Help Instruction File:

Anoxic Limestone Drain Module

Provided by the Office of Surface Mining Reclamation and Enforcement (OSMRE), the Pennsylvania Department 

of Protection (PADEP), the U.S. Geological Survey's (USGS) and the West Virginia Department of Environmental 

Protection (WVDEP).
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Anoxic Limestone Drain Module Overview

Table of Contents

1.0 Objective 3

2.0 Treatment Component Overview 3

2.1 Physical and Hydraulic Description 3

2.2 Application and Treatment Chemistry 4

2.3 Anoxic Limestone Bed Conceptual Treatment Module 5

2.3.1 Soil Cover Layer 5

2.3.2 Limestone Layer 5

3.0 Anoxic Limestone Bed Module Overview 6

3.1 Layout and Workflow 6

3.2 Module Inputs 7

3.2.1 Water Quality and Flow Input 7

3.2.2 Sizing Methods 7

3.2.2.1 Retention Time 7

3.2.2.2 Bureau of Mines 8

3.2.2.3 User-Specified Limestone Quantity (tons) 9

3.2.2.4 User-Specified Dimensions 9

3.2.2.5 Volumetric Calculations 9

3.2.2.6 Calculations for Retention Time, Bureau of Mines, and User-Specified Limestone Sizing 

Methods 10

3.2.2.7 Calculations for User-Specified Dimensions Sizing Method 12

3.2.3 System Properties 13

3.2.4 Layer Materials 13

3.2.4.1 Soil Cover Layer 13

3.2.4.2 Limestone Layer 13

3.2.4.3 Pipe Layer 13

3.2.4.4 Liner Layer: 14

3.2.5 Other Items 14

3.3 Module Outputs 14

3.3.1 Sizing Summary 14

3.3.2 Capital Cost 14

3.3.3 Annual Cost 15

3.3.4 Net Present Value 15
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3.3.4.1 Financial Variables 15

3.3.4.2 Cost Categories 15

3.3.4.3 Rationale for Recapitalization Recommendations 17

3.4 Assumptions of Design Sizing and Costs 18

4.0 References 19

5.0 Figures 20

Figure 1: Typical Inverted Trapezoidal Prism 20

Figure 2: Typical Cross Section View of an Anoxic Limestone Drain (ALD) with a water layer. 20

Figure 3: Plan View of the ALD to Illustrate Common Piping Layout and Components (Used in 

AMDTreat Piping Calculator) and the Layer Surface Areas 21

Figure 4: Excavator unrolling Non-woven Geotextile to act as a protective barrier between soil and 

synthetic liner as part of constructing an ALD. 22

Figure 5: Excavator unrolling synthetic liner overtop of a layer of non-woven Geotextile. 23

Figure 6: Triaxle truck dumping limestone on top of synthetic liner during ALD construction. 24

Figure 7: Long narrow ALD. Workers wrapping the limestone with synthetic liner before burial. 

Vertical pipes are used to measure head loss caused by the gradual accumulation of precipitate within 

the porosity of the limestone. 25

Figure 7: Photograph of completed and operating ALD. 26
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1.0 Objective 

An Anoxic Limestone Drain (ALD) is a passive treatment system (meaning that no machinery or daily 

chemical addition is required for its operation) that is typically used to treat net-acidic, anoxic mine 

drainage of low to moderate flow with low to moderate acidity and dissolved metals.

The objectives of this overview are to: (1) Provide a basic understanding of the theory and application of 

ALD and (2) Provide an overview of the ALD module to guide users in developing an estimate of the cost 

to construct, operate, and maintain ALDs for treating mine drainage. Therefore, this overview will be 

presented in two sections, Treatment Component Overview and ALD Module Overview. 

2.0 Treatment Component Overview

The following section is organized into three sections, 1.) Physical and Hydraulic Description, 2.)

Application and Treatment Chemistry, and 3.) Conceptual Treatment Module. The first section provides a 

physical description and hydraulic profile for a ALD to provide context for users of AMDTreat. The 

second section describes the common application of the ALD component. The third section describes a 

generalized conceptual model describing the treatment chemistry to help familiarize users with terms and 

concepts presented throughout the AMDTreat modules. 

