Department of Earth Sciences

Permanent URI for this community

https://scholar.sun.ac.za/handle/10019.1/380

Browse

Now showing 1 - 10 of 10

36 Chlorine isotope systematics in saline groundwater in the Buffels River Valley
(Stellenbosch : Stellenbosch University, 2018-03) Van Gend, Jani; Miller, Jodie A.; Clarke, Catherine E.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: It is typical for coastal aquifers in arid regions to be affected by salinisation, populations in these areas that rely upon groundwater resources are often directly afflicted by this phenomenon. Large parts of the western coast of South Africa are affected by variably saline groundwater as a result of varying degrees of salinisation, primarily driven by evaporative processes. Initial stable δ2H and δ18O isotopic investigations suggest rainfall and groundwaters carry evaporative signatures, further investigation has suggested that there are other significant salt contributors in the region, although these sources and pathways are poorly constrained. One such alternate salt source may arise from paleo-termite mounds, called heuweltjies, that are found in areas along the west coast. These structures typically consist of salt- and nutrient-rich sediments and the recorded sediment EC values for these structures are an order of magnitude higher than that of the adjacent interheuweltjies. Furthermore, sediment EC values of heuweltjies increase with depth and the difference between deep and shallow EC values in heuweltjies are between 1000 and 3000 μS/cm, with the deeper samples having the higher EC value. The possibility of heuweltjie salts entering the groundwater system and contributing to groundwater salinisation in this area has not been investigated as yet. The town of Buffels Rivier in the western region of the Northern Cape has limited surface water resources and is dependent on local groundwater for subsistence. For this reason, it is an ideal site to investigate these atypical salinisation drivers. In order to effectively isolate the additional salt sources, several geochemical, isotopic and geophysical methods were implemented. Highly variable soil and groundwater EC values were observed in the field area. Furthermore, 36Cl isotope ratios together with noble gas data provides some evidence that aquifer mixing occurs. This is further evidenced by the spatial discontinuity in groundwater 87Sr/86Sr ratios. The geology of this area consist of basement granite gneisses and the contribution of rock salt to the groundwater cannot be excluded. 87Sr/86Sr ratios in groundwater in the Buffels River Valley are elevated, ranging between 0.73030 and 0.78240, which is typically associated with the water-rock interaction granitic rocks. The extent of the salt contribution from the various systems in the Buffels River Valley is still not fully understood but it is evident that conventional, semi-arid salinisation through evaporation is not the only driver in this regional system.
The effect of winery wastewater irrigation on the properties of selected soils from the South African wine region
(Stellenbosch : Stellenbosch University, 2016-12) Mulidzi, Azwimbavhi Reckson; Clarke, Catherine E.; Myburgh, P. A.; Roychoudhury, A. V.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: Due to an increase in wine production as well as an intensification of environmental legislation in South Africa, the need for guidelines for sustainable management of winery wastewater has increased. To address this, the first part of the study focused on the seasonal dynamics of the volumes and quality of undiluted winery wastewater. The soil chemical dynamics were monitored in two different soils that were irrigated with undiluted winery wastewater for three years. Over-irrigation with undiluted winery wastewater in combination with winter rainfall caused large amounts of cations, particularly K+ and Na+, to leach beyond 90 cm soil depth. Consequently, the leached elements are bound to end up in natural water resources over time. Irrigation with undiluted winery wastewater did not have a pronounced effect on soil pH(KCl). This was probably due to the decomposition of organic matter and the fact that the applied salts as well as dissolved organic or mineral acids leached beyond 90 cm depth. The practical application of irrigation with diluted winery wastewater was assessed in a pot experiment. Irrigations were applied under a rain shelter over four simulated irrigation seasons. Four soils varying in texture were irrigated with winery wastewater that was diluted to 3000 mg/L chemical oxygen demand (COD). The four soils were irrigated with municipal water as a control. The rate of K+ increase in the soil containing 20% clay was higher than in soils containing 13% clay, or less. This suggested that heavy soils will aggravate the risk of high K+ levels. The risk of Na+ accumulation increased linearly with the clay content in the soil. Low Ca2+ and Mg2+ concentrations in the diluted wastewater had no effect on the soil, irrespective of clay content. Irrigation with diluted winery wastewater increased soil pH(KCl) substantially in all soils over four simulated seasons. The soil pH increase was attributed to the addition of organic and mineral salts via the diluted winery wastewater to the soil. The effect of simulated rainfall on soils irrigated with winery wastewater was also assessed in a pot experiment. Six soils with different clay content were irrigated with winery wastewater diluted to 3000 mg/L over one simulated irrigation season. Thereafter, good quality river water simulating winter rainfall was added to the pots. The rainfall was simulated according to the long term averages of the regions were the soils originated. Leaching of cations, particularly K+ and Na+ occurred only from four of the six soils when winter rainfall was simulated. In one of the sandy soils, the simulated rainfall was too low to allow leaching. In the case of other soil where there was no leaching, high clay content of 35% in combination with low rainfall prevented leaching. Where three soils received the same amount of rainfall, more cations leached from the sandy soils compared to the two heavier soils. These trends indicated that leaching of cations was a function of soil texture and rainfall.
