Delineation of Groundwater Region 65 : Zululand Coastal Plain Aquifer, KwaZulu-Natal

Abstract

The Zululand Coastal Aquifer or Groundwater Region 65 is the largest primary porosity aquifer in South Africa. Despite the veneer of well rounded, medium size sand grains, the subsurface environment comprises geological units with unique hydrogeological properties. Utilising Vegter’s (2001) methodology, nine laterally delineated groundwater regions (Q and Qb; Qm; Qpd; Kz, Pv and Pvo; JI and Zn; Nhl, Nng and ZB; Tu and Ntu) were identified however, data was a major shortcoming. Therefore to gain clarity, the hydrostratigraphic units were then vertically delineated using geological data derived from borehole logs and chronologically aligned with the regional geology to produce four hydrostratigraphic units. Surficial sands of the Sibayi Formation constitute hydrostratigraphic unit 1 which has the highest permeability, porosity and hydraulic conductivity (vertical and horizontal). The shallow to unconfined groundwater table facilitates abstraction (yield of <0.4 L/s) in the rural communities. However, it was recently reported that the cover sands are capable of generating higher yields (10 L/s to >25 L/s). Hydrostratigraphic unit 2 (Kwabonambi Formation) and 3 (Kosi Bay and Port Dunford Formations) are considered aquitards on account of incessant vertical leakage. Hydrostratigraphic unit 2 represents the most prominent perched aquifer in the study area while hydrostratigraphic unit 3 is illustrated by several expansive wetlands. Hydrostratigraphic unit 4 (Uloa Formation) is a leaky, semi-confined to confined aquifer. It is often utilised for production purposes on account of high borehole yields (6.7 to 28 L/s) which are a function of lithology thickness and karstification. The impermeable Zululand Group represents the hydrogeological basement for the aforementioned hydrostratigraphic units. Marked by reduced hydrogeological properties, low borehole yields (<0.1 L/s) and highly saline water, the Zululand Group is unfeasible to exploit as a potable resource. The discussions above attest to the presence of a shallow (hydrostratigraphic unit 1, 2 and 3) and deep aquifer (hydrostratigraphic unit 4). Hydrostratigraphic unit 1 and 2 are extensive while the remaining hydrostratigraphic units are limited and erratically distributed across the study area. Therefore, boreholes are unlikely to intercept all four hydrostratigraphic units including the hydrogeological basement in a vertically, sequential manner. The degree of surface water – groundwater interaction was quantified using the Herold’s Curve Fitting and the Saturated Volume Fluctuation Methods. The results confirmed that groundwater sustains major lakes and smaller streams and that there is constant interaction between the shallow aquifer and the surrounding surface water bodies. Anthropogenic activities were delineated on the basis of land use. Forestry and commercial sugar cane farming were the dominant anthropogenic activities occurring on a regional scale while mining, urban and or industrial land use, rural practises and salt water intrusion were localised. On account of its hydrogeological properties and shallow to unconfined groundwater table, the Zululand Coastal Aquifer is extremely vulnerable to pollution.