Applying of Modified Constant Rate of Supply Model to Lake Sediments in 210 Pb Dating and Assessment of Some Heavy Metals

In present study, Lead-210 (Polonium-210) activity concentrations were indirectly obtained by utilizing alpha spectrometry. Sediment chronology was determined by using modified CRS model. Average sediment accumulation rates are 0.351, 0.324, 0.222 g cm -2 y -1 in S-1, S-2, S-3 stations respectively. With reference to atmospheric 210 Pb flux (29 mBq cm -2 y -1 ) Lake Karagol has extra outer 210 Pb input (soil erosion) from the catchment area. In terms of Enrichment Factor (EF), heavy metal concentrations are lower than the anthropogenic values. So it can be said that the elements except Au originate from the continental supply. Au has unexpectedly too high level EF value (58.22) in northern station (S-1) of the Lake Karagol. It corresponds to time interval from 2004 to nowadays. However Pollution Load Index (PLI) says different consequences from the EF values. PLI value has only baseline levels of pollutants in northern part of the Lake Karagol as it shows progressive deterioration of the site for southern part of the Lake Karagol.


Introduction
The naturally occurring radioactive nuclide Lead-210 ( 210 Pb) has often been used in environmental studies because Polonium ( 210 Po) and 210 Pb (half-life is 22.3 y) are well known to be particle reactive in marine environment [1]. 210Po (half-life is 138 d) is an alpha particle emitter in the Uranium-238 ( 238 U) decay chain with high energy and it is a naturally occurring radionuclide formed by the beta decay of its grandparent 210 Pb by the agency of Bismuth-210 ( 210 Bi). 210Pb and 210 Po have been used extensively as an important tracer for particle transport, studying processes of chemical scavenging, atmospheric analysis, describing the sediment type for pollution point of view ( 210 Po/ 210 Pb ratio), geochronology and dating sediments in marine environment [2].Especially 210 Po is important tool for determining the sediment characteristics and it provides the foresight to radionuclide sorption capability of the sediment. 210Po/ 210 Pb ratio also gives other information on water and sediment columns in lakes, seas and oceans.Determination of this ratio in different type of samples such as in sediment profiles, shallow and deep waters, is convenient to obtain the relations between study area and impacts of study area.Investigation of radioactive disequilibria about the pairs 210 Po -210 Pb and 210 Pb - 226 Ra within the oceanic water column determines the rates of scavenging processes [3].There are two different type of 210 Pb supply in coastal (Radon-222) and shelf (Radium-226) waters.Scavenging is important tool to transfer of 210 Pb from water column to the sediment layers. 210Pb is also important tool for determining the chronology.Lead dating models [4][5][6][7][8][9] have entirely applied in lake sediments and they have prosperously improved by new kind of approaches [10][11][12][13][14][15][16][17][18][19][20][21] to the models for advance correctness of the chronology.Models are commonly derived from the profile distribution of 210 Pb activity concentrations.Thus character of the 210 Pb distribution is highly descriptive for model choice.Another significand parameter is atmospheric 210 Pb flux.Profile distribution of 210 Pb is directly affected by the atmospheric 210 Pb flux in sediment core [4,8,11,15].If high supply of 210 Pb concentration is existing in any station, it will indicate sediment focusing or significant amount of contribution from the catchment area [22].Role of the heavy metals which is originating from human activities or from natural contributions is an important impact on environment.Thus pollution investigations are crucial part of geochemical studies [23][24][25].According to Nieboer and Richardson [26], some heavy metals are normal constituents of the marine environment.Fe, Cu, Zn, Co, Mn, Cr, Mo, V, Se and Ni are known to be essential to marine organisms, they always function in combination with organic molecules in general proteins.However, Ag, Hg, Cu, Cd and Pb are particularly toxic [27].
The first goal of the present study is to determine the recent chronology in Lake Karagöl by using modified CRS (Constant Rate of Supply) model.Second one is to evaluate the state of lake in term of potential erosion or any other similar effects.Last one is the clarification of metal contamination degree by utilizing the some pollution indexes such as Enrichment Factor (EF), Contamination Factor (CF) and Pollution Load Index (PLI).
There is not any research about sediment dating or sediment accumulation rate especially using modified CRS model, heavy metal pollution degree via Pollution Load Index for Lake Karagöl in literature.To understand the accumulation pattern, the relation between sediment accumulation rates and metal load rates was investigated. 210Pb flux to the lake surface was determined by using core inventories to compare with the global mean 210 Pb flux and to detect whether there is erosion.Correlations among metal concentrations were calculated and correlation also was clarified between northern station and southern station in lake.

