Frequency And Extent Of Dispenser

Releases At Underground Storage

Tank Facilities In South Carolina

Printed on recycled paper
This document was prepared by ICF, Incorporated and submitted 

in support of Contract No. 68-W0-0065. 

Lynn DePont 

EPA Project Officer 

U.S. Environmental Protection Agency 

Washington, D.C. 20460 
Contents 

Page 

Introduction 1 

Background 1 

Purpose Of This Study 3 

Methodology 3 

 Data Collection 3 

 Data Screening 4 

 Data Recording 4 

Quality Assurance/Quality Control 8 

Results 9 

Conclusions 13 

 

Appendix 1 – South Carolina Dispenser Data Gathering Form 

For First Phase 15 

Appendix 2 – South Carolina Dispenser Data Gathering Form 

 For Second Phase 16 

Appendix 3 – South Carolina RBCA Site Priority 

 Classification System 17 

Appendix 4 – Summary Of Results 19 

Appendix 5 – Statistical Analysis 22 
Tables 

Number Page 

1 Summary Of Quality Control Data 8 

2 Summary of Contamination At Dispenser 

Islands And Facilities 9 

3 Numbers Of Sampled Facilities, Dispenser 

Islands, And Samples By Pump Type 10 

4 Frequency Of Contamination And Contamination 

Above RBSLs At Dispenser Islands 11 

5 Frequency Of Contamination Above RBSLs 

At Facilities 12 

6 Affected Media And South Carolina RBCA 

Classification Comparison 13 

Figures

Number Page 

1 Contamination At 474 Dispenser Islands 10 

 2 Contamination At 370 Facilities 10 

3 Contamination For Pressure Piping Systems – 

 Dispenser Islands 11 

4 Contamination For Suction Piping Systems – 

 Dispenser Islands 11 

5 Contamination For Pressure Piping Systems – Facilities 12 

 

6 Contamination For Pressure Piping Systems – Facilities 12 
1

Introduction

 In this document, EPA presents the results of information collected and analyzed from 

underground storage tank (UST) closure and assessment reports at sites in South Carolina. The 

report discusses the background, purpose, methodology used, quality assurance and quality 

control procedures applied, results of the study, and conclusions. Supporting information is in 

the appendices. 

Background

 In 1984, Congress responded to the increasing threat to groundwater posed by leaking 

underground storage tanks by adding Subtitle I to the Resource Conservation and Recovery Act 

(RCRA). This legislation required the U.S. Environmental Protection Agency (EPA) develop a 

comprehensive regulatory program for USTs storing petroleum or certain hazardous substances 

in order to protect the environment and human health from releases. EPA’s Office of 

Underground Storage Tanks (OUST) developed regulations for owners and operators of UST 

systems covering leak detection, addressing release prevention, and minimizing the potential for 

USTs to corrode. (The regulations also cover additional requirements, such as financial 

responsibility and corrective action, not discussed in this report.) The regulatory requirements 

were phased in over ten years; the final deadline was December 1998. 

 Federal and state regulations helped EPA achieve its mandate. As a result of these 

regulatory changes, USTs are now significantly more protective of human health and the 

environment than those installed before the requirements were implemented. Despite these 

improvements, however, releases from new and upgraded UST systems continue. To respond to 

this, OUST initiated an effort to evaluate the performance of UST systems. The specific 

objectives of the effort are: 

z Identify the most common sources and causes of releases. 

z Determine how successful leak detection methods are at detecting releases. 

z Determine what, if any improvements are needed in the UST program. 

EPA has identified releases from UST dispensing systems as a potential concern for the 

national UST program and state UST regulatory agencies. EPA has gathered anecdotal evidence 

from representatives of regulatory agencies; the regulated community; UST equipment sales, 

repair, maintenance, and consulting companies; and UST remediation firms which repeatedly 

cited fuel dispensers as a source of contamination at UST sites. 

 Current federal UST leak prevention and leak detection regulations do not address 

dispensers. In addition, the federal regulations do not explicitly require assessment under 

dispensers at the time of UST system closure. The requirements for assessment at closure are 

established in 40 CFR Part 280, Subpart G (280.70-280.74, Out-of-Service UST Systems and 

Closure). This subpart specifies that assessment is required for the places where contamination 
2

is most likely present at an UST site.1

 Initially, the most likely sites of release were usually 

considered to be under the tank or along the piping system. 

 Some states now specify requirements for sampling under UST dispenser systems 

because of this evidence of dispenser releases. For example, Florida now requires sampling near 

dispensers during site investigations and under dispenser containment (UDC) to contain releases 

that do occur. Industry has also recognized the potential for dispenser releases and, as a result, 

installing under dispenser containment is becoming increasingly common for new installations. 

 South Carolina’s Department of Health and Environmental Control (DHEC) first began 

requiring dispenser sampling at the time of UST system closure2 in 1990. Additionally, South 

Carolina determined sampling under a dispenser may also be required if contamination is 

observed during a compliance inspection.3

 The requirement to sample near or under a dispenser 

is specified in DHEC’s 2000 guidance document Underground Storage Tank Assessment 

Guidelines for Permanent Closure and Change-In-Service. This current guidance specifies: 

 

 All dispenser islands should be sampled. If the dispenser island is located above 

or immediately adjacent (less than five feet) to the UST, the sample for the island 

can be incorporated into the sample for that UST. Otherwise, dispenser islands 

should be individually sampled. Samples should be collected approximately two 

feet below the bottom of the associated piping. 4 

 

 EPA determined that South Carolina’s history of sampling under dispensers provided an 

excellent source of data for examining the frequency of under dispenser contamination. In order 

to better evaluate the frequency and severity of releases under dispensers, EPA partnered with 

DHEC, which has a fairly robust set of data on petroleum contamination found during UST 

system inspections and closures, including contamination under dispensers. 

 As mentioned above, it has become common practice in recent years to install UDC (e.g. 

dispenser pans/sumps) at the time an UST system is installed. Previously, dispenser pans were 

very rarely installed at the time of installation, so most older dispensers do not have UDC. 

Because the facilities covered in this study were all closed by 1999, EPA assumed that all of 

these systems were installed before it was common practice to install dispenser pans. Therefore, 

the results of this study are assumed to reflect systems without UDC, although a good portion of 

the current UST population does have UDC, particularly new facilities. 

 1

 40 CFR Part 280 Technical Standards and Corrective Action Requirements for Owners and Operators 

of Underground Storage Tanks (USTs), Subpart G 280.70-280.74, Out-of-Service UST Systems and 

Closure. U. S. Environmental Protection Agency. 2 Procedures for Abandonment of Underground Storage Tanks. July 25, 1990. Underground Storage 

Tank Program, South Carolina Department of Health and Environmental Control, Columbia, South 

Carolina. 