2.1 Physical and Hydraulic Description An Anoxic Limestone Drain (ALD) is essentially a buried trench or basin filled with limestone overlain 

by a soil cover. The trench can be rectangular in shape with vertical side walls or can be constructed as 

an inverted trapezoidal prism (basin). The limestone is wrapped like a “burrito” in a synthetic liner to 

prevent water leaking, minimize the introduction of air and to prevent the surrounding soil from migrating 

into the limestone layer (Figure 7). However, the influent and effluent (upstream and downstream) ends 

of the ALD may or may not be wrapped with liner (only non-woven geotextile) and left open in order to 

accept the water to be treated depending on the type of hydraulic connection to the source of water. After 

the ALD is filled with limestone and wrapped in liner, a layer of soil (soil cover) is placed over the liner 

covering the limestone layer to bury or enclose the ALD. An influent pipe transports water into one end 

or side of the ALD and an effluent pipe discharges water from the opposite downstream end or side. The 

effluent pipe carries water to the surface and is positioned at an elevation to ensure the limestone layer 

remains fully submerged and anoxic (minimal or no headspace within the ALD). 

An ALD can be conceptualized as having four distinct “material layers”: soil cover, limestone, piping, 

and liner layers (Figures 2 and 3). Typically, an ALD is filled with limestone ranging from 1.0 to 3.5 

inches in size (AASHTO #1 or #3) and is commonly 3.0-4.0 ft thick (AMDTreat default value is 4.0 ft). 

The soil cover is typically 1.0 to 3.0 ft thick on average. It is recommended to construct the soil cover 

with a peak or crown over the middle of the ALD and gently slope the final surface down and away from 

the crown to encourage surface runoff from precipitation events to flow off and away from the ALD. 

Historically ALDs were constructed as long narrow trenches but many modern ALDs are shorter and 

wider in more of a pond or basin shape with some even constructed in a square shape. The long narrow 

trench-like ALDs contain a small cross-sectional surface area (perpendicular to flow) and tend to quickly 

clog with metal precipitates and sediment resulting in short-circuiting or overall plugging and bypass of 

the ALD. 

ALDs can be installed on underground or surface mine discharges. For underground mine discharges, 

typically an excavator is used to expose the underground mine void along a hillside and a pipe is installed 

as part of a wet seal structure to convey the water from the mine to the ALD. In many cases, a pressure 
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relief pipe is installed at a higher elevation than the ALD inlet pipe so if the ALD clogs the rise in water 

within the mine will be controlled and allowed to discharge to the surface to prevent building up the head 

pressure that could create a mine blowout of the outcrop coal barrier. For ALDs installed on surface mine 

discharges, an excavator is used to excavate the discharge back into the hillside to collect the water and 

construct the ALD. In either case, water flows horizontally through the drain. The piping network is used 

to transport water in and out of the ALD and designed to provide pressure relief as the ALD clogs but is

not typically used for flushing. ALDs are typically discharged to an oxidation and settling pond. 

Figures 2 and 3 provide a perspective of the physical layout of an ALD and Figures 4 through 7 show 

various stages during construction of an ALD to treat mine water. 

2.2 Application and Treatment Chemistry An Anoxic Limestone Drain (ALD) is a type of “passive” treatment component used to treat 

circumneutral pH net-acidic mine drainage. The term “passive” refers to the inability to exert operational 

control to adjust treatment in response to changing influent or effluent conditions. Strictly speaking, 

circumneutral pH is defined as having a pH of 7, but the term is commonly used to describe mine 

drainage with a pH between 5.5 and 8.0. Net-acidic mine drainage describes solutions that contain more 

acid-producing than acid-consuming species when the water is “treated”. Therefore, a net-acidic solution 

is one where the acidity of the solution is greater than the alkalinity (net acidity = acidity – alkalinity). 

Given the pH constraint and net acidic condition, circumneutral net acidic mine drainage typically 

contains ferrous iron and/or manganese in the Appalachian coal fields. ALDs are primarily used to treat 

circumneutral pH net-acidic mine drainage containing ferrous iron. The ferrous iron concentration will be 

at least 1.7 times the concentration of alkalinity (as CaCO3) to create the net acidic condition. 

