Pasos previos al proceso metastático: Formación del tumor

The phenomenon of dryland salinity in the Western Cape (South Africa) was first investigated by Malherbe (1953). However the issue has recently (1990’s) received widespread attention due to the pronounced salinisation of the Berg River. This has culminated in a series of Water Research Commission (WRC) projects, aimed to understand and address the issue.

Malherbe (1953) identified the presence of fossil salts as a cause of dryland salinity. Fossil salts are salts deposited in marine sediments of ancient seas. These sediments are buried, lithified, then uplifted and become parent material for the soil. Evaporation of groundwater concentrates these salts at the surface thereby degrading the soil. The present hard pans and soils of the North- western coastal area of the Western Cape developed as a result of inland sea water intrusion (Malherbe, 1953).

Sedimentary rocks in South Africa (e.g. Dwyka Series, the Malmesbury shale and the Enon conglomerate) are rich in soluble salts, which if weathered to soil material may cause an accumulation of salts under low rainfall conditions (Malherbe, 1953). These salts may remain in the original soils resulting in the area becoming saline. During the wet winter, flood and seepage water transport salts from the higher- to lower-lying areas where the water evaporates and the salts are left to concentrate at the soil surface. The salts in the districts of Malmesbury and Picketberg in the Western Cape are believed to have originated from the sea as well as from the weathering of the underlying bedrock (Malherbe, 1953).

According to Fourie (1976), the West Coast of South Africa is a semi-arid region in which dryland salinity is expected. The Department of Water Affairs (DWA) has monitored the water quality in the Berg River since the mid-1970s. Natural soil salinity has been identified as a source of salts affecting the water quality of the Berg River (Fourie and Steer, 1971; Fourie and Görgens, 1977). Fourie (1976) assessed the salinity of the Berg River in 1976 by focusing on certain Berg River tributaries and reported them to exhibit elevated salt concentrations. Fourie and Görgens (1977) investigated the mineralization of the Berg River and it was reported that the salinity increase of the river could be the result of increasing irrigation practices along the river. Flügel (1995) extensively studied river salinisation due to dryland agriculture between 1985 and 1986 in the 150 km2 catchmentof the Sandspruit River, a tributary of the Berg River. Water bodies within the catchment were investigated with the aim of identifying and quantifying their salinity dynamics. Flügel (1995) reported that dryland agriculture contributed to river salinisation based on the findings that the bulk annual atmospheric deposition accounted for only a third of the total salt output for 1986. The mean annual rainfall in the Sandspruit River catchment area is approximately 400 mm a-1, and was reported to have a salt concentration of 37 mg L-1. Sodium and chloride, transported by wind and rain from the Atlantic Ocean, were reported to be the dominant ions. Flügel (1995) stated that the balance of the total salt output was delivered by groundwater and interflow from the weathered shale and the soils within the catchment.

35 Görgens and de Clercq (2006) assessed the influence of irrigation return-flow on the water quality of the Berg River and it was reported that its contribution to the salt levels in the Berg River was minimal when compared to the consequences of dryland salinisation. A need for considerable improvement of monitoring systems for point and non-point source pollution in the Berg River catchment was highlighted.

Fey and de Clercq (2004) undertook a pilot study to determine whether a more extensive investigation is required of dryland agricultural impacts on river salinity in the Berg River catchment. It was reported that dryland salinity is extensive and that it is likely to have a significant impact on the water quality of the Berg River. Extensive patchiness in croplands, especially in wheat fields, which dominate the land use in the Berg River catchment, was identified. Ground truthing of these patches confirmed that they are associated with soil salinity. The soils were found to be sufficiently saline to affect wheat growth. The findings of this study suggested the need for a more detailed survey of salt distribution in the soils, regolith, and ground- and surface waters coupled with a fundamental study of salt mobilisation in response to climate, topography and land use practice in a small scale catchment. The results would serve as a prelude to extrapolation and calculation of the extent of the problem through hydrological modelling.

de Clercq et al. (2010) and Bugan (2008) investigated the hydrosalinity dynamics in the soil and vadose zone of a small scale catchment (SSC), i.e. Goedertrou, exhibiting evidence of dryland salinity and which is representative of semi-arid conditions in the Berg River catchment. The study not only examined salt sources and storage but also groundwater fluxes and catchment runoff with the view of informing future large-scale modelling and to guide the development of on-farm management practices. An experimental site was also established at Voëlvlei Dam to allow for a comparison of hydrology and salt balances to be made between winter wheat and restored Renosterveld. Monitoring and modelling of runoff under different vegetation scenarios (winter-wheat and Renosterveld) suggested that land use changes have a potential impact on salt release from the regolith into surface water. Salt and water discharge into the Berg River was also monitored at the medium scale Sandspruit catchment (de Clercq et al., 2010). This study has provided valuable information concerning the water and salt fluxes in overland flow and the vadose zone from different land uses. de Clercq et al. (2010) also suggested that the salts inducing salinisation of areas in the Berg River catchment are of marine origin as opposed to being products of rock weathering. Aqueous extraction of various regional Malmesbury shale powders provided evidence that these contribute insufficient quantities of inorganic salts to account for those discharging from catchments such as the Sandspruit. Resultantly it was interpreted that the salts present in the regolith have accumulated meteorically over a long period. This contradicts an initial theory that the salts are contributed from rocks, which are of marine origin. de Clercq et al. (2010) also calculated the input and output of salts from the Sandspruit catchment, a significantly saline tributary of the Berg River. The net salt discharge (a maximum of

20 000 tons a-1, however averaging at 6 700 tons a-1_{) when calculated per unit area of the catchment is}

close to 0.5 ton ha-1_a-1_{. The chemical signature of the discharge reflected a marine origin as the}

salts, i.e. chloride, sulphate and bicarbonate, concentrations match those of seawater. de Clercq et al. (2010) suggest that the salts discharging from catchments such as the Sandspruit have accumulated aerially over a long period during which either the climate was more arid and/or there was a vegetation cover (renosterveld and, before that, a speculated Olea-Rhus savanna) with a larger water use potential when compared to wheat. Recommendations from the study highlighted a need for continued monitoring in order to describe typical salinity patterns in

36 similar environments, to inform hydrological models and to establish management measures and guidelines.