Sean Mix

1/28/04

Physics-C

Group 8C
 
Sweet Home Project: Final Paper

The Effects of Arsenic on Freshwater Snails

 

I. Abstract

In this study we assessed the relationship between environmental arsenic levels in the Sweet Home, OR, area and the population density and shell color of aquatic snails living in the area’s bodies of water. Previous studies suggest that arsenic could affect snail populations and that snail color is easily altered through a variety of environmental factors. Based on this, we expected to see lower population density in high arsenic areas and some difference in shell color between the areas. To determine population density we selected small areas in three different streams (one of high arsenic, two of lower arsenic). In these sites we counted and removed snails from streambed rocks, photographed them to document color, and took sediment samples to determine arsenic concentration using Instrumental Neutron Activation Analysis. In INAA, the atoms of samples are bombarded with neutrons to create radioactive isotopes. The energy of the gamma rays emitted by these isotopes during decay are measured to determine the concentration of arsenic in the sample. Our results showed that the area with the highest population density (avg.: 23 snails/m2) had the lowest arsenic (trace amt.) and the area with lowest population density (avg.: 0.33 snails/m2) had the highest arsenic. Our photographs revealed no connection between arsenic concentration and snail shell color. These results led us to conclude that arsenic does have an adverse effect on snails. From this conclusion, the obvious next step would be to determine whether the arsenic has any effect on other species in the area, including humans.

II. Introduction

The goal of this study was to determine the relationship between arsenic concentration in streambed sediment and the population density and shell color of the freshwater snails living in local bodies of water. We chose specific sites for our study based on previous arsenic testing. We chose one site that has high arsenic levels and two sites with normal amounts of arsenic. Within these sites we collected data on population density of snails, took photographs to examine the range of shell colors, and collected soil samples to be processed for arsenic content using neutron activation analysis at Oregon State University. The results of this study will be entered into a GIS database that will improve the characterization of arsenic over time.

            We decided to perform this study because it has been established that there is an abnormally high concentration of arsenic in the Sweet Home area (Kirsch, 2003), and it is important that we find whether this has a negative effect on the human population living there. By studying the wildlife that inhabits areas that are known to have high arsenic levels and comparing the characteristics of these animals with similar animals in areas that have normal arsenic concentrations, we can determine whether arsenic is having an impact on wildlife. We chose to study aquatic rather than terrestrial animals because animals that need to be constantly submerged in water are not able to leave the arsenic-rich environment as easily as non-aquatic animals. Snails are constantly in contact with soil and rocks that can absorb arsenic from groundwater. Therefore, we can be more certain that the animals we are studying have been exposed to high-arsenic conditions.

In 1996, the U.S. Geological Survey determined that there was an anomalous area of high arsenic concentration around the town of Sweet Home. Starting in 2002, Crescent Valley High School students began an investigation of the arsenic issue. (Kirsch, 2003) As a part of this study we looked at the effects of arsenic on animal populations of the area. In a previous study not in Sweet Home, populations of pond snails (Lymnaea emarginata) began to die when exposed to arsenic pentoxide and arsenic trioxide in concentrations of 900-1000 mg/L in a laboratory setting. (Orme, 2002) This information implies that the arsenic in Sweet Home could have an impact on the area’s wildlife, and led us to our first hypothesis, that snail populations in the field will be adversely affected by environmental arsenic.

There has been no study of snail shell color in relation to arsenic concentration, but it has been studied in relation to other factors, including food supply and environment. (Machala, 2002) The species studied was Littorina obtusata, a saltwater snail. In the study cited, its color ranged from light yellow to dark brown and olive green. Based on this information, we hypothesized that the abnormally high arsenic levels could affect the color of snails in Sweet Home.

The results of this study will be valuable to other scientists as well as to the citizens of Sweet Home and the surrounding areas. Scientists will be interested in this study because our results could be used in future studies in similar fields. Our data will be useful for future study of the arsenic problem in Sweet Home and its effects on wildlife and the human population. The local government may use our results and the results of other groups to devise a plan of action to solve any problems that are detected. In this study, we expected to see a correlation between the arsenic concentration of streambed sediment and the population density and shell color of snails.

