# Frequency And Extent Of Dispenser Releases At Underground Storage

Tank Facilities In South Carolina

Printed on recycled paper

### This document was prepared by IC F, 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 Contaminati on At Dispenser

Islands And Facilities 9

3 Numbers Of Sampled Facilities, Dispenser Islands, And Samples By Pump Type 10

4 Frequency Of Contaminati on 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 Pipi ng Systems – Facilities 12

6 Contamination For Pressure Pipi ng Systems – Facilities 121 

## Introduction

In this document, EPA presents the results of information collected and analyzed from underground storage tank (UST) cl osure and assessment reports at sites in South Carolina. The report discusses the background, purpose, met hodology used, quality as surance and quality control procedures applied, results of the study, and conclusions. Supporting information is in the appendices.



## Background

In 1984, Congress responded to the increasi ng 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. E nvironmental Protection Agency (EPA) develop a comprehensive regulatory program for USTs stori ng petroleum or certain hazardous substances in order to protect the envi ronment and human health from releases. EPA’s Office of Underground Storage Tanks (OUS T) developed regulations for owners and operators of UST systems covering leak detection, addressing release prevention, a nd minimizing the potential for USTs to corrode. (The regulations also cove r additional requirements, such as financial responsibility and corrective action, not discussed in this report.) The regulatory requirements were phased in over ten years; th e final deadline was December 1998.

Federal and state regulations helped EPA achieve its manda te. 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 th e 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 dispensi ng systems as a potential concern for the national UST program and state UST regulatory agen cies. 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 regula tions do not address dispensers

. In addition, the federal regulations do not explicitly require assessment under dispensers at the time of UST sy stem 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 assessmen t is required for the places where contamination2 is most likely present at an UST site. 1 Initially, the most likely si tes 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 investiga tions and under dispenser containm ent (UDC) to contain releases that do occur. Industry has also recognized the potential for disp enser releases and, as a result, installing under dispenser containment is becomi ng increasingly common for new installations.

South Carolina’s Department of Health a nd Environmental Control (DHEC) first began requiring dispenser sampling at th e time of UST system closure 2 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 is land is located above or immediately adjacent (less than five f eet) 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 hist ory of sampling under dispensers provided an excellent source of data for examining the freque ncy of under dispenser contamination. In order to better evaluate the frequenc y and severity of releases under dispensers, EPA partnered with DHEC, which has a fairly robust set of data on petroleum contamin ation found during UST system inspections and closures, in cluding contamination under dispensers.

As mentioned above, it has become common pr actice 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 tim e 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 dispen ser pans. Therefore, the results of this study are assumed to refl ect 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. Enviro nmental 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. Th e 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 gath er and analyze dispenser sampling data 5 from South Carolina DHEC underground storage tank assessment and closure files to determine whether the data showed any patterns of disp enser releases and to identify the likelihood that releases from dispensers will contaminate subsurface media (s oil and groundwater). These data provide information about the frequency and imp act of releases under dispensers.



## Methodology

This report reflects the results of two se parate study phases during which data about petroleum releases from dispensers associated wi th USTs was collected and analyzed. The data came primarily from closure reports, 95 percen t 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 US T system. The closure report mu st address all components of the tank system, including tanks, piping, and dispensers . The sites with clos ure 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 unde r 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 ma intained by DHEC’s Underground Storage Tank Program. The first data collection phase wa s conducted by DHEC in January 2002 to pilot the data collection techniques and forms. EPA, with contractor assist ance, 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, identif y 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 Ma rch 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 determ ined to require no further action (NFA); sites

with confirmed releases that had been rehab ilitated 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 conditi ons 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 th e purposes of this study) for any of the following

reasons:

z No dispenser samples taken/analysis performed.

z UST was within five feet of dispen ser(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, th e following additional criteria were also applied to identify information that was cons idered invalid (for purpos es 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 genera tor fuel, motor oil, or waste oil were

considered invalid in the data screening for both phases. For th e 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 f eet 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 w hole facility was considered valid. In such cases, dispensers with valid data were included in the popul ation of data collected.



