22-27 September 2019
Trade Fairs and Congress Center (FYCMA)
Europe/Madrid timezone

Saline groundwater generation from paleo-termite mounds in the Buffels River valley, South Africa

26 Sep 2019, 15:00
15m
Conference room 1.B ()

Conference room 1.B

Oral Topic 8 - Groundwater quality and pollution processes Parallel

Speaker

Mrs Jani van Gend (Department of Earth Science, Stellenbosch University)

Description

Salinisation of groundwater under semi-arid to arid conditions is generally attributed to a three-step process: (1) evaporation rates exceed mean annual precipitation (MAP), leading to (2) formation of a salt crusts on the surface, followed by (3) formation and downward percolation of brines when episodic rainfall is received. Groundwater in the north west of South Africa is variably saline with electrical conductivity (EC) in excess of 4 000 μS/cm. However, much of the coastal zone of southern Africa (including Namibia) has similar MAP profiles and associated high evaporation rates and yet groundwater does not suffer from the same level of salinisation. Moreover, saline groundwater is not uniformly distributed and together these features suggest that other factors leading to salinization are at play. The Buffels River catchment extends over an area of approximately 9250 km2 from the elevated interior in the north of the country running down to the coastal plain at Kleinzee on the north-west coast. The river itself is ephemeral and while a few larger towns receive piped water from the Orange River situated to the north, but most of the smaller local communities rely on groundwater to sustain themselves. Palaeo-termite mounds, known as heuweltjies (meaning little hills), are common surface features along the west coast of South Africa, covering between 14 and 25% of the land surface. They are highly concentrated in zones in the Buffels River catchment. These structures typically consist of salt- and nutrient-rich sediments and the recorded sediment EC values for these structures are an order of magnitude higher than that of the surrounding sediments. One hypothesis is that heuweltjie salts contribute to salinisation of groundwater in areas where the distribution of heuweltjies are particularly abundant. Distinct sulphate rich layers have recently been found within the heuweltjies and may be related to highly variable sulphate concentrations in the groundwater in this area (2 mg/L to 453 mg/L). Groundwater 87Sr/86Sr ratios in this area are extremely elevated, up to 0.78240, suggesting interaction between the groundwater and local radiogenic granitic gneisses. However, groundwater 87Sr/86Sr ratios decrease to the west which could indicate interaction or mixing between different water sources. Highly variable 36Cl/Cl ratios (25.94 x10-15 and 156.19x10-15) provide an indication that groundwater recharge occurs inland and mixing between multiple sources. Radiocarbon activity in groundwater in the Buffels River Catchment range between 22.60 and 100 pMC, providing an indication of varying groundwater residence times and could indicate mixing between multiple sources. 34S and 18O in sulphates as well 14C activity in groundwater and sediments are being used to constrain the source or origin and transport mechanisms of the salts to understand the possible link between heuweltjie salts and groundwater salinity.

Primary authors

Mrs Jani van Gend (Department of Earth Science, Stellenbosch University) Prof. Jodie Miller (Department of Earth Science, Stellenbosch University) Dr Catherine Clarke (Department of Soil Science, Stellenbosch University) Dr Laszlo Palcsu (Institute for Nuclear Research, Hungarian Academy of Sciences) Dr Michele Francis (Department of Soil Science, Stellenbosch University)

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