numerical modelling

Groundwater numerical models are commonly used to determine the impact that groundwater abstraction has on the ability of surrounding areas to supply water, and thus to inform Water Use Licence (WUL) Applications. However, data available is often limited to that generated by relatively short-term geohydrological studies and pumping tests. In most cases this data and the degree of defined uncertainty in the model results are sufficient.

The Elandsfontein Phosphate Mine is situated midway between the Langebaan Lagoon and the town of Hopefield. It is located on the Cape West Coast, within the Saldanha Bay Municipality. The mine is positioned within the Elandsfontein Aquifer Unit – which comprises an upper and lower aquifer separated by an aquitard. The economic phosphate layer is situated within the saturated zone of the Upper Aquifer Unit. There are fresh water inflows into the Langebaan Lagoon and all measures must be taken to ensure the natural geohydrological flows are not impacted.

Three dimensional numerical flow modelling has become one of the best tools to optimise and management wellfields across the world. This paper presents a case study of simulating an existing wellfield in an alluvial aquifer directly recharged by a major perennial river with fluctuating head stages. The wellfield was originally commissioned in 2010 to provide a supply of water to a nearby Mine. Ten large diameter boreholes capable of abstracting ±2 000 m3 /hour were initially installed in the wellfield.

A coal mine in South Africa had reached decant levels after mine flooding, where suspected mine water was discharging on the ground surface. Initial investigations had indicted a low-risk of decant, but when ash-backfilling was performed in the defunct underground mine, decant occurred. Ash-backfilling was immediately suspended as it was thought to have over-pressurised the system and caused decant. Contrariwise, a number of years later decant was still occurring even though ash-backfilling had been terminated.

Pope Gregory defined the seven deadly sins in order to guide the Catholic Church in the 6th century. The past 20 odd years in the industry has shown that there are several mistakes that are repeatedly made by numerical modelers. Although we all acknowledge that any numerical model is a non-unique solution, and that there exists and infinite number of solutions, there are several sins that will prevent the model from giving an accurate representation.

Imrie, S.

Monitoring groundwater storage is conducted in the study. World Health Organisation estimates, about 55 million people affected by drought yearly. However, Surface water holds 0.3 percent of the freshwater, and groundwater holds 30.1 percent of the freshwater. Hence, monitoring groundwater storage is vital.

The City of Cape Town (CoCT) metropolitan municipality seeks to identify and develop alternative water resources for the augmentation of surface water to ensure more robust and sustainable water supply to the CoCT and its inhabitants. A 3-D finite-element numerical model of the Atlantis Aquifer was developed using the commercial code FEFlow 7.1 to support the assessment of the impact of groundwater abstraction from the aquifer which has been identified as one of the target zones to develop sustainable alternative water resources as part of the CoCT water reconciliation strategy.