Draft Peer Review Document – Do Not Cite Or Quote 

Evaluation Of Releases From 

New And Upgraded 

Underground Storage Tank Systems 

Peer Review Draft – August 2004 

EPA Office Of Underground Storage Tanks 

Draft Peer Review Document– Do Not Cite Or Quote 

1

Table Of Contents 

Introduction................................................................................................................................... 3

Background ................................................................................................................................... 3

Purpose Of This Report................................................................................................................ 3

Methodology .................................................................................................................................. 4

Data Gathering................................................................................................................................ 4 

Use Data.......................................................................................................................................... 6 

Statistical Analysis.......................................................................................................................... 6 

Limitations Of The Data .............................................................................................................. 7

Results ............................................................................................................................................ 7

Number Of Data Points................................................................................................................... 7 

Sources Of Releases........................................................................................................................ 8 

Causes Of Releases......................................................................................................................... 9 

Tank Releases ............................................................................................................................... 11 

Relationship Between Tank Construction, Use, And Releases ................................................ 11 

Causes Of Tank Releases.......................................................................................................... 13 

Piping Releases ............................................................................................................................. 15 

Relationship Between Piping Construction, Use, And Releases .............................................. 15 

Piping Product Delivery And Sumps........................................................................................ 16 

Causes Of Piping Releases........................................................................................................ 17 

Media Affected And Extent Of Releases...................................................................................... 19 

Method Of Release Identification................................................................................................. 21 

Age Distributions Of Tank And Piping Releases Versus Age Distributions Of Tank And Piping 

Use ................................................................................................................................................ 23 

Summary And Conclusions........................................................................................................ 27

Releases And Causes .................................................................................................................... 27 

Tanks............................................................................................................................................. 27 

Piping ............................................................................................................................................ 28 

Method Of Discovery And Leak Detection.................................................................................. 29 

Extent Of Releases And Media Affected...................................................................................... 30 
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List Of Figures And Tables 

Figure 1 - States Participating In The Data Gathering.................................................................... 4 

Table 1 - Information On Location, Begin-End Dates Of Data Gathering, And Number Of Data 

Points Gathered....................................................................................................................... 5 

Figure 2 - Valid And Invalid Data Points ....................................................................................... 8 

Figure 3 - Sources Of Releases....................................................................................................... 9 

Figure 4 – Known Causes Of Releases By Source....................................................................... 10 

Figure 5 - Use And Release Frequencies For Different Tank Designs......................................... 11 

Figure 6 - Use And Release Frequencies For Single Walled And Double Walled Tank Designs 12 

Figure 7 - Use And Release Frequencies For Single Walled And Double Walled Tanks............ 13 

Figure 8 - Causes Of Releases From Steel Tanks......................................................................... 14 

Figure 9 - Causes Of Releases From Single And Double Walled Steel Tanks ............................ 14 

Figure 10 - Causes Of Releases From Single And Double Walled Fiberglass Tanks.................. 15 

Figure 11 - Use And Release Frequencies For Different Piping Designs .................................... 16 

Figure 12 - Use And Release Frequencies For Single Walled And Double Walled Piping......... 16 

Figure 13 - Causes Of Releases From Metal, Flexible Plastic, And Fiberglass Piping................ 18 

Figure 14 - Causes Of Releases From Single Walled And Double Walled Fiberglass Piping 

Systems ................................................................................................................................. 18 

Figure 15 - Extent Of Releases For Different Sources ................................................................. 19 

Figure 16 – Media Affected For Different Sources...................................................................... 20 

Figure 17 - Extent Of Releases To Soil Only For Different Sources ........................................... 20 

Figure 18 - Extent Of Releases To Water For Different Sources................................................. 21 

Figure 19 - Methods Of Release Identification............................................................................. 22 

Figure 20 - Success Rates For Leak Detection Technologies....................................................... 23 

Figure 21 – Age Distribution Of Tank And Piping Use And Releases ........................................ 24 

Figure 22 – Examples Of Age Distribution Of Tanks And Tank Releases.................................. 25 

Figure 23 - Age Distribution Of Piping And Piping Releases...................................................... 26 

Figure 24 – Comparison Of Piping Use And Releases By Material Of Construction For Piping 

Less Than Or Equal To Ten Years Old ................................................................................ 26 
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Introduction 

 In this document EPA presents the results of the collection and analysis of release 

information from underground storage tank (UST) systems. The report discusses the 

background, purpose, methodology used, data limitations, results, and summary 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) to 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 and prevention, including 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 

regulations, 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 to: 

• Identify the most common sources and causes of releases. 

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

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

 EPA needs to know where problems are occurring, why they are occurring, whether leak 

detection is quickly identifying the problems, and the extent of problem. In 1999, two EPA 

regional offices began gathering data from their state partners when a new release occurred. 

Two additional regions then joined in the data-gathering effort and one State Fund offered to 

provide data on new releases. EPA collected the data from the regions and the State Fund and 

merged and summarized it in this report. 

Purpose Of This Report 

 The purpose of this report is for EPA to develop quantitative information on the sources 

and causes of releases from UST systems so that future work to improve UST system 

performance is appropriately focused. In addition, the report examines the performance of 

release (leak) detection methods in detecting releases at UST facilities. When tank or piping is 
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4

the source of a problem, the report evaluates the construction details of the tank or piping and 

compares the releases from different construction types to their frequency of use. The report also 

looks at UST system age at the time of release, the extent of the release, and secondary 

containment. 

Methodology 

Data Gathering

Twenty-two states from EPA Regions 1, 3, 4, and 6 and the Missouri Petroleum Storage 

Tank Insurance Fund (MO Fund) provided data on 687 UST system problems for this report. 

EPA determined 580 of the data points contained useful information. Figure 1 shows a map 

highlighting the states that provided data for this report. The data gathered in the regions 

represents releases discovered between September 1999 and February 2003. Each region began 

gathering data at different times. The MO Fund provided data from the December 1998 deadline 

through July 2002. Table 1 shows the regional data source, dates of data gathering, state where 

the data was gathered, and the number of data points gathered. 

Figure 1 - States Participating In The Data Gathering 

ALASKA

CALIFORNIA

IDAHO

OREGON

WASHINGTON

MO NT ANA

WYOMING

UTAH

COLORADO

A RIZONA

TEXAS

KANS AS

NEBRASKA

SOUTH DA KOT A

MINNESOTA

WISCONS IN

IOWA

ILLINOIS

OHIO INDIANA

K ENTUCKY

WV

VIRGINIA

NO. CAROLINA

FLORIDA

ALABAM A

MISSOURI

A RKANSAS HAW AII

MICHIGAN

PENNSYLVANIA NJ

NEW YORK C T

MA

TE NNESSE E

CAROL INA

SO.

MD

DE

RI

- DC

- PR

- VI

- Guam

- NMI

- American

Samoa

NEW M EXICO

OKLAHOM A

NORTH DA KO TA

MS GEORGIA

LA

NH

MAINE

VT

NEV ADA

 = Participating State
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Table 1 - Information On Location, Begin-End Dates Of Data Gathering, And 

Number Of Data Points Gathered 

Data Source Begin-End Dates Of 

Data Gathering State Data Points Gathered Total Useful 

CT 11 9 

MA 141 95 

ME 91 84 

NH 8 8 

RI 1 1 

Region 1 

September 1999 - 

January 2003 

VT 11 10 

DE 10 5 

MD 3 3 

PA 54 49 

VA 9 9 

Region 3 

October 2001 - 

October 2002 

WV 13 10 

AL 12 12 

GA 16 14 

KY 18 18 

MS 11 11 

NC 7 6 

SC 50 46 

Region 4 

October 2000 - 

February 2003 

TN 12 12 

AR 59 48 

LA 6 6 

NM 6 6 

Region 6 

January 2000 - 

July 2002 

OK 79 60 

MO Fund December 1998 - 

July 2002 

MO 59 58 

Totals: 23 687 580 

The data in Table 1 does not represent all releases for these states over the time periods 

designated. The reasons for this are as follows: 

• Data gatherers were not able to get to every release to document the appropriate details. 

• Reliable or sufficient information was not available for many of the releases. 

• Not all states gathered data over the full length of the data-gathering. 

• Not all personnel in the participating states gathered information. 

EPA regions 1, 3, 4, and 6 worked with their states to develop information-gathering 

forms. States then voluntarily provided the specific information to their region when a release 

occurred in their state. Each region’s form was slightly different, but contained the same base 

information (Appendix A contains the information gathering forms for each region). The regions 

provided OUST with the data; OUST merged the data and performed the analysis. The MO 

Fund volunteered to provide EPA with information on releases that have occurred since 

December 1998. EPA added the MO Fund data to the data provided by the regions. 
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 States gathered the following data for UST systems: 

• material of construction 

• corrosion protection 

• secondary containment 

• spill protection 

• overfill prevention 

• age 

• product delivery system type 

• leak detection 

In addition, the following information was gathered for the release: 

• release discovery date 

• date of initial release 

• source of release 

• cause of release 

• method of identification 

• media affected 

• extent of release 

Some states attempted to gather some information on specific manufacturer or vendor, 

but that effort did not yield useful information. There was a comments box on the forms for 

additional pertinent information. 

Use Data

Based on the 580 useful data points, EPA developed information which compared the 

magnitude of problems to how often each component was used. For this report, EPA determined 

whether a problem with a specific type of tank, piping, cathodic protection, leak detection, or 

piping delivery method was significant by comparing how often there was a component problem 

with how often the component was used. To remove potential bias from the data, EPA did not 

count a tank data point if the tank was the source of release, nor did EPA count a piping data 

point if piping was the source of release. 

Statistical Analysis

Appendix B contains the statistical methods applied in the analysis and explains the 

meaning of the statistical values. Statistical methods included the z-Test for difference of 

proportions, Fisher’s Exact Test/Yates-Corrected Chi-Squared Test, and Kolmorgorov-Smirnov 

Test. Appendix C contains the data and statistical analysis. Significant and marginally 

significant differences in proportions are highlighted in the statistical analysis tables. Important 

results of the data and statistical analysis are incorporated into the results of this report. 
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Limitations Of The Data 

EPA identified the following limitations with the data: 

• Some problems may be easier to identify and gather data on than others. For example, a 

person can see a problem if it occurs from an above ground component of the UST system. 

However, it is more difficult to gather information on problems from buried UST 

components. 

• When a release occurred, the rate of release was unknown. Therefore, EPA does not know 

whether the threshold for leak detection equipment was met. This limitation is especially 

important when looking at whether automatic line leak detectors identified piping problems. 

• If a release occurred and was identified between leak detection checks, the leak detection 

method would not have detected the release. Under this circumstance, EPA considered leak 

detection as not detecting the release. 

• When analyzing piping construction, releases from metal piping components in a nonmetal 

piping system were attributed to the overall nonmetal piping system. For example, some 

fiberglass piping releases did not occur in the fiberglass, but rather at the metal connector 

associated with the piping system. 

• It is often difficult to determine when faulty installation is the cause of a release. Other 

causes may be indicated when faulty installation might have been a contributing factor. 

• Because 23 different organizations collected data, the quality of the data could be affected 

(and possibly vary). To address this, EPA reviewed each data point and associated 

comments to ensure the highest quality of data. If EPA found a discrepancy, the Agency 

contacted the collector for clarification. If EPA could not clarify the discrepancy, the 

Agency did not use that data point. 

Results 

Number Of Data Points

EPA received 687 data points (or records) from 22 states and the MO Fund. Of these, 

EPA determined 580 data points were useful. EPA excluded a data point if the source was 

unknown or the component that exhibited the problem did not meet the new or upgraded UST 

system requirements. Figure 2 shows the breakdown of valid and invalid data points. 
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Figure 2 - Valid And Invalid Data 

Points 

Useful Data 

Points

85%

Source 

Unknown

12%

Component 

That Leaked 

Did Not Meet 

New Or 

Upgrade 

Requirements

3%

Sources Of Releases

Figure 3 summarizes the sources of releases for the data. A total of 608 releases were 

reported from 580 records (some records contained multiple sources). Dispensers and piping 

each represent more than 30 percent of releases and are the two most frequent sources of 

releases. Releases from tanks1

 and delivery problems each represent 16 percent of releases. 

Releases from submersible turbine pump (STP) areas (including line leak detector problems) and 

“others” together account for the remaining 5 percent of the total. Other releases include those 

from vent lines, fill pipes (other than delivery problems), and vapor recovery lines. 

The number of release sources is large enough to support statistical analyses of the 

distribution of releases across sources. The proportions of releases from dispensers and piping 

are significantly greater than the proportions of releases from all other sources. The statistical 

analysis indicates that the difference between the number of piping and dispenser releases is not 

large enough to conclude that dispenser releases occur more frequently than piping releases. 

Delivery problems and tank releases are significantly more common in the data set than releases 

from STP areas and other releases. 

 

1

 Eight of 97 tank releases were contained by secondary containment and are included as part of the tank release 

sources and causes. However, contained problems were not the intended focus of this effort and these cases were 

generally not documented. For use and age analysis, these eight contained problems were removed, resulting in 89 

tank releases. 
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Figure 3 - Sources Of Releases

Dispenser

32%

Tank

16%

STP Area

4%

Others

1%

Delivery 

Problems

16%

Piping

31%

Causes Of Releases

Figure 4 shows the distribution of known causes of releases from the different sources. 

The distribution of causes is different depending on the source of release. Some release sources 

had multiple known causes. The cause was unknown for more than 20 percent of releases. Of 

known causes of releases, physical/mechanical damage2

 is the most common, representing 50 

percent of all causes. Spills/overfills (S/O)3

 are attributed to 34 percent of releases and corrosion 

to 12 percent. Delivery problems at the tank fill area always came from a spill or overfill. 

Examples of delivery problems include tank overfills, uncoupling a delivery hose containing 

product, and deliveries to monitoring wells. Faulty installations are attributed to 4 percent of 

releases, but this value is likely undercounted because one may not know whether a problem 

occurred as a result of improper installation. The differences in values for each of the known 

causes (physical/mechanical damage, spills/overfills, corrosion, and faulty installation) are 

significantly different. 

 

2

 Physical/mechanical damage includes causes such as joint failures, seal/gasket problems, ruptures, punctures, 

cracking, dimensional change, vehicle accidents, or any other problems resulting from physical or mechanical 

action. 

3

 There were 14 spills/overfills that occurred either at the dispenser or during delivery but the specific location was 

unknown. For the purposes of this analysis, EPA assumed that the distribution of spills/overfills where the location 

could not be determined was similar to the distribution of those where the specific source was known. Therefore, 

EPA attributed eight spills/overfills to delivery and six to the dispenser. 
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Figure 4 – Known Causes Of Releases By Source 

0%

20%

40%

60%

80%

100%

Piping Dispenser Tank STP Area Others Delivery

Problems

All Sources

Release Source

Percent Of Known Release Causes

Corrosion

Phys/Mech Damage

Faulty Install

Spill/Overfill

For piping, most known causes (74 percent) are due to physical/mechanical damage. 

Corrosion accounts for 16 percent of piping releases and faulty installation for 10 percent. For 

piping, physical/ mechanical damage was significantly greater than both corrosion and faulty 

installation. Corrosion was marginally greater than faulty installation. The cause of 30 percent 

of piping releases is unknown. 

As with piping, physical/mechanical damage was the most common cause, representing 

60 percent of known dispenser releases. The cause of spill/overfill represented 38 percent of 

known dispenser releases. For dispensers, physical/mechanical damage was significantly greater 

than spills/ overfills, and spills/overfills was significantly greater than both corrosion and faulty 

installation. The cause was unknown for 14 percent of dispenser releases. 

More than half of the causes of tank problems are unknown. Because of the way the data 

was gathered, some of these unknowns could be spills/overfills associated with delivery. For the 

known causes, 67 percent are associated with corrosion, 25 percent are due to physical or 

mechanical damage, and 8 percent are attributed to faulty installation. For tanks, corrosion was 

significantly greater than physical/mechanical damage and physical/mechanical damage was 

significantly greater than faulty installation. 

All of the releases from STP areas for which a cause is known are attributed to physical 

or mechanical damage. The cause was unknown for six out of seven other release sources, with 

the one known cause attributed to physical/mechanical damage. 
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11

Tank Releases

Relationship Between Tank Construction, Use, And Releases 

Figure 5 shows a comparison between tank material of construction, tank releases, and 

tank use. There were 89 tank releases to the environment reported in the data. In addition, there 

were eight tank problems that came from double walled steel tanks and were contained.4 The 

number of walls was not reported for one tank release. Steel tanks were the most common 

source of tank releases at 60 percent followed by fiberglass tanks at 32 percent. The most 

commonly used tanks in the data are fiberglass tanks, followed closely by steel tanks. 

Figure 5 - Use And Release Frequencies For Different Tank Designs 

12%

43% 45%

32%

9%

60%

0%

20%

40%

60%

80%

100%

All Steel Composite Fiberglass

Type Of Tank

Percent 

Proportion Of Tanks

Proportion Of Tank Releases

Figure 6 shows an overall comparison between single walled (SW) and double walled 

(DW) tank releases and use. Single walled steel tanks retrofit with cathodic protection (CP) were 

the most common tank source at 40 percent, with single walled fiberglass at 18 percent and 

double walled fiberglass at 14 percent. All other tank designs had releases under 10 percent. 

