United States Office of Ground Water EPA/816-R-99-014j

Environmental and Drinking Water (4601) September 1999

Protection Agency 

The Class V Underground Injection

Control Study

Volume 10

Mining, Sand, or Other Backfill Wells
September 30, 1999

Table of Contents

Page

1. Summary ............................................................... 1

2. Introduction ............................................................. 2

3. Prevalence of Wells ....................................................... 3

4. Backfill Characteristics And Injection Practices ................................... 3

4.1 Injectate Characteristics .............................................. 3

4.1.1 Mill Tailings ................................................. 6

4.1.2 Coal Combustion Ash ........................................ 10

4.1.3 Flue Gas Desulfurization Sludge ................................. 19

4.1.4 Coal Cleaning Waste ......................................... 19

4.1.5 Mine Drainage Precipitate Waste ................................ 24

4.2 Well Characteristics ................................................ 24

4.3 Operational Practices ............................................... 26

4.3.1 Placement Methods .......................................... 26

4.3.2 Integrated Mining and Backfilling ................................ 31

4.3.3 Retroactive Backfilling ........................................ 33

4.3.4 Well Maintenance and Closure .................................. 33

5. Potential And Documented Damage to USDWs ................................. 33

5.1 Injectate Constituent Properties ....................................... 33

5.2 Observed Impacts ................................................. 35

5.2.1 Metal Mines ................................................ 35

5.2.2 Coal Mines ................................................ 37

6. Alternative And Best Management Practices .................................... 41

6.1 Injectate Characteristics ............................................. 41

6.2 System Design and Construction ....................................... 41

6.3 Well Operation .................................................... 42

6.4 Well Closure ..................................................... 43

7. Current Regulatory Requirements ............................................ 45

7.1 Federal Programs .................................................. 45

7.1.1 SDWA .................................................... 45

7.1.2 SMCRA .................................................. 46

7.2 State and Local Programs ........................................... 48

Attachment A: State and Local Program Descriptions .................................. 50

References ................................................................... 68
September 30, 1999 1

MINING, SAND, OR OTHER BACKFILL WELLS

The U.S. Environmental Protection Agency (USEPA) conducted a study of Class V

underground injection wells to develop background information the Agency can use to evaluate the risk

that these wells pose to underground sources of drinking water (USDWs) and to determine whether

additional federal regulation is warranted. The final report for this study, which is called the Class V

Underground Injection Control (UIC) Study, consists of 23 volumes and five supporting appendices. 

Volume 1 provides an overview of the study methods, the USEPA UIC Program, and general findings. 

Volumes 2 through 23 present information summaries for each of the 23 categories of wells that were

studied (Volume 21 covers 2 well categories). This volume, which is Volume 10, covers Class V

mining, sand, or other backfill wells.

1. SUMMARY

Mine backfill wells are used in many mining regions throughout the country to inject a mixture of

water and sand, mill tailings, or other materials (e.g., coal combustion ash, coal cleaning wastes, acid

mine drainage (AMD) treatment sludge, flue gas desulfurization sludge) into mined out portions of

underground mines. On occasion, injection (in low porosity grout form) also occurs into the rubble

disposal areas at surface mining sites. Mine shafts and pipelines in an underground mine, as well as

more “conventional” drilled wells, used to place slurries and solids in underground mines are considered

mine backfill. Such wells may be used to provide subsidence control (the most common purpose),

enhanced ventilation control, fire control, reduced surface disposal of mine waste, enhanced recovery

of minerals, mitigation of AMD, and improved safety.

The physical characteristics and chemical composition of the materials that are injected into

backfill wells vary widely depending on the source of the backfill material, the method of injection, and

any additives (e.g., cement) that may be included. Data from leaching tests (e.g., USEPA Method

1311 Toxicity Characteristic Leaching Procedure (TCLP)) of backfill materials indicate that

concentrations of antimony, arsenic, barium, beryllium, boron, cadmium, chromium, lead, mercury,

molybdenum, nickel, selenium, thallium, sulfate, and zinc frequently exceed primary maximum

contaminant levels (MCLs) or health advisory levels (HALs). Concentrations of aluminum, copper,

iron, manganese, total dissolved solids (TDS), and sulfate, as well as the pH, frequently exceed

secondary MCLs.

At sites where water is present in the injection zone (the previously mined ore body), the mine

water may already exceed MCLs or HALs prior to injection either as a result of mining activity or

natural conditions. At such sites, one objective of injection often is to improve the already poor quality

of the mine water by reducing the availability of oxygen in the mine workings and/or neutralizing AMD. 

In other areas, water from coal beds may be used to supply domestic wells.

No incidents of contamination of a USDW have been identified that are directly attributable to

injection into mine backfill wells. Although ground water contamination is not uncommon at mining
September 30, 1999 2

sites, it is generally difficult to identify the specific causes. The chance that backfill injection will

contribute to ground water contamination is highly dependent onsite conditions, including mine

mineralogy, site hydrogeology, backfill characteristics, and injection practices. Some studies of the

effects of backfill injection on mine water quality show that concentrations of some cations and anions

can increase in mine water following injection, whereas concentrations of trace metals generally are

relatively unaffected or decline over time. Other studies (at other sites) show an increase in selected

metal concentrations.

The vulnerability of mine backfill wells to receiving spills or illicit discharges also depends on

site-specific conditions and practices. For example, if coal ash is hauled to a mine site, slurried with

water, and then injected, the likelihood of contamination of the injected material resulting from a spill or

illicit discharge is relatively low. On the other hand, if mill tailings are collected in a tailings pond along

with site runoff and other facility wastes prior to injection, then the likelihood of contamination of the

backfill material by spills would be higher.

According to the state and USEPA Regional survey conducted for this study, there are

approximately 5,000 documented mine backfill wells and more than 7,800 wells estimated to exist in

the United States A total of 17 states report having mine backfill wells. More than 90 percent of the

documented wells reported are in four states: Ohio (3,570), Idaho (575); West Virginia (401), and

North Dakota (200). In truth, there may be more due to the broad scope of this well type and the fact

that some state inventories may count these wells as subsidence control wells while others did not. 

Also, the number of active wells at any given time varies widely due to their generally short life span,

most often a few days or less. The number of mine backfill wells has the potential to grow in the future

due to the growing movement to decrease surface disposal and control ground subsidence.

State regulations pertaining to mine backfill wells vary significantly in their scope and stringency. 

Some states impose few restrictions while others require permitting, or impose requirements by contract

rather than regulation. Some of these approaches include permit by rule (e.g., West Virginia, Idaho,

North Dakota), general or area permits (e.g., Wyoming), and individual permits (e.g., Ohio). In

addition, federal requirements for planning and approval of mining activities include mine backfill

activities. These requirements apply in states that have not obtained primacy under the Surface Mining

Control and Reclamation Act and to activities on federal and Native American tribal lands.

2. INTRODUCTION

Under the existing UIC regulations, Class V injection wells include “sand backfill and other

backfill wells used to inject a mixture of water and sand, mill tailings or other solids into mined out

portions of subsurface mines whether what is injected is a radioactive waste or not” (40 CFR

146.5(e)(8)). Piping systems within mine shafts and workings, as well as more “conventional” drilled

wells, used to place slurries/solids in underground mines are considered mine backfill wells under the

USEPA’s UIC regulations. Similarly, mine shafts are considered backfill wells if backfill is injected into

the shaft.
September 30, 1999 3

Backfill injection is extremely diverse. Although subsidence control is a common objective of

backfilling, injection can be performed for a wide range of reasons, as noted above. The types of

materials that are injected are similarly diverse, and include materials (and various mixtures of materials)

resulting from coal mining and combustion, primary and precious metal mining, uranium mining, and

non-metal mining. The environmental settings in which the mines are located and injection occurs are

similarly diverse. This volume only provides an overview and a general categorization of mine

backfilling activities.

3. PREVALENCE OF WELLS

For this study, data on the number of Class V mining, sand, or other backfill wells were

collected through a survey of state and USEPA Regional UIC Programs. The survey methods are

summarized in Section 4 of Volume 1 of the Class V Study. Table 1 lists the numbers of Class V

mining, sand, or other backfill wells in each state, as determined from this survey. The table includes

the documented number and estimated number of wells in each state, along with the source and basis

for any estimate, when noted by the survey respondents. If a state is not listed in Table 1, it means that

the UIC Program responsible for that state indicated in its survey response that it did not have any

Class V mining, sand, or other backfill wells.

In 1998, a total of approximately 5,000 mine backfill wells were reported nationwide, all of

which are reported to be in 17 states. As indicated in Table 1, several states estimated that the actual

number of mine backfill wells is greater than the number reflected in their documented inventory. In

addition, some states did not provide inventory information, although it is likely that wells exist in some

of these states. Thus, the actual number of operating mine backfill wells in 1998 is estimated to be at

least 7,800. The fact that they often exist for a relatively short operating time (in some cases, a few

days or less) complicates development of a precise count of mine backfill wells in use during a given

year, as exemplified by the information provided by Pennsylvania, Texas, Illinois, and West Virginia and

summarized in Table 1. 

4. BACKFILL CHARACTERISTICS AND INJECTION

PRACTICES

4.1 Injectate Characteristics

A wide assortment of materials are used for backfilling of underground mines. These materials

may include waste rock, mining and ore beneficiation wastes (e.g., mill tailings, coal cleaning wastes),

coal combustion ash and flue gas desulfurization (FGD) sludge resulting from coal combustion, or

sludge from AMD treatment operations. Mill tailings have been reported to be the most commonly

used mine backfill materials, because they are inexpensive and abundant (Underground Injection

Council Research Foundation, 1988).
September 30, 1999 4

Table 1. Inventory of Mine Backfill Wells in the U.S.*

State Documented 

Number of Wells

Estimated Number of Wells

Number Source of Estimate and Methodology1

USEPA Region 1 -- None

USEPA Region 2 -- None

USEPA Region 3

MD 6 6 N/A

PA NR NR Injection for subsidence control is common, but no wells were

reported to be active at the time of the survey. A total of 1,123

wells are planned as part of four projects awaiting approval. 

PA only includes wells used for subsidence control in the

“backfill injection” category.

VA NR NR USEPA Region reports that backfill wells exist in VA, but the

number of wells is not documented by the Region and was not

available from the state.

WV 401 <401 Best professional judgement. Most backfill wells are used for

fire control and are closed when the fire is extinguished, so state

staff believe that most of these wells have been closed. Backfill

wells used for subsidence control (73) are also included in the

inventory.

USEPA Region 4

AL 22 22 N/A

KY NR NR State staff report that backfill wells exist in KY, but none are

documented.

TN 2 2 N/A

USEPA Region 5

IL 19 17 2 of the 19 wells may not have commenced operations. UIC

inventory shows 34 wells, but state personnel believe many

have been closed and abandoned.

IN 98 (UIC)

83 (Region)

2 (DNR)

NR Combination of state and regional information: state does not

routinely distinguish mine backfill wells from some other

categories of Class V wells. The 1997 UIC inventory is a

compilation from the region and an 1988 EEI study. DNR

believes that at least 2 wells are not included in the region’s

inventory. 

OH 3,570 6,400 Best professional judgement, based on knowledge of areas

containing mines and installation frequency of backfilling wells.

USEPA Region 6

TX 61 61 Although 61 wells are in the UIC inventory, all of these wells

may be closed.
Table 1. Inventory of Mine Backfill Wells in the U.S. 

(continued)

State Documented 

Number of Wells

Estimated Number of Wells

Number Source of Estimate and Methodology1

September 30, 1999 5

USEPA Region 7

KS 48 48 N/A

MO 15 15 N/A

USEPA Region 8

CO 2 NR N/A

M T NR NR

USEPA Region 8 Montana Operations Office staff believe that

backfill wells may exist in MT. However, neither the USEPA

Region nor the state has inventory data on such wells.

ND 200 200 N/A

SD 1 1 N/A

UT

0 2 State database shows that 2 wells are under construction, but

have never been completed due to economic factors.

WY 20 >20 Best professional judgement. The documented 20 wells do not

include subsidence prevention wells. No information was

available for subsidence prevention wells.

USEPA Region 9

CA 17 17 State personnel did not provide estimate but indicated that they

suspect that more than the documented number of wells exist.

USEPA Region 10

AK 1 >1 N/A

ID 575 575 N/A

All USEPA Regions

All States 5,060 >7,890 Total estimated number counts the documented number when

the estimate is NR.

1

 Unless otherwise noted, the best professional judgement is that of the state or USEPA Regional staff completing the survey

questionnaire. 

N/A Not available.

NR Although regional, state and/or territorial personnel reported the presence of the well type, the number of wells was not

reported, or the questionnaire was not returned.

* Backfill wells regulated by states primarily under other UIC categories are not included. For example, Kansas applies Class III

requirements (in addition to Class V requirements) to wells used to backfill solution-mined salt caverns.
1

 The particle size of mill tailings depends primarily on the beneficiation and processing techniques

employed.

2

 These terms may also be used to refer to materials other than mill tailings, such as materials used in

backfilling of underground coal mines.

3

 Material that reacts at ambient temperature with moisture to form a slow-hardening cement.

September 30, 1999 6

Backfill materials may also contain cementing agents and other additives, such as cement. 

These agents normally are added to increase the suitability of the material for providing structural

support. The use of a particular backfill material depends on its availability, cost, and properties after

placement (Karfakis, 1996). Backfill material needs to be physically, hydrologically, chemically, and

mineralogically stable, especially when subsidence control is one of the objectives of backfilling. To

provide long-term stability, fill material must resist infiltration and conductance of ground water, because

water migration can weaken backfills by promoting chemical reactions. Further, low permeability

reduces the potential for contaminants to leach into ground water (Jude, 1995).

The characteristics of the backfill materials most commonly injected into underground mines are

discussed below. The information presented is not an exhaustive compilation given the wide range of

materials and practices. Examples of site-specific operations are provided in Section 4.3.

4.1.1 Mill Tailings

Mill tailings typically consist of a finely ground mixture of processed ore, disaggregated host

rock, and traces of the solutions used (if any) in ore beneficiation operations.1 In some backfill

applications, mill tailings (with or without size classification) are slurried with water and injected into

underground mines in what is often referred to as a “hydraulic sandfill” or “sand backfill” operation (see

Section 4.3) (Levens, 1993; Sutter Gold Mining Company, 1998; Scheetz Mining Company, 1999).2

In other applications, mill tailings are mixed with cement or other pozzolanic material3 to form a

pumpable material with relative low (10 to 25 percent) moisture content this is often referred to as

paste backfill. When mixed with cement or a similar additive, the resulting backfill may also be referred

to as cemented sandfill. The fine solid particles may consist of naturally occurring metamorphic and

igneous clay-sized to sand-sized material and metamorphic rock fragments (Brackebusch, 1994).

Available data on the chemical composition of mill tailings sandfill slurry or backfill paste

injected into mines are limited. Leachate data for mill tailings, however, are available and are included. 

Table 2 provides information on tailings used as backfill at several facilities. As shown, these materials

may contain significant quantities of iron and trace metals.

The chemical characteristics of tailings used for backfill are determined primarily by the

characteristics of the ore body and host rock and to a lesser extent the extraction processes used. 

Thus, in many cases the chemical characteristics of the mill tailings injected into underground
September 30, 1999 7

Table 2. Chemical Characteristics of Selected Metal Mine Tailings Backfill

Constituent

Pb-Zn Mine Tailings (ppm)* Zn Mine (ppm)**

(1) (2) (3) (4) (5)

Aluminum 19,000 15,000 14,000

Arsenic <300 500 1,500

Barium 200 80 44

Cadmium 16 8 13 6.00 <0.006

Calcium 2,300 3,300 3,700

Chromium 90 80 100

Cobalt 10 20 30

Copper 210 250 620 33.0 <0.05

Iron 53,000 57,000 54,000

Lead 1,800 1,500 1,500 9.96 <0.04

Manganese 5,600 5,000 1,600

Magnesium 1,900 2,400 1,400

Mercury <0.05 <0.0002

Molybdenum <50 <50 <50

Nickel 200 330 240

Potassium 5,500 5,800 5,100

Silicon 334,000 348,000 358,000

Sodium 400 600 500

Zinc 4,300 2,500 5,800 1,100 0.654

 (1) Cemented tailings backfill collected from a test stope.

 (2) Uncemented tailings backfill collected from a stope about 10 years after placement.

 (3) Uncemented tailings backfill collected from new tailings.

 (4) Sample of solids from tailings impoundment.

 (5) Sample of water from tailings impoundment.

 * Source: Levens, 1996

 ** Source: ASARCO, 1998

mines are similar to the ore body before it was mined even though the physical characteristics (i.e.,

particle size) have changed. As shown in Table 3a, a variety of leaching tests have been performed to

evaluate the potential effect of the change in physical characteristics on the release of metals from

backfilled mill tailings at a lead and zinc mine. For the constituents analyzed, concentrations frequently

exceeded the primary drinking water standards (MCLs) or HALs in both of the acidic leaching tests. 

When leaching tests were performed using deionized water, only lead concentrations exceeded a

health-based standard. The type of leaching test used varies depending on the conditions anticipated in

the mine. Laboratory leaching data from gold mines that also backfill tailings as part of mining

operations are shown in Table 3b. (Information on mine water analyses from backfilled stopes and

other field monitoring of leachate quality is discussion in Section 5.) As shown, concentrations of

arsenic, barium, chromium, lead, and nickel exceed MCLs in USEPA
September 30, 1999 8

Table 3a. Chemical Characteristics of Leachates from Selected Mill Tailings

Constituent

Drinking Water

Standard

Health Advisory

Level Pb-Zn Mine Tailings Leachate Concentrations (mg/l)

mg/l

Primary

or

Secondary mg/l

Cancer

or

Noncance

r

HCl/HNO3* Deionized water** H2SO4***

(1) (2) (3) (4) (5) (6) (4) (5) (6)

Aluminum 0.05 to

0.2

S -- 2.1 2.0 1.9 0.2 2.2 0.2 54.0 0.44 80.1

Arsenic 0.05 P 0.002 C 1.2 6.0 10.4 -- -- -- -- -- --

Barium 2 P 2 N 0.3 0.03 0.02 0.17 0.08 0.06 0.01 0.003 ND

Boron -- 0.6 N -- 0.84 0.51 -- -- -- -- -- --

Cadmium 0.005 P 0.005 N 0.17 0.21 0.34 -- -- -- 0.01 0.32 1.8

Calcium -- -- 23.0 35.4 38.8 54.2 96.1 183 640 716 623

Chromium 0.1 P 0.1 N -- 0.40 0.25 -- -- -- -- -- --

Cobalt -- -- -- 0.13 0.17 -- -- -- -- -- --

Copper 1.3 P -- 0.69 0.56 0.69 0.04 0.09 ND 0.10 0.07 0.68

Iron 0.3 S -- 306 312 186 0.2 0.2 0.1 6.6 6.8 97.6

Lead 0.015 P -- 32.0 21.6 27.8 0.06 ND 0.02 0.90 1.6 3.9

Manganese 0.05 S -- 32.4 29.4 9.5 ND 0.1 ND 52.8 65.3 64.8

Magnesium -- -- 11.2 12.4 6.4 -- -- -- 95.2 125 110

Nickel 0.1 P 0.1 N -- 0.16 0.21 -- -- -- -- -- --

Potassium -- -- 0.9 0.9 0.8 7.3 15.5 26.6 3.9 1.4 3.0

Silicon -- – 2.2 2.2 1.7 6.6 6.7 8.5 14.6 34.7 157

Silver 0.1 S 0.1 N 0.04 0.02 0.03 -- -- -- -- -- --

Sodium -- -- 0.52 0.12 0.41 7.9 14.7 14.0 8.0 8.7 9.4

Sulfur -- -- 7.8 37 86 -- -- -- -- -- --

Sulfate 250 S -- -- -- -- 86.1 130 366 1,807 2,050 3,622

Zinc 2/5 P/S 2 N 21.9 14.9 35.4 0.06 ND 0.05 0.97 33.7 336

ND = Not detected.

