Risk Management Guidance for 

Evaluating Arsenic Concentrations 

in Soil 

Hazardous Materials and Waste Management Division 

(303) 692-3300 

Second Edition 

June 2011 
Regulatory Limitation 

This guidance does not modify, replace, or pre-empt any existing statutory or regulatory requirements, enforcement 

actions, agreements, policies or other legal mechanisms that may govern actions within the Hazardous Materials 

and Waste Management Division’s (the “Division’s”) various remedial programs. In the event of a conflict 

between this guidance and existing risk assessment guidance and other programmatic requirements, this guidance 

defers to the various legal and operating mechanisms of those remedial programs. 

This guidance was developed with the Division’s remedial programs in mind. Other State and Federal agencies are 

not obligated to use the process outlined herein, although the same analysis could apply to other sites undergoing 

investigation and cleanup where testing for arsenic is required and it may be present in sampled environmental 

media. Parties wanting to use this guidance at their site must seek approval to do so from the regulatory agency 

responsible for overseeing their remedial activities. 

Purpose 

The Division has prepared this guidance for the purpose of making preliminary determinations when screening data 

collected from sites that don’t necessarily have a reason to believe arsenic contamination may be present, such as a 

routine Phase II investigation conducted prior to a property transaction. This guidance is simply meant to inform 

the regulated community of their responsibilities in managing arsenic risks: it is not regulation, nor does it 

constitute an enforceable standard that must be complied with. 

Background 

Arsenic is naturally occurring in some geologic environments in Colorado due to weathering and erosion of 

bedrock and soil, including highly mineralized areas that are mined for metal ores. It is present in more than 200 

different minerals, the most common of which is called arsenopyrite. It may also be present in the environment due 

to a number of anthropogenic activities including: military operations and firing ranges; mining, especially sulfide 

ores; smelting copper, gold and lead ores; preservation of wood (CCA); chicken feed operations and associated 

manures (CAFO) due to arsenic-containing growth promoters; tanning and taxidermy operations; coal-burning 

emissions and ash-derived residues from power plants; and may be present in landfills and landfill-derived leachate. 

Arsenic may also be found due to the manufacture, use and disposal of: ammunition; fireworks; pigments (paint, 

paper, ceramics, etc.); older herbicides, insecticides, and pesticides (examples: monosodium methanearsonate 

(MSMA), disodium methanearsonate (DSMA) and lead-arsonate); electronics containing Gallium-ArsenideSelenium (GAS) semi-conductors; lead acid battery plates; glass; and some pharmaceuticals. Other anthropogenic 

arsenic sources may likely exist. Arsenic contamination in soil is of public health concern due to its toxic effects as 

a carcinogen and a non-carcinogen. Making risk management decisions about arsenic can be difficult because 

natural occurring concentrations in soil often exceed carcinogenic risk based exposure values. 

This guidance was prepared by the Division using a data set of background arsenic concentrations developed by the 

U.S. EPA Region 8. The data set includes over 2,700 samples from 44 counties in Colorado. The areas sampled 

included: native grasslands; agricultural areas; urban mixed land use; and mining. A summary of the data set is 

presented in the table below. The complete data set may be found on the U.S. EPA Region 8’s website at 

http://www.epa.gov/region8/r8risk/hh_exposure.html . 

Region 8 U.S. EPA 95% UCLM Background Soil Arsenic Concentrations in Colorado 

Land Use Concentration (mg/kg) 

Native Grassland, Rangeland, or Agriculture 3-14 

Urban Mixed Use 19 

Mining 10 

Average of all land uses 11 

Division Guidance Regarding Background Arsenic Concentration 

The Division’s approach to evaluating arsenic in soil is depicted in the following flowchart. This guidance assumes 

that, based upon the size, history and environmental concerns associated with a particular site, an adequate amount 

of arsenic data has been obtained to make a determination regarding arsenic concentrations in soil. It isn’t meant to 

be a guide on how to conduct a background study for risk assessment and/or site closure purposes. Guidance on the 
subject of data collection and analysis needs for conducting a background study should be sought from other 

published sources. Soil samples should be collected and analyzed for arsenic if the site history suggests it may be 

present as a result of anthropogenic activities. However, since arsenic is one of the chemicals included as part of a 

standard “metals” analysis package from a laboratory, you may already have obtained arsenic data for your site. 

The current residential/unrestricted land use remedial objective for arsenic is 0.39 mg/kg (i.e., Colorado Soil 

Evaluation Value, which can be found at http://www.cdphe.state.co.us/hm/csev.pdf). If arsenic concentrations at 

your site are lower than 0.39 mg/kg, the Division will require no further action to address arsenic in soil. If arsenic 

concentrations are lower than 11 mg/kg (the average of the 95% UCLM of background concentrations found by the 

U.S. EPA in Colorado), and releases of arsenic could not have occurred at the site, based on historical data or 

process knowledge, the Division will require no further action to address arsenic in soil. If arsenic concentrations 

are greater than 0.39 mg/kg, and the available information suggests that a release of arsenic could have occurred at 

the site, the Division will require additional evaluation of the data and possibly additional sampling to determine 

whether corrective measures for arsenic are required. This evaluation may include a site specific background study 

with sampling from offsite locations, and/or additional sampling in areas of the site where activities that could have 

contributed to environmental contamination never occurred. Please consult with the Division prior to performing 

any background study. If it can be demonstrated that arsenic concentrations in soil are unrelated to site activities, 

the Division will require no further action regarding arsenic. It should be noted that material such as arsenicbearing mine tailings or oil and gas drill cuttings, although derived from a naturally occurring source material, are 

not considered to be naturally occurring background once they have been generated through human activity. 

Therefore, mine tailings and drill cuttings may be subject to remediation if ecological or health-based 

concentrations are exceeded. 

For more information please contact: 

Colorado Department of Public Health and Environment 

Hazardous Materials and Waste Management Division 

4300 Cherry Creek Drive South 

Denver, Colorado 80246-1530 

Customer Technical Assistance Line: 

 (303) 692-3320 

 (888) 569-1831 ext. 3320 toll-free 

E-mail: comments.hmwmd@state.co.us 

Website: http://www.cdphe.state.co.us/hm/

START 

Arsenic is not a chemical of concern. 

Is there any reason to suspect that arsenic or 

arsenic containing materials may have been 

used onsite? 

Do sampling results 

exceed 11 mg/kg? 

Arsenic may be a chemical of concern. 

Contact the Division and coordinate 

with the appropriate remedial program 

to further evaluate the site. 

Do sampling results 

exceed 0.39 mg/kg? 

Arsenic is not a chemical of concern because 

concentrations are below Residential/Unrestricted 

remedial values

No

Yes

Yes

 Yes 

 No 

 No