Single walled fiberglass was the most commonly used tank in the database at 27 percent 

followed by single walled steel tanks retrofit with CP and double walled fiberglass each at just 

under 20 percent. 

 

4

 Problems with UST systems that did not result in a release to the environment were not the intended focus of this 

effort. These cases were generally not documented. However, in the cases where they were reported, EPA included 

them as part of the data. However, since the component did what it was intended to do (contain the release), EPA 

removed these cases from the use and age to release analyses. 
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12

Figure 6 - Use And Release Frequencies For Single Walled And Double Walled Tank 

Designs 

3% 1%

5% 2%

6%

27%

18%

6%

19%

14%

2%

18%

2%

14%

5% 7%

3%

40%

8%

1%

0%

20%

40%

60%

SW Steel

(Manufacturer's

CP)

DW Steel

(Manufacturer's

CP)

SW Steel

(Retrofit CP)

DW Steel

(Retrofit CP)

SW Steel

(Lined)

SW Steel

(Lined, CP)

SW Composite

DW Composite

SW Fiberglass

DW Fiberglass

Type Of Tank

Percent

Proportion Of Tanks

Proportion Of Tank Releases

Overall, the data show that steel tanks account for over 40 percent of use and 60 percent 

of releases. This difference is statistically significant. However, to get a better picture of steel 

tank performance, a detailed breakdown of the number of walls and type of corrosion protection 

is necessary. Single walled steel tanks with manufacturer’s cathodic protection represent 14 

percent of the tank population and 5 percent of releases. The proportion of releases from single 

walled steel tanks with manufacturer’s cathodic protection is significantly lower than use of 

these tanks in the database. Conversely, single walled steel tanks retrofit with cathodic 

protection make up almost 20 percent of the tank population, but account for 40 percent of 

releases. Releases from single walled steel tanks retrofit with cathodic protection are 

significantly higher than the use of these tanks. Releases from double walled steel tanks with 

cathodic protection (both retrofit and manufacturer’s) and lined steel tanks were not significantly 

different than their use. 

Overall, fiberglass tanks are used 45 percent of the time, but account for only 32 percent 

of releases. The proportion of releases from all fiberglass tanks is significantly lower than use of 

these tanks. Releases from single walled fiberglass tanks are marginally lower than use of these 

tanks. Although double walled fiberglass tanks have a lower incidence of problems than their 

use, the statistical analysis does not show these values to be significantly different. 

Composite tanks5

 are used 12 percent of the time and experienced releases 9 percent of 

the time. Single walled and double walled composite tanks are each used 6 percent of the time 

while single walled releases for this tank type are 7 percent and double walled releases are 2 

percent. None of these differences are statistically significant.

 

5

 Composite tanks are steel tanks clad with or jacketed with a non-corrodible material. 
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Figure 7 shows a comparison between single walled and double walled tank releases and 

use. Overall, double walled tanks are used 28 percent of the time and represent 22 percent of 

tank releases. Although double walled tanks have a lower incidence of problems than its 

corresponding use, the statistical analysis does not show these values to be significantly 

different. 

Figure 7 - Use And Release Frequencies For Single Walled And Double Walled Tanks 

28%

72%

78%

22%

0%

20%

40%

60%

80%

100%

Single Walled Tanks Double Walled Tanks

Type Of Tank

Percent

Proportion Of Tanks

Proportion Of Tank Releases

Causes Of Tank Releases 

The proportion of tank releases due to different causes varies significantly with the 

material of construction. For steel tanks, nearly 80 percent of all known release causes are 

attributed to corrosion (Figure 8), 12 percent to physical/mechanical damage, and 10 percent to 

faulty installation. Causes are unknown for 36 percent of releases from steel tanks. The 

proportion of steel tank releases due to corrosion is significantly greater than releases due to all 

other known causes. 

For fiberglass tanks, the cause is unknown for over 70 percent of releases. All of the 

known release causes from fiberglass tanks are attributed to physical/mechanical damage. Seven 

of the eight causes for composite tank releases are unknown. The one known cause was due to 

corrosion. 

The distribution of causes of releases from single walled steel tanks is different from that 

for double walled steel tanks (Figure 9). 50 of 65 causes for steel tanks where the number of 

walls is known (77 percent) were from single walled tanks. 30 of 36 known release causes (83 

percent) from single walled steel tanks were attributed to corrosion. 21 of the 30 corrosion 

causes for steel tanks were from single walled steel tanks retrofit with cathodic protection and 6 

were from lined steel tanks. The cause of nearly 30 percent of single walled steel tank releases 

was unknown. In contrast, the cause of release was unknown for 60 percent of double walled 

steel tanks. Of known release causes from double walled steel tanks, 3 of 6 (50 percent) were 
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14

attributed to corrosion, 2 of 6 were due to improper installation, and 1 of 6 were physical or 

mechanical damage. These data suggest that the causes of releases are inherently harder to 

determine for double walled steel tanks. In addition, double walled steel tanks appear to be less 

prone to corrosion failure than single walled steel tanks (the difference is marginally significant). 

 

Figure 8 - Causes Of Releases From Steel Tanks 

Corrosion

78%

Faulty 

Installation

10% Physical/ 

Mechanical 

Damage

12%

Figure 9 - Causes Of Releases From Single And Double Walled Steel Tanks 

30

2

14

2

50

4

9

1 3

15

0

10

20

30

40

50

60

Total Steel

Tank Causes

Corrosion Physical/

Mechanical

Damage

Faulty

Installation

Unknown

Steel Tank Causes

Number Of Causes .

Single Walled

Double Walled

The pattern of releases from single and double walled fiberglass tanks is shown in Figure 

10. The total numbers of reported releases are similar for single walled fiberglass tanks and 

double walled fiberglass tanks. The causes of releases from single walled fiberglass tanks are 

also almost equally divided between physical/mechanical damage and unknown. As with double 

walled steel tanks, the causes of releases from double walled fiberglass tanks are generally 

unknown, perhaps because the origin of releases from these tanks are again harder to diagnose. 
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For composite tanks, the cause is unknown for all but one release. The number of known 

causes from composite tanks is not sufficient to support further analysis. 

Figure 10 - Causes Of Releases From Single And Double Walled Fiberglass Tanks 

0 0

9

0

7

16

12

0 1

11

0

5

10

15

20

25

Total

Fiberglass

Tank Causes

Corrosion Physical/

Mechanical

Damage

Faulty

Installation

Unknown

Fiberglass Tank Causes

Number Of Causes . 

Single Walled

Double Walled

Piping Releases

Relationship Between Piping Construction, Use, And Releases 

Figure 11 shows a comparison of piping material of construction, piping releases, and 

piping use. Fiberglass piping accounted for 64 percent of piping releases, metal piping for 27 

percent, and flexible plastic piping for 9 percent. 9 percent of all piping releases came from 

metal connectors directly associated with the piping (such as flexible connectors and swing 

joints).6

Flexible plastic piping systems are used 18 percent of the time and account for 9 percent 

of releases. Releases from flexible plastic piping are significantly lower than flexible plastic 

piping use. Although the percentage of releases was higher than use for single walled fiberglass 

and single walled metal piping, the differences were not statistically significant. There were six 

double walled metal piping systems in the data set and no releases occurred from double walled 

metal piping. This difference is marginally significant. A breakout of single walled and double 

walled flexible plastic piping is not shown in Figure 11 because nearly all flexible plastic piping 

in the database was double walled. 

Figure 12 shows a comparison between single walled and double walled piping releases 

and use. Overall, double walled pipes are used 36 percent of the time and represent 26 percent of 

piping releases. Releases from double walled piping are significantly lower than its 

corresponding usage. 

 

6

 Note that a release from a piping type that was the result of a metal connector is considered a release from and 

counted as a problem with the piping type when compared to piping type use. 
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Figure 11 - Use And Release Frequencies For Different Piping Designs 

18% 18% 17%

41%

22% 27%

9%

47%

0%

20%

40%

60%

80%

100%

SW Metal Flexible Plastic SW Fiberglass DW Fiberglass

Type Of Piping

Percent

Proportion Of Piping

Proportion Of Piping Releases

Figure 12 - Use And Release Frequencies For Single Walled And Double Walled Piping 

36%

64%

74%

26%

0%

20%

40%

60%

80%

100%

All SW All DW

Type Of Piping

Percent

Proportion Of Piping

Proportion Of Piping Releases

Piping Product Delivery And Sumps 

No significant differences were found between use and release frequencies for the 

different types of piping product delivery -- pressurized (82 percent of systems, 86 percent of 

releases); “safe” suction (7 percent of systems, 7 percent of releases); and “American” or “US” 

suction (11 percent of systems, 8 percent of releases). “Safe” suction piping delivery has the 

following characteristics: 

• there is only one check valve located immediately below the suction pump; 

• piping slopes so that it drains back to the tank; and 

• below grade piping operates at less than atmospheric pressure. 
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“American” or “US” suction is any suction system that does not meet the “safe” suction 

criteria. In addition, differences in release frequencies relative to use are not statistically 

significant for dispensers and turbines with and without sumps. 

Causes Of Piping Releases

The identified causes of piping releases differ across different materials of construction. 

Figure 13 shows a breakout of the causes of releases by piping construction. Of known causes, 

physical or mechanical damage was identified as the cause of nearly 75 percent of piping 

releases. Over 15 percent are attributed to corrosion, and 10 percent to faulty installation. The 

causes of 30 percent of piping releases are unknown. Physical/mechanical damage occurs 

significantly more frequently than any other known piping cause. 

For metal piping, physical/mechanical damage was the most common known cause (61 

percent). The other major cause of release from metal piping was corrosion at 39 percent. The 

proportion of metal piping releases due to physical/mechanical damage was significantly greater 

than the proportion of metal piping releases due to corrosion. The cause was unknown for 35 

percent of releases from metal piping. Metal piping had two releases where the metal flexible 

connector was the problem – one swing joint problem and one case where the piping was 

accidentally punctured. 

Physical/mechanical damage was identified as the cause of over 70 percent of releases 

from flexible plastic pipe, and 80 percent of releases from fiberglass pipe. Physical/mechanical 

damage was significantly greater than any other cause for both fiberglass and flexible plastic 

piping. For flexible plastic pipe, the second most common cause of releases was improper 

installation at 24 percent. The cause was unknown for 6 percent of flexible piping releases. For 

fiberglass piping, the cause was unknown for over 30 percent of releases. Flexible plastic piping 

had one release where a metal flexible connector corroded; fiberglass piping had 13 releases 

where a metal connector was the problem (seven of which were due to corrosion). There were 

also nine releases included under physical/mechanical damage where humans punctured 

fiberglass piping. 

Since all of the releases from metal piping came from single walled systems and all but 

one release from flexible plastic piping came from double walled systems, there is no basis on 

which to compare the causes of releases from single and double walled piping systems 

constructed from these materials. In contrast, the numbers of releases from single walled and 

double walled fiberglass piping systems are sufficient to allow a statistical comparison of the 

causes of releases (Figure 14). For known causes, physical/mechanical damage was identified 

for 54 of 62 releases (87 percent) for single walled fiberglass piping and 11 of 16 (69 percent) for 

double walled fiberglass systems. This difference is marginally significant. In contrast, faulty 

installation was identified as a cause of release from 4 of 16 (25 percent) double walled 

fiberglass piping systems, but only 3 of 62 (5 percent) single walled systems. This difference is 

statistically significant. 
Draft Peer Review Document– Do Not Cite Or Quote 

18

Figure 13 - Causes Of Releases From Metal, Flexible Plastic, And Fiberglass Piping 

39%

6% 8%

16%

61%

71%

80%

74%

0%

24%

12% 10%

0%

20%

40%

60%

80%

100%

Metal Flexible Plastic Fiberglass All Materials

Piping Construction

Number Of Known Release Causes

Corrosion

Physical/Mechanical Damage

Faulty Installation

 

Figure 14 - Causes Of Releases From Single Walled And Double Walled 

Fiberglass Piping Systems 

5 3

22

4

84

54

16 11

1

32

0

10

20

30

40

50

60

70

80

90

100

Total

Fiberglass

Causes

Corrosion Physical/

Mechanical

Damage

Faulty

Installation

Unknown

Fiberglass Piping Causes

Number Of Causes 

Single Walled

Double Walled
Draft Peer Review Document– Do Not Cite Or Quote 

19

Media Affected And Extent Of Releases 

The autopsy data also included information on the extent of releases (whether releases 

were local or large)7 and media affected (soil or water)8. The patterns of releases from different 

sources in terms of local or large are shown in Figure 15. 

Figure 15 - Extent Of Releases For Different Sources 

83% 83%

72%

54%

65%

17%

35%

18%

46%

28%

0%

20%

40%

60%

80%

100%

Piping Dispenser Tank STP Area Delivery

Problems

Release Source

Percentage

Local Large

Among the 415 records where the extent of release was recorded, 71 percent were 

characterized as local and 29 percent as large. When releases to all media are considered, the 

majority of releases from all of the individual types of sources are local. The proportion of large 

releases ranges from under 20 percent for dispensers and STP areas to 46 percent for tanks. The 

proportion of large releases from dispensers is significantly less than the proportion of large 

releases from both tanks and piping. The proportion of large releases from tanks is significantly 

higher than large releases from dispensers, delivery problems, and STP areas. These findings are 

consistent with the presence of relatively larger volumes of product to be released when tanks are 

damaged than when releases occur from other sources. 

Figure 16 shows media affected information for the different sources. The proportion of 

releases to water from piping is significantly greater than the proportions of releases to water 

from dispensers and delivery problems. Also, the proportion of releases to water from dispensers 

is significantly smaller than the proportion of releases to water from tanks, the STP area, and 

delivery problems. 

 

7

 Extent of release data was gathered on local, large, and off-site releases. EPA combined large and off-site into one 

category, representing releases that were not considered local. EPA believes this is a reasonable grouping because 

the extent of release categories are intended to represent increasingly severe releases. 8

 For media affected, a release to soil means that only soil was impacted. Releases to water means that either 

groundwater or surface water was impacted. 
Draft Peer Review Document– Do Not Cite Or Quote 

20

Figure 16 – Media Affected For Different Sources 

77%

50%

61%

48% 45% 50% 55%

23%

52%

39%

0%

20%

40%

60%

80%

100%

Piping Dispenser Tank STP Area Delivery

Problems

Release Source

Percentage

Soil Water

Releases to different media tend to differ in terms of extent (Figures 17 and 18). For all 

sources combined, nearly 90 percent of releases to soil are local. For the individual types of 

sources, the proportion of local releases to soil ranges from 64 percent for tanks to 100 percent 

for the STP area. Similarly, 90 percent of releases to soil from piping are local, as are 93 percent 

of releases from dispensers and 89 percent of releases from delivery problems. In contrast, just 

over 40 percent of releases affecting water are local, and the proportion of local releases to water 

ranges from 34 percent for tanks to 63 percent for releases associated with STP areas. The 

proportion of local releases to soil from all sources is significantly higher than the proportion of 

local releases to water. Likewise, the proportion of large releases to soil from all sources is 

significantly lower than the proportion of large releases to water. These patterns confirm that 

large releases tend to reach water while local releases tend to only affect soil. 

Figure 17 - Extent Of Releases To Soil Only For Different Sources 

93% 100% 97%

0%

64%

90%

10% 7%

36%

3%

0%

50%

100%

Piping Dispenser Tank STP Area Delivery

Problems

Release Source

Percentage

Local Large
Draft Peer Review Document– Do Not Cite Or Quote 

21

Figure 18 - Extent Of Releases To Water For Different Sources 

63%

42% 43%

34% 40% 38%

60% 58%

66%

57%

0%

20%

40%

60%

80%

100%

Piping Dispenser Tank STP Area Delivery

Problems

Release Source

Percentage

Local Large

EPA also examined media affected and extent of release for suction and pressurized 

piping delivery methods. For this analysis, only the release sources of piping, STP area, and 

dispenser (excluding spills/overfills) were evaluated. There were no significant differences in 

proportions between suction and pressurized delivery for local releases, large releases, releases to 

soil, or releases to water. 

Method Of Release Identification 

Figure 19 shows how releases were identified. Nearly one-half of releases were 

identified through someone seeing or smelling the problem (visual or olfactory). Other 

identification methods included leak detection at 17 percent and closure at 13 percent. Releases 

were occasionally detected during property transfer or through the observation of water in the 

tank. The method of release identification was identified as other for 15 percent of releases.9

 

Other methods of identification included tightness testing performed for reasons other than leak 

detection and soil/groundwater sampling results. The frequency of visual or olfactory release 

identification was significantly greater than identification by any other means. The frequency of 

release identification by leak detection and closure were likewise significantly greater than the 

other less frequent methods of identification. The method of discovery was unknown for 4 

percent of releases. 

 

9

 The data contain 18 cases where tightness testing was identified as the method of release discovery, but tightness 

testing was not chosen as a method of leak detection for the UST system. EPA assumed that these tightness tests 

were performed in reaction to a potential problem rather than as part of regularly scheduled leak detection testing. 