* Overnight shaking of 1 g of backfill with mixture of HCl (2 cm3), HNO3 (4 cm3), and 20 cm3 water with filtering prior to analysis.

** Washing with deionized water for 7 days.

*** Washing with H2SO4 for 227 days.

(1) Cemented tailings backfill collected from a test stope.

(2) Uncemented tailings backfill collected from a stope about 10 years after placement.

(3) Uncemented tailings backfill collected from new tailings.

(4) Same tailings material as (1) with cement added in the laboratory.

(5) Same tailings material as (2) with cement added in the laboratory.

(6) Same tailings material as (3) with cement added in the laboratory.

Source: Levens, 1993; 1996
September 30, 1999 9

Table 3b. Chemical Characteristics of Laboratory Leachates from Tailings Backfill

in Underground Gold Mines (concentrations in mg/l)

Constituent Units Drinking Water

Standards

Health Advisory Levels (1) (2) (3) (4)

Aluminum mg/l 0.05-0.2 S -- -- -- -- --

Antimony mg/l 0.006 P 0.003 N -- -- -- <0.00976

Arsenic mg/l 0.05 P 0.002 C 0.030 0.025 0.06 0.0244

Barium mg/l 2 P 2 N 0.623 1.15 8.2 0.0415

Beryllium mg/l 0.004 P 0.0008 C -- -- -- <0.0021

Cadmium mg/l 0.005 P 0.005 N <0.001 0.0012 0.0011 <0.00275

Chromium mg/l 0.1 P 0.1 N 0.004 0.30 0.25 <0.0033

Cobalt mg/l – – -- -- -- <0.0025

Copper mg/l 1.3 P -- -- -- -- <0.0030

Gold mg/l -- -- -- -- --

Iron mg/l 0.3 S -- -- -- --

Lead mg/l 0.015 P -- 0.001 0.030 0.007 <0.00505

Mercury mg/l 0.002 P 0.002 N 0.0009 0.0003 0.0002 <0.00004

Molybdenum mg/l – 0.04 N -- -- -- <0.0043

Nickel mg/l 0.1 P 0.1 N -- 0.25 0.10 <0.00415

Selenium mg/l 0.05 P -- <0.005 0.002 0.002 <0.0108

Silver mg/l 0.1 S 0.1 N <0.001 0.0021 0.0022 <0.0030

Thallium mg/l 0.002 P 0.0005 N -- -- -- <0.0118

Vanadium mg/l -- -- -- -- -- <0.0052

Zinc mg/l 5 S 2 N -- 0.030 0.33 <0.00525

(1) TCLP extraction analysis of sand backfill from Homestake Mine

(2) EP extraction analysis of backfill tailings from Homestake Mine open cut and fill stope 20 years after backfill

(3) EP extraction analysis of backfill tailings from Homestake Mine stope 2 years after backfill

(4) Underground fill material from Sutter Gold Mine

Source: Scheetz, 1999; Righettini, 1999

Method 1310 Extraction Procedure (EP) leachate from sand backfill but not in TCLP leachate

(USEPA Method 1311).

Backfilling of tailings also occurs in association with non-metal mining activities. For example,

backfilling of tailings occurs at a soda ash and caustic production facility in Wyoming. Available data

on chemical characteristics of the injected tailings slurry indicate that pH and presumably the dissolved

solids content exceed secondary MCLs of 6.5 to 8.5 for pH and 500 mg/l for total dissolved solids, as

shown below (Tg Soda Ash, 1997).
September 30, 1999 10

Parameter

No. of

Samples Minimum Maximum Average Mean

pH 207 9.84 12.49 10.51 10.48

% Solids 207 0.00 29.58 14.66 15.16

4.1.2 Coal Combustion Ash

Coal combustion ash and cement (when needed) are mixed (mass ratio on the order of 9:1) and

slurried with water to produce a high-volume, low-strength fluid material that is used to fill mined-out

sections of underground mines. Gradual hardening of the slurry after injection will occur without

bleeding (Vlasak, 1993). In addition, this material can be used in related mine applications such as

construction of packwalls and filling of abandoned entries (Jude, 1995).

Coal combustion ash characteristics depend primarily on the characteristics of the coal burned

and the type of combustion technology utilized. For example, fly ash from conventional pulverized coal

combustion (PCFA) is a powder-like substance typically collected from flue gas exhaust ducts using

electrostatic precipitators or fabric filter units. PCFA derived from burning subbituminous coal and

lignite produced in the Western U.S. typically has a calcium oxide content greater than 10 percent (on a

weight basis), making it a self-hardening and pozzolanic material when in the presence of water. PCFA

derived from bituminous or anthracite coal produced in the Eastern U.S., on the other hand, generally

has a much lower calcium oxide content and, thus, requires the addition of either cement or lime and

water to achieve hardening properties (Jude, 1995).

Another type of fly ash results from fluidized bed combustion (FBC). This type of fly ash is

derived from crushed coal and limestone burned in a “bed” of ash particles suspended upward by

blowing air in the combustion chamber. FBC ash is made up of larger particles composed mainly of

coal mineral matter, calcium sulfate and unreacted lime that result from the sulfation and calcination of

the limestone (Jude, 1995).

Available data on the chemical composition of coal combustion ash slurries injected into mines

are limited. Data on coal ash (prior to slurrying and/or mixing with other materials) and leachate data

for coal ash are available and are included for reference, although the leachate characteristics of

mixtures of ash and cement or other materials may differ. Tables 4a through 4e summarize information

from selected studies that provide information on the chemical characteristics of coal combustion ash. 

As shown, trace metal composition varies over a wide range for each type of material (e.g., bottom

ash) and among types of materials. Table 4d further illustrates the variability by type of material and

type of coal based on data from a power plant in Kentucky. Table 4e provides a comparison of the

metal content of ash from a Pennsylvania facility to soil.
September 30, 1999 11

Table 4a. Coal Combustion Ash Characteristics from Selected Studies

Data Source Analyte Number of

Samples

Number of

Non-Detected

Values

Concentration (ppm)

Mean Minimum Maximum Median

Mechanical Arsenic n/a n/a n/a 3.3 160 25.2

Hopper Ash (a) Barium n/a n/a n/a 52 1152 872

Boron n/a n/a n/a 205 714 258

Cadmium n/a n/a n/a 0.40 14.3 4.27

Chromium n/a n/a n/a 83.3 305 172

Cobalt n/a n/a n/a 6.22 76.9 48.3

Copper n/a n/a n/a 42.0 326 130

Fluorine n/a n/a n/a 2.50 83.3 41.8

Lead n/a n/a n/a 5.2 101 13.0

Manganese n/a n/a n/a 123 430 191

Mercury n/a n/a n/a 0.008 3.00 0.073

Selenium n/a n/a n/a 0.13 11.8 5.52

Silver n/a n/a n/a 0.08 4.0 0.70

Strontium n/a n/a n/a 396 2430 931

Vanadium n/a n/a n/a 100 377 251

Zinc n/a n/a n/a 56.7 215 155

Fine Fly Ash (b) Arsenic n/a n/a n/a 2.3 279 56.7

Barium n/a n/a n/a 110 5400 991

Boron n/a n/a n/a 10.0 1300 371

Cadmium n/a n/a n/a 0.10 18.0 1.60

Chromium n/a n/a n/a 3.6 437 136

Cobalt n/a n/a n/a 4.90 79.0 35.9

Copper n/a n/a n/a 33.0 349 116

Fluorine n/a n/a n/a 0.40 320 29.0

Lead n/a n/a n/a 3.10 252 66.5

Manganese n/a n/a n/a 24.5 750 250

Mercury n/a n/a n/a 0.005 2.50 0.10

Selenium n/a n/a n/a 0.60 19.0 9.97

Silver n/a n/a n/a 0.04 8.0 0.501

Strontium n/a n/a n/a 30.0 3855 775

Vanadium n/a n/a n/a 11.9 570 248

Zinc n/a n/a n/a 14.0 2300 210

1993 Data (c) Antimony 46 35 10.5 0.2 205 4.6

Arsenic 81 3 76.4 0.0003 391.0 43.4

Barium 74 3 1589 0.02 10850 806.5

Beryllium 12 0 201.8 0.200 2105 5.0

Boron 27 0 469.5 2.98 2050 311

Cadmium 66 41 6.1 0.0100 76.0 3.4

Chromium 83 8 129 0.19 651 90

Copper 78 1 123 0.20 655 112

Lead 76 2 67.0 0.02 273 56.8

Mercury 27 7 4.3 0.013 49.5 0.1

Nickel 71 0 117.5 0.1 1270 77.6

Selenium 81 16 8.7 0.0003 49.5 7.7

Silver 62 42 3.7 0.01 49.5 3.2

Thallium 11 4 19.2 0.15 85.0 9.0

Vanadium 61 5 397 43.5 5015 252

Zinc 79 0 286.5 0.28 2200 148

Source: USEPA, 1993b (a) Mechanical hopper fly ash data from Tetra Tech’s 1983 Study and presented in the 1988 RTC.

(b) Fine fly ash data from Tetra Tech’s 1983 Study and presented in the 1988 RTC.

(c) Statistics calculated assuming that values below the detection are equal to 1⁄2 the detection limit.
September 30, 1999 12

Table 4b. Coal Combustion Ash Characteristics from Pennsylvania Facilities

Concentration (ppm)

Elements No. of Values Minimum Maximum Mean

Media

n

Major Elements

Aluminum 199 12 156000 24661 19160

Calcium 23 3 400000 59114 6200

Iron 200 8 130000 20872 13663

Magnesium 25 .5 3840 1501 1140

Manganese 191 .07 2980 153 70

Sulfate 189 4 10500 770 447

Trace Elements

Antimony 80 .01 142 35.6 28.5

Arsenic 195 .03 22320 271.24 17.05

Barium 109 .16 2960 303.5 194

Boron 144 .016 3995 160 40

Cadmium 91 .02 30 3.3 1.42

Chromium 201 .05 360 46.4 34

Cobalt 21 4.32 82.6 21.96 15

Copper 194 .04 474 48.09 32.5

Lead 179 .04 225 37.1 27.3

Mercury 134 .0003 5.44 .56 .4

Molybdenu

m

103 .23 108 20.4 16

Nickel 190 .015 753 44.8 22

Selenium 138 .0022 7540 63 3.4

Silver 73 .015 22 3.7 1.2

Zinc 199 .05 841 61.5 26

Total number of values in solids data set = 242.

 Source: Kim, 1997
September 30, 1999 13

Table 4c. Summary of Fluidized Bed Combustion Ash Composition

(concentrations in ppm)

Material Num.of Minimum 25th 50th 75th 90th 95th Maximum

Type Constituent Values Value Percentile Percentile Percentile Percentile Percentile Value

Bed Ash Aluminum 34 9.000 3825.00 10682.00 30446.00 56700.00 68800.00 104300.00

 Antimony 58 0.100 2.50 3.50 28.00 62.00 111.40 1775.00

 Arsenic 62 0.250 3.50 9.93 34.70 58.00 82.00 119.70

 Barium 68 0.050 7.00 62.15 172.00 274.00 316.10 453.00

 Beryllium 38 0.500 0.50 1.10 8.00 15.00 17.00 31.00

 Boron 52 0.050 1.50 3.15 22.74 41.38 118.00 304.00

 Cadmium 61 0.003 0.50 0.50 1.50 3.60 6.75 14.00

 Chromium 68 3.700 5.00 16.18 41.85 56.10 74.10 259.80

 Cobalt 47 0.125 1.40 3.90 14.00 37.90 51.40 128.40

 Copper 65 0.500 1.70 8.90 18.50 26.00 42.70 50.00

 Iron 33 6.200 9570.00 13010.00 15640.00 18534.00 21111.10 31500.00

 Lead 67 0.050 1.50 2.50 26.00 56.00 66.00 89.90

 Manganese 33 34.500 62.00 110.00 379.00 610.00 719.40 892.90

 Mercury 54 0.000 0.01 0.10 0.10 0.43 1.10 208.90

 Molybdenum 52 0.050 3.90 13.00 19.60 27.00 48.00 190.00

 Nickel 63 2.000 22.50 66.70 735.00 1000.00 1270.00 1440.00

 Potassium 41 1.300 100.00 150.00 240.00 1340.00 4700.00 11950.00

 Selenium 56 0.001 0.52 2.00 3.50 3.50 13.40 45.00

 Silver 55 0.005 0.50 0.50 1.00 5.00 7.00 338.00

 Thallium 29 0.250 2.50 3.50 5.00 20.00 25.00 50.00

 Vanadium 37 12.000 1150.00 3820.00 5700.00 7550.00 8700.00 10000.00

 Zinc 65 1.000 20.00 26.00 33.10 52.70 147.50 399.00

Fly Ash Aluminum 42 20.000 23495.60 32835.65 53415.00 88900.00 105920.00 176300.00

 Antimony 66 0.100 2.86 3.50 36.00 63.55 151.70 1370.00

 Arsenic 73 0.100 3.50 17.00 39.22 93.70 115.00 176.00

 Barium 73 0.100 17.00 177.00 320.33 540.00 940.00 7700.00

 Beryllium 39 0.500 0.50 1.20 6.00 11.00 15.00 16.00

 Boron 60 0.050 1.50 6.98 50.00 101.95 606.00 2473.00

 Cadmium 72 0.003 0.50 0.60 2.10 4.00 7.00 13.00

 Chromium 76 0.500 6.05 29.50 56.45 77.60 104.00 211.10

 Cobalt 47 0.125 2.00 5.00 19.00 33.90 75.30 178.50

 Copper 71 0.500 2.00 28.10 47.00 73.35 73.35 99.00

 Iron 46 22.170 18620.00 26530.00 32722.00 50900.00 55962.00 81318.00

 Lead 75 0.500 1.50 17.50 44.80 65.00 73.00 129.50

 Manganese 42 0.100 86.00 126.40 196.70 470.00 661.60 57700.00

 Mercury 73 0.000 0.10 0.31 0.95 1.68 7.35 384.20

 Molybdenum 67 0.050 3.10 9.00 21.10 28.50 48.64 143.60

 Nickel 75 12.500 32.80 51.20 529.00 825.00 900.00 1270.00

 Potassium 44 1.125 150.00 214.50 3132.00 8332.49 11478.80 14680.00

 Selenium 69 0.001 2.05 3.50 5.40 23.00 39.00 166.00

 Silver 64 0.005 0.50 0.50 2.00 3.40 5.00 38.50

 Thallium 34 0.500 2.50 3.50 5.00 20.00 25.00 39.01

 Vanadium 39 36.333 160.00 2880.00 3840.00 4830.00 5430.00 10000.00

 Zinc 73 1.000 28.00 36.00 54.50 79.77 114.40 167.90

Combined Ash Aluminum 48 1.090 14617.50 24585.00 32950.00 44300.00 64000.00 75850.00

 Antimony 45 0.003 0.50 10.00 26.00 43.87 51.70 142.00

 Arsenic 60 0.140 7.08 13.05 32.49 68.90 106.15 115.50

 Barium 57 0.100 120.00 180.00 253.00 457.70 650.00 690.00

 Beryllium 12 0.295 0.99 1.91 2.51 5.00 9.50 9.50

 Boron 45 0.904 14.40 21.10 31.95 45.00 49.00 1670.00

 Cadmium 50 0.000 0.25 0.69 1.34 3.49 5.00 7.00

 Chromium 58 8.000 19.30 34.50 47.30 53.70 56.00 1906.00

 Cobalt 30 1.200 2.84 4.60 8.00 9.80 12.54 18.70

 Copper 56 1.900 19.10 26.10 37.45 71.00 249.00 408.10

 Iron 48 850.000 8042.50 12765.00 18175.00 26600.00 28074.70 51600.00

 Lead 57 0.714 13.00 23.00 33.80 52.30 67.00 89.00

 Manganese 47 20.000 49.00 61.80 91.00 133.00 170.40 905.00

 Mercury 57 0.000 0.06 0.26 0.61 0.80 2.78 29.00

 Molybdenum 50 0.050 2.50 9.96 16.00 24.00 27.00 41.00

 Nickel 59 0.500 11.35 15.40 23.00 70.60 530.00 985.00

 Potassium 26 2.820 2950.00 4140.00 5400.00 6362.00 6600.00 9163.00

 Selenium 59 0.003 1.25 4.00 9.80 16.00 22.97 27.00

 Silver 48 0.005 0.35 0.75 1.70 2.45 5.00 21.80

 Thallium 8 0.180 1.88 5.19 18.55 25.00 25.00 25.00

 Vanadium 11 19.570 21.50 38.00 838.00 1700.00 5000.00 5000.00

 Zinc 57 6.100 14.40 19.90 26.00 48.10 257.00 90619.00

Source: Council of Industrial Boiler Owners (CIBO), 1997
September 30, 1999 14

Table 4d. Comparison of Coal and Ash Content for Selected Metals

at a Kentucky Power Plant

Element

High-sulfur Unit Low-sulfur Unit

Feed Coal

Mean*

(Whole

Coal)

Feed Coal

Mean*

(Ash Basis)

Fly Ash

Mean*

Botto

m

Ash

Mean*

Feed Coal

Mean*

(Whole

Coal)

Feed Coal

Mean*

(Ash Basis)

Fly Ash

Mean*

(Fine)

Fly Ash

Mean*

(Coarse)

Botto

m

Ash

Mean*

As 12 120 170 11 3.3 37 91 54 54

Be 1.5 15 19 14 2.4 27 27 22 16

Cd 0.4 3.6 5.5 0.8 0.1 0.8 1 0.8 --

Co 4.6 45 59 49 11 120 150 97 61

Cr 15 150 170 150 19 210 230 190 200

Hg 0.07 0.69 0.39 0.02 0.03 0.31 0.02 0.02 0.24

Mn 25 250 270 330 14 150 230 210 480

Ni 18 170 220 210 17 190 220 160 140

Pb 11 110 150 46 11 120 170 100 380

Sb 0.9 8.7 13 3.5 0.7 7.9 15 8.9 10

Se 2.5 26 8.9 0.59 5.6 52 1.1 0.82 1.7

Th 2 20 22 21 2.9 32 31 30 29

U 1.6 16 19 14 1.4 16 21 15 10

* Notes: 

All elements are in parts per million and are presented on the whole coal and as-determined basis for the feed coal, and on as-determined

basis for the fly ash and bottom ash.

Leaders (--) indicate statistics could not be calculated owing to an insufficient number of analyses above the lower detection limit.

Source: Affolter, 1997.
September 30, 1999 15

Table 4e. Comparison of Ash and Soil Composition for Selected Elements

at a Pennsylvania Power Plant

Constituents

Comparison of Ash to U.S. Soils Comparison of Ash to Local Soils

Ash

(mg/kg)*

Soil

Average

(ppmw)*

Soil

Range

(ppmw)*

Ash

(mg/kg)*

Residential

Soil

(mg/kg)*

(North)

Residential

Soil

(mg/kg)*

(South)

Aluminum 24100 66000 700 - >100,000 24100 12,000 15,500

Antimony 25 0.67 <1 - 8.8 25 54 <20.0

Arsenic 24 7.2 <0.1 - 97 24 1.9 2.6

Barium 1142 580 10 - 5,000 1142 32 71

Boron <10.0 34 <20 - 300 <10.0 <10.0 <10

Cadmium <0.5 0.06 0.01 - 0.7 <0/5 <0.5 <0.5

Chromium 28 54 1.0 - 2,000 28 11 13

Cobalt 7 10 <3 - 70 7 23 25

Copper 23 25 <1 - 700 23 32 34

Iron 9590 25,000 100 - >100,000 9590 32,000 33,700

Lead 25 19 <5 - 70 25 27 18

Manganese 38 660 <1 - 7,000 38 1,020 400

Mercury <0.02 0.089 <0.01 - 4.6 <0.02 <0.02 <0.02

Molybdenum <10 <3 - 7 <10 <10.0 <10

Nickel 14 19 <5 - 700 14 39 47

Potassium 5000 23,000 50 - 70,000 5000 670 460

Selenium 2.4 0.39 <0.1 - 4.3 2.4 <0.5 <0.5

Silver 1 1 <1.0 <1.0

Zinc 13.8 60 <5 - 2,900 13.8 80 110

* Notes:

C Ash analysis from Northampton Generating Plant (July 1996).

