Hydrogeology of Fractured-Rock Aquifers and Related Ecosystems within the QoQodala Dolerite Ring and Sill Complex, Great Kei Catchment, Eastern Cape
The present project is a follow-up of a previous Water Research Commission project No 937 on the hydrogeology of Karoo dolerite rings and sills (Chevallier et al., 2001). These saucer-shaped intrusions have induced a complex network of fractures inside the surrounding sediments and are responsible for the formation of numerous shallow and deep seated aquifers. Dolerite ring systems also control to a very large extent the morphology, the recharge and the drainage pattern at surface and influence the emergence of many springs and seepages. It becomes obvious from the hydro-morpho-tectonic model that many of these springs and related ecosystems may be vulnerable to large scale groundwater abstraction from this complex plumbing system of interconnected sills, dykes and fractures.
In the handbook on Hydrogeology of the Karoo Basin (Woodford and Chevallier, 2002b), needs for future research were identified and the study of the relation between groundwater, springs and ecosystems was highly recommended. It was suggested that future project should for that matter adopt a multidisciplinary programme-based approach including hydrostratigraphy, spring census, geomorphology, flow dynamics, biosystems and use of remote sensing.
The study area, i.e. the Qoqodala ring system is situated within the Great Kei river catchment, in the Eastern Cape. It covers four 1: 50 000 sheets and includes Queenstown. In these Eastern Karoo regions, precipitations are higher than in the Western Karoo, runoff is more important and a large part of the population depends on the numerous springs and seeps coming out from the mountains. Boreholes are very few in the study area (former Transkei) but the numerous springs are associated to dolerite rings.
AIM AND OBJECTIVES
The aim of the project is to investigate ecosystem and spring/seepage dependency on shallow or deep fractured rock aquifers related to dolerite rings and their possible vulnerability to abstraction.
The two objectives as indicated in the memorandum of agreement are:
Dolerite sill and ring systems of the Great Kei catchment control to a large extent the morphology, the rain fall, the drainage pattern, the occurrence of many springs and therefore the geographic distribution of the local communities and their economic and social life. The Qoqodola sill and ring system comprises several coalescing and overlapping saucer-shape dolerite rings (inner sill, outer sill, inclined sheet), forming an intrusive network which should be conducive to high yielding fractured aquifers at various depths and the emergence of many springs. The natural vegetation seems to be preserved on the slopes (ring) where it mainly comprises of aloes and shrubs, and few succulent plants. In contrast the middle flat area of the saucer-shape structure, corresponding to the communal land, is severely overgrazed.
Morpho-tectonic and elevation analysis of the sills completed with drilling results have shown at least six levels of sill intrusion over the study area. Inner sills and outer sills are all interconnected. The outer sill of a specific ring can becomes the inner sill of another ring. For instance one of the outer sill of the Qoqodala ring forms the inner sill of the Bonkolo ring.
The inclined sheet forms the structural back bone of the ring system and the transition between the inner sills inside the ring and the outer sills outside the ring. Structural analysis over the study area showed that the ring part is densely fractured following major regional trends (dyke intrusion). At the drilling site across the Qoqodala ring, NW fractures are very prominent; the inclined sheet, feeding several sills, is probably fed by a regional dyke.
From March 2002 to June 2003, the Department of Water Affairs and Forestry (DWAF) drilled a total of 12 exploration percussion boreholes across the SW rim of the Qoqodala ring, along the road from Qoqodala to Zingqutu. A total drilling depth of 2655m was attained. The inclined sheet and the inner sill of the southwest portion of the Qoqodala ring were the main targets. This was completed by geological logging, video camera, geophysical down the hole logging and Time Domain Electromagnetic survey (TDEM), especially for deep structure detection. The profile shows a very thick dolerite inclined sheet feeding two outer sills. The back of the inclined sheet between the two outer sills is structurally very complex with several dolerite offshoots, confirming results obtained at Victoria West. The Qoqodala inner sill and the deeper sill were not intercepted by drilling but detected by TDEM.
Three aquifer-testing schedules, each consisting of a step-drawdown test followed by a 72-hour constant-discharge and waterlevel-recovery test, were conducted on boreholes BH-1, BH-7 and BH-11 over the period 27th June to 22nd July 2003. Qoqodala dolerite ring-system exhibits a typical multi-layered aquifer system, where at least three hydraulically distinct aquifer units are evident.
