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Hydrogeology Journal

, Volume 27, Issue 3, pp 827–840 | Cite as

A typical groundwater storage assessment in the Tugela area, South Africa

  • Haili LinEmail author
  • Lixiang Lin
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Abstract

Water storage assessment is an important component of feasibility studies for prospective mining areas. As required by national mineral resources and environmental Acts, this may include assessment of both exploitable and sustainable storage; the former relates to the amount of groundwater stored within the exploitable aquifer depth and the latter is defined as the groundwater that can be sustainably extracted without producing unacceptable environmental and economic problems. A simplified method is proposed to assess the groundwater storage in a typical mine area, Tugela in South Africa. In the area, five aquifers (Natal Group, Coastal plain deposits, Basement aquifer, Ecca Group and Dwyka Group) have better harvest potential compared with others on the basis of borehole yield. The study area was divided into four subareas (A, B, C and F) based on proposed mining boundaries. Both exploitable and sustainable groundwater storage were estimated. The estimated exploitable groundwater storage for subareas A, B, C and F are 20.66, 5.78, 43.12, 36.90 Mm3, respectively, on the basis of current median exploitation depths of each aquifer or geological formation. The calculated sustainable groundwater storage for subareas A, B, C and F are 3.31, 0.89, 6.67 and 6.01 Mm3, respectively, with a total of 16.88 Mm3. Groundwater recharge of the subareas was also estimated for subareas A, B, C and F as 31.92, 11.44, 43.38 and 29.78 Mm3/annum, respectively, with a total of 116.53 Mm3/annum. The assessment method can be applied to other areas with similar hydrogeological settings with the available datasets.

Keywords

Groundwater storage Sustainability Groundwater recharge/water budget South Africa Sub-Saharan Africa 

Une évaluation des réserves en eau souterraine dans la région de Tugela, Afrique du sud

Résumé

L’évaluation des réserves en eau est une composante importante d’études de faisabilité dans des régions d’intérêt minier. Tel que demandé dans les lois nationales sur les ressources minérales et l’environnement, l’évaluation peut inclure les réserves exploitables, correspondant à la quantité d’eau souterraine emmagasinée dans la tranche de profondeur exploitable de l’aquifère, et les ressources durables, correspondant à l’eau souterraine qui peut être exploitée sans générer de problèmes environnementaux et économiques inacceptables. Une méthode simplifiée est proposée pour évaluer les réserves en eau souterraine dans la zone minière typique de Tugela en Afrique du Sud. Dans cette zone, cinq aquifères (Groupe de Natal, les dépôts de la plaine côtière, aquifère de base, groupe d’Ecca et groupe de Dwyka) montrent le meilleur potentiel pour l’exploitation en comparaison des autres aquifères, sur la base de la productivité des forages. La zone d’étude a été divisée en quatre secteurs (A, B, C et F) sur la base des limites proposées pour l’activité minière. Les réserves en eau souterraine exploitables et durables ont été toutes deux estimées. Les réserves exploitables pour les quatre secteurs A, B, C et F sont respectivement 20.66, 5.78, 43.12, 36.90 Mm3, sur la base des profondeurs médianes des exploitations en cours dans chaque aquifère ou formation géologique. Les réserves durables pour les secteurs A, B, C et F sont respectivement 3.31, 0.89, 6.67 and 6.01 Mm3, pour un volume total de 16.88 Mm3. La recharge des aquifères pour les quatre secteurs A, B, C et F a également été estimée à respectivement 31.92, 11.44, 43.38 and 29.78 Mm3/an, soit une recharge totale de 116.53 Mm3/an. La méthode d’évaluation utilisée peut être appliquée à d’autres régions ayant des conditions hydrogéologiques similaires avec des données disponibles.

Una evaluación característica del almacenamiento de agua subterránea en el área de Tugela, Sudáfrica

Resumen

La evaluación del almacenamiento de agua es un componente importante en los estudios de factibilidad para la prospección de áreas mineras. Como lo exigen los recursos minerales nacionales y las leyes ambientales, esto debe incluir la evaluación de almacenamientos tanto explotables como sostenibles; el primero se relaciona con la cantidad de agua subterránea almacenada dentro de la profundidad del acuífero explotable y el segundo se define como el agua subterránea que se puede extraer de manera sostenible sin producir inaceptables problemas ambientales y económicos. Se propone un método simplificado para evaluar el almacenamiento de agua subterránea en un área de un área minera típica: Tugela en Sudáfrica. En el área, cinco acuíferos (grupo Natal, depósitos de planicies costeras, acuífero del basamento, grupo Ecca y grupo Dwyka) tienen un mejor potencial de extracción en comparación con otros en base al rendimiento de los pozos. El área de estudio se dividió en cuatro subáreas (A, B, C y F) en función de los límites mineros propuestos. Se estimaron tanto el almacenamiento de agua subterránea explotable como el sostenible. El almacenamiento de agua subterránea explotable estimado para las subáreas A, B, C y F es 20.66, 5.78, 43.12, 36.90 Mm3, respectivamente, sobre la base de las profundidades medias actuales de explotación de cada acuífero o formación geológica. El almacenamiento de agua subterránea sostenible calculado para las subáreas A, B, C y F es 3.31, 0.89, 6.67 y 6.01 Mm3, respectivamente, con un total de 16.88 Mm3. La recarga de agua subterránea de las subáreas también se estimó para las subáreas A, B, C y F como 31.92, 11.44, 43.38 y 29.78 Mm3/año, respectivamente, con un total de 116.53 Mm3/año. El método de evaluación se puede aplicar a otras áreas con configuraciones hidrogeológicas similares con los conjuntos de datos disponibles.