C U.S. soil analysis data based on results from a survey performed by the USGS team on native soils across the United States.

C Local soil analysis data from a local survey performed by Hawk Mtn. Labs in early August 1996. Samples were taken near the

housing development on the north side of the quarry and along Chestnut Street on the south side.

Source: Ramsey, 1999

Tables 5a, 5b, 5c and 5d provide similar data from laboratory leaching studies using EP,

TCLP, and Synthetic Precipitation Leaching Procedure (SPLP) methods (USEPA Methods 1310,

1311, and 1312, respectively). As shown, the leachate concentrations vary depending on the study

and leaching method used, but in general antimony, arsenic, beryllium, cadmium, lead, molybdenum,

selenium, and thallium typically (at least 50 percent of the time) exceed MCLs or HALs. Leachate

concentrations of barium, boron, chromium, copper, iron, mercury, nickel, silver, and zinc also exceed

MCLs or HALs in some cases. Because laboratory leaching data are not always predictive of leachate

chemistry in the field (Robl, 1999), data from field studies are discussed in Section 5.
September 30, 1999 16

Table 5a. Coal Combustion Ash Leachate Characteristics from Selected Studies

Data Source Analyte

Number

of Samples

Number of

Non-Detected

Values

Concentration (mg/l)*

Drinking Water

Standard

Health Advisory

Level

Mean Minimum Maximum Median mg/l

Primary

or

Secondary mg/l

Cancer or

Noncance

r

Tetra Tech

(a)

Arsenic n/a n/a 0.012 <0.004 1.46 n/a 0.05 P 0.002 C

Barium n/a n/a 0.222 0.003 7.6 n/a 2 P 2 N

Cadmium n/a n/a 0.0047 0.0001 1.4 n/a 0.005 P 0.005 N

Chromium n/a n/a 0.036 0.001 0.68 n/a 0.1 P 0.1 N

Lead n/a n/a 0.005 <0.0001 0.25 n/a 0.015 P --

Mercury n/a n/a 0.00042 <0.0001 0.007 n/a 0.002 P 0.002 N

Selenium n/a n/a 0.01 <0.0001 0.17 n/a 0.05 P --

Silver n/a n/a 0.00064 <0.0001 0.20 n/a 0.10 S 0.1 N

ADL (b) Arsenic n/a n/a 0.08 0.002 0.410 n/a 0.05 P 0.002 C

Barium n/a n/a 0.34 0.1 0.7 n/a 2 P 2 N

Cadmium n/a n/a 0.03 0.002 0.193 n/a 0.005 P 0.005 N

ChromiumVI n/a n/a 0.16 0.008 0.930 n/a 0.1 P --

Lead n/a n/a 0.01 0.003 0.036 n/a 0.015 P --

Mercury n/a n/a <0.002 <0.002 --- n/a 0.002 P 0.002 N

Selenium n/a n/a 0.05 0.002 0.340 n/a 0.05 P --

Silver n/a n/a <0.001 <0.001 --- n/a 0.10 S 0.1 N

1993 Data (c) Antimony 1 1 --- 0.0495 0.0495 --- 0.006 P 0.003 N

Arsenic 76 19 0.393 0.001 16.4 0.038 0.05 P 0.002 C

Barium 76 16 1.22 0.005 22.5 0.28 2 P 2 N

Beryllium 5 3 0.0187 0.001 0.0495 0.002 0.004 P 0.0008 C

Boron 8 0 4.01 0.126 17.1 0.955 --- 0.6 N

Cadmium 78 21 0.0342 0.0003 0.548 0.01 0.005 P 0.005 N

ChromiumVI 78 25 0.249 0.001 8.37 0.0405 0.1 P --

Copper 8 1 0.888 0.0036 6.3 0.17 1.3 P --

Lead 77 39 0.0968 0.008 1.83 0.01 0.015 P --

Mercury 74 67 0.0023 0.00004 0.0495 0.0007 0.002 P 0.002 N

Nickel 7 1 4.54 0.0495 29.4 0.45 0.1 P 0.1 N

Selenium 77 18 0.0698 0.0005 0.376 0.027 0.05 P --

Silver 75 59 0.0161 0.0001 0.520 0.005 0.10 S 0.1 N

Thallium 1 1 --- 0.0495 0.0495 --- 0.002 P 0.0005 N

Vanadium 14 3 4.47 0.005 26.9 0.665 --- --

Zinc 16 1 10.82 0.009 111.0 0.372 5 S 2 N

* All data were obtained using the Extraction Procedure (EP) Toxicity leaching procedure.

(a) Data from Tetra Tech's 1983 Study and presented in the 1988 RTC. Tetra Tech's results are for coal ash in general.

(b) Data from Arthur D. Little's 1985 Study and presented in the 1988 RTC.

(c) Statistics calculated assuming that values below the detection are equal to 1⁄2 the detection limit

---: data not available

Source: USEPA, 1993b
September 30, 1999 17

Table 5b. Coal Combustion Ash Leachate Characteristics from Selected Studies

Constituent

Drinking

Water

Standard

(mg/l)

Health Advisory Levels Kincaid Station

TCLP

10/18/94

(mg/l)

Coffen Station

TCLP

8/22/97

(mg/l)

Kincaid (50%)/Coffen (50%)

Fly Ash Leachate (mg/l)

mg/l

Cancer or

Noncancer

TCLP

3/31/98

TCLP

4/30/98

TCLP

5/7/98

TCLP

8/20/98

TCLP

11/17/98

Deionized

Water

4/30/98

Antimony 0.006 P 0.003 N -- 0.140 0.07 0.0332 0.03 0.06 0.09 --

Arsenic 0.05 P 0.002 C 0.40 0.075 0.049 0.0267 0.20 0.10 0.28 --

Barium 2 P 2 N 2 0.47 0.641 0.376 <1 <1 <1 --

Beryllium 0.004 P 0.0008 C -- 0.015 ND ND 0.019 <0.0002 <0.0001 --

Boron -- 0.6 N -- 34.2 51.68 92.1 49 35 23 --

Cadmium 0.005 P 0.005 N 0.30 0.163 0.168 ND 0.21 0.0005 0.0052 --

Chloride 250 S -- -- -- -- -- -- -- -- 3.4

Chromium 0.1 P 0.1 N 1.4 0.083 0.166 0.39 0.31 0.023 0.043 --

Cobalt -- -- -- 0.11 ND ND 0.11 0.007 0.004 --

Copper 1.3 P -- -- 0.43 ND ND 0.67 0.023 0.028 --

Cyanide, total 0.2 P 0.2 N -- -- -- -- -- -- -- 0.011

Fluoride 4 P -- -- -- -- -- -- -- -- 8.1

Iron 0.3 S -- -- 0.17 ND ND 10 0.02 0.09 --

Lead 0.015 P -- 0.4 0.04 ND ND 0.07 <0.01 <0.06 --

Manganese 0.05 S -- -- 0.88 ND ND 1.9 0.10 0.1 --

Mercury 0.002 P 0.002 N <0.0002 ND ND ND <0.0002 <0.0002 0.0022 --

Nickel 0.1 P 0.1 N -- 0.97 ND ND 0.80 0.090 0.094 --

Selenium 0.05 P -- 0.046 0.011 0.451 0.869 0.05 0.29 0.3 --

Silver 0.1 S 0.1 N <0.05 ND ND ND <0.005 <0.005 0.012 --

Sulfate 500 P -- -- ND -- -- -- -- -- 1210

Thallium 0.002 P 0.0005 N -- 0.021 ND ND <0.4 0.006 0.008 --

Zinc 5 S 2 N -- 3.21 0.045 0.177 3.1 <1 <1 --

Source: Crislip, 1999

Table 5c. Coal Combustion Ash Leachate Characteristics from Pennsylvania Facilities

Elements

Mean/Median Leachate Concentrations

(in mg/l, by method)

Drinking Water

Standard

Health Advisory

Level

ASTM(4)* EPTOX(10)* SPLP(20)* TCLP(200)* mg/l P/S* mg/l C/N*

Antimony BDL .15/.15 .13/.06 .28/.11 0.006 P 0.003 N

Arsenic 01/.008 .06/.02 .18/.09 .10/.03 0.05 P 0.002 C

Barium .40/.20 .20/.24 .29/.22 .40/.30 2 P 2 N

Boron BDL 1.09/1.02 2.1/.19 1.30/.50 -- 0.6 N

Cadmium .009/.009 .012/.004 .003/.003 .04/.02 0.005 P 0.005 N

Chromium .22/.22 .11/.02 .05/.06 .14/.08 0.1 P 0.1 N

Cobalt BDL BDL .01/.01 .22/.05 -- --

Copper .06/.05 .03/.01 .06/.04 .09/.05 1.3 P --

Lead .02/.02 .02/.02 BDL .17/.15 0.015 P --

Mercury BDL .001/.001 .001/.001 .01/.003 0.002 P 0.002 N

Molybdenum .02/.02 .10/.08 .39/.09 .23/.19 -- 0.04 N

Nickel .06/.04 .17/.15 .09/.08 .15/.12 0.1 P 0.1 N

Selenium .015/.015 .03/.05 .13/.11 .12/.05 0.05 P --

Silver BDL .001/.001 .003/.003 .03/.02 0.1 S 0.1 N

Zinc .18/.18 .35/.29 .04/.03 .65/.14 5 S 2 N

*( ) Indicates number of samples per method; P=primary; S=secondary; C=cancer; N=non-cancer.

BDL=Below Detection Limit

Source: Kim, 1997
September 30, 1999 18

Table 5d. Summary of Fluidized Bed Combustion Ash Leachate Test Results*

(concentrations in ppm)

Material Num.of Minimum 25th 50th 75th 90th 95th Maximum

Type Constituent Values Value Percentile Percentile Percentile Percentile Percentile Value

Bed Ash Aluminum 26 0.05000 0.13000 0.3250 1.6000 10.5000 13.140 20.600

 Antimony 26 0.00250 0.05000 0.3350 0.5000 0.7100 0.920 1.250

 Arsenic 69 0.00100 0.01000 0.0500 0.0500 0.1250 0.180 0.300

 Barium 67 0.02500 0.05000 0.2000 0.4520 0.9000 1.000 8.400

 Beryllium 11 0.00008 0.00500 0.0250 0.0500 0.0500 0.280 0.280

 Boron 23 0.00300 0.10000 0.1200 0.5500 2.6000 2.800 3.950

 Cadmium 63 0.00100 0.01600 0.0250 0.0300 0.0500 0.090 0.500

 Chromium 68 0.00500 0.02500 0.0250 0.0550 0.1770 0.220 0.320

 Cobalt 15 0.05000 0.12500 0.1400 0.1750 0.2500 0.310 0.310

 Copper 30 0.01000 0.02000 0.0495 0.0600 0.1340 0.158 0.184

 Iron 29 0.04000 0.09900 0.1900 0.5100 2.7900 3.200 38.800

 Lead 69 0.00500 0.02500 0.0500 0.2500 0.3600 0.418 0.710

 Manganese 28 0.00400 0.03350 0.0545 0.1900 0.7800 7.600 10.900

 Mercury 61 0.00010 0.00030 0.0010 0.0010 0.0014 0.010 0.100

 Molybdenum 23 0.05000 0.12500 0.1600 0.2400 0.6100 0.940 1.200

 Nickel 54 0.00500 0.02500 0.0500 0.1600 0.2360 0.250 2.500

 Potassium 13 0.12500 2.00000 5.6000 8.4000 11.0000 18.600 18.600

 Selenium 64 0.00050 0.00250 0.0500 0.0500 0.1000 0.134 2.500

 Silver 63 0.00150 0.02400 0.0250 0.0430 0.1000 0.125 0.310

 Thallium 7 0.00500 0.04500 0.0500 0.3250 0.5000 0.500 0.500

 Vanadium 32 0.02500 0.10500 0.3300 0.4550 1.6400 3.400 40.000

 Zinc 34 0.00250 0.02000 0.0650 0.1110 0.5100 1.040 4.460

Fly Ash Aluminum 35 0.04000 0.22000 0.5000 8.4500 23.9000 111.000 120.800

 Antimony 37 0.00250 0.03070 0.1000 0.5000 1.1700 1.290 1.520

 Arsenic 81 0.00050 0.01100 0.0500 0.0500 0.1190 0.250 0.600

 Barium 90 0.02500 0.08000 0.3000 0.6250 1.5500 6.500 42.000

 Beryllium 14 0.00008 0.00330 0.0105 0.0500 0.0500 0.050 0.050

 Boron 33 0.03000 0.10000 0.2800 0.6000 0.9800 1.400 23.317

 Cadmium 76 0.00100 0.02000 0.0250 0.0400 0.0600 0.100 0.500

 Chromium 83 0.00500 0.02500 0.0500 0.1200 0.2000 0.260 0.910

 Cobalt 18 0.00500 0.04500 0.0980 0.1370 0.2500 0.270 0.270

 Copper 39 0.00500 0.02000 0.0580 0.0850 0.1330 0.160 0.183

 Iron 38 0.01000 0.09000 0.1800 0.5000 0.7600 0.900 7.790

 Lead 80 0.00100 0.02500 0.0500 0.2685 0.4450 0.518 0.700

 Manganese 37 0.00250 0.03000 0.0500 0.3300 0.7300 1.100 1.130

 Mercury 76 0.00010 0.00025 0.0010 0.0010 0.0040 0.010 0.290

 Molybdenum 35 0.02320 0.07000 0.2000 0.3200 0.5900 0.610 0.720

 Nickel 65 0.00500 0.02500 0.0500 0.1600 0.2500 0.330 1.200

 Potassium 20 1.21000 4.63000 17.7000 39.3000 54.2500 63.400 66.800

 Selenium 81 0.00050 0.00700 0.0500 0.1000 0.2000 0.266 0.420

 Silver 74 0.00400 0.02000 0.0250 0.0400 0.0520 0.100 0.240

 Thallium 9 0.00500 0.04500 0.0500 0.0500 0.5000 0.500 0.500

 Vanadium 35 0.00750 0.09000 0.1300 0.2040 0.7000 1.640 3.200

 Zinc 42 0.00500 0.02000 0.0568 0.1400 0.3700 1.040 4.460

Combined Ash Aluminum 44 0.01000 0.53500 1.8650 4.0550 8.8900 10.700 18.670

 Antimony 42 0.00010 0.00500 0.0950 0.2700 0.5000 0.590 1.200

 Arsenic 62 0.00230 0.01000 0.0250 0.0500 0.2500 0.350 0.890

 Barium 60 0.00500 0.05600 0.1700 0.5950 1.1585 3.925 37.000

 Beryllium 6 0.00200 0.00200 0.0095 0.0500 7.8000 7.800 7.800

 Boron 43 0.00500 0.09000 0.1600 0.4600 0.6000 0.650 26.700

 Cadmium 51 0.00250 0.00250 0.0050 0.0130 0.0500 0.050 0.130

 Chromium 60 0.00330 0.02500 0.0500 0.1150 0.2450 0.280 0.600

 Cobalt 24 0.00070 0.00500 0.0180 0.0250 0.0315 0.250 0.400

 Copper 52 0.00250 0.01000 0.0225 0.0855 0.4400 1.860 6.100

 Iron 46 0.00005 0.01500 0.0700 0.1780 0.3100 0.360 2.045

 Lead 54 0.00100 0.02500 0.0500 0.1290 0.2500 0.430 1.540

 Manganese 47 0.00250 0.00500 0.0500 0.3100 0.4700 0.619 0.660

 Mercury 51 0.00010 0.00010 0.0002 0.0010 0.0020 0.100 0.100

 Molybdenum 46 0.02500 0.05000 0.0865 0.2000 0.4100 0.540 1.200

 Nickel 48 0.00500 0.02000 0.0250 0.0920 0.2640 0.420 0.900

 Potassium 23 1.55400 7.50000 14.5000 20.0000 24.0000 27.200 45.300

 Selenium 63 0.00100 0.00800 0.0200 0.0500 0.2400 0.256 0.350

 Silver 51 0.00250 0.00500 0.0050 0.0150 0.0400 0.130 0.250

 Thallium 5 0.00100 0.00100 0.0500 0.4600 0.5000 0.500 0.500

 Vanadium 6 0.00500 0.08599 0.1535 1.0000 2.2000 2.200 2.200

 Zinc 54 0.00250 0.00500 0.0215 0.1340 0.3000 0.480 2.400

* Leachate date obtained primarily using EP and TCLP procedures, with some SPLP and other methods also used.

Source: Council of Industrial Boiler Owners (CIBO), 1997
4

 Available data for the injectate are for dissolved rather than total concentrations, which may be

higher and, thus, in some cases could exceed MCLs or HALs. It seems unlikely that the total values

would be greater than MCLs in this case, however, because the total values measured for the injectate

were less than the relevant health-based benchmarks.

September 30, 1999 19

4.1.3 Flue Gas Desulfurization Sludge

Flue gas desulfurization (FGD) sludge is generated by flue gas scrubber units at electric power

plants. The material is primarily composed of anhydrite, sulfite, and small amounts of unreacted calcium

oxide and calcium carbonate (Jude, 1995). Exact constituent concentrations, including metals, vary with

the type of coal burned and the technologies used. When FGD is injected as backfill in underground mines,

it is usually mixed with fly ash and quicklime, slurried, and pumped down surface boreholes into abandoned

mine workings (Jude, 1995).

Available data on the chemical composition of FGD slurries that are injected into mines are limited. 

Table 6 presents chemical characteristics information for FGD sludge. Table 7a provides similar data from

EP toxicity testing of FGD sludge. As shown, median concentrations in the EP leachate exceed the MCL

or HAL for four constituents -- antimony, arsenic, boron, and thallium. Mean values exceed the relevant

MCL or HAL for these constituents plus five others (beryllium, cadmium, lead, mercury, and selenium),

while maximum values also exceed the applicable reference level for barium, chromium, nickel, and silver. 

The extent to which leachate from FGD sludge under field conditions (i.e., injected into an underground

mine) will be similar to these laboratory EP leaching test results will vary depending on a variety of factors,

such as pH.

Table 7b provides data on leachate from mixtures of FGD sludge and coal combustion ash

obtained with a modified version of the TCLP procedure (USEPA Method 1311) that used mine water

(also shown in Table 7b) for the leaching solution. As shown, concentrations of arsenic, boron, sulfate, and

total dissolved solids measured in leachate were above the levels in the mine water and above MCLs or

HALs. In contrast, concentrations of beryllium, iron, and manganese water in the mine water were above

the relevant benchmark but were reduced to levels below the relevant benchmark in the leachate.