The primary objective of ALDs is to impart alkalinity and turn the net-acidic mine water to net-alkaline 

(alkalinity > acidity) thereby allowing the ferrous iron to oxidize in a subsequent oxidation/settling pond 

and the pH will be buffered sufficiently and remain > 6.0. The combination of an ALD to impart 

alkalinity followed by an oxidation/settling pond(s) for iron removal has proven to be very effective and 

one of the lowest maintenance passive treatment strategies for mine water. In fact, some ALDs have 

operated for more than 20 years without intervention and consistently achieve water quality limits. 

The primary reason why ALDs are so successful when applied to the appropriate mine water chemistry, 

i.e. mine water containing low dissolved oxygen and low aluminum concentration, is because the ferrous 

iron remains soluble in the ALD. The circumneutral pH water quality criteria avoids aluminum and ferric 

iron-rich waters that would precipitate within the ALD and cause hydraulic failure. If properly designed 

and constructed, the ALD will be anoxic (free of oxygen) and ferrous iron will remain dissolved as 

limestone dissolves and the pH and alkalinity increases. In addition, manganese treatment/removal is not 

expected or observed in properly functioning ALD’s as anoxic conditions prevent the presence of 

manganese oxidizing bacteria and that pH constraints limit abiotic manganese removal mechanisms 

Since ALDs are primarily utilized on circumneutral pH mine water and ferrous iron remains soluble, 

ALDs lack pH and mineral acidity to drive limestone dissolution like in most passive treatment systems. 

Instead, dissolved carbon dioxide is the primary driving force of limestone dissolution and the amount of 

alkalinity that can be generated from the ALD is directly related to the carbon dioxide concentration in 

the raw mine water. Thus, it is important to characterize the raw mine water carbon dioxide concentration 

and calculate whether the ALD can generate the alkalinity to produce net alkaline conditions as part of the 

design evaluation process. Otherwise, a net acidic condition leaving the ALD or within the subsequent 
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oxidation/settling pond will result in iron oxidation consuming all the available alkalinity and the excess 

iron acidity will decrease the pH below 6.0 and decrease the rate of iron oxidation. As a result, the settling 

pond will discharge low pH and net-acidic water containing unoxidized (dissolved) ferrous iron. 

The solubility of aluminum is largely pH dependent, so the geochemical behavior of aluminum will be 

similar in Vertical Flow Ponds, Limestone Beds, and ALDs. As the mine water pH is increased to ~ 5.5, 

most of the aluminum will precipitate and cause hydraulic issues by clogging the limestone void spaces. 

However, Vertical Flow Ponds and Limestone Beds are much more suitable for treating aluminum-rich 

mine water since they have much larger cross-sectional area perpendicular to flow and are also capable of 

periodically flushing some of the precipitates as part of their normal operating conditions. Aluminum 

will still precipitate, but the larger surface area and flushing will allow the beds to function longer before 

hydraulic failure. In addition, Vertical Flow Ponds and Limestone Beds contain freeboard that allows the 

build up of hydraulic head to continue to force the flow of water through the bed as aluminum precipitate 

accumulates in the limestone layer and impact the permeability of the bed. Since ALDs have no freeboard 

layer they lack the ability to increase hydraulic head without causing the rerouting of water around the 

ALD in surface mine discharges or increasing the mine pool elevation in underground mine discharges. 

Thus, ALDs are better suited for treating waters void of ferric iron, aluminum, and dissolved oxygen. 

2.3 Anoxic Limestone Bed Conceptual Treatment Module

The following discussion presents a conceptual treatment model for ALDs treating net acidic 

circumneutral pH mine drainage containing ferrous iron and manganese. 

2.3.1 Soil Cover Layer: The soil cover layer is the top layer and is to serve as a barrier to keep 

atmospheric oxygen from entering the limestone layer. This is done in order to create anoxic 

conditions necessary for the limestone drain to function properly. 

2.3.2 Limestone Layer: The following mine drainage flows from an underground coal mine and 

into an ALD: pH = 5.8, Alkalinity = 31.8 mg/L as CaCO3, Ferrous iron = 170 mg/L, Dissolved 

Oxygen < 0.1 mg/L, and Temperature = 11 °C. Performing chemical speciation of the water 

shows the mine drainage contains 52 mg/L of CO2(aq) and 21 mg/L of Total Inorganic Carbon 

(TIC).

