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Applied Geochemistry 23 (2008) 2496–2511

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AppliedGeochemistry

Major and trace element geochemistry in Zeekoevlei,South Africa: A lacustrine record of present and past processes

Supriyo Kumar Das a, Joyanto Routh a, Alakendra N. Roychoudhury b,*,J. Val Klump c

a Department of Geology and Geochemistry, Stockholm University, Stockholm 10691, Swedenb Department of Geological Sciences, University of Cape Town, Rondebosch 7700, South Africa

c Department of Biological Sciences and Department of Geosciences, Great Lakes WATER Institute, University of Wisconsin-Milwaukee,

600 East Greenfield Avenue, Milwaukee, WI 53204, USA

Received 17 October 2006; accepted 5 February 2008Editorial handling by R. Fuge

Available online 29 February 2008

Abstract

This study reports a multi-parameter geochemical investigation in water and sediments of a shallow hyper-eutrophicurban freshwater coastal lake, Zeekoevlei, in South Africa. Zeekoevlei receives a greater fraction of dissolved majorand trace elements from natural sources (e.g., chemical weathering and sea salt). Fertilizers, agricultural wastes, raw sew-age effluents and road runoff in contrast, constitute the predominant anthropogenic sources, which supply As, Cd, Cu, Pband Zn in this lake. The overall low dissolved metal load results from negligible industrial pollution, high pH and elevatedmetal uptake by phytoplankton. However, the surface sediments are highly polluted with Pb, Cd and Zn. Wind-inducedsediment resuspension results in increased particulate and dissolved element concentrations in bottom waters. Low C/Nratio (10) indicates primarily an algal source for the sedimentary organic matter. Variation in sedimentary organic C con-tent with depth indicates a change in primary productivity in response to historical events (e.g., seepage from wastewatertreatment plant, dredging and urbanization). Primary productivity controls the enrichment of most of the metals in sed-iments, and elevated productivity with higher accumulation of planktonic debris (and siltation) results in increased elementconcentration in surface and deeper sediments. Aluminium, Fe and/or Mn oxy-hydroxides, clay minerals and calcareoussediments also play an important role in adsorbing metals in Zeekoevlei sediments.� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Trace and major elements have a dynamic distri-bution and behaviour in lakes. Some elements are

0883-2927/$ - see front matter � 2008 Elsevier Ltd. All rights reserveddoi:10.1016/j.apgeochem.2008.02.011

* Corresponding author. Fax: +27 21 650 3783.E-mail address: alakendra.roychoudhury@uct.ac.za (A.N.

Roychoudhury).

necessary micronutrients for living organismsalthough many of them (e.g., Cd, Cu, Pb and Zn)are toxic over a certain concentration and canadversely affect aquatic organisms and human health(Bowen, 1966; Nor, 1987; Timmermans, 1992; Silvaet al., 2000). Their sources in lakes can be natural oranthropogenic. Urban lakes, in particular, aremore vulnerable to trace element pollution because

.

S.K. Das et al. / Applied Geochemistry 23 (2008) 2496–2511 2497

of widespread cultural activities surrounding thelake.

There is a distinct lack of data on seasonal vari-ations in trace and major element concentrations inurban lakes (Balistrieri et al., 1992; Murray, 1987).Moreover, most trace metal studies have been donein deep lakes with a seasonal or permanent anoxicdeep layer (e.g., Seyler and Martin, 1989; Michardet al., 1994; Viollier et al., 1995; Mogollon et al.,1996; Widerlund et al., 2002; Granina et al., 2004).In contrast, shallow lakes remain oxic throughoutthe year (e.g., Elbaz-Poulichet et al., 1997). Theselakes are generally characterised by high primaryproductivity, and their relatively shallow depthenables considerable sediment resuspension (e.g.,Schallenberg and Burns, 2003). Metal enrichmentin shallow lake sediments is controlled by severalfactors, for example, pH, phytoplankton abundanceand organic matter (see Bilali et al., 2002). Anyminor changes in these controlling factors can initi-ate a series of geochemical processes, which affectmetal accumulation in sediments. More impor-tantly, a decipherable sedimentary record of thesegeochemical changes can be used to interpret palae-oenvironmental processes (e.g., Sternbeck et al.,2000; Routh et al., 2004, 2007).

The rich ecological importance (for diverse avi-faunal composition) of Zeekoevlei (vlei meaninglake in Afrikaans), and its proximity to Cape Townand the coast (2 km) has prompted this study ontrace and major element deposition and cycling.The lake is hyper-eutrophic and is believed to bewidely affected by anthropogenic inputs, which haveresulted in enhanced productivity in recent years(Southern Waters Ecological Research and Consult-ing, 2000). To the best of the authors’ knowledge,there has been no published work on elementcycling in Zeekoevlei. Moreover, the use of trace ele-ments as geochemical proxies (e.g., Sternbeck et al.,2000) to interpret different biogeochemical processesin the lake sediments is rare. Hence, the main objec-tives of the study are to: (1) use sedimentary organicmatter and element records as proxies to interpretrecent historical changes in the lake and its catch-ment, (2) investigate the seasonal variation in con-centrations of selected elements and their sourcesin the water column and sediments, and (3) studythe sources of dissolved major inorganic ions. Theexistence of any possible correlation betweenorganic matter sources and trace and major elementinput with particular emphasis on the lake’s primaryproductivity is also investigated. This study demon-

strates the use of both organic and inorganic geo-chemical proxies in exploring the historicalchanges and environmental perturbations in Zee-koevlei. Many aspects of this study can thus beextended to interpret analogous observations inother lacustrine systems.

2. Study area

Zeekoevlei is a well-mixed, alkaline (pH 9) fresh-water (salinity 0.3‰) coastal lake. The lake is situ-ated on the sandy coastal plains of Cape Flats nearCape Town in the Western Cape Province, and isthe largest of the shallow (maximum depth 5 m)urban lakes in South Africa (surface area =2.56 km2, longest dimension = 2.3 km; Harding,1997). Zeekoevlei is a warm water lake and temper-ature varies between 10.3 �C and 28.5 �C (mean18 �C) in the water column. The prevailing climateis sub-tropical Mediterranean type with hot drysummers and cold wet winters. Zeekoevlei lacks anatural outlet and the man-made Zeekoe canal con-nects the lake to the Atlantic Ocean near False Bay(Fig. 1). The lake is partially divided into two basinsby a peninsula (Fig. 1), and is fed by the Great andLittle Lotus Rivers. These rivers together drain acatchment of approximately 80 km2 covering resi-dential, industrial, agricultural and horticulturalareas. It is estimated that over 400,000 people livein the catchment of the water bodies (Grobicki,2001). The rivers are polluted with agricultural andurban runoff containing high levels of P (SouthernWaters Ecological Research and Consulting, 2000).In addition, these rivers carry excessive silt resultingfrom intensive erosion caused by poor urban andagricultural practices in the catchments. Inadequateor non-existent sanitation conditions and overflow-ing raw sewage effluents from the poor settlementsand fertilizers used in the Philippi Horticulture areain the Great Lotus River catchments supply NO�3 ,NO�2 and PO3�

4 to the lake (Water Research Com-mission, 2001). Ground water (TDS < 150 mg L�1;Day and King, 1995), which contributes 15% ofthe total annual inflow, is also contaminated by agri-cultural practices. It is estimated that an average of34,000 kg of P enters the lake annually; groundwater and seepage from the wastewater treatment(WWT) plant located on the south-eastern shoreline(Fig. 1) contribute towards the major fraction of thisexternal P input (Southern Waters EcologicalResearch and Consulting, 2000). Extensive nutrientinput has resulted in the hyper-eutrophic condition,

S

E18˚ 30'

34˚CAPE TOWN

LITTLE LOTUS RIVER

GREAT LOTUS RIVER

ZEEKOEVLEI

PHILIPPI HORTICULTURE AREA

WASTE WATER TREATMENT PLANT

ATLANTIC OCEAN

0 2000 m

SCALE

500

N N

LITTLE LOTUS RIVER

GREAT LOTUS RIVER

Sediments and waters

waters

HOME BAY

0 500 m

SCALE

5 Bathymetric contour in meters above sea level

Sampling positions

5 43

2

54

30

3

3

345

210

22 1

ZEEKOE CANAL

FALSE BAY

S

E18˚ 30'

34˚CAPE TOWN

LITTLE LOTUS RIVER

GREAT LOTUS RIVER

ZEEKOEVLEI

PHILIPPI HORTICULTURE AREA

ATLANTIC OCEAN

0 2000 m

SCALE

500

N N

LITTLE LOTUS RIVER

GREAT LOTUS RIVER

Sediments and waters

waters

HOME BAY

0 500 m

SCALE

5 Bathymetric contour in meters above sea level

Sampling positions

5 43

2

54

30

3

3

345

210

22 1

ZEEKOE CANAL

FALSE BAY

Fig. 1. Bathymetry map of Zeekoevlei showing the position of the lake in relation to the Cape Peninsula (inset), influent Lotus Rivers,Zeekoe Canal and the coast (False Bay), and sampling positions (modified from Harding, 1992; Harding and Quick, 1992).

2498 S.K. Das et al. / Applied Geochemistry 23 (2008) 2496–2511

and Cyanophyta, Chlorophyta and Bacillariophyta(diatoms, division Chrysophyta) constitute the totalphytoplankton assemblage in this lake. The Cya-nophyta (Microcystis spp.) dominates over the othertwo species, and diatom development takes placeonly during the winter months (Harding, 1992).

Unconsolidated Quaternary sands of LangebaanLimestone (calcareous sand), Springfontein (cleanquartz sand) and Graafwater Formations (fine tomedium marine silty sand with coarse shelly gravel)of the Sandveld Group underlie Zeekoevlei (South-ern Waters Ecological Research and Consulting,2000). The sands overlie granitic base rock of theCape Granite Suite and shales of the MalmesburyGroup (Theron et al., 1992). The thickness of thesandy unit varies from 10 to 25 m from north tosouth of Zeekoevlei (Southern Waters EcologicalResearch and Consulting, 2000).

The lake has been a popular destination for recre-ational activities (e.g., sailing, canoeing, water-ski-

ing, fishing, swimming and bird watching), andseveral modifications have been made to improvethe water quality since the 1940s (Harding andQuick, 1992; Southern Waters Ecological Researchand Consulting, 2000; Grobicki, 2001). Permanentand artificial water level regulation (stabilization)was implemented by constructing small dams (51 mwide and 5.19 m high) in 1948, which reduced flush-ing during winter and increased the water retentiontime. This changed the natural hydraulic cycle, anddecreased the ability of the lake to dilute nutrientsand algal population by periodic discharge. Thiswas followed by the eradication of pondweed, whichused to obstruct yachts, by spreading sodiumarsenite over the lake in 1951. This triggered large-scale algal bloom in the following years and thelake became permanently eutrophic. Small scaledredging was conducted in the northern basin nearHome Bay (Fig. 1) in 1983 to reduce the internal P

load, and approximately 20,000 m3 of sediment

S.K. Das et al. / Applied Geochemistry 23 (2008) 2496–2511 2499

was removed (Southern Waters Ecological Researchand Consulting, 2000). More recently, an annuallake water pump out from 1997 until 2000 (lateApril to end of June) causing 0.2 m drop in waterlevel was done to allow the early winter flows tomove through the system, and to reduce the pollu-tant loading.

3. Methods

3.1. Water sampling and chemistry

Surface water samples were collected in Januaryand August 2004 (Austral summer and winter,respectively) from an inflatable boat. Divers col-lected water samples from the lake bottom in winter.All sample containers and laboratory glassware werepre-cleaned with detergent, soaked in HNO3 (10%)for 24 h and rinsed with deionised distilled waterbefore use. Acids and standard solutions used duringthe digestion and analyses were of suprapure grade.All bottles and equipment used to collect and pro-cess the samples were rinsed at least three times withnative water prior to sample collection. Water sam-ples for dissolved trace and major element analysiswere filtered using 0.45 lm Sarstedt non-pyrogenicsterile filters. The filters were cleaned with deioniseddistilled water before use. Waters for total trace andmajor element analysis were not filtered. Through-out this paper, the term ‘dissolved’ refers to materialthat passes through a 0.45 lm filter, including trulydissolved species as well as colloids. The term ‘total’includes the particulate, colloidal and dissolvedconcentration. Both filtered and unfiltered watersamples were acidified to pH 2 using concentratedHNO3 (suprapure grade) immediately after collec-tion, and stored at 4 �C until analysis. DissolvedO2 (DO), specific conductivity (EC), temperatureand pH were measured during sampling using aWTW� multi-parameter meter and associated elec-trodes. Each electrode was calibrated every day priorto the start of sampling. Total dissolved solid (TDS)was calculated by multiplying the specific conductiv-ity by 0.65 (Ronald et al., 1999). The biological oxy-gen demand (BOD) was determined by comparingthe DO level in water samples measured immedi-ately, and after incubating the sample in the darkfor five days. The difference between the DO mea-surements represented the amount of O2 requiredfor decomposition of organic matter in the sample.The BOD tests were performed in BOD bottles usinga YSI dissolved oxygen electrode.

Water samples for dissolved organic C (DOC)analysis were filtered through 0.45 lm pre-washednon-pyrogenic sterile filters, and kept frozen in thedark until analysis. DOC was analysed on a Shima-dzu TOC-5000 analyser using the TOC (totalorganic carbon)–IC (inorganic C) mode. The detec-tion limit was 0.2 mg L�1 with a precision of ±20%at the detection limit (three times the standard devi-ation). Trace and major elements (As. Ba, Cd, Cr,Cu, Fe, Mn, Mo, Ni, Pd, Si and Zn) in acidifiedsamples were determined on a Perkin Elmer/SciexElan 6000 inductively coupled plasma mass spec-trometer (ICP-MS) equipped with an acid-resistantcross flow nebuliser, a Ryton TM Scott-type spraychamber and a Perkin Elmer AS 90 auto sampler.Standard reference material SRM 1640 was run inbetween every two samples. Particulate elementswere calculated by subtracting the dissolved valuefrom the total element concentrations. Major ions(Na+, K+, Mg2+, Ca2+, Cl� and SO2�

4 ) were deter-mined by ion chromatography (Dionex� DX-300).A standard reference material (SPS-NUTR WWIBatch 102; SPS, Oslo, Norway) was run in betweenevery 10–15 samples. Typical precision was 64%. Astandard deviation of 0.038–0.221 mg L�1 wasderived for the above major ions by running 17 rep-licates of the standard.