These cases were included in the other category of method of release identification. 
Draft Peer Review Document– Do Not Cite Or Quote 

22

Figure 19 - Methods Of Release Identification 

17%

13%

2% 2%

47%

15%

4%

0%

10%

20%

30%

40%

50%

60%

Visual/

Olfactory

Leak

Detection

Other Closure Property

Transfer

Water In

Tank

Unknown

Method Of Discovery

Percentage

Success rates for leak detection technologies are summarized in Figure 20. Leak 

detection identified releases only 17 percent of the time in the data. However, there are some 

releases in the data where the leak detection method was not capable of detecting the release. 

For example, many leak detection methods will not detect a delivery problem or dispenser 

problem. EPA eliminated those releases where leak detection was not capable of detecting the 

release to obtain the success rates in Figure 20. Releases were eliminated based on the following 

criteria: 

• Line tightness testing (LTT) and line leak detectors (LLD) would not detect a tank problem. 

• Manual tank gauging (MTG), tank tightness testing (TTT), and automatic tank gauging 

(ATG) would not detect a piping problem or a STP area problem. 

• MTG, ATG, interstitial monitoring, TTT, and line leak detectors (LLDs) would not detect a 

dispenser problem. 

• Only groundwater or vapor monitoring would be able to detect spills/overfills, fill pipe, vent 

line, and vapor recovery problems. 

After eliminating those releases, the overall leak detection success rate was 30 percent 

(detecting 100 of 330 release opportunities). 

Also, some releases had multiple leak detection methods that could have detected the 

release. For example: 

• If piping was the release source, both the LLD plus line tightness testing or monthly 

monitoring would have had the opportunity to detect the one release. 

• Some UST systems were using multiple forms of leak detection at the time of release. 

Therefore, there were a total of 473 opportunities to detect a release. See Appendix C for 

details about opportunities for leak detection to detect a release. 
Draft Peer Review Document– Do Not Cite Or Quote 

23

Figure 20 - Success Rates For Leak Detection Technologies 

32%

26% 24% 24%

19%

9%

34%

29%

20%

0%

10%

20%

30%

40%

Vapor

Groundwater

ATG

LTT

Interstitial

LLD

MTG

SIR

IC & TTT

Release Detection Method

Success Rate

Success rates (the number of instances where releases were detected divided by number 

of opportunities to detect them) ranged from 34 percent for vapor monitoring to 9 percent for 

inventory control and tank tightness testing (IC&TTT). The success rate for IC&TTT was 

significantly lower than the success rates for all the other leak detection methods except manual 

tank gauging (MTG) and statistical inventory reconciliation (SIR) where there was no statistical 

significance. None of the other success rates were significantly different from one another. 

The apparently poor success rate of the leak detection systems must be interpreted 

cautiously. The success rates are calculated only for those releases in the data set where leak 

detection was reportedly in use and where the leak detection method was specifically designed to 

detect releases from the specified release source. In addition, if facilities have more than one 

leak detection method in place, then it is only necessary for one method to succeed for a leak to 

be detected. Also, the rate of each release was not known, making it impossible to know whether 

the threshold value was met for the leak detection equipment. Finally, it may be that some leak 

detection methods would ultimately have detected releases that were instead identified by visual 

or olfactory methods because the problem was immediately discovered and occurred between 

leak detection checks. Since there is no way to identify or extract these cases, the analysis 

counts these data points as missed detections. 

Age Distributions Of Tank And Piping Releases Versus Age 

Distributions Of Tank And Piping Use10

Overall age patterns of use and releases from tanks and piping are somewhat different 

(Figure 21). The proportion of releases from tanks at least 10 years old (88 percent) is 

significantly greater than the proportion of tanks in use at least 10 years old (68 percent). In 

contrast, the proportion of releases from piping at least 10 years old (60 percent) is not 

significantly different than the proportion of piping in use at least 10 years old (52 percent). In 

 

10 Not all data points from tank and piping releases and use had age information. This analysis includes those data 

that had age information associated with the data point. 
Draft Peer Review Document– Do Not Cite Or Quote 

24

summary, the data indicate the likelihood of release from the tank population generally increases 

with age while age does not have a significant effect on the likelihood of releases from piping. 

Figure 21 – Age Distribution Of Tank And Piping Use And Releases 

0%

20%

40%

60%

80%

100%

<= 5 Years <= 10 Years <= 15 Years All

Age

Cumulative Proportion .

Of Usage/Releases

Tank Usage

Tank Releases

Piping Usage

Piping Releases

Understanding the relative performance of different materials of construction for tanks 

and piping over time requires a comparison of the age distributions of releases with the age 

distributions use. For example, releases from a particular tank design may predominantly be 

reported in older systems, but this does not mean the design is more reliable if this type of tank is 

no longer used and the ones remaining in service are also predominantly old. Similarly, if a 

piping design or material has been in widespread use for only a relatively short time, determining 

its performance long-term will not be possible. 

EPA compared the use and release age distributions for different tank constructions. 

Examples of this comparison for two tank designs are provided in Figure 22. 

For both single walled steel tanks and all composite tanks, there are higher proportions of 

relatively young tanks (10 years old or less) in use than there are releases from young tanks. 

Therefore, releases from these types of tanks tend to occur more frequently in tanks greater than 

10 years old (old). This is consistent with the idea that they are relatively reliable early in their 

service lives, with reliability decreasing with length of use. The age distributions of release for 

these types of tanks are both significantly different from the age distribution of tanks of the same 

construction. For all other tank constructions, the differences in use and release patterns over 

time are not significantly different. Full results of the statistical tests are in Appendix C. 
Draft Peer Review Document– Do Not Cite Or Quote 

25

Figure 22 – Examples Of Age Distribution Of Tanks And Tank Releases 

0%

20%

40%

60%

80%

100%

<= 5 Years <= 10 Years <= 15 Years All

Tank Age

Cumulative Proportion .

Of Usage/Releases

Fiberglass Usage

Fiberglass Releases

Steel Usage

Steel Releases

For piping, the age distribution of releases closely parallels the age distribution of use 

(Figure 23). This holds true for all fiberglass, all metal, and all flexible plastic piping combined. 

The age distributions of use and releases are not significantly different from one another, 

although the distributions of age and releases are very different across materials of construction. 

These data indicate that, for these three construction materials, releases are not concentrated 

either in young piping or in old piping. Rather, the release rates are relatively constant across 

piping, independent of age. 

The proportions of releases from tanks and piping less than 10 years old (young) were 

compared for trends across material of construction using the Kolmorgorov-Smirnov statistical 

test (see appendix C). For tanks, the proportion of releases from young tanks is always lower 

than corresponding tank use. For steel tanks with corrosion protection, the proportion of young 

releases (5 percent) is significantly lower than the proportion of young tanks (19 percent). This 

difference is mainly associated with single walled tanks, which account for the majority of the 

steel tanks and releases in the data set. The proportion of young releases from composite tanks 

(25 percent) is also significantly lower than the proportion of young composite tanks (74 

percent). For piping, the proportions of releases in the first 10 years of service life are very close 

to the proportions of use in the same age category for the various piping constructions. These 

data do not indicate that any type of piping has disproportionate rates of early failure; the 

differences in early release rates across the various types of piping are not statistically 

significant. 

EPA also performed statistical analyses on releases and use for different tank and piping 

constructions less than 10 years old and older than 10 years using the z-test and Fisher’s Exact 

test. Figure 24 shows a comparison of use versus releases for the various young piping 

constructions. The analysis reaffirmed that releases from young flexible plastic piping were 
Draft Peer Review Document– Do Not Cite Or Quote 

26

significantly lower than corresponding use and releases from young fiberglass piping were 

significantly greater than corresponding use. Releases from double walled piping less than 10 

years old were significantly lower than corresponding use. The converse is true for releases from 

young single walled piping. There were no significant differences between releases and use for 

piping constructions greater than 10 years old. 

For tanks, the only notable difference for tank construction based on young or old age 

was for fiberglass tanks older than 10 years. Releases from old fiberglass tanks were lower than 

corresponding use. However, the more robust (Fisher’s Exact) statistical test did not indicate the 

difference was significant. 

Figure 23 - Age Distribution Of Piping And Piping Releases 

0%

20%

40%

60%

80%

100%

<= 5 Years <= 10 Years <= 15 Years All

Pipe Age

Cumulative Proportion .

Of Usage/Releases

Fiberglass Usage

Fiberglass Releases

Steel Usage

Steel Releases

Flex. Plastic Usage

Flex. Plastic Releases

Figure 24 – Comparison Of Piping Use And Releases by Material Of Construction For 

Piping Less Than Or Equal To Ten Years Old 

5%

37%

54%

43%

6%

100%

49%

44% 48%

93%

0%

20%

40%

60%

80%

100%

Metal (CP) Flexible Plastic Fiberglass (SW) Fiberglass (DW) All Fiberglass

Type Of Pipe

Percentage

Piping Usage <= 10 Years Old

Piping Releases <= 10 Years Old
Draft Peer Review Document– Do Not Cite Or Quote 

27

Summary And Conclusions 

Releases And Causes 

EPA gathered information on 687 releases from more than 20 states. Of these, EPA 

determined 580 data points contained useful release information from a new or upgraded UST 

system. The number of useful data points is large enough to support statistical analyses using 

simple, robust statistical methods. EPA analyzed the data for sources, causes, methods of 

identification, media affected, release extent, and age. 

Piping and dispensers are the two most common sources of releases. Fuel deliveries and 

tanks are the next most common sources of releases, with releases from the STP area as the other 

notable source. Piping has been a leading source of releases since before the federal regulations 

went into effect, and given the various junctions and connections in a piping run, it is not 

surprising that piping remains a common source of releases. Dispensers have emerged as a 

leading concern, and neither EPA nor most states have release prevention or detection programs 

to address releases at the dispenser. 

Physical and mechanical damage is the most common overall cause of releases, followed 

by spills and overfills. Corrosion accounts for only 12 percent of releases with a known cause, 

though certain components are still particularly susceptible to corrosion (see tanks and piping 

sections below for further discussion of corrosion). Improper installation is only identified as a 

cause in a small number of cases. However, due to the difficulty of determining problems at the 

time of installation as the cause of a problem, EPA believes the contribution of improper 

installation to releases is under-represented. While physical and mechanical damage emerged as 

the leading cause of releases, EPA was often unable to identify the true cause of physical or 

mechanical damage, be it faulty material, improper operation and maintenance, improper 

installation, natural causes, or some combination of these. Generally only sudden events, such as 

a puncture of a tank or piping, had obvious root causes. Despite specific regulatory requirements 

to prevent spills and overfills during delivery, these activities still occur far too often. 

 

Looking more closely at causes from specific release sources, physical and mechanical 

damage are the leading causes of releases for dispensers and the STP area, with a significant 

secondary cause for dispensers being spills/overfills. Delivery problems in this study are always 

due to spill or overfill. Causes were unknown for more than 20 percent of all releases. Causes 

of tank and piping releases are discussed in more detail below. 

Tanks 

Tanks are a significantly less frequent source of releases than dispensers or piping, but 

still represent a significant proportion of releases. Fiberglass tanks are the most commonly used 

type of tank, followed by steel, and then composite tanks. The various types of tank 

constructions performed differently from each other. Steel tanks with cathodic protection were 

significantly more likely to leak relative to their use, while fiberglass tanks were significantly 

less likely to have releases. Composite tanks were neither more nor less likely to have a release 

given their frequency of use. Looking more closely at steel tanks with cathodic protection, there 
Draft Peer Review Document– Do Not Cite Or Quote 

28

was a distinct difference between single walled tanks that were retrofit with cathodic protection 

and single walled tanks installed with manufacturer’s cathodic protection. Single walled retrofit 

steel tanks are significantly more likely to release given their frequency of use, whereas single 

walled steel tanks with manufacturer’s cathodic protection are significantly less likely to have a 

release. These results indicate that newly installed tanks of all types tend to perform fairly well, 

but raise concerns about either the quality or durability of single walled steel tanks retrofit with 

cathodic protection. 

Double walled tanks were less likely to have a release, relative to their use, than single 

walled tanks. However, partially due to the relatively low number of data points, that difference 

was not statistically significant. This analysis was largely influenced by single walled retrofit 

steel tanks, which performed significantly worse than expected given their frequency of use. 

Single walled fiberglass tanks actually performed marginally better than expected given their 

frequency of use, while the improvement with double walled fiberglass tanks was not statistically 

significant. For composite tanks, the improvement in double walled versus single walled tanks 

was not statistically significant. 

While the cause of releases for tanks was often unknown, corrosion is the most 

commonly identified known cause. This result is driven largely by the large number of 

corrosion-related releases of retrofit steel tanks, as well as lined steel tanks. Physical and 

mechanical damage accounted for all known causes of releases for fiberglass tanks. These 

results highlight the need for proper operation and maintenance of corrosion protection for steel 

tanks retrofit with cathodic protection and the need for care when working around UST systems. 

For tanks that met the new tank standards when installed, proper diligence when working in and 

around the tank field is critical, given the number of physical and mechanical problems due to 

human error, such as puncturing tanks. The large number of unknown causes highlights the 

challenge of determining the cause of a release for buried tanks (except for sudden events such as 

punctures). 

For all tanks taken together, the release rate increases as tanks get older. This holds true 

for all tank types, but the relationship was only statistically significant for composite tanks and 

single walled steel tanks. These results are not surprising and support a common hypothesis that 

tank reliability decreases with tank age. 

Piping 

Piping remains a leading source of releases, despite concentrated efforts to reduce piping 

releases. Fiberglass piping is the most common piping material used and also accounts for the 

majority of piping releases. In comparing materials of construction, flexible plastic piping was 

found to leak less frequently than expected given its use, while the release rates for steel and 

fiberglass piping were not significantly different from their use. For all piping types, these 

results relate to actual releases to the environment, not incidents corrected before releases occur. 

Double walled piping significantly outperformed single walled piping. Double walled 

piping performed better for both steel and flexible plastic piping relative to their use. 
Draft Peer Review Document– Do Not Cite Or Quote 

29

Physical/mechanical damage was the most common cause of piping releases for each 

type of piping. Corrosion was the second most common cause for piping and was attributed to 

flexible plastic and fiberglass piping as well, due to corrosion of metal connectors used with 

those types of piping. As indicated above, faulty installation is likely undercounted as a cause of 

release because it can be difficult to identify faulty installation as the cause of releases that occur 

well after a system is installed, even though mistakes at installation can lead to releases well into 

the future. 

No significant differences were found between use and the release frequencies for the 

different types of piping delivery systems (suction and pressure). 

EPA found no distinguishable reliability differences with respect to piping age – both 

young and old piping are experiencing problems. With regard to specific materials of 

construction, flexible plastic piping has not been used long enough to compare its short-term and 

long-term reliability. Conversely, almost all steel piping is more than 10 years old, so 

comparisons between short-term and long-term reliability of steel piping were not possible. 

Fiberglass piping, which has been used for a long time and is still commonly installed today, 

shows no distinguishable pattern in terms of changes in reliability over time. The other notable 

age-related finding is that releases from young double walled piping systems were significantly 

less than corresponding use. 

Method Of Discovery And Leak Detection 

Nearly half of all releases were discovered by someone either seeing or smelling the 

problem, despite comprehensive leak detection requirements in place since 1993. Leak detection 

detected only 17 percent of all releases reported. Part of the reason for this low rate of leak 

detection success is because leak detection systems are generally designed to detect releases 

from tanks and piping, not from delivery spills/overfills or problems at the dispenser. Since 

dispenser and delivery problems were such common sources of releases, leak detection 

equipment often did not have an opportunity to detect the release. To better gauge the success of 

the actual leak detection system, EPA narrowed the analysis to releases the leak detection 

systems should have been able to detect. In this subset of releases, leak detection systems 

detected 30 percent of releases. While this result must still be interpreted cautiously, leak 

detection is clearly not detecting releases at the rate envisioned at the time EPA developed the 

regulations. 

In comparing the relative performance of various types of leak detection equipment, the 

only significant result was that inventory control combined with tank tightness tests performed 

worse than most other leak detection equipment. This tends to support EPA’s decision to phase 

out the use of inventory control and tank tightness testing. 

Some leaks are still discovered at the time of closure and/or property transfer. This 

suggests there still may be existing releases not yet identified. 
Draft Peer Review Document– Do Not Cite Or Quote 

30

Extent Of Releases And Media Affected 

Only about 30 percent of all releases were characterized as large. However, this varied 

significantly by source. Nearly half of tank releases were characterized as large, whereas less 

than 20 percent of releases from dispensers or STP areas were characterized as large. Given the 

volume of product contained in tanks, it is not surprising that releases from tanks tend to be 

larger than most other releases. 

More than 40 percent of all releases affected either groundwater or surface water 

(collectively considered “water”). Releases from piping affected water more than half the time, 

which was significantly more than releases from dispensers or delivery problems. Dispenser 

releases affected water less than 25 percent of the time, significantly less than all other sources. 

For pressurized and suction piping delivery systems, there were no significant differences 

in proportions between the two delivery systems for local releases, large releases, releases to soil, 

or releases to water. 

Larger releases and releases to water are generally more problematic from an 

environmental perspective. Using these two indicators, individual tank and piping releases 

appear to be most significant while releases from dispensers appear to be less significant. The 

large number of piping releases coupled with the relatively large size and impact of piping 

releases indicates that piping is the most significant source of releases from UST systems. 
Draft Peer Review Document – Do Not Cite Or Quote 

Appendix A – Regional Forms 

 Appendix A contains instructions and forms used by EPA Regions 1, 3, 4, and 6 

to capture and report information about releases from underground storage tank systems. 