3

 Communication with Mark K. Berenbrok. April 2002. Underground Storage Tank Program, South 

Carolina Department of Health and Environmental Control. 

4 Underground Storage Tank Assessment Guidelines for Permanent Closure and Change-In-Service. 

August 2000. Underground Storage Tank Program, South Carolina Department of Health and 

Environmental Control, Columbia, South Carolina. The term dispenser island is used to refer to the 

raised (usually approximately 6 inches high) concrete platform on which one or more fuel dispensers are 

installed. Dispenser islands are normally 2 to 4 feet wide and may be any length. 
3

 

 

Purpose Of This Study

 The purpose of this study is to gather and analyze dispenser sampling data5

 from South 

Carolina DHEC underground storage tank assessment and closure files to determine whether the 

data showed any patterns of dispenser releases and to identify the likelihood that releases from 

dispensers will contaminate subsurface media (soil and groundwater). These data provide 

information about the frequency and impact of releases under dispensers. 

 

 

Methodology

 This report reflects the results of two separate study phases during which data about 

petroleum releases from dispensers associated with USTs was collected and analyzed. The data 

came primarily from closure reports, 95 percent of which were received by DHEC between 1996 

through 1999.6

 South Carolina’s regulations require a closure report for the permanent closure 

or change in service of an UST system. The closure report must address all components of the 

tank system, including tanks, piping, and dispensers. The sites with closure reports on file did 

not necessarily have a confirmed release. Excluding observations made by DHEC personnel, a 

release is confirmed when analytical results indicate petroleum constituents are present in excess 

of detection limits. Sampling under dispensers was part of site investigations to determine 

whether any soil or groundwater contamination existed at the UST facility as a result of a release 

from the UST system. These files are maintained by DHEC’s Underground Storage Tank 

Program. The first data collection phase was conducted by DHEC in January 2002 to pilot the 

data collection techniques and forms. EPA, with contractor assistance, conducted the second 

data collection phase in February and March 2002. Below are more details about the data 

collection effort. See page 8 for descriptions of the results. 

Data Collection

 DHEC performed the first phase of data collection in January 2002 to test the 

effectiveness of the data collection form, identify unanticipated issues or problems, and gauge 

the level of effort required to review the files. The first phase encompassed review of a set of 

closure and assessment reports collected in March and April 1996. The reports included: 

closure and assessment files which described sites with no contamination; contaminated sites that 

had not been actively remediated but were determined to require no further action (NFA); sites 

with confirmed releases that had been rehabilitated and determined to require NFA; and 

confirmed releases that are still being remediated. 

 5

 In this study, the terms dispenser sampling data and dispenser data are used to refer collectively to 

measured concentrations of common petroleum components, such as benzene, in samples collected under 

a dispenser. 

6

 In a few cases, assessment reports were the source of the site data. An assessment report can be a 

voluntary submittal prepared as a result of a pending property sale or loan application, or may be 

requested if an inspector observes site conditions that indicate a release may have occurred. 
4

Data Screening

The first step in collecting the data was to determine whether an UST site report was 

valid or invalid for purposes of this study. File information reviewed during the first phase was 

determined to be invalid (i.e., not useful for the purposes of this study) for any of the following 

reasons: 

z No dispenser samples taken/analysis performed. 

z UST was within five feet of dispenser(s) with sampling/analysis data. 

z Contaminant levels were unknown. 

z No dispensers were present (e.g., UST used to store emergency generator fuel, motor oil, 

or waste oil). 

During the second data collection phase, the following additional criteria were also 

applied to identify information that was considered invalid (for purposes of this study): 

z Contamination levels of all chemicals were non-detect, but detection limits for one or 

more samples were greater than state Risk Based Screening Levels (RBSLs). 

z File was incomplete and the reviewer could not determine whether samples were 

collected under the dispenser and/or whether contamination was present. 

z Tank to dispenser distance was unknown. 

z Sample locations were unclear. 

z Location of dispensers was unknown.

Sites that only contained emergency generator fuel, motor oil, or waste oil were 

considered invalid in the data screening for both phases. For the most part, such sites were 

excluded from detailed review because dispensers are generally not used with tanks storing these 

substances. If a dispenser was within five feet of an UST at a facility, but samples were 

collected at other dispensers located more than five feet from an UST (and otherwise meeting the 

above criteria), then the whole facility was considered valid. In such cases, dispensers with valid 

data were included in the population of data collected. 

Data Recording

During the first phase of data collection and as each report was reviewed, the reviewer 

completed a data form and recorded selected information about the facility and the release (see 

Appendix 1 on page 13 for a copy of the blank form). During the second phase, data from 

closure reports were entered directly into an electronic form created in an Access database. Data 

were entered at DHEC offices in Columbia, South Carolina. 

More information was gathered about each report during the second phase of data 

collection (see Appendix 2 on page 14) compared to the first data collection. Descriptions of the 

data elements collected during both phases are provided below. Data elements that were only 

included in the second phase are identified in the descriptions that follow. 
5

z Facility ID – Unique site identification number assigned by DHEC. 

z Date Closed – Date the UST(s) on site were closed and/or removed. 

z Number of Dispensers – Total number of dispensers at the site. 

z Invalid File – Box checked if report was invalid, followed by reason(s) why it was 

invalid. 

z Information on samples taken at the dispenser: 

- Dispenser Island ID – If more than one sample was taken on one island, the 

sample was labeled as “1a”, “1b,” “2a,” “2b,” etc. If one sample was taken at 

each island, the sample number correlated to the numbers of islands (e.g., sample 

from island #1 would be “1,” sample from island #2 would be ”2,” etc.). 

- Piping Type – Second phase only. One of three types of piping product delivery 

methods: suction (S), pressurized (P), or unknown (U); additional details about 

piping are addressed in the “Note on Piping Delivery Types” section below. 

- Contamination – If contamination was present, data recorded as "Yes"; specific 

contamination levels addressed below. 

- Above State RBSLs (Risk Based Screening Levels) – Entered as "Yes" or "No" 

in accordance with contamination levels compared to South Carolina RBSLs.7

- Benzene, Ethylbenzene, Toluene, Xylenes (total), and Naphthalene 

Contamination Levels – If above detectable limits, recorded in ppm (mg/kg); if 

non-detect, recorded as less than detection limit or non-detect, as appropriate. 

z Contaminants above RBSLs for the dispenser only – Second phase only. If 

contamination was present at levels above RBSLs at the dispenser only (and nowhere 

else at the site), this box was checked. Then EPA obtained a summary report from 

DHEC’s corrective action database and recorded the following information: 

- Contaminant Medium – Select whether the contamination reached soil or 

groundwater, both, or unknown. 

- South Carolina RBCA Classification – If the site was classified, enter the South 

Carolina Risk Based Corrective Action Classification (SCRBCA) level (see 

Appendix 3 on page 15 for details about the SCRBCA classification levels). 