### Data Recording

During the first phase of data collection and as each report was re viewed, the reviewer completed a data form and recorded selected in formation about the facility and the release (see Appendix 1 on page 13 for a copy of the blank fo rm). During the second phase, data from closure reports were entered direct ly into an electronic form create d 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 pr ovided below. Data elements that were only included in the second phase are identifie d in the descriptions that follow.5 z Facility ID – Unique site identificati on 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 thre e types of piping product delivery methods: suction (S), pressurized (P), or unknown (U); addi tional details about piping are addressed in the “Note on Pi ping 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 detec tion 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 r ecorded the following information:

- Contaminant Medium – Select whether the contamination reached soil or groundwater, both, or unknown.

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

Carolina Risk Based Corrective Acti on Classification (SCRBCA) level (see Appendix 3 on page 15 for details about the SCRBCA classification levels).

- Receptor type – Multiple choices, includi ng public well, creek, property boundary, etc.

South Carolina’s risk-based approach to pr ioritizing cleanups uses RBSLs established for two types of soils – sand and clay. In or der 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 be tween two South Carolina RBSLs, then EPA determined the soil type from a map indicati ng 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 correc tive 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 correc tive action database. DHEC provided the statewide summary discussed in the re sults section on page 8.

If a report was determined to be invalid, the “S ite ID,” “Date Closed,” “Invalid File,” and the appropriate “Reason File Inva lid” boxes were the only fields completed on the form. Valid files included sites with at least one soil or groun dwater 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 ot her 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 samp les 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 re quirement. DHEC requir es that samples be collected approximately two feet below the botto m of the piping of the dispenser chosen for sampling and that the dispenser wi th the highest anticipated leve l 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 samp les taken at the dispenser(s). If the various species of xylene were reported separately (i.e., xylene [m, o, a nd p]), then the total of those results was recorded as a single value in th e database (i.e., concentration of m-xylene + concentration of o-xylene + concen tration of p-xylene). In the case of other contaminants when two analytical results were provided, the higher concentratio n was always used. If no contamination was present (reporte d as a “non-detect”), then the re sult 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 stan dard laboratory analyses for the detection and quantification of common petrol eum constituents. The accuracy (i.e., the closeness of the measured value to the true value) of the labor atory analysis data depe nds 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 potenti al effects of these accu racy limitations on the results of this study, EPA identified the situations where the characterizati on 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 percen t of the time. The percentage of measurements characterized as “above RBSLs” that might actually be below RBSLs (assuming a +/- 30 percent marg in 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 draw n from the data is thought to be small.

There has been increasing concern about cont amination 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 closur e 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 cons tituents found in gasolin e – into groundwater.

Note On Piping Product Delivery Types

EPA collected data on the types of pumps us ed 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 th e 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 su ction systems would increase or decrease the

likelihood of a release. For those sites at wh ich 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 determ ine 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 coll ection 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 wa s 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 ot her. During the first w eek 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. Ev ery 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 th e 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 wa s performed at the end of each workday or the beginning

of the next. Each data collect or kept a separate list of th e 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 f ile selected for QC review in detail. Where necessary, the South Carolina corrective action data base was consulted. If a discrepancy or error was discovered, this was noted and discussed th at 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 review ed were revised in the da tabase as a result of this review process.

While no QA/QC strategy was used during the fi rst 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 iden tified issues and provided recommendations about the report. EPA evaluated the comments and in corporated 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 clos ure 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 f iles reviewed are assumed to be generally representative of closur e 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” (f or 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 da ta and 515 (61 percent) were invalid because dispenser(s) with sampling/analysis data were with in 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 mo re 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 disp ensers at nearly half of all facilities and dispenser islands with valid da ta and dispenser contamination that exceeded RBSLs was found at just over 25 percent of al l facilities and dispenser islands. The distribution of these data by pump type is provided in the tables that follow. Of the 454 dispense r 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 leve ls in the multiple samples fall in the same contaminant grouping (i.e., not detected, at least one contaminant det ected, 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 de pict 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 cont aminant detected, and at least one contaminant above the RBSL. The bottom portion shows the binomial confidence limits for these proportions. As shown, the proporti ons of pressure and suction pi ping systems with and without detected contamination appear similar. This is generally confirmed by the results of three statistical significance tests – Pear son’s Chi-Squared, Fisher’s Exac t 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 re sults of other supporting statistical analysis, indicate that the small differences in proporti ons 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

23911 Fi g ure 4 - Contamination For Suction Pipin g Systems - Disp enser Islands

Below RBSL

41 Above RBSL

59 No

Contamination

118 Contamination

100 dispenser islands without cont amination, where the difference between pressure and suction piping systems is marginally signi ficant (p values approach 0.05).