4.1.4 Coal Cleaning Waste

Coal cleaning waste that results from the wet cleaning of raw coal is comprised of extremely fine

solids, including coal particles and coal associated minerals, suspended in water. At some mines, coal

cleaning wastes are injected into underground mine workings. The chemical composition of the injected

material depends primarily on the characteristics of the coal, the associated rock, and the quality of the

water used in the coal cleaning process. At the New Elk Mine in Colorado, for example, the injected slurry

is comprised of a slightly alkaline, sodium bicarbonate water and as much as 30 percent coal, shale, and

sandstone solids. Data shown in Table 8 indicate that coal cleaning wastes and injected slurry at this facility

do not exceed the relevant primary or secondary MCLs or HALs for the constituents tested, with the

exception of arsenic and TDS4 (USEPA, 1995a; Lopez, 1995). Table 8 also indicates, however, that coal

cleaning waste slurry and slurry leachate from the Kindall 3 mine in Indiana exceed the
September 30, 1999 20

Table 5e. Leachate Characterization Data for Fly Ash Grout Mixtures

Constituent

Leachate Concentration 1 (mg/l) Drinking Water

Standard

Health

Advisory

Level

Grout Mixture (% Cement/% Fly Ash/% Sand)

Curing Time (Days)

mg/l P/S* mg/l C/N*

5/45/50

(28)

5/55/40

(28)

5/65/30

(28)

5/75/20

(28)

5/85/10

(28)

7/55/38

(28)

7/65/28

(28)

7/75/18

(28)

7/85/8

(28)

5/95/0

(41)

7/93/0

(41)

9/91/0

(91)

Arsenic 0.004 0.009 0.014 0.017 0.019 <0.004 <0.004 <0.004 0.004 0.033 0.016 <0.004 0.05 P 0.002 C

Barium 0.860 0.898 0.731 0.832 0.834 1.040 1.250 1.340 1.090 0.632 0.767 1.140 2 P 2 N

Cadmium <0.0005 <0.0005 <0.0005 <0.000

5

<0.0005 <0.0005 <0.0005 <0.000

5

<0.0005 <0.0005 <0.0005 <0.0005 0.005 P 0.005 N

Chromium 0.022 0.033 0.033 0.033 0.028 0.019 0.017 0.020 0.022 0.027 0.031 0.019 0.1 P 0.1 N

Lead <0.001 <0.002 0.004 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 <0.002 0.015 P --

Mercury <0.0002 <0.0002 <0.0002 <0.000

2

<0.0002 <0.0002 <0.0002 <0.000

2

<0.0002 <0.0002 <0.0002 <0.0002 0.002 P 0.002 N

Selenium 0.017 0.020 0.013 0.020 0.020 0.006 0.008 0.010 0.014 0.051 0.035 0.009 0.05 P --

* P = primary; S = secondary; C = cancer; N = non-cancer

1

 Analyzed by the modified TCLP Method using deionized water as the extractant.

Source: Pappas, 1994

Table 6. FGD Sludge Characteristics from 1993 Study 

Analyte

Number of

Samples

Number of

NonDetected

Values

Concentration (ppm)

Mean Minimum Maximum Median

Antimony 31 25 15.8 3.65 90.0 6.0

Arsenic 36 5 53.6 0.0075 341.0 32.5

Barium 35 3 352.1 0.08 2280 162.5

Beryllium 14 7 27.7 0.900 49.5 29.3

Boron 18 11 144.8 5.00 633.0 60.0

Cadmium 36 22 19.2 0.005 81.9 3.9

Chromium VI 36 5 90.7 0.17 312.0 73.0

Copper 36 0 62.4 0.04 251.0 46.1

Lead 34 2 121.7 0.01 527.0 25.3

Mercury 15 7 5.2 0.073 39.0 4.8

Nickel 35 1 72.5 3.7 191.0 68.1

Selenium 34 9 12.1 0.0150 162.0 4.5

Silver 29 20 3.5 0.01 10.3 3.3

Thallium 6 6 9.0 9.00 9.0 9.0

Vanadium 33 16 104.9 0.01 302.0 65.0

Zinc 36 1 921.0 0.01 5070 90.9

Statistics calculated assuming that values below the reported detection limit equal 1⁄2 the

detection limit.

 Source: USEPA, 1993b
September 30, 1999 21

Table 7a. FGD Sludge Leachate Characteristics from Selected Studies*

Data

Source Analyte

Number of

Samples

Number

of NonDetected

Values

Concentration (mg/l)

Drinking Water

Standard Health Advisory Level

Mean Minimum Maximum Median mg/l

Primary or

Secondary (mg/l)

Cancer or

Noncancer

ADL (a) Arsenic n/a n/a 0.20 0.002 0.065 n/a 0.05 P 0.002 C

Barium n/a n/a 0.18 0.15 0.23 n/a 2 P 2 N

Cadmium n/a n/a 0.01 0.002 0.020 n/a 0.005 P 0.005 N

ChromiumVI n/a n/a 0.02 0.011 0.026 n/a 0.1 P 0.1 N

Lead n/a n/a 0.01 0.005 --- n/a 0.015 P --

Mercury n/a n/a <0.002 <0.002 --- n/a 0.002 P 0.002 N

Selenium n/a n/a 0.020 0.008 0.049 n/a 0.05 P --

Silver n/a n/a <0.001 <0.001 --- n/a 0.10 S 0.1 N

1993

Data (b)

Antimony 10 6 0.129 0.010 0.570 0.030 0.006 P 0.003 N

Arsenic 25 9 0.11 0.001 1.60 0.030 0.05 P 0.002 C

Barium 23 5 0.448 0.075 2.80 0.230 2 P 2 N

Beryllium 10 6 0.0013 0.0005 0.003 0.0005 0.004 P 0.0008 C

Boron 12 1 9.60 0.050 36.0 5.95 --- 0.6 N

Cadmium 25 17 0.066 0.0003 1.50 0.0025 0.005 P 0.005 N

ChromiumVI 23 8 0.075 0.0055 0.200 0.050 0.1 P 0.1 N

Copper 11 1 0.040 0.005 0.120 0.022 1.3 P --

Lead 22 19 0.056 0.0005 0.680 0.009 0.015 P --

Mercury 23 18 0.002 0.00005 0.013 0.0003 0.002 P 0.002 N

Nickel 11 3 0.043 0.0015 0.220 0.006 0.1 P 0.1 N

Selenium 25 9 0.051 0.0015 0.230 0.040 0.05 P --

Silver 22 10 0.037 0.0005 0.200 0.0195 0.10 S 0.1 N

Thallium 10 8 0.070 0.045 0.170 0.045 0.002 P 0.0005 N

Vanadium 11 0 0.126 0.030 0.270 0.074 --- --

Zinc 12 2 0.040 0.0015 0.172 0.007 5 S 2 N

* Leachate data based on Extraction Procedure (EP) testing.

(a) Data from Arthur D. Little's 1985 Study and presented in the 1988 RTC.

(b) Statistics calculated assuming that values below the detection are equal to 1⁄2 the detection limit.

---: data not available.

Source: USEPA, 1993b
September 30, 1999 22

Table 7b. FGD Sludge and Ash Mixture Leachate Characteristics Data 

Constituent

Drinking Water Standard Health Advisory Level

Mine Water (1)

(mg/l)

Modified TCLP Leachate (1) (mg/l)

mg/l

Primary or

Secondary mg/l

Cancer or

Noncancer Low Ash Grout * High Ash Grout **

Acidity -- -- 158 <1 <1

Alkalinity -- -- <1 77 79

Aluminum 0.05 to 0.2 S -- 2.88 0.89 1.12

Arsenic 0.05 P 0.002 C <0.004 0.017 0.018

Barium 2 P 2 N 0.021 0.148 0.196

Beryllium 0.004 P 0.0008 N 0.001 0.0003 0.0002

Boron -- 0.6 N 0.16 1.16 1.14

Cadmium 0.005 P 0.005 N <0.0005 <0.0005 <0.0005

Calcium -- -- 54.1 210 207

Chloride 250 S -- <1 65 63

Chromium 0.1 P 0.1 N 0.002 0.005 0.005

Cobalt -- -- 0.015 <0.002 <0.002

Copper 1.3 P -- 0.004 0.002 0.003

Cyanide 0.2 P 0.2 N <0.02 <0.02 <0.02

Iron 0.3 S -- 45.3 <0.01 0.01

Lead 0.015 P -- 0.003 <0.002 <0.002

Lithium -- -- <0.1 0.4 0.4

Magnesium -- -- 22.3 3.0 1.7

Manganese 0.05 S -- 3.65 <0.01 <0.01

Mercury 0.002 P 0.002 N <0.0002 <0.0002 <0.0002

Molybdenum -- 0.04 N <0.003 0.064 0.090

Nickel 0.1 P 0.1 N 0.039 <0.003 <0.003

Nitrate-Nitrite 10 P -- <0.05 <0.05 <0.05

pH 6.5 to 8.5 S -- 3.67 -- --

Phosphorus -- -- 0.04 <0.01 <0.01

Potassium -- -- 3.1 33.6 34.1

Selenium 0.05 P -- <0.005 0.008 0.011

Silicon -- -- 24.6 5.75 5.98

Silver 0.1 S 0.1 N 0.002 <0.0002 <0.0002

Sodium -- -- 23.0 35.4 35.8

Sulfate 500 P -- 429 557 541

TDS 500 S -- 626 977 959

Turbidity 5 NTU P -- 14.0 NTU – –

Zinc 5 S 2 N 0.073 0.012 0.015

 * TCLP text on low ash group (1.0:1.0 - Fly Ash: FGD sludge ratio by dry weight) using mine water as the leaching solution.

 ** TCLP test on high ash grout (1.25:1.0 - Fly Ash: FGD sludge ratio by dry weight) using mine water as the leaching solution.

 

(1) Roberts-Dawson Mine

Source: Whitlatch, 1998
September 30, 1999 23

Table 8. Coal Cleaning Waste and Injectate Characteristics

Constituent

Drinking Water

Standards

Health Advisory

Levels

New Elk Mine(1) Kindall 3 Mine(2)

Coal Cleaning Waste

 ( mg/l) Injectate Slurry

(dissolved 

conc. in mg/l)

 Coal Cleaning

Waste (total

conc. in mg/l)

Injectate

mg/l P/S* mg/l C/N* Total Dissolved Slurry (mg/l)

Aluminum 0.05 to 0.2 S -- -- <0.05 0.04 -- --

Antimony 0.006 P 0.003 N -- 0.002 -- -- --

Arsenic 0.05 P 0.002 C 0.006 0.004 0.001 <5 <0.050

Barium 2 P 2 N -- 0.6 0.02 0.4 0.76

Beryllium 0.004 P 0.0008 C -- <0.005 -- -- --

Boron -- 0.6 N -- 0.03 0.02 0.14 --

Cadmium 0.005 P 0.005 N <0.005 <0.005 0.0013 0.01 0.0062

Chloride 250 S -- 8 -- 72 11 --

Chromium 0.1 P 0.1 N <0.01 <0.01 -- <0.03 <0.0050

Copper 1.3 P -- <0.01 <0.01 0.01 0.02 --

Cyanide, free 0.2 P 0.2 N <0.1 -- -- -- --

Fluoride 4 P -- 2 -- 1.5 0.51 --

Hardness (CaCO3) -- -- 27 -- 49 994 --

Iron 0.3 S -- 0.35 0.08 0.03 0.02 ** --

Lead 0.015 P -- <0.02 <0.02 -- <0.05 0.058

Lithium -- -- -- 0.04 -- -- --

Manganese 0.05 S -- <0.01 <0.01 -- 0.46 --

Magnesium -- -- 3 -- -- -- --

Mercury 0.002 P 0.002 N <0.0002 <0.0002 -- <0.3 <0.00020

Molybdenum -- 0.04 N 0.02 -- -- <0.7 --

Nickel 0.1 P 0.1 N <0.02 <0.02 0.01 0.09 --

Nitrate as N 10 P -- 0.02 -- 0.38 -- --

Nitrite as N 1 P -- 0.02 -- 0.3 -- --

Nitrogen,

ammonia

-- -- 1.32 -- 0.16 -- --

pH 6.5 to 8.5 S -- 8.1 -- 8.2 7.83 7.5

Selenium 0.05 P -- <0.001 0.001 0.007 <5 <0.050

Silver 0.1 S 0.1 N <0.01 <0.01 -- <0.01 <0.0050

Sodium -- -- 518 -- 39 230 --

Sulfate 500 P -- 128 -- 130 1175 --

Thallium 0.002 P 0.0005 N -- <0.002 -- -- --

Thiocyanate -- -- <0.1 -- -- -- --

Total alkalinity -- -- -- -- 215 210 --

TDS 500 S -- 1280 -- -- -- --

Vanadium -- -- -- <0.01 -- -- --

Zinc 5 S 2 N 0.02 0.02 -- 0.1 --

(1) Source: Lopez, 1995

(2) Source: Endress, 1996

* P=primary; S=secondary; N=non-cancer, C=cancer 

** dissolved
5

 This approach might be used when the goal is to fill the mineshaft to prevent access rather than to

fill substantial portions of the mine workings.

September 30, 1999 24

primary MCLs for arsenic, cadmium, and lead and the secondary MCLs for TDS, sulfate, and manganese.

4.1.5 Mine Drainage Precipitate Waste

Mine drainage precipitate waste, or sludge, resulting from treatment (generally neutralization) of

AMD is composed of ferric oxide, gypsum, hydrated aluminum oxide, variable amounts of sulfates, calcium

salts, carbonates, bicarbonates and trace amounts of silica, phosphate, manganese, copper, and zinc

compounds (Smith, 1987). Additional metals (e.g., lead, arsenic, selenium) may be present depending on

the mineralogy of the coal and associated rocks of the drainage area.

4.2 Well Characteristics

Mine backfill materials are typically injected into underground mines through one or more drilled

wells or through a pipeline installed in the mine shaft and appropriate portions of the underground workings. 

In some situations, injection may be directly into a mineshaft without a pipeline for distributing the injected

material within the mine workings.5 The specific injection method(s) selected by a facility depends primarily

on the backfilling objectives and method (see Section 4.3) to be used. 

If drilled wells are used, the details of well construction (e.g., diameter, casing, cementing) are

determined by site-specific factors such as depth to the mine workings to be backfilled and the geology of

the overlying strata, as well as backfilling practices. If the mine workings to be backfilled are mostly

horizontal and relatively shallow (e.g., a few hundred feet or less below ground surface), backfilling may be

accomplished by injecting backfill material down a well until the well will not accept any additional material,

plugging the well, and then drilling and “filling” additional wells located at appropriate locations throughout

the mine workings. For example, the underground workings at the abandoned Roberts-Dawson

underground coal mine in Ohio were backfilled with 23,000 cubic yards of fly ash and FGD sludge injected

through 318 injection points (DOE, 1998a). Similarly, 227 injection points were used to backfill a 23-acre

portion of the Omega underground coal mine in West Virginia (DOE, 1998b).

In this type of backfill operation, each injection well is used for a relatively short period of time

(e.g., days). Such wells may have a casing run from the surface to the top of the mine workings (see Figure

1). Conductor pipe may also be used depending on the stability of near-surface rock and soil (Crislip,

1998). Alternatively, material may be injected down the drill pipe and a casing does not need to be

installed.
September 30, 1999 25

Source: Endress, 1996

Figure 1. Example of Shallow Mine Backfill Injection Well Construction
September 30, 1999 26

When the mine workings to be backfilled are deeper, fewer injection points may be needed but

more piping and related distribution equipment in the mine workings may be required. When deeper wells

are used, well construction may more typically involve the use of multiple casings, as illustrated in Figure 2. 

When pipelines are used, as illustrated in Figure 3, they are used to convey the backfill to the desired

location in the mine. Distribution of backfill material by piping within a mine is common when active mines

are backfilled, especially mines that use mining methods dependent on on-going backfilling of mined-out

stopes. 

4.3 Operational Practices

The operational practices for mine backfill wells vary depending on how the backfill material is

placed in the mine and the relationship between mining and backfilling activities. In general, operational

practices do not appear to make mine backfill wells particularly vulnerable to accidental contamination of

the injectate or to misuse, although monitoring of the injectate and ground water help minimize the potential

for misuse. In addition, backfill injection often occurs into zones already affected by prior activities (e.g.,

AMD formation following mining), sometimes with the primary objective of reducing existing contamination

problems.

4.3.1 Placement Methods

Injection of backfill into underground mines may be accomplished using hand, gravity, mechanical,

pneumatic, and hydraulic placement methods. The most popular methods are pneumatic and hydraulic

(Underground Injection Council Research Foundation, 1988). Hand and mechanical methods, such as belt

or sling packing machines, are restricted to construction of selected supports from within a mine.

Pneumatic Backfilling

In pneumatic backfilling operations, backfill material is transported into a mine through a well or

pipeline in a stream of continually flowing air, either in a vacuum or under pressure (see Figure 4). When a

“dense phase” approach is used, the pipeline is nearly filled with material that is moved as a fluid with low

velocity air pressure in slugs. When the more common “dilute phase” approach is used, an air/backfill

mixture typically consisting of less than 5 percent fill material is moved through a pipeline at relatively high

velocity as a fluid. Both approaches tend to be used where water is scarce, the mine is dry, or where water

would interfere with mining or the backfill process (Walker, 1993; Sand, 1990).

Hydraulic backfilling

Hydraulic backfilling, which is more common than pneumatic backfilling, is the practice of filling

mine voids with backfill material by washing or pumping the backfill material as a slurry through a well or

pipeline into the mine (see Figure 5). Hydraulic backfilling is normally accomplished by one of three

methods: controlled flushing, blind flushing, and pumped slurry injection.
September 30, 1999 27 Figure 2. Example of Deep Mine Backfill Injection Well Construction Source: Tg Soda Ash, 1997
September 30, 1999 28

Source: Rouse, 1979

Figure 3. Example of Sandfill Injection Using a Mine Shaft Pipeline
September 30, 1999 29

Source: Sands, 1990

Figure 4. Pneumatic Backfilling Schematics
September 30, 1999 30

Source: Sutter Gold Mining, 1998

Figure 5. Example Schematic of Stope Backfilling Using Hydraulic Sandfilling
September 30, 1999 31

Controlled flushing is used in mines where workers can safely enter and gain access to key areas

during the filling operations. When this approach is used, bulkheads may be built in mine passages around

the perimeter of the area to be filled. One or more wells are constructed and cased from the surface to the

upper portion of the mine workings to be filled. At the base of the vertical portion of the well, additional

piping may be used to aid in distributing the slurry into the mine workings. Horizontal dispersal from the

point of discharge ranges from 300 to 1,000 or more feet, depending on the vertical distance from the

ground surface to the mine opening and the solids concentration of the slurry. Because controlled flushing

provides relatively uniform distribution of backfill material, it generally provides better structural support

than the other methods and so is preferred where conditions permit (Whaite, 1975).

Blind flushing is used when the mine workings are inaccessible to workers. With this approach, a

slurry of backfill material is gravity fed through a well (either a drilled well or a mine shaft) into the mine until

the well will not accept any additional backfill material. The quantity that can be injected down a single well

depends on the conditions underground, such as the slope, height, and the proximity of pillars in the mine

workings. Usually, hundreds of injection points are required (Whaite, 1975).

Pumped slurry injection is similar to blind flushing except that the slurry is pumped down a well

rather than injected by gravity. With this approach, increased distribution of the fill material within the mine

can be achieved due to the increased velocity at which the slurry is injected. As shown in Figure 6, solid

particles settle out near the borehole when the slurry is first delivered and the velocity of the injected slurry

drops as it enters the mine workings. As more material is injected, the fluid velocity increases in the mine

workings and the solid materials are transported farther from the borehole (Whaite, 1975).