- Receptor type – Multiple choices, including public well, creek, property 

boundary, etc. 

South Carolina’s risk-based approach to prioritizing cleanups uses RBSLs established for 

two types of soils – sand and clay. In order to determine whether contamination under a 

dispenser exceeded South Carolina RBSLs for each of the contaminants examined in this study, 

the soil type present at the site had to be identified. If the soil type was not provided in the report 

 

7 South Carolina Risk-Based Corrective Action for Petroleum Releases, May 15, 2001. Risk-based screening levels 

for groundwater, soil, and air are listed in Appendix B. 
6

and the contamination level reported fell between two South Carolina RBSLs, then EPA 

determined the soil type from a map indicating the approximate location of the geologic fall 

line.8 Sites to the northwest of the fall line were classified as clays and sites to the southeast 

were considered to be sands for the purpose of determining whether the dispenser sample 

concentration was above or below the RBSL for each contaminant. 

Collection of information from DHEC’s corrective action database was added as part of 

the second phase data collection effort to allow additional comparisons of the characteristics of 

the facilities with dispenser releases that may require corrective action with a statewide summary 

of facility releases available from the corrective action database. DHEC provided the statewide 

summary discussed in the results section on page 8. 

If a report was determined to be invalid, the “Site ID,” “Date Closed,” “Invalid File,” and 

the appropriate “Reason File Invalid” boxes were the only fields completed on the form. Valid 

files included sites with at least one soil or groundwater sample (and analytical data) taken at a 

dispenser. The analytical results are likely to be representative of contamination, or lack of 

contamination, that can reasonably be attributed to the operation of the dispenser itself and not 

attributed to a release from other portions of the UST system. 

Note On Dispenser Islands 

An UST facility may have one or more dispenser islands and each dispenser island may 

have one or more dispensers. DHEC regulations require that the facility closure process include 

collection of at least one soil sample at each dispenser island. However, sampling at each 

dispenser is not required. Results from samples collected for more than one dispenser on a 

dispenser island may be submitted voluntarily. If the dispenser island is located above or 

immediately adjacent to (less than five feet from) the UST, then DHEC allows a single sample to 

meet both a dispenser island and UST sampling requirement. DHEC requires that samples be 

collected approximately two feet below the bottom of the piping of the dispenser chosen for 

sampling and that the dispenser with the highest anticipated level of contamination be sampled. 

Note On Contaminant Data 

Information from the assessment and closure reports for soil contamination levels of 

benzene, ethylbenzene, toluene, xylene, and naphthalene were recorded in milligrams per 

kilogram (mg/kg) (parts per million [ppm]) for samples taken at the dispenser(s). If the various 

species of xylene were reported separately (i.e., xylene [m, o, and p]), then the total of those 

results was recorded as a single value in the database (i.e., concentration of m-xylene + 

concentration of o-xylene + concentration of p-xylene). In the case of other contaminants when 

two analytical results were provided, the higher concentration was always used. If no 

contamination was present (reported as a “non-detect”), then the result was recorded as less than 

the reported laboratory detection limit for each contaminant (e.g., "<0.0005 ppm"). If both 

results for a contaminant were non-detect and the detection limits varied, then the higher 

detection limit was reported. 

 

8

 The boundary zone between older, resistant rocks and younger Coastal Plain sediments. 
7

As noted above at sites with detected contamination, EPA used the available data to 

determine whether the contamination levels were above or below RBSLs. The data available in 

the closure reports are based on the results of standard laboratory analyses for the detection and 

quantification of common petroleum constituents. The accuracy (i.e., the closeness of the 

measured value to the true value) of the laboratory analysis data depends on a wide range of 

factors including the contaminant measured, the concentration level, the type of sample (e.g., 

soil, water), the analytical method, and the equipment used. EPA’s “Test Methods for 

Evaluating Solid Wastes” (SW-846) reports single-laboratory accuracy for Method 5035 

(appropriate for the contaminants included in this study) applied to soil samples that approach 

+/- 20 percent. Use of multiple laboratories (as is the case here) introduces further variation in 

the measured results. To evaluate the potential effects of these accuracy limitations on the 

results of this study, EPA identified the situations where the characterization of a data point as 

being “above” or “below” the RBSL was based on a reported concentration that is within 30 

percent of the RBSL. EPA found this occurred between 5 and 10 percent of the time. The 

percentage of measurements characterized as “above RBSLs” that might actually be below 

RBSLs (assuming a +/- 30 percent margin of error) is similar to the percentage characterized as 

“below RBSLs” that actually might be above RBSLs. Therefore, the effect of this inherent 

analytical uncertainty on conclusions drawn from the data is thought to be small. 

There has been increasing concern about contamination of groundwater sources from the 

gasoline additive methyl tertiary butyl ether (MTBE). Data on MTBE were not available for this 

study because DHEC did not require analysis of MTBE in soil samples from UST sites. (Since 

1995, DHEC has required groundwater encountered during tank closure or assessment activities 

to be sampled for MTBE.) However, the results of this analysis may be useful in trying to 

control the release of MTBE – and other constituents found in gasoline – into groundwater. 

Note On Piping Product Delivery Types

EPA collected data on the types of pumps used to deliver product through the piping at 

the sites selected for inclusion in the database. The term “Piping Type” was used on the form in 

the second phase of data gathering to indicate the type of pump used to deliver product from the 

tank to the dispenser. The piping type (pump) information was collected to determine whether 

the presence of the pump in dispensers using suction systems would increase or decrease the 

likelihood of a release. For those sites at which the pump type was clearly identified, EPA 

recorded the pump type under piping type on the form. For those sites with multiple tanks and 

different types of pumps, and where reviewers could not determine which type of pump applied 

to the dispenser island sampled, the piping type on the form was marked as unknown. The pump 

type generally was indicated as unknown for the assessment and closure reports reviewed during 

the first phase of the data collection effort because information on pump type was not collected 

during that phase. For sites reviewed in the first phase that appeared to have dispenser 

contamination, information on pump type was collected during the second phase. 
8

Quality Assurance/Quality Control 

The following Quality Assurance (QA) strategy was used during the second phase data 

collection effort, which last two weeks. As part of this strategy, Quality Control (QC) 

procedures were developed to ensure the quality of the information gathered. While on site, 

EPA gathered data from DHEC’s existing files and recorded the information in an Access 

database. 

Because two individuals entered data from DHEC files into the database, each individual 

performed a QC check of the data entered by the other. During the first week of data collection, 

each individual checked every fifth file entered into the database by the other individual, for a 

review of 20 percent of the data entries. Every field was checked for accuracy on every file 

reviewed. During the second week of data collection, EPA assessed this procedure and 

determined it to be more extensive than needed because of the experience of the data collectors 

and because few errors were detected during the first week. EPA decided that from then on, 

every tenth file would be checked, for a review of 10 percent of the data entries. This change 

was implemented for the second week of the two-week data collection effort. Overall, 15.5 

percent of the data entries were reviewed for QC (see Table 1). 

