| dc.description.abstract | Study area
The study area centres on the newly developed Vergenoegd well field (VWF) on the farm Vergenoegd 60 JO west of Zeerust in the North West Province. The area is underlain by dolomiteswhich are part of the Malmani Sub-Group in the Chuniespoort Group of the Transvaal Supergroup.
The majority of the dolomite is located in quaternary catchment A31C. It forms part of the Malmani River drainage, a part of the Marico catchment which drains northward to join the Limpopo River system. This catchment’s western boundary also forms the westernmost boundary of primary catchment “A”. West of this divide is the Molopo Catchment (primary catchment “D”). The Molopo Eye is also located on the same dolomite as the Malmani Eye (eight kilometres southwest), but drains towards the Atlantic Ocean via the Molopo drainage system whereas the Malmani Eye drains towards the Indian Ocean via the Malmani-Limpopo drainage system.
The study area forms part of the Crocodile West-Marico Water Management Area.
According to the hydrogeological map series of South Africa, the dolomite is described as a Karst Type aquifer. Dolomite is known as a rock type with significant groundwater potential due to the occurrence of groundwater in open cavities. Depending on the siting of production boreholes, dolomite aquifers can yield in excess of 20 L/s. Dolomite aquifers are classified as Major Aquifers according to the South African Aquifer Classification System. The development of the Rietpoort, Uitvalgrond and Vergenoegd well fields are testimony to the significant water potential of the Zeerust Dolomites (Botha, 1993).
Historical work
For more than a century the groundwater potential of the dolomites west of Zeerust has been known and investigated. The fluctuations in spring-flow emanating from the dolomite were the most readily observable effects of change in the aquifer recharge. As far back as pre-1906 there have been reports of spring-flow declining in the area, and states of low water levels. It was mostly attributed to below average rainfall.
In order to optimally exploit the potential of the dolomite aquifers on a sustainable basis (without compromising spring-flow), attempts have been made since the 1960’s to quantify the volume of water available. This is done by quantifying the volume of water being added annually to the aquifer as a percentage of recharge.
As more farmers tapped into the groundwater potential of the dolomite aquifers, the government saw it necessary to protect the groundwater by means of declaring a Groundwater Control Area, effectively limiting and controlling abstraction to ensure sustainability.
In 1965 the first case of alleged over-abstraction was investigated. A farmer alleged that the ‘extreme weakening of the flow from the spring at Vergenoegd No. 3’ was the direct result of excessive pumping of nearby boreholes. The report concluded that a cone of depression could be observed around the points of abstraction, but that this did not influence the spring-flow at that time. The weakening of the spring was due to a combination of below average natural recharge and increased abstraction. The Zeerust boreholes were subsequently decommissioned in 1967.
Data and interpretation
Existing data were obtained from the NGA (National Groundwater Archive), WARMS (Water Resource Management System) containing water use data, and HYDSTRA monitoring data. The HYDSTRA data proved the most valuable in terms of monitoring data.
New data were sourced from the hydrocensus and monitoring runs undertaken in 2012 and form part of this study. A fieldwork trip including a monitoring run of all identified boreholes was initiated in November 2012, while the monitoring data for the Vergenoegd monitoring boreholes were sourced.
Although the Vergenoegd and Tweefontein Dolomite Compartment Units (DCU) have been split by the Vergenoegd Dyke in previous literature, it was grouped together as the Paardevlei Groundwater Management Unit (GMU) in 2009. Since the VWF boreholes occur on both sides of the Vergenoegd Dyke, the new GMU is seen as the study area. Three springs occur here: the Vergenoegd Spring (eyes) and the Paardenvallei Springs. The Paardenvallei Springs’ data were more complete and did not reveal any impact from the VWF. The fluctuations seem to be more controlled by climatic conditions, mainly rainfall recharge. The monitoring data for the Vergenoegd Spring were insufficient to draw any significant conclusions, but the fact that farmers downstream complained about a reduction in their irrigation water from the spring, is indicative of a reduction in flow prior to the aquifer being recharged in 2011-12. Although the reduction in spring-flow was predicted during the numerical modelling of the borehole field, it was not identified during the EIA preceding the development as having a cumulative impact on the farmers irrigating from the canal.