 

III. Methods

            Using results from previous tests of arsenic levels, we chose three different areas to collect samples. At Ames Creek in Sweet Home’s Sankey Park we expected to find high arsenic levels in streambed sediment. We also collected samples at Quartzville Creek and in the Santiam River in Sweet Home, where we predicted less arsenic in the soil. At each of these sampling areas we chose three sites in shallow water less than 30 cm deep. These sites were approximately 15m away from each other and had an approximate area of 1m2.

We measured the population density of snails by picking up rocks from the streambed within our 1m2 area and scraping all the snails from these rocks into a container. After an entire 1m2 site had been searched for snails we counted them and laid them out on a white background to be photographed. We used these photographs later to examine any differences in shell color between our three sites. The snails were then returned to the stream.

After all snails had been examined, we used a trowel to scoop streambed sediment from each of our sub-sites. These scoops were combined into a single bag for each sampling area (one bag for Santiam, one for Quartzville). At Ames Creek, sampling limitations forced us to use a soil sample from another group of students. All of these soil samples were prepared according to the following procedure and taken to Oregon State University’s nuclear reactor to undergo Neutron Activation Analysis. (see detailed description of INAA below)

Sample Preparation Procedure

 

Drying

  1. Put on a pair of latex gloves.
  2. Select a sample
  3. Prepare the sample for drying.
  4. If the sample is soil:  obtain a quantity that is representative of all material in the original sample bag.
  5. Obtain a Petri dish.  Label the bottom of the Petri dish using a marker with your group number and the sample number (in red on the plastic bag).
  6. Obtain the mass of the Petri dish.  Record this number.
  7. Obtain the “wet” mass of your sample.  Record this number.
  8. Place the sample in one of the two drying ovens.

 

Placing samples in small vials

  1. Prepare 2/5 dram polyvial for irradiation. The vials are cut to length (11-12 mm). Remove plastic lip from lid. Save lid and container in separate boxes.
  2. Weigh both container and lid on the balance. Record weight in lab book.
  3. Carefully, fill about 250 to 500 mg of sample in the vial.
  4. Place lid on vial.
  5. Use ‘Dust-Blaster’ to remove particles from vial (hold away from any open sample containers!)
  6. Place closed vial on balance and record gross weight.
  7. Record mass. Use a calculator (or Excel) to obtain sample weight.
  8. Clean the spatula with a KimWipe and acetone (avoid contact with acetone, as it is a carcinogen).
  9. Seal the small vial using a soldering iron.
  10. Proceed with next sample.

 

Placing small vials in larger vials

  1. Two smaller vials will be placed in each larger vial, one on top of the other.
  2. Two smaller vials with vegetation samples should be placed in the same larger vial.

 

Wear latex gloves throughout this procedure to avoid contamination of containers and samples.  DO NOT USE WATER TO CLEAN ANYTHING!

 

  1. Obtain the mass of the dried bulk sample plus its drying dish.  Record this mass.
  2. Prepare sample by grinding organic matter with the coffee grinder or using a mortar and pestle to break up rocks and soil to fine particles.
  3. Trim off the tab connecting the lid to the small vial.
  4. Obtain the mass of BOTH THE LID AND THE EMPTY SMALL VIAL.  Record the mass.
  5. Carefully place between 250 and 1000 mg of ground sample into the small vial. Place lid on vial.
  6. Use ‘Dust-Blaster’ to remove particles from vial (hold away from any open sample containers!)
  7. Place closed vial on balance and measure mass of small vial + sample.  Record this mass.
  8. Clean the spatula with a KimWipe and acetone.
  9. Seal the lid to the small vial with a soldering iron.
  10. Label the small vial as follows:

·        On the bottom of the vial, write just the Sample Number -- "01".

  1. Obtain the mass of BOTH THE LID AND THE EMPTY LARGE VIAL.  Record this mass.
  2. Prepare a second sample.

WHEN YOU HAVE TWO SAMPLES IN SMALL VIALS:

  1. Place the two samples within the large vial.  Record the number of the large vial as well as the two sample numbers you are placing within the larger vial and enter these numbers into the appropriate spreadsheet.  WE NEED TO KNOW WHICH SAMPLES ARE IN EACH LARGE VIAL!!  Close the large vial.
  2. Label the large vial on the sides using the same notation you used for the small vial except vial numbers will range from 0 to 40.