Table 1 - Summary Of Quality Control Data 

Total number of files reviewed 1,218 

Number of files double-checked for QC 189 

Percentage of files double-checked for QC 15.5 

Percentage of files double-checked with errors 3 

Double-checking of the files was performed at the end of each workday or the beginning 

of the next. Each data collector kept a separate list of the files she double-checked throughout 

the data collection period. 

To verify the information recorded in the database was correct, the individual who had 

not originally entered the data examined each file selected for QC review in detail. Where 

necessary, the South Carolina corrective action database was consulted. If a discrepancy or error 

was discovered, this was noted and discussed that day with the other data collector. All 

discrepancies or errors were resolved and any necessary changes were made in the database. 

Overall, approximately 3 percent of the files reviewed were revised in the database as a result of 

this review process. 

While no QA/QC strategy was used during the first phase of the study, 12 of the 77 files 

were reviewed again during the second phase to determine if any files contained samples with 

contamination above RBSLs at the dispenser only. 
9

 EPA provided copies of the draft report to EPA regional offices, states, and industry 

experts for peer review. The reviewers identified issues and provided recommendations about 

the report. EPA evaluated the comments and incorporated many of the recommendations into 

this final version of the report. 

Results 

For both phases of this study, a combined total of 1,218 files were reviewed, each 

representing an individual closure action (or in a few cases a voluntary site assessment). DHEC 

reviewed 77 files in the first phase, of which 54 were invalid; EPA reviewed 1,141 files in the 

second phase, of which 794 were invalid. The files reviewed are assumed to be generally 

representative of closure actions undertaken in South Carolina during the period of interest. Of 

the 1,218 files reviewed, 370 (30.4 percent) provided at least one “valid” (for purposes of this 

study) sampling result. Of the 848 files without a valid sampling result, 269 (32 percent) were 

invalid because they lacked dispenser sampling data and 515 (61 percent) were invalid because 

dispenser(s) with sampling/analysis data were within five feet of an UST. The other 64 files 

were invalid because they did not have dispensers (17); had all contaminants reported non-detect 

but with detection limits above RBSLs for one or more contaminants (16); or were incomplete in 

some way (31) (e.g., sample locations unclear, tank to dispenser distance unknown). Data for the 

370 facilities with at least one valid sample are summarized in Appendix 4 on page 17 and 

discussed below. 

The 370 facilities with valid data had a total of 454 dispenser islands with valid data. As 

shown in Table 2, contamination was found under dispensers at nearly half of all facilities and 

dispenser islands with valid data and dispenser contamination that exceeded RBSLs was found at 

just over 25 percent of all facilities and dispenser islands. The distribution of these data by pump 

type is provided in the tables that follow. Of the 454 dispenser islands with valid data, 56 had 

data for more than one soil sample. The data for 40 (71 percent) of these 56 dispenser islands 

indicate that contaminant levels in the multiple samples fall in the same contaminant grouping 

(i.e., not detected, at least one contaminant detected, or at least one contaminant above RBSL). 

Of the 40 dispenser islands with contaminant levels in multiple samples in the same contaminant 

grouping, 27 had no detectable contamination in all samples. For 13 of the remaining 16 

dispenser islands from the group of 56 with data for more than one soil sample, at least one 

sample had a detected contaminant while one or more samples had no detected contaminants. 

Table 2 - Summary Of Contamination At Dispenser Islands And Facilities 

With At Least One 

Contaminant Detected 

With At Least One Contaminant 

Above RBSL Total 

Number

(#) # 

% (95% Confidence 

Limits) # 

% (95% Confidence 

Limits) 

Dispenser 

Islands 454 215 47.4 (42.8 - 51.9) 116 25.6 (21.5 - 29.6) 

Facilities 370 179 48.4 (43.3 - 53.5) 98 26.5 (22.0 - 31.0) 
10

 Figures 1 and 2 visually depict the number of dispenser islands and facilities, 

respectively, where contamination was discovered, and the number of times the contamination 

exceeded RBSLs. 

 Table 3 shows the distribution of the numbers of sampled facilities, dispenser islands, and 

individual samples by pump type. 

Table 3 - Numbers Of Sampled Facilities, Dispenser Islands, 

And Samples By Pump Type 

Pump Type Facilities Dispenser 

Islands Samples 

All 370 454 534 

Pressure 135 183 211 

Suction 198 218 261 

Unknown 37 53 62 

 Table 4 shows the frequency of detected contamination and contamination above RBSLs 

for the 454 dispenser islands. The top portion of Table 4 shows the number of dispenser islands 

with no detected contamination, at least one contaminant detected, and at least one contaminant 

above the RBSL. The bottom portion shows the binomial confidence limits for these 

proportions. As shown, the proportions of pressure and suction piping systems with and without 

detected contamination appear similar. This is generally confirmed by the results of three 

statistical significance tests – Pearson’s Chi-Squared, Fisher’s Exact Test, and a parametric test 

based on the standard error of the differences in proportions. The results of these tests, which are 

presented in Appendix 5 on page 25 along with results of other supporting statistical analysis, 

indicate that the small differences in proportions are not significant, with the exception of 

Figure 2 - Contamination At 370 Facilities

Below RBSL

81

Above RBSL

98

No 

Contamination 

191

Contamination 

179

Figure 1 - Contamination At 454 Dispenser

Islands

Below RBSL

99

Above RBSL

116

Contamination 

215

No 

Contamination 

239
11

Figure 4 - Contamination For Suction Piping

Systems - Dispenser Islands

Below RBSL

41

Above RBSL

59

No 

Contamination 

118

Contamination 

100

dispenser islands without contamination, where the difference between pressure and suction 

piping systems is marginally significant (p values approach 0.05). 

Table 4 - Frequency Of Contamination And Contamination Above RBSLs 

At Dispenser Islands 

Number Of Islands 

Pump Type With Valid 

Data 

With No Detected 

Contamination 

With At Least One 

Contaminant 

Detected 

With At Least One 

Contaminant 

Above RBSL 

All 454 239 215 116 

Pressure 183 84 99 49 

Suction 218 118 100 59 

Unknown 53 37 16 8 

Proportion Of Islands, Percent (95% Confidence Limits) 

Pump Type Percent With 

Valid Data 

Percent With No 

Detected 

Contamination 

Percent With At 

Least One 

Contaminant 

Detected 

Percent With At 

Least One 

Contaminant 

Above RBSL 

All 100.0 52.6 (48.0 - 57.2) 47.4 (42.8 - 51.9) 25.6 (21.5 - 29.6) 

Pressure 40.3 45.9 (38.7 - 53.1) 54.1 (46.9 - 61.3) 26.8 (20.4 - 33.2) 

Suction 48.0 54.1 (47.5 - 60.7) 45.9 (39.3 - 52.5) 27.1 (21.2 - 33.0) 

Unknown 11.7 69.8 (57.4 - 82.1) 30.2 (17.8 - 42.5) 15.1 (5.4 - 24.7) 

 Figures 3 and 4 visually depict the number of dispenser islands with pressure and suction 

piping systems, respectively, where contamination was discovered under the dispenser, and the 

number of times the contamination exceeded RBSLs. 