A shallow, laterally extensive, unconfined to semi-confined aquifer unit developed in the weathered / uplifted layers above the Inner Sill, where the main water-bearing fractures are predominantly sub-horizontal to horizontal. The depth to the waterlevel varies between 9 and 11 m.bgl. When flowing, the river is effluent into the groundwater system. The deeper, sub-horizontal fracture zone associated with the relatively thin dolerite 'offshoot' may only be weakly connected to the overlying fracture network and, if this is the case, could be considered as separate aquifer layer.
A shallow, semi-confined to confined aquifer developed within the intensely fractured sediments behind Inclined Sheet and Outer Sill of the ring system. The confined nature of the undisturbed aquifer resulted in boreholes BH-4 and BH-5 becoming artesian soon after drilling as well as the spring near BH-5 becoming dry. At this point, it is also likely that the aquifer was effluent to the river.
A deeper-seated, confined aquifer associated with discrete, open, fractures in the dolerite and meta-sediments at the base of the Outer Sill and Inclined Sheet, as well as presumably the Inner Sill. High yields were struck in a fracture system at the base of the Outer Sill. The temperature of the groundwater in borehole BH-7 is slightly elevated at 24º C, indicating upward movement of groundwater from a greater depth. The water is generally of good quality and varies around 44 mS/s. No chemical data was available yet from DWAF when the present report was compiled.
The ecosystem mapping part of the project aimed to produce a high resolution vegetation map by using remote sensing technique. Secondly, any wetlands present in the study area were mapped using moisture and greenness indices derived from multi-temporal satellite imagery. Finally, the spectral properties of a known seep were used to map this groundwater dependant ecosystem.
The results of the vegetation mapping revealed that it is possible to use medium resolution satellite image for broad vegetation mapping however it is not suitable for mapping at community level. The nature of the landscape lends itself more to land use mapping than vegetation mapping.
The wetland mapping proved that large wetlands are not present on the upland dolerite regions where rain fall and moisture are important, but instead vast grassland areas are present. Small seeps on the slope of the dolerite rings are such that they were not successfully mapped using either Aster or Landsat imagery.
Finally, using the spectral properties of a known seep to map other seeps in the area revealed interesting trends. When the results of the seep mapping are compared with the slope analysis and dolerite elevation classes, it was seen that in general, the density of seeps decreases with elevation and with slope steepness. The optimal slope on which the 'seeps' occur at these altitudes is on slopes of less than eight degrees closely followed by slopes of between eight and 14 degrees.
Ecosystem studies have shown that grassland is very common on high lying outer dolerite sills in recharge areas but do not however host large wetlands that could act as water reservoir. Aloe dominated shrubs and succulent thicket occurs on North facing and East facing slopes of the dolerite rings being adapted to more arid conditions that the grassy highlands. None of these plants however are likely to be groundwater dependant. Lush tall shrubs with a tree component dominate in the kloofs and at the foot of dolerite cliffs and seem to prefer South and West facing steep exposures. These luxuriant thickets may be terrestrial to phreatophytic i.e. a combination of moist conditions in valley or shadowy places and periodic use of non-perennial seeps at the base of dolerite cliffs. Groundwater dependent plants occur around perennial seeps where the vegetation consists mainly of grass and sedges growing on peaty black soil forming small wet areas from few meters to several tens of meters wide. Shrubs adapted to harsher conditions are unable to grow. They typically occur in depressions along fractures or created by morphological breaks along lithological contact zone i.e mudstone-sandstone or dolerite sediment. They also tend to occur at low elevation and on slopes no more than 14º. These seeps and small wetlands might not significantly contribute to the overall water recharge but they are very vulnerable to change in groundwater regime via drilling or abstraction as proven at Qoqodala drilling site. In the middle of the ring extreme land degradation occurs; bare soil and overgrazed grassland dominate. This does not mean that wetland vegetation did not exist in the past.
The most vulnerable eco-hydrogeological system corresponds to the upper unconfined aquifer above the low outer sill and to the seeps occurring at low elevation. The bulk of the groundwater recharged during rainfall events is stored in this aquifer layer. Because of the high density of fracturing of the ring and the high connectivity of these shallow fractures, badly planned drilling can induced water flow regime and deplete the aquifer and therefore affect these ecosystems. The location of wetlands or seeps at low elevation, the direction and density of fracturing, the slope of the inclined sheet, and the presence of an outer sill at depth are factors that should be taken into account when developing dolerite ring related groundwater.