Arsenic from mine tailings can become dangerous as it has in Sutter Creek in a variety of ways. It can become airborne as a result of:
• wind
• being spun up by ATVs and other vehicles driving on the tailings themselves or on dirt roadbeds where mine tailings have been used for gravel
• children's play in developments where tailings have been used for fill
Arsenic is also freed from naturally occurring mineral sources like pyrite through interaction with sulfuric acid that is often present in water pooled in mine shafts and in tailing ponds at deserted mine sites. In this form it can be leached into groundwater, although its concentration is reduced as it moves away from its source.
To Start You Thinking -
Examine the Average Arsenic Levels map of California. Print a copy you can use as you work through the assignment. The map shows test results for arsenic from sites in California averaged across counties. You need to keep in mind as you read the map that the results are averaged across large areas. Therefore, you cannot draw conclusions about specific locations - cities or towns, for example. A fact to keep in mind as you consider the questions below is that the U.S. Environmental Protection Agency safe drinking water standard for arsenic is 10 parts per billion or less.2
1) Mine tailing sites are common throughout the Mother Lode. As a result runoff from old mine shafts and from tailing ponds are considered potential sources of arsenic groundwater contamination. Complete a table showing the average arsenic levels in the nine Mother Lode counties.
| County |
Arsenic Level |
County |
Arsenic Level |
Yuba
|
|
Amador |
|
| Sierra |
|
Calaveras |
|
| Nevada |
|
Tuolomne |
|
| Placer |
|
Mariposa |
|
| El Dorado |
|
|
|
2) Do the arsenic levels in the Mother Lode counties seem different from those in other counties around the state? Explain.
3) Describe any geographic pattern you see to the arsenic levels within the Mother Lode.
4) As you roll your mouse over the map a layer of known gold mine sites is presented. Describe the relation between mine sites and the levels of arsenic in counties around the state.
Arsenic & Cancer
Is there a relationship between ground water levels of arsenic in California and the rate at which cancer occurs? Epidemiologists ask this sort of question all the time searching for risk factors for all sorts of medical conditions. Open the El Dorado GIS file and check to see that the following layers are active:
- California
- World Street Map
All other layers should be off. Use the bookmark and zoom in on California.
Prepare side-by-side county level maps that will allow you to study the correlation between arsenic levels and cancer.
The arsenic levels in the file range from 0 to 6 and correspond to the number of parts per billion (ppb) of arsenic in the groundwater indicated in the table.
My World GIS
|
ArcGIS |
• Double click on the California Counties layer to bring up the Edit Appearance window pictured below:
• You can choose the Colorscheme you want to use. Select to make a Custom classification. Enter 4 as the Number of Fill Colors and enter the Arsenic Level values indicated in the To (not including) column. Set the level of Transparency to 40% if you would like to see the terrain through the new map layer
.Click on the Apply button and Close the window.
|
• Right click the California - Counties layer in the table of contents and select Properties....
• Click on the Symbology tab and select Quantities.
• In the Fields pane select Arsenic Level as the Value.
• In the Classification pane click the Classify button. You will see a new window. Select to make a Custom classification with the Break Values pictured below:
• Click OK
• You can define your own labels to make the map easier to read by editing the values in the Labels column:
• Click the Display tab and set Transparency to 50% if you would like to see the terrain through the new map layer.
• Click OK again to leave the Properties window. |
To Start You Thinking -
1) Is the correlation between ground water levels of arsenic in California and cancer incidence rates positive or negative? That is, are higher levels of arsenic generally associated with higher rates of cancer (positive) or lower rates (negative)?
2) How strong is the correlation? Is there a distinct pattern (strong) or does the relationship appear random (weak) or somewhere in between?
3) The County layer in the El Dorado GIS file contains fields for the death rates per 100,000 population for All Cancer and for Lung, Bladder, and Skin cancers for each of the California counties. There is a moderate degree of correlation between rates of cancer and arsenic levels in California counties. However, is the relationship stronger for some types of cancer than others? Prepare a comparison using the data available for lung, bladder and skin cancers. As you study the results keep the following national and state averages in mind:
Average Annual Cancer Incidence Rates/100,000 Population 2002-2006
| |
All Cancers |
Lung |
Bladder |
Skin |
| United States |
467.5 |
53.7 |
21.6 |
17.9 |
| California |
434.3 |
69.0 |
19.2 |
19.7 |
You may also want to refer to the intensity of mining activity as reflected in the California Gold Mines layer.
Write a short summary discussing your conclusions.