Figure 3 - Contamination For Pressure Piping

Systems - Dispenser Islands

Below RBSL

50

Above RBSL

49

No 

Contamination 

84

Contamination 

99
12

Figure 5 - Contamination For Pressure Piping

Systems - Facilities

Contamination 

70

No 

Contamination 

65

Above RBSL

34

Below RBSL

36

 Table 5 is organized identically to Table 4, but shows the frequency of detected 

contamination and contamination above RBSLs for the 370 facilities. The results of the three 

statistical significance tests, which are presented in Appendix 5 on page 25 along with results of 

other supporting statistical analysis, indicate that the small differences in proportions are not 

significant. 

Table 5 - Frequency Of Contamination And Contamination Above RBSLs At Facilities 

Numbers Of Facilities

Pump Type With Valid 

Data 

With No Detected 

Contamination 

With At Least One 

Contaminant 

Detected 

With At Least One 

Contaminant Above 

RBSL 

All 370 191 179 98 

Pressure 135 65 70 34 

Suction 198 103 95 57 

Unknown 37 23 14 7 

Proportions Of Facilities, Percent (95% Confidence Limits)

Pump Type Percent With 

Valid Data 

Percent With No 

Detected 

Contamination 

Percent With At 

Least One 

Contaminant 

Detected 

Percent With At 

Least One 

Contaminant Above 

RBSL 

All 100.0 51.6 (46.5 - 56.7) 48.4 (43.3 - 53.5) 26.5 (22.0 -31.0) 

Pressure 36.5 48.1 (39.6 - 56.2) 51.9 (43.4 – 60.3) 25.2 (17.9 - 32.5) 

Suction 53.5 52.0 (45.0 - 58.9) 48.0 (41.0 - 54.3) 28.8 (22.5 - 35.1) 

Unknown 10.0 62.2 (46.6 - 77.8) 37.8 (22.2 - 53.5) 18.9 (6.2 - 31.5) 

Figures 5 and 6 visually depict the number of facilities with pressure and suction piping 

systems, respectively, where contamination was discovered under the dispenser, and the number 

of times the contamination exceeded RBSLs. 

Figure 6 - Contamination For Suction Piping

Systems - Facilities

Below RBSL

38

Above RBSL

57

No 

Contamination 

103

Contamination 

95
13

Table 6 provides a comparison of the media affected and the South Carolina RBCA site 

priority classification (see Appendix 3 on page 15) for the 19 sites found to have contamination 

above RBSLs only at the dispenser and data for all releases in South Carolina for the period 

January 1996 through June 2002. As shown, the proportions of facilities that affected 

groundwater (approximately three-quarters) or only soil (approximately one-quarter) are similar 

for the facilities with dispenser-only contamination and for all facilities with releases. Although 

there are some differences with respect to the South Carolina RBCA classifications, the 

relatively small number of facilities with dispenser only contamination makes it difficult to 

conclude that these differences are significant. 

As discussed on page 3 in the methodology section, information on the type of receptor 

potentially affected by contamination was included in the data collection effort. Because these 

data do not indicate whether receptors have actually been affected or not, EPA later decided 

comparison of these data for the facilities studied with other facilities in the state would not yield 

meaningful results. 

Table 6 - Affected Media And South Carolina RBCA 

Classification Comparison 

Facilities With DispenserOnly ContaminationAll Facilities With Release From 

January 1996 Through June 2002

 Number Percent* Number Percent* 

Total # Of Facilities 19 1581 

Affected Media 

Groundwater 14 74 1103 70 

Soil Only 5 26 478 30 

South Carolina RBCA** 

1 1 5 39 3 

2B 2 10 309 20 

3B 6 32 433 27 

4B 1 5 61 4 

5 9 47 639 40 

Other 0 0 100 6 

*Totals may not add to 100 due to rounding. 

** See Appendix 3 (page 15) for descriptions of these categories. 

Conclusions 

 For closed facilities in South Carolina, which EPA assumes had no under dispenser 

containment, releases to the environment near dispensers appear to be quite common; nearly onehalf (48 percent) of all facilities included in the study show contamination detected under one or 

more dispenser islands. This result is likely somewhat lower than actual contamination rates 

because data was often not available for every dispenser or even every dispenser island at a 

facility. The frequency of these releases appears to be affected little, if at all, by the type of 
14

pump delivery system (pressure or suction) used at the facility. At about 10 percent (19) of the 

facilities with contamination above RBSLs detected under a dispenser island, the dispenser 

appears to be the only source of release. Based on a comparison with corrective action data for 

all releases in South Carolina, the consequences of these dispenser releases in terms of the media 

affected and the need for corrective action (as measured by the South Carolina RBCA 

classification) appear to be comparable to those from other sources of release, such as tanks and 

piping. 
Appendix 1 - South Carolina Dispenser Data Gathering Form For First Phase 

□ INVALID FILE - Check here & indicate the reason below: 

□ No dispenser samples taken/analysis performed. □ UST was within five (5) feet of the dispenser. □ Contaminant levels are unknown. 

□ Other: _____________________________________ 

Facility ID: ___________________ Date Closed: __

Number Dispen

Assessment Report / Closure Report (Circle One)

Dispenser Contamination Above State RBSLs Benzene Ethylbenzene Toluene Xylenes 

Island ID (Yes/No) (mg/kg)

Instructions: 1. Dispenser Island ID - If more than 1 sample is taken at an island, ID the islands as 1a and 1b. 

2. If more than one constituent is reported [i.e., for xylenes - xylene (m,p) and zylene (o)], add those numbers together for re

3. If value is not reported, indicate with N/A. 

4. If value below detection level, indicate the detection limit by using “<“ followed by the detection limit value. SC RBSLs in Clays (mg/kg): Benzene 0.008; Ethylbenzene 6.168; Toluene 1.167; Xylenes 22.495; Naphthalene 0.069 SC RBSLs in Sands (mg/kg): Benzene 0.007; Ethylbenzene 1.150; Toluene 1.450; Xylenes 14.500; Naphthalene 0.036
Appendix 2 - South Carolina Dispenser Data Gathering Form For Second Phase 

Site ID: ____________________ Date Closed: __________________ Number of Dispensers: 

□ INVALID FILE - Check here & indicate the reason below: 

□ No dispenser samples taken/analysis performed. □ UST was within five (5) feet of the dispenser. 