Geochemical and isotopic tracing of salinity loads into the Ramsar listed Verlorenvlei freshwater estuarine lake, Western Cape, South Africa
(Stellenbosch : Stellenbosch University, 2018-03) Sigidi, Nthabeliseni Thendo; Miller, Jodie A.; Clarke, Catherine E.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: The Verlorenvlei estuarine lake is a natural, semi-fresh lake along the West Coast of South Africa. The lake is situated in a semi-arid region, and is subject to high evaporation rates of up to 2400mm, and average rainfall of less than 300mm/per year. The lake is also an important host for diversified species of flora and fauna. Due to normal functions of the lake, variation in salinity of the lake is due to several factors. These include: (1) ingress and mixing with sea water; (2) floods, droughts and evaporation due to climate change and (3) salt transported into the lake from terrestrial sources. For the long term health and management of the lake, it is required to understand the role of terrestrial salinity and transport processes into the lake. Therefore, the study focuses on four main tributaries which are the Krom Antonies, Hol, Bergvallei, and the Kruismans. These four tributaries join at the Verloren confluence and thereafter form a single river system called the Verloren River. The Verloren River and its tributaries drain on predominantly fine- grained rocks of the Malmesbury group (contain high concentration of salts) and of the Table Mountain Group. The geology of this area has an important bearing on the composition of the water that enters the lake. To understand the origin and processes of salinity for surface water and shallow groundwater to the Verloren River, analysis of coupled major ion hydrochemistry and environmental isotopes (δ18O, δ2H, 87Sr/86Sr and 3H) has been undertaken. To fully evaluate the contribution from each tributary, the TDS values in both the surface and shallow groundwater are quantified, the daily overall discharge (surface runoff, interflow and baseflow) in each tributary has been established. Interaction with the aquifer matrix, evaporation, and precipitation-dissolution control the hydrochemistry of surface water and shallow groundwater. Due to the dominance of Na and Cl ions surface water and shallow groundwater can be characterised as Na-Cl water type. The presence of Mg and Ca ions in surface water and shallow groundwater corresponds to dissolution of dolomite limestones. The decrease in Ca2+ and increase in Na+, is a result of cation exchange processes and weathering of feldspars and micas. High sulphate concentrations correspond to dissolution of gypsum and nitrate concentrations can be related to the use of fertilisers. Evaporation for surface water and shallow groundwater in the Hol, Bergvallei and Kruismans is a dominant source for salt concentration, therefore increasing Na and Cl ions in water. The use of strontium isotopes indicates mixing between the Hol and the Bergvallei as source of observed salinity and water composition for surface and shallow groundwater in the Verloren confluence. The results show that the Hol contributes the least salt load (5.3%), followed by the Krom Antonies (6.2%), then the Bergvallei (35.0%) and the Kruismans contribute the most salt load (53.5%), into the Verloren confluence.
