Reason for Study

By learning more about the amount and the distribution of trace elements in water and the sediment, we can learn more about the connection between water and sediment chemistry. We can also learn about the fate of the elements that enter the lake, and therefore help to trace elements that might be toxic to the biota in the lake. The goal of this report is to investigate the distribution of trace elements in the Lake Louise, a 5.4-hectare lake located in Lowndes County, Georgia.

Lake Inputs

There are four sources of water input to Lake Louise:

1. Input from rivers
2. Input from rain water
3. Runoff from surrounding wetlands
4. Interflow from shallow groundwater.

There are 3 main uptake mechanisms are (Hart, 1982):

1. Physico-chemical adsorption
2. Biological uptake
3. And the sedimentation of particulate matter enriched in metals.

Physico-chemical adsorption is the adsorption of metals onto solid organics, clay and metal oxides.

Adsorption onto Solid Organics: Solid organic materials strongly adsorb metal ions. The solid organics surfaces that can adsorb metal ions can originate in four different ways:

• from organisms like algae and bacteria
• by the breakdown of aquatic plant material
• by the adsorption of organic material onto clay minerals or metal oxides
• terrestrial organic detritus (Lerche, 1998)

The processes of adsorption of trace elements are not completely understood (Hart, 1982). One possibility is that the metals bond to the surface in the same way that they bond to ligands. Ligands are ions that make complex bonds with the metal atoms. The number of ligands that are able to attach are determined be the size of the ligand and not the valence of the metal atom as you would expect (Jensen; 1974). Factors that influence the processes are:

• the constant of formation of the complexing ligands
• the concentration of the species involved
• pH (maximal adsorption occurs when pH=5)
• ionic strength and
• the concentrations of competing ions.

2. Biological Uptake

The metals are adsorbed onto the algae cell or into the algae cell.

Different kinds of algal species have different of adsorption potentials (Hasset et. al. 19). This means that a small percentage of the total algal biomass may account for a much bigger percentage of the total amount of adsorbed trace elements.

A number of variables affect the ability of algae to adsorb trace elements (Hasset et. al.19):

• type of element
• oxidation state of element
• pH
• salinity
• the presence of organic pollutants
• and the length of exposure time (dependent on the discharge in the lake)
• species of algae

3. Sedimentation of Particulate Matter Enriched in Metals

Trace metals may come from the sedimentation of allochthonous or autochthonous inorganic matter. The amount of metals accumulated in this way depends on the magnitude and type of the lake water input. A big input and output means a short residence time and therefore a smaller percentage of the dissolved species on particulate matter join the biochemical cycles. A small input and output means a longer residence time and a bigger percentage of the incoming dissolved species on particulate matter will enter the biochemical cycles.
 
 

The Biochemical Cycles of the Trace Elements

The biochemical cycles of the trace elements are the continuous reactions that store or move trace elements among the different reservoirs.

The 3 main trace elements reservoirs are:

1. the dissolved phase: free ions and complexed and colloidally bound trace elements
2. the particulate phase = biotic (phytoplankton and bacteria) and abiotic (inorganic and organic particulates)
3. bottom sediments

Figure 1: The 2 major biochemical cycles.

The input and output are also a part of the overall cycle. It is also possible for the elements to follow other chemical paths between the reservoirs, than the ones suggested in figure 1. Figure 3 shows additional pathways of the elements, which include the two cycles mentioned earlier.

Figure 2: Pathways of elements that enter Lake Louise

Organic matter and biogenic silica plus a minor amount of clay dominate the sediment constituents of Lake Louise. The grain size may vary greatly. The smallest particles play the biggest role in the adsorption of trace elements, because of their bigger surface area. The surfaces of the particles are generally negatively charged, which is the reason why they are very good at adsorbing the positively charged trace element species.
 
 

The samples were collected in February 1999 and can be divided into two main groups:

1. Water samples
2. Sediment samples

Collecting the Water Samples

We collected water samples from six different depths:

1. From the surface water
2. From 4.0 meters depth
3. From 5.0 meters depth
4. From 5.5 meters depth
5. From 5.75 meters depth
6. and from 6.0 meters depth

2. The 4-meter Water: The water from 4 meters depth was collected from a sediment trap, where the material had accumulated for about 30 days prior to collection.