南非图盖拉(Tugela)地区典型地下水储量评估

摘要

地下水储水量评估是新矿区可行性研究的重要组成部分。根据南非矿产资源和环境法案的要求,这项评估包括两部分:可开采储量及可持续储量。前者定义为可开采含水层深度内储存的地下水量,后者定义为在不会造成环境和经济问题的情况下,含水层内可供持续开采的地下水量。本文以南非的图盖拉地区为例,提出了一种简化的方法来评估一个矿区的地下水储量。根据钻孔资料,在该地区有五个含水层(Natal组,沿海平原沉积,基底岩,Ecca组和Dwyka组)的产水量较高。根据拟定的采矿边界,研究区域分为四个子区域(A,B,C和F)。分别在四个子区域对上述两种地下水储量进行评估。根据目前每个含水层的中位开采深度,分区A,B,C和F的可开采地下水储量估算为20.66, 5.78, 43.12, 36.90 Mm3,总计106.46Mm3。分区A,B,C和F的可持续地下水储量估算为3.31, 0.89, 6.67和6.01 Mm3,总计16.88 Mm3。分区A,B,C和F的地下水补给量也分别估算为31.92, 11.44, 43.38和29.78 Mm3 /年,总计116.53 Mm3 /年。此评估方法可应用于具有相似数据来源及水文地质条件的其他区域。

Uma avaliação típica de armazenamento de água subterrânea na área de Tugela, África do Sul

Resumo

A avaliação do armazenamento de água é um componente importante dos estudos de viabilidade para áreas de mineração prospectivas. Conforme exigido pelos recursos minerais nacionais e leis ambientais, isso pode incluir a avaliação de armazenamentos exploráveis ​​e sustentáveis; a primeira diz respeito à quantidade de água subterrânea armazenada na profundidade explorável do aquífero e a última é definida como a água subterrânea que pode ser extraída de maneira sustentável sem produzir problemas ambientais e econômicos inaceitáveis. Um método simplificado é proposto para avaliar o armazenamento de água subterrânea em uma área de mina típica - Tugela na África do Sul. Na área, cinco aquíferos (Grupo Natal, Planície Costeira, Aquífero de Base, Grupo Ecca e Grupo Dwyka) apresentam melhor potencial de produção em comparação com outros, com base no rendimento do poço. A área de estudo foi dividida em quatro subáreas (A, B, C e F) com base nos limites de mineração propostos. Tanto o armazenamento de água subterrânea explorável como sustentável foi estimado. O armazenamento estimado de água subterrânea explorável para as subáreas A, B, C e F é de 20.66, 5.78, 43.12, 36.90 Mm3, respectivamente, com base na profundidade de exploração média atual de cada aquífero ou formação geológica. O armazenamento sustentável de águas subterrâneas calculado para as subáreas A, B, C e F é 3.31, 0.89, 6.67 e 6.01 Mm3, respectivamente, com um total de 16,88 Mm3. A recarga de águas subterrâneas das subáreas foi também estimada para as subzonas A, B, C e F em 31.92, 11.44, 43.38 e 29.78 Mm3/ano, respectivamente, com um total de 116.53 Mm3/ano. O método de avaliação pode ser aplicado a outras áreas com configurações hidrogeológicas semelhantes aos conjuntos de dados disponíveis.

Introduction

Water is perhaps South Africa’s most critical resource, one of low abundance and growing needs. The country is located in an arid to semi-arid region where less than 10% of rainfall is available as surface water, and groundwater resources are equally limited (DWA 2011). As the use of surface water has reached the upper limit of the resource in this country, it is hence important to have a better understanding of the status quo of groundwater resources and their availability. According to relevant governmental Acts in mineral resources, water and environment management, in any new developing area, assessment of water resources availability needs to be carried out compulsorily, of which the issues of groundwater storage, recharge and sustainable yield are three key components in determining groundwater availability (DWA 2013).

Many scientific efforts have been made to the study of groundwater recharge, discharge and associated sustainability (ASCE 1972; Bredehoeft 1997; Xu et al. 2003; Sophocleous and Devlin 2004; Alley and Leake 2004; Zhou 2009). Very limited work has been done in terms of the determination of groundwater storage since its concept and methods were proposed by Pulotenikov (1959). Groundwater storage is the basis of sustainable development which is presented as the volume of water stored in an aquifer or aquifer system with the dimension of liters (L3). It is accumulated in a long-term dynamic process of the interaction of recharge, flow, and discharge. In the long run, when the balance between groundwater recharge and discharge stays relatively stable, the account of groundwater storage is also stable. However, if the recharge varies periodically, for example, with the seasonal variation of rainfall, the balance will be disturbed and the storage and discharge will change accordingly. For some aquifers such as fractured porous and dolomitic aquifers and alluvial deposits, while the amount of recharge is less than expected, part of the storage will be released to satisfy the groundwater discharge including natural discharge and artificial abstraction in a specific area. On the other hand, excessive recharge will enhance storage. Generally, storage can be regarded as a buffer zone in the hydrogeological cycle to adjust between recharge and discharge and, therefore, has much significance in evaluating groundwater resources.

In South Africa, research on the quantification of groundwater resources on a regional scale has come a long way since 1980s. For example, the publication Groundwater Resources of South Africa, consisting of a report and maps as the output of the first national hydrogeological mapping program, was published by the Water Resource Commission (Vegter 1995). The maps present South Africa’s groundwater resources with seven components, including saturated indices, depth of groundwater level, mean annual groundwater recharge, groundwater component of river flow, groundwater quality and hydrochemical types. In 1998, the Groundwater Harvest Potential map of South Africa was published (Baron et al. 1998), which estimated the sustainable rates of groundwater abstraction in South Africa on a country-wide basis. Regional estimates of storage and recharge were used to calculate this sustainable yield (Woodford et al. 2009). Based on all the previous work, Directorate: Geohydrology, Department of Water Affairs (DWA) initiated the development of a series of hydrogeological maps on a scale of 1:500,000 and the work was completed in 2003 (Woodford and Rosewarne, 2006). The produced Hydrogeological Map Series is regarded as the first full set of regional hydrogeological information of South Africa. However, no groundwater resource quantification was carried out in Groundwater Resources Assessment project phase 1 (GRA1). The Groundwater Resources Assessment project phase 2 (GRA2) began in 2003 with the aim to quantify aquifer storage, recharge, baseflow and the groundwater reserve, which was completed in 2005. The outputs of GRA2 include a country-wide 1 km × 1 km grid showing average groundwater storage and a country-wide 1 km × 1 km grid showing current groundwater storage based on the previous year’s input data (especially recharge and abstraction). The methodology is described in a series of DWA reports (Witthuser et al. 2009).