***DO NOT USE IF BARESTEEL*** Region 1

LUST Site Release Report

Agency Information

___________________________________ ___________________________________ __________________________

 (Inspector) (Agency) (Date)

Site Information

__________________ _______________________________________________________________________________________

(Facility ID #) (Site Name/Street Address) 

 _____________________________________________ Facility type: “Retail Gasoline Outlet

(City / State / County / Zip Code) “Other _______________

__________________________________________________________________________________________________________________

System Information

TANK

Material: “Galv. Steel “Fiberglass “Composite “Unprotected Baresteel “Other ____________________

C.P./lining (/ all that apply): “Lined “Sacrificial Anode “Retrofit C.P/Impressed Current.

Walls: “Single wall “Double wall (same material) “Double wall (jacketed) “Unknown

Product: “Gasoline “Diesel “Used oil “Haz. Substance “Heating oil “Other

Age: “0-5yr “6-10yr “11-15yr “>15yr “Unknown

Upgrade: “Spill bucket “Overfill protection “None “Unknown

------------------------------------------------------------------------------------------------------------------------

PIPING

Material: “Galv. steel “C.P. “Rigid fiberglass “Flexible plastic “Other____________

Walls: “Single wall “Double wall (rigid) “Double wall (trench liner)

Age: “0-5yr “6-10yr “11-15yr “>15yr “Unknown

Containment:“Dispenser sump “Turbine sump “None “Unknown

Pump type: “Pressure “Exempt European suction “Conventional suction “None “Other______________

__________________________________________________________________________________________________________________

Leak Detection Check all method(s) used at time of release identification

“Manual Inventory Reconciliation “Manual Tank Gauging

“SIR (Brand/Model________________________) “Dispenser sump LD

“TTT (Brand/Model________________________) “LTT test (Brand/Model_____________________)

“ATG (Brand/Model________________________) “Interstitial (Brand/Model____________________)

“GW (Brand/Model_________________________) “Vapor (Brand/Model_____________ __________)

“Mechanical LLD (Brand/Model_______________) “Electronic LLD (Brand/Model _______________)

“No leak detection was used “Unknown 

Last TTT? “Pass “Fail “Inconclusive Date:_____ ______________ 

Last LTT? “Pass “Fail “Inconclusive Date:_____________________

Last SIR? “Pass “Fail “Inconclusive Date:_____________________

Release Information
Date release discovered___________________ Estimated date of initial release ___________________

Source of release: “Tank “Piping “Dispenser “Spill/Overfill “Unknown

Cause: “Physical damage “Corrosion “Mechanical failure “Spill/Overfill

“Faulty installation “Unknown “Other____________________________

How identified? “LD method(s) specify _____________________ “Closure/Removal “Property transfer

“Visual/olfactory “Water in tank “Unknown “Other________________

Estimated age of spill/release: “Recent (< 1yr) “Old (>1yr) “Unknown

Estimated extent: “Localized tank “Localized piping “Localized Dispenser

“Large (beyond excavated area) “Off-site

Medium affected “Soil “Groundwater “Surface water

__________________________________________________________________________________________________________________

Additional Information:

_____________________________________________________________________________________________

_____________________________________________________________________________________________

_____________________________________________________________________________________________

_________________________________________________________________________________________

_____________________________________________________________________________________________

_____________________________________________________________________________________________

_____________________________________________________________________________________________

_________________________________________________________________________________________

_____________________________________________________________________________________________

_____________________________________________________________________________________________

_____________________________________________________________________________________________

_________________________________________________________________________________________

_____________________________________________________________________________________________

_____________________________________________________________________________________________

_____________________________________________________________________________________________

_________________________________________________________________________________________

Key to abbreviations

C.P. Cathodic Protection TTT Tank Tightness Test (precision test)

LD Leak Detection LTT Line Tightness Test

Galv. Galvanized ATG Automatic Tank Gauging System

SIR Statistical Inventory Reconciliation Vapor Vapor Monitoring System

GW Groundwater Monitoring System LLD Line Leak Detector
***DO NOT USE IF BARESTEEL*** Region 3 Version 1.01

LUST Site Release Report

Agency Information

___________________________________ ___________________________________ __________________________

 (Inspector) (Agency) (Date)

Site Information

__________________ _______________________________________________________________________________________

(Facility ID #) (Site Name/Street Address) 

 _____________________________________________ Facility type: “Retail Gasoline Outlet

(City / State / County / Zip Code) “Other _______________

__________________________________________________________________________________________________________________

System Information

TANK

Material: “ Steel “Fiberglass “Composite “Other _____________________________________

C.P./lining (/ all that apply): “Lined “Sacrificial Anode “Retrofit C.P/Impressed Current.

Walls: “Single wall “Double wall (same material) “Double wall (jacketed) “Unknown

Product: “Gasoline “Diesel “Used oil “Haz. Substance “Heating oil “Other

Age: “0-5yr “6-10yr “11-15yr “>15yr “Unknown

Upgrade: “Spill bucket “Overfill protection “None “Unknown

------------------------------------------------------------------------------------------------------------------------

PIPING (Note: if nonmetal piping has metal connectors, and these are source of release, please provide details in the comments field)

Material: “Steel “C.P. “Rigid fiberglass “Flexible plastic “Other____________

Walls: “Single wall “Double wall (rigid) “Double wall (trench liner)

Age: “0-5yr “6-10yr “11-15yr “>15yr “Unknown

Containment:“Dispenser sump “Turbine sump “None “Unknown

Pump type: “Pressure “Exempt European suction “Conventional suction “None “Other______________

__________________________________________________________________________________________________________________

Leak Detection Check all method(s) used at time of release identification

“Manual Inventory Reconciliation “Manual Tank Gauging

“SIR (Brand/Model________________________) “Dispenser sump LD

“TTT (Brand/Model________________________) “LTT test (Brand/Model_____________________)

“ATG (Brand/Model________________________) “Interstitial (Brand/Model____________________)

“GW (Brand/Model_________________________) “Vapor (Brand/Model_____________ __________)

“Mechanical LLD (Brand/Model_______________) “Electronic LLD (Brand/Model _______________)

“No leak detection was used “Unknown 

Last TTT? “Pass “Fail “Inconclusive Date:_____ ______________ 

Last LTT? “Pass “Fail “Inconclusive Date:_____________________

Last SIR? “Pass “Fail “Inconclusive Date:_____________________

Release Information

Date release discovered___________________ Estimated date of initial release ___________________

Source of release:

“Tank “Piping “Dispenser “Other ___________________________

Cause: “Physical damage “Corrosion “Mechanical failure “Spill/Overfill

“Faulty installation “Unknown “Other____________________________

How identified? “LD method(s) specify _____________________ “Closure/Removal “Property transfer

“Visual/olfactory “Water in tank “Unknown “Other________________

Estimated extent: “Localized tank “Localized piping “Localized Dispenser

“Large (beyond excavated area) “Off-site

Medium affected “Soil “Groundwater “Surface water

__________________________________________________________________________________________________________________

Additional Information:

_____________________________________________________________________________________________

_____________________________________________________________________________________________

_____________________________________________________________________________________________

_________________________________________________________________________________________

_____________________________________________________________________________________________

_____________________________________________________________________________________________

_____________________________________________________________________________________________

_________________________________________________________________________________________

_____________________________________________________________________________________________

_____________________________________________________________________________________________

_____________________________________________________________________________________________

_________________________________________________________________________________________

_____________________________________________________________________________________________

_____________________________________________________________________________________________

_____________________________________________________________________________________________

_________________________________________________________________________________________

Key to abbreviations

C.P. Cathodic Protection TTT Tank Tightness Test (precision test)

LD Leak Detection LTT Line Tightness Test

ATG Automatic Tank Gauging System Vapor Vapor Monitoring System

SIR Statistical Inventory Reconciliation LLD Line Leak Detector

GW Groundwater Monitoring System
Instructions

Agency and Site Information -Fill out details as appropriate.

System Information - Fill out details regarding the Tank, Piping and Leak Detection at the time of the release. 

Check all appropriate boxes. If the tank or piping are constructed of a metal other than steel,

please indicate that information in the “Other” box.

NOTE: It is most important to have specific details on the component that leaked. For example, if the piping was the

source of the release and the cause was corrosion of a metal connector, detailed information about the piping,

including information in the comments about details of the cause is needed. 

Release Information - Fill out details regarding the release. The information most important in this section are:

Source: This is WHERE the release occurred from. Useful forms need to indicate the source of release. If the

source of release is unknown, the form will not be useful. Some samples follow:

Use Other = Delivery vehicle if a release occurred when the delivery driver unhooked the

delivery hose from the fill pipe.

Use Source = Tank if there is an overfill at the tank.

Use Other = Fill Pipe if there is a release due to some failure of the fill pipe.

Use Source = Dispenser if a customer spills gas from the nozzle at the dispenser.

Use Other = Turbine is the turbine pump is the source of the release.

Use Source = Pipe AND Other = Metal Flex Connector OR Metal Swing Joint if source is

metal connector on nonmetal pipe.

Use Other = LLD if the line leak detector was source.

Cause: This is WHY the release occurred. Cause is very good to know, but does not make the data unusable

if this information is unknown. If you do not think one of the categories fits under cause, choose

Other and indicate specific details. Some samples follow:

Use Cause = Spill/Overfill if there is a spill or overfill at the dispenser or at the tank.

Use Cause = Corrosion if the release was a result of corrosion.

Use Other = loose component if a loose component was the cause.

Use Other = Filter if the filter is the cause (you can be more specific too, such as loose filter or

broken filter...)

How Identified: The key information here is whether or not the release detection identified the release (and if so,

specifically what leak detection. For example, the data should indicate specifically what method

identified the release – such as the LLD or a tightness test, or the ATG) 

Estimated Extent and medium affected are good to know, but not crucial.

Additional Information - Fill out ANY additional information you have about the leak. For example, if fiberglass

piping was the source, and the cause was corrosion of a flexible connector, the additional

information area is the only place where we can identify the specific problem.

U.S. Environmental Protection Agency

Region 4

UST SYSTEM INTEGRITY FAILURE CHECKLIST

Refer to UST SYSTEM INTEGRITY FAILURE CHECKLIST INSTRUCTIONS

Report only on systems in compliance with post 1988 construction requirements (i.e. corrosion, spill and overfill

protection are in place). DO NOT report on nonupgraded systems. Please see INSTRUCTIONS.

 Check here if filling out more than one form for this site. Number of forms for this site _____

 I. State Information

 State_______________________________________________ Date form completed ______________________

 Person completing form _____________________________________ Phone number ______________________

 Employer ________________________________________________

II. Site Information

 Site name ___________________________________________________ Facility ID ______________________

 Address __________________________________________________ ______________________ _________

 (Street address) (City) (Zip Code)

 Facility type:  retail gasoline outlet  other ___________________________________________________

 Status of tank:  active  temporary closure  permanent closure  abandoned, unclosed

 In compliance with:  release detection  operational requirements

III. Description of system integrity failure

 Describe the tank failure: ____________________________________________________________________

 ____________________________________________________________________

 Describe the piping failure: ____________________________________________________________________

 ____________________________________________________________________

 IV. System Information

 Tank Material:  steel, dielectric coated  fiberglass reinforced plastic  fiberglass clad steel

 Design:  single wall  double wall  single wall in lined pit or vault  lined steel

 Corrosion protection:  galvanic, manufacturer installed  impressed current retrofit (date __________) 

 Lining:  lined (date _______________) date of last inspection _______________________________ 

 Date installed: ____________ years Capacity: ____________ gallons

 Product:  gasoline  diesel  used oil  kerosene  aircraft fuel  heating oil  hazardous

substance

 Piping Material:  corrosion protected steel  rigid fiberglass  flexible plastic  other

__________________

 Design:  single wall  double wall  single wall in lined trench

 Containment:  dispenser sump  turbine sump  none  unknown

 Pump type:  pressure  suction, check valve at dispenser  suction, check valve at tank
 V. Release Detection

 Method of tank release detection used:  inventory control/tank tightness testing  automatic tank gauging

  statistical inventory control  groundwater wells  vapor monitor wells

  interstitial monitoring  manual tank gauging

 Last tank tightness test:  pass  fail  not applicable

 Method of piping release detection used:  automatic line leak detector and annual line tightness test

(pressurized)

  automatic line leak detector and monthly monitoring (pressurized)

  line tightness test every three years (suction)

 monthly monitoring (suction)

 not required (suction)

 Last line tightness test:  pass  fail  not applicable

VI. Release information, if a release occurred

 Date release discovered: ________________________ Estimated date of release: _________________________

 Source: (Check all that apply)  tank  piping  dispenser  unknown  other ________________________

 Cause: (Check all that apply)  physical damage  corrosion  mechanical failure  spill  overfill  other

 Who identified release? (Check all that apply)

  owner/operator  third party  service contractor  state inspector  other ___________________

 How was release identified? (Check all that apply)

  release detection  during closure/removal  property transfer  visual/smell  water in tank

  ground water contamination  surface water contamination  unknown  other _________________

 Estimated extent (Check all that apply)

  tank pit  piping trench  dispenser area  beyond construction excavations  off site

 Media affected (Check all that apply)

  soil  ground water  surface water

 Estimated quantity of product released into environment: ___________________ gallons

 Estimated cleanup cost, if cleanup is required: $_________________________

 Additional comments/information:

UST SYSTEM INTEGRITY FAILURE CHECKLIST INSTRUCTIONS

Background

The purpose of this form is to document the effectiveness of federal and state UST regulations now that all interim deadlines for release detection and

upgrading have past. By analyzing facilities that are in compliance with construction and operation and maintenance requirements yet have had an integrity

failure that might or has caused a release it can be determined where program improvements and priorities should be. Submit this form only when the means

of integrity failure at such a site is known. Do not submit information on sites that have not met the 1998 upgrade requirements (i.e. bare steel tanks,

whether still in operation or temporary closure). Begin with sites for which information is available on or after October 1, 2000.

Multiple forms

Whenever a site contains multiple failed systems complete one form for each different failed system and check the box indicating that more than one form

has been completed for the site. Indicate the total number of forms to be filed for the site. Do not complete a form for other unfailed UST systems at the site.

 I. State Information

Indicate the name, employer, and phone number of the person completing the form or, alternatively, the best person to contact should additional information

or clarification be needed.

 II. Site Information

Provide information about the site as requested. If the state assigns each site a unique identifying number please include that number in the Facility ID

space, otherwise leave it blank. If checking box for abandoned, unclosed please insure the site is otherwise compliant with 1988 construction standards or

upgraded. Operational requirements includes annual line leak detector calibration, periodic corrosion protection system inspection, periodic lining

inspection, etc.

III. Description of system integrity failure

Describe the tank or the piping failure in as much detail as is available, e.g., numerous tank perforations to lower third of tank due to corrosion, tank cracked

due to improper installation, piping leak at dispenser flex joint due to faulty seal.

 IV. System Information

Tank material: Note the material of the failed tank’s construction. 

Tank design: Note design type that best fits the failed tank.

Tank corrosion protection: If applicable, note whether the failed tank has manufacturer installed galvanic or a retrofit impressed current system. Some

systems may be lined and use a corrosion protection system. Check all that apply.

Tank lining: If applicable, note whether the failed tank has met upgrade requirements by internal lining. If known, indicate when the tank was lined and, if

more than ten years have elapsed, when its owner/operator last inspected it. Some systems may be lined and use a corrosion protection system. Check all

that apply.

Date installed: Note the failed tank’s date of installation.

Tank product: Note all known contents of the failed tank.

Piping material: Note the material of failed piping in contact with liquid product (not vent lines).

Piping design: Note design type that best fits failed piping.

Piping containment: This section refers to the presence of containment sumps under the dispenser or around the pump turbine. Check all those that apply.

Piping, pump type: Note the type of pumping system.

 _V. Release Detection

Note the method of release detection practiced at the failed tank or piping. If available and applicable, indicate the results of the latest tank and/or line

tightness test.

 VI. Release Information, if a release occurred

If a reportable, confirmed release occurred as a result of the system integrity failure provide an estimated (or actual, if known) date the release began and

when it was discovered.

Source: Identify the portion of the system that leaked or indicate that the release came from a spill or overfill. A tank release is any failure of a tank surface

that is not directly a part of any piping connection. A piping release is a failure of any portion of the piping (except as defined as a dispenser leak) up to and

including connections to the tank. Dispenser releases are those from those portions of the piping above the shear valve. Check all that apply.

Cause: Indicate the reason for the release. Check all that apply.

Who identified release?: Indicate all those that identified the release.

How was release identified?: Indicate any and all means that were used to identify the release.

Estimated extent: Estimate the extent of the release based on the best current analysis.

Media affected: Indicate which environmental media have been affected based on the best current analysis. Check all that apply.

Estimated quantity of product released into the environment: Based on the best current analysis how much product has been released?

Estimated cleanup cost: Based on the best current analysis, should a clean up be necessary, how much will it cost to cleanup the release to state standards?
Region 6 - RELEASE DETECTION CHECKLIST

If you have questions about this form or need further explanation please refer to the “Instructions” page.