Geochemical and mineralogical characterisation of Vaalputs palaeosols : inference of paleoclimates
(Stellenbosch : Stellenbosch University, 2013-03) Majodina, Thando Olwethu; Clarke, Catherine E.; Mikes, Daniel; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: Vaalputs radioactive waste disposal facility is situated in an arid region of Bushmanland currently with evapotranspiration potential that far exceeds precipitation. Dominant soil features in Vaalputs are palimpsests of climates under which they formed. Particle sizes vary drastically between horizons which suggest different modes of sediment transport. Petrographic analyses revealed euhedral habits of primary mineral feldspar within the soils of Vaalputs. This suggests a proximal source of sediments and minimal primary mineral weathering under an arid climate where euhedral grains of feldspar are maintained. The surface horizon of the soils is covered by an equigranular coarse sand of residual aeolian origin. The transition from the surface horizon to the subsurface horizons is widely marked by a pebble sized stone-line. The pebble sized material of the stone-line suggests residual accumulation during the weathering of a previously surface exposed horizon. Since deposition of subsurface sediments (15 Ma) pedogenic alteration has been active in Vaalputs. This has resulted to a complex soil system which displays varied forms of thick dorbank horizons including massive polygonal peds and platy horizons. The polygonal peds are defined by desiccation cracks where vertical and horizontal laminations are hosted. Slaking tests as well as bulk chemistry confirmed that the laminations are composed largely of secondary calcite, however elemental mapping revealed numerous illite bands alternating with calcite layers. It is proposed that calcite layers represent solution features rather than cutanic features. Signs of hydromorphy are commonly observed within the dorbank horizons, since an arid climate currently prevails in Vaalputs such hydromorphic features may indicate formation under past climates. The occurrences of palygorskite, sepiolite and dorbank horizons in Vaalputs require high soil pH (generated by high concentrations of Na) for their formation. Vaalputs soils, however, measured circumneutral pH and relatively low Na concentrations which suggests that palygorskite, sepiolite and dorbank horizons are relic features. Salt casts of lenticular texture occur between polygonal peds of massive dorbank horizons and their enveloping vertical and horizontal laminations. Scanning Electron Microscope analyses indicate high concentrations of Mg, Al, Si and O which suggests sepiolite and palygorskite accumulation through a replacement of gypsum. Elemental maps in conjunction with x-ray tomography and bulk chemical analyses revealed that high concentrations of secondary barite occur along the contact surfaces between dorbank horizons and the laminations. The solution chemistry of all horizons show supersaturation with respect to barite suggesting that the Ba accumulation adjacent to the laminations is likely to have taken place at lower sulphate conditions than those present in the soils today. Evidence shows that Vaalputs soils have experienced at least one climate shift. The preserved soil mottles are indicative of soil environments that remain wet for an extended period. A fine textured platy dorbank horizon is an extensive feature in Vaalputs. The presence of this horizon indicates that the sediments were deposited from a low energy fluvial system. The large polygonal ped units in the lower dorbank units as well as the barite enrichments in pore spaces suggests a climate shift from wet to dry began after the sediments were deposited.
Investigation of heuweltjie structure and soil chemistry in the Buffels River valley and implications for transfer of salts to groundwater
(Stellenbosch : Stellenbosch University, 2019-12) Vermooten, Marli; Miller, Jodie A.; Clarke, Catherine E.; Francis, Michele Louise; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: Globally, soil salinization is a major soil degradation problem. Saline soils damage the land by altering various Earth cycles and are especially a threat to farming. Saline soils can cause salts to leach into the groundwater, causing an increase in groundwater salinity. The Buffels River catchment, situated in the Northern Cape Province of South Africa, is affected by saline groundwater. Various locations along the coastal zone of southern Africa show similar mean annual precipitation profiles as the Buffels River catchment, yet do not suffer from the same salinization levels. Saline waters within this area are also not evenly distributed, suggesting that other factors may play a role in the groundwater salinization. Heuweltjies, circular earth mounds that can be up to 2 m in height and 32 m in width, are abundant in the Buffels River catchment, and have salinity levels an order of a magnitude higher than the surrounding soils, suggesting that heuweltjie salts may be contributing to this groundwater salinization. Detailed analyses were done on two heuweltjies, through excavation, to examine the salt profile of the heuweltjies, as well as other chemical and physical analyses of the sediments. This was done to see if heuweltjies could be contributing to groundwater salinity in this area. In order to gather this information, a range of analytical techniques were employed with the aim of determining whether the heuweltjies are leaching salts into the groundwater system. Amongst the analytical techniques available for this type of profiling, electrical conductivity, pH, mineralogy, particle size analysis, anion and cation profiling, and dissolved silica were analysed. Results showed that heuweltjies have different soil compositions compared to the interheuweltjie sediments. More clay was present on heuweltjie than off heuweltjie. Heuweltjies had elevated salt concentrations compared to the interheuweltjie sediments, and minerals such as gypsum, calcite, and calcium oxalate were only present in the heuweltjie soils. The source of salt is possibly from marine origin. This could be caused by evaporative concentration, transfer of marine blown salts, or termites bringing saline plant materials into the heuweltjies. Calcite was found in the top soils, and gypsum deeper down, indicating that the water is moving downwards. This was concluded since gypsum is a more soluble salt than calcite and moves further down the profile. Preferential flow pathways were observed through granular soils in one heuweltjie, and termite nests and tunnels within the second heuweltjie, aiding in the transfer of salts to the groundwater.