Material and Method
Lake Karagöl is a crater lake and has had the area of 2 km 2 .Lake lies between at the 38° 57' 30.8''N latitude and 26° 50' 55.6'' E longitude.It is about 55 km away from the center of Izmir province.Depth has reached the maximum value at southeastern part with 7.5 meter however it has fallen into 1.8 meter towards north and west part of the lake.Catchment area is closed by native forest which is consists of the pine trees [28].Sampling stations provide the well representation of the lake in terms of sedimentation pattern, catchment impacts and human activities.These factors were considered in choosing the sampling stations.S-1 is taken place in the northern section of the lake and it is vulnerable to the catchment impacts such as erosion and human activities.S-2 station is taken place in the southeastern part of the lake and it is protected from catchment impacts besides it has maximum water depth.S-3 is taken place in the southwestern section of the lake and it is also protected from catchment impacts and it has minimum water depth.

Radionuclides
In present work three sediment cores obtained from the Lake Karagöl, one of them is from northern and others are southern part of the Lake in 2015.Figure 1.shows the working area.Depths of the cores vary between 27 and 44 cm.There are two stages in preparation of samples for analysis.First stage is physical preparation.It consists of the following steps.The cores were displaced from the PVC pipes and cut in one cm intervals.Before sediment samples were oven dried, wet weights of the samples noted.Then they were ground and passed through a 63 µm mesh to reach homogenization.Second stage which is consists of the chemical steps is used for the radiochemical analysis.One gram sediment sample and standardized 0.1 Bq mL -1 amount of 209 Po (halflife: 103 years) internal tracer (National Institute of Standards and Technology, ˂500 Bq in 7% hydrochloric acid) were completely dissolved in HF:HNO3 (1:1) and HCl in Teflon beaker.After the ascorbic acid addition, polonium was spontaneously plated onto copper discs in 0.5 M HCl solution.Ascorbic acid was used to reduce ferrum ions [29].To measure 210 Po concentrations 5.30 MeV alpha particle emission which comes from 210 Po discs were counted in alpha dedectors (Ortec Octete Plus with 450 mm 2 ULTRA-AS Detectors).After the first deposition of polonium, residual 0.5 M HCl solutions were kept for about sixt months to form of 210 Po that was supported 210 Pb [30].Polonium which is formed from 210 Pb in the kept 0.5 HCl solution, was plated again onto discs to determine the 210 Pb activity.Counting period was adjusted to keep the standard error under the 10% (relative standard error was approximately 5%).The recovery rates of standardized tracer for the sediment samples varied 73% and 86%.Bateman equations and recovery were used to determine 210 Po ( 210 Pb) concentrations [31,32].
In calculation of the sediment accumulation rates, modified CRS (Constant Rate of Supply) model was utilized.Modified CRS model bases on the ratio of the cumulative residual unsupported 210 Pb for a given depth to the total unsupported 210 Pb activity in the sediment column.The inventory is calculated as below: Where () (g cm -3 ), () (Bq g -1 ) are the sediment bulk density and excess 210 Pb activity at depth  (cm) and () (Bq cm -2 ) is the cumulative unsupported 210 Pb below depth .For a given depth  the age  (y) of the sediment is calculated like below, Where  0 (Bq cm -2 ) denotes the total inventory of the core.Prevailing sediment accumulation rate (g cm -2 y -1 ) at depth  is calculated as below [33].