 

IV. Description of Neutron Activation Analysis

            In order to determine the amount of arsenic present in each of our sediment samples, we used a process called Instrumental Neutron Activation Analysis. This process requires the use of a nuclear reactor to irradiate samples and equipment to measure the radiation released by the irradiated samples.

            After the samples are prepared according to the procedure above, they are inserted into the nuclear reactor, where they are bombarded with neutrons that are products of uranium fission within the reactor. These neutrons collide with the nuclei of atoms in the sample, creating a compound nucleus that is a different isotope of the original element. The compound nucleus is in an excited and unstable state because the strong nuclear force is not strong enough to hold the nucleus together. To achieve a more stable condition, the nucleus emits beta particles and gamma radiation, creating a new product nucleus. Gamma rays are released at a particular energy level that is specific to each element.

            Because each isotope releases gamma rays of different energy levels, the amount of radiation at a particular energy level tells us how much of a particular element is present in a sample. In analyzing an irradiated sample, detectors measure the frequency of gamma ray emission (counts/time elapsed). This is commonly referred to as activity (A), and is measured in Becquerels. When the activity of a certain element in a test sample is compared to that of a standard sample of known element concentration, the following ratio can be used to determine the concentration of the desired element in the test sample:

 

 


           

This process can be used to determine the concentration of any element in a sample, but for our study we only looked at the arsenic values in our samples.

V. Results

A. Population density

The table below is a summary of the numeric data from our collections in Sweet Home. The “site#” column refers to the sampling site. There are three separate entries for Ames Creek, Quartzville Creek, and the Santiam River. The column “Arsenic” shows the results from Neutron Activation Analysis of our samples of streambed sediment. Because we were only able to process one sample for each of our three areas, the sub-sites within each area all have the same value for arsenic concentration.

TABLE 1

site#

Arsenic (ppm)

Snails/m2

Avg. snails

Ames Cr. 1

11.4

0

0.3

Ames Cr. 2

11.4

1

0.3

Ames Cr. 3

11.4

0

0.3

Quartzville Cr. 1

3.8

2

1

Quartzville Cr. 2

3.8

0

1

Quartzville Cr. 3

3.8

1

1

Santiam R. 1

<1

19

23

Santiam R. 2

<1

25

23

Santiam R. 3

<1

24

23

 

Our sediment sample from the Santiam River had such a small amount of arsenic in it that it could not be detected using INAA. The column “snails/m2” shows the population density of snails at each of our sites. The final column, “avg. snails”, shows the average population density for the 3 sites in each area. From this data, it is obvious that snails grow much more easily in the Santiam River than in either of the other two areas.

            It is easier to understand this data in graphical form. The following charts show our data for population density vs. arsenic concentration, both in raw and compiled form. In the graph of compiled data (averages of values), it becomes apparent that this data fits a power fit function. The power line is drawn over the points of data to show its fit.

 

 

 

 

 

 

 

 

B. Snail Color

We also obtained qualitative data on the shell color of snails vs. arsenic content. We recorded our observations of shell color from the snails we collected, and also took photographs as a record of our observations.

Table 2
 
 


site#

Arsenic (ppm)

Colors

ames1

11.4

N/A

ames2

11.4

black

ames3

11.4

N/A

quartz1

3.8

lt. brown-black

quartz2

3.8

N/A

quartz3

3.8

dk. Brown

sant1

<1

dk. brown-black

sant2

<1

dk. brown-black

sant3

<1

dk. brown-black

 

 

 

 

 

 

V. Discussion

 

A.     Our data supported some aspects of our hypothesis, but did not support other parts. We observed a definite correlation between streambed sediment arsenic concentration and population density of aquatic snails. Our observations did not support our prediction of a relationship between snail shell color and arsenic concentration.

According to our data, there was an inverse relationship between population density and arsenic concentration (see table 1). In areas with low arsenic, snail populations thrived. As arsenic concentration went up, population density went down. It is not likely coincidental that the average population density in an area with trace amounts of arsenic was nearly 23 snails/m2, while the average for Ames Creek, where the arsenic concentration was over 11 ppm, was less than one snail/m2. We observed a non-linear trend of arsenic vs. population density. Our data fit a power function, which implies that as arsenic reaches a certain point in a population, it quickly takes effect. The power function is quite common when modeling populations. 