Groundwater resource quantification on a national scale has many benefits by providing:
  • an general overview of the groundwater storage/availability in the country;

  • a baseline for any groundwater research or supply project; and

  • ready-to-use datasets if there are no requirements on resolution of data.

However, a local-scale study, such as groundwater quantification for an urban or a mining area, requires a more accurate and applicable assessment. For example, in GRA2, all results have the resolution of 1 km × 1 km, which is apparently low for a local or site study and ranges from well-field scale to several thousand square kilometres. Some datasets have been developed from previous project results so that specific boundaries i.e. quaternary drainage and national groundwater regions, were used to derive the average data such as aquifer thickness and storativity values over the bounded area. However in a small scale study, a geological/hydrogeological boundary is relatively easy to identify and provides more accurate estimates when assessing aquifer properties. According to statistics of National Groundwater Archive (NGA) boreholes, the average aquifer thickness used in GRA2 in South Africa is 154 m, which is largely biased by deep exploration boreholes and is much higher than 90% of the borehole depths in South Africa, thus not applicable for most groundwater supplying projects. Based on all above, the groundwater assessment results on a national scale cannot be directly used and it is necessary to develop an easy and applicable way for groundwater quantification at a local scale.

This paper selects the Tugela new mining area as a typical study area and attempts to propose a convenient method to evaluate groundwater storage at a local scale. Geological and hydrogeological settings in the study area are discussed, on which the aquifers are classified. Two forms of groundwater storage, exploitable storage and sustainable storage, are defined and calculated for each aquifer using geological information and borehole data from NGA.

Materials and methods

Study area

The selected study area, with a size of 5621 km2, is located in the central-eastern part of KwaZulu-Natal Province mostly along the Tugela River catchment. It has been identified as one of the new mining areas for heavy minerals and gems. Based on proposed mining boundaries, the area was divided into four subareas, i.e. A, B, C and F, as shown in Fig. 1. It comprises two primary drainage basins (V and W), six secondary basins, seven tertiary and over 60 quaternary basins (Middleton and Bailey 2008). The western part of the area (subareas A and B) includes the low-lying wide Tugela River Valley, the high-elevation forest (the Qudeni Forest Reserve), as well as a forested area to the north. Subareas C and F in the east include the drainage areas of the Mhlatuze, Mlalazi and Matigulu Rivers. The elevation of the study area reaches around 1800 m above sea level in the Middle Veld area from where the topography slopes down to the Indian Ocean (Fig. 2). The lower reaches of the Tugela River and its tributaries constitute the major river system in this area. It originates from the high-lying Drakensberg Mountains and drains eastwards to the Indian Ocean. Other rivers in the area include the Mhlatuze, Mlalazi and Matigulu Rivers.
Fig. 1

a, b Location and c digital elevation model of the Tugela area, South Africa

Fig. 2

Simplified geology for the demarcation of aquifer systems in the Tugela area

Mean annual precipitation (MAP) in this area ranges from 700 to 1200 mm, which is much higher than average annual rainfall in South Africa (450 mm). Approximately 75% of the rainfall takes place during the summer months from October to March, while only 25% occurs in winter months. High annual rainfall (>1000 mm) mostly occurs in the coastal area in subareas of C and F, and in the mountainous areas in A and B as well. Mean annual evaporation (MAE) ranges from 1200 to 1500 mm from the south to the north in the study area.

Geologically, the area is located in the Natal sector of the Namaqua-Natal Belt, which is subdivided into three tectonostratigraphic terranes, from north to south, the Tugela Terrane, the Mzumbe Terrane and the Margate Terrane, and this subdivision is based on different rock types and metamorphic grades (Cornell et al. 2006). The study area falls in the Tugela Terrane. Several stages of deformation and metamorphism have taken place in the Tugela Terrane (Cornell et al. 2006), resulting in the current complex dense distribution of structures as shown in Fig. 2. The lithology and geological structure of the area have been studied in detail by many researchers (Du Toit 1954; Ryan and Whitfield 1978; McCathy and Rubidge 2005; Cornell et al. 2006; Voordouw and Rajesh 2012; Botha and Singh 2012; Van der Walt 2012; Hicks et al. 2014), which forms the basis of aquifer classification and delineation, which will be discussed later. According to these results, the main stratigraphy underlying the area includes Archaean granite and gneiss Suite, Nsuze Group of the Pongola Supergroup, Namaqua and Natal Metamorphic Provinces, Natal Group of Cape Supergroup, Dwyka and Ecca Groups of Karoo Supergroup, Drakensberg and Lebombo Groups and Quaternary deposits (Algoa Group) (Fig. 2).

Regional hydrogeology

The whole study area receives higher precipitation than the average in South Africa and rainfall is strongly topographically related, with the elevated area experiencing higher rainfall than the low-lying areas, except along the coast. Groundwater flow generally follows topography according to the groundwater level contour created from NGA boreholes.

Hard-rock secondary porosity aquifers are dominant in the study area, including Natal Group sandstones, Karoo Supergroup sediments, basement rocks and metamorphic rocks. Faults, joints and Karoo dolerite intrusions in these rocks may locally increase the groundwater occurrence. Primary aquifers mainly present along the coast with a thickness up to about 30 to 50 m and also along the river valley for around 5 m. According to the borehole yield analysis, Dwyka tillite in Karoo Supergroup and Namaqualand-Natal metamorphic rocks are the poorest aquifers while Natal Group is the best aquifer in terms of yield.

According to DWAF (2004), groundwater recharge in the area varies from 1 to 5% of the MAP with a better rate found in high-rainfall parts. Groundwater quality in the study area is generally good, also with the best quality groundwater found in the higher-rainfall areas. By 2004, groundwater usage in the Tugela Water Management Area is 100 m3/km2/a on average which is only some 0.4% of the mean annual recharge over the area. It was suggested that the groundwater usage could be safely increased cautiously without adverse impact on the resource.