As this mine water flows into the ALD, the dissolved carbon dioxide will react with the limestone 

and produce bicarbonate alkalinity and increase pH. The dissolution of limestone will increase the 

TIC concentration along the flow path within the ALD. Ferrous iron will remain soluble since 

the water lacks an electron acceptor, such as dissolved oxygen, to facilitate oxidation. Assume 

the ALD is constructed with enough retention time to bring the water into equilibrium with 

calcite (CaCO3). At equilibrium, the pH will have increased to 7.2 and the alkalinity will be 118 

mg/L (as CaCO3). It is important to note that very few ALDs are constructed large enough to 

bring the water into equilibrium (saturation index = 0) with calcite and many only achieve a 

saturation index of -1.5 to -0.5. Nevertheless, for the purpose of this discussion assume the water 

discharged from the ALD is in equilibrium with calcite. 
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The water flows from the ALD down a turbulent rock-lined ditch to an oxidation/settling pond. The 

turbulence within the ditch dissolves 6.0 mg/L of dissolved oxygen (O2) into the water and the oxic-rich 

and increased pH environment facilitates ferrous iron oxidation. The 170 mg/L of ferrous iron will 

generate 300 mg/L of acidity (as CaCO3) after oxidation and hydrolysis of the ferrous iron. Thus, even if 

the ALD is constructed large enough to bring the water into equilibrium with calcite, the generation of 

118 mg/L of alkalinity will be insufficient to neutralize the 300 mg/L of iron acidity and the water will 

remain net acidic. Approximately 69 mg/L of the 170 mg/L of ferrous iron would oxidize and precipitate 

before turning the water net acidic again. Further iron oxidation will produce excess acidity and cause the 

pH to decrease and minimize further oxidation. The settling pond will discharge low pH water (< 4.0) 

with a lower but still elevated dissolved ferrous iron concentration. 

3.0 Anoxic Limestone Bed Module Overview

3.1 Layout and Workflow

In general, inputs are on the left-hand side of the module and calculated outputs are on the right. The 

module inputs on the left-hand side are arranged into five sections: (1) Water Quality and Flow Input, (2) 

Sizing Methods, (3) System Properties, (4) Layer Materials, and (5) Other Items. The workflow for the 

module is for users to start at the top left-hand side. Enter the Design Flow and Net Acidity, then select a 

method to determine the amount of limestone contained in the limestone layer of the ALD. Next, users 

specify design parameters such as the depth of the soil cover and limestone layers, and unit costs for items 

affecting the capital cost to construct the ALD. Finally, users can enter any capital cost not captured by 

the module into the Other Items section. 

Module output is provided on the right-hand side of the module. Module outputs on the right-hand side 

are arranged into four sections: (1) Sizing Summary, (2) Capital Cost, (3) Annual Cost, and (4) Net 

Present Value. The sizing summary provides dimensional details for the system. The estimated cost to 

construct and operate the ALD is provided under the Capital Cost and Annual Cost headings. Lastly, 

users can opt to conduct a Net Present Value (NPV) to obtain the total cost to operate and maintain a 

treatment system for a defined time period. 
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A general overview of the module input and output sections are presented below, however, users are 

directed to the numerous tool tips located in the module that provide additional detailed information, such 

as definitions of terminology. In most cases, the tool tips are accessed by clicking on the information icon 

( ) in each of the subheadings in the module. 

3.2 Module Inputs

3.2.1 Water Quality and Flow Input: The Water Quality and Flow Input section is where users specify 

the design flow and net acidity to size the passive treatment system component. The definitions for the 

required input can be found in the tool tip for this section. Click on the information icon ( ) on the right 

side of the Water Quality and Flow subheading. 

3.2.2 Sizing Methods: Users can select from five different methods to size the ALD: Retention Time, 

Bureau of Mines, User-specified Limestone Quantity, and User-specified Dimensions. The module offers 

two different approaches to sizing ALDs. The first approach calculates the mass of limestone (tons) in the 

limestone layer before determining other dimensions. The Retention Time, Bureau of Mines, and UserSpecified Limestone Quantity methods utilize this approach. The first approach is the most common as it 

relates the sizing of the treatment system based on flow and water quality characteristics. The second 

approach requires users to specify the length and width of the freeboard layer (User-specified Dimensions 

method) before calculating the remaining dimensions of the ALD. The second approach is often used 

when the land area available for treatment is restricted, when reverse engineering, or when developing a 

cost estimate for an existing treatment system. 