While observing snail shell colors we found an apparently random variation of colors ranging from light brown to black. Color variations did not seem to have any systematic relation to arsenic concentration.

B.     Arsenic has been used throughout history as a poison because it has proven effective in killing humans, rats, and any number of other animals. Because arsenic is known to have such harmful effects to many other animals, and because our data reveals lower population of snails at higher arsenic concentration, it is reasonable to conclude that arsenic has a similar effect on invertebrates such as snails.

Because there were still some snails present in the high-arsenic areas, there was apparently not a lethal dose of arsenic in the environment, but it did apparently cause an obstacle to the snails’ livelihood. There was obviously some factor that made snails in arsenic-rich areas less likely to survive and reproduce, and it is possible that arsenic contributed to this.

Our field study supports previous lab studies that documented arsenic toxicity in snails. (Orme, 2002) We have shown that the arsenic level near Sweet Home is high enough to have an adverse effect on wildlife. As stated earlier, we did observe variations in snail shell color from our different sampling areas. However, these differences did not seem to correspond to arsenic level, so it is more likely that they were caused by genetic traits in the snail populations. A previous study has shown that differences of shell color within a species of snail can be attributed to genetics. (Rausher, 2003)

C.     Based on the results of this study, we can conclude that the elevated level of arsenic in the Sweet Home area does have an adverse effect on the wildlife of the area. We have observed that snails are affected, and it follows that other animals in the area could be affected in a similar manner. Although we did not measure arsenic concentration within the snails themselves, we speculate that concentrations within the organisms are proportional to the values measured from sediment. If so, we could hypothesize that animals that eat snails and other animals that are in close contact with high-arsenic soil and sediment may experience similar effects. Further study would be needed to verify this hypothesis.

D.     In such a small study it is difficult to obtain definite results. If we had had more time to conduct our research and to collect data and observations from more sites, we could have had more conclusive results. It would have been interesting to do a comprehensive investigation of the Sweet Home area, looking at snails from more bodies of water of different sizes, temperatures, elevations, etc. Our graph of population density vs. arsenic concentration resembled a power function. With a greater variety of data points, it could be shown more clearly whether arsenic really does cause a rapid drop in population density as predicted by this preliminary curve fit.

E.      From our study, we have observed that the arsenic levels in Sweet Home are high enough to affect invertebrates. If this is true, what effect might arsenic have on other animals, including humans? While there have been no documented cases in Sweet Home of people dying from arsenic contamination, it is possible that arsenic in groundwater and soil could have less severe, but still serious, effects on people. The answer to this question would be extremely important to the residents of Sweet Home and would logically follow our results.


 

Sources

  1. Harvey, Charles F. et al.  “Arsenic Mobility and Groundwater Extraction in Bangladesh.” Science 2002 November 22; 298: 1602-1606.
  2. Kirsch, Adam. “Preparing Samples for Drying.” http://www2.corvallis.k12.or.us/cvhs/science/sampledryingprocedure.doc. 2003.
  3. Kirsch, Adam. “Sample Preparation Procedure for INAA.” http://www2.corvallis.k12.or.us/cvhs/science/samplepreparationprocedure forINAA.doc. 2003.
  4. Kirsch, Adam. Sweet Home Arsenic Project. 2003.
  5. Machala, Ellen. “Littorina Obtusata and Color Morphology.” http://www.clarku.edu/departments/biology/biol201/emachala/. 2002.
  6. Meisner, Craig A. “The Arsenic Hazard in Bangladesh Agriculture.” http://www.cimmyt.org/bangladesh/Arsenic/arsenic_in_bangladesh.htm. 2002.
  7. Orme, S. and Kegley, S. “PAN Pesticides Database – Chemical Toxicity Studies on Aquatic Organisms.” http://www.pesticideinfo.org/PCW/List_AcquireAll.Jsp?Species =2172.
  8. Rausher, Mark D. “Banding Patterns in Snail Shells.” http://www.biology.duke.edu/rausher/cepea.htm. 2003.
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