□ Contaminant levels are unknown. 

□ Other: _____________________________________ 

□ Contaminants are above RBSLs (If yes, print out a corrective action ssite ID)

Contaminant Medium _____SC RBCA Classification ___Receptor Type ___________

Contamination Above State 

RBSLs

Dispenser Benzene Ethylbenzene Toluene 

Island ID 

Piping Type 

(S) = Suction 

(P) = Pressurized 

(U) = Unknown (Yes/No) (mg/kg)

Additional Comments: SC RBSLs in Clays (mg/kg): Benzene 0.008; Ethylbenzene 6.168; Toluene 1.167; Xylenes 22.495; Naphthalene 0.069 SC RBSLs in Sands (mg/kg): Benzene 0.007; Ethylbenzene 1.150; Toluene 1.450; Xylenes 14.500; Naphthalene 0.036 
17

Appendix 3 – South Carolina RBCA Site Priority Classification System 

1. Sites are placed in Classification 1 if: 

1A an emergency situation exists 

1B a fire or explosion hazard exists 

1C vapors or free product exists in a structure or utility 

1D concentrations of chemical of concern (CoC) have been detected in a potable 

water supply or surface water supply intake 

1E free product exists on surface waters 

1F CoC exists in surface water 

2. Sites are placed in Classification 2 if: 

Classification 2a:

2AA a significant near term (0 to 1 year) threat to human health, safety, or sensitive 

environmental receptors exists 

2AB potable supply wells or surface water supply intakes are located < 1 year ground 

water travel distance downgradient of the source area 

Classification 2b:

2BA free product exists in a monitoring well at a measured thickness > 1 foot 

2BB potable supply wells or surface water supply intakes are located < 1000 feet 

downgradient of the source area (where groundwater velocity data is not 

available) 

3. Sites are placed in Classification 3 if: 

Classification 3a:

3AA a short-term (1 to 2 years) threat to human health, safety, or sensitive 

environmental receptors exists 

3AB potable supply wells or surface water supply intakes are located > 1 year and < 2 

years groundwater travel distance downgradient of the source area 

3AC sensitive habitats or surface water exist < 1 year groundwater travel distance 

downgradient of the source area and the groundwater discharges to the sensitive 

habitat or surface water 

Classification 3b:

3BA free product exists in a monitoring well at a measured thickness > 0.01 foot 

3BB concentrations of CoC above the RBSL have been detected in a non-potable water 

supply well 

3BC hydrocarbon containing surface soil (< 3 feet below grade) exists in areas that are 

not paved 
18

3BD sensitive habitats or surface water used for contact recreation exist < 500 feet 

downgradient of the source area (where groundwater velocity and discharge 

location data are not available) 

3BE the site is located in a sensitive hydrogeologic setting, determined based on the 

presence of fractured or carbonate bedrock hydraulically connected to the 

impacted aquifer 

3BF groundwater is encountered < 15 feet below grade and the site geology is 

predominantly sand or gravel 

4. Sites are placed in Classification 4 if: 

Classification 4a:

4AA a long term (> 2 years) threat to human health, safety, or sensitive environmental 

receptors exists 

4AB potable supply wells or surface water supply intakes are located > 2 years and < 5 

years groundwater travel distance downgradient of the source area 

4AC non-potable supply wells are located < 1 year ground water travel distance 

downgradient of the source area 

Classification 4b:

4BA free product exists as a sheen in any monitoring wells 

4BB non-potable supply wells are located < 1000 feet downgradient of the source area 

(where groundwater velocity is not available) 

4BC the ground water is encountered < 15 feet and the site geology is predominantly 

silt or clay 

5. Sites are placed in Classification 5 if: 

5A there is no demonstrable threat, but additional data are needed to show that there 

are no unacceptable risks posed by the site 

assessment data for the site indicate concentrations in some samples are above the 

RBSL or Site-Specific Target Level (SSTL), as appropriate, and further 

assessment is needed 

5B assessment data for the site indicate concentrations in samples are below the 

RBSL or SSTL, as appropriate, but the samples are determined not to be 

representative; therefore, further assessment is needed 
19

Appendix 4 - Summary Of Results 

Total number of facilities reviewed 1,218 

Number of facilities with valid data 370 

Number of dispenser islands with valid data 454 

Number of facilities with invalid data 848 

Number of facilities with contamination above RBSLs at dispenser only 19 

Number of facilities with detectable contamination under dispensers* 179 

Number of facilities with detectable contamination above RBSLs 98 

Number of facilities with detectable contamination below RBSLs 81 

Number of facilities with no detectable contamination under dispensers 191 

Number of dispenser islands with detectable contamination under 

dispensers* 215 

Number of dispenser islands with detectable contamination above

RBSLs 116 

Number of dispenser islands with detectable contamination below

RBSLs 99 

Number of dispenser islands with no detectable contamination under 

dispensers 239 

Pressurized Pumping System Data 

Number of facilities with pressurized pumping 135 

Number of facilities with pressurized pumping systems and 

contamination under dispensers* 70 

Number of facilities with pressurized pumping systems and 

contamination under dispensers above RBSLs 34 

Number of facilities with pressurized pumping systems and 

contamination under dispensers below RBSLs 36 

Number of facilities with pressurized pumping systems and no 

detectable contamination under dispensers 65 

Number of dispenser islands with pressurized pumping 183 

Number of dispenser islands with pressurized pumping systems and 

contamination under dispensers* 99 

Number of dispenser islands with pressurized pumping systems and 

contamination under dispensers above RBSLs 49 

Number of dispenser islands with pressurized pumping systems and 

contamination under dispensers below RBSLs 50 

Number of dispenser islands with pressurized pumping systems and no 

detectable contamination under dispensers 84 
20

Suction Pumping System Data 

Number of facilities with suction pumping 198 

Number of facilities with suction pumping systems and contamination 

under dispensers* 95 

Number of facilities with suction pumping systems and contamination 

under dispensers above RBSLs 57 

Number of facilities with suction pumping systems and contamination 

under dispensers below RBSLs 38 

Number of facilities with suction pumping systems and no detectable 

contamination under dispensers 103 

Number of dispenser islands with suction pumping 218 

Number of dispenser islands with suction pumping systems and 

contamination under dispensers* 100 

Number of dispenser islands with suction pumping systems and 

contamination under dispensers above RBSLs 59 

Number of dispenser islands with suction pumping systems and 

contamination under dispensers below RBSLs 41 

Number of dispenser islands with suction pumping systems and no 

detectable contamination under dispensers 118 

Unknown Pumping System Data 

Number of facilities with unknown pumping 37 

Number of facilities with unknown pumping systems and 

contamination under dispensers* 14 

Number of facilities with unknown pumping systems and 

contamination under dispensers above RBSLs 7 

Number of facilities with unknown pumping systems and 

contamination under dispensers below RBSLs 7 

Number of facilities with unknown pumping systems and no detectable 

contamination under dispensers 23 

Number of dispenser islands with unknown pumping 53 

Number of dispenser islands with unknown pumping systems and 

contamination under dispensers* 16 

Number of dispenser islands with unknown pumping systems and 

contamination under dispensers above RBSLs 8 

Number of dispenser islands with unknown pumping systems and 

contamination under dispensers below RBSLs 8 

Number of dispenser islands with unknown pumping systems and no 

detectable contamination under dispensers 37 

* Number in shaded box indicates total of two subsets of data below, which include contamination detected above 

and below state RBSLs.
21

 

