of the reserve is mainly made up of dolomite and chert. The central portions, from north to south consist of andesitic lava with agglomerate, shale and tuff layered in between. The western ridges portion is mainly shale, siltstone and some places quartzite and grit stone on top with layers of tillite and limestone. Between the lava and dolomite there are also strips of shale and quartzite ridges. Many sinkholes can be found in the eastern dolomite portions of the reserve.

The most prominent feature of the soils on the reserve is the fact that it is so shallow. The limiting layers are mainly lava, quartzite, diabase or dolerite. The soils are also very prone to erosion.

Due to the fact of the nearby sinkholes in the dolomite area, the water of the springs was not completely subjected to ground filtering and thus resulted in the fact that the water is not as clean as what it used to be many years ago before the sinkholes appeared.

Only one of the springs in the Rietvlei Nature Reserve is Perennial (continuous) and the other three are Ephemeral (intermittent) and will deliver water in the high rainfall season.

Natural Springs:
A spring is a point where groundwaterflows out of the ground, and is thus where the aquifersurface meets the ground surface. Dependent upon the constancy of the water source, a spring may be ephemeral (intermittent) or perennial (continuous). Water issuing from an artesian spring may rise to a higher elevation than the top of the aquifer from which it issues. When water issues from the ground it may form into a pool or flow downhill, in surface streams.

Minerals become dissolved in the water as it moves through the underground rocks. This gives the water flavor and even carbon dioxidebubbles, depending upon the nature of the geologythrough which it passes. Springs that contain significant amounts of minerals are sometimes called 'mineral springs'. Springs that contain large amounts of dissolved sodiumsalts, mostly sodium carbonate, are called 'soda springs'.

Types of springs:
Water which emerges at the surface naturally with a distinct current is called a spring. When a distinct current is not present, the flow is called a seep. Most springs and seeps represent water from rain or snow on some nearby higher ground which moves underground to where it comes up out of the ground. Its underground course depends on the type of soil it moves through. In some springs, the water bubbles up with a measurable force, indicating that it is under pressure, these are called artesian springs. Any spring having a temperature higher than the yearly average temperature for a given region is termed a thermal spring. This indicates a source of heat other than that of the surface climate, of which magmatic heat is an example. Based upon the pressure of the emergent water, any spring or seep which is not artesian may be classified as the gravity type. Gravity springs and seeps are those in which subsurface water flows by gravity from a high point of intake to a lower point of issue. The two most important type’s are;
• Water table springs and seeps, which occur where the water table comes near or intersects the surface of the ground.
• Contact springs and seeps, which occur along an exposed contact point, like along a hillside. Water table springs and seeps are normally found around the margin of depressions, along the slope of valleys, and at the foot of alluvial fans. Contact springs appear along slopes but may be found at almost any elevation, depending on the position of the rock formations.

Classifications:
In the past all the springs in the Rietvlei Reserve delivered second Magnitude, but due to the extra bore holes that were drilled, only one spring still delivers water today at a rate of third Magnitude. A second spring close to the road inside the reserve only produces water in the summer and dries out during the long winter months. Springs are often classified by the volume of the water they discharge. The largest springs are called "first-magnitude," defined as springs that discharge water at a rate of at least 100 cubic feet per second. The scale for spring flow is as follows:
• 1st Magnitude - > 2800 L/s
• 2nd Magnitude – 280 to 2800 L/s
• 3rd Magnitude – 28 to 280 L/s
• 4th Magnitude - 6.3 to 28 L/s
• 5th Magnitude - 0.63 to 6.3 L/s
• 6th Magnitude - 63 to 630 mL/s
• 7th Magnitude - 8 to 63 mL/s
• 8th Magnitude - 8 mL/s
• 0 Magnitude – no flow
The strange values in Liters per second are due to the original scale was done in gallons and feet per second having 1s 10s and 100s; thus round decimal figures.

Please note the rules on the reserve:
• The public is requested to stay in their vehicles and they are only allowed to get out at the picnic site and bird hides at their own risk.
• The speed limit is 30 km/h and some of the roads are closed to the public mainly because they go to the fences or are inaccessible with normal vehicles.
• No pets are allowed.
• Fires are only permitted at the picnic sites.

Acknowledgements:
City of Tshwane Agriculture and environmental Management department.

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