Table 4 - Frequency Of Contaminatio n 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 isla nds with pressure and suction piping systems, respectively, where contaminati on was discovered under the dispenser, and the

number of times the contamination exceeded RBSLs.

Fi g ure 3 - Contamination For Pressure Pipin g Systems - Dispenser Islands

Below RBSL

50 Above RBSL

49 No

Contamination

84 Contamination

9912 Fi g ure 5 - Contamination For Pressure Pipin g Systems - Facilities

Contamination

70 No

Contamination

65

Above RBSL

34

Below RBSL

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

contamination and contamination above RBSLs for the 370 facilities. The results of the three statistical significance tests, wh ich are presented in Appendix 5 on page 25 along with results of other supporting statistical anal ysis, indicate that the small di fferences in proportions are not significant.

Table 5 - Frequency Of Contamination And Contamination Above RB SLs 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, Pe rcent (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.

Fi g ure 6 - Contamination For Suction Pipin g Systems - Facilities

Below RBSL

38 Above RBSL

57 No

C ontamination

103 Contamination

9513 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 dispense r 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-qu arters) or only soil (approximately one-quarter) are similar for the facilities with dispenser-only contaminati on 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 recepto rs have actually been affect ed 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 Contamination All 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 Caroli na, which EPA assumes had no under dispenser containment, releases to the envi ronment 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 so mewhat lower than actual contamination rates

because data was often not availa ble for every dispenser or even every dispenser island at a facility. The frequency of these re leases appears to be affected li ttle, if at all, by the type of14 pump delivery system (pressure or suction) used at the facility. At about 10 percent (19) of the

facilities with contamination above RBSLs de tected under a dispense r 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 acti on (as measured by the South Carolina RBCA classification) appear to be comparable to those from other sources of release, such as tanks and piping.15Appendix 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 Dispensers: _________________Assessment Report / Closure Report (Circle One)ContaminationAbove State RBSLsBenzeneEthylbenzeneTolueneXylenesNaphthaleneDispenserIsland ID(Yes/No)(mg/kg)Instructions:1.Dispenser Island ID- If more than 1 sample is taken atan 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 reporting purposes.3. If value is not reported, indicate withN/A.4. If value below detection level, indicate the detectionlimit 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 0.069SC RBSLs in Sands (mg/kg): Benzene 0.007; Ethylbenzene1.150; Toluene 1.450; Xylenes 14.500; Naphthalene 0.03616Appendix 2 - South Carolina Dispenser Data Gathering Form For Second PhaseSite 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 for the dispenser only(If yes, print out a correctiveaction summary report using thesite ID)Contaminant Medium __________________________SC RBCA Classification ________________________Receptor Type ________________________________ContaminationAbove StateRBSLsBenzeneEthylbenzeneTolueneXylenesNaphthaleneDispenserIsland IDPiping 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 0.069SC RBSLs in Sands (mg/kg): Benzene 0.007; Ethylbenzene1.150; Toluene 1.450; Xylenes 14.500; Naphthalene 0.03617 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 concer n (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 downgrad ient of the source area

Classification 2b:

2BA free product exists in a monitoring we ll at a measured thickness > 1 foot 2BB potable supply wells or surface wate r 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 s upply 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 we ll at a measured thickness > 0.01 foot

3BB concentrations of CoC above the RBSL ha ve been detected in a non-potable water supply well 3BC hydrocarbon containing surface soil (< 3 feet below grade) exists in areas that are not paved18 3BD sensitive habitats or surface water used for contac t recreation ex ist < 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 hydr ogeologic setting, determined based on the presence of fractured or carbonate be drock 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, safe ty, or sensitive environmental receptors exists 4AB potable supply wells or surface water s upply intakes are locate d > 2 years and < 5 years groundwater travel distance downgradient of the source area 4AC non-potable supply wells are located < 1 year gro und water travel distance downgradient of the source area