Aquifer delineation

Based on geology, the major aquifers in the area are Natal Group Aquifer, Coastal Plain Aquifer, Karoo Supergroup Aquifer,Basement Aquifer and Metamorphic Rock Aquifer, described as follows.

Natal Group aquifer

As shown in Fig. 3, the Natal Group covers around 642 km2 or 11.8% of the Tugela area and mostly occurs in subarea C. Most of sediments of the Natal Group are fluviatile and were deposited by an extensive braided river system in a northeast–southwest-trending lowland trough or rift-basin, resulting in a sandstone sequence with the thickness of between 500 and 600 m (Demlie and Titus 2015).
Fig. 3

Groundwater (GW) occurrence and aquifer types of the Tugela area (after Vegter 2001)

Natal Group is very similar to Table Mountain Group (a regional fractured aquifer with high productivity in South Africa) in deposition and lithology, and is often interpreted as part of the Cape Super Group in the Kwazulu-Natal province (de Beer 2002; Knight and Grab 2016). There have been few research projects on the hydrogeology of Natal Group sandstones. However, well-field studies have shown that the Natal Group sandstone can be a very productive aquifer with high borehole yield and low chances of dry boreholes (Demlie and Titus 2015). In addition, the extensive faulting and fracturing has largely increased the permeability and storativity of the sandstone, making it a reliable fractured aquifer. A previous analysis show that the borehole yield ranges from 0.17 to 25 L/s with a median yield of about 3 L/s, with groundwater recharge ranging from 25 to 130 mm per annum (Demlie and Titus 2015). The storativity of the Natal Group sandstone has been reported by Bell and Maud (2000) as of 0.001 and transmissivity can vary from 1.5 to 60 m2/s.

Coastal plain aquifer

The Cenozoic deposits of Algoa Group consist of sediments, sand and Quaternary calcrete rocks, distributing along streams and over the coastal area. These rocks cover 255 km2 or 4.7% of the study area and are mainly exposed in the coastal area. These unconsolidated primary aquifers are characterised as follows: 1) the topography is flat with highly permeable sand/soils so that the recharge can take place quickly after raining (Kelbe et al. 2001); 2) recharge of groundwater falls in the range from 5% and 18% of mean annual rainfall (EMATEK-CSIR 1995); 3) the shallow primary aquifer has a maximum depth of about 30 m (Kelbe and Germishuyse 2010); and 4) storativity of the aquifer system can be up to 0.1 (Botha and Singh 2012). However, there are some risks related to the development of the coastal aquifer, including salt water intrusion, climate change and groundwater vulnerability to pollution.

Karoo Supergroup aquifer

Natal Group is unconformably overlain by the Late Carboniferous to Early Permian Dwyka Group and the Permian Ecca Group of the Karoo Supergroup (Demlie and Titus 2015). The Ecca Group sediments reach an estimated maximum thickness of 3000 and 1200 m in the Karoo and Natal troughs respectively (Ryan and Whitfield 1978). Arenite, shale and coal seams of the Vryheid and Volksrust formations of Ecca Group, and tillites, mudstones and shales of the Dwyka Group, are the major Karoo sediments present in the area, while the Emakwezini Formation of the Beaufort Group comprising mudstone, shale and arenite, has a very limited exposure in the study area.

The Dwyka and Ecca Groups collectively cover 1672 km2 or 30.7% of the study area, mainly occurring in subarea A and the western part of B and having very sparse exposure in C and F. Covering around two thirds of the country, the shallow aquifer (<300 m depth) of the Karoo Supergroup has received extensive detailed groundwater research and exploration in the last several decades (Rosewarne et al. 2013). The Karoo basin is characterised by fractured rock aquifers with numerous dolerite intrusions (Woodford and Chevallier 2002). It was also observed that the primary porosity of Karoo sediments is higher in the weathered part of the Karoo Aquifer (≤30 m) and decreases with depth, while the secondary porosity may develop at depth where breccia plugs, fractures or dolerite intrusions occur (Wooford and Chevallier, 2002). The major drilling target for water supply currently is still the weathered part of the Karoo Aquifer, while 90% of the boreholes have depth less than 50 m. Because of the highly heterogeneous nature of the aquifer, borehole yield in the Karoo Aquifer may vary greatly from site to site, in the range of 0 (dry) to >15 L/s, but generally lower than 2 L/s. Storativity values between 10−3 to 10−6 have been estimated and used for resource estimation (Wooford and Chevallier, 2002).

Basement aquifer

Crystalline basement rocks are extensively exposed in South Africa, constituting 15% of the total surface area. In the study area, the basement rocks are mainly granite, gneiss and greenstone. Weathered or fractured basement rocks can be very important aquifers for areas of high population density with few or no alternative water sources (Holland 2011). The top of the weathered bedrock has sufficient permeability to support successful boreholes for small-scale village water supply (Chilton and Foster 1995; Reboucas 1993). The fractured basement rock is characterised by high transmissivity and low storativity values. Basement aquifer occurs in subarea A, C and F in the study area.

Metamorphic rock aquifer

Metamorphic rocks in the area, mainly the Tugela Group, are classified as intergranular and fractured aquifers in the groundwater occurrence map. They generally show very low productivity according to borehole yield statistics. However metamorphic rocks are extensively exposed and their groundwater yield cannot be simply ignored. In the western part of the study area, the groundwater yield of the Tugela Group is predominantly 0.1–0.5 L/s and in the eastern part of the study area, it is 0.5–2.0 L/s.