Calculate Mass of Limestone in Limestone Layer

3.2.2.1 Retention Time – The user enters the desired time to retain the water in the void 

spaces between the individual pieces of limestone. The mass of limestone required to achieve 

the user-specified retention time is calculated by multiplying the Design Flow rate by the 

Retention Time to determine the volume required to retain the water (Equation 1). The 

calculated volume is then divided by the porosity of the limestone layer to calculate the total 

volume of the limestone layer (Equation 2). 

The porosity represents the percent (%) of void space in the limestone layer (volume of the 

voids in between individual pieces of limestone/total volume of the voids and the limestone). 
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Users can specify a value for porosity to match the gradation of limestone they plan to use in 

the limestone layer. Users can specify the porosity value under the Limestone section, which 

is located under the Layer Materials subheading. Based on the user-specified porosity value, 

AMDTreat calculates the Bulk Density of the limestone. The mass of the limestone required 

in the limestone layer is calculated by multiplying the limestone layer volume by the Bulk 

Density.

Results from bench-scale testing indicate that a retention time of 16 hours is a good balance 

between maximizing alkalinity production and the size of the treatment system (mass of 

limestone). Some professionals conduct “cubitainer” testing to determine the amount of time 

required to achieve net alkaline conditions. A cubitainer is filled with limestone and mine 

drainage, with care taken to prevent off-gassing of carbon dioxide (CO2) from the mine

drainage, and alkalinity is measured over time. Some professionals find it appropriate to add 

a safety factor to the retention time since the retention time and contact between limestone 

and mine drainage will change as soon as limestone starts to dissolve. 

See Section 3.2.2.5 to 3.2.2.7 for volume calculations for the other layers.

3.2.2.2 Bureau of Mines - The Bureau of Mines was an agency in the U.S. Department of 

Interior that is now retired but was instrumental in conducting some of the early research in 

how to design passive treatment systems (Hedin and Watzlaf, 1994). A method they 

developed to determine the mass of limestone needed in passive treatment systems was to 

perform two calculations. The first calculation is to determine the mass of limestone required 

to retain the water for a given retention time. This calculation is the same as Equation 3,

described above for the retention time sizing method. 

A second calculation (Equation 4) is used to determine the mass of limestone required to 

neutralize the acid loading of the mine drainage for a specified time frame (Neutralization 

Period). The mass of limestone required to neutralize the acid loading is determined by 

multiplying the user-specified Design Flow by the Net Acidity and the Neutralization Period. 

The resultant value is divided by both the Purity and Dissolution Efficiency of the limestone 

to correct for the portion of limestone that will not dissolve to produce alkalinity and account 

for the fact that larger pieces of limestone are not likely to efficiently dissolve due to a loss of 

surface area from weathering and mineral precipitate coatings (i.e. iron hydroxide) that form 

on the surface and disrupt dissolution. A Dissolution Efficiency of less than 100% will 

increase the mass of limestone required in the LB to compensate for the mass of limestone 

that is unlikely to dissolve. Users can specify the Purity and Dissolution Efficiency of 

limestone under the Limestone section, which is located under the Layer Materials

subheading. 
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The mass of limestone required to retain the water for a specified time (Retention Time) is 

added to the mass of limestone required to neutralize the acidity loading for the 

Neutralization Period to determine the total mass of limestone required for the ALD

limestone layer (Equation 5). This method ensures the user-specified Retention Time is 

preserved, to generate maximum alkalinity, at the end of the user-specified Neutralization 

Period when the limestone mass dedicated to neutralizing the acidity load is nearly depleted.

See Section 3.2.2.5 to 3.2.2.7 for volume calculations for the other layers.

3.2.2.3 User-Specified Limestone Quantity (tons) - If users have their own method for 

determining the mass of limestone, the user-specified limestone method can be used to 

calculate the dimensions of the limestone layer and remaining layers of the ALD. The 

mass of limestone is entered, and the user-specified porosity value for the limestone layer 

is used to calculate the volume and retention time of the limestone layer. See Section 

3.2.2.7 for volume calculations for the other layers.