4.3.2 Integrated Mining and Backfilling

At some mines, backfill activities are closely integrated with mining activities. For example, in the

case of base metal and uranium mining, a common operational practice is to develop a mine using a cutand-fill method, illustrated in Figure 3. This method involves the following steps:

C a cross cut is driven from the main access shaft to the ore vein;

C a raise, later to be used as an ore pass for delivering the broken ore to the haulage level, is driven

upwards to intersect the vein;

C the raise is used as a platform to excavate a “slice” of ore to create a stope or excavated room

above the level of the raise;

C after each horizontal “slice” is cleaned of ore, the orepass is extended upward; and
September 30, 1999 32

Source: Whaite, 1975

Figure 6: Schematic Depiction of Pumped Slurry Backfill Injection

C the void is slurry backfilled and capped with cement which provides a floor from which the next

“slice” of ore, and process is repeated (Underground Injection Practices Council Research

Foundation, 1988).

Alternatively, an “underhand” cut-and-fill method involving a “top down” rather than “bottom up” sequence

of cut and fill, such that the ceiling rather than the floor of the stope is comprised of cemented backfill, is

used. An example is the Bulldog Mountain Mine in the Creede Mining District of Colorado. In both

“bottom up” and “underhand” operations, backfill material is normally delivered through a pipeline down the

mine shaft to the stope being filled.
September 30, 1999 33

4.3.3 Retroactive Backfilling

When backfilling is performed after mining is complete or largely complete, blind gravity or pumped

slurry injection are common approaches. As discussed above in Section 4.2, such applications may

involved hundreds of injection wells, each of which may be operated for only a few days, in an effort to get

thorough distribution of the fill material within the mine workings.

4.3.4 Well Maintenance and Closure

As mentioned, most backfill wells are used for short periods of time (e.g., days or weeks) and,

thus, little maintenance is required. When a backfill injection well is used on an on-going basis, periodic

integrity testing may be performed, as discussed in more detail in Section 6. When injection is through a

pipeline down a mine shaft, as would be typical at a site where backfilling is an integral part of mining

activity, maintenance would normally be a part of mine operations and would include inspection and repair

or replacement of piping as needed.

Available information provides few descriptions of well closure and abandonment practices. 

Where wells are used to backfill slurries that are similar in may respects to grouts (i.e., self-cementing), it

appears that the injection borehole is simply grouted to the surface. In some cases, cementing of the

borehole may occur either near the surface or from the injection zone to the surface.

5. POTENTIAL AND DOCUMENTED DAMAGE TO USDWs

5.1 Injectate Constituent Properties

The primary constituent properties of concern when assessing the potential for Class V mining,

sand, or other backfill wells to adversely affect USDWs are toxicity, persistence, and mobility. The toxicity

of a constituent is the potential of that contaminant to cause adverse health effects if consumed by humans. 

Appendix D to the Class V UIC Study provides information on the health effects associated with

contaminants found above MCLs or HALs in the injectate of mining, sand, or other backfill wells and other

Class V wells. Based on the information presented in Section 4, the following constituents that were found

to routinely or frequently exceed health-based standards in TCLP or other leachate from one or more types

of backfill material: antimony, arsenic, barium, beryllium, boron, cadmium, chromium, lead, mercury,

molybdenum, nickel, selenium, thallium, and zinc. Aluminum, copper, iron, manganese, TDS, sulfate, and

pH have been measured above secondary MCLs in TCLP or other leachate and are also discussed,

although these standards are designed to minimize aesthetic (taste) effects not adverse health effects (healthbased standards do not exist for these parameters).

Persistence is the ability of a chemical to remain unchanged in composition, chemical state, and

physical state over time. Appendix E to the Class V UIC Study presents published half-lives of common

constituents in fluids released in mining, sand, or other backfill wells and other Class V wells. All of the

values reported in Appendix E are for ground water. Caution is advised in interpreting these values

because ambient conditions have a significant impact on the persistence of both inorganic and organic
September 30, 1999 34

compounds. Appendix E also provides a discussion of mobility of certain constituents found in the injectate

of mining, sand, or other backfill wells and other Class V wells.

The persistence of constituents that leach from mine backfill following injection will depend on

complex solution-mineral equilibria that will be determined by site-specific conditions such as leachate and

ground water characteristics, host rock characteristics, and oxygen availability in the mine workings and the

surrounding formation. Because the point of injection for most backfill wells is typically within a permeable

unit, or into a zone where prior mining activity has created a preferential (as compared to adjacent

undisturbed formations) flow pathway, physical conditions are relative conducive to mobility. It should be

noted, however, that in some situations backfilling occurs under dry conditions, while in others the primary

objective of backfilling is to reduce the mobility of metals and other constituents in mine water by altering

the physical and chemical conditions in the mine.

For injected backfill, mobility of metals in the mine environment is primarily dependent on their

tendency to dissolve rather than remain in a solid form, which generally increases as pH decreases for most

metals. For iron, for example, the solubility decreases abruptly when pH increases above 6.5 due to the

oxidation of ferrous iron (Fe+2) to the much less soluble ferric iron (Fe+3), which readily precipitates as iron

oxide or iron hydroxide. This oxidation to ferric iron can also occur, with the resulting marked decrease in

solubility, under acidic conditions created by oxidation and hydrolysis reactions that occur when mine water

from a strongly reducing environment is exposed to an oxygen supply. In either case, the resulting decrease

in dissolved iron concentrations also reduces the concentrations of many other metals, notably arsenic and

selenium, that co-precipitate with iron and/or adsorb onto the iron oxides and hydroxides. Mine water is

frequently acidic due to the oxidation of sulfide minerals, with the result that the mobility of most metals is

generally relatively high absent measures that limit oxygen availability, such as backfill injection (Levens,

1996; EPRI, 1998; Freeze and Cherry, 1979; Robl, 1999).

Unlike most metals, the solubility of chromium in the +6 form is not especially dependent on pH. 

Chromium in the +3 form is much more common in coal mines, however, and shows decreasing solubility

with increasing pH. 

Some other constituents present in injected backfill and backfill leachate, such as boron and sulfate,

do not have solubilities controlled by pH. Most sulfate is likely to remain in solution although some

precipitation of sulfate may occur when enough calcium or magnesium is present. In addition, adsorption

may occur at very low pH values. Similarly, boron is also likely to be present in backfill leachate (EPRI,

1998).

As discussed in more detail below in Section 5.2, injection of backfill material often occurs at sites

where low pH water is present or in contact with the backfill injection zone. At these sites, mobility of most

metals present in the backfill will be greater than if injection occurred under neutral or alkaline pH

conditions. Nevertheless, backfill injection under these conditions can result in a decrease in total (injectate

plus in-situ sources) metal mobility in the mine for several reasons. First, some backfills can reduce water

flow rates through the mine. Second, backfill can reduce the oxidation of sulfides by reducing or eliminating
6

 Because the diffusion of oxygen in water is about four orders of magnitude less than in air, flooding

of a mine to eliminate direct air contact with pyrite or other sulfide minerals greatly reduces acid

generation and, thus, metal solubility and mobility (Sutter Gold Mining Company, 1998).

7

 Where ground water contamination has been identified at mining sites, it has not been clearly

attributable to backfilling.

8

 It should be noted, however, that backfill of tailings occurs in other contexts as well. For example,

at Solvay Soda Ash Joint Venture in Green River, Wyoming, tailings are injected along with processing

plant wastewater and fly ash from coal fired boilers. The injected tailings consist of shale breaks from

within the ore itself, calcium carbonate from the caustic soda plant, and low grade oil shale (Wyoming,

1996). The available data do not include specific studies related to ground water impacts from this or

other similarly “unique” situations and, thus, they are not discussed in this section.

9

 Total water production from 4,450 feet of mine workings ranges from 5 gallons per minute (gpm) in

summer to 20 gpm in winter, with most of this water entering the mine from the surface through

ventilation boreholes.

September 30, 1999 35

direct contact with air through sealing or flooding of the mine voids.6 Third, some backfill materials that are

alkaline in nature, such as coal ash and FGD sludge, can at least temporarily increase the pH of acidic mine

water present in the injection zone (Levens, 1996; Whitlatch, 1998; Kim, 1998).

5.2 Observed Impacts

None of the 23 states included in Table 1 indicated documented cases in which mine backfill wells

have caused contamination of a USDW.

7

 Studies have been conducted, however, by government,

industry, and universities that examine the effects of backfill injection on ground water quality, in part

because coal beds in some areas supply water to domestic wells (University of Kentucky, 1998).

This section summarizes studies of the effects of backfill injection on ground water quality. It is

organized in two parts. The first part discusses backfill in metal mines and the second discusses backfill in

coal mines.8

5.2.1 Metal Mines

The potential or observed impacts of underground mine backfilling on ground water quality have

been evaluated in several studies of metal mines. For the Lincoln Mine Project, a gold mine in California,

the leachability of backfill material was measured and evaluated in the context of the conditions that occur

naturally at the mine. Ground water at the site is limited, occurring in the weathered bedrock (generally not

deeper than 20 to 30 feet) and limited bedrock fractures.9 Because the gold deposit contains arsenopyrite

(FeAsS) (0.2 to 0.65 percent by weight of the ore), the focus of ground water quality investigations has

been on arsenic. Analysis of samples from bedrock ground water monitoring wells over a four year period

shows naturally occurring arsenic concentrations that range from 0.0014 to 0.185 mg/l, with well-specific

averages ranging from 0.008 to 0.063 mg/l for the six wells examined (Sutter Gold Mining Company,

1998). Maximum concentrations in three wells and the average concentration in one well exceed the MCL
September 30, 1999 36

of 0.05 mg/l. The minimum concentration in five of the six wells exceeds the HAL of 0.002 mg/l. In

addition, assessment of the acid formation potential of the backfill material indicates that it was quite low,

with an acid neutralization/acid generation potential ratio of 98:1, due to the presence of carbonate minerals

that yield a mine water pH of 8.3+. Deionized water leaching of the backfill (sandfill) material showed an

arsenic concentration of 0.13 mg/l compared to a reported average concentration of 0.2 mg/l for ground

water in the ore zone (Sutter Gold Mining Company, 1998). Thus, it appears that the potential for release

of arsenic from backfill material to degrade ground water is low. In addition, suitability of ground water for

use as drinking water is low due to limited availability and naturally occurring arsenic.

The U.S. Bureau of Mines (USBM) also examined the impact of mine backfill material on ground

water quality. At a moderately deep underground lead-zinc mine located in the Coeur d’Alene Mining

District of northern Idaho, samples of ground water both before and after contact with a sandfilled stope

showed an increase in electrical conductance. Increased concentrations of Ca, Mg, SO4

-2, and HCO3-

account for most of the increase. Sulfate levels increased from levels slightly below the MCL of 500 mg/l

to levels that ranged from 797 to 1,171 mg/l. Changes in other metal concentrations were generally mixed,

with relatively small increases observed for some sampling events and decreases observed for others. Zinc

was an exception, showing consistently higher levels after contact with the sandfill (at levels consistently less

than the non-cancer lifetime HAL of 2 mg/l). Lead and arsenic levels were present at levels above the

MCL before and after contact with the sandfill. For all of the metals, the levels observed both before and

after contact with backfill material were well below the maximum leachability values measured in the

laboratory, as shown above in Tables 2 and 3 (Levens, 1993).

In a related investigation, USBM also examines the effect of cemented sandfill on ground water by

analyzing samples collected both before and after contact with the backfill. In this investigation, samples

from exploratory boreholes represent water quality within the native rock, whereas samples from a sump

and seeps that contact the backfill within the mine represent water quality affected by backfilling. 

Comparison with the uncemented backfill examined above showed a much greater acid neutralization

capacity. This is consistent with the higher pH values observed after contact with the cemented backfill

(6.5 to 9.3) as compared to the pH measured in the native rock boreholes (6.29 to 7.98). 

Concentrations of Ca, K, Mg, and SO4

-2 are also higher after backfill contact, with SO4-2

concentrations increasing to levels that in most cases exceeded the primary MCL. Concentrations of zinc

and lead also increased. Zinc levels remained well below the non-cancer lifetime HAL of 2 mg/l. Lead

levels, in contrast, were typically several times the MCL action level of 0.015 mg/l before contact with the

backfill. After backfill contact, levels were sometimes increased and sometimes decreased, but generally

were above the MCL. Secondary MCLs were also exceeded both before and after contact with backfill

for Iron and Mn. Iron concentrations were generally lower after backfill contact while Mn concentrations

showed both increases and decreases following backfill contact (Levens, 1996).

Notable increases in the concentrations of Ca, K, and SO4

-2 were also observed in the laboratory

for three cemented backfill samples after washing with deionized water (see Table 3). Further laboratory

exposure (for 227 days) to a sulfuric acid wash showed additional increases in Ca, SO4

-2, and metal

concentrations. Concentrations of most constituents measured in the laboratory were notably higher than
10 Mine water pH at the mines examined was 7 or greater, so the results indicated by these sites may

not apply to mines with acidic water (Smith, 1987).

September 30, 1999 37

those measured in the field following contact with backfill. Notable exceptions are Ba, K, and Na, which

showed lower concentrations in the laboratory than were observed in the field. Cadmium, lead, and SO4

-2

concentrations exceeded primary MCLs while concentrations of iron, manganese, silver, and zinc exceeded

secondary MCLs in the laboratory leaching tests (Levens, 1996).

Observed differences in leachate concentrations appear to be due to differences in the particle-size

distribution of the backfill (tailings) material, with generally lower concentrations for backfill containing a

wide size distribution. In addition, the presence of sulfide minerals and Ca and Mg carbonates in the

backfill appeared to be important in determining the acid neutralizing capacity of the cemented material and,

thus, the release of some metals. The data also indicate that backfill cementing may help reduce metal

release and that ground water, to which releases occur, may contain metals at concentrations above MCLs

(Levens, 1996).

At the Homestake mine in South Dakota, samples were collected from drainage from the sand

backfill that was placed in stopes. Results indicate that the pH fluctuates around 8.0. As shown in Table 9,

concentrations of arsenic, iron, and nickel in exceed MCLs or HALs in some samples of drainage from a

backfilled stope. Arsenic and iron concentrations also exceed MCLs in mine water collected at a variety of

locations in the mine (Scheetz, 1999).

Backfill injection has also been widely used in uranium mines. Field sampling of tailings and backfill,

and studies of water discharging from where backfill was used suggest that short- and long-term effects on

ground water quality are negligible both during and following completion of mining (Levens, 1996).

5.2.2 Coal Mines

A study conducted in 1987 assessed the injection of coal slurry wastes from coal preparation and

sludge from treatment of AMD into underground coal mines in West Virginia. The study examined water

quality using samples from 9 mines that had received injection of slurry or AMD treatment sludge. Slurry

injection (at 6 mines) was found to improve the already degraded water quality by increasing alkalinity and

pH10, and decreasing concentration of iron and manganese. Sulfate concentrations also increased,

however. Only minor changes in trace
September 30, 1999 38

Table 9. Backfill Drainage At Homestake Mine, South Dakota

Constitue

nt Units

Drinking Water

Standards

Health Advisory

Levels (1)

(2)

(3)

(4)

a b c d a b c

Aluminum mg/l 0.05-0.2 S -- -- 0.229 0.346 0.649 0.309 -- -- -- --

Arsenic mg/l 0.05 P 0.002 C 0.063 0.032 0.019 0.028 0.056 <0.005 0.001 0.001 0.003

Cadmium mg/l 0.005 P 0.005 N <0.001 -- -- -- -- <0.001 0.001 0.001 0.001

Chromium mg/l 0.1 P 0.1 N <0.001 -- -- -- -- -- <0.001 NF <0.001

Copper mg/l 1.3 P -- 0.006 0.037 0.015 0.018 0.020 <0.005 0.01 0.092 0.035

Gold mg/l -- -- <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 0.001 0.002

Iron mg/l 0.3 S -- 0.59 -- -- -- -- 0.348 0.06 0.08 0.43

Lead mg/l 0.015 P -- <0.001 -- -- -- -- <0.001 0.005 NF 0.01

Mercury mg/l 0.002 P 0.002 N -- -- -- -- -- <0.0002 0.0 0.0 0.0

Nickel mg/l 0.1 P 0.1 N 0.148 0.010 0.005 0.005 0.005 0.008 NF NF NF

Selenium mg/l 0.05 P -- -- <0.005 <0.005 <0.005 <0.005 <0.005 -- -- --

Zinc mg/l 5 S 2 N 0.016 0.054 0.056 0.061 0.055 <0.050 NF NF 0.016

CN Total mg/l -- -- 13.71 -- -- -- -- -- -- -- --

CN WAD mg/l -- -- 0.40 -- -- -- -- -- -- -- --

NF = not found (not detected)

(1) Water issuing from sand backfill on the 7,100 foot level of the mine

(2) Water from selected underground sumps (labeled a-d) in the mine

(3) A combination of surface water and ground water from the upper underground levels of the mine

(4) Three samples (labeled a-c) of mine water (stope drainage) prior to treatment

Source: Scheetz, 1999

element concentrations were apparent following injection, and these changes do not appear to be threats to

drinking water (Smith, 1987). 

Sludge injection (at 3 mines) appeared to increase alkalinity, pH, sulfate, and total suspended solids

(TSS). However, pH and alkalinity decreased when injected sludge had a much lower pH than the native

mine water. Sludge injection into mines with high (> 7) pH water resulted in lower iron and manganese

concentrations, except where the concentration of these elements were very low (<0.1 mg/l) prior to

injection. Injection into mines with low pH resulted in a great increase of iron concentrations, presumably

as a result of dissolution from the sludge. Most changes in trace element concentrations were negligible,

with the possible exception of arsenic, which showed a large apparent increase at one mine. However,

concern about potential analytical error complicate assessment of this result (Smith, 1987). 

A recent study also examined the impact of injection of a grouting material composed of fly ash,

FBC, and FGD sludge into an abandoned underground mine near Friendsville, Maryland. At this site,

ground water (AMD) seeping from the mine was a known cause of surface water quality degradation, and

injection was initiated with the intention of improving both ground and surface water quality. Seepage water

quality measured before injection indicated Fe and Mn concentrations above secondary MCLs, SO4

-2

concentrations generally above the primary MCLs, and Zn concentrations sometimes above the HAL. 
11 About 5,600 cubic yards of fly ash, FBC ash, and FGD sludge were injected as a slurry via 38

boreholes in October and November 1996.

12 Water to make the slurry was pumped from the main mine pool, which temporarily lowered the

level by about 2 feet.

13 Ground water is being monitored in the overlying Freeport Sandstone, the Middle Kitanning No. 6

coal seam, and the underlying Clarion Sandstone. Two perched aquifers exist: one in the Freeport

Sandstone, caused by the underlying claystone and siltstone which immediately overlies the No. 6 coal;

(continued...)

September 30, 1999 39

0

50

100

150

200

250

300

350

400

CALCIUM, mg/L 

0

500

1000

1500

2000

2500

SULFATE, mg/L

Calcium

Sulfate

0

5

10

15

20

25

30

35

1/6/95 7/25/95 2/10/96 8/28/96 3/16/97 10/2/97 4/20/98

CALCIUM LOAD, kg/day 

0

20

40

60

80

100

120

140

160

180

SULFATE LOAD, kg/day

Grout 

Injection

 Period

Figure 7. Calcium and Sulfate Concentrations

and Loads at an Abandoned Maryland Coal

Mine (from Aljoe, 1999)

Immediately after injection11, acidity of the AMD

increased markedly along with dissolved iron and

aluminum concentrations, and then decreased to preinjection levels by the following summer. Ca and

SO4

-2 concentrations also increased in the AMD

following injection and remain high (as of April 1998),

apparently due to dissolution of these materials from

the injected grout (see Figure 7) (Aljoe, 1999).