Isotopic and elemental ratios to assess the relationship between heuweltjies and saline groundwater in the Northern Cape of South Africa.
(Stellenbosch : Stellenbosch University, 2021-12) Van Gend, Jani; Miller, Jodie A.; Clarke, Catherine E.; Francis, Michele Louise; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: Globally, groundwater is becoming one of the most important resources. This is specifically the case in semi-arid to arid southern Africa where surface water resources are limited. In the Buffels River catchment, part of a coastal desert and global diversity hotspot in Namaqualand, South Africa, many communities and the local economy are largely dependent on groundwater as the only source of potable water in the region. However, the groundwater is variably saline. In this study, hydrochemistry and stable and radiogenic isotopes from groundwater in the Buffels River catchment is used to determine the origin of salts in the groundwater as well as the mechanism of salinisation. In order to do this, a better understanding of the aquifer systems was required. Basic cation and anion data together with δ¹⁸O, δ²H and ⁸⁷Sr/⁸⁶Sr data indicated that evaporative concentration of salts is not the main contributor to salinisation as previously thought, but that dry deposition of marine aerosols and water-rock interaction are the main sources of salts. Heuweltjies are prominent features in this region and it was known that they generally consist of nutrient rich soils, but geophysics data revealed that these soils are extremely saline with the salinity increasing with depth and towards the centre of the heuweltjie. Thus, heuweltjies are zones where salts accumulate and given that heuweltjies consist of aerated soils and contain tunnels which could act as preferential flow paths, their contribution to salinisation was further investigated. A new groundwater recharge model for was conceptualised which include recharge through heuweltjies, and total mean groundwater ages were calculated using a combination of ¹⁴C and ³H and a lumped parameter approach to understand when recharge has been taking place. The age of groundwater in the Buffels River catchment range between modern and ~18 000 years, with modern fraction of up to 80 %. The relationship between heuweltjie salts and saline groundwater was further investigated by determining the relative depths and ages of the different carbonate horizons. Heuweltjies are up to ~30 000 years old and three distinct wetting fronts, which is an indication of mean annual rainfall amounts, are seen. This proved that heuweltjies act as preferential flow paths and that salts are transported downwards through the centre of the heuweltjies. δ¹⁸O SO4 and δ S SO4 isotope signatures of heuweltjie soils indicated that the salts in 2- 34 2- heuweltjies is directly related to dry deposition of aerosols containing both marine and non-marine- salts. δ¹⁸O SO4 signatures of groundwater hosted in the granitic gneisses are similar to that of the 2- heuweltjies, suggesting that the mechanism of formation of these salts are the same, while the δ34S 2- SO4 signature indicate a “granitic gneiss”-influence. In contrast to this. In areas were the heuweltjie density is high, the δ³⁴S SO4 and δ O SO4 signatures of groundwater and heuweltjie soils are 2- 18 2- comparable indicating that salts stored in heuweltjies are flushed into the aquifer system and that heuweltjies play a role in salinisation of groundwater and have been doing so for thousands of years.