Dispenser Data 

Number of dispensers at facilities with valid data 1,251 

Average number of dispensers per facility 3.38 

Values within 30% of RBSL value 

Number of facilities with contamination above RBSLs within 30% of 

sand RBSL value 5* 

Number of facilities with contamination below RBSLs within 30% of 

sand RBSL value 8 

Number of dispenser islands with contamination above RBSLs within 

30% of sand RBSL value 5* 

Number of dispenser islands with contamination below RBSLs within 

30% of sand RBSL value 9 

* An additional 6 facilities/dispenser islands were within 30% of the clay RBSL only

Data For Facilities With Contamination Above RBSLs At Dispenser Only 

Number Of 

Records Contaminant Medium

14 Groundwater 

5 Soil 

Number Of 

Records South Carolina RBCA Classification 

1 1D Chemicals detected in water 

1 2BA Free product >1 ft 

1 2BB Water supply wells <1000 ft downgrade 

1 3BA Free product >0.01 ft thick 

1 3BE Sensitive hydrologic setting 

4 3BF GW < 15 ft in sand or gravel 

1 4BC GW < 15 ft in silt of clay 

3 5A No pending threat, additional data 

6 5B Assessment data NOT conclusive 
22

Appendix 5 - Statistical Analysis 

This appendix presents the results of statistical analyses done to investigate whether 

variation in facility closure date, size, or other characteristics might affect contamination patterns 

and, thus, the conclusions that can be drawn from the data collected. 

Characteristics Of Closures And Closed Facilities

 Valid data files included closures that took place between 1982 and 1999. The primary 

focus of the data collection effort was on closures that occurred between 1995 and the present, 

but some earlier closures were also included (see Table A). As shown, almost half of the 

closures (162, or 44 percent) occurred in 1998. (The regulatory deadline for replacing or 

upgrading tank systems was December 1998.) The table also shows that the relative proportions 

of closures at facilities with different piping delivery types changed somewhat over the time 

period covered in the database, with the proportion of pressure (P) closures generally increasing 

and the proportion of suction (S) facilities decreasing. 

Table A - Distributions Of Closures By Year And Pump Type 

Year Total Closures Pressurized 

Facilities Closed

Suction Facilities 

Closed 

Unknown 

Facilities Closed

Pre-1996 20 1 (5) 11 (55) 8 (40) 

1996 52 14 (27) 31 (60) 7 (13) 

1997 80 21(26) 53 (66) 6 (8) 

1998 162 75(46) 79 (49) 8 (5) 

1999 56 24(43) 24 (43) 8 (14) 

All 370 135 (37) 198 (54) 37 (10) 

Note: Values outside of parentheses represent total number of facilities in the specific category; values inside 

parentheses represent the percent of facilities in the specific category. 

 The size of closed facilities, as indicated by the numbers of dispensers present, also 

varied (Table B).1

 The bulk of the closures took place at small facilities; slightly fewer than 30 

percent of the facilities had only one dispenser, and about 84 percent had four dispensers or 

fewer. The largest facility had 50 dispensers. There was a slight tendency for the largest 

facilities that were closed (e.g., those with more than six dispensers) to have pressure piping. 

Facilities with three or fewer dispensers were also more likely to have suction piping. There was 

no significant correlation between closure date and size of facility. 

 

1

 The data collection effort did not include collection of data on the number of dispenser islands at the closed 

facilities. 
23

Table B - Size Distribution Of Facilities Closed (Number Of Dispensers) 

Dispensers Total Closures Pressurized 

Facilities Closed

Suction Facilities 

Closed 

Unknown 

Facilities Closed

1 109 (29) 44 (40) 55 (51) 10 (9) 

2 84 (23) 24 (29) 50 (60) 10 (12) 

3 74 (20) 26 (35) 43 (58) 5 (7) 

4 45 (12) 15 (33) 24 (53) 6 (13) 

5 13 (4) 3 (23) 8 (62) 2 (15) 

6-10 31 (8) 16 (52) 13 (42) 2 (7) 

>10 14 (4) 7 (50) 5 (36) 2 (14) 

All 370 (100) 135 (37) 198 (54) 37 (10) 

Note: Values outside of parentheses represent total number of facilities in the specific category; values inside 

parentheses represent the percent of facilities in the specific category. 

Analyses of Soil Contamination Data 

 Valid data collected included 534 samples. Of the 370 facilities in the data set, 273 (74 

percent) included only one sample result. A total of 454 dispenser islands were sampled at least 

once. Multiple dispenser islands were sampled at 57 facilities, including 41 facilities with results 

from two islands. The bulk of dispenser islands (398, or 88 percent) were sampled only once; 56 

islands were sampled two, three, or four times each. As evident from these data, the number of 

dispensers at the facility is not necessarily representative of the number of dispenser islands 

sampled. It is possible there were some islands at a given facility that were not sampled, or 

which were not recorded on the data form; but for each facility, at least one island had a valid 

sample. 

 Analytical results were reported for benzene, toluene, ethylbenzene, xylenes, and 

naphthalene (Table C). Individual analytes were reported above detection limits in between 15.7 

percent (benzene) and 34.6 percent (xylenes) of the chemical analyses. Detection limit values 

were not reported for 28 samples (5 percent) of the samples. Among the analytes, only 

benzene’s RBSLs are close to the typical analytical detection limits. 

 Concentrations were highly variable, with relatively few high results (> 1 mg/kg). 