The initial increase in acidity of the AMD

following grouting most likely resulted from changes in

mine pool hydrology, including a drop in mine pool

elevation that occurred when water was pumped from

the mine to prepare the grout.12 This drop in

elevation would have exposed an estimated 10,500

cubic feet of highly-fractured, previously-submerged

material within the mine to atmospheric oxygen. 

Another change could have been re-routing of flow

through new areas of the mine workings, thereby

mobilizing acidic products that had previously been

stored in stagnant zones within the mine. The

intention of the backfilling effort had been to entirely fill the mine voids with grout, thereby isolating the

pyritic surfaces from air and water and reducing the AMD production rate. This did not occur, however,

because limited information available on the size of the mine workings led to an underestimate of the

quantity of grout needed (Aljoe, 1999).

In a similar project initiated in 1997, grout comprised of a 1.25:1.00 (by dry weight) mixture of fly

ash and FGD filter cake with 5 percent added lime was injected through 318 drilled grout holes into an

abandoned underground coal mine near Conesville, OH. The objective of the injection project was to

inject grout that would seal old mine entries and coat the floor and walls of the abandoned mine chambers,

thereby reducing the amount of oxygen available and slowing the process of acid formation. Because

grouting was only completed in early 1998, it is too early to determine the net effect on water quality, which

will be monitored for three additional years.13 For all surface and ground water monitoring locations
13 (...continued)

and one in the No. 6 coal caused by claystone, siltstone and limestone layers between it and the

underlying Clarion Sandstone. The lowest and most extensive aquifer is in the Clarion Sandstone and

intersects major hydrologic boundaries, such as Wills Creek.

September 30, 1999 40

(through September 1998), no measurable increase in arsenic, chromium, boron, or pH levels have been

observed since grout injection (Whitlatch, 1998).

Laboratory studies of the reaction between the grout and AMD showed a significant increase in

solution pH to approximately 8 and in the concentration of a number of ions, including arsenic and boron. 

Only 1.32 percent of the arsenic present in the grout was released over the 168-day reaction time and the

resulting arsenic levels were below the primary MCL. Because further solubilization could result in higher

concentrations, both arsenic and boron will receive particular attention in the on-going monitoring program

(Whitlatch, 1998).

The reason for the apparent difference in the reaction between the grout and AMD in the

laboratory and the field is not yet clear, but it appears that conditions within the mine are unfavorable for the

development of acid neutralization reactions. This may be due to the fact that longer contact times are

required than are achieved in the mine. It should be noted, however, that the project is designed to achieve

AMD reduction through successful sealing of the mine and does not rely on acid neutralization by the grout.

Observations to date show that the water level has risen in the mine, indicating that the grout was of

sufficient strength to provide effective seals. In-situ core samples taken after about nine months indicate

that the grout is highly impermeable and has weathered very little (Whitlatch, 1998).

At sites in Greene and Clinton counties in Pennsylvania and Upshur county in West Virginia,

injection of mixtures of fly ash, FBC ash, lime, AMD treatment sludge, and/or cement with water has been

used in attempt to reduce AMD from reclaimed surface mined areas as well. At these sites, grout was

injected at relatively shallow (< 30 feet) depths in an attempt to fill voids in the spoil material, thereby

reducing contact between the buried pyrite and air and water (Kim, 1998).

At the Greene county site, the pH in wells in the injection area increased by 0.5 pH units and the

pH of the seep (discharge from the spoil area) increased slightly from 3.2 to 3.3 following grouting. 

Acidity was unchanged in the injection area but decreased in the seep. In the injection area and the seep,

little difference in trace metal concentrations was observed before and after grouting (Kim, 1998).

At the Clinton county site, pH increased and acidity decreased in samples from the injection area

and discharges. The pH remained less than 3, however. Average trace metal data from about two years

after injection indicate higher concentrations of As, Co, Cu, Ni, and Zn in the injection area, but

concentrations in the discharge were closer to the discharge levels from areas without injection (Kim,

1998).

At the Upshur county site, the average pH of the water in the injection area decreased initially after

injection, but had increased when the water was sampled five years later. This increase, however, could
14 Recent experimental work indicates injection of alkaline FBC ash in dilute slurry form may be

more effective in reducing AMD than ash injected in the form of low-permeability grout. In some

situations, the benefits from increased neutralization may outweigh the potential increase in the release of

trace metals from the ash (Canty, 1999). 

September 30, 1999 41

be due to the similar increases observed in the inflows and the ungrouted area. Acidity decreased in the

injection area, the inflows, and outflows immediately following injection. Five years after injection, acidity

had decreased further in the injection area and increased in the discharger, although it remained below the

pre-injection level. Also, concentrations of Ba, Be, Cd, Co, Cr, Cu, Ni, Pb, Sb, and Zn were generally

higher in the injection area and discharge than the inflow or control areas, but were still less than primary

and secondary MCLs (Kim, 1998).

6. ALTERNATIVE AND BEST MANAGEMENT PRACTICES

A number of best management practices (BMPs) can be implemented to provide increased benefits

and protection of USDWs with mine backfill wells. As discussed above, the effect of mine backfill

operations on ground water quality depends to a large extent on the characterisitics of the backfill and the

mine, and the interaction between the two. Thus, selecting the appropriate backfill materials for a mine and

selecting appropriate BMPs require characterization of the the backfill materials, including the potential to

cause AMD, and an understanding of where the backfill will be placed (especially with respect to the water

table) and how the backfill is expected to react over time in this environment.

BMPs are discussed below in relation to injectate characteristics, design and construction,

operation, and closure. The discussion is neither exhaustive nor represents an USEPA preference for the

stated BMPs. Each state, USEPA Region, and federal agency may require certain BMPs to be installed

and maintained based on that organization’s priorities and site-specific considerations.

6.1 Injectate Characteristics

Some injected backfill materials have cement-like properties that cause them to harden following

injection. Although the importance of the cementing properties of the injected backfill will vary with site

conditions and the objectives, cement-like characteristics are generally, but not always, desirable.14

Cementing properties are intrinsic to some backfilled materials and can be created in all materials commonly

used for backfilling through the use of appropriate mixtures and additives. Cementing properties provide

both increased structural support (where backfill is provided to control subsidence) and reduced

permeability. Reduced permeability generally serves to reduce dissolution of constituents from the injected

material. In addition, flow rates through the backfilled zone are reduced, thereby reducing the availability of

oxygen and the release of constituents from the mine surfaces as well as the backfill material.

6.2 System Design and Construction

As discussed above, backfill injection is sometimes used with the intention of preventing or abating

AMD. Although injection is intended to increase pH and reduce acidity, the acidic conditions present in
15 An expandable plug used in a well’s casing to isolate producing zones; also to isolate a section of

the borehole to be filled with cement when a well is plugged.

16 An instrument for measuring the inside diameter of a well.

September 30, 1999 42

these applications will lead to the dissolution of some constituents from the injected backfill. This

dissolution can be reduced, however, by ensuring that the mine voids are filled as completely as possible

with the injected material, thereby reducing the availability of oxygen and the potential for additional acid

formation. To achieve this objective requires adequate distribution of the backfill within the mine workings,

which in turn requires appropriate quantities of injection material, appropriate well spacings, and injection

pressures that will distribute the material and effectively fill the mine voids. In addition, bonding agents

and/or materials to reduce the porosity of the injected backfill can help to reduce the residual void space

remaining after backfill injection. These considerations warrant special attention when injection occurs in

inactive mines where access and knowledge of the size and geometry of the mine workings are limited.

BMPs for well construction may vary significantly with site conditions and the type of backfill

injected. Backfill injection often occurs in abandoned mines or other settings where injection is performed

in an effort to improve existing poor ground water quality. In these situations, injection through uncased

boreholes may be appropriate if the wells are relatively shallow, the strata have sufficient integrity, the

injectate is grout-like in nature, and injection only occurs for a short period, such as a few days. At other

injection sites, however, casing is clearly needed to ensure that the injected material reaches the intended

formation. This is particularly true when the injection well is relatively deep, operates on an on-going basis,

passes through a USDW, and/or injects a low solids content fluid. In some cases, the site setting or the

nature of the injected material may make the use of tubing appropriate.

6.3 Well Operation

For backfill wells that operate on an on-going basis, which most often occurs in association with

backfilling that is integrated with mining operations, BMPs include mechanical integrity tests (MIT) before

the well is put into service and periodically during use. A variety of MITs may be run on backfill wells to

check casing integrity. For example, pressure tests may be run prior to initial well operation and

subsequently at periodic intervals, generally ranging from one to five years. Because mine backfill injection

is normally done without a tubing string, pressure testing requires that a temporary, retrievable bridge plug15

be set near the bottom of the casing. This retrievable plug approach is used at least at one site in Wyoming. 

The appropriate pressure for a pressure test depends on anticipated well operating conditions. At the

Wyoming facility, testing at a pressure 10 percent greater than the maximum pressure reached during the

previous year (or maximum of 200 psi) is required (State of Wyoming 1988, 1996).

Due to the relatively high solids content of injected backfill as compared to other types of injected

fluids, abrasion can threaten casing integrity. Thus, caliper logs16 may be run periodically on the entire

length of the surface. A casing log run prior to well use provides a baseline against which subsequent logs

can be compared. In Wyoming, casing logs are repeated at nine month intervals unless the results show
September 30, 1999 43

more than 20 percent reduction in the wall thickness of the casing, in which case the log is repeated every

six months (Wyoming 1988, 1996).

Other examples of MIT used in backfilling operations include Multifrequency Electromagnetic

Thickness (MET) logs and cement bond logs. MET has been used by Tg Soda Ash, Inc. during the

operation of an underground tailings disposal and mine backfilling system to evaluate corrosion and metal

loss (Tg Soda Ash, 1997). Cement bond logs (and caliper logs) have been used in the Big Island Trona

Mine to evaluate the integrity of cement to pipe and cement to formation bonding (Wickersham, 1995).

6.4 Well Closure

Appropriate well abandonment is important to ensure that the well does not provide a pathway

through which contamination of USDWs could occur. As with well construction and operation,

appropriate closure practices depend onsite conditions. In some situations, abandonment may be as simple

as allowing the injected material to fill the well bore after the mine stops accepting additional injected

material. This approach is most likely to be appropriate for shallow wells used for short periods to inject

self-cementing grouting materials into closed or abandoned mines.

In other situations, protection of USDWs may require plugging and abandonment using more

“conventional” cementing techniques. For example, in Wyoming abandonment includes setting a cast iron

bridge plug approximately 50 feet above the bottom of the casing. Cement is placed in the casing by

pumping through a tubing string in stages as the tubing is withdrawn. This is performed until the entire

casing is cemented to the surface. At the surface, the casing is cut off 5 feet below the ground surface and

the land surface reclaimed in accordance with mine abandonment permits (Wyoming 1988, 1996).

In another example, all injection wells, water withdrawal wells, and monitoring boreholes at the

Kindall 3 Mine in Indiana were required to be plugged and sealed with cement from the bottom to the

surface. Figure 8 provides example cross sections showing borehole plugging methods. In this example, a

cement basket is used instead of a bridge plug to establish the location in the casing at which cementing

begins.
September 30, 1999 44

Source: Endress, 1996

Figure 8. Cross Sections of Typical Borehole Plugging Methods
September 30, 1999 45

7. CURRENT REGULATORY REQUIREMENTS

As discussed below, several federal, state, and local programs exist that either directly manage or

regulate Class V mining, sand, and other backfill wells. On the federal level, management and regulation of

these wells falls primarily under the UIC program authorized by the Safe Drinking Water Act (SDWA). 

Some states and localities have used these authorities, as well as their own authorities, to extend the

controls in their areas to address concerns associated with mining, sand, and other backfill wells.

7.1 Federal Programs

7.1.1 SDWA

Class V wells are regulated under the authority of Part C of SDWA. Congress enacted the

SDWA to ensure protection of the quality of drinking water in the United States, and Part C specifically

mandates the regulation of underground injection of fluids through wells. USEPA has promulgated a series

of UIC regulations under this authority. USEPA directly implements these regulations for Class V wells in

19 states or territories (Alaska, American Samoa, Arizona, California, Colorado, Hawaii, Indiana, Iowa,

Kentucky, Michigan, Minnesota, Montana, New York, Pennsylvania, South Dakota, Tennessee, Virginia,

Virgin Islands, and Washington, DC). USEPA also directly implements all Class V UIC programs on

Tribal lands. In all other states, which are called Primacy States, state agencies implement the Class V UIC

program, with primary enforcement responsibility.

Mining, sand, and other backfill wells currently are not subject to any specific regulations tailored

just for them, but rather are subject to the UIC regulations that exist for all Class V wells. Under 40 CFR

144.12(a), owners or operators of all injection wells, including mining, sand, and other backfill wells, are

prohibited from engaging in any injection activity that allows the movement of fluids containing any

contaminant into USDWs, “if the presence of that contaminant may cause a violation of any primary

drinking water regulation . . . or may otherwise adversely affect the health of persons.”

Owners or operators of Class V wells are required to submit basic inventory information under 40

CFR 144.26. When the owner or operator submits inventory information and is operating the well such

that a USDW is not endangered, the operation of the Class V well is authorized by rule. Moreover, under

section 144.27, USEPA may require owners or operators of any Class V well, in USEPA-administered

programs, to submit additional information deemed necessary to protect USDWs. Owners or operators

who fail to submit the information required under sections 144.26 and 144.27 are prohibited from using

their wells.

Sections 144.12(c) and (d) prescribe mandatory and discretionary actions to be taken by the UIC

Program Director if a Class V well is not in compliance with section 144.12(a). Specifically, the Director

must choose between requiring the injector to apply for an individual permit, ordering such action as closure

of the well to prevent endangerment, or taking an enforcement action. Because mining, sand, and other

backfill wells (like other kinds of Class V wells) are authorized by rule, they do not have to obtain a permit
17 May 2003 is the deadline including an 18-month extension.

September 30, 1999 46

unless required to do so by the UIC Program Director under 40 CFR 144.25. Authorization by rule

terminates upon the effective date of a permit issued or upon proper closure of the well. 

Separate from the UIC program, the SDWA Amendments of 1996 establish a requirement for

source water assessments. USEPA published guidance describing how the states should carry out a source

water assessment program within the state’s boundaries. The final guidance, entitled Source Water

Assessment and Programs Guidance (USEPA 816-R-97-009), was released in August 1997.

State staff must conduct source water assessments that are comprised of three steps. First, state

staff must delineate the boundaries of the assessment areas in the state from which one or more public

drinking water systems receive supplies of drinking water. In delineating these areas, state staff must use

“all reasonably available hydrogeologic information on the sources of the supply of drinking water in the

state and the water flow, recharge, and discharge and any other reliable information as the state deems

necessary to adequately determine such areas.” Second, the state staff must identify contaminants of

concern, and for those contaminants, they must inventory significant potential sources of contamination in

delineated source water protection areas. Class V wells, including mining, sand, and other backfill wells,

should be considered as part of this source inventory, if present in a given area. Third, the state staff must

“determine the susceptibility of the public water systems in the delineated area to such contaminants.” State

staff should complete all of these steps by May 2003 according to the final guidance.17

7.1.2 SMCRA

The Office of Surface Mining Reclamation and Enforcement (OSM) in the U. S. Department of the

Interior oversees state mining regulatory and reclamation activities under the Surface Mining Control and

Reclamation Act (SMCRA), or directly implements mining programs in states that have not obtained

primacy under SMCRA. The Office also directly regulates coal mining and reclamation activities on

federal and Indian lands. The Bureau of Land Management (BLM), also a part of the U. S. Department of

the Interior, is responsible for the management of public lands, including minerals leasing and oversight for

the development of energy and mineral leasing and compliance with regulations governing the extraction of

mineral resources. It is also responsible for subsurface resource management where mineral rights, but not

the land surface, are federally owned.

Regulations promulgated by the Office of Surface Mining in Title 30 Chapter 7 apply to mine

backfill wells if the well is located in a state that has not accepted primacy under SMCRA. In some

Primacy States, the federal requirements have also served as the model for the state regulations. 

Part 784 of Chapter 7 addresses underground mining permit application requirements, and contains

minimum requirements for the reclamation and operation plan that must be submitted as part of the permit

application. Section 784.25 provides that each underground mining permit application must supply a plan

describing the design, operation, and maintenance of any proposed coal processing waste disposal facility,

including flow diagrams and any other necessary drawings and maps, for the approval of the state
September 30, 1999 47

regulatory authority and the Mine Safety and Health Administration under 30 CFR 817.81(f), the

permanent program performance standards for underground disposal of coal mining waste. The section

provides further that: 

C Each plan shall describe the sources and quality of waste to be stowed, area to be backfilled,

percent of the mine void to be filled, method of constructing underground retaining walls, influence

of the backfilling operation on active underground mine operations, surface area to be supported by

the backfill and the anticipated occurrence of surface effects following backfilling;

C The applicant shall describe the source of the hydraulic transport mediums, method of dewatering

the backfill that is emplaced, retention of water underground, treatment of water if released to

surface streams, and the effect on the hydrology; and 

C The plan shall describe each permanent monitoring well to be located in the backfilled area, the

stratum underlying the mined coal, and gradient from the backfilled area except where pneumatic

backfilling operations are exempted from hydrologic monitoring (30 CFR 784.25).

Both §817.81(f), the permanent program requirements on underground disposal of coal mine waste

described above, and §817.71(j), the permanent program requirements on underground disposal of excess

spoil, provide that coal mine waste or excess spoil may be disposed of in underground mine workings “only

in accordance with a plan approved by the regulatory authority and MSHA under §784.25.”

SMCRA also authorizes the promulgation of regulations addressing the surface effects of

underground coal mining operations. The statute provides in 30 U.S.C.A. §1266 that with respect to

surface disposal of mine wastes, tailings, coal processing wastes, and other wastes in areas other than the

mine workings or excavations, permitted mining operations are required to stabilize all waste piles and

ensure that the leachate from such piles will not fall below the water quality standards established under

federal or state law for surface or ground waters. The provision does not specify that leachate must result

from precipitation, and could be applied to leachate from injection into a surface rubble pile (which could

be defined as a Class V well). 

BLM regulations establish performance standards for coal mining and for solid minerals other than

coal under federal leases and licenses. The rules pertaining to underground coal mining specify that

backfilling of exploratory drill holes, openings, and excavations must be in accordance with sound

engineering practices and an approved plan (43 CFR 3484.2). Non-coal mineral mining also must be

conducted according to an approved plan, which must address backfilling of drill holes (see e.g., 43 CFR

3522.3-3(c)(3)). In addition, BLM rules provide that the operator/lessee must dispose of all wastes

resulting from the mining, reduction, concentration, or separation of mineral substances in accordance with

the terms of the lease, approved mining plan, applicable federal, state, and local law and regulations and the

directions of the authorized officer (43 CFR 3596.2).
September 30, 1999 48

7.2 State and Local Programs

Ninety-eight percent of the documented mine backfill wells and 99 percent of estimated wells in the

United States exist in 10 states: Idaho, Illinois, Indiana, Kansas, North Dakota, Ohio, Pennsylvania, Texas,

West Virginia, and Wyoming. Attachment A of this volume describes how each of these states current

regulate mining, sand, and other backfill wells. 

In Indiana and Pennsylvania, USEPA Regions 5 and 3, respectively, directly implement the UIC

Class V program. The USEPA Regions apply inventory requirements and use permit by rule to ensure

non-endangerment of USDWs. Indiana, in addition, has enacted state regulations that apply to

underground mining operations, including backfilling of mines, that are implemented by the state’s

Department of Natural Resources. Indiana’s requirements for backfilling plans parallel requirements

established in 30 CFR 817.81(f) under SMCRA. Pennsylvania, in addition, regulates mine backfill well

projects through regulations implemented by the Bureau of Mining and Reclamation.