3.2.2.4 User-Specified Dimensions – Mine drainage discharges often emerge at the base of 

hillsides. The available land area for constructing a treatment system between the point of 

emergence and the receiving stream can be limited. The use of other sizing methods may 

produce an ALD that is larger than the available land area. A smaller ALD can be 

designed for the site, but it will most likely require more maintenance and prematurely 

plug due to high metal loading per unit area/volume. Alternatively, a smaller ALD could 

be designed to treat only a portion of the mine drainage while bypassing the rest. To use 

this sizing method, enter the top length and width of the limestone layer. AMDTreat then 

uses the user-specified side slopes and depths to calculate the dimensions and volumes of 

the ALD layers. 

Volumetric Calculations 

3.2.2.5 Volumetric Calculations –AMDTreat models all passive treatment systems, 

including ALDs, as an “inverted” trapezoidal prism (Figure 1). AMDTreat calculates the 

overall dimensions of a passive treatment system by first breaking up the system into 

individual soil cover and limestone layers. Each individual layer and the geometric shape 

of the overall system is modeled as an inverted trapezoidal prism. AMDTreat sizing 
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methods fall into one of two categories: (1) users input information that is used to 

calculate a layer volume or surface area is calculated or users specify the mass of 

limestone, or (2) users specify the top width and length of the limestone layer. In either 

case, AMDTreat uses the information generated by the sizing method, along with userspecified depths, side slopes, and length-to-width ratios to determine the dimensions of a 

layer shaped as an inverted trapezoidal prism. What differs is the approached used to 

calculate the geometry. The two approaches are provided below

3.2.2.6 Calculations for Retention Time, Bureau of Mines, and User-Specified Limestone 

Sizing Methods –

The equation for an inverted trapezoidal prism is: 

Where:

V = Volume of trapezoidal prism

LTWT =Length & Width of top of prism

LBWB =Length & Width of bottom of prism

D = Depth

These sizing methods determine the volume of the limestone layer. Therefore, AMDTreat 

calculates the volume of the limestone layer and users are required to specify the depth of 

the limestone layer, inside slope, and length-to-width ratio for the bottom of the 

limestone layer. The unknowns are the length and width of the top and bottom 

dimensions of the trapezoidal prism for the limestone layer. Since the depth and inside 

side slope of the ALD is known, the top width and length of the limestone layer can be 

written in terms of the bottom length and width to reduce unknowns in the equation. 

Furthermore, since the bottom length-to-width ratio is known, the bottom width can be 

written in terms of the bottom length. Now Equation (8) is rearranged to solve for the 

bottom length of the limestone layer. The resultant equation is a quadratic: 

Where:

V = Volume of Limestone Layer 

LBLL = Bottom Length of Limestone Layer

D = Depth of Limestone Layer

S = Inside Side Slope of ALD

R = Length to Width Ratio of Bottom of Limestone Layer
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Once the bottom length is known, the bottom width of the limestone layer (WBWL) is 

calculated by dividing the bottom length by the user-specified value for length-to-width 

ratio for bottom of the ALD:

The two remaining unknown dimensions of the limestone layer are the top length and 

width. The top length of the limestone layer is calculated by: 

Where:

LTLL =Top Length of Limestone Layer

LBLL = Bottom Length of Limestone Layer 

DLL = Depth of Limestone Layer

S =Inside Side Slope of ALD (rise/run)

The top width of the limestone layer is determined by the same approach. Now that all 

the dimensions of the limestone layer are known, the dimensions of the water and 

freeboard layers can be determined using a similar approach: 

Soil Cover Layer

The bottom length of the soil cover layer (LBSCL) is equal to the top length of the 

limestone layer (LTLL). 

The top length of the soil cover layer (LTSCL) is calculated by: 

Where:

S =Inside Side Slope of ALD (rise/run)

DSCL = Depth of Soil Cover Layer

Equation (8) is used to calculate the volume of the soil cover layer. 
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In addition to assuming all passive treatment systems are constructed as an inverted 

trapezoidal prism, an additional assumption is that the excavation volume is equal to the 

summation of the limestone and soil cover layer volumes:

3.2.2.7 Calculations for User-Specified Dimensions Sizing Method – When users specify 

top width and length of the limestone layer for a system, AMDTreat uses the userspecified values for inside side slope, and the soil cover and limestone layer depths to 

calculate the dimensions and volumes of the individual layers of the ALD.