 Check here if filling out more than one form for this site Number of forms for this site______

I. Agency Information Inspector ___________________________ Agency ______________ Date__________

 If not a State inspector Name _____________________________________________ Date__________

 Company _______________________________ Phone number ___________________ 

II. Site Information Site name _____________________________________ Facility ID # ________________

 Address ______________________________ _________________ ____________ 

 (Street Address) (City) (Zip Code)

 Facility Type:  Retail Gasoline Outlet  Other ____________________

 Status of tank:  Active  Temporary Closure  Permanent Closure  Abandoned

 

 In Compliance with:  1998 Equipment Requirements

  Release Detection Equipment

 Operational Requirements

 None of the above

III. System Information

 Tank Material Galv. Steel Fiberglass Composite Other __________________

 C.P./Lining Manufacturer’s C.P. Retrofit C.P. Lined

 Walls Single Wall Double Wall (same material) Double Wall (jacketed) Unknown

 Product Gasoline Diesel Used Oil Haz. Substance Heating Oil Other

 Age ______________ years

Capacity ______________ gallons

 Piping Material Galv. Steel Galv. Steel w/C.P. Rigid fiberglass Flexible Plastic Other

_______

 Walls Single wall Double Wall Trench Liner 

Age _____________ years

Containment Dispenser sump Turbine sump None Unknown

Pump Type Pressure Safe suction Conventional suction None 

Other______________

IV. Leak Detection

 (Check all that apply)

Method Tank(T)

 Pipe(P)

Vendor/Model Working

(y/n) Method Tank(T)

Pipe(P)

Vendor/Model Working

(y/n)

 Inventory Control Manual Tank Gauging

Tank Tightness Test (TTT) Dispenser sump LD

Automatic Tank Gauging

(ATG)

 Hydrostatic line test

(Line Tightness Test) 

Statistical Inventory

Reconciliation (SIR)

Electronic LLD 

Ground Water Monitoring Wells

(GWMW)

Mechanical LLD 

Vapor Monitoring No leak detection 

Interstitial Monitoring Unknown

(Continued)
Release Detection Checklist

Page 2

IV. Leak Detection - Continued

Last LLD? Pass Fail Inconclusive Date:______________ Company:_____________________

Last TTT? Pass Fail Inconclusive Date:______________ Company:_____________________

Last LTT? Pass Fail Inconclusive Date:______________ Company:_____________________ 

Last SIR? Pass Fail Inconclusive Date:______________ Company:_____________________

V. Release Information

 Release to environment Release to secondary containment

Date release discovered _____________________ Estimated date of initial release _____________________

Source Tank Piping Dispenser Spill/Overfill Unknown Other_________________

Cause Physical damage Corrosion Mechanical failure Spill/Overfill 

Other________________

Who identified? If more than one entity signaled the existence of the release, indicate (1) for primary and (2) for secondary.

 Owner/Operator 3rd Party Contractor State Federal Other________________

How identified? If more than one item signaled the existence of the release, indicate (1) for primary and (2) for secondary.

 LD method(s) specify_________________ Closure/Removal Property transfer

 Visual/Olfactory Water in tank Ground water contamination Surface water contamination

 Unknown Other_____________________ 

Estimated extent Localized tank Localized piping Localized dispenser Large (beyond excavated area) Off-site

Medium affected Soil Groundwater Surface water

 (check all that apply)

Estimated quantity of product released into environment _______________ gallons.

Estimated cleanup cost $__________________. Comments:_______________________________________________

 (Attach additional comments on a separate page)

---------------------------------------------------------------------------------------------------------------------------------------------

By my signature affixed below, I certify that I have reviewed this report for accuracy and completeness of information. 

I acknowledge that if I intentionally or knowingly make false statements, representations, or certifications in this report

I may be subject to administrative, civil, and/or criminal penalties. 

Signature: ______________________________________________ Date: ___________________________

Title: __________________________________________________
RELEASE DETECTION CHECKLIST INSTRUCTIONS

Multiple Forms:

Whenever a site contains multiple tank or piping configurations, multiple products or varied release detection methods, complete one form for

each different configuration and check the box indicating that more than one form has been completed for the site. Indicate the total number of

forms to be filed for the site.

I. Agency Information:

Some states may allow someone other than a State inspector to fill out this form. If State inspector is completing this form please fill in relevant

data. If a contractor or responsible party is completing form, please supply requested data following the statement “If not a State inspector”.

II. Site Information:

Provide information about the site as requested. If your agency assigns each site a unique identifying number, please include that number in the

Facility ID# space, otherwise leave it blank. If checking box for abandoned tanks, fill out as much information as is available. If no

information is available or is highly suspected as being unreliable, leave those sections blank.

III. System Information:

Tank Material: Note the material of tank construction. Composite means fiberglass bonded to steel.

C.P./lining: Note whether the tank(s) have manufacturer installed or retrofit cathodic protection and/or internal lining. Check all boxes that

apply (e.g., retrofit C.P. and lined)

Tank Walls: Note whether the tank(s) are single walled, double walled, or jacketed with an interstitial space.

Product: Note the last known contents of the tanks.

Tank Age: Note the age of the tanks.

Upgrade: Note whether the tanks(s) have spill and overfill devices in place at the time of inspection.

Piping Material: Note the material of the piping in contact with liquid product (not vent lines).

Piping Walls: Note whether the pipes are single walled, double walled or single walled contained in a trench liner.

Piping Age: Note the age of the piping. If the site has piping of different ages, please note that.

Containment: This section refers to the presence of containment sumps under the dispenser or around the pump turbine. Check the

appropriate box(es) if the sump(s) are present.

Pump type: Note the type of pumping system. Safe suction (European Suction) refers to suction lines with only one check valve located under

dispenser. Conventional suction is any other suction system.

IV. Leak Detection:

Note the type of leak detection equipment at the site, and indicate whether it was working (properly functioning) at the time of the inspection

(this can be indicated with a check mark or yes/no). Check ALL that apply. Also denote whether it is for Tank (T) or Piping (P).

Last LLD: Note the result, date and testing company of last Line Leak Detector test, if any.

Last TTT: Note the result, date and testing company of last Tank Tightness Test, if any.

Last LTT: Note the result, date and testing company of last Line Tightness Test, if any.

Last SIR: Note the result, date and testing company of last Statistical Inventory Reconciliation, if any.

V. Release Information:

Indicate whether there appears to have been no release at the site, a release to the environment, or a release that was contained by the secondary

containment system. Provide an estimate of the date the release began and/or the date the release was discovered, if known. If the release was

discovered during your inspection, please use the date of the inspection.

Source: Identify the portion of the system that leaked or indicate that the release came from a spill or overfill. A tank leak is any failure of a

tank surface that is not directly a part of any piping connection. A piping leak is a failure of any portion of the piping (except as

defined as a dispenser leak) up to and including connections to the tank. Dispenser leaks are releases from those portions of the piping

which, if the piping is double walled, would be on the dispenser side of the terminus of the double walled condition, or if single

walled, those portions of the piping that are above grade under a dispenser.

Cause: Indicate the reason for the release. Mark as many boxes as apply.

Who Identified: Specify who identified the presence of a release. Can indicate more than one.

How Identified: Specify how the system owner/operator became aware of the presence of a release. Can indicate more than one.

Estimated Extent: Estimate the extent of the release, based upon any and all information available to you at the time of the inspection and your

best professional judgement.

Medium Affected: Indicate which environmental compartments have been affected by the release. Check as many as apply.

Other Abbreviations (not previously clarified or spelled out on form):

C.P. Cathodic Protection Galv. Galvanized TTT Tank Tightness Test (precision test)

LD Leak Detection LLD Line Leak Detector LTT Line Tightness Test (hydrostatic line test)
Draft Peer Review Document – Do Not Cite Or Quote 

B-1

Appendix B: Statistical Methods

EPA used three statistical tests to evaluate the significance of differences in the 

numbers and proportions of releases, releases as a function of use, and the differences 

between the patterns of facility age and age of facilities with releases. The first two 

procedures, the z-test and Fisher’s Exact Test/Yates-Corrected Chi-Squared, were used to 

evaluate differences in proportions in two independent samples and to measure the 

probability that a given pattern of results in a two-way comparison of outcomes would 

occur by chance, respectively. The Kolmorgorov-Smirnov test is a non-parametric test 

that compares cumulative distributions (in this case, the cumulative age of all facilities or 

facilities with releases) and indicates the probability that the observed difference between 

the distributions could arise by chance by sampling a single population. 

B-1. The z-Test For Difference Of Proportions 

The z-test for difference of proportions tests the hypothesis that two independent 

proportions, p1 and p2, are equal. The statistic (z) is calculated as (Sheskin, p. 379): 

 

 (B-1) 

 (B-2) 

Where: n1, n2 = number of observations (e.g., facilities or records) in the 

sample 

X1, X2 = number of positive successes (e.g., number of releases) in 

the sample 

 p1, p2 = sample proportions = X1/n1, X2/n2 

P = weighted average of the sample proportions 

Using a two-sided test, the hypothesis that the proportions are equal can be 

discarded with 95 percent certainty (or p < 0.05) if (z) exceeds the critical value of 1.96. 

This value corresponds to the point on the normal (Gaussian) distribution which excludes 

5 percent of the total probability density (2.5 percent on each end). The difference in 

proportions is characterized as marginally significant if the z value is between 1.65 and 

1.96. The z value indicating a 90 percent certainty is 1.65. The two-tailed test is used in 

non-directional situations where there is no reason to believe that either p1 or p2 is greater 

or less than the other; it is more demanding (less likely to identify a difference in 

proportions as significant) than the one-tailed test. 

z p p P Pn P Pn ( )

P X X nn

=− − +−

=+ +

12 1 1 1 2

1 2 12

/ ( )/ ( )/

( )/( )
Draft Peer Review Document – Do Not Cite Or Quote 

B-2

In some cases, the proportions being compared are both drawn from the same set 

of observations (all records). In that case, the z-calculation is: 

 (B-3) 

 (B-4) 

where: N is the total number of observations (records) 

 P is the average of the sample proportions 

The underlying assumption of the z-test is that the two proportions are independent of 

one another and are the result of sets of Bernoulli trials having a constant probability of 

success. 

B-2. Fisher’s Exact Test/Yates-Corrected Chi-Squared

When the number of observations is small, Fisher’s Exact Test is considered to be 

superior for judging the difference in frequencies of outcomes.1 This non-parametric 

(distribution-independent) procedure tests the hypothesis that two sets of results 

(successes) in a number of observations (trials) are equal. Put another way, the procedure 

provides an estimate of the probability that the observed difference in proportions could 

have arisen by chance, assuming both samples came from the same population. A low 

value (generally, p<0.05) indicates that the proportions being compared are significantly 

different. 

Computation of the test statistic (p-value) is computationally intensive, involving 

exact evaluations of the probability that the specific permutations of results (n1 successes 

in X1 trials, n2 successes in X2 trials) could have arisen by chance, relative to all other 

permutations. (See Sheskin, pp. 371-376, for a complete description of the calculations.) 

In this analysis, p-values were calculated using the Fisher’s Exact Test algorithm in the 

STATISTICA® package, Version 5.0. P-values less than 0.05 were characterized as 

indications that two proportions were significantly different from one another. P-values 

between 0.05 and 0.10 were characterized as marginally different from one another. 

 

1 Sheskin (p. 368) states that most sources recommend that parametric tests (Chi-Squared, z-test) 

not be used when the total number of observations (N, or n1 + n2 as defined in equations B-1 

through B-4) is less than approximately 20. Some suggest that parametric tests are acceptable for 

N (or n1 + n2) between 20 and 40, but only if the expected values for all cells (the groups being 

compared) are at least five. 

z p p P PN ( )

PXX N

=− −

= +

12 1

1 22

/ ( )/

( )/
Draft Peer Review Document – Do Not Cite Or Quote 

B-3

Fisher’s Exact Test is also theoretically valid for large samples (hundreds of 

observations). However, because the test involves factorials, beyond a certain number of 

observations (on the order of a few hundred) the calculation exceeds the precision 

achievable with a PC. Thus for larger numbers of observations, EPA employed either 

the z-test described above, or the Yates-Corrected Chi-Squared test for the difference of 

proportions. The Yates-Corrected Chi-Squared statistic (χ

2

) is calculated as: 

 

 (B-5) 

In the equation, “i” and “j” refer to the two groups being compared and “O” and 

“E” are analogous to the numbers of successes (e.g, releases, etc.) observed in one group 

versus the other. In a large sample from a homogeneous distribution, O and E would be 

equal, so the value of χ

2

 would be zero. The calculated χ

2

 value is compared to a table of 

critical values, and again, the difference in proportions is considered significant if the pvalue is less than 0.05 and marginally significant if the p-value was between 0.05 and 

0.10. As for the Fisher’s Exact Test, the Yates-Corrected Chi-Squared values were 

calculated using STATISTICA®. Examples of STATISTICA® outputs for the Fisher’s 

Exact Test and Yates-Corrected Chi-Squared are given in Appendix C. 

B-3. Kolmorgorov-Smirnov Test

As noted above, the Kolmorgorov-Smirnov (KS) test evaluates the probability 

that two cumulative distributions (such as cumulative numbers of releases over time) 

might have arisen by chance, rather than as a result of a real difference in the 

distributions. The KS test is simple and robust, and performs reasonably well even when 

there are relatively few observations. The test statistic for the KS test is simply the 

largest vertical distance between two cumulative distributions. The calculation of the 

statistic can be illustrated using the data in Table B-1 below that compares the 

distribution of the age of single wall tanks with corrosion protection with the age 

distribution of releases from this type of tanks. 

Table B-1. Age Distribution Of Single Wall Tanks And Releases 

Age Of 

Facility

Number Cumulative Release Cumulative 

Proportion 

"M" (Difference)

 Use Releases Use Releases Use Releases 

0-5 yr 10 0 10 0 0.088 0.000 0.088 

6-10 yr 7 0 17 0 0.150 0.000 0.150 

11-15 yr 36 6 53 6 0.469 0.194 0.275

>15 60 25 113 31 1.000 1.000 0 

 

The first columns of the table show the numbers of tanks in the data set in 

specified age groups (use) and the numbers of releases from the same kinds of tanks in 

the same age groups (releases). The numbers of tanks and releases are then re-expressed 

( )

x

O E

E

ij ij

i j ij

2

2

05

=

− − 



















∑ ∑ .
Draft Peer Review Document – Do Not Cite Or Quote 

B-4

in cumulative form and are converted to cumulative probability distributions 

(proportions), starting at zero (no tanks, no releases) and ending at 1.0 (all tanks, all 

releases). 

The KS test then involves finding the maximum difference between the 

proportions of tanks and proportion of releases at any point in the age distribution. It can 

be seen in the final column of the table that the maximum difference occurs at 11-15 

years, which corresponds to about 47 percent on the use distribution, but only about 19 

percent on the release distribution. This difference (0.275) is “M,” the value that is used 

in the probability calculation. 

The p-values associated with different values of “M” for given degrees of 

freedom are tabulated in statistics texts. However, when the number of observations is 

large enough, the p-value can also be approximated using the Chi-Squared 

Approximation as follows:2 

 

 (B-6) 

 

 

Where: χ

2

 = Chi-squared value (two degrees of freedom) 

 M = Maximum vertical distance from KS test 

 n1, n2 = number of observations (tanks, releases) 

The p-value can thus be calculated using the CHIDIST function in Excel®; two 

distributions are characterized as significantly different if the resulting p-value is less 

than 0.05 and marginally different if the p-value was between 0.05 and 0.10. For the 

example data set, the chi-squared value corresponding to M = 0.275 is 7.38, and the 

corresponding p-value is 0.025. Thus, the two distributions are significantly different. 

In this analysis, the KS test was used to compare cumulative age distributions of tank and 

piping designs and tank and piping releases. The Fisher’s Exact Test described above 

was also used to determine if the proportion of use and releases for tanks and piping less 

than 10 years old, compared to all use/releases in the data set, are significantly different. 

The results of the two tests give roughly equivalent results for the data that have been 

examined. 

References:

Sheskin, David J., Handbook of Parametric and Nonparametric Statistical Procedures, 

Second Edition, Chapman and Hall, 2000. 

 

2 When (n1 and n2) are suitably large, the quantity of the right-hand side equation B-6 is known to 

have a chi-squared distribution with two degrees of freedom (Sheskin, p 326). Above n1 = n2 = 

25, the KS test critical values can be approximated by well-behaved functions that are consistent 

with the Chi-squared distribution to approximately one percent accuracy. (Sheskin, Table A-23). 

x M

n n

n n

2 2 1 2

1 2

= 4

+





 



* 
Draft Peer Review Document – Do Not Cite Or Quote 

Appendix C – Data And Statistical Analysis 

 Appendix C contains the release data and the statistical analysis of that data. 