Groundwater occurrence

The study area comprises the following five groundwater regions, Kwazulu-Natal coastal foreland, Northeastern Middleveld, Southern Lebombo, Northwestern Middleveld and Northern Zululand coastal plain (Vegter 2001). The principal water-bearing rocks varied in each region in the study area. The groundwater occurrence map shows that aquifer types in the study area are mostly fractured rock (Type b) and intergranular and fractured rock (dual medium, Type d). Intergranular (Type a) aquifer type occurs along the coast in the subarea F (Vegter 2001). According to the groundwater occurrence map, relatively productive aquifers with yield of 0.5–2.0 L/s include the fractured rock of the Natal Group and intergranular rock of the Cenozoic and Quaternary deposits along the coast; this delineation is clearly reflected by the borehole yield data. Metamorphic rocks of the Tugela Group in subarea F are also marked as intergranular and fractured rock with good yield (Type d3) but the low borehole yield does not support this classification. Metamorphic rocks and Karoo sedimentary rocks in subareas A and B are mainly classified as Type b2 and d2 in the same low yield range (0.1–0.5 L/s). Fig. 3 shows the groundwater occurrence and aquifer types of the study area extracted from the national groundwater occurrence map (Vegter 2001).

Borehole yield and aquifer ranking

For the Tugela are, 1362 groundwater boreholes (BH) were extracted from the NGA. The depths of 969 of the boreholes are recorded as ranging from 1 m to 200 m. Of the boreholes, 95% are shallower than 150 m and one third are shallower than 50 m. Yield data are available for 571 boreholes, ranging from 0.01 L/s up to 25 L/s. Around 58.1% of the boreholes are dry, 13.4% of the boreholes have yield higher than 1 L/s and 28.5% produce less than 1 L/s.

To evaluate the performance of each aquifer in the study area, the geology information was appended to the borehole data using the spatial join function in ArcGIS. The borehole yields were then reclassified by aquifers. In Table 1, the percentage of medium to high yield boreholes (> = 1 L/s) were calculated for each aquifer and aquifer performance were ranked based on the results. In Table 2, the density of medium to high yield boreholes (> = 1 L/s) was calculated for each geological formation and aquifer performances were ranked based on the calculation results.
Table 1

Ranking of aquifer performance based on yield

Aquifer

Total no. of BHs

No. of BHs with yield

> = 1 L/s

No. of BHs with yield

<1 L/s

% of BHs with yield

> = 1 L/s

Ranking

Natal Group

231

64

167

27.71%

1

Coastal deposits

132

23

109

17.42%

2

Karoo Supergroup

335

50

285

14.92%

3

Basement rocks

164

23

141

14.02%

4

Metamorphic rocks

407

19

388

4.67%

5

Other

93

4

89

4.30%

6

Sum

1362

183

1179

13.44%

(Percentage of boreholes (BHs) with yield > = 1 L/s)

Table 2

Ranking of aquifer performance based on yield

Aquifer

Total no. of BHs

No. of BHs

with yield > = 1 L/s

No. of BHs with yield

<1 L/s

Area

(km2)

No. of BHs with yield > = 1 L/s,

per 100 km2

Ranking

Natal Group

231

64

167

641.69

9.97

1

Coastal deposits

132

23

109

255.53

9.00

2

Basement rocks

164

23

141

558.48

4.12

3

Karoo Supergroup

335

50

285

1676.3

2.98

4

Other

93

4

89

362.62

1.10

5

Metamorphic rocks

407

19

388

2126.58

0.89

6

Sum

1362

183

1179

5621.2

3.26

(Borehole density with yield > = 1 L/s per 100 km2)

In Table 1 and Table 2, the Natal Group aquifer is ranked as the most productive aquifer by criteria “percentage” and “density of high yield boreholes”. Coastal plain aquifer type ranks the second in both tables. Karoo aquifer and basement aquifer/rocks have similar results and rank the third and fourth in Table 1, respectively, and their ranking sequence flips in Table 2. The metamorphic and igneous rocks in the Namaqualand-Natal Metamorphic Province, Pongola Group, Drankensberg Group and Lebombo Group have very low aquifer performance in the study area according to Table 1 and Table 2.

Groundwater storage

Groundwater storage is simply the volume of groundwater stored in aquifer systems. In GRA2, a detailed methodology was developed for estimating the thickness of various aquifer zones or groundwater levels and storativity values required for determining the groundwater storage in an aquifer system(s) on a national scale. Dy defines two crucial aquifer levels, and two corresponding groundwater storage types were proposed: the “weathered-jointed” or WZ type and (2) the underlying, “fractured” or FZ aquifer zone. Borehole data in NGA and datasets from previous projects were extensively used to carry out the assessment and results were presented for each groundwater region.

To estimate groundwater storage, the geometry, volume and storativity of the aquifers need to be evaluated first. Furthermore, the condition of groundwater recharge/discharge and their dynamic characteristics should be well understood in that they have a close relationship with storage. It is generally acknowledged that static groundwater storage is usually calculated by multiplying the aquifer volume by the aquifer storativity (Freeze and Cherry 1979), which can be expressed as
$$ Q=\mu \cdot V\ \left( Unconfined\ aquifer\right)\ or\ Q=F\cdot S\cdot h\ \left( Confined\ aquifer\right) $$
(1)
where Q is the groundwater storage quantity, μ the specific yield of the unconfined aquifer, V the volume of the aquifer, F the area of the confined aquifer, S the storage coefficient of the confined aquifer and h the pressure head of the confined aquifer.

In the present study, to make the groundwater assessment more meaningful and practical, two types of groundwater storage were proposed and estimated, exploitable groundwater storage and sustainable groundwater storage. Exploitable storage is defined as the amount of groundwater stored within the currently exploitable depth of aquifer. The “currently exploitable depth” had been under consideration and discussion and was finally determined to be the median aquifer thickness of the aquifer that can be obtained from borehole data in NGA (National Groundwater Archive). The choice of using median aquifer thickness from existing or historical boreholes other than a full aquifer thickness is because this study intends to provide a practically exploitable aquifer storage value over a theoretically or technically exploitable storage value. Average thickness was not used because the result may be considerably skewed by the existing of a few exploration boreholes. The steps to calculate the median aquifer thickness are given in the next section.