The bottom length of the limestone layer (LBLL) is calculated by: 

LLBBBBBB = LLTT − 2 ∗ SS ∗ (DDFF ) (1111)

Where:

LBLL = Bottom Length of Limestone Layer 

LTLL =User-Specified Top Length of Limestone Layer

DFL = Depth of Freeboard Layer

S =Inside Slope of ALD (Run/Rise)

The bottom width of the limestone layer is calculated using the same approach. 

The volume of the limestone layer is determined by: 

Where:

VLL = Volume of trapezoidal prism-shaped Limestone Layer

WTLL = Top Width Limestone Layer

LTLL = Top Length Limestone Layer

WBLL = Bottom Width Limestone Layer

LBLL = Bottom Length Limestone Layer

DLL = Depth of Limestone Layer

Soil Cover Layer

The bottom length of the soil cover layer (LBSCL) is equal to the top length of the 

limestone layer (LTLL). 
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Now that the bottom width and length of the soil cover layer is known, the user-specified 

values for inside slope and depth of soil cover layer are used in Equation (13) to calculate 

the top width and length of the soil cover layer. Equation (14) is used to calculate the 

volume of the soil cover layer. 

Lastly, the excavation volume is determined by adding the volumes for the soil cover and 

limestone layers (Equation 12). The model assumes that the excavated earth excess 

material is disposed of on-site. 

3.2.3 System Properties: The Systems Properties section allows the user to specify the inside slope of 

the ALD, the limestone layer bottom length-to-width ratio, and the excavation unit cost. 

3.2.4 Layer Materials: Unlike active treatment systems which require the continuous addition of 

chemicals to maintain treatment of mine drainage, passive treatment systems typically consist of

impoundments filled with enough reagent (e.g., limestone) to provide treatment for many years. For 

some passive treatment system components, such as ALDs, limestone is used within the impoundment to 

manipulate the geochemistry and control oxidation/reduction and solubility reactions as the mine drainage 

flows through the system. AMDTreat considers each reagent or material as a “layer” in the 

impoundment. Both “reagent” and “material” are used to describe layers because not all layers are 

comprised of reagents or materials meant to manipulate water chemistry. For example, the liner layer is 

designed to prevent leakage from the constructed impoundment feature and also to provide a separation 

material between the excavated soil face and the treatment media (e.g., limestone). The Layer Materials

section is where users can control the depth and other characteristics of the materials used in an ALD. 

3.2.4.1 Soil Cover Layer: Users specify the depth of the soil used to cover the ALD. A typical soil 

depth is between 1.0 and 3.0 feet. The soil cover layer is most likely not a uniform depth in order 

to construct a crown in the middle of the ALD and then to gradually slope down away (<1%) 

from the crown to promote positive drainage of surface runoff off of the ALD to minimize 

infiltration.

3.2.4.2 Limestone Layer: The limestone layer is the main source of alkalinity generation in an 

ALD. Users can specify the depth, purity, dissolution efficiency, porosity and unit cost for both 

the material and placement of limestone in this section. A typical limestone depth is 4.0 feet and 

limestone used in passive treatment system components such as ALDs normally have purity 

greater than 85% calcium carbonate (CaCO3) by weight. The dissolution efficiency is used to 

compensate for limestone losing surface area and becoming less reactive as it dissolves/weathers 

and as a safety factor for additional limestone to compensate for potential plugging or coating 

(a.k.a. armoring) issues. 

3.2.4.3 Pipe Layer: The piping layer considers the piping required to convey the water into and/or 

out of the ALD. The piping layer may be situated between the limestone and liner layers or 

within the limestone layer and is necessary to distribute the water and promote flow horizontally 

and vertically through the treatment media and collect the water and discharge it out of the ALD. 