Appendix C contains the following: 

Summary Of Sources.......................................................................... C-1 

Causes Of Release From Each Source...................................................... C-2 

Tank Use And Releases For Tanks With A Known Number Of Walls................. C-3 

Tank Release Details........................................................................... C-4 

Piping Use And Releases For Piping With A Known Number Of Walls............... C-5 

Piping Release Details......................................................................... C-6 

Extent Of Contamination By Source, Media............................................... C-7 

Media And Extent Of Contamination For Suction And Pressure Piping............... C-9 

Methods Of Release Identification.......................................................... C-10 

Leak Detection Success Rate Evaluation................................................... C-11 

Leak Detection ID.............................................................................. C-12 

Releases Versus Age Of Tanks, Piping (All Designs, Construction Materials)....... C-13 

Age Patterns Of Use, Releases, For Tank Designs, Construction Materials........... C-15 

Age Patterns Of Use, Releases For Piping, Designs, Construction Materials......... C-16 

Comparison Of Piping Types <10 Years Old and Piping Types >10 Years Old...... C-17 

 Comparison Of Tank Types <10 Years Old and Tank Types >10 Years Old......... C-18 
Summary Of Sources

Source Number of Releases

Percentage of 

Records

Piping 189 31.1%

Dispenser 195 32.1%

Tank 97 16.0%

STP Area (Including LLD) 25 4.1%

Others 7 1.2%

Delivery Problem 95 15.6%

TOTAL1 608 100.0%

1. There were 580 records in the data set; 28 records identified two sources each.

Calculation of z-statistics: z = (p1 - p2) / sqrt (P * (1-P) * (1 / n))

Source (1) Source (2) p1 p2 p1-p2 P 1-P 1/n z p value

Piping Versus Dispenser 0.31 0.32 -0.01 0.32 0.68 0.002 0.52 0.60

Tank 0.31 0.16 0.15 0.24 0.76 0.002 8.80 0.00

STP Area 0.31 0.04 0.27 0.18 0.82 0.002 17.47 0.00

Others 0.31 0.01 0.30 0.16 0.84 0.002 20.07 0.00

Delivery Problem 0.31 0.16 0.15 0.23 0.77 0.002 9.01 0.00

Dispenser Versus Tanks 0.32 0.16 0.16 0.24 0.76 0.002 9.30 0.00

STP Area 0.32 0.04 0.28 0.18 0.82 0.002 17.91 0.00

Others 0.32 0.01 0.31 0.17 0.83 0.002 20.49 0.00

Delivery Problem 0.32 0.16 0.16 0.24 0.76 0.002 9.52 0.00

Tank Versus STP Area 0.16 0.04 0.12 0.10 0.90 0.002 9.72 0.00

Others 0.16 0.01 0.15 0.09 0.91 0.002 13.05 0.00

Delivery Problem 0.16 0.16 0.00 0.16 0.84 0.002 0.22 0.82

STP Versus Others 0.04 0.01 0.03 0.03 0.97 0.002 4.56 0.00

Delivery Problem 0.04 0.16 -0.12 0.10 0.90 0.002 9.52 0.00

Other Versus Delivery Problem 0.01 0.16 -0.14 0.08 0.92 0.002 12.87 0.00

significant = 

marginally significant = 

Appendix C C-1
Causes Of Release From Each Source

Source

Total 

Releases

Total Known 

Causes Corrosion

Phys/Mech 

Damage Faulty Install S/O Unknown

Piping 189 135 21 100 14 0 57

Dispenser 195 178 4 106 1 67 28

Tank 97 51 34 13 4 0 51

STP Area (Including LLD) 25 22 0 22 0 0 3

Others 7 1 0 1 0 0 6

Delivery Problem 95 95 0 0 0 95 0

TOTAL1 608 482 59 242 19 162 145

1. There were 580 records in the data set; 28 records identified 2 sources each. Some sources had multiple causes.

Calculation of z-statistics: z = (p1 - p2) / sqrt (P * (1-P) * (1 / n))

Cause (1) Cause (2) p1 p2 p1-p2 P 1-P 1/n z p - value

Corrosion 0.502 0.122 0.380 0.31 0.69 0.00207 17.99 0.00

Spill/Overfill 0.502 0.336 0.166 0.42 0.58 0.00207 7.39 0.00

Faulty Install 0.502 0.039 0.463 0.27 0.73 0.00207 22.86 0.00

Spill/Overfill 0.122 0.336 -0.214 0.23 0.77 0.00207 11.16 0.00

Faulty Install 0.122 0.039 0.083 0.08 0.92 0.00207 6.68 0.00

Spill/Overfill Faulty Install 0.336 0.039 0.297 0.19 0.81 0.00207 16.68 0.00

Cause Comparison By Source

Cause (1) Cause (2) p1 p2 p1-p2 P 1-P 1/n z p-value

Corrosion Versus Phys/Mech 0.16 0.74 -0.59 0.45 0.55 0.007 13.67 0.00

 Faulty Install 0.16 0.10 0.05 0.13 0.87 0.007 1.79 0.07

Phys/Mech. Versus Faulty Install 0.74 0.10 0.64 0.42 0.58 0.007 14.99 0.00

Corrosion Versus Phys/Mech 0.02 0.60 -0.57 0.31 0.69 0.006 16.55 0.00

 Faulty Install 0.02 0.01 0.02 0.01 0.99 0.006 1.91 0.06

 Spill/Overfill 0.02 0.38 -0.35 0.20 0.80 0.006 11.82 0.00

Phys/Mech. Versus Faulty Install 0.60 0.01 0.59 0.30 0.70 0.006 17.16 0.00

Spill/Overfill 0.60 0.38 0.22 0.49 0.51 0.006 5.85 0.00

Faulty Install Versus Spill/Overfill 0.01 0.38 -0.37 0.19 0.81 0.006 12.58 0.00

Corrosion Versus Phys/Mech 0.67 0.25 0.41 0.46 0.54 0.020 5.90 0.00

 Faulty Install 0.67 0.08 0.59 0.37 0.63 0.020 8.69 0.00

Phys/Mech. Versus Faulty Install 0.25 0.08 0.18 0.17 0.83 0.020 3.38 0.00

Phys/Mech. Versus Corrosion 1.00 0.00 1.00 0.50 0.50 0.045 9.38 0.00

Faulty Install 1.00 0.00 1.00 0.50 0.50 0.045 9.38 0.00

Corrosion Versus Faulty Install 0.00 0.00 0.00 0.00 1.00 0.045 0.00 1.00

General Cause Comparison

Physical/Mechanical Damage

Corrosion

Piping

significant = 

marginally significant = 

Dispenser

Tank

STP

Appendix C C-2
Tank Use And Releases For Tanks With A Known Number Of Walls

Tank Type

Number In Data 

Set1

Proportion In Data 

Set

Number Of 

Releases2

Proportion Of 

Releases Tank Type

Number In 

Data Set

Proportion In 

Data Set

Number Of 

Releases

Proportion Of 

Releases

Steel SW 184 39.2% 47 53.4%

SW 64 13.6% 4 4.5% Steel DW 19 4.1% 5 5.7%

DW 16 3.4% 3 3.4% All Steel 203 43.3% 52 59.1%

Total 80 17.1% 7 8.0% All SW 339 72.3% 69 78.4%

All DW 130 27.7% 19 21.6%

SW 91 19.4% 35 39.8% All Tanks 469 100.0% 88 100.0%

DW 3 0.6% 2 2.3%

Total 94 20.0% 37 42.0%

Steel w/Lining (SW) 22 4.7% 7 8.0%

Steel w/Lining And CP (SW) 7 1.5% 1 1.1%

SW 27 5.8% 6 6.8%

DW 27 5.8% 2 2.3%

Total 54 11.5% 8 9.1%

SW 128 27.3% 16 18.2%

DW 84 17.9% 12 13.6%

Total 212 45.2% 28 31.8%

Total All Tanks 469 88

1. Excludes records with tank release

2. Excludes 8 cases where problem was contained by secondary containment and 1 steel tank w/mfgr. CP where number of walls was not known

Including the steel tank with an unknown number of walls, 60 percent (53/89) of tank releases came from steel tanks

Calculation of z-statistics: z = (p1 - p2) / sqrt (P * (1-P) * (1 / n)) where 1/n = 1/n1+1/n2 if proportions are independent

p1 (Use) p2 (Releases) p1-p2 P 1-P 1/n z p - value

SW 13.6% 4.5% 9.1% 12.2% 87.8% 0.0135 2.39 0.02

DW 3.4% 3.4% 0.00% 3.4% 96.6% 0.0135 0.00 1.00

Total 17.1% 8.0% 9.1% 15.6% 84.4% 0.0135 2.16 0.03

SW 19.4% 39.8% -20.4% 22.6% 77.4% 0.0135 4.19 0.00

DW 0.6% 2.3% -1.6% 0.9% 99.1% 0.0135 1.49 0.14

Total 20.0% 42.0% -22.0% 23.5% 76.5% 0.0135 4.47 0.00

Steel w/Lining (SW) 4.7% 8.0% -3.3% 5.2% 94.8% 0.0135 1.26 0.21

Steel w/Lining, CP (SW) 1.5% 1.1% 0.4% 1.4% 98.6% 0.0135 0.26 0.80

All SW Steel 39.2% 53.4% -14.2% 41.5% 58.5% 0.0135 2.48 0.01

All DW Steel 4.1% 5.7% -1.6% 4.3% 95.7% 0.0135 0.69 0.49

All Steel 43.3% 59.1% -15.8% 45.8% 54.2% 0.0135 2.73 0.01

SW 5.8% 6.8% -1.1% 5.9% 94.1% 0.0135 0.39 0.70

DW 5.8% 2.3% 3.5% 5.2% 94.8% 0.0135 1.35 0.18

Total 11.5% 9.1% 2.4% 11.1% 88.9% 0.0135 0.66 0.51

SW 27.3% 18.2% 9.1% 25.9% 74.1% 0.0135 1.79 0.07

DW 17.9% 13.6% 4.3% 17.2% 82.8% 0.0135 0.97 0.33

Total 45.2% 31.8% 13.4% 43.1% 56.9% 0.0135 2.33 0.02

SW 72.3% 78.4% -6.1% 73.2% 26.8% 0.0135 1.19 0.23

DW 27.7% 21.6% 6.1% 26.8% 73.2% 0.0135 1.19 0.23

Steel Retrofit w/CP

marginally significant =

Steel w/ Mfgr. CP

Composite

Steel w/ Mfgr. CP

Fiberglass

Composite

Steel Retrofit w/CP

All Tanks

significant =

Fiberglass

Appendix C C-3
Tank Release Details

Material Walls Total Releases Corrosion

Physical/ 

Mechanical 

Damage

Faulty 

Installation Unknown

Total Known 

Causes

SW 4 2 2 0 1 4

DW 10 2 1 2 7 5

Unknown 1 0 0 0 1 0

Total 15 4 3 2 9 9

SW 35 21 2 2 12 25

DW 3 1 0 0 2 1

Total 38 22 2 2 14 26

SW 7 6 0 0 1 6

DW 0 0 0 0 0 0

Total 7 6 0 0 1 6

SW 1 1 0 0 0 1

DW 0 0 0 0 0 0

Total 1 1 0 0 0 1

61 33 5 4 24 42

SW 6 1 0 0 5 1

DW 2 0 0 0 2 0

81 0 0 71

SW 16 0 7 0 9 7

DW 12 0 1 0 11 1

28 0 8 0 20 8

97 34 13 4 51 51

Calculation of z-statistics: z = (p1 - p2) / sqrt (P * (1-P) * (1 / n)) where 1/n = 1/n1+1/n2 if proportions are independent

Comparison 1 Comparison 2 p1 p2 p1-p2 P 1-P 1/n z p - value

Man. CP Versus Retrofit CP 0.2830 0.7170 -0.4340 0.5000 0.5000 0.0189 6.32 0.00

SW Corrosion Versus DW Corrosion 0.8333 0.5000 0.3333 0.7857 0.2143 0.1944 1.84 0.07

Steel Tanks: Causes Of Release

Phys./Mech. 0.7857 0.1190 0.6667 0.4524 0.5476 0.0238 8.68 0.00

Faulty Install 0.7857 0.0952 0.6905 0.4405 0.5595 0.0238 9.01 0.00

Phys./Mech. Versus Faulty Install 0.1190 0.0952 0.0238 0.1071 0.8929 0.0238 0.50 0.62

Fisher's Exact Test - Comparison Of Corrosion Releases From Single- And Double-Wall Steel Tanks

Configuration Corrosion Not Corrosion

SW 30 6

DW 3 3

0.10

Note: No analysis was performed on composite tanks - only 1 known cause; for fiberglass tanks - all 8 known causes were physical/mechanical damage.

Total All Composite 

Cause comparison is for known causes only. Unknown causes are not included in the totals.

Fisher's Exact Test p-value: 

Corrosion Versus

significant = 

Value of N is low for z-test (N=6) = marginally significant = 

Cause

Steel With 

Manufacturer Cathodic 

Protection

Steel Retrofit With 

Cathodic Protection

Steel With Internal 

Lining

BOLD

Fiberglass

Total All Fiberglass 

Total All Tanks

Steel Tanks: Corrosion Protection

Steel Tanks: Single Wall Versus Double Wall

Steel With Lining And 

Cathodic Protection

Total All Steel 

Composite

Appendix C C-4
Piping Use And Releases For Piping With A Known Number Of Walls

Material

Number In Data 

Set1

Proportion In Data 

Set

Number Of 

Releases2

Proportion Of 

Releases Material

Number In 

Data Set

Proportion In 

Data Set

Number Of 

Releases

Proportion Of 

Releases

SW 85 22.1% 48 27.0% SW 248 64.4% 132 74.2%

DW 6 1.6% 0 0.0% DW 137 35.6% 46 25.8%

Total 91 23.6% 48 27.0% Total 385 178

SW 7 1.8% 1 0.6%

DW 63 16.4% 15 8.4%

Total 70 18.2% 16 9.0%

SW 156 40.5% 83 46.6%

DW 68 17.7% 31 17.4%

Total 224 58.2% 114 64.0%

Total All Piping 385 178

1. Excludes records with piping releases

2. Excludes 11 piping releases where number of walls was not known

Calculation of z-statistics: z = (p1 - p2) / sqrt (P * (1-P) * (1 / n)) where 1/n = 1/n1+1/n2 if proportions are independent

p1 (Use) p2 (Releases) p1-p2 P 1-P 1/n z p - value

SW 22.1% 27.0% -4.9% 23.6% 76.4% 0.008 1.27 0.20

DW 1.6% 0.0% 1.6% 1.1% 98.9% 0.008 1.67 0.09

Total Metal w/CP 23.6% 27.0% -3.3% 24.7% 75.3% 0.008 0.85 0.39

SW 1.8% 0.6% 1.3% 1.4% 98.6% 0.008 1.17 0.24

DW 16.4% 8.4% 7.9% 13.9% 86.1% 0.008 2.53 0.01

Total Flex. Plastic 18.2% 9.0% 9.2% 15.3% 84.7% 0.008 2.82 0.00

SW 40.5% 46.6% -6.1% 42.5% 57.5% 0.008 1.36 0.17

DW 17.7% 17.4% 0.2% 17.6% 82.4% 0.008 0.07 0.94

Total Fiberglass 58.2% 64.0% -5.9% 60.0% 40.0% 0.008 1.32 0.19

SW 64.4% 74.2% -9.7% 67.5% 32.5% 0.008 2.29 0.02

DW 35.6% 25.8% 9.7% 32.5% 67.5% 0.008 2.29 0.02

Piping Delivery And Dispenser/Turbine Sumps

Number In Data 

Set1

Proportion In Data 

Set

Number Of 

Releases

Proportion Of 

Releases

Pressure 391 82.1% 244 85.6%

Safe Suction 32 6.7% 19 6.7%

US Suction 53 11.1% 22 7.7%

Totals 476 285

Yes 40 27% 16 29%

No 110 73% 40 71%

Totals 150 56

Yes 58 30% 6 40%

No 135 70% 9 60%

Totals 193 15

Note: a significant number of dispenser/turbine sump Use and releases were unknown

p1 (Use) p2 (Releases) p1-p2 P 1-P 1/n z p - value

Pressure 82% 86% -3% 83% 17% 0.0056 1.25 0.21

Safe Suction 7% 7% 0% 7% 93% 0.0056 0.03 0.98

US Suction 11% 8% 3% 10% 90% 0.0056 1.53 0.13

Yes 27% 29% -2% 27% 73% 0.0245 0.27 0.78

No 73% 71% 2% 73% 27% 0.0245 0.27 0.78

Yes 30% 40% -10% 31% 69% 0.0718 0.80 0.42

No 70% 60% 10% 69% 31% 0.0718 0.80 0.42

Value of N is low for z-test (N=18) = 

Fisher's Exact Test - Use-Release Comparison For Tanks With/Without Turbine Sumps 

Configuration Releases No Releases

Sump 6 52

No Sump 9 126

0.39

Turbine Sumps

Fisher's Exact Test p-value: 

All Piping

Piping Delivery Type

Dispenser Sumps

BOLD significant = 

marginally significant = 

Piping Delivery Type

Dispenser Sumps

Metal w/CP

Turbine Sumps

All Piping

Flexible Plastic

Fiberglass

Metal w/CP

Flexible Plastic

Fiberglass

Appendix C C-5
Piping Release Details

Material Walls Total Releases Corrosion

Physical/ 

Mechanical 

Damage

Faulty 

Installation Unknown

Total Known 

Causes

SW 48 13 19 0 16 32

DW 0 0 0 0 0 0

Unknown 3 0 1 0 2 1

51 13 20 0 18 33

SW 1 0 1 0 0 1

DW 15 1 11 4 1 16

16 1 12 4 1 17

SW 83 5 54 3 22 62

DW 31 1 11 4 16 16

Unknown 8 1 3 3 1 7

122 7 68 10 39 85

189 21 100 14 58 135

Calculation of z-statistics: z = (p1 - p2) / sqrt (P * (1-P) * (1 / n)) where 1/n = 1/n1+1/n2 if proportions are independent