Heavy metals
Contamination investigations are of great importance in terms of study area evaluations.To realize the this type of evaluations, normalization is crucial.Thus pollution degrees and enrichment factors (EF) of elements are computed by employing normalization in study area sediments.In general, Al, Sc, Fe, Zr are widely employed in normalization process.Al is commonly utilized choice [31,[34][35] to clarify EFs.It also compensates the fluctuation on both particle size and sediment matrix in coastal sediments [23].
Although Fe is also used in normalization [36][37][38][39][40][41][42], Din [43] and Rubio et al. [44] are mentioned that Fe is originated from anthropogenic pollution [45].Beside of this Sc and Zr are successfully utilized in normalization [46].EF value is calculated via the formula which is suggested below [47][48][49]: Where  is any element,     is the concentration ratio of X to , index S is surface layer of sediment column, index C is Earth's crust.Beside of this the background values in general could be immediately obtained from the bottom layer [23].Nolting et al. [50] argues that if EF is higher than 10, it will indicate that sediment is polluted.However some authors [51][52][53] argue that If EFs varied 0.5 and 2.0 sediments will be independent of human impacts.However they are over 2.0 this implies significant anthropogenic inputs [45].Anthropogenic factor (AF) values were obtained by using formula [49].Below; Here C s is the surface layer concentration and C d is the bottom layer concentration of sediment column [54].The other name of Anthropogenic factor is Contamination Factor (CF) [40,55].In here, C d value is identified as the 30 th layer of sediment column [31].Contamination factor generally is used in combination with Pollution Load Index (PLI) in evaluation of metal pollution.According to Tomlinson et al. [56]; For a site where  contamination factor and  number of metals, Where,  is the number of sites for a zone [40].PLI provides an idea to evaluate a site or zone quality.If PLI is 0.0, it shows perfect quality, 1.0 shows that there is only starting level pollution present and >1 shows acute corruption of the zone.Metal concentrations of the investigated stations were measured via ICP-MS technique in laboratory (Acme) and some standards were (DS9, NIST-981-1Y, NIST-983-1Y) utilized.
In this study, SPSS package program was used to realize the statistical analysis.Variables were the profile distributions of the metals' concentrations.
Bivariate correlation analysis was applied to the vertical distributions of metals.Bivariate correlation analysis is a method which points out the magnitude, significance and trend of relation between two variables.In this method, it is unnecessary whether both variables are dependent or independent.Therefore bivariate correlation (two-tailed, pearson correlation) was applied to vertical distributions of metal' concentrations.

Results
The total 210 Pb activities are ranged from 109.6 ± 8.