The ALD module offers users two methods to estimate the cost to purchase and install the piping 

network, the AMDTreat Piping Calculator or User-Specified Piping Layout. The default design of 
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the AMDTreat Piping Calculator for ALDs is different than for Vertical Flow Pond, however, it 

is very similar to Limestone Beds. Similar to the Limestone Bed piping default design, the ALD 

default piping layout includes a perforated header pipe installed at both the influent and effluent 

ends of the limestone layer to transport water in and out of the ALD. The perforated header pipes 

are intended to evenly distribute the raw mine water across the width of the limestone bed and to 

draw water both horizontally and vertically through the treatment media into the effluent pipe for 

discharge from the LB. The exact placement of the influent and effluent perforated header pipes 

within the limestone layer of an ALD is at the discretion of the designer, but Figures 1 and 2 

show typical locations for both header pipes. The header pipe is attached to the influent/effluent 

pipes using a “Tee” or Elbow coupler depending upon the pipe orientation. Users can also opt to 

enter custom pipe lengths, diameter, and unit costs to develop a custom piping design by selecting 

the User-Specified Piping Layout.

3.2.4.4 Liner Layer: Many mine sites lack the soil characteristics required to prevent leaky passive 

treatment system components. AMDTreat offers three types of liner systems that prevent water 

from leaking out of the impoundment including Clay, Synthetic (i.e. PVC), and Geosynthetic 

Clay (GCL) liners (Figure 4, 5, and 6). The volume of clay required is calculated from the entire 

inside sloped area of the limestone layer and multiplying it by the user-specified thickness 

(typically 0.5 to 1.0 foot after compaction) and unit cost to purchase and install the clay liner. 

However, since an ALD has a soil cover over the limestone layer, no clay liner is placed between 

the soil cover and the limestone so it is highly recommended that if the user selects clay liner that 

they also select the non-woven geotextile in order to at a minimum provide the separation 

material (non-woven geotextile) between the soil cover and limestone layers as well as around the 

bottom and sides of the ALD to keep the limestone from penetrating into the clay liner. The area 

required for Synthetic or Geosynthetic Clay (GCL) liners is determined by calculating the inside 

sloped area of the limestone layer and the bottom area of the ALD similar to the clay liner and 

adding the top surface area of the limestone layer in order to provide sufficient line material to 

enclose the entire limestone layer and also provide separation between the soil cover and 

limestone. AMDTreat provides the option of adding Non-woven Geotextile as a separate material 

for all liner systems. Non-woven geotextile separation material is commonly used to protect the 

Synthetic or GCL liner from the excavated inside surface of the pond and/or the limestone layer. 

The non-woven geotextile is also important to provide separation between limestone layer and the 

soil cover for an ALD.

3.2.5 Other Items: The Other Items section allows users to consider the capital cost of inlet and outlet 

structures, valves, and flow distribution structures. AMDTreat suggests default quantities and unit 

costs for construction of a single ALD component. 

3.3 Module Outputs

3.3.1 Sizing Summary: The Sizing Summary section contains all the calculated module outputs, such 

as the dimensions, volumes, and plug flow retention times for the applicable layers. The mass of 

limestone required for the limestone layer is provided on the right-hand side of the Sizing Summary 

heading.

3.3.2 Capital Cost: This section provides the estimated costs for the various user-specified components 

and the total estimated cost to construct the entire ALD component of the passive treatment system.
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3.3.3 Annual Cost: Since an ALD is a passive treatment system component, the only annual cost 

considered is the cost for maintenance. Annual costs, such as sampling, labor, and access road 

maintenance are “project-wide” costs since they apply to an entire treatment site, not each individual 

treatment component. Sampling, Labor, and Site Development (i.e. snow plowing) modules are located 

under the “Project Modules” section of the Main User Interface and capture these types of annual costs. 

Annual maintenance costs for ALDs can include periodic “flushing” of the piping network to evacuate 

metal hydroxide precipitates from the limestone layer, maintaining clear influent and effluent piping, and 

manipulating the water level control structure (e.g. Agri drain) to change the water level in the ALD. 

However, it is important to note that it is not recommended to flush ALDs due to the introduction of air 

into the limestone layer, which will compromise the anoxic conditions for some period within the ALD, 

which could result in iron precipitation and plugging of the limestone. It is assumed that some years will 

not require the full amount of funds set aside for maintenance but will eventually be used over the system 

life. Users can estimate the annual maintenance costs by assuming a percentage of the capital cost (more 

expensive the system, the more expensive the maintenance