3. The 5.0, 5.5 and 5.75-meter Water: A well sampler was used to collect the water from 5.0, 5.5 and 5.75 meters depth. 4. The 6.0-meter Water: A Kemmer bottle was used to take water samples from 6.0 meters depth. Filtering of the Water samples: Transportation of water samples from the lake to the lab may cause a rise of water temperature. This may alter the ratio of dissolved species to adsorbed species. It is therefore preferable if the filtering is done at the lake and that the samples are stored in a cooler. Filtering on the lake is obvious more difficult than in a lab and there are therefore a couple of things that should be taken care of before going to the lake. Preparation of equipment before going to the lake: The bottles and the filtering devises must be washed properly. First leave them in a 2% nitric acid bath for at least one hour. Then wash them 3 times in RO-pure water, followed by rinsing them two times with ">18 W water". We did not put the rubber rings from the filtering devises in the acid bath, thinking that they might be destroyed. The filtering device should be checked for its ability to hold a pressure applied by the hand pump. We added a small amount of vacuum grease to the rubber o-rings on the filtering devise to make them seal better. It is a good idea to bring at least four prepared filtering devices to the lake so you do not have to change filters at the lake. It is also important to check if water can run through the filter. Sometimes, an air bubble seems to form under the filter, which makes it difficult for even clean water to run through. This will obviously make it almost impossible for lake water with suspended particles in it, to run through! It is also necessary to weigh the dry filter before filtering water through it because it is needed for later calculations.

All these preparations may seem insignificant but they can save a lot of time and tears. All the water samples were filtered through a 0.45 m m or a glass-fiber filter. We used a hand pump to pump air out of the bottle, which made the lake water run through faster. After about 500 ml, the filtering speed went down because of the accumulation of sediment on the filter. It is tempting to start on another filter bud filter. We used a hand pump to pump air out of the bottle, which made the lake water run through faster. After about 500 ml, the filtering speed went down because of the accumulation of sediment on the filter. It is tempting to start on another filter but a fair amount of sediment is needed on the filter to measure trace element concentrations, so patience is needed.

Preparing the Water Samples for the Trace Element Analysis

The samples were examined for two different kinds of elements:

1. Major Elements, like Ca, Mg, Na, S etc. (ME)
2. Trace elements (TE), which include rare earth elements (REE)

The preparation of the samples for metal and trace element analysis is fairly simple. To each of the filtered water samples from the four different depths where added 2.0 ml of concentrated nitric acid (HNO₃) per 100 ml sample. As an internal standard, 20 ppb Indium (In) was added. Indium is a very rare element in the nature, and the can therefore compare concentrations of In before and after measurement and detects variations in the percentage of the original samples being measured.

The Rare Earth Elements analysis:

The preparations of the samples for rare earth element analysis are much more complicated. We prepared a column with Bio-Rad AG50-X8 cation exchange resin, which were kept wet at all time to avoid bubbles to form in the resin. If bubbles formed anyway, they were removed by putting the column in an ultrasonic bath for a couple of hours. The procedure for preparing a water sample for rare earth elements analysis is:

1. Evaporate 1 liter of filtered lake water down to 50 ml by heating it up in an Erlenmayer flask. When approaching 50 ml, a brown precipitate may form in the sample. If this happens, add either:

• H₂O₂, but it may destroy the resin in the column or
• 4.2 ml concentrated HCl followed by centrifuging. Remove water from tube and centrifuge again with new water. Repeat 3 times in total, which should get all of the elements out of the brown precipitate.

1. Make up sample as 50 ml, with a 1 M HCL concentration.
2. Load onto column; let it run through (~65 min.) and discard elutant.
3. Load 50 ml of 2 M HCl onto the column; let it run through (~55 min.) and discard elutant.
4. Load 50 ml of 2 M HNO₃ onto the column; let it run through (~50 min.) and discard elutant.
5. Load 50 ml of 6 M HNO₃ onto the column; let it run through (~45 min.) and keep elutant.
6. Load 50 ml of 8 M HNO₃ onto the column; let it run through (~45 min.) and keep elutant.
7. Combine the elutants from 6 and 7 in an Erlenmayer flask and evaporate to dryness under a fume hood.
8. Re-dissolve in 5.0 ml 20% HNO₃. Hating may help the dissolving.
9. Transfer the solution to a 50.0 ml volumetric flask and add the internal standard solution. Then dilute to the mark. This is the analytical solution and it is know ready to run!
10. To regenerate the column, run 50 ml of 8 M HNO₃ through the column twice and discard the elutant. The column is now ready to run the next sample.

Collecting the Sediment Samples

We collected sediment samples from 3 different depths:

• suspended sediment from the surface water
• suspended sediment from water at 4 meters depth
• sediment from the bottom of the lake

The 4-meter Sediment: The sediment from 4 meters depth was collected by an Eckmann sediment trap, which previously where placed in the lake.

The Bottom Sediment: A shovel, specially designed to scoop sediment from the bottom of the lake, collected the bottom sediment.

Preparing the Sediment Samples for Trace Elements Analysis

Preparation of the Suspended Sediment samples from the Surface Water: The filters contain the sediment that we wish to examine. Because it is impossible to get the sediment of the filters after we have dried them, additional methods must be used:

1. Weigh the filter
2. By comparing with the weight before filtering, calculate the weight of the sediment on the filter.
3. Put filter into a Teflon vial.
4. Add 5.0 ml concentrated HNO₃ and 5.0 ml de-ionized water
5. Cap tightly and place in oven until both filter and sediment is dissolved.