Representative concentrations of toluene (geometric mean 0.085 mg/kg) and benzene (0.088 

mg/kg) were the lowest, followed by ethylbenzene (0.159 mg/kg), with xylenes (0.294 mg/kg) 

and naphthalene (0.352 mg/kg) having the highest geometric mean concentrations in samples 

where contamination was detected. 
24

Table C - Summary Of Analytical Results 

Analyte 

Frequency Of 

Detection 

(percent) 

Representative 

Concentration, 

Geometric 

Mean/Standard 

Deviation 

(mg/kg) 

Representative 

Detection Limit 

(mg/kg) 

RBSLs For 

Sand/Clay 

(mg/kg) 

Benzene 15.7 0.088 / 2.87 0.001 - 0.005 0.007 / 0.008 

Ethylbenzene 24.9 0.159 / 3.05 0.001 - 0.005 1.15 / 6.17 

Toluene 29.2 0.085 / 3.46 0.001 - 0.005 1.45 / 1.17 

Xylenes 34.6 0.294 / 3.65 0.001 - 0.015 14.5 / 22.5 

Naphthalene 31.8 0.352 / 3.11 0.002 - 0.005 0.036 / 0.069 

 Preliminary analysis suggested that average contaminant concentrations and frequencies 

of detection were higher in samples from large facilities (i.e., those with larger number of 

dispensers), and that contaminant concentrations were generally lower in samples from more 

recent closures. EPA found the latter trend to be driven primarily by higher concentrations from 

a relatively few early closures and there were no significant trends in contaminant concentrations 

in closures after 1995. Results of the preliminary analysis prompted more detailed evaluation of 

detection frequencies and exceedences of RBSLs discussed below. 

Analysis Of Frequency Of Detection And Frequency Of Occurrence Above RBSLs 

 The distribution of the numbers of sampled facilities, dispenser islands, and individual 

samples, as well as the frequency of contaminant detection and the frequency of occurrence 

above RBSLs are presented in Tables 2 through 4 in Section VI of this report. 

 ICF used three significance tests to evaluate the differences in proportions of facilities 

with detected contamination and with contamination above RBSLs: Pearson’s Chi-Squared, 

Fisher’s Exact Test, and a parametric test based on the standard error of the difference in 

proportions.2

 All give results that are generally consistent, but vary slightly depending on the 

size of the data set and the magnitude of the difference in proportions. 

 Both the Fisher’s exact test and Chi-Squared procedure assume that individual 

observations are not correlated (are independent). Fisher’s Exact Test is the most robust and 

reliable test for small sample sizes, while the Chi-Squared generally is considered to be reliable 

as long as the number of observations in the smallest group is not too small. The results of these 

 

2

 Sheskin, DJ, Parametric and Nonparametric Statistical Procedures, Chapman and Hall, New York, 200, p.381. 
25

two tests may differ slightly because the Chi-Squared test uses an approximation of the 

distribution of difference between proportions, while Fisher’s exact test calculates the difference 

between proportions explicitly. The parametric test assumes that the difference between the 

proportions is normally distributed, and may therefore be somewhat less robust than the other 

two procedures. EPA included this test as a contingency in case the data set resulted in Fisher’s 

calculations that exceeded the capability of a typical computer (which did not occur).

 Tables D and E present the frequencies of having at least one sample with detected 

contamination or one sample with contamination above RBSLs at dispenser islands and 

facilities, respectively, with pressure (P) and suction (S) piping systems. The modest differences 

in the proportions of P and S islands with contamination are marginally significant (p - values 

approach 0.05) by two of the three significance tests (Table D). The very small difference in the 

proportions of P and S islands that have contamination above RBSLs is, as expected, not 

significant. Similarly, the small differences in the frequencies of contamination seen at the P and 

S facilities also are not significant by any of the statistical tests. 

Table D - Significance Test Results For Difference In Proportions Of Dispenser Islands 

With Contamination 

Chi^2 Fisher’s Exact 

Test 

Diff. in 

Proportions 

Proportions of Islands With, Without Detected Contamination 

P vs S NS P ~ 0.05 p ~ 0.05 

Proportions of Islands with Contamination > RBSLs 

P vs S NS NS NS 

Table E - Significance Test Results For Differences In Proportions Of Facilities 

With Contamination 

Chi^2 Fisher’s Exact 

Test 

Diff. in 

Proportions 

Proportions of Facilities With, Without Detected Contamination 

P vs S NS1

 NS NS 

Proportions of Facilities with Contamination Above one or More RBSLs 

P vs S NS NS NS 

1. NS = not significant at p = 0.05 

Frequency Of Contamination Above RBSLs As A Function Of Facility Size

 Table F shows the patterns of contaminant occurrence (detection and presence above 

RBSLs) in facilities of different sizes. When analyzed as a whole, the data generally indicate 

weak, insignificant, positive relationships between the frequency of contamination and the 
26

number of dispensers. This means the frequency of detected contamination increases with the 

number of dispensers. However, this relationship is inconsistent. The smaller facilities (one 

dispenser) have more frequent contamination than the middle-size facilities (four dispensers). 

To the extent there is a positive trend, it appears to be limited to the largest facilities (>10 

dispensers). The high frequency of contamination at these facilities (71.4 percent) and their 

dispenser islands (64.9 percent) are driven by a relatively high frequency of contamination in 

individual samples (61.9 percent). The reasons for the apparent higher frequency of 

contamination in samples from the largest facilities are not clear; they may reflect real 

differences, or may be a statistical artifact associated with the relatively small number of samples 

(42) from this size range. 

Table F - Frequency Of Contamination As A Function Of Facility Size (In Percent) 

Number Of Facilities Dispenser Islands Samples 

Dispensers Detected >RBSL Detected >RBSL Detected >RBSL 

1 51.4 28.4 51.8 28.2 51.3 27.4

2 45.2 19.0 44.3 18.2 42.9 16.3 

3 40.5 24.3 42.4 28.2 38.7 26.1 

4 46.7 22.2 42.2 15.6 37.5 13.8 

5 53.8 30.8 46.7 26.7 44.4 22.2 

6-10 56.7 41.9 45.5 34.5 43.1 34.7 

>10 71.4 42.9 64.9 32.4 61.9 31.0 

 The proportion of facilities with contamination above RBSLs also increases with 

increasing facility size (Table F). However, in this case, there is no corresponding increase in 

contamination above RBSLs at individual dispenser islands or in individual samples. Thus, the 

increase at the facility level may be a consequence of more samples and/or more dispensers per 

facility at larger sizes. In such a case, a constant proportion of independent samples above 

RBSLs would naturally translate into larger numbers of dispenser islands and facilities with at 

least one sample above RBSLs. (For example, if the probability of any sample being above an 

RBSL were 50 percent, a facility with two samples would have a 75 percent chance of having at 

least one sample above an RBSL.) The fact that the proportion of dispenser islands and facilities 

with values above RBSLs is only slightly higher than the proportion of samples above RBSLs 

illustrates that sampling results from the same islands and/or facilities are highly correlated. 

United States Solid Waste And EPA 510-R-04-004

Environmental Protection Emergency Response September 2004

Agency 5401G www.epa.gov/oust/pubs