Evaluation of factors influencing transmissivity in fractured hard-rock aquifers of the Limpopo Province

Evaluation of factors influencing transmissivity in fractured hard-rock aquifers of the Limpopo Province Martin Holland* Delta h, PO Box 66662, Woodhill 0076, South Africa Abstract Geologically-complex fractured aquifers underlie large parts of the semi-arid Limpopo Province where some of the greatest groundwater needs in South Africa occur. It is important to identify potentially high-yielding zones that can be targeted for water supply. The study covered 7 distinct groundwater regions within Limpopo Province, together covering about 63 500 km 2. Results from over 4 000 pumping-test analyses indicated that geological setting (e.g. aureole of granitoids), proximity and orientation of dykes and lineaments and proximity of surface-water drainages may exert an influence on borehole productivity. Although dykes are poor groundwater targets, drilling dykes composed of dolerite may prove to be more successful. Lineaments striking perpendicular to the current maximum horizontal stress seem to be more favourable targets, which is inconsistent with the predicted regime. Due to the complex geological history, it is difficult to link open discontinuities to a distinct recent or past tectonic event. Regional stress-field data, as in this case, may not account for local, possibly highly significant, stress-field variations. The hydrogeological importance of several factors related to groundwater occurrence, can be used as a working reference for future groundwater-development programmes. Keywords: Limpopo Province, groundwater, transmissivity, lineaments, borehole productivity Introduction The development of hard-rock aquifers as a reliable source of rural water supply is notoriously complicated, and transmissivities are spatially highly variable (Wright, 1992; Chilton and Foster, 1995; Banks and Robins, 2002). Due to the low intrinsic primary permeability and porosity of the hard bedrock, these aquifers differ in important ways from other aquifer types, and demand specific knowledge and techniques if groundwater is to be extracted and managed efficiently. Depending on the hydrogeological and climatic conditions either the magnitude of natural groundwater resources or hydraulic parameters of rocks represent the limits of groundwater development (Krâsny, 1997). Transmissivity data describe the general ability of an aquifer to transmit water and is vital for developing an understanding of the controls on groundwater occurrence. In many instances the magnitude of transmissivity (aquifer permeability) affords a notion about the water-bearing characteristics of hydrogeological bodies and is a decisive factor for groundwater-abstraction possibilities. However, numerous independent or interrelated factors, such as geomorphology (topography), lithology, brittle (neo-) tectonics, and surface-water hydrology, all play a significant role in the occurrence of groundwater in hard-rock environments, because together they control the: • Nature and depth of the regolith • Development of fracture and fault zones • Presence of higher porosity material (or adjacent alluvium) This paper was originally presented at the International Conference on Groundwater: Our Source of Security in an Uncertain Future, Pretoria, 19-21 September 2011. * To whom all correspondence should be addressed.  +27 82 497 9088; e-mail: The influence of topography on borehole yield has been shown by many (e.g. McFarlane et al., 1992; Henriksen, 1995; Mabee, 1999) with the common result that wells located in valleys and flat areas show generally higher yields compared to wells located on slopes and hilltops. Although specific rock types (e.g. granite, gneiss and schist) are in many cases the obvious factor in explaining the variations in borehole yields (Gustafson and Krásný, 1994; Neves and Morales, 2007), the influence is often supplanted by secondary features such as faults, fracture zones and dykes. Throughout the past decade the optimisation of the location of boreholes (wells) in tectonically fractured areas throughout Africa, India and Brazil has focused mainly on assessing the relationship between bedrock structure and groundwater production by analysing the position of wells in relation to lineaments (e.g. Fernandes and Rudolph, 2001; Solomon and Quiel, 2006; Henriksen and Braathen, 2003; Owen et al., 2007; Ranganai and Ebinger, 2008). Despite the unresolved relationship between lineaments and subsurface permeability (e.g. Greenbaum, 1992; Gustafson and Krásný, 1994), the use of lineament identification in groundwater exploration will continue to be an important initial guide to borehole target selection at the regional level. This is especially true for the Limpopo Province where some of South Africa’s greatest groundwater needs occur. Groundwater is the only dependable source of water for many users but is available in varying quantities depending upon the hydrogeological characteristics of the underlying aquifer. Heterogeneous fractured aquifers are distributed throughout the Province and transmissivity may vary by an order of magnitude from one borehole to the next. Knowledge of the variation and distribution of transmissivity provides a basis for future groundwater development and protection. It is important to identify potentially high transmissivity zones that can be targeted for water supply to sustain areas of high population density with few or no alternative water sources. In order to enhance the current understanding of groundwater occurrence in the Limpopo Province the variation http://dx.doi.org/10.4314/wsa.v38i3.3 Available on website http://www.wrc.org.za ISSN 0378-4738 (Print) = Water SA Vol. 38 No. 3 International Conference on Groundwater Special Edition 2012 ISSN 1816-7950 (On-line) = Water SA Vol. 38 No. 3 International Conference on Groundwater Special Edition 2012 379 Figure 1 Area of investigation in South Africa (adapted from Boshoff et al., 2006) of transmissivity is studied in terms of the following factors: • Hydrogeological and geological setting • Dykes and linear anomalies (including their orientation) and proximity to rivers. and 1 000 mm and orographic rains occur frequently along the escarpment and mountain ranges. This paper uses data obtained during the Limpopo Province GRIP (Groundwater Resources Information Project) Programme, and provides a statistical analysis of the relationship between transmissivity and geological structures. This study builds on the knowledge base set up by Holland and Witthüser (2011) who outlined the relationship of 5 influencing factors on groundwater productivity focusing on the crystalline rocks of the Limpopo Province. These factors included: • The geological and topographic setting • Dykes and linear anomalies (including their orientation) • Regional tectonics (maximum horizontal stress) • Weathering thickness • Proximity to surface water drainages. Several major shears (crustal transforms) subdivide the Limpopo Mobile Belt and the Kaap (...truncated)