Comparison 1 Comparison 2 p1 p2 p1-p2 P 1-P 1/n z p - value

Phys./Mech. 0.1556 0.7407 -0.5852 0.4481 0.552 0.03030 6.76 0.00

Faulty Install 0.1556 0.1037 0.0519 0.1296 0.870 0.03030 0.89 0.38

Phys./Mech. Versus Faulty Install 0.7407 0.1037 0.6370 0.4222 0.578 0.03030 7.41 0.00

Phys./Mech. 0.3939 0.6061 -0.2121 0.5000 0.500 0.03030 2.44 0.01

Faulty Install 0.3939 0.0000 0.3939 0.1970 0.803 0.03030 5.69 0.00

Phys./Mech. Versus Faulty Install 0.6061 0.0000 0.6061 0.3030 0.697 0.03030 7.58 0.00

Phys./Mech. 0.0588 0.7059 -0.6471 0.3824 0.6176 0.0588 5.49 0.00

Faulty Install 0.0588 0.2353 -0.1765 0.1471 0.8529 0.0588 2.05 0.04

Phys./Mech. Versus Faulty Install 0.7059 0.2353 0.4706 0.4706 0.5294 0.0588 3.89 0.00

SW Phys./Mech. Versus DW Phys./Mech. 0.8710 0.6875 0.1835 0.8333 0.1667 0.0786 1.76 0.08

SW Faulty Install Versus DW Faulty Install 0.0484 0.2500 -0.2016 0.0897 0.9103 0.0786 2.52 0.01

Phys./Mech. 0.0824 0.8000 -0.7176 0.4412 0.5588 0.0118 13.33 0.00

Faulty Install 0.0824 0.1176 -0.0353 0.1000 0.9000 0.0118 1.08 0.28

Phys./Mech. Versus Faulty Install 0.8000 0.1176 0.6824 0.4588 0.5412 0.0118 12.62 0.00

Fisher's Exact Test - Comparison Of Releases From Flexible Plastic Causes

Configuration Yes No Configuration Yes No

Phys/Mech 12 5 Phys/Mech 12 5

Corrosion 1 16 Faulty Install 4 13

0.00 0.02

Configuration Yes No

Corrosion 1 16

Faulty Install 4 13

0.34

Fisher's Exact Test - Comparison Of Releases From SW And DW Fiberglass Piping

Configuration Yes No Configuration Yes No

SW Phys/Mech 54 8 SW Faulty Inst. 3 59

DW Phys/Mech 11 5 DW Faulty Inst. 4 12

0.13 0.03

Cause comparison is for known causes only. Unknown causes are not included in the totals.

significant = 

marginally significant = 

Value of N is low for z-test (N=17) = BOLD

Value of N is low for z-test (N=16) = BOLD

Corrosion Versus

Flexible Plastic Piping: Causes

Total All Flexible Plastic

Fiberglass

Total All Fiberglass

Cause

Metal With Cathodic 

Protection

Total All Metal 

Flexible Plastic

Total

General Piping: Causes Of Release

Corrosion Versus

Metal Piping: Causes Of Release

Corrosion Versus

Corrosion Versus

Fiberglass Piping: Single Wall Versus Double Wall

Fisher's Exact Test p-value: Fisher's Exact Test p-value: 

Fisher's Exact Test p-value: Fisher's Exact Test p-value: 

Fisher's Exact Test p-value: 

Fiberglass Pipes: Causes Of Release

Appendix C C-6
Extent Of Contamination By Source, Media

Local Large Unknown Total Media

Soil 44 5 5 54

Water 25 37 4 66

Unknown 11 2 56

Total 80 44

Soil 79 6 1 86 

Water 10 14 2 26

Unknown 29 5 43

Total 118 25

Soil 18 10 3 31

Water 11 21 2 34

Unknown 8 0 16

Total 37 31

Soil 9 0 0 9

Water 5 3 1 9

Unknown 1 0 6

Total 15 3

Soil 2 0 0 2

Water 2 1 0 3

Unknown 0 0 2

Total 4 1

Soil 30 1 4 35

Water 9 12 1 22

Unknown 2 3 25

Total 41 16

295 120

Total Soil 217

Total Water 160

Note: For the purposes of this analysis, off-site releases were considered to be "large"

Source Media Affected

Extent

Piping

Dispenser (Including 

Spills/Overfills)

Tank (8 cases where release 

was contained are not 

included in this table)

Others (Vent Line, Fill 

Pipe, Vapor Recovery)

STP Area

Delivery Problem

Total Extent

Appendix C C-7
Calculation of z-statistics: z = (p1 - p2) / sqrt (P * (1-P) * (1 / n)) where 1/n = 1/n1+1/n2 if proportions are independent

Media (1) Media (2) p1 p2 p1-p2 P 1-P 1/n z p - value

Proportion Of Releases: Soil Versus Water For Each Source

Piping Soil Versus Piping Water 0.450 0.550 -0.100 0.500 0.500 0.00833 2.19 0.03

Dispenser Soil Versus Dispenser Water 0.768 0.232 0.536 0.500 0.500 0.00893 11.34 0.00

Tank Soil Versus Tank Water 0.477 0.523 -0.046 0.500 0.500 0.01563 0.74 0.46

STP Area Soil Versus STP Area Water 0.500 0.500 0.000 0.500 0.500 0.05556 0.00 1.00

Others Soil Versus Others Water 0.400 0.600 -0.200 0.500 0.500 0.20000 0.89 0.37

Deliv. Prob. Soil Versus Deliv. Prob. Water 0.614 0.386 0.228 0.500 0.500 0.01754 3.44 0.00

All Soil Versus All Water 0.576 0.424 0.151 0.500 0.500 0.00265 5.87 0.00

Extent (1) Extent (2) p1 p2 p1-p2 P 1-P 1/n z p - value

Proportion Of Releases: Local Versus Large

Piping Local Versus Large 0.645 0.355 0.290 0.500 0.500 0.00806 6.47 0.00

Dispenser Local Versus Large 0.825 0.175 0.650 0.500 0.500 0.00699 15.55 0.00

Tank Local Versus Large 0.544 0.456 0.088 0.500 0.500 0.01471 1.46 0.15

STP Area Local Versus Large 0.833 0.167 0.667 0.500 0.500 0.05556 5.66 0.00

Other Local Versus Large 0.800 0.200 0.600 0.500 0.500 0.20000 2.68 0.01

Deliv. Prob Local Versus Large 0.719 0.281 0.439 0.500 0.500 0.01754 6.62 0.00

Total Local Versus Large 0.711 0.289 0.422 0.500 0.500 0.00241 17.18 0.00

Source (1) Source (2) p1 p2 p1-p2 P 1-P 1/n z p - value

Dispenser 0.550 0.232 0.318 0.397 0.603 0.01726 4.95 0.00

Tank 0.550 0.523 0.027 0.541 0.459 0.02372 0.35 0.73

STP Area 0.550 0.500 0.050 0.543 0.457 0.06389 0.40 0.69

Others 0.550 0.600 -0.050 0.552 0.448 0.20833 0.22 0.83

Deliv. Prob. 0.550 0.386 0.164 0.497 0.503 0.02588 2.04 0.04

Tank 0.232 0.523 -0.291 0.339 0.661 0.02431 3.94 0.00

STP Area * 0.232 0.500 -0.268 0.269 0.731 0.06448 2.38 0.02

Others 0.232 0.600 -0.368 0.248 0.752 0.20893 1.86 0.06

Deliv. Prob. 0.232 0.386 -0.154 0.284 0.716 0.02647 2.10 0.04

STP Area 0.523 0.500 0.023 0.518 0.482 0.07094 0.17 0.86

Others 0.523 0.600 -0.077 0.529 0.471 0.21538 0.33 0.74

Deliv. Prob. 0.523 0.386 0.137 0.459 0.541 0.03293 1.52 0.13

Others 0.500 0.600 -0.100 0.522 0.478 0.25556 0.40 0.69

Deliv. Prob. 0.500 0.386 0.114 0.413 0.587 0.07310 0.86 0.39

Others Deliv. Prob. 0.600 0.386 0.214 0.403 0.597 0.21754 0.94 0.35

Source (1) Source (2) p1 p2 p1-p2 P 1-P 1/n z p - value

Dispenser 0.361 0.175 0.186 0.260 0.740 0.01519 3.44 0.00

Tank 0.361 0.456 -0.095 0.395 0.605 0.02290 1.29 0.20

STP Area 0.361 0.167 0.194 0.336 0.664 0.06375 1.63 0.10

Others 0.361 0.032 0.328 0.294 0.706 0.04045 3.58 0.00

Deliv. Prob. 0.361 0.281 0.080 0.335 0.665 0.02574 1.06 0.29

Tank 0.175 0.456 -0.281 0.265 0.735 0.02170 4.32 0.00

STP Area 0.175 0.167 0.008 0.174 0.826 0.06255 0.09 0.93

Others 0.175 0.032 0.143 0.149 0.851 0.03925 2.02 0.04

Deliv. Prob. 0.175 0.281 -0.106 0.205 0.795 0.02454 1.67 0.09

STP Area * 0.456 0.167 0.289 0.395 0.605 0.07026 2.23 0.03

Others 0.456 0.032 0.424 0.323 0.677 0.04696 4.18 0.00

Deliv. Prob. 0.456 0.281 0.175 0.376 0.624 0.03225 2.01 0.04

Others 0.167 0.032 0.134 0.082 0.918 0.08781 1.66 0.10

Deliv. Prob. 0.167 0.281 -0.114 0.253 0.747 0.07310 0.97 0.33

Others Deliv. Prob. 0.032 0.281 -0.248 0.193 0.807 0.04980 2.82 0.00

Media/Extent (1) Media/Extent (2) p1 p2 p1-p2 P 1-P 1/n z p - value

Soil Versus Water Releases

Local To Soil Versus Local To Water 0.891 0.408 0.483 0.688 0.312 0.01175 9.61 0.00

LargeTo Soil Versus Large To Water 0.109 0.592 -0.483 0.312 0.688 0.01175 9.61 0.00

Fisher's Exact Test - STP Area Comparisons For Asterisks Above

Configuration Water Yes Water No Configuration Large Yes Large No

Dispenser 26 86 Tank 31 37

STP Area 9 9 STP Area 3 15

0.02 0.03

Tank Versus

Dispenser Versus

Tank Versus

STP Area Versus

Proportion Of Large Releases - Source Comparison

Piping Versus

Dispenser Versus

STP Area Versus

Proportion Of Releases To Water - Source Comparison

Piping Versus

Fisher's Exact Test p-value: Fisher's Exact Test p-value: 

marginally significant = 

Value of N is low for z-test (N=18) = significant = 

Value of N is low for z-test (N=5) = 

BOLD

BOLD

Note: Others - Small value of observations (N=5) makes all statistical tests somewhat unreliable.

Appendix C C-8
Media And Extent Of Contamination For Suction And Pressure Piping

Releases from piping, dispensers, and the STP area were included in this data. All spills/overfills were excluded.

Local Large Unknown Total Media

Soil 18 2 0 20

Water 6 5 0 11

Unknown 1 1

Total 25 8

Soil 76 5 3 84 

Water 29 29 5 63

Unknown 15 1

Total 120 35

Note: For the purposes of this analysis, off-site releases were considered to be "large"

Calculation of z-statistics: z = (p1 - p2) / sqrt (P * (1-P) * (1 / n)) where 1/n = 1/n1+1/n2 if proportions are independent

Extent(1) Extent(2) p1 p2 p1-p2 P 1-P 1/n z p - value

Suction Local Versus Pressure Local 0.758 0.774 -0.017 0.771 0.229 0.03675 0.21 0.84

Suction Large Versus Pressure Large 0.242 0.226 0.017 0.229 0.771 0.03675 0.21 0.84

Media(1) Media(2) p1 p2 p1-p2 P 1-P 1/n z p - value

Suction Soil Versus Pressure Soil 0.645 0.571 0.074 0.584 0.416 0.03906 0.76 0.45

Suction Water Versus Pressure Water 0.355 0.429 -0.074 0.416 0.584 0.03906 0.76 0.45

significant = 

marginally significant = 

Pressure

Piping Delivery Media Affected

Extent

Suction

Appendix C C-9
Methods Of Release Identification

Method Of Identification Number Of Releases Percent

Leak Detection 100 16.8%

Closure 76 12.8%

Property Transfer 12 2.0%

Visual Or Olfactory 279 47.0%

Water In Tank 12 2.0%

Other 89 15.0%

Unknown 26 4.4%

Total Known Method of ID 594

Calculation of z-statistics: z = (p1 - p2) / sqrt (P * (1-P) * (1 / n))

Method (1) Method (2) p1 p2 p1-p2 P 1-P 1/n z p - value

Leak Detection Versus Closure 0.168 0.128 0.040 0.148 0.852 0.002 2.77 0.01

Property Transfer 0.168 0.020 0.148 0.094 0.906 0.002 12.36 0.00

Visual Or Olfactory 0.168 0.470 -0.301 0.319 0.681 0.002 15.76 0.00

Water In Tank 0.168 0.020 0.148 0.094 0.906 0.002 12.36 0.00

Closure Versus Property Transfer 0.128 0.020 0.108 0.074 0.926 0.002 10.03 0.00

Visual Or Olfactory 0.128 0.470 -0.342 0.299 0.701 0.002 18.20 0.00

Water In Tank 0.128 0.020 0.108 0.074 0.926 0.002 10.03 0.00

Property Transfer Versus Visual Or Olfactory 0.020 0.470 -0.449 0.245 0.755 0.002 25.47 0.00

Water In Tank 0.020 0.020 0.000 0.020 0.980 0.002 0.00 1.00

Visual Or Olfactory Versus Water In Tank 0.470 0.020 0.449 0.245 0.755 0.002 25.47 0.00

significant = 

marginally significant = 

Appendix C C-10
Leak Detection Success Rate Evaluation

Leak Detection Method

Opportunities To 

Dectect Leaks

Proportion Of 

Opportunites

Leaks 

Detected

Proportion 

Detected

Leaks Not 

Detected

Proportion Not 

Detected

Vapor 29 6% 10 34% 19 66%

Groundwater 25 5% 8 32% 17 68%

ATG 28 6% 8 29% 20 71%

LTT 78 16% 20 26% 58 74%

LLD 103 22% 25 24% 78 76%

Interstitial 51 11% 12 24% 39 76%

MTG 5 1% 1 20% 4 80%

SIR 78 16% 15 19% 63 81%

IC&TTT 76 16% 7 9% 69 91%

Total 473 100% 106 22% 367 78%

LD Method Vapor Groundwater ATG LTT LLD Interstitial MTG SIR IC&TTT Total

Vapor -- p=1.00 p= 0.77 p= 0.47 p= 0.34 p= 0.31 p=1.00 p= 0.12 p= 0.01 p= 0.20

Groundwater -- p=1.00 p= 0.61 p= 0.45 p= 0.58 p=1.00 p= 0.27 p= 0.01 p= 0.33

ATG -- p= 0.80 p= 0.63 p= 0.79 p=1.00 p= 0.30 p= 0.02 p= 0.60

LTT -- p= 0.86 p= 0.84 p=1.00 p= 0.44 p= 0.01 p= 0.63

LLD -- p=1.00 p=1.00 p= 0.47 p= 0.01 p= 0.78

Interstitial -- p=1.00 p=0 .66 p= 0.04 p= 0.99

MTG -- p=1.00 p= 0.41 p=1.00

SIR -- p= 0.11 p= 0.63

IC&TTT -- p= 0.01

Total --

Shading: = significant

Typical STATISTICA Fisher's Exact Test, Chi-Squared Output (Vapor Versus ATG)

Frequencies, row 1 10 19 29

Percent of total 17.544% 33.333% 50.877%

Frequencies, row 2 8 20 28

Percent of total 14.035% 35.088% 49.123%

Column totals 18 39 57

Percent of total 31.579% 68.421%

Chi-square (df=1) 0.23 p= .6312

V-square (df=1) 0.23 p= .6342

Yates corrected Chi-square 0.04 p= .8454

Phi-square 0.00404

Fisher exact p, one-tailed p= .4231

two-tailed p= .7771

McNemar Chi-square (A/D) 2.7 p= .1004

Chi-square (B/C) 3.7 p= .0543

Significance Of Differences In Detection Success Rates (p-values)1

All p-values are calculated based on Fisher's Exact Test, except for bold values, which are Yates-corrected Chi-squared values;

Column 1 Column 2 Row Totals

Appendix C C-11
Leak Detection ID

 - some LD not designed to detect problem beyond tank and piping while GW and vapor could - these areas BLACKED out - LD method associated with component that leaked (e.g., TTT is for tank only, LTT for piping only) 

MIR MTG SIR TTT LTT ATG Interstitital GW V

Source Total LD ID'd ID Opp ID Opp ID Opp ID Opp ID Opp ID Opp ID Opp ID Opp ID OPiping 162 57 3 31 8 39 19 65 2 22 3 12 4 