Table 1. Sediment accumulation rates and the chronology by means of the modified CRS model
Element concentrations have measured as Fe > Ca > K > Ti (%), V > B > Tl > Cd (ppm), Au (ppb) in northern part, as Fe > Ca > K > Ti (%), V > B > Tl > Cd (ppm), Au (ppb) in southwestern part of the lake (Table 2.).K, Cd, Tl, and Au are anthropogenically enriched in upper layers of core S-1 but Ca and K are anthropogenically enriched in upper layers of core S-3.Rice of above elements from bottom layer to upper one is not continuous.Table 3. shows correlation coefficients.Figure 3. and Figure 4. display that elevated element values were observed, especially at 1-7, 14 cm in northern station.It was also observed especially at 5, 16 cm in southwestern station sediment profile.Sediment accumulation rates ranged from 0.034 ± 0.002 to 0.592 ± 0.025 g cm -2 y -1 along the core S-1.
Sediment accumulation rates (SAR) were given in Table 1.SAR did not reach to hill values.They could be examined in two parts (1,2).Part 1 has first nine layers of the core.Part 2 has the remaining layers of the core.Lower SARs are seen in Part 2 and range from 1861 to 1991 years but higher SARs are in Part 1 and also range from 2001 to 2013 years.According to the core inventory (941.54 mBq cm -2 ) 210 Pb flux was calculated as 29.28 mBq cm -2 y -1 .Mean SAR is 0.351 ± 0.016 g cm -2 y -1 .
S-2 station shows that SAR ranged from 0.029 ± 0.001 to 0.590 ± 0.025 g cm -2 y -1 in core.SAR displayed noncontinuous decrease from top to bottom levels especially upper eight levels of the core. 210Pb flux and inventory are 21.65 mBq cm -2 y -1 and 696.13 mBq cm -2 in core S-2 respectively.Mean SAR is 0.324 ± 0.015 g cm -2 y -1 in S-2.
Sediment accumulation rate is lower than the others in S-3 station.It ranged from 0.021 ± 0.001 to 0.359 ± 0.010 g cm -2 y -1 along the core S-3.SAR also displayed noncontinuous diminish from top to bottom layers especially upper fifteen layers of the core. 210Pb flux and inventory are 19.35mBq cm -2 y -1 and 622.20 mBq cm -2 in core S-3 respectively.Mean SAR is 0.222 ± 0.008 g cm -2 y -1 .It is the lowest one in sampling area.SARs have presented in Table 1. for all stations.
Similarly it has differences on 210 Pb fluxes which are measured in stations of study area.Krishnaswami and Lal [57], denoted the global mean value of atmospheric 210 Pb flux as 165 Bq m -2 y -1 but Appleby and Oldfield's [58] estimation was 185 Bq m -2 y -1 .
Fluxes which were measured in study area are slightly greater than both the global mean value and Mediterranean field value (75 Bq m -2 y -1 ) [59].
Sediment accumulation rates were determined by using Lead-210 model.When glimpsed to the profile distribution of 210 Pb concentrations it is seen that R 2 values are lower than the 0.950 for northern and southeastern stations.In addition atmospheric flux is not the dominant factor on controlling the 210 Pb supply.Thus in clarifying the sediment chronology modified CRS model was utilized in study area.SAR is generally similar in northern and southeastern stations and higher than it in southwest station.They do not show regular diminish along the cores.By means of Table 3. it is seen that the trend for Fe has a strict correlation with the trend for Ca (0.735**), K (0.493*), V (0.737**), Au (0.540*) in northern station(** p<0.01, * p<0.05).However the trend for Fe has a strict correlation with the trend for only V (0.832*) in southern station.Similarly Ca has been followed by K (0.522*), Au (0.580*) in S-1 station but it has been followed by only K (0.831*) in S-3 station.Ti has been followed by V (0.725**) in northern station but it has no correlation with other elements in southwestern station.Cd has been followed by Au (0.781**) in S-1 station but it has also no correlation with other elements in S-3 station.V has been negatively followed by Au (-0.873**) in southwestern station.According to above statements it can be said that mentioned elements could be originated from the identical sources in both S-1 and S-3 stations of the lake.
Figure 3. and Figure 4. show that elevated element values in northern and southwestern stations were seen especially at 1-7, 14 cm in S-1 those levels correspond to 1989  There is no relation between sediment accumulation rates and enrichment of metals concentrations inasmuch as metals' concentrations display fluctuation.However sediment accumulation rates do not display similar character with metal concentrations along the core in northern station.
Core S-3 has elevated metal values too.They are especially at 5 th , 16 th layers of sediment column (Figure 4.) and correspond to 2000, 1956 years respectively.Ti and V show hill values at fifth layer of column.They correspond to 2000 year with concentrations of 0.14 (%) and 66.00 (ppm) respectively.V shows increased value at eleventh layer of column too.It corresponds to 1980 year.Ti also shows increased value at sixteenth layer of column.It corresponds to 1958 year.Neither sediment accumulation rate nor profile distribution of elements is systematic.Therefore it could be said that, the variations of the element concentrations are free from the SARs at southwestern section of the lake too.
Enrichment Factor (EF) is a entirely utilized approach to identify the grade of human impacts on environment [40].It is a suitable measure to compare pollution with geochemical trends.To fulfil this task there is three reference scale in literature.According to the Nolting et al. [50], an area is polluted by the metals when EFs of the metals are higher than 10.Next, EF values are greater than 5 samples will be considered as polluted [66].Zhang and Liu [67] argue that, if EF values range from 0.5 to 1.5 metals will originated from Earth's crust.However if EF values are higher than 1.5, metals will probably be related to human activities.
In core S-1, EF values ranged from 1.04 (V) to 58.22 (Au).They are lower than 2 except Au.Like it is mentioned above, there is no input in term of metal pollution except Au.In present work, EF values are evaluated in terms of the successions of Nolting et al. [50].However PLI value (1.70) indicates progressive deterioration of the site for northern section of the lake.
In core S-3, EF values for Ca (1.18) and K (1.20) indicate that sediments were not contaminated by Ca and K but were rich.PLI value (1.02) also indicates only baseline levels of pollutants present for S-3 station.
In terms of same metal northern station has no statistical correlation with southwestern station in the lake.However, Fe which belongs to S-1 station has a correlation with the B (0.81*) which belongs to S-3.Similarly Ca has a correlation with K (0.71*) and Cd (0.72*), next Ti has a negative correlation with Ca (-0.79*) and K (-0.80*), lastly Tl has a negative correlation with Cd (-0.85**).
Sediment radioisotope dating was realized and element contamination grade of the lake was clarified in present study.To provide the answers for identified tasks, profile distributions of 210 Pb activity concentrations and metal concentrations were evaluated.The results show that by means of the profile distribution of unsupported 210 Pb, it is seen that 210 Pb depositions to the lake is not systematic along the line which lies from northern station to southern station.Similarly 210 Pb fluxes calculated from core inventory display the changing character amongst stations and indicate the presence of soil erosion [22].Because, mean global 210 Pb flux is lower than the calculated one.It is known that if measured 210 Pb flux is higher than the atmospheric supply rates there will be outer contributions from the catchment area.Apart from this, as to sediment accumulation rates, they are not clearly systematic along the core profiles in all stations (S-1, S-2 and S-3).Mean SAR is 0.299 ± 0.013 g cm -2 y -1 in study area.Lake has no relation between SAR and enrichment of elements.Since sediment accumulation rates and metal concentrations along the cores act independent of each other in both northern and southern parts of the lake.According to EF values metals are originated from the crustal (natural) sources in northern and southern sections of the lake.It may be related to the crater lake feature of the Lake Karagöl.Since lake is located the Yamanlar Mountain and Yamanlar volcano has intense internal tectonic deformation [68].Similarly, AF values indicate that sediments are getting rich on account of some metals but not polluted.Au has interestingly too high level (58.22)EF value in northern station (S-1).It corresponds to time interval from 2004 to nowadays.According to PLI value, northern part of the lake has only baseline levels of pollutants as it indicates progressive deterioration of the site for northern part of the lake.This result is interesting because Lake Karagöl is a natural park and it is protected by natural park rules.The baseline level pollution may be originated from tectonic deformations or human activities because lake catchment has hosted the camping activities.So some more protective precautions should be taken for sustainable environment in Lake Karagöl.