1. Weigh a 0.45m m Nuclepore filter
2. Filter sediment onto the 0.45 m m filter.
3. Dry the filter and weigh it again
4. Put filter with sediment into a Teflon vial
5. Add 5.0 ml trace grade HNO₃ and 5.0 ml >18 MW Water.
6. Put Teflon vial into the oven and heat for 48 hours at 100 °C.
7. Transfer the content to centrifuge tube 3 times, removing the content each time.
8. Evaporate to dryness
9. Add >18 MW Water until the volume is 10 ml.
10. Spilt the sample into two samples; one for testing trace elements and one for REE.

The samples were analyzed by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) at Georgia State University by Dr. A. M. Ghazy. As described earlier, water and sediment samples were taken from Lake Louise at different depths. The concentrations of Ba, Co, Cr, Cu, Nb, Pb, Rb, Sc, Sr, Th, U, V, Y, Zn and Zr at different depths are shown in the table 1.

The "avg. deep" (average deep water) is calculated by adding the "5m wtr" (5 meter water), "5.5m wtr", "5.75m wtr" and then dividing by 3.

The "Enrichment Factor" is calculated by dividing the "avg deep" by the "surf wtr", which are the two highlighted columns on graph #x:

1) 

The enrichment factor (E) can take values from 0 to 10⁶. When E<1, the surface water concentration is bigger than the deep water, which means that the element is depleted with depth.

If E = 1 the concentration is the same at the surface and at greater depth.

If E>1, then the element is enriched with depth.
 
 

The "bottom sed D" is the D-Value at the bottom of the lake. The D-value is the ratio of the concentration of an element in the sediment over the concentration of the element in the water.

Like E, D can also theoretically assume values from 0 to 10⁶. The D-value is usually a large number, in this case from 1,500-166,000. That means that elements are usually found in much greater concentrations in the sediment than in the water.

It is calculated by the following equation:

2) 

To try to find relationships between enrichment, D-value and type of element, I sorted the data by different parameters. One of them is Ionic Potential. The ionic potential of an element Te^x+ is calculated by:

3) 

, where Z is the charge and r is the ionic radius.

Enrichment versus Charge, Ionic Radius and Ionic Potential

Figure 3-5 shows the Enrichment of different elements versus charge, ionic radius and Ionic potential of the elements. The elements listed on the x-axis are sorted by the 3 parameters on the 3 different graphs. The 3 graphs show that there is no obvious relationship between enrichment and charge, ionic radius and ionic potential of the ion.

What the graphs do show is:

Bottom D-value vs. Charge, Ionic Radius and Ionic Potential

Graph 5, 6 and 7 shows the D-Values of different elements versus charge, ionic radius and Ionic potential of the different elements. The elements listed on the x-axis are sorted by the 3 parameters on the 3 different graphs. The 3 graphs show that there is no obvious relationship between D-value and ionic radius and ionic potential of the ion. But the D-value seems to show a more pronounced dependency of charge. There is a general increase in D-value with increasing charge. Zr⁴⁺ and Th⁴⁺ have too low values to fir into the pattern. They are found in very small concentrations in the water and sediment, which means that it can be hard to get very good measurements of those elements. A very small increase in concentration could cause the D-value to rise the 3-4 times that it needed to fit into the pattern. The same reason could be the reason for the too low value of Sc³⁺!

So what the graphs do show is:

It is stated is this report, that there are no clear relationships between the enrichment and D-values of the elements and the charge, ionic radius and ionic potential of the elements. It should be made clear, that this does not mean that the 3 factors are irrelevant to the enrichment and D-value of the element. It just means that other parameters influence the process too. Examples of other processes could be the formation of complex compounds, which would influence the solubility of the salts of the ions, and therefore the concentrations of the ions in the water. Also variation in Eh with depth could influence the ionic concentrations, which would weaken the relative importance of the charge of the ions and the correlation between charge and E and D-values will not show clearly.

The elements in Lake Louise go through adsorption and release cycles, where physico-chemical adsorption, biological uptake and the sedimentation of particulate matter are the 3 main processes.

The sampling technique is very important to eliminate errors, caused by the change in environment from lake to lab. Time is also an important factor when the type of sampling technique is to be selected.

The enrichment factors and the D-values calculated for each element were set up on graphs as a function of charge, ionic radius and ionic potential of the elements. The graphs did not show any simple and clear relationships, except for D-value vs. charge. The D-value shows a general increase with increasing charge.

Other processes may cause the relationship between enrichment and D-value, and charge to seem less clear. This, however, does not mean that charge is not an important factor. This just show, that Lake Louise is a complex environment, where a lot of processes interact.