3.2. Potential loading

The potential loading (PL) of pollutants was cal-culated (Berner and Berner, 1996), which gave anestimate of the pollutant input into the lake.

Potential loadingðP LÞ

¼ per capita energy consumption ðEÞlake volume ðV Þ ð1Þ

where average E for year 2000, 2001 and 2002 was2752 tons of oil equivalent per capita (UN commondatabase, 2005) and V was 5.0 � 106 m3 (SouthernWaters Ecological Research and Consulting,2000). It was assumed that wasteful consumptionor pollution was roughly proportional to the energyconsumption per capita.

3.3. Sediment sampling and sediment chemistry

Two sediment cores, located within a 1 m radiusof each other, were collected by pushing in PVCtubes through the water column in January 2004(Fig. 1). The tubes were carefully pulled out without

Table 1Certified Values for Elements in Reference Material, SL-1 fromthe International Atomic Energy Agency (1979) compared to theones measured in this study

Element Acceptedvalues(mg kg�1)

Degree ofconfidence foraccepted value

Measuredconcentration: thisstudy (mg kg�1)

Cr 104 ± 9 Rb 96Zn 223 ± 10 Ha 248Ni 44.9 ± 8 R 53.1Cu 30.0 ± 5 R 29.6Co 19.8 ± 1.5 H 17.5Pb 37.7 ± 7.4 R 33.5As 27.5 ± 2.9 H 38.3Cd 0.26 ± 0.05 R 0.18Hg 0.13 Value not

certified1.52

Mo 1.3 Value notcertified

0.15

Se 2.9 Value notcertified

1.03

a H is a certified value with a high degree of confidence.b R is a certified value with a reasonable degree of confidence.

All certified values are shown with the 95% confidence limits.

2500 S.K. Das et al. / Applied Geochemistry 23 (2008) 2496–2511

disturbing the upper layers. A gravity corer couldnot be used because of the shallow depth and strongwinds in the lake. Sampling was done in the deeperpart of the lake with the assumption that it would beleast disturbed by human activity, and could bedescribed as an accumulation bottom. The dredgedsection of the lake near Home Bay (Fig. 1) wasavoided during sediment sampling. The undisturbedsediment-water interfaces in the core, which wereroughly perpendicular to the core axis before extru-sion, were distinct. Samples from the first core werefreeze-dried, whereas sediment samples from thesecond core were centrifuged at 10,000 rpm(1064g) for 30 min to extract the pore water priorto freeze-drying. The pore water samples were fil-tered through 0.45 lm non-pyrogenic filters and dis-solved organic C (DOC) concentrations weredetermined on a Shimadzu TOC-5000 analyser.The sand and silt percentage in sediments was esti-mated by dry sieving the freeze-dried sedimentsthrough 0.63 mm sieve. Element oxides (Al2O3,CaO, Na2O and K2O) in freeze-dried sedimentswere analysed by using a Philip PW 1480 X-rayfluorescence (XRF) spectrometer with a Mo/Sc X-ray tube.

A small portion (50 mg) of the freeze-dried sedi-ment was digested with HNO3–HF in sealed Teflonbeakers for trace and major element (Al, As, Ba,Ca, Cd, Cr, Cu, Fe, K, Mg, Mn, Mo, Na, Ni, P,Pb, Si, Sr, Ti, Zn and Zr) analyses. After totaldigestion, the solution was evaporated to near dry-ness. The residue was redissolved in HNO3 andthen diluted with sub-boiled deionised distilledwater prior to analysis. Trace and major elementswere analysed using a Perkin Elmer/Sciex Elan6000 ICP-MS. A Hawaiian basalt (BHVO-1) fromthe United States Geological Survey and an internalsediment standard (SL 1), which were treated thesame way as the samples, were run in between thesamples. Typical precision was 63% (n = 21 forBHVO-1). The certified and measured values forSL 1 are given in Table 1.

TOC, N and total S (TS) contents were measuredusing a Carlo Erba 2500 elemental analyser. Sedi-ment samples were treated with HCl to remove car-bonates and then analysed for their TOC content. Adetection limit of 100 lg C with standard deviation<1% was derived by running 15 replicates of theVienna PeeDee Belemnite limestone standard (V-PDB). The MINITAB 14 software was used forgenerating the correlation matrix for TOC, metalsand Ca concentrations.

Total reduced (inorganic) S (TRS) fractions wereextracted by using the digestion apparatus andmethodology described by Canfield et al. (1986)and later modified by Herbert et al. (2000). Briefly,the TRS analysis involved hot extraction of 4�5 gfreeze-dried sample with 1 M CrCl2 and 12 NHCl. The chromous solution was produced by pass-ing CrCl3 through a Jones reductor. The TRS frac-tion was determined by iodimetric titration using a1% starch indicator solution. Sample reproduc-ibility was ca. ±5%. A standard deviation of <5%(detection limit of 0.02% S) was determined byanalysing three replicates of a pyrite standard. A90% recovery of S was obtained from the standardin the analysis.

3.4. Sediment quality

Chemical index of alteration (CIA; Nesbitt andYoung, 1982) together with upper continental crust(UCC) normalised element concentrations (CM/CUCC, where CM = concentration of element insample, and CUCC = concentration of the same inUCC; Yanes et al., 2006) were used to interpretthe degree and type of weathering of the parentmaterial. CIA was expressed as

CIA ¼ Al2O3

Al2O3 þ CaO� þNa2OþK2O� 100 ð2Þ

S.K. Das et al. / Applied Geochemistry 23 (2008) 2496–2511 2501

where CaO* represents the CaO fraction incorpo-rated with SiO2.

The anthropogenic factor (Af), enrichment factor(Ef) and geoaccumulation index (Igeo) for metals insediments were calculated. Af (Szefer and Skwarzec,1988) was the quotient of the means of metal con-centrations in surface sediments (CTM; 0�6 cm)and the mean values of the corresponding elementsin deeper layers (DTM; below 22 cm).

Af ¼CTM=CZrðsampleÞ

DTM=DZrðdeeper layersÞð3Þ

Ef (Sinex and Helz, 1981), on the other hand,compared metal concentration (CTM) with the cor-responding background concentration (BTM) basedon average crustal composition (Wedepohl, 1995).

Ef ¼CTM=CZrðsampleÞ

BTM=BZrðUCCÞð4Þ

To account for the variable mobility of differentmetals they were normalized to Zr, because Zr is lessmobile and is normally associated with the residualfraction (Roychoudhury and Starke, 2006).