Classification 4b:

4BA free product exists as a sh een in any monitoring wells 4BB non-potable supply wells ar e located < 1000 feet downgrad ient of the source area (where groundwater veloc ity 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 con centrations 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 th e samples are determined not to be representative; therefore, further assessment is needed19 Appendix 4 - Summary Of Results

Total number of facilities reviewed 1,218

N umber of facilities with valid dat a 370

N umber of dispenser isla nds with valid data 454

N umber of facilities with invalid data 848

N umber of facilities with contamination above RBSLs at d ispenser only 19

N umber of facilities with detectable contamination under dispensers* 179

N umber of facilities with detectable contamination above RBSLs 98

N umber of facilities with detectable contamination below RBSLs 81

N umber of facilities with no detectable contamination under dispensers 191

N umber of dispenser islands with detectable contamination under dispensers* 215

N umber of dispenser islands with detectable contamination above

RBSLs 116

N umber of dispenser islands with detectable contamination below

RBSLs 99

N umber of dispenser islands with no detectable contamination under dispensers 239

Pressurized Pumping System Data

N umber of facilities with pressurized pumping 135

N umber of facilities with pressurized pumping systems and contamination under dispensers* 70

N umber of facilities with pressurized pumping systems and contamination under dispensers above RBSLs 34

N umber of facilities with pressurized pumping systems and contamination under dispensers below RBSLs 36

N umber of facilities with pressurized pumping systems and no detectable contamination under dispensers 65

N umber of dispenser islands with pressurized pumping 183

N umber of dispenser islands with pressurized pumping systems and contamination under dispensers* 99

N umber of dispenser islands with pressurized pumping systems and contamination under dispensers above RBSLs 49

N umber of dispenser islands with pressurized pumping systems and contamination under dispensers below RBSLs 50

N umber of dispenser islands with pressurized pumping systems and no detectable contamination under dispensers 8420 Suction Pumping System Data

N umber of facilities with suction pumping 198

N umber of facilities with suction pumping systems and contamination under dispensers* 95

N umber of facilities with suction pumping systems and contamination under dispensers above RBSLs 57

N umber of facilities with suction pumping systems and contamination under dispensers below RBSLs 38

N umber of facilities with suction pumping systems and no detectable contamination under dispensers 103

N umber of dispenser islands with suction pumping 218

N umber of dispenser islands with suction pumping systems and contamination under dispensers* 100

N umber of dispenser islands with suction pumping systems and contamination under dispensers above RBSLs 59

N umber of dispenser islands with suction pumping systems and contamination under dispensers below RBSLs 41

N umber of dispenser islands with suction pumping systems and no detectable contamination under dispensers 118

Unknown Pumping System Data

N umber of facilities with unknown pumping 37

N umber of facilities with unknown pumping systems and contamination under dispensers* 14

N umber of facilities with unknown pumping systems and contamination under dispensers above RBSLs 7

N umber of facilities with unknown pumping systems and contamination under dispensers below RBSLs 7

N umber of facilities with unknown pumping systems and no detectable contamination under dispensers 23

N umber of dispenser islands with unknown pumping 53

N umber of dispenser islands with unknown pumping systems and contamination under dispensers* 16

N umber of dispenser islands with unknown pumping systems and contamination under dispensers above RBSLs 8

N umber of dispenser islands with unknown pumping systems and contamination under dispensers below RBSLs 8

N umber 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

N umber of dispensers at facilities with valid dat a 1,251 Average number of dispensers per facility 3.38

Values within 30% of RBSL value

N umber of facilities with contamination above RBSLs within 30% of

sand RBSL value 5*

N umber of facilities with contamination below RBSLs within 30% of

sand RBSL value 8

N umber of dispenser islands with contamination above RBSLs within

30% of sand RBSL value 5*

N umber 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 Contaminat ion 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 conclusive22 Appendix 5 - Statistical Analysis

This appendix presents the results of stat istical analyses done to investigate whether variation in facility closure date , size, or other characteristics mi ght 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 clos ures that occurred betw een 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 tabl e also shows that the relative proportions

of closures at facilities w ith different piping delivery type s changed somewhat over the time period covered in the database, w ith the proportion of pressure (P ) closures generally increasing and the proportion of sucti on (S) facilities decreasing.