In the eight states that are Primacy States for Class V UIC wells, state regulations pertaining to

mine backfill wells vary significantly in their scope and stringency. 

• Mine tailing backfill wells are authorized by rule in Idaho, unless use of such a well results in

exceedance of water quality standards, when the well is required to obtain an individual permit or

close.

• Illinois has enacted UIC Class V requirements that are identical to those of the USEPA. The state

applies inventory requirements and uses permit by rule to ensure non-endangerment of USDWs. In

addition, the state has enacted a Groundwater Protection Act and ground water quality regulations

that require responses to ground water contamination before it exceeds specified ground water

quality standards.

• Kansas has incorporated the federal UIC Class V regulations by reference. (Mine backfill wells

that are designed to backfill salt caverns are covered by the state’s Class III UIC requirements. and

by water well requirements and are not discussed in this report.) Class V mine backfill wells are

permitted by rule. 

• North Dakota uses inventory requirements and permit by rule to ensure non-endangerment of

USDWs from mine backfill wells. In addition, the state places special requirements for siting,

construction, and operation of backfill wells into the contracts that the state enters into for

backfilling to address highway subsidence, the predominant backfilling activity that takes place in

the state.

• Ohio authorizes by rule Class V UIC mine backfill wells and requires drilling and operating permits

for some types of wells, including backfill wells. In addition, Ohio’s rules pertaining to underground

coal mines, administered by the Division of Mines and Reclamation, require the development of a

reclamation plan, including ground water monitoring, and specify that discharge of water into
September 30, 1999 49

underground mines is prohibited unless approved by the Division of Mines. Ohio’s requirements

for backfilling plans parallel the requirements in 30 CFR 817.81(f) under SMCRA. 

• In Texas, mine backfill wells are authorized by rule. The state’s mining regulations exempt shafts

and boreholes authorized under the UIC program. The state requirements for Class V wells,

however, include siting, construction, and closure standards. 

• West Virginia issues individual permits or area permits to mine backfill wells.

• Wyoming covers mine backfill wells under the general permit provisions of the state’s Class V UIC

requirements. In addition to requiring the submission of information, the general permit

requirements also include a well operator to establish a monitoring program. In addition, the state’s

regulations pertaining to underground coal mining require a permit to return coal-mining waste to

abandoned underground workings. 
September 30, 1999 50

ATTACHMENT A

STATE AND LOCAL PROGRAM DESCRIPTIONS

This attachment does not describe every state’s control programs; instead it focuses on the ten

states where relatively large numbers of mine backfill wells are known to exist: Idaho, Illinois, Indiana,

Kansas, North Dakota, Ohio, Pennsylvania, Texas, West Virginia, and Wyoming. Altogether, these ten

states have a total of 4,992 documented mine backfill wells, which is almost 99 percent of the documented

well inventory for the nation.

Idaho

Idaho is a UIC Primacy State for Class V wells and has promulgated regulations for its UIC

program in the Idaho Administrative Code (IDAPA), Title 3, Chapter 3.

Permitting

Idaho’s rules state that mine tailings backfill wells are authorized by rule as part of mining operations

“because federal studies show the threat of endangerment from use of these wells is low. They are

therefore exempt from the MCLs and permitting requirements of the UIC rules, provided that their use is

limited to the injection of mine tailings only.” For rule-authorized mine backfill wells inventory information

must be supplied (37.03.03.030.01 IDAPA). 

The rules provide that the use of a well shall not result in water quality standards at points of

beneficial use being exceeded or otherwise affect a beneficial use. If water quality standards are exceeded

or beneficial uses affected, the rules state that the well may be put under the permit requirements of Title 3,

Chapter 3, or the well may be required to be remediated or closed (37.03.03.025.03.g IDAPA (Rule

25)).

If a mining backfill well is placed under the permitting requirements, detailed permit application

information is required. It includes information on location and construction of the proposed well; proposed

injectate; local features such as topography, wells producing water, surface waters, residences, and

geology; and maps and cross sections depicting all USDWs within a quarter mile radius of the injection

well, their location relative to the injection zone, and the direction of water movement. Corrective action

and contingency plans must be prepared and submitted, and proof of financial responsibility must be

supplied.
18 “Ground water” is defined as “any water that occurs beneath the surface of the earth in a

saturated geological formation of rock or soil.” A “Drinking Water Source” is defined as “an aquifer

which contains water having less than ten thousand (10,000) mg/l total dissolved solids” that has not been

exempted from that designation by the Director of the Department of Water Resources.

September 30, 1999 51

Siting and Construction

Class V wells may be required to be located at a distance from a point of diversion for beneficial

use sufficient to minimize or prevent ground water18 contamination resulting from unauthorized or accidental

injection (37.03.03.050.03.a IDAPA)

Operating Requirements

Idaho’s requirements for use of Class V wells are based on the premise that if the injected fluids

meet MCLs for drinking water at the wellhead, and if ground water produced from adjacent points of

diversion for beneficial use meets the water quality standards found in Idaho’s “Water Quality Standards

and Wastewater Treatment Requirements,” 16.01.02 IDAPA, administered by the Idaho Department of

Health and Welfare, the aquifer will be protected from unreasonable contamination. The state may, when

necessary “to protect the ground water resource from deterioration and preserve it for diversion to

beneficial use,” require specific injection wells to be constructed and operated in compliance with

additional requirements (37.03.03.050.01 IDAPA (Rule 50)). Rule-authorized wells “shall conform to the

MCLs at the point of injection and not cause any water quality standards to be violated at the point of

beneficial use” (37.03.03.050.04.d. IDAPA).

Monitoring, recordkeeping, and reporting may be required if the state finds that the well may

adversely affect a drinking water source or is injecting a contaminant that could have an unacceptable effect

upon the quality of the ground waters of the state (37.03.03.055 IDAPA (Rule 55)). As a condition of use

of mine tailings backfill wells, the owner or operator may be required to monitor ground water

(37.03.030.025.03.g IDAPA (Rule 25)).

Financial Responsibility

No financial responsibility requirement exists for rule-authorized mine backfill wells. Operators of 

permitted wells are required to demonstrate financial responsibility through a performance bond or other

appropriate means (the rule does not specify the amount(s) required) to abandon the injection well

according to the conditions of the permit (37.03.03.35.03.e IDAPA).

Plugging and Abandonment

The Idaho Department of Water Resources (IDWR) has prepared “General Guidelines for

Abandonment of Injection Wells,” which are not included in the regulatory requirements. IDWR expects to

approve the final abandonment procedure for each well. The General Guidelines recommend the following:
September 30, 1999 52

C The casing should be pulled, if possible, or cut a minimum of two feet below the land surface.

C The total depth of the well should be measured.

C If the casing is left in place, it should be perforated and neat cement with up to 5 percent bentonite

can be pressure-grouted to fill the hole. As an alternative, when the casing is not pulled, course

bentonite chips or pellets may be used. If the well extends into the aquifer, the chips or pellets must

be run over a screen to prevent any dust from entering the hole. Perforation of the casing is not

required under this alternative

C If the well extends into the aquifer, a clean pit-run gravel or road mix may be used to fill the bore up

to ten feet below the top of the saturated zone or ten feet below the bottom of the casing,

whichever is deeper, and cement grout or bentonite clay used to the surface. Gravel may not be

used if the lithology is undetermined or unsuitable.

C A cement cap should be placed at the top of the casing if the casing is not pulled, with a minimum of

two feet of soil overlying the filled hole/cap.

C Abandonment of the well must be witnessed by an IDWR representative.

Illinois

Illinois is a UIC Primacy State for Class V wells. The Illinois Environmental Protection Agency

(IEPA), Bureau of Land, has promulgated rules establishing a Class V UIC program in 35 Illinois

Administrative Code (IAC) 704. These rules are identical in substance to USEPA rules in 40 CFR 144

(704.101 IAC). In addition, Part 702, “RCRA and UIC Permit Programs,” establishes requirements for

those UIC wells required to obtain a permit, while Part 705 describes the procedures for issuing UIC

permits. Finally, 35 IAC Part 730 sets out technical criteria and standards for the UIC program. Part 730

Subpart F currently does not specify technical criteria and standards for siting, construction, operating,

monitoring and reporting, mechanical integrity, or closure for Class V UIC wells, although other subparts of

Part 730 do so for other classes of UIC wells (730.151 IAC). 

Permitting

Any underground injection, except into a well authorized by permit or rule, is prohibited. The

construction of any well required to have a permit is prohibited until the permit has been issued (704.12.

IAC). However, injection into Class V wells is authorized by rule (704.146 IAC). Owners or operators

of wells authorized by rule must submit inventory information (704.148 IAC). In addition, IEPA may

require submission of other information deemed necessary by IEPA (704.149 IAC). In addition to the

inventory information required from all Class V wells, certain categories of wells, including sand or other

backfill wells as defined by 35 IAC 730.105(e)(8), are required to submit additional information, including

the following:
19 “Potable” is defined as “generally fit for human consumption in accordance with accepted water

supply principles and practices.”

September 30, 1999 53

C Location of each well by township, range, section, and quarter-section;

C Date of completion of each well,

C Identification and depth of the formation(s) into which each well is injecting,

C Total depth of each well,

C Casing and cementing record, tubing size, and depth of packer,

C Nature of the injected fluids,

C Average and maximum injection pressure at the wellhead,

• Average and maximum injection rate, and

• Date of the last MITs, if any (704.148(b)(2) IAC).

Operating Requirements

No operating requirements are specified for Class V UIC wells permitted by rule. Such wells,

however, are subject to the state’s ground water protection requirements. Under Illinois’ Ground Water

Quality regulations, found in 35 Ill. Adm.Code Part 620, a classification system is established for the

State’s ground waters. The regulations also enact a nondegradation provision, establish standards for

quality of ground waters, and create procedures for the management and protection of ground waters. The

regulation defines “potential route” of ground water contamination to include, among others, abandoned and

improperly plugged wells of all kinds, drainage wells, and all injection wells. The regulation provides that no

person shall cause a violation of the state’s Environmental Protection Act, the Groundwater Protection Act,

or regulations adopted under those Acts, including the Ground Water Quality regulations.

The fours classes of ground water established by the classification system are (I) potable resource

ground water;19 (II) general resource ground water, which cannot easily be tapped to supply drinking

water; (III) special resource ground water, which is “demonstrably unique (e.g., irreplaceable”), vital for a

particularly sensitive ecological system (not further defined), or contributes to a dedicated nature preserve;

and (IV) other ground water, which is naturally saline, contaminated, or is limited in its resource potential

(e.g., within a zone of attenuation for a solid waste landfill, under a coal mine refuse disposal area, under a

potential contaminant source, within a previously mined area, or ground water that has been designated as

an exempt aquifer under the underground injection policy of 730.104 IAC (620.201 - 240 IAC). (Under

730.104 IAC an aquifer or portion of an aquifer that otherwise meets the criteria for a USDW may be

determined to be an exempted aquifer if it does not currently serve as a source of drinking water and it

cannot now and will not in the future serve as a source of drinking water.)

The Ground Water Quality regulations prohibit impairment of resource ground water and require

preventive notice and response procedures to detect and address contaminants before they exceed the

ground water quality standards for Class I and III ground waters. The regulations also include ground

water quality standards for each class of ground water, as well as ground water quality restoration

standards. The latter include coal reclamation ground water quality standards, addressing inorganic

chemical constituents and pH in ground water, within an underground coal mine, or within the cumulative
September 30, 1999 54

impact area of ground water for which the hydrologic balance has been disturbed from a permitted coal

mine (620.450(b). These requirements also address coal mine refuse disposal areas, but do not explicitly

address mine backfill activities.

Indiana

USEPA Region 5 directly implements the UIC program for Class V injection wells in Indiana. In

addition, however, state regulations found in Title 310 Indiana Administrative Code (IAC) administered by

the Indiana Department of Natural Resources (DNR), Division of Reclamation, apply to mine backfill wells.

Permitting

The DNR permitting rules require applications for underground mining operation permits to

describe proposed disposal methods and sites for placing underground development waste and excess spoil

generated at surface areas (310 IAC 12-3-86). Each plan also must describe the design, operation, and

maintenance of any proposed coal processing waste disposal facility, including the source and quality of

waste, the area to be backfilled, the method of constructing underground retaining walls, the source of the

hydraulic transport mediums, the method of dewatering the emplaced backfill, the retention of water

underground, the effect on the hydrology, each monitoring well to be located in the backfilled area, the

stratum underlying the mined coal, and the gradient from the backfilled area (310 IAC 12-3-91).

Regulations of the Water Pollution Control Board also specify that if an applicant for an National

Pollutant Discharge Elimination System (NPDES) permit proposes to dispose of pollutants by underground

injection as part of the overall effort to meet the requirements of the NPDES program, the application shall

be denied, unless conditions can be placed in the NPDES permit that will control the proposed discharge to

prevent pollution of ground water resources of such character and degree as would endanger or threaten to

endanger the public health and welfare (327 IAC 5-4-2).

Siting and Construction

Approval must be obtained for return of coal processing waste to abandoned underground

workings (310 IAC 12-3-91). Plans submitted to the Division of Reclamation must identify the locations of

the wells. 

Operating Requirements

Coal processing waste may be returned to underground mine workings only in accordance with the

waste disposal program approved under 310 IAC 12-3-91 (310 IAC 12-5-46 and 310 IAC 12-5-110). 

The Division of Reclamation specifies operating requirements on a case-by-case basis. Quarterly ground

water analyses must be submitted, and must continue to be submitted following completion of injection

activities until a demonstration can be made that no adverse effects on the hydrologic balance have

occurred. Each drilled hole, well, or other exposed underground opening identified in the approved permit
September 30, 1999 55

application for use to return coal processing waste or water to underground workings must be temporarily

sealed before use and protected during use (310 IAC 12-5-9).

Mechanical Integrity Testing

Not specified by statute or regulation.

Financial Responsibility

Operators are required to post a performance bond with DNR’s Division of Reclamation. The

bond is released upon a showing that following cessation of injection activities no adverse effects to the

hydrologic balance have occurred. 

Plugging and Abandonment

 If no longer in use, a drilled hole or well must be cased, sealed, or otherwise managed to prevent

acid or other toxic drainage from entering ground or surface waters and to minimize disturbance to the

prevailing hydrologic balance (310 IAC 12-5-74 and 310 IAC 12-5-76).

Kansas

Kansas is a UIC Primacy State for Class V wells. It has incorporated the federal UIC regulations

by reference in Kansas Administrative Regulations (KAR) Article 28-46.

Permitting

Mine backfill wells, except for wells backfilling salt caverns, are permitted by rule under KAR 28-

46. Mine backfill wells that are designed to backfill salt caverns are covered by regulations for Class III

salt solution mining wells (KAR 28-43) and also are covered by KAR 28-30. 

Siting and construction

There are no siting or construction requirements for mine backfill wells, except for wells backfilling

salt caverns. 

Operating requirements

There are no operating requirements for mine backfill wells, except for wells backfilling salt caverns. 

The state requires salt cavern backfill well operators to prepare a closure plan and to fill wells with grout,

relying upon requirements for abandonment in 28-30 KAR.
September 30, 1999 56

North Dakota

North Dakota is a UIC Primacy State for Class V wells. Regulations establishing the UIC program

are found in Article 33-25 of the North Dakota Administrative Code (NDAC).

Permitting

Underground injection is prohibited, unless authorized by permit or rule (33-25-01-03 NDAC).

Injection into a Class V well is authorized by rule indefinitely, subject to the requirements of subsections 4

(evidence of financial responsibility), 5 (maintenance of records until 3 years after plugging and

abandonment), and 6 (reporting within 24 hours of any endangerment of a USDW and any noncompliance

with a permit condition or malfunction of the injection system that could cause fluid migration into or

between USDWs) of § 33-25-01-10 and subsection 3 (notice to the Department of Health before

conversion or abandonment of the well) of §33-25-01-12 NDAC. The operator of a Class V well

authorized by rule may be required to apply for and obtain an individual or area permit under specific

circumstances, including cases in which protection of a USDW requires the injection operation to be

regulated by requirements not contained in the rules (33-25-01-16 NDAC).

Siting and Construction

Although not explicitly called for by the Class V requirements, siting and construction requirements

are imposed on mine backfill wells by the Abandoned Mine Lands Division of the Public Service

Commission on a case-by-case basis through the contract terms that the Division includes in its contracts

for backfilling services.. The wells are sited where abandoned underground mines that lie beneath towns or

highways have caused subsidence. The contracts call for the wells to be constructed as 5 inch diameter

holes cased with 3 inch I.D. Schedule 40 PVC pipe., and to be 50-70 feet deep. None inject into

USDWs. 

Operating Requirements

Operating requirements are established by contract. The contractors who construct and operate

the wells are required to inject grout in conformance with contract specifications, including specifications

concerning grout pressure at the well head, flow rates, pumping rates, and cumulative volume pumped;

grout constituents and consistency; records and recordkeeping; and permitting. 

Subsection 5 of § 33-25-01-10 requires records to be maintained concerning the nature and

composition of injected fluids for three years after plugging and abandonment of the well. A single type of

injectate, which has been approved by the Department of Health, is utilized by the Abandoned Mines

Division of the Public Service Commission. Operations are generally concluded within 24 hours. A

qualified inspector is on-site whenever injection occurs.

Subsection 6 of § 33-25-01-10 requires a report within 24 hours of any monitoring or other

indication that any contaminant may cause an endangerment to a USDW, or any noncompliance with a
September 30, 1999 57

permit condition or malfunction of the injection system that may cause fluid migration into or between

USDWs.

Mechanical Integrity Testing

Not specified by statute or regulation.

Financial Responsibility

Subsection 4 of § 33-25-01-10 requires operators to have sufficient financial responsibility and

resources to close, plug, and abandon the underground injection operation in a manner prescribed by the

Division of Water Supply and Pollution Control of the Department of Health. A surety bond, or other

evidence of adequate assurance, in an amount satisfactory to the Department, must be provided.

Plugging and Abandonment

Subsection 3 of § 33-25-01-12 requires notice before conversion or abandonment of the well. 

Ohio

Ohio is a UIC Primacy State for Class V wells. Regulations establishing the UIC program are

found in Chapter 3745-34 of the Ohio Administrative Code (OAC). In addition, the Ohio Department of

Natural Resources, Division of Mines and Reclamation, regulates active and abandoned mines.

Permitting

Class V injection wells are defined to include sand backfill and other backfill wells used to inject a

mixture of water and sand, mill tailings or other solids into mined out portions of subsurface mines (3745-

34-04 OAC). Any underground injection, except as authorized by permit or rule, is prohibited. The

construction of any well required to have a permit is prohibited until the permit is issued (3745-34-06

OAC).

Injection into Class V injection wells is authorized by rule (3745-34-13 OAC).

However, a drilling and operating permit is required for injection into a Class V injection well of sewage,

industrial wastes, or other wastes (including backfill), as defined in § 6111.01 of the Ohio Revised Code,

into or above a USDW (3745-34-13 OAC and 3745-34-14 OAC). 

Permit applications must include a description of the activities conducted by the applicant; facility

location, listing of other permits under specified programs, where the well is to be drilled, name of the

geological formation to be used and the proposed total depth of the well, type of drilling equipment to be

used, plan for disposal of water and other waste substances, composition of the substance to be injected,

topographic map indicating specified features, and description of the business (3745-34-16 OAC).
September 30, 1999 58

Class V injection well permits may be issued on an area basis. The permit will specify requirements

for construction, monitoring, reporting, operation, and abandonment (3745-34-18 OAC).