The Groundwater Resource Potential datasets are similar to the Department of Water Affairs and Forestry’s (DWAF) Harvest Potential coverage in that they provide estimates of the maximum volumes of groundwater that are potentially available for abstraction on a sustainable basis, and only take into consideration the volumes of water held in aquifer storage and the recharge from rainfall. The feasibility of abstracting this water is limited by many factors due mainly to the physical attributes of a particular aquifer system and economic and/or environmental considerations.

Sustainable groundwater storage is defined as the groundwater that can be extracted in a sustainable manner without producing unacceptable negative effects to the environment (Alley et al. 1999). This definition is largely equivalent to the Utilisable Groundwater Exploitation Potential (UGEP) in GRA2. It takes into consideration not only the volumes of water physically stored in an aquifer and the recharge from rainfall, but also the feasibility of abstracting this water such as the physical attributes of a particular aquifer system, and economic and/or environmental considerations (DWA 2006). Conrad and Van De Voort (1999) also proposed a similar concept of Sustainable Utilisable Potential (SUP) which considers the Reserve when determining the volumes of groundwater available for development. The SUP was defined as the “volume of groundwater that can be abstracted on a sustainable basis after the requirements of the Reserve have been met” while the Reserve, as stipulated in the National Water Act (1998), comprises two basic components, namely water requirements for Basic Human Needs and the ecosystem. The concept and average value of maximum allowable groundwater-level drawdown as a management constraint is adopted and slightly modified in this study from GRA2. According to Appendix 1 in the GRA2 report (DWA 2006), maximum allowable groundwater-level drawdown in most of the quaternary catchments in the study area has been determined as 2 m. Therefore, groundwater stored in the top 2 m of aquifer is considered as the sustainable groundwater storage.

Based on these definitions, both exploitable and sustainable groundwater storage were estimated in subareas A, B, C and F of the study area.

Results and discussion

Aquifer volume

Aquifer volume can be estimated by aquifer area and thickness. The Tugela area is divided into four sub-areas (A, B, C and F) based on prospective mining, and groundwater storage evaluation is also based on these four sub-areas. The surface area for each aquifer is captured in ArcGIS in each subarea (Table 3).
Table 3

Aquifer area in each subarea in the Tugela area

Aquifer

Subarea A

Subarea B

Subarea C

Subarea F

Total (km2)

Natal Group

69.95

52.02

519.72

0

641.69

Coastal deposits

0

0.9

9.65

244.98

255.53

Basement rocks

157

0

221.62

179.86

558.48

Karoo

Supergroup

Ecca

653.14

59.42

103.67

129.32

945.55

Dwyka

564.99

67.87

74.3

23.59

730.75

Metamorphic rocks

318.84

833.66

563.49

410.59

2126.58

Other

231.54

15.66

0

115.41

362.61

Sum

1995.46

1029.53

1492.45

1103.75

5621.19

Borehole data such as borehole depth and depth to water table in NGA were used to generate the aquifer thickness for this study. To simplify the process, a single aquifer is assumed to have the same saturated thickness over the study area. It is expected that the inaccuracy induced from this assumption can be partly counterbalanced by the use of median values of water levels, borehole depths and aquifer thicknesses. All boreholes were grouped by their target aquifers, and the median borehole depth and depth to water table were calculated from the boreholes associated with each type of aquifer. The median aquifer thickness for each type of aquifer was then calculated by subtracting the median depth to water table from median borehole depth. The resultant values have been compared with the GRA2 data and it is found that this estimation is generally much lower than the aquifer thickness (weathered thickness + fractured thickness), which is between 100 m and 200 m in GRA2.

To consider the decrease of storativity over the vertical depth of the aquifer due to the extent of weathering, the aquifer thicknesses are divided into top (the most weathered), middle and bottom thickness for each aquifer. To simplify the process, the top and middle thicknesses are fixed as 10 m, as showed in Table 4. This division necessarily avoided the complicated functions to calculate the scale effect of storativity over depth and it is more acceptable than an over-simplified average storativity value for the whole thickness. This method of division is not compatible with the weathered thickness (WZ) and fractured thickness (FZ) in GRA2 in most areas, as the total thickness adopted in GRA2 in the study area is dominantly greater than 150 m, which is far above the exploitable depth of the aquifer in the study area according to the borehole depth statistics held in NGA.
Table 4

Median aquifer thickness for each type aquifer in Tugela area (unit: m)

Aquifer

Median depth to water table

Median

borehole depth

Median aquifer thickness

Top thickness

Middle thickness

Bottom thickness

Natal Group

21

96

75

10

10

55

Coastal deposits

16

43

27

10

10

7

Basement rocks

19.5

51

31.5

10

10

11.5

Karoo Supergroup

Ecca

18

60

31

10

10

11

Dwyka

30

77

47

10

10

27

Metamorphic rocks

26

100

74

10

10

54

Other

8

72

64

10

10

44

Sum

21

72

51

10

10

31

Storativity

Storativity values for each aquifer have been summarized from previous publications or reports together with analysis based on aquifer performance reflected by borehole yield.

In the Durban Brochure of groundwater maps published by DWA (1998), the storativity of Natal Group and the Dwyka Group is estimated between 0.0001 and 0.005. With a combined consideration of the aquifer ranking in Table 1 and Table 2, the storativity value of 0.005 is assigned to the top 10 m of Natal Group aquifer and the value of 0.0005 is assigned to the top 10 m of Dwyka aquifer.

Quaternary sand in the Richards Bay area has been reported to have a storativity value of 0.01 (Germishuyse 1999) and this value is assigned to the top 10 m of the Coastal plain aquifer in the sub-area F.

Then a coarse interpolation was done to obtain the storativity values of the Basement aquifer and Ecca Group Aquifer based on the aquifer performance ranking in this study. The storativity values of 0.002 and 0.001 are assigned to the top 10 m of Basement aquifer and Ecca Group aquifer, respectively.

Though the metamorphic rocks associated with the Namaqualand-Natal province and other geological formations in the study area have very low productivity (Table 1 and Table 2), they collectively cover approximately 2500 km2 of the surface area. There are 500 boreholes, or around 37% of the boreholes drilled on these formations. Therefore, groundwater storage in these formations should also be estimated and a low storativity value of 0.0001 is assigned to these formations.

Aquifer storativity decreases as the degree of weathering decreases with depth and pressure increases with depth. Exponential functions have been widely used to determine the storativity along depth for a specific aquifer. However, it is not always easy to define the parameters/coefficients in these functions, especially in the study area with a variety of aquifers. Therefore, for the convenience of calculation, one assumes that the storativity value for the middle thickness is one half that of the top thickness of the Coastal plain aquifer and one fifth for other aquifers. Then the storativity value for the bottom thickness is one fifth of the middle thickness for the Coastal plain Aquifer and 1/10 of the middle thickness for other aquifers, as shown in Table 5.
Table 5

Storativity values used for each type of aquifer in Tugela area

Aquifer

Top thickness storativity

Middle thickness storativity

Bottom thickness storativity

Natal Group

5.00E-03

1.00E-03

1.00E-04

Coastal deposits

1.00E-02

2.00E-03

4.00E-04

Basement rocks

2.00E-03

4.00E-04

4.00E-05

Karoo Supergroup

Ecca

1.00E-03

2.00E-04

2.00E-05

Dwyka

5.00E-04

1.00E-04

1.00E-05

Metamorphic rocks

1.00E-04

2.00E-05

2.00E-06

Other

1.00E-04

2.00E-05

2.00E-06

Exploitable groundwater storage

Based on the previous definition and analysis, exploitable groundwater storage is calculated by multiplying exploitable aquifer volume and storativity. Exploitable groundwater storage was estimated for subareas A, B, C and F in the Tugela area (Fig. 1), as shown in Tables 6, 7, 8 and 9. The estimated exploitable groundwater storages are 20.66 million cubic meters (Mm3) in subarea A, 5.78 Mm3 in subarea B, 43.12 Mm3 in subarea C and 36.90 Mm3 in subarea F, with a total of 106.46 Mm3 in the study area.
Table 6

Exploitable groundwater storage in Subarea A

Aquifer

Subarea

(km2)

Top thickness

storage

Middle thickness

storage

Bottom thickness

storage

Total storage (Mm3)

Natal Group

69.95

3.50

0.70

0.38

4.58

Coastal deposits

0

0.00

0.00

0.00

0.00

Basement rocks

157

3.14

0.63

0.07

3.84

Karoo Supergroup

Ecca

653.14

6.53

1.31

0.14

7.98

Dwyka

564.99

2.82

0.56

0.15

3.54

Metamorphic rocks

318.84

0.32

0.06

0.03

0.42

Other

231.54

0.23

0.05

0.02

0.30

Sum

1995.46

16.54

3.31

0.81

20.66

(Unit: Mm3 )

Table 7

Exploitable groundwater storage in Subarea B

Aquifer

Subarea B

(km2)

Top thickness

storage

Middle thickness

storage

Bottom thickness

storage

Total storage (Mm3)

Natal Group

52.02

2.60

0.52

0.29

3.41

Coastal deposits

0.9

0.09

0.02

0.00

0.11

Basement rocks

0

0.00

0.00

0.00

0.00

Karoo Supergroup

Ecca

0.59

0.12

0.01

0.73

0.86

Dwyka

0.34

0.07

0.02

0.43

0.52

Metamorphic rocks

833.66

0.83

0.17

0.09

1.09

Other

15.66

0.02

0.00

0.00

0.02

Sum

1029.53

4.47

0.89

0.41

5.78

Table 8

Exploitable groundwater storage in Subarea C

Aquifer

Subarea C

(km2)

Top thickness

storage

Middle thickness

storage

Bottom thickness

storage

Total storage (Mm3)

Natal Group

519.72

25.99

5.20

2.86

34.04

Coastal deposits

9.65

0.97

0.19

0.03

1.19

Basement rocks

221.62

4.43

0.89

0.10

5.42

Karoo Supergroup

Ecca

1.04

0.21

0.02

1.27

1.50

Dwyka

0.37

0.07

0.02

0.47

0.56

Metamorphic rocks

563.49

0.56

0.11

0.06

0.73

Other

0

0.00

0.00

0.00

0.00

Sum

1492.45

33.36

6.67

3.09

43.12

Table 9

Exploitable groundwater storage in Subarea F

Aquifer

Subarea F

(km2)

Top thickness

storage

Middle thickness

storage

Bottom thickness

storage

Total storage (Mm3)

Natal Group

0

0.00

0.00

0.00

0.00

Coastal deposits

244.98

24.50

4.90

0.69

30.08

Basement rocks

179.86

3.60

0.72

0.08

4.40

Karoo Supergroup

Ecca

1.29

0.26

0.03

1.58

1.87

Dwyka

0.12

0.02

0.01

0.15

0.18

Metamorphic rocks

410.59

0.41

0.08

0.04

0.54

Other

115.41

0.12

0.02

0.01

0.15

Sum

1103.75

30.03

6.01

0.86

36.90

Sustainable groundwater storage

As discussed, sustainable groundwater storage in the study area is defined as the groundwater stored in the top 2 m of aquifer. The results can be extracted from the exploitable groundwater storage values and equivalent to 3.31 Mm3 in subarea A, 0.89 Mm3 in subarea B, 6.67 Mm3 in subarea C, and 6.01 Mm3 in subarea F, and totally 16.88 Mm3 in the study area. The estimated values in subareas C and F are much higher than the others, because there are more good aquifers such as Natal Group sandstones and Coastal plain aquifer in these two subareas.

Recharge

Groundwater recharge in the study area was roughly estimated. The national Mean Annual Precipitation (MAP: mm) contour map was used as the annual rainfall data. Empirical recharge rate for each aquifer was summarized and modified from previous documents and reports (DWAF 2004; Kirchner 2009; Rosewarne et al. 2013; Titus et al. 2009; Turner 2000; Woodford and Chevallier 2002; Wright 1992; Davies and Partners 1995; Demlie and Titus 2015).

The calculated recharge for subareas A, B, C and F are 31.92, 11.44, 43.38 and 29.78 Mm3 /annum and totally 116.53 Mm3/annum. The recharge rate and yearly recharge volume for each aquifer in each subarea is listed in Table 10.
Table 10

Estimated recharge in the study area and subareas (Unit: Mm3/annum)

Aquifer

Recharge Rate

Annual recharge volume

A

B

C

F

Total

Natal Group

5%

2.80

2.01

24.99

0.00

29.79

Coastal deposits

5%

0.00

0.03

0.39

12.22

12.63

Basement rocks

4%

5.20

0.00

7.25

6.87

19.32

Karoo

Supergroup

Ecca

2.5%

12.90

0.94

2.39

3.13

19.37

Dwyka

1.5%

6.32

0.68

0.96

0.35

8.31

Metamorphic rocks

1.5%

2.92

7.69

7.42

6.15

24.18

Other

1%

1.77

0.10

0.00

1.06

2.93

Sum

N/A

31.92

11.44

43.38

29.78

116.53

Comparison of results

All estimated values in this study are compared with similar assessments in GRA2, such as GEP (Groundwater Exploitable Potential), UGEP (Utilisable Groundwater Exploitable Potential) and Recharge. The national recharge map in the unit of mm/year has been created in GRA2 in two conditions, normal and dry. The tool “Zonal Statistics” in ArcGIS was used to extract these values for the study area. The comparison is listed in Table 11. It can be noted that the GRA2 assessment is generally higher than the estimates in this study except the exploitable groundwater storage in subarea C and F and sustainable groundwater storage in subarea C. Recharge in GRA2 is about five to ten times higher than the recharge estimation in this study.
Table 11

Comparison of results of recharge and storage

Assessment

A

B

C

F

Total

Exploitable groundwater storage (Mm3)

22.66

5.78

43.12

36.90

108.46

GRA2: GEP ((Mm3)

26.58

11.62

34.08

38.06

110.35

Sustainable groundwater storage (Mm3)

3.31

0.89

6.67

6.01

16.88

GRA2: UGEP (Mm3)

24.40

10.

29

31.19

35.73

101.61

Recharge (Mm3/annum)

31.92

11.44

43.38

29.78

116.53

GRA2: Recharge (Mm3/annum)

118.24

48.94

126.42

142.90

436.50

The estimated exploitable groundwater storage is over six times the sustainable storage in the study area, which means only less than one sixth of the groundwater storage can be abstracted on a yearly basis which would not induce any adverse environmental impact. However, the estimation in GRA2 shows a small difference between these two assessments. Groundwater recharge is about 6.9 times more than the sustainable storage.

Limitations

Though easy to understand and apply, the proposed methodology has the following limitations, which can be improved in the future:
  • The aquifer delineation was exclusively based on geological formations, and the impacts of faults, joints and dolerites have not been considered. A more detailed borehole yield analysis can be done using a large-scale geological structure map to study the impact of structures on the aquifer.

  • The division of aquifer thickness is simplified by applying the same thickness (10 m) to the top and middle of the aquifer, which can be improved by studying borehole logs to summarize a more realistic division for each aquifer subject to data availability.

  • Specific functions can be developed to represent the decrease of storativity with depth within each aquifer.

  • The maximum allowed groundwater-level drawdown of 2 m has been adopted from GRA2, which may be modified with respect to local groundwater use conditions.

  • Average recharge rates have been adopted and modified from the DWAF (2004) at aquifer level without considering rainfall magnitude and other factors.

Conclusions

In this study, a simple groundwater storage assessment for the proposed Tugela mining area is presented, with the intention of assessing groundwater storage at a local scale. Borehole data have been extracted from the National Groundwater Archive to analyse the borehole yields. Borehole yield data were further studied in relation to the following geological formations: Natal Group, Coastal deposits, Basement rocks, Ecca and Dwyka Group of Karoo Supergroup, Metamorphic rocks of Namaqua-Natal Province and small areas of Pongola Supergroup, Lebombo Group and Drakensberg Group. Results showed that in the study area, Natal Group Aquifer and Coastal Plain aquifer are the most productive aquifers. The Basement aquifer has a moderate performance, followed by Ecca and Dwyka aquifers. Though covering around 38% of the surface area, the Namaqua-Natal province, mainly Tugela Group in the study area, however, showed very poor performance in terms of borehole yield.

Two types of groundwater storage were estimated in the study area, exploitable groundwater storage and sustainable groundwater storage. Both of the groundwater storage types in the Tugela area were estimated for four subareas, A, B, C and F. The estimated exploitable groundwater storage is 22.66, 5.78, 43.12 and 36.90 Mm3 for subareas A, B, C and F, respectively. This is based on the current median aquifer thickness of 75 m (for the Natal Group aquifer), 27 m (for the Coastal plain aquifer), 31.5 m (for the Basement aquifer), 42 m (for the Ecca Group aquifer), 47 m (for the Dwyka Group aquifer), 74 m (for Namaqua-Natal Province) and 64 m (for the rest geological formations). Assuming that the top 2 m of the aquifer storage can be extracted in a sustainable way on a yearly basis without causing adverse environmental impacts, the sustainable groundwater storage (or groundwater availability) for subareas A, B, C and F is estimated as 3.31, 0.89, 6.67 and 6.01 Mm3 respectively.

Groundwater recharge was estimated in the study area with empirical recharge rate applied to each aquifer. The estimated recharge volumes for subareas A, B, C and F are 31.92, 11.44, 43.38 and 29.78 Mm3/annum and totally 116.53 Mm3/annum.

Results of this work were compared with the national GRA2 assessment. It was found that the GRA storage estimates are generally higher than in this study except for a few estimates.

Notes

Acknowledgements

The Department of Mineral Resources has funded the Mine Water Management Project. This work is under the task: Proactive Solutions. The authors would like to thank all reviewers and editors for their valuable time and input to this manuscript.

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Council for GeosciencePretoriaSouth Africa

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