Table A - Distributions Of Clos ures 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 onl y one dispenser, and about 84 pe rcent 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 di spensers were also more likely to have suction piping. There was no significant correlation between cl osure 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 Facili ties 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 tota l 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 ea ch. As evident from these data, the number of dispensers at the facility is not necessarily representative of the number of di spenser 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 fo r each facility, at least one island had a valid sample.

Analytical results were reported for ben zene, toluene, ethylbenzene, xylenes, and naphthalene (Table C). Indivi dual analytes were reported abov e 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 ty pical 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 ethylben zene (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 averag e contaminant concentrations and frequencies of detection were higher in samples from large facilities (i.e., those with larger number of dispensers), and that contaminan t 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 prompt ed more detailed evaluation of detection frequencies and exceedence s of RBSLs discussed below.

Analysis Of Frequency Of Detection And Frequency Of Occurrence Above RBSLs

The distribution of the numbers of sampled facilities, dispenser is lands, and individual samples, as well as the frequency of contamin ant detection and the frequency of occurrence above RBSLs are presented in Tables 2 thr ough 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 contam ination above RBSLs: Pearson’s Chi-Squared, Fisher’s Exact Test, and a parame tric test based on the standa rd error of the difference in proportions. 2 All give results that ar e generally consistent, but va ry slightly depending on the size of the data set and the magnit ude of the difference in proportions.

Both the Fisher’s exact test and Chi- Squared procedure assume that individual observations are not correlated (a re independent). Fisher’s Exact Test is the most robust and reliable test for small sample sizes, while the Chi- Squared generally is cons idered to be reliable as long as the number of observations in the smalle st 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 ca lculates the difference between proportions exp licitly. The parametric test assumes that the difference between the proportions is normally distribute d, and may therefore be somewhat less robust than the other two procedures. EPA included this test as a conti ngency in case the data se t 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 contamin ation above RBSLs at dispenser islands and facilities, respectively, with pressure (P) and suct ion (S) piping systems. The modest differences in the proportions of P and S islands with contam ination are marginally significant (p - values approach 0.05) by two of the thr ee significance tests (Table D). Th e very small difference in the proportions of P and S islands that have cont amination above RBSLs is, as expected, not significant. Similarly, the small differences in th e frequencies of contamin ation seen at the P and S facilities also are not significant by any of the statistical tests.

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

With Contamination

Chi^2 Fisher’s Exact

Test Diff. in Proportions
Proportions of Islands With, Wi thout Detected Contamination

P vs S NS P ~ 0.05 p ~ 0.05
Proportions of Islands wi th Contamination > RBSLs

P vs S NS NS NS

Table E - Significance Test Results For Di fferences In Proportions Of Facilities

With Contamination

Chi^2 Fisher’s Exact

Test Diff. in Proportions

Pro p ortions of Facilities With, Wi thout Detected Contamination

P vs S NS 1 NS NS
Proportions of Facilities with Co ntamination Above one or More RBSLs

P vs S NS NS NS

1. NS = not significant at p = 0.05

Frequency Of Contamination Above RBSL s 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 whol e, the data generally indicate weak, insignificant, positive re lationships between the frequency of contamination and the26 number of dispensers. This means the frequenc y of detected contamina tion 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 appe ars to be limited to the largest facilities (>10 dispensers). The high frequency of contaminatio n at these facilities ( 71.4 percent) and their dispenser islands (64.9 percent) are driven by a re latively high frequency of contamination in individual samples (61.9 percent). The r easons for the apparent higher frequency of contamination in samples from the largest fac ilities are not clear; th ey may reflect real differences, or may be a statistical artifact associ ated 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)

Facilities Dispenser Islands Samples Number Of 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 contamin ation 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 conseque nce of more samples and/or more dispensers per facility at larger sizes. In such a case, a constant proporti on of independent samples above RBSLs would naturally translate into larger numbe rs 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 sample s would have a 75 percent chance of having at least one sample above an RBSL.) The fact that the proportion of dispense r islands and facilities with values above RBSLs is only slightly hi gher 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