Physiochemical controls on the formation and stability of atacamite in the soil surrounding the Spektakel mine, Northern Cape Province, South Africa
(Stellenbosch : Stellenbosch University, 2013-03) Le Roux, Stephan Gerhard; Clarke, Catherine E.; Roychoudhury, Alakendra N.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: The Northern Cape Province of South Africa has played host to numerous mining activities for over a century. To date, most of the mining activity has ceased, leaving the area laden with derelict mine sites and unlined tailings dumps. One such site is the Spektakel mine situated to the west of the town of Springbok. The unlined copper and sulphide rich tailings at the site have the potential to leach elevated concentrations of copper and acidic water into the Buffels River downslope of the site. This poses a threat to the surrounding communities that rely mainly on the river to supply water for drinking, livestock and irrigation. The soil surrounding the tailings dumps was characterised in terms of its mineralogical and chemical properties. The results indicate that the soil contains elevated concentrations of Cu2+, which is bound in the soil in the form of the secondary copper hydroxy mineral atacamite (Cu2(OH)3Cl). No other secondary copper minerals were identified at the site. Analysis of the solution present on the surface of the tailings dumps indicate that the tailings are the main source of the high Cu2+, Mg2+ and SO42- concentrations observed in the surrounding soils. As this solution migrates through the tailings dumps, into the soil, it accumulates Cl- through halite dissolution. The resulting acidic Cu2+, Mg2+, SO42- and Cl- solution reacts with the calcite in the soil, replacing it with atacamite. To determine why only a copper chloride mineral formed in the sulphate rich environment a synthetic solution with the composition of a solution in equilibrium with the soil was evaporated, both in the presence and absence of calcite. The results indicate that when the solution comes into contact with calcite, atacamite immediately precipitated, removing the Cu2+ from the solution. In the absence of calcite Cu2+ remains conservative, accumulating in the solution without precipitating a copper sulphate mineral. This establishes that the elevated Mg2+ concentration of the solution induces the formation MgSO4 aqueous complexes that reduce the activity of free sulphate, thus restricting copper sulphate mineral formation. The results from the soil characterization indicate that the atacamite stabilization mechanisms (circumneutral pH, high Cl- concentration and calcite) in the soil are diminishing. During sporadic rain events the acidic tailings solutions dissolve the calcite and temporarily reduce the Cl- concentration of the soil. To determine how these decreases will influence Cu2+ mobility in the soil, the stability of atacamite was tested by reducing the pH both in the presence and the absence of chloride. The results indicate that an elevated Cl- concentration and a pH > 6 stabilizes atacamite. A decrease in either of these parameters destabilizes atacamite and favours its dissolution. The study concludes that the current chemical conditions in the soil at Spektakel favour the stability of atacamite. However, continued sporadic rain events will reduce the Cl- concentration in the soil by increasing the SO42- concentration. This acidic solution will dissolve the calcite in the soil, thus reducing the buffering capacity of the soil, leading to the instability of atacamite, resulting in the leaching of large quantities of Cu2+ into the surrounding water bodies.
Re-purposing of tailings from a sulphidic based ore : an extra source of soil filler material or an additional source of acidity
(Stellenbosch : Stellenbosch University, 2021-03) Walker, Ruairi; Miller, Jodie A.; Clarke, Catherine E.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: The mine of Spektakel, situated in the Northern Cape of South Africa, is an old copper mine with a long history of owners. This turbid history has led to the mismanagement of the site which has resulted in the generation of AMD. Recently the mine has restarted operations and has begun re-processing some of the old tailings materials found on the site in hopes of extracting further copper. To do this the mine constructed leach pads and leach ponds to facilitate the process. In doing so, the mine looked to re-purpose some tailings material that was determined to have too little copper for extraction. The re-purposed tailings material was then used as extra soil material to help construct service features for the new operation such as roads and berms. Detailed analyses of the soils on the mine site were completed to determine whether the practice of re-purposing tailings material, although deemed to have too little copper for re-processing, would pose as an additional source of AMD. Soil samples from the surface, 5 cm and 30 cm were collected from a total of 17 samples locations. The range of analyses included testing the soils pH, EC, examining their elemental composition and mineralogy as well as conducting an acid-base accounting test. It was also discovered that there was a substantial amount of water soluble material present which was then examined further by completing a sequential leach extraction. From these analyses it was determined that the soils contained a high amount of leachable S, Mg and Cu. The semi-arid climate of the region has resulted in the precipitation of secondary soluble minerals at the surface, which are susceptible to dissolution and migration during heavy rain events.
