Background Concentrations of Copper and Zinc in
Soils and Waters of the U.S.
Mark Jones
American River College, Geography 26: Data Acquisition in GIS; Fall 2002

Abstract

This project involves the collection of data to develop maps showing background concentrations of copper and zinc in the soil and waters of the U.S. These maps represent a comprehensive effort to collect data on background concentrations, and can be used to perform a wide variety of spatial analyses.

Introduction

Assessing the likelihood of adverse ecological effects posed by metals in the environment differs from similar assessments of synthetic organic compounds in two important ways. First, naturally occurring elements such as cadmium, copper, lead, and zinc are fundamental constituents of the environment and, as such, organisms have evolved tolerance or avoidance behaviors to these substances. Second, some elements such as zinc are required micronutrients for plants and animals (McClendon, 1976). Increasing the awareness and understanding of the role that metals and minerals play in defining natural systems requires a mechanism to display information in a presentation format that is easy to comprehend. This project focuses on determining naturally occurring ('background') concentrations of copper and zinc in the soil and waters of the U.S. Maps showing concentrations of metals in a spatially explicit manner, either at local or national scales, are effective tools for analyzing and describing differences in background metal concentrations among geographical regions. Additional information can be added that demonstrates the relationships among soil and water geochemistry, human activity, and ecological communities.

Background

While a great deal of information is available about the natural background or ambient levels of metals in both aquatic and terrestrial systems, it is buried in files, data tables, and difficult-to-read charts and graphs. There has been limited effort to define “background” or “ambient” concentrations of metals in water and soils on either a national or local basis. Connor and Shacklette (1975) published a consolidation of studies of 69 elements from more than 8,000 samples of rock, soil, and plant material collected over the previous 10 years, and Shacklette and Boerngen (1984) summarized the U.S. Geological Survey (USGS) soil analyses in an effort to define national background concentrations. However, these studies are 15 to 25 years old and do not provide sufficient information for assessing concentrations at different spatial scales or specific localities. There have been a few attempts to characterize background concentrations of metals over large areas in the U.S. Many different organizations have performed water and soil testing in different areas for various reasons. The U.S. Environmental Protection Agency (EPA) has a centralized location for reporting levels of various constituents of soil and water across the country. The USGS and other state agencies also have reported soil and water quality data.

Methods

A search for metal concentrations in soil and water samples from the U.S. was undertaken. Government databases, private databases, academia, and published literature were searched via the web. Web leads were followed up by calling officials of various government offices in the U.S. The criteria for data acceptance for this project were as follows:
  • Detection of copper and zinc in the database.
  • Samples must have georeferencing information.
  • The samples must not have been derived from sediments.
  • The data must not be derived from obvious anthropogenic contamination (e.g. EPA Brownfield or Superfund sites).
The list of organizations contacted is shown below.

Organization
 Metals Data Received
USGS
 Yes
EPA STORET
 Yes
USDA
 No

Results

U.S. water data were retrieved from a computerized database utility maintained by the EPA for the STOrage and RETrieval (STORET) of chemical, physical, and biological data pertaining to the quality of the waterways within and contiguous to the U.S (EPA, 2000; link below). STORET includes water quality measurements for 45 million samples collected at 800,000 stations across the U.S. between the mid 1960’s and 1999. Only total metal concentrations were used (dissolved metal concentrations were not used). All data in STORET are geo-referenced. This database basically is a storehouse for any information that people or organizations want to include. The data therefore are heavily qualified, as there is no consistent sampling technique or QA/QC protocol.