Ohio's rules on underground coal mines, administered by the Division of Mines and Reclamation,

require the development of a reclamation plan that must be submitted as part of an application for a permit

to conduct coal mining (1501:13-4-14 OAC). The plan must include a description of the measures to be

used to seal or manage mine openings and to plug, case or manage exploration holes, other bore holes,

wells, and other openings within the proposed permit area (1501:13-4-14(D)(2)(g) OAC). It must include

a ground water monitoring plan, including identification of the monitoring parameters, sampling frequency,

and site locations, sufficient to monitor the suitability of the ground water for current and approved postmining land uses and the objectives for protection of the hydrologic balance established in the permit

(1501:13-4-14(F)(1)(a) OAC).

The rules require submission of a subsidence control plan, including a detailed description of the

subsidence control measures that will be taken to prevent or minimize subsidence, such as backfilling voids

(1501:13-4-14(M)((2)(e) OAC). The Ohio rules also adopt the MSHA requirements concerning return of

coal mine wastes to abandoned underground workings. They require the application to contain a plan that

describes the design, operation, and maintenance of any proposed coal processing waste disposal facility. 

The plan must describe the source and quality of waste to be stowed, area to be backfilled, percent of the

mine void to be filled, method of constructing underground retaining walls, influence of the backfilling

operation on active underground mine operations, surface area to be supported by the backfill, and the

anticipated occurrence of surface effects following backfill. The application is required to

 describe the source of the hydraulic transport mediums, method of dewatering the emplaced backfill,

retention of water underground, treatment of water if released to surface streams, and the effect on the

hydrology. The plan must describe each permanent monitoring well to be located in the backfilled area, the

stratum underlying the mined coal, and the gradient from the backfilled area. Pneumatic backfilling

operations are covered, except they may be exempted from requirements specifying hydrologic monitoring

(1501:13-4-14(N)(1)-(5) OAC).

Siting and Construction

The permit applicant must submit plans for testing, drilling, and construction, and no construction

may commence before permit issuance. Permits will contain conditions specifying construction

requirements (3745-34-27 OAC).

The mining rules specify that each exploration hole, other drill or borehole, shaft, well, or other

exposed mine opening must be cased, sealed, or otherwise managed as approved by the Division of Mines. 

Each well or other opening identified in the approved permit application for use to return coal processing

waste or water to underground workings must be temporarily sealed before use and protected during use

by barricades, fences, or other protective devices. When no longer needed, they must be capped, sealed,

backfilled, or otherwise properly managed as required by the Division of Mines (1501:13-9-02 OAC).
September 30, 1999 59

Operating Requirements

Permits contain conditions specifying operation requirements, including maximum injection volumes

and/or pressures and monitoring and reporting requirements (3745-34-27 OAC). Permittees are required

to maintain records of the nature and composition of all injected fluids for three years. Reports of any

noncompliance that may endanger health or the environment, including any monitoring or other information

that indicates that any contaminant may cause an endangerment to a USDW, or any noncompliance with a

permit condition or malfunction of the injection system that may cause fluid migration into or between

USDWs must be reported within 24 hours (3745-34-26 (J) and (K) OAC).

Wells must be inspected before commencing injection. The permittee must provide notice before

conversion or abandonment of the well (3745-34-26 (M) and (N) OAC).

The mining rules also establish a general requirement for protection of the hydrologic system from

mining activities. Backfilled materials are required to be placed so as to minimize contamination of ground

water systems with acid, toxic, or otherwise harmful mine drainage, and to minimize adverse effects of

mining on ground water systems outside the permit area (1501:13-9-04(K) OAC). Discharge of water

into underground mines is prohibited, unless specifically approved by the Division of Mines and by the

federal MSHA, and such discharges are limited to water, coal processing waste, fly ash from a coal-fired

facility, sludge from an acid-mine drainage treatment facility, flue-gas desulfurization sludge, inert material

used for stabilizing underground mines, and underground mine development wastes (1501:13-9-04(Q)

OAC).

Mechanical Integrity Testing

Permits may include a condition prohibiting injection operations until the permittee shows that the

well has mechanical integrity, as specified under § 3745-34-34 OAC ((3745-34-27 OAC). Detailed

specifications for mechanical integrity are included in § 3734-34-34.

Financial Responsibility

Permittees are required to maintain financial responsibility and resources sufficient to close, plug,

and abandon the underground injection operation (3734-34-27 OAC). 

Plugging and Abandonment

Permits may include conditions to ensure that plugging and abandonment of the well will not allow

the movement of fluids either into or between USDWs (3745-34-27 OAC). There is no established

closure guidance or policy that lists specific materials or procedures to be employed. Site-specific closure

plans are reviewed by Ohio USEPA. 
September 30, 1999 60

Pennsylvania

USEPA Region 3 directly implements the UIC program for Class V injection wells in Pennsylvania. 

However, the Bureau of Mining and Reclamation in the Department of Environmental Protection approves

mine backfill well projects. The Department has no specific regulations pertaining to mine backfill wells.

Technical specifications are provided to drilling contractors as part of the contract for mine backfill

projects. The technical specifications vary depending on the well location (i.e., anthracite coal regions or

bituminous coal regions).

Permitting

The drilling contractor is required to obtain all necessary permits, and to comply with all existing

laws, ordinances, rules and regulations relating to the contractor’s operations.

Siting and Construction

The Department determines well siting. All work is required to be done under the direction of a

Resident Engineer or the Technical Specifications of the contract. Technical Specifications address

overburden drilling, drilling in material other than overburden, and casing with steel or PVC pipe.

Operating Requirements

Technical specifications address supply, delivery, and injection of grout material. A Department

inspector is onsite during operations. 

Mechanical Integrity Testing

Not specified by statute or regulation.

Financial Responsibility

Not specified by statute or regulation.

Plugging and Abandonment

A technical specification addresses sealing of boreholes. The contractor is required to seal

boreholes according to the directions of the Department’s representative. Sealing is required by means of a

minimum of 10 feet of cement backfill below the overburden/rock interface or below the bottom of the

smaller casing pipe, whichever is deeper. In the event that “significant” quantities of water are encountered,

the contractor may be required to set the plug below the aquifer and build the seal from that elevation. 
September 30, 1999 61

Texas

Texas is a UIC Primacy State for Class V wells. The Injection Well Act (Chapter 27 of the Texas

Water Code) and Title 3 of the Natural Resources Code provide statutory authority for the UIC program. 

Regulations establishing the UIC program are found in Title 30, Chapter 331 of the Texas Administrative

Code (TAC).

Permitting

Underground injection is prohibited, unless authorized by permit or rule (331.7 TAC). Injection

into a Class V well is authorized by rule, although the Texas Natural Resources Control Commission

(TNRCC) may require the owner or operator of a well authorized by rule to apply for and obtain an

injection well permit (331.9 TAC). No permit or authorization by rule is allowed where an injection well

causes or allows the movement of fluid that would result in the pollution of a USDW. A permit or

authorization by rule must include terms and conditions reasonably necessary to protect fresh water from

pollution (331.5 TAC). Sand backfill wells used to inject a mixture of water and sand, mill tailings or other

solids into mined out portions of subsurface mines are specifically defined as Class V wells (331.11

(a)(4)(H) TAC). The state’s mining regulations require permits for the construction, use, or operation of a

new shaft, but exempt penetrations or boreholes authorized by the TNRCC under the underground

injection control program and penetrations authorized by the TNRCC whose purpose is the transmission of

concrete slurries, muds, or bulk materials to underground mine workings (329.4 TAC). Therefore the

state’s mining backfill wells are regulated under the UIC program and not the mining program. 

Siting and Construction

All Class V wells are required to be completed in accordance with the following specifications in

the rules, unless otherwise authorized by the TNRCC:

C A form provided either by the Water Well Drillers Board or the TNRCC must be completed.

C The annular space between the borehole and the casing must be filled from ground level to a depth

of not less than 10 feet below the land surface or well head with cement slurry. Special

requirements are imposed in areas of shallow unconfined ground water aquifers and in areas of

confined ground water aquifers with artesian head.

C In all wells where plastic casing is used, a concrete slab or sealing block must be placed above the

cement slurry around the well at the ground surface (the rules include additional specifications

concerning the slab).

C In wells where steel casing is used, a slab or block will be required above the cement slurry, except

when a pitless adaptor is used. The rules contain additional requirements concerning adaptors.
September 30, 1999 62

C All wells must be completed so that aquifers or zones containing waters that differ significantly in

chemical quality are not allowed to commingle through the borehole-casing annulus or the gravel

pack and cause degradation of any aquifer zone.

C The well casing must be capped or completed in a manner that will prevent pollutants from entering

the well.

C When “undesirable” water is encountered in a Class V well, the undesirable water must be sealed

off and confined to the zone(s) of origin (331.132 TAC).

Operating Requirements

Not specified by statute or regulation. 

Mechanical Integrity Testing

Injection may be prohibited for Class V wells that lack mechanical integrity. The TNRCC may

require a demonstration of mechanical integrity at any time if there is reason to believe mechanical integrity

is lacking. The TNRCC may allow plugging of the well or require the permittee to perform additional

construction, operation, monitoring, reporting, and corrective actions which are necessary to prevent the

movement of fluid into or between USDWs caused by the lack of mechanical integrity. Injection may

resume on written notification from the TNRCC that mechanical integrity has been demonstrated (331.4

TAC).

Financial Responsibility

Chapter 27 of the Texas Water Code, “Injection Wells,” enacts financial responsibility

requirements. However, the requirement, unless incorporated into a individual permit for a Class V well,

applies specifically only to Class I and Class III wells (331.142 TAC).

Plugging and Abandonment

Plugging and abandonment of a well authorized by rule is required to be accomplished in

accordance with §331.46 TAC (331.9 TAC). In addition, closure standards specific to Class V wells

provide that closure is to be accomplished by removing all of the removable casing and filling the entire well

with cement to land surface. Alternatively, if the use of the well is to be permanently discontinued, and if the

well does not contain undesirable water, the well may be filled with fine sand, clay, or heavy mud followed

by a cement plug extending from the land surface to a depth of not less than 10 feet. If the use of a well

that contains undesirable water

is to be permanently discontinued, either the zone(s) containing undesirable water or the fresh water zone(s)

must be isolated with cement plugs and the remainder of the wellbore filled with sand, clay, or heavy mud to

form a base for a cement plug extending from the land surface to a depth of not less than 10 feet (331.133

TAC).
September 30, 1999 63

West Virginia

West Virginia is a UIC Primacy State for Class V wells. Regulations establishing the UIC program

are found in Title 47-13 West Virginia Code of State Regulations (WVAC). The state regulates sand

backfill and other backfill wells used to inject a mixture of water and sand, mill tailings or other solids into

mined out portions of subsurface mines as Class V wells (47-13-3.4.5.b. WVAC).

Permitting

Class V injection wells are authorized by rule unless the Office of Water Resources of the Division

of Environmental Protection (DEP) requires an individual permit (47-13-12.4.a. and 47-13-13.2 WVAC). 

All backfill wells in the state are required to have either individual permits or, if a group of wells in close

proximity injects into the same abandoned mine, an area permit (Parsons, 1999).

Siting and Construction

Individually permitted wells are subject to case-by-case construction requirements, based on plans

for testing, drilling, and construction submitted as part of the permit application. Wells subject to area

permits also are subject to construction requirements for all wells authorized by the permit (47-13-13.6, .7,

and .4.b.2 WVAC).

Operating Requirements 

Owners or operators of Class V wells are required to submit inventory information describing the

well, including its construction features, the nature and volume of injected fluids, alternative means of

disposal, the environmental and economic consequences of well disposal and its alternatives, operation

status, location, and ownership information (47-13-12.2 WVAC).

Individual and area permits specify requirements for monitoring, reporting, and operation for all

wells authorized by the permit (47-13-13.4, .6, and .7 WVAC). Owners and operators must meet the

requirements for monitoring and records (requiring retention of records pursuant to 47-13-13.6.b. WVAC

concerning the nature and composition of injected fluids until 3 years after completion of plugging and

abandonment); immediate reporting of information indicating that any contaminant may cause an

endangerment to USDWs or any malfunction of the injection system that might cause fluid migration into or

between USDWs.

The rules enact a general prohibition against any underground injection activity that causes or allows

the movement of fluid containing any contaminant into USDWs, if the presence of that contaminant may

cause a violation of any primary drinking water regulations under 40 CFR Part 142 or promulgated under

the West Virginia Code or may adversely affect the health of persons. If at any time a Class V well may

cause a violation of the primary drinking water rules the well may be required to obtain a permit or take

other action, including closure, that will prevent the violation (47-13-13.1 WVAC). Inventory requirements
September 30, 1999 64

for Class V wells include information regarding pollutant loads and schedules for attaining compliance with

water quality standards (47-13-13.2.d.1 WVAC).

If protection of a USDW requires, the injection operation may be required to satisfy requirements

for corrective action, monitoring, and reporting, or operation, that are not contained in the UIC rules (47-

13-13.2.c.1.C. WVAC).

Mechanical Integrity

Backfill wells required to obtain an individual permit will be required to demonstrate that the well

has mechanical integrity (47-13-13.7.h WVAC). Wells permitted by rule or subject to an area permit may

be required to demonstrate mechanical integrity.

Financial Responsibility

A Class V well required to obtain an individual permit will be required to demonstrate financial

responsibility and resources for plugging and abandonment. Evidence of financial responsibility includes

submission of a surety bond or other adequate assurance such as a financial statement of other material

acceptable to DEP.

Plugging and Abandonment

Backfill wells required to obtain an individual permit will be subject to permit conditions pertaining

to plugging and abandonment to ensure that the plugging and abandonment of the well will not allow the

movement of fluids into or between USDWs. A plan for plugging and abandonment will be required (47-

13-13.7.f WVAC). Wells permitted by rule or subject to an area permit may be required to submit a plan

for plugging and abandonment.

Wyoming

Wyoming is a UIC Primacy State for Class V wells and the Wyoming Department of

Environmental Quality (DEQ) Water Quality Division, has promulgated regulations pertaining to its Class V

UIC program in Chapter 16, Water Quality Rules and Regulations (WQRR). Rules on ground water

pollution control permits are promulgated in Chapter 9, WQRR, but Class V wells are specifically

exempted from coverage by Chapter 9 (Chapter 9 Section 3(a) WQRR). The DEQ Land Quality Division

has promulgated requirements pertaining to coal and non-coal mining.

Permitting

Mining, sand, and backfill facilities (category 5B1) are covered by the General Permit provisions of

the state’s Class V rules (Chapter 16 Section 7 WQRR). A general permit is a permit issued to a class of

operators, all of which inject similar types of fluids for similar purposes. General permits require less

information to be submitted by the applicant than individual permits, and do not require public notice for a
September 30, 1999 65

facility to be included under the authorization of a general permit (Chapter 16 Section 2 (l) WQRR). 

General permits specify the subclass of injection facility covered, the geographic area covered, the general

nature of the fluids discharged, and the location of the receiver where the discharge will be allowed.

The state’s coal mining regulations (CRR) provide that a permit applicant who is proposing to

return coal-mining waste to abandoned underground workings must:

C Describe the design, operation, and maintenance of any proposed coal-processing waste facility,

including flow diagrams and any other necessary drawings and maps, for the approval of the DEQ

and the Mine Safety and Health Administration;

C Describe the sources and quality of waste to be stowed, area to be backfilled, percent of the mine

void to be filled, method of constructing underground retaining walls, influence of the backfilling

operation on active underground mine operations, surface area to be supported by the backfill and

the anticipated occurrence of surface effects following backfilling;

C Describe the source of the hydraulic transport mediums, method of dewatering the placed backfill,

retainment of water underground, treatment of water if released to surface streams, and the effect

on the hydrologic regime;

C Describe each permanent monitoring well to be located in the backfilled area, the stratum

underlying the mined coal, and gradient from the backfilled area except where pneumatic backfilling

operations are exempted from hydrologic monitoring; and 

C Be approved by MSHA as well as DEQ prior to implementation (Chapter 7 Section 2(b)(xv)

CRR).

Permit applicants for underground coal mines must describe in their permit application measures to

be taken in the mine to prevent or minimize subsidence, including backfilling of voids (Chapter 7 Section

1(a)(v)(C) CRR).

Siting and Construction

Class V facilities may not be located within 500 feet of any active public water supply well,

regardless of whether or not the well is completed in the same aquifer. This minimum distance may increase

or the existence of a Class V well may be prohibited within a wellhead protection area, source water

protection area, or water quality management area (Chapter 16 Section 10(n) WQRR).

A separate permit to construct is not required under Chapter 3 of the WQRR for any Class V

facility. Construction requirements are included in the UIC permit issued under Chapter 16 (Chapter 16,

Section 5 (v) WQRR). In order to be covered by a general permit, an operator must submit the

information required by Chapter 16 Section 6 (i), (ii) and (iii), which includes a brief description of the

nature of the business and activities to be conducted, information about the operator, and the location of the
September 30, 1999 66

facility. Additional information also may be required as a condition of the general permit. The rules specify

that certain construction and operating requirements must be included (see operating requirements)

(Chapter 16 Section 10(d) WQRR). 

A facility is covered by a general permit as soon as the DEQ has issued a general statement of

acceptance to allow the construction and operation of the facility (Chapter 16 Section 7 WQRR). The

facility must meet construction requirements in Chapter 16 Section 10 WQRR, submit notice of completion

of construction to the DEQ, and allow for inspection upon completion of construction prior to commencing

any injection activity (Chapter 16 Section 5(c)(I)(U) WQRR).

Operating Requirements

The general permit conditions include a requirement that the permittee properly operate and

maintain all facilities and systems, furnish information to the DEQ upon request, allow inspections, 

establish a monitoring program pursuant to Chapter 16 Section 11 WQRR and report monitoring results, 

give prior notice of physical alterations or additions, and orally report confirmed noncompliance resulting in

the migration of injected fluid into any zone outside of the permitted receiver within 24 hours and follow up

with a written report within 5 days. Detailed information requirements also are included in the general

permit, including a requirement to monitor the injectate at a specified frequency and report the information

to DEQ (Chapter 16 Section 7WQRR). A continuous monitoring program normally will not be required,

but monitoring frequency will depend on the ability of the facility to cause adverse environmental damage or

affect human health (Chapter 16 Section 7(e)(v) WQRR).

The rules (Chapter 16 Section 10(d) WQRR) also specify that the permittee must demonstrate:

C Mechanical integrity of any well designed to remain in service for more than 60 days;

C Provision for controlling the type of material injected and to insure that no hazardous waste is

injected;

C Leak detection in all surface piping; 

C Provision for insuring that the backfill remains within the permitted area of injection; and

C Provision to ensure that the injection does not cause a ground water standards violation for the

class of use of the receiver. 

The mining regulations further provide that surface entries and accesses to underground workings

must be located, designed, constructed, and utilized to prevent or control gravity discharge of water from

the mine in excess of state or federal water quality standards (Chapter 7 Section 2(b)(ii) CRR).

Public notice must be given of any proposed measures to prevent or control adverse surface

effects, such as subsidence (Chapter 7 Section 3(a)(ii) CRR).
September 30, 1999 67

Mechanical Integrity

Permittees are required to adopt measures to insure the mechanical integrity of any well designed to

remain in service for more than 60 days. No specific regulatory requirements on MIT have been enacted;

the specific tests to be used will depend on the specific well conditions.

Financial Responsibility

Not specified by statute or regulation. 

Plugging and Abandonment

Wells may be abandoned in place if it is demonstrated to DEQ that no hazardous waste or

radioactive waste has ever been discharged through the facility, all piping allowed for the discharge has

either been removed or the ends of the piping have been plugged in such a way that the plug is permanent

and will not allow for a discharge, and all accumulated sludges are removed from holding tanks, lift stations,

or other waste handling structures prior to abandonment (Chapter 16 Section 12 (a) WQRR).
September 30, 1999 68

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