Igeo (Muller, 1981; Ruiz, 2001; Roychoudhuryand Starke, 2006) uses the background matrix cor-relation factor (1.5) to compensate for the differencein metal concentrations due to lithogenic variationand demonstrates the degree of pollution in surface

Table 3Dissolved and particulate element concentrations (lg L�1) in Zeekoevl

AS Ba Cd Cr

Dissolved surface Summer 3 2 bdl bdlWinter 7 33 bdl 5

Dissolved Bottom Winter 7 50 bdl 3Winter particulate Surface bdl bdl bdl bdl

Bottom bdl bdl bdl 9

US EPA 150 – 0.2 74–11WHO 10 700 3 50

bdl = below detection limit; nm = not measured.Concentrations under the US EPA water quality criteria represent ‘theaquatic community can be exposed indefinitely without resulting in anand Cr (VI), respectively.

Table 2Water quality and dissolved major ions in Zeekoevlei

pH EC (lS cm�1) TDS DO BOD DOC

10(9) 758a 493a 11(14) 10 25(16)

Values are in mg L�1, except for pH and if not mentioned otherwise; va Represents data from both summer and winter.

sediments (CTM) in comparison to the background(BTM; Ruiz, 2001).

Igeo ¼ log2

CTM

1:5� BTM

ð5Þ

Igeo classified pollution levels into 6 grades,namely, unpolluted (grade 1, Igeo < 1), very lightlypolluted (grade 2, 1 < Igeo < 2), lightly polluted(grade 3, 2 < Igeo < 3), moderately polluted (grade4, 3 < Igeo < 4), highly polluted (grade 5, 4 < Igeo <5) and very highly polluted (grade 6, Igeo > 5).

4. Results

4.1. Water quality characteristics and major ions

The Zeekoevlei waters had high pH, BOD andDO levels, moderate to low concentrations of dis-solved major ions, low EC and TDS (Table 2).DOC concentrations in the different seasons variedwithin a narrow range. The calculated PL was0.0005 ton of oil equivalent m�3.

4.2. Trace and major elements in the water column

Seasonal and depth wise variation in dissolvedand particulate element concentrations are pre-sented in Table 3. Dissolved element concentrations

ei water

Cu Fe Mn Mo Ni Pb Si Zn

bdl 19 2 bdl 3 bdl 80 989 320 2 bdl 3 2 220 1288 250 6 2 3 19 720 109nm bdl 9 bdl bdl 2 bdl 6nm 2700 60 bdl 4 60 360 130

9.0 1000 – – 52 2.5 – 1202000 – 400 – 20 10 – –

highest concentration of a material in surface water to which anunacceptable effect’. Chromium concentrations represent Cr (III)

Na+ K+ Mg2+ Ca2+ Cl� SO2�4

(194) 17(13) 25(12) 46(59) 203 79

alues in parenthesis are from winter.

2502 S.K. Das et al. / Applied Geochemistry 23 (2008) 2496–2511

in surface water showed an increase in winter versusthe summer samples. Dissolved element (except Pband Zn) concentrations in winter did not showany significant variation between bottom and sur-face water. However, particulate metal (e.g., Fe,Mn, Pb and Zn) and Si concentrations in wintershowed a considerable increase in the bottom com-pared to surface water (Table 3).

4.3. Sediment and pore water characteristics

Silt and clay content (Fig. 2) dropped from �63%near the bottom of the core to 44% at 24 cm depth.Silt and clay content steadily increased upward from22 cm depth and reached nearly 88% near the top ofthe core. The TS concentration (0.09%) and TRSfraction (0.06%) were low in surface sediments,and decreased further to 0.01% and 0.02%, respec-tively in the deeper layers (data not shown).

TOC concentration was �9% by weight near thebottom of the core before it increased to 18% from18 to 20 cm (Fig. 2). Between 8 and 15 cm, TOCconcentrations were low. Notably, the TOC concen-trations dramatically decreased to �1% at 12 cm.TOC started to increase above 8 cm, and peakedat 14% near the top of the core.

Atomic Corganic/Ntotal ratio in sedimentaryorganic matter varied from 10 to 12 from the bot-tom of the core up to 15 cm (Fig. 2). C/N ratioreached a maximum value of 19 around 14 cm

0

10

20

30

40

DOC (mg L-1)

00 125 250

Dep

th (

cm)

0 125 250 0 8 16

SO4

(mg L-1)NO3

(mg L-1)Silt + Clay

(%)

0 50 100

Zon

e I

Zon

e II

Zon

e II

I

0

10

20

30

40

DOC (mg L-1)

00 125 250

Dep

th (

cm)

0 125 250 0 8 16

SO4

(mg L-1)NO3

(mg L-1)Silt + Clay

(%)

0 50 100

Zon

e I

Zon

e II

Zon

e II

I

Fig. 2. Variation in pore water DOC, SO2�4 and NO�3 concentrations, s

(atomic) ratio and total P concentration with depth in Zeekoevlei sedim

depth; the values decreased to �10 near the top ofthe core. Nitrogen/P (molar) ratio was close to zero(0.01–0.09) throughout the core (data not shown).

The pore-water DOC and SO2�4 concentrations

varied from 140–219 to 55–200 mg L�1, respectively(Fig. 2). Nitrate concentrations were generally low,and varied from 0 to 2 mg L�1. However, NO�3 con-centrations increased sharply between 20 and 22 cmand reached up to 13 mg L�1.

4.4. Elemental trends, quality and zonation in

sediments

The distribution of Al, As, Ba, Cd, Cr, Cu, Fe,Mn, Mo, Ni, P, Pb, and Zn, and ratios (Fe/Aland Zr/Al) with depth in the Zeekoevlei sedimentsare presented in Fig. 3. Arsenic, Ba, Cr and Mnshowed an upward decreasing trend and reachedtheir lowest concentrations between 12 and 14 cm.In contrast, Al, Cd, Cu, Fe, Ni, Pb and Zn showedalmost a consistent trend up to 20–24 cm. Concen-trations of all elements increased upwards above20 cm and reached their maximum values (exceptAl and Fe) near the top of the core. The Fe/Al ratiowas high (0.7–0.8) from the bottom of the core to 18cm before it decreased to 0.2 between 12 and 14 cm.The Fe/Al ratio increased up-core from 10 cm andreached 0.5 in surface sediments. The Zr/Al ratiowas consistent (0.001–0.003) throughout the coreexcept for an increase to �0.005 in between 8 and

TOC (%)

10 20

% N

0 1 2

C/N(atomic)

0 10 20

P(102 mg kg-1)

1 11 22

1983

1963

1998

1970

1950

2004

1990

1981

1964

1956

1945

Age

TOC (%)

10 20

% N

0 1 2

C/N(atomic)

0 10 20

P(102 mg kg-1)

1 11 22

1983

1963

1998

1970

1950

2004

1990

1981

1964

1956

1945

Age

ilt and clay content, TOC concentration, % N concentration, C/Nents. Sediments ages are based on 137Cs dating (Das et al., 2007).