Groundwater level monitoring data from various sources were used to assess the possible impact of the abstraction on the aquifer. The monitoring boreholes located in the Paardenvallei GMU did not reveal any adverse effect of the VWF on the water table in the long term. In fact, the water levels prior to the borehole field development were on par with what was measured in November of 2012. It must be noted that monitoring data were not available for the entire period covered since the inception of the field of boreholes, and therefore fluctuations on a shorter term is likely, as might have been the case in 2010 when irrigation farmers complained about a decline in the Vergenoegd Spring-flow.
The EIA Audit report submitted in 2011 (Masilo & Associates, 2011) indicated that no correlation exists between groundwater levels and the abstraction from the borehole field.
The calculations in this study contradict these findings. The EIA report however concluded that the abstraction volumes from some of the production boreholes of the VWF exceeded the recommended levels in 2010 indicating poor management of the borehole field. It also confirmed that monitoring data are lacking and concluded that the monitoring protocol was not fully in accordance with the 2005 modelling update and EMP recommendations.
Conclusions
Due to the lack of monitoring data for the area, short term fluctuations cannot be accurately be predicted. When the available monitoring data is examined, it be deduced that there is a seasonal fluctuation in water levels as well as spring-flows and that these two components are in relation. This fluctuation is due to the precipitation variations between wet and dry months that affect recharge. From the recharge calculations and subsequent modelling undertaken during this study, it can be deduced that there is a high correlation between the recharge, abstraction and spring-flow factors in this area. It is also apparent that these factors influence one another greatly and that variance in one of the inputs will have an adverse effect on the rest and will change the system’s response significantly, this is especially so when abstraction rates are increased as well as when drought conditions are simulated.
Based on the available monitoring data, including spring-flow and groundwater level data, it can be concluded that the VWF might show a long term impact on the aquifer.
It is likely that, the abstraction from the VWF reduces the spring-flow of the Vergenoegd Springs, which in turn has a cumulative impact on the irrigation farmers receiving water from the spring via a canal. This is especially so in the dry months when recharge is limited. This reduction was predicted during the modelling phase. The reduction in water levels then causes the secondary effect of sinkholes forming in the area due to weakened dolomite stability, especially in areas where there are contributing factors, such as pipeline leaks.
Recommendations
Monitoring of the spring-flow from the Vergenoegd Eyes must be reinstated as a matter of priority. The irrigation farmers downstream claim that the VWF leads to a reduction in their irrigation water from the spring, which in turn leads to loss of production, income loss and creation of sinkholes. Other than this study, there are no data to prove that the abstraction does not adversely affect the spring-flow from the Vergenoegd Eyes, and the farmers might be entitled to compensation due to loss of income. This issue must be investigated further on a legal basis.
Ngaka Modiri Molema District Municipality (NMMDM) as the Water Services Authority and DWA as the custodians of water resources in South Africa are mandated to ensure that proper monitoring are done. This includes monitoring of the abstraction volumes, groundwater levels and spring-flows from the affected compartment. To ensure the continuous monitoring of the borehole fields, the monitoring can be outsourced on a tender basis to external contractors. The monitoring reports must be audited annually.
The management of the VWF must be audited on a catchment and national level as per the ‘dolomite guideline’. The lack of proper and continuous monitoring data being the most important factor hindering proper management, must be addressed. The recommendations pertaining to the operation and maintenance of the well field made in the EIA and numerical modelling report must be adhered to (Sections 8.1.18 and 8.1.15). Similarly the findings and recommendations of the 2011 Audit report (Section 8.2) must be implemented. In essence these recommendations are the modelled sustainable yields (that were determined as being exceeded in the 2011 audit). The 2011 audit then goes on to recommend that the abstraction rates immediately be reduced to their modelled rates and that proper monitoring be instated.
Once monitoring data collection has been reinstated and a sufficient amount of time series data has once again been recorded, a similar study to this should be completed. A comprehensive hydrocensus should be conducted in which important data and observations should be made. Any changes to the area should be noted and recorded. It would be beneficial to conceptualise the system if changed and based thereupon, run another analytical model to determine if the correlation between recharge, abstraction and spring-flow has changed and if so, to what extent. | en_US |