Dispenser 113 2 0 15 1 1 1 1 0 1 0 Tank 83 24 3 20 1 3 4 18 1 14 7 23 9 18 0 3 0 STP Area (Including LLD) 25 4 1 6 1 12 0 4 1 4 1 1 0 Piping And Dispenser 14 2 0 2 0 4 0 4 1 1 0 3 0 0 0 Tank And Piping 13 1 0 2 0 2 1 3 0 1 0 4 0 4 0 4 0 2 0 Vent Line 2 1 1 10 Fill Pipe 3 1 1 10 Vapor Recovery 2 0 0 00 Tank And Dispenser 1 00 00 0 0 10 0 0 0 0 00 00 S/O At Tank (Delivery) 87 4 0 24 S/O At Dispenser 61 0 0 00 S/O At Tank Or Dispenser 14 4 2 22 

100 7 76 1 5 15 78 1 15 20 78 8 28 12 51 8 25 10 

Total Opportunities = 473ID - Number of leak detection identificationsOpp = Opportunities to detect the problem6 releases had multiple methods of leak detection identificationOpportunities are greater than total that LD should have caught because some sources used multiple forms of release detectionOpportunites exclude 15 TTT cases because those are assumed to be part of 76 MIR -- this is a combination LD method

Appendix C C-12
Shaded values are significant at p < 0.05

Use Releases Use Releases

0-5 yr 49 4 53 21

6-10 yr 64 6 59 27

11-15 yr 121 36 70 30

>15 120 38 52 42

N 354 84 234 120

Cumulative Distributions

0-5 yr 13.8% 4.8% 22.6% 17.5%

6-10 yr 31.9% 11.9% 47.9% 40.0%

11-15 yr 66.1% 54.8% 77.8% 65.0%

>15 100.0% 100.0% 100.0% 100.0%

Differences

9.1% 5.1%

20.0% 7.9%

11.3% 12.8%

0.0% 0.0%

M 20.0% 12.8%

Chi-squared 10.88 5.18

p - value 0.00 0.08

Significance Of Difference In Use/Releases Less Than/Greater Than Ten Years (Fisher's Exact Test)

N <=10 113 10 112 48

N >10 241 74 122 72

P <= 10 32% 12% 48% 40%

P > 10 68% 88% 52% 60%

p -value 0.00 0.18

marginally significant =

Releases Versus Age Of Tanks, Piping (All Designs, Construction Materials)

Kolmorgorov-Smirnov Test For Difference In Age Patterns (Chi-Squared Approximation)

significant = 

Tanks (All) Piping (All)

Tanks (All) Piping (All)

Appendix C C-13
Tanks

Column 1 Column 2 Totals

Frequencies, row 1 113 10 123

Percent of total 25.799% 2.283% 28.082%

Frequencies, row 2 241 74 315

Percent of total 55.023% 16.895% 71.918%

Column totals 354 84 438

Percent of total 80.822% 19.178%

Chi-square (df=1) 13.47 p= .0002

V-square (df=1) 13.44 p= .0002

Yates corrected Chi-square 12.5 p= .0004

Phi-square 0.03075

Fisher exact p, one-tailed p= .0001

two-tailed p= .0001

McNemar Chi-square (A/D) 7.72 p= .0055

Chi-square (B/C) 210.76 p=0.0000

Piping

Column 1 Column 2 Totals

Frequencies, row 1 112 48 160

Percent of total 31.638% 13.559% 45.198%

Frequencies, row 2 122 72 194

Percent of total 34.463% 20.339% 54.802%

Column totals 234 120 354

Percent of total 66.102% 33.898%

Chi-square (df=1) 1.98 p= .1594

V-square (df=1) 1.97 p= .1600

Yates corrected Chi-square 1.68 p= .1956

Phi-square 0.00559

Fisher exact p, one-tailed p= .0976

two-tailed p= .1765

McNemar Chi-square (A/D) 8.27 p= .0040

Chi-square (B/C) 31.35 p= .0000

Example STATISTICA® Outputs (Use Versus Releases Greater/Less Than 10 Years)

Appendix C C-14
Age Patterns Of Use, Release For Tank Designs, Construction Materials

Kolmorgorov-Smirnov Test For Difference In Age Patterns (Chi-Squared Approximation)

Use Releases Use Releases Use Releases Use Releases Use Releases Use Releases Use Releases Use Releases Use Releases Use Releases Use Releases Use 

0-5 yr 10 0 2 0 12 0 1 0 0 0 11 0 2 0 13 0 3 0 14 1 17 1 6 6-10 yr 7 0 5 2 12 2 0 1 0 0 7 1 5 2 12 3 1 0 24 1 25 1 8 11-15 yr 36 6 9 11 45 17 3 2 0 0 39 8 9 11 48 19 10 4 3 0 13 4 32 >15 60 25 0 0 60 25 20 2 0 0 80 27 0 0 80 27 2 2 0 0 2 2 36 N 113 31 16 13 129 44 24 5 0 0 137 36 16 13 153 49 16 6 41 2 57 8 82 

Cumulative Distributions

0-5 yr 8.8% 0.0% 12.5% 0.0% 9.3% 0.0% 4.2% 0.0% -- -- 8.0% 0.0% 12.5% 0.0% 8.5% 0.0% 18.8% 0.0% 34.1% 50.0% 29.8% 12.5% 7.3% 6-10 yr 15.0% 0.0% 43.8% 15.4% 18.6% 4.5% 4.2% 20.0% -- -- 13.1% 2.8% 43.8% 15.4% 16.3% 6.1% 25.0% 0.0% 92.7% 100.0% 73.7% 25.0% 17.1% 

11-15 yr 46.9% 19.4% 100.0% 100.0% 53.5% 43.2% 16.7% 60.0% -- -- 41.6% 25.0% 100.0% 100.0% 47.7% 44.9% 87.5% 66.7% 100.0% 100.0% 96.5% 75.0% 56.1% 

>15 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% -- -- 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 

Differences

8.8% 12.5% 9.3% 4.2% -- 8.0% 12.5% 8.5% 18.8% 15.9% 17.3% 

15.0% 28.4% 14.1% 15.8% -- 10.4% 28.4% 10.2% 25.0% 7.3% 48.7% 27.5% 0.0% 10.3% 43.3% -- 16.6% 0.0% 2.8% 20.8% 0.0% 21.5% 

0.0% 0.0% 0.0% 0.0% -- 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 

M 27.5% 28.4% 14.1% 43.3% -- 16.6% 28.4% 10.2% 25.0% 15.9% 48.7% Chi-squared 7.38 2.31 2.59 3.11 -- 3.14 2.31 1.55 1.09 0.19 6.65 p - value 0.02 0.32 0.27 0.21 -- 0.21 0.32 0.46 0.58 0.91 0.04 

Significance Of Difference In Use/Releases Less Than/Greater Than Ten Years (Fisher's Exact Test)

N <=10 17 0 7 2 24 2 1 1 0 0 18 1 7 2 25 3 4 0 38 2 42 2 14 N >10 96 31 9 11 105 42 23 4 0 0 119 35 9 11 128 46 12 6 3 0 15 6 68 P <= 10 15% 0% 44% 15% 19% 5% 4% 20% -- -- 13% 3% 44% 15% 16% 6% 25% 0% 93% 100% 74% 25% 17% P > 10 85% 100% 56% 85% 81% 95% 96% 80% -- -- 87% 97% 56% 85% 84% 94% 75% 100% 7% 0% 26% 75% 83% p -value 0.02 0.13 0.02 0.32 -- 0.13 0.13 0.10 0.54 1.00 0.01 

Significant =

Example STATISTICA® Output (Use Versus Releases Greater/Less Than 10 Years, Steel w/CP , SW)

Row

Column 1 Column 2 Totals

Frequencies, row 1 17 0 17Percent of total 11.806% 0.000% 11.806%Frequencies, row 2 96 31 127Percent of total 66.667% 21.528% 88.194%Column totals 113 31 144Percent of total 78.472% 21.528%Chi-square (df=1) 5.29 p= .0215V-square (df=1) 5.25 p= .0219Yates corrected Chi-square 3.94 p= .0471Phi-square 0.03672Fisher exact p, one-tailed p= .0122two-tailed p= .0237McNemar Chi-square (A/D) 3.52 p= .0606Chi-square (B/C) 94.01 p= .0000

Steel w/CP (SW) Steel w/CP (DW) Steel w/CP (All) Steel w/Lining (SW) Steel W/Lining (DW) Composite (SW) Composite (DW) SW Steel DW Steel All Steel Composite (All)

SW Steel DW Steel All Steel

Fibergla

Steel w/CP (SW) Steel w/CP (DW) Steel w/CP (All) Steel w/Lining (SW) Steel W/Lining (DW) Composite (SW) Composite (DW) Composite (All) Fibergla

Appendix C C-15
Age Patterns Of Use, Releases For Piping Designs, Construction Materials

Kolmorgorov-Smirnov Test For Difference In Age Patterns (Chi-Squared Approximation)

Use Releases Use Releases Use Releases Use Releases Use Releases Use Releases Use Releases U

0-5 yr 1 0 0 0 1 0 1 0 26 3 27 3 12 14 16-10 yr 1 2 0 0 1 2 1 1 22 4 23 5 23 13 111-15 yr 14 5 1 0 15 5 1 0 2 0 3 0 35 14 1>15 22 25 1 0 23 25 0 0 1 0 1 0 24 14 N 38 32 2 0 40 32 3 1 51 7 54 8 94 55 4

Cumulative Distributions

0-5 yr 2.6% 0.0% 0.0% -- 2.5% 0.0% 33.3% 0.0% 51.0% 42.9% 50.0% 37.5% 12.8% 25.5% 286-10 yr 5.3% 6.3% 0.0% -- 5.0% 6.3% 66.7% 100.0% 94.1% 100.0% 92.6% 100.0% 37.2% 49.1% 5411-15 yr 42.1% 21.9% 50.0% -- 42.5% 21.9% 100.0% 100.0% 98.0% 100.0% 98.1% 100.0% 74.5% 74.5% 91>15 100.0% 100.0% 100.0% -- 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100

Differences

2.6% -- 2.5% 33.3% 8.1% 12.5% 12.7% 1.0% -- 1.3% 33.3% 5.9% 7.4% 11.9% 

20.2% -- 20.6% 0.0% 2.0% 1.9% 0.1% 

0.0% -- 0.0% 0.0% 0.0% 0.0% 0.0% 

M 20.2% -- 20.6% 33.3% 8.1% 12.5% 12.7% Chi-squared 2.84 -- 3.03 0.33 0.16 0.44 2.23 p - value 0.24 -- 0.22 0.85 0.92 0.80 0.33 

Significance Of Difference In Use/Releases Less Than/Greater Than Ten Years (Fisher's Exact Test)

N <=10 2 2 0 0 2 2 2 1 48 7 50 8 35 27 2N >10 36 30 2 0 38 30 1 0 3 0 4 0 59 28 2P <= 10 5% 6% 0% -- 5% 6% 67% 100% 94% 100% 93% 100% 37% 49% 54P > 10 95% 94% 100% -- 95% 94% 33% 0% 6% 0% 7% 0% 63% 51% 46p -value 1.00 -- 1.00 1.00 1.00 1.00 0.17 

Significant =

Example STATISTICA® Output (Use Versus Releases Greater/Less Than 10 Years, Fiberglass, SW)

Row

Column 1 Column 2 Totals

Frequencies, row 1 35 27 62Percent of total 23.490% 18.121% 41.611%Frequencies, row 2 59 28 87Percent of total 39.597% 18.792% 58.389%Column totals 94 55 149Percent of total 63.087% 36.913%Chi-square (df=1) 2.01 p= .1565V-square (df=1) 1.99 p= .1579Yates corrected Chi-square 1.55 p= .2132Phi-square 0.01347Fisher exact p, one-tailed p= .1068

two-tailed p= .1716McNemar Chi-square (A/D) 0.57 p= .4497Chi-square (B/C) 11.17 p= .0008

Metal w/CP (SW) Metal w/CP (DW) Metal w/CP (All) Flexible Plastic (SW) Flexible Plastic (DW) Flexible Plastic (All) Fiberglass (SW) F

Steel w/CP (SW) Steel w/CP (DW) Steel w/CP (All) Flexible Plastic(SW) Flexible Plastic(DW) Flexible Plastic (All) Fiberglass (SW) F

Appendix C C-16
Comparison Of Piping Types <10 Years Old And Piping Types >10 Years Old

Material

Number In 

Data Set1

Proportion In Data 

Set

Number Of 

Releases

Proportion Of 

Releases

Metal w/CP 2 1.8% 2 4.2%

Flexible Plastic 50 44.6% 8 16.7%

Fiberglass 60 53.6% 38 79.2%

Total 112 48

Single Wall 39 34.8% 30 62.5%

Double Wall 73 65.2% 18 37.5%

Metal w/CP 38 31.1% 30 41.7%

Flexible Plastic 4 3.3% 0 0.0%

Fiberglass 80 65.6% 42 58.3%

Total 122 72

Single Wall 96 78.7% 58 80.6%

Double Wall 26 21.3% 14 19.4%

1. Excludes records with piping releases

Calculation of z-statistics: z = (p1 - p2) / sqrt (P * (1-P) * (1 / n)) where 1/n = 1/n1+1/n2 if proportions are independent

p1 (Use) p2 (Releases) p1-p2 P 1-P 1/n z p - value

Metal w/CP 1.8% 4.2% -2.4% 2.5% 97.5% 0.030 0.88 0.38

Flexible Plastic 44.6% 16.7% 28.0% 36.3% 63.8% 0.030 3.37 0.00

Fiberglass 53.6% 79.2% -25.6% 61.3% 38.8% 0.030 3.05 0.00

Single Wall 34.8% 62.5% -27.7% 43.1% 56.9% 0.030 3.24 0.00

Double Wall 65.2% 37.5% 27.7% 56.9% 43.1% 0.030 3.24 0.00

Metal w/CP 31.1% 41.7% -10.5% 35.1% 64.9% 0.022 1.48 0.14

Flexible Plastic 3.3% 0.0% 3.3% 2.1% 97.9% 0.022 1.55 0.12

Fiberglass 65.6% 58.3% 7.2% 62.9% 37.1% 0.022 1.01 0.31

Single Wall 78.7% 80.6% -1.9% 79.4% 20.6% 0.022 0.31 0.76

Double Wall 21.3% 19.4% 1.9% 20.6% 79.4% 0.022 0.31 0.76

significant = 

marginally significant = 

Piping >10 Years Old

Piping <10 Years Old

Piping >10 years old

Piping <10 Years Old

Appendix C C-17
Comparison Of Tank Types <10 Years Old And Tank Types >10 Years Old

Material

Number In 

Data Set1

Proportion In 

Data Set

Number Of 

Releases

Proportion Of 

Releases

Steel 25 22.1% 3 30.0%

Composite 42 37.2% 2 20.0%

Fiberglass 46 40.7% 5 50.0%

Total 113 10

Single Wall 36 31.9% 2 20.0%

Double Wall 77 68.1% 8 80.0%

Steel 128 53.1% 46 62.2%

Composite 15 6.2% 6 8.1%

Fiberglass 98 40.7% 22 29.7%

Total 241 74

Single Wall 199 82.6% 56 75.7% (Yates-corrected Chi2)

Double Wall 42 17.4% 18 24.3%

1. Excludes records with piping releases

Calculation of z-statistics: z = (p1 - p2) / sqrt (P * (1-P) * (1 / n)) where 1/n = 1/n1+1/n2 if proportions are independent

p1 (Use) p2 (Releases) p1-p2 P 1-P 1/n z p - value

Steel 22.1% 30.0% -7.9% 22.8% 77.2% 0.109 0.57 0.57

Composite 37.2% 20.0% 17.2% 35.8% 64.2% 0.109 1.09 0.28

Fiberglass 40.7% 50.0% -9.3% 41.5% 58.5% 0.109 0.57 0.57

Single Wall 31.9% 20.0% 11.9% 30.9% 69.1% 0.109 0.78 0.44

Double Wall 68.1% 80.0% -11.9% 69.1% 30.9% 0.109 0.78 0.44

Steel 53.1% 62.2% -9.1% 55.2% 44.8% 0.018 1.37 0.17

Composite 6.2% 8.1% -1.9% 6.7% 93.3% 0.018 0.57 0.57

Fiberglass 40.7% 29.7% 10.9% 38.1% 61.9% 0.018 1.69 0.09

Single Wall 82.6% 75.7% 6.9% 81.0% 19.0% 0.018 1.32 0.19

Double Wall 17.4% 24.3% -6.9% 19.0% 81.0% 0.018 1.32 0.19

Tank <10 years old

Tank >10 years old

Tank <10 Years Old

0.51

0.81

0.73

0.77

Note: The Fisher's Exact Test for fiberglass tanks >10 years old shows no statistical difference while the z-test shows a marginal difference.

Fisher's Exact Test 

p-value

0.33

0.74

0.30

0.60

0.73

0.71

Tank >10 Years Old

(see Fisher's Exact Tests above) 

significant = 

marginally significant = 

Value of N is low for z-test (N=10) = BOLD

Appendix C C-18