U.S. soil data were obtained from the USGS open file report 81-197 (Boerngen and Shacklette, 1981). This data set contains geochemical data from soils and other regoliths collected and analyzed by Hans Shacklette and colleagues beginning in 1958 and continuing until about 1976. Geochemical point-symbol maps were plotted from these data and published as USGS Professional Paper 1270 entitled "Element Concentrations in Soils and Other Surficial Materials of the Conterminous United States." The samples were collected at a depth of about 20 cm from sites that, insofar as possible, had surficial materials that were very little altered from their natural condition and that supported native plants. These data provide a low-density geochemical baseline for soils and other surficial materials in the conterminous U.S. The data set contains 1,323 samples for a sampling density of approximately one sample per 6,000 square kilometers. The data are most appropriately used to provide information on background concentrations of elements in soil. The major drawback with the data set is its extremely low number of samples for the entire conterminous U.S. However, it is a more representative sampling than the water data, due to the fact that the sampling effort followed a standard protocol and a statistically based sampling design.

Analysis

All soil and water data were entered into spreadsheets for inclusion in an overall database. The data then were used to generate ArcView maps. A base map was developed using USGS digital line graphs at 1:250,000 scale. These files contain line data such as streams and are digitized from USGS topographic maps. All other geo-referenced data were registered to these maps to make sure that all locations were accurately cited on the maps. Land Use and Land Cover (LULC) data were also used from the USGS, registered to the 1:250,000 scale base maps. Point maps based on the collected data were overlaid on this base map. Each of the point maps are categorized and presented based on metals concentrations.

Figure 1 shows the results for copper in soil. The southern and southeastern regions of the U.S. generally have low levels of copper. There is a strip across the country, close to where Interstate 80 runs, where copper values are slightly elevated. The highest concentrations are found near the Appalachian mountain range, in the Rocky Mountains, and along the west coast. 

Figure 2 shows the results for zinc in soil. The values for zinc are varied and carry less spatial continuity than copper. The most apparent spatial trend is that the southeast U.S. has levels of low zinc, while the west and northwest U.S. have a greater concentration of high zinc. 

Figure 3 shows the results for total copper in water. Due to the large number of heterogeneous samples in the eastern U.S., it is difficult to determine large regions of similar metal concentrations based on the point map. The southeastern U.S. has low concentrations of total copper, but no other extrapolations are supported by the data.

Figure 4 shows the results for total zinc in water. The eastern U.S. has many samples of differing concentrations, with the only area of clear  low detection being Maine. Areas of high concentrations are Kansas, California, and most of the east coast.

Conclusions

This project and the accompanying maps represent a comprehensive effort to collect information on background concentrations of copper and zinc in the U.S. Soil data are representational, but are nearly 25 years old, while water data are considerably more sparse, but cover a larger range of years. Use of ArcView maps provides a valuable tool for visualization of the data as well as for grouping data into areas of similar background levels. This effort provides the data in a spatially explicit framework for further refinements of the database and development of ecologically and geochemically based metalloregions.

For the U.S. waters, further information may be available from individual states, but are not readily retrievable in electronic forms or in a manner consistent with this spatially-dependant database. It would be useful for states to coordinate their reporting of routine water quality monitoring data in a manner that makes such results readily retrievable.  For example, significant amounts of water data for California were missing from the STORET database.

Similar data and maps could be developed for other metals of interest. In addition, the data could be spatially correlated with urban areas to remove locations potentially impacted by anthropogenic sources. This would provide a better representation of natural background levels. The data could also be used to perform spatial analysis with vegetation types, etc.

References

Boerngen, J.G., and H.T. Shacklette, 1981. Chemical analyses of soils and other surficial materials of the conterminous United States: U.S. Geological Survey Open-File Report 81-197. U.S. Geological Survey, Denver, CO.

Connor, J.J. and H.T. Shacklette, 1975. Background geochemistry of some rocks, soils, plants, and vegetables in the conterminous United States. United States Government Printing Office, Washington, D.C.

McClendon, J.H., 1976. Elemental abundance as a factor in the origins of mineral nutrient requirements. Journal of Molecular Evolution, 8:175-195.

Shacklette, H.T. and J.G. Boerngen, 1984. Element concentrations in soils and other surficial materials of the conterminous United States. United States Government Printing Office, Washington, D.C.

U.S. Environmental Protection Agency, 2000. The Storage and Retrieval of Water Quality Data.  Retrieved from: http://www.epa.gov/storet/.
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