0 60 120

Ba(mg kg-1)

0 0.3 0.6

Cd(mg kg-1)

40

0 2.5 50

10

20

30

Dep

th (

cm)

As(mg kg-1)

0 12 24

Cr(mg kg-1)

0 16 32

Cu(mg kg-1)

Zon

e I

Zon

e II

Zon

e II

I

1998

1970

1950

1990

1981

1964

1956

1945

2004

Age

0 2010

Al103 (mg kg-1)

0 8 16

Fe103 (mg kg-1)

0 60 120

Ba(mg kg-1)

0 0.3 0.6

Cd(mg kg-1)

40

0 2.5 50

10

20

30

Dep

th (

cm)

As(mg kg-1)

0 12 24

Cr(mg kg-1)

0 16 32

Cu(mg kg-1)

Zon

e I

Zon

e II

Zon

e II

I

1998

1970

1950

1990

1981

1964

1956

1945

2004

Age

0 2010

Al103 (mg kg-1)

0 8 16

Fe103 (mg kg-1)

0 0.5 1

Dep

th (

cm)

Mo(mg kg-1)

0 6 12

Ni(mg kg-1)

0 60 120

Pb(mg kg-1)

0 125 250

Zn(mg kg-1)

Zon

e I

Zon

e II

Zon

e II

I

0 60 1200

10

20

30

40

Mn(mg kg-1)

0 0.6 1.2

Fe/Al Zr/Al(10-3)

0 3 6

1998

1970

1950

2004

1990

1981

1964

1956

1945

Age

0 0.5 1

Dep

th (

cm)

Mo(mg kg-1)

0 6 12

Ni(mg kg-1)

0 60 120

Pb(mg kg-1)

0 125 250

Zn(mg kg-1)

Zon

IZ

one

IIZ

one

III

0 60 1200

10

20

30

40

Mn(mg kg-1)

0 0.6 1.2

Fe/Al Zr/Al(10-3)

0 3 6

1998

1970

1950

2004

1990

1981

1964

1956

1945

Age

Fig. 3. Distribution of trace and major elements, and ratios (Fe/Al and Zr/Al) with depth in Zeekoevlei sediments. Sediments ages arebased on 137Cs dating (Das et al., 2007).

S.K. Das et al. / Applied Geochemistry 23 (2008) 2496–2511 2503

16 cm. Silicon/Al values ranged between (0.02–1.58;data not shown). Aluminium, CaO, Na2O and K2Ovalues ranged between 0.5–2.9, 25.7–2.0, 0.0–0.4and 0.1–0.6, respectively (data not shown). CIA val-ues ranged between 64 and 76 (average 67, data notshown). CM/CUCC values were 0.1–0.5, 0–0.01, 0.1–0.4 and 0.09–0.12, respectively for Cr, Si, Ti and Zr(data not shown). In contrast, the CM/CUCC valueswere 1–5, 1–5 and 1–4, respectively for Ca, K and Sr(data not shown). Af and Ef values are shown inTable 5. Af values were >1 for As, Cd, Cu, Pband Zn. Ef values were high for As, Cd, Cu, Pband Zn. Igeo values (not shown) were <1 exceptfor Cd (4.22), Pb (5.22) and Zn (4.11) in sediments

at depths of 0–8 cm. Calcium (14,300–154,000 mgkg�1), K (800�136,000 mg kg�1) and Mg (355�6000 mg kg�1) values were high throughout the core(data not shown).

The TOC, C/N ratio and metals showed a dis-tinct changes in concentration between 8 and24 cm depth in the Zeekoevlei sediments. Mostlikely this was due to sedimentation and in-lake pro-cesses occurring at these depths which were mark-edly different from processes above and below it.Hence, a depth zonation was imposed in the sedi-ment core (see Figs 2 and 3), namely zone I (0–8 cm), zone II (8–24 cm) and zone III (24–38 cm),for ease of explaining the observed variations in

2504 S.K. Das et al. / Applied Geochemistry 23 (2008) 2496–2511

geochemical trends. The sediment geochemistry isdiscussed as per the observed zonation in the follow-ing sections.

5. Discussion

5.1. Sources of major ions and processes affecting

water quality

The results of the study reveal that several ongo-ing physical (e.g., rain and wind) and chemical pro-cesses (e.g., surface weathering) influence the waterquality in Zeekoevlei. The dominance of Na+ andCl� ions in the lake water results in high Na+/(Na+ + Ca2+) and Cl�/Na+ (0.8 and 1.05, respec-tively) mass ratios. Sea salts, which are supplied bymist, salt-laden on-shore winds and coastal rainfall,are the major source of Na+ and Cl� (Gibbs, 1970;Mphepya et al., 2004). This finding is supported bysimilar observations in other wetlands in the south-western Cape (e.g., Silberbauer and King, 1991),and in rainwater collected on the premises of theGeology Department at University of Cape Town(unpublished data). Smith and Compton (2004)reported K+/Cl�, Mg2+/Cl�, Ca2+/Cl� and SO2�

4 /Cl�mass ratios of 2, 10, 2 and 5, respectively, in sea-water. The low K+/Cl� (0.09), Mg2+/Cl� (0.13),Ca2+/Cl� (0.23) and SO2�

4 /Cl� (0.4) mass ratiosimply that seawater is not the predominant sourceof these ions in Zeekoevlei. Instead, ground waterand the calcareous sandy sub-stratum supply Ca2+

and Mg2+ ions, whereas soil dust could be the mainsource for SO2�

4 (Mphepya et al., 2004). The overalllow concentration of major ions results in a low TDSvalue (Gibbs, 1970; Day et al., 1998) in the lake.

The pH of the influent Lotus Rivers and groundwater percolating into the lake is high (pH 8; South-ern Waters Ecological Research and Consulting,2000). Further, photosynthetic consumption of dis-solved CO2 by algae can result in less carbonic acidformation thereby raising the pH in Zeekoevleiwaters. The DOC concentrations (Table 2), how-ever, are less than expected for this hyper-eutrophiclake (Thurman, 1986). Elevated bacterial consump-tion could cause these low DOC concentrations(e.g., Arvola and Tulonen, 1998; Søndergaard andWorm, 2001). Consistent with this idea, the highnumbers of faecal coliform bacteria in the influentLotus River (Grobicki, 2001; Harding, 1992), andhigh lake water BOD levels measured in this studyindicate high microbial activity in the water column.Finally, the high DO values (Table 2) could result

from algal photosynthesis and strong winds, whichkeep the water column well-mixed and oxygenatedthroughout the year.

5.2. Sources and distribution of metals in the water

column

Dissolved metal concentrations in Zeekoevlei(Table 3) are generally high in comparison to otherfresh water systems (Forstner and Wittmann, 1998;Hamilton-Taylor and Davison, 1995; Chen et al.,2000; An and Kampbell, 2003; Nguyen et al.,2005). In particular, dissolved Cu concentration(�89 lg L�1) during winter exceeds the US EPAwater quality criteria (Table 3) recommended foraquatic life forms (Environmental ProtectionAgency, 2005). However, metal concentrations arelower than the drinking water guidelines proposedby the World Health Organization (2004) and USEPA (Environmental Protection Agency, 2003;Table 3). One of the reasons behind the low dis-solved metal concentration is low pollutant inputfrom local industries in the surrounding areas. Thisis indicated by the low PL value (0.0005 ton of oilequivalent/m3), and is not surprising given the factthat there are few industries located within thecatchments of the influent Lotus Rivers. Instead,the most likely anthropogenic sources of metalsare fertilizers, sewage input and road runoff (Grob-icki, 2001). In addition, natural sources (i.e., chem-ical weathering) could also contribute towards thedissolved metal load. Below the primary factors thataffect the distribution of metals in the water columnare discussed.

Phytoplankton are good scavengers of metalsdue to their fast growth rate and high surfacearea/volume ratio (e.g., Fisher, 1985; Twiss et al.,1996; Baeyens et al., 1998). Several studies havedescribed metal uptake by phytoplankton as oneof the main processes involved in transport andremoval of metals in different environments (e.g.,Noriki et al., 1985; Gonzalez-Davila, 1995; Markertet al., 1997; Shaked et al., 2004). Incorporation ofmetals by primary producers in the surface waters,and their rapid sedimentation (e.g., Twiss et al.,1996) can result in low concentrations of dissolvedmetals. Notably, zooplankton can regenerate dis-solved metals in surface waters by grazing phyto-plankton (e.g., Lee and Fisher, 1994; Yu andWang, 2002). However, zooplankton are not com-pletely dependent on phytoplankton for foodbecause of their poor nutritional value and potential

S.K. Das et al. / Applied Geochemistry 23 (2008) 2496–2511 2505

toxicity (Combrink, 1994) and therefore, wouldhave limited control on the dissolved metal dynam-ics in this lake.

Restricted mobility of metals under high pH con-ditions results in low dissolved metal concentrations(Stumm and Morgan, 1996). High pH favourshydrolysis and can generate Fe and Mn oxy-hydroxides, which serve as scavengers (Stumm andMorgan, 1996), and results in co-precipitation ofother metals. High pH also results in increased com-plexation of metals with organic matter (colloidal orparticulate fraction; >0.45 lm; Benoit, 1995), andthus decrease the dissolved unbound metal fractionin the water column. High dissolved Ba, Cu, Fe andSi concentrations in winter result from increasederosion (chemical weathering) and supply of silt-sized particles during the rainy winter season. HighSi concentrations, which further increase duringwinter, suggest that these waters are oversaturatedwith respect to quartz (equilibrium solubility ofquartz is 2.8–6.5 lg L�1 at surface temperature;Yanes et al., 2006). This also implies that diatoms,siliceous algae and siliceous flagellates (>0.45 lm)are less important primary producers in Zeekoevlei(see Harding, 1992).

Proximity to the Atlantic Ocean and unrestrictiveterrain results in strong wind (mean annual windspeed exceeds 6 m s�1; Harding, 1997) with a deepwave base that induces sediment resuspension (Har-ding, 1996). Sediment resuspension can increase thecolloidal fraction of elements (<0.45 lm) and resultin relatively high concentrations of dissolved Ba,Mn, Pb, Si and Zn in the bottom lake waters. Thepartition coefficient (Kd) for individual elements iscalculated after Zhang (1995).

Kd ¼CTM � 103

Cw

ð6Þ

where Cw and CTM represent the concentration(lg L�1) of metals in the dissolved and particulatephase, respectively. Kd values measured in summerand winter show very different signatures with gen-erally higher values in summer than winter for sur-face waters (except Mn and Ni). Lower Kd values(Table 5) of metals in winter surface water indicatea higher supply of dissolved metal rather than theweak association between the dissolved and particu-late fractions of metals. However, relatively highlogKd values (P3.5) for Cr, Fe, Mn, Ni and Pb (Ta-ble 5) in bottom water in winter indicate high parti-cle reactivity (Benoit et al., 1994), which results inincreased sorption of these metals to the solid phase,

and explains their high particulate concentration inbottom water (Table 3). In contrast, elements withlow Kd values (e.g., As and Mo) have low particlereactivity and stronger potential to form stable com-plexes, and remain in the dissolved phase (Benoitet al., 1994). However, the high particulate Zn con-centration (130 lg L�1) observed during winter inbottom waters cannot be explained by the aboveKd concept alone.

5.3. Preservation of sedimentary geochemical record

Several in situ post-depositional processes (e.g.,changing redox conditions and microbial activity)can affect the organic and elemental record in sedi-ments (e.g., Cornwell, 1986). Low Fe/Al ratio(Lyons et al., 2003) and low P concentration couldindicate reducing conditions in zone II (Figs 2 and3). Under such conditions, Fe2O3 will dissociateand generate Fe2+ ions causing the release of metalsinto pore water. The released metals should eithermigrate upward and re-precipitate with Fe andMn oxy-hydroxides, or diffuse downward and re-precipitate with Fe monosulphides (FeS) in deeperlayers (Davison, 1993). The increase in Fe concen-tration in zone III (Fig. 3) cannot be associated withprecipitation of FeS because insufficient S is presentat these depths. Furthermore, a decrease in metalconcentrations with greater depth will be expectedif reducing conditions persist below zone II. Thisis however not evident in Zeekoevlei sediments.Rather, metals show a downward increasing trendwith depth (Fig. 3). These observations suggest thatthe sediments are oxic and the cores have notreached the redox boundary. Therefore, variationsin element concentrations are not necessarily associ-ated with a change in redox condition; rather theyare controlled by other factors as discussed below.

The steady pattern of pore water DOC, and thepersistence of pore water SO2�

4 (55–200 mg L�1)and NO�3 (1.0–0.2 mg L�1; see Fig. 2) concentra-tions with depth suggest an absence of significantmicrobial activity (e.g., Routh et al., 2007). Thelow TS (<0.09%) content and negligible TRS values(<0.06%) further support the lack of microbial SO4

reduction. Consistent with this, the low Mo/Alratios (67 � 10�5), which fall within the range ofaverage shale and upper continental crust (Taylorand McLennan, 1985; Wedepohl, 1995), imply envi-ronmental settings with high O2 and low sulphidelevels (Lyons et al., 2003). Because of the prevailingoxygenated condition, electron donors available at

2506 S.K. Das et al. / Applied Geochemistry 23 (2008) 2496–2511

these depths cannot be coupled to bacterial SO2�4

and NO�3 reduction. The high concentration ofNO�3 (13 mg L�1) at 20–22 cm depth could howeverresult from ground water seepage (Southern WatersEcological Research and Consulting, 2000). Finally,there was no evidence of bioturbation such as bur-rows, chironomid mounds or epibenthic fauna inthese sediments. Hence, the depositional history ofelements in these sediments is evidently well-preserved.

5.4. Organic matter sources and productivity

TOC concentrations have widely been used todescribe the abundance of sedimentary organic mat-ter (e.g., Meyers and Teranes, 2001; Meyers, 2003)and changes in lake palaeoproductivity (e.g., Schel-ske and Hodell, 1991; Routh et al., 2004, 2007).Atomic Corganic/Ntotal ratios, on the other hand,have been used to study the source of organic mat-ter (e.g., Meyers and Teranes, 2001; Meyers, 2003).The steady increase in TOC concentrations from thebottom of the core up to 20 cm, and further up-corein zone I suggests higher input of organic matter(Fig. 2). The C/N ratios around 10 (Fig. 2) furtherindicate algae as the dominant source of organicmatter (e.g., Meyers and Ishiwatari, 1993; Meyers,1994) at these depths.

Archival records from this lake indicate anincrease in lake productivity in recent years.Enhanced productivity due to the nutrient seepagefrom the WWT plant since the 1920s has deliveredhigh levels of organic matter into zone III. Anaggressive pondweed eradication programme hasreportedly increased primary productivity in thelake since 1951 (Southern Waters EcologicalResearch and Consulting, 2000). This results inthe steady upward increase of TOC concentrationin zone III sediments. However, zone III sedimentsdo not seem to contain organic matter derived frompondweed that sank after eradication (Das et al.,2007). This is probably because of the fact thatpondweeds only existed in the shallow parts of thelake, whereas the cores were collected from thedeepest section. Furthermore, rapid urbanizationwithin the catchment of the Lotus Rivers since thelate 1990 post-apartheid period has greatlyenhanced nutrient (raw sewage) delivery into thelake (Grobicki, 2001). Finally, accelerated farmingin the Philippi Horticulture area over the last twodecades has supplied agricultural waste, and Nand P based fertilizers into the lake (Water Research

Commission, 2001). This has resulted in enhancedprimary productivity and organic C accumulationin zone I. The high TOC concentration (14%) in sur-face sediments represents freshly deposited organicmatter. Notably, both% N and P show an increasingupward trend in zone I; P concentration is severalfold higher than in deeper sediments. The similaritybetween N and P profiles in zone I suggests a com-mon source i.e., input from sewage and fertilizers.However, N does not follow the P trend in zone IIand III sediments; this implies N and P do not sharea common source at these depths. The N/P ratio hasbeen widely used to identify nutrient limitation forprimary productivity (e.g., Lee et al., 1996). Theclose similarity between% N and TOC along withthe low N/P (molar) ratio (0.01–0.07) clearly indi-cate that productivity in the lake is more likely N-limited (Harding, 1996).

The sedimentary P record (Fig. 2) does not showany evidence of closure of seepage channels fromthe WWT plant between 1970 and 1980. This isbecause the WWT plant is probably not the majorsource of P at these depths. Instead, ground waterand PO3�

4 based fertilizers used in nearby agricul-tural farms are probably the major sources of Pinput (Southern Waters Ecological Research andConsulting, 2000). Moreover, dredging (1983)related re-suspension and sub-aerial exposure oforganic-rich fine sediments cause rapid degradationof organic matter and further decrease the TOCcontent in zone II (Das et al., 2007). The absenceof significant microbial degradation and grain sizevariation at these depths, support the above obser-vation. Consistent with this, an increase in the Zr/Al ratio (Fig. 3) and high silt + clay content(Fig. 2) indicate an increase in siltation in zone IIas expected from dredging generated resuspension.High winds can keep the sediments suspended forseveral years (Harding, 1997) and thereby increasethe turbidity in lake water. High turbidity candiminish algal biomass by limiting photosynthesis,and this in turn, decreases the net TOC input.

5.5. Element source, distribution and toxicity insediments

Chromium, Si, Ti and Zr show CM/CUCC ratiolower than 0.5. These elements are considered rela-tively immobile during chemical weathering due totheir low solubility, and are generally transportedin refractory minerals (quartz, ilmenite, chromiteand zircon; Yanes et al., 2006). Therefore, their

S.K. Das et al. / Applied Geochemistry 23 (2008) 2496–2511 2507

low ratios imply the insignificance of physicalweathering process supplying trace and major ele-ments in Zeekoevlei sediments. Moreover, low Zr/Al ratios throughout the core indicates the domi-nance of chemical a weathering product (Fig. 3).Consistent with this, low Si/Al (0–1.6) and highCM/CUCC ratios for Ca (1–5), K (1–5) and Sr (1–4) throughout the core represents the dominanceof feldspathic fraction. Low Na/Al (0.18–0.34),Na/K (0.02–0.80) and Na/Ti (1.19–2.47) ratios withhigh concentrations of Al, Fe, K, Mg and Mn insediments (Fig. 3) support the above observation,and suggests chemical weathering products as themajor natural source of metals (e.g., Ba, Cr, Fe,Mn, Mo and Ni) in Zeekoevlei sediments. Nearunity Af values for Ba, Cr, Mn, Mo and Ni indicatetheir natural source and further support the finding(Table 5). The major anthropogenic sources of met-als (e.g. Cd, Cu and Zn) and As, on the other hand,can be from phosphate based fertilizers, agriculturalwaste, sewage input and road runoff as these are themajor source of pollutants in the lake. Likewise,higher than unity Af values for As, Cd, Cu, Pband Zn (Table 5) in zone I sediments suggest a sig-nificant anthropogenic input of these elements.Rapid urbanization in catchments during the post-apartheid (1994) period is primarily responsiblefor this increase. The Igeo values suggest that zoneI sediments are very highly polluted with Pb(Igeo > 5, grade 6) and highly polluted with Cdand Zn (4 < Igeo < 5, grade 5). Leaded petroleumwas used in South African automobiles until 2005,and can constitute a major source of Pb in Zeekoev-lei. The relatively high Pb concentrations in surficialsediments (114 mg kg�1), which is substantiallyhigher than the average Pb concentration in uncon-

Table 4Correlation matrix of TOC, trace and major elements in Zeekoevlei se

TOC Ca Al As Ba Cd

Ca 0.89Al 0.88 0.86As 0.54 0.48 0.34Ba 0.81 0.94 0.86 0.58Cd 0.33 0.25 0.09 0.94 0.34Cr 0.79 0.79 0.81 0.72 0.92 0.50Cu 0.09 0.06 �0.14 0.86 0.14 0.95Fe 0.85 0.85 0.98 0.24 0.83 �0.03Mn 0.72 0.88 0.75 0.66 0.95 0.43Mo 0.74 0.81 0.57 0.66 0.72 0.48Ni 0.66 0.68 0.49 0.92 0.76 0.81Pb 0.10 0.07 �0.13 0.85 0.16 0.95Zn 0.14 0.06 �0.05 0.85 0.23 0.91

taminated soils of South Africa (12 mg kg�1; Adri-ano, 2001), could result in toxicity to sedimentdwelling organisms (e.g., Peltier et al., 2003). Thesediments are unpolluted with respect to the remain-ing measured metals (Igeo < 1; grade 1). Further-more, the Igeo values suggest that Zone II and IIIare unpolluted. Although sodium arsenite wasspread in 1951, the overall low As concentration(Fig. 3) in sediments did not reflect this activity.

Trace metal accumulation in sediments resultsfrom a combination of complex and concurrentlyoccurring processes. Accumulation is often domi-nated by adsorption on the finer fraction of sedi-ments, precipitation and/or co-precipitation on Al,Fe and Mn oxy-hydroxides and complexation withorganic matter (e.g., Davis and Leckie, 1978). Ingeneral, trace metal-sediment interaction canbroadly be divided into a fast surface sorption pro-cess that takes place during a relatively short periodfollowed by redistribution between different bindingsites until equilibrium is reached (Standring et al.,2002). Sediment characteristics (e.g., clay fraction,Al, Fe, Mn and TOC) affect these interactions(Horowitz, 1991; Standring et al., 2002). Significantcorrelation (r P 0.75) of Ba and Cr with Al andFe, and Ni with Mn (r P 0.85) implies adsorptionof these trace metals to Al, Fe and/or Mn oxy-hydroxides in oxygenated sediments (Dong et al.,2000; Widerlund et al., 2002). The average CIA ofsediments (67; range 64–76) is very close to the aver-age shale value (Nesbitt and Young, 1982), andreflects the dominance of clay minerals, which fur-ther contribute towards trace metal (e.g., Cd, CuPb and Zn) sorption. Finally, significant correlation(r P 0.68; Table 4) of Ca with Al, Ba, Cr, Fe, Mn,Mo and Ni most likely indicates sorption of metals

diments

Cr Cu Fe Mn Mo Ni Pb

0.310.79 �0.260.88 0.28 0.710.66 0.35 0.52 0.830.79 0.69 0.41 0.85 0.850.32 0.98 �0.25 0.30 0.36 0.710.39 0.94 �0.16 0.34 0.38 0.70 0.92

Table 5logKd values in waters and sediment quality in ZeeKoevlei

Al As Ba Cd Cr Cu Fe Mn Mo Ni Pb Zn

logkd Summer nm 30 4.7 nc nc nc 5.4 4.6 nc 3.4 nc 4.3Wintera nm 2.7 3.4 nc 3.5 2.4 4.2 5.6 nc 3.5 3.7 4.2Winterb nm 2.7 3.2 nc 3.7 2.4 4 3 4.1 2.5 3.5 3.6 3.2

Af 0.6 2.3 0.9 4.0 0.9 6.4 0.4 1.0 1.4 1.6 11.2 5.5Ef Zone I 0.7–1 5 7.6–24 0.9–1.6 18–48.8 3.1–5.9 8.6–22.4 0.8–2 0.9–1.9 2.6–5.8 3.7–5 16.3–74.8 15.9–43.7

Zone II 0.6–2 1.3–5.6 0.8–1.4 56–13.2 1.3–5 1.6–5.6 0.4–38 0.5–1.4 0.7–4.5 1.6–4.1 2.3–20.4 2.8–10.9Zone III 1.9–2.3 4.8–7.7 1.4–1.5 5.8–16.5 4.8–5.6 1.5–2.3 3.9–4.5 1.5–1.8 2.7–3.5 3.2–3.9 3.7–4.5 3.9–4.8

a Surface water.b bottom water; nm = not measured; nc = not calculated.

2508 S.K. Das et al. / Applied Geochemistry 23 (2008) 2496–2511

in calcareous sediments (e.g., Adriano, 2001) as animportant mechanism for metal enrichment inZeekoevlei.

Hamilton-Taylor and Davison (1995) suggestalgal production and degradation of algal organicmatter as one of the main processes affecting ele-ment cycling in lakes. Likewise, metal enrichmentin marine sediments has been related to primaryproductivity (e.g., Sternbeck et al., 2000; Monteiroand Roychoudhury, 2005). The role of primary pro-ducers in scavenging metals from the water columnhas already been discussed. The release of metals atthe sediment-water interface during decompositionof dead planktonic biomass (e.g., Balistrieri et al.,1992) is another aspect that needs to be consideredin this shallow lake. The moderate (r = 0.54) to highcorrelation (r = 0.66–0.88) between elements (Al,As, Ba, Cr, Fe, Mn, Mo and Ni) and TOC (Table4) results from the observed close match betweenelement concentration and TOC depth profiles,and indicates the important role of primary produc-ers in element enrichment in sediments. Hence, itseems likely that increase in primary productivityand deposition of planktonic debris could contrib-ute towards greater accumulation of different ele-ments such as Al, As, Ba, Cr, Fe, Mn, Mo and Niin zone I and III. In contrast, low productivityresults in low element concentration in zone II(Fig 3). Interestingly, Cd, Cu, Pb and Zn have nosignificant correlation with TOC (r 6 0.33). Thesemetals are toxic to phytoplankton (Sunda andHuntsman, 1998) and hence, their enrichment isindependent of productivity in the lake. In addition,their enrichment except for As, is not related tosorption by Fe or Al oxy-hydroxides.

Ef values show significantly higher enrichment ofAs, Cd, Cu and Pb (Table 5) in zone I compared tothe deeper sediments. Several ongoing processes canresult in high element (Al, As, Ba, Cd, Cr, Cu, Fe,

Mn, Mo, Ni, Pb and Zn) accumulation in surfacesediments. These processes probably occur concur-rently and they are: (1) increased productivity andhigher accumulation of planktonic debris on thelake floor, (2) hydrolysis of metals under alkalinecondition (e.g., Borg et al., 2001; Wallstedt andBorg, 2004), (3) increased anthropogenic input and(4) increased siltation. Excessive siltation due tohigh erosion from urban and agricultural activitiesin the catchments has been reported in Zeekoevlei(Southern Waters Ecological Research and Consult-ing, 2000). This is supported by the observed increasein silt and clay content to 88% in surface sediments(0–2) from an average of 60% in deeper sediments(zone III; Fig. 2). In addition, although vehiculartraffic is relatively limited in the area, deposition ofmodern roadside dust cannot be ruled out.

6. Conclusions

Zeekoevlei demonstrates the affect of culturalactivities over the natural in-lake processes in shal-low lake environment. Although, the greater frac-tion of trace and major elements supplied in thislake have natural sources (e.g., chemical weatheringand sea salt), an increase in anthropogenic inputs(e.g., fertilizers, agricultural wastes, sewage and roadrunoff) in recent years is observed from the higheraccumulation of As, Cd, Cu, Pb, Zn and P in surfacesediments. Moreover, the dissolved element concen-tration is seasonally controlled and sediment resus-pension affects the dissolved and particulateelement concentration in bottom waters. Phyto-plankton act as scavengers of dissolved elementsfrom the water column and primary productivitycontrols the enrichment of most elements in Zee-koevlei sediments. Besides, metal oxy-hydroxides,clay minerals and calcareous sediments play animportant role in adsorption of metals. Historical

S.K. Das et al. / Applied Geochemistry 23 (2008) 2496–2511 2509

events (e.g., pondweed eradication and dredging)have affected primary productivity, and these humaninduced modifications within the lake and catch-ments are reflected in the sedimentary geochemicalprofiles.

Acknowledgements

The authors gratefully acknowledge the financialsupport of SIDA–NRF and SMF for this study.Heather Sessions, Geof Bailey of Marine and Coast-al Management, South Africa and Roger Herbert ofUppsala University are acknowledged for theirtechnical support. Dalton Gibbs of Rondevlei Nat-ure Reserve is acknowledged for providing informa-tion on Zeekoevlei. Eric H. De Carlo and KevinTaylor are acknowledged for reviewing the manu-script. This is an AEON contribution #50.

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