3
Bq kg -1 to 54.2 ± 3.2 Bq kg -1 along the core in S-1 station.Similarly, they are ranged from 103.3 ± 4.7 Bq kg -1 to 59.7 ± 3.4 Bq kg -1 , from 119.5 ± 5.6 Bq kg -1 to 59.5 ± 3.6 Bq kg -1 in S-2 and S-3 stations respectively.226Ra activities were designated as 54.2 ± 3.2 Bq kg -1 , 59.7 ± 3.4 Bq kg -1 and 59.5 ± 3.6 Bq kg -1 from the deepest layers of the cores.Unsupported 210 Pb activities and their error bars were plotted vs depths and inclination curves were added to the graphs.Fitness between profile distribution of the unsupported 210 Pb concentrations and inclination curves was clarified by the regression coefficients in Figure2.R 2 value is 0.907 in S-1, 0.891 in S-2 and 0.984 in S-3.Therefore in calculation of sediment accumulation rates CRS (Constant Rate of Supply), CIC (Constant Initial Concentration) and CF; CS (Constant Flux; Constant Sedimentation) models are not compatible with each other.Sediment accumulation rates were determined by the modified CRS model.It was preferred rather than the others in all cores due to the R 2 values are lower than the 0.950.

Table 2 .
Element concentrations in stations