Relationship of historical copper mining to the transport and accumulation of trace metals and salts in semi-arid environments : an example from Buffels River, Northern Cape, South Africa
(Stellenbosch : Stellenbosch University, 2016-03) Dunford, Alexander; Miller, Jodie A.; Clarke, Catherine E.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: The town of Kleinzee, located in the Northern Cape, South Africa exists solely to mine alluvial diamonds found in the Buffels River. Given the very low precipitation of the area (generally <150mm/yr) groundwater is the principal supply of domestic and industrial water to the town. In an effort to increase the water supply needed, the town installed an underground membrane across the Buffels River approximately 5.5km upriver from the town. However, climatic conditions, long term copper and diamond mining and poor groundwater management practices have combined to produce very poor quality groundwater within the Buffels River Valley. The purpose of this study is to look at the impact of the membrane on the quality of both the shallow groundwater system as well as the soils within the river bed. To do this, 46 sites were selected for soil analysis starting below the membrane and continuing above the membrane upriver for a distance of approximately 5 km. Additionally, a further 15 groundwater samples were collected at the end of the wet season in October when groundwater was available. Cation and anion analysis of the shallow groundwater and the saturated paste extracts shows significant peaks in SO4 2- and Mg2+ immediately above the membrane in the sediments in addition to Na+ and Cl-, diminishing upstream away from the membrane. Cu2+ and Zn2+ did not show an elevated concentrations in the groundwater or sediments above the membrane as originally thought. Experiments on atacamite, a Cu2+-hydroxide, which forms naturally in the Spektakel soils within the Buffels River Valley, indicates that although generally stable at moderate pH, large rain events may increase the solubility of atacamite resulting in its transport downstream. However, the source of sulphate is probably linked to processing of Cu2+ ores further up the river valley. The transport mechanism of the groundwater salts and trace metals is via dissolution mobilization. The membrane is accumulating trace metals and salts in sediments behind the membrane and not in groundwater found behind the membrane. Thus accumulation is due principally to evaporative concentration.
Salinisation source(s) and mechanism(s) in shallow alluvial aquifers along the Buffels River, Northern Cape Province, South Africa
(Stellenbosch : Stellenbosch University, 2015-04) Nakwafila, Albertina Nuusiku; Miller, Jodie A.; Clarke, Catherine E.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
ENGLISH ABSTRACT: Several areas in Namaqualand are affected by elevated levels of salinity; including the shallow alluvial aquifers along the Buffels River catchment. The chemistry of rainwater that recharges these aquifers has low salt-loads, but the groundwater system is very saline. The shallow alluvial aquifers are an important source of water for several communal areas along the river both for domestic and agricultural purposes. Groundwater is also used for the copper and diamond mining activities along the river. Prior to this study, the sources of salinity to the alluvial aquifers and whether salinity can be remediated has not been determined. Possible salinity sources included seawater intrusion, evaporitic salts dissolution, concentrations by evaporation, deep aquifer brines, dissolution of minerals from the aquifer geology as well as salts from anthropogenic activities (i.e. mining). The source(s) of salinity to the groundwater has been determined using the geochemical as well as the isotopic tracers. The ratios of major ions (i.e. Na/Cl) as well the oxygen (18O), hydrogen (2H), 36Cl/Cl, and 87Sr/86Sr isotopes were used in determining the possible solute sources. In addition to these, the chloride mass balance (CMB) method was used to determine the recharge rates to the alluvial aquifers. Furthermore, the groundwater age of the alluvial aquifers was determined using the tritium (3H) isotope. The groundwater samples have high 87Sr/86Sr ratios, which are in the same ranges as the 87Sr/86Sr ratios in the surrounding granitic rocks indicating water-rock interactions. The 36Cl/Cl ratios are low and indicate the dissolution of old salts. The 18O and 2H stable isotopes of groundwater show an evaporative trend with respect to the LMWL, which indicate that groundwater has been evaporated before or during infiltration. Salinity to the shallow alluvial aquifers was found to be mainly derived from the dissolution of rock mass and concentration by evaporation process. The groundwater is relatively young, but there is mixing between old water recharged prior to bomb testing and recently recharged groundwater in some boreholes. The Chloride mass balance (CMB) method assumes atmospheric chloride is the only source of chloride to the aquifers. However, there is additional chloride to these alluvial aquifers from rock mass dissolution. The estimated recharge rates by CMB method (0.1-3.4mm/a) are therefore underestimated. A simulation model was used to determine the recharge rates based on the annual precipitation that will yield the current measured chloride concentrations in the groundwater. Recharge was found to range from 1-5% of the annual precipitation, which is also low. The CMB method therefore gives significant recharge rate estimates, but they are not accurate and need to be supported by another method.

Browse

Browsing Department of Earth Sciences by browse.metadata.advisor "Clarke, Catherine E."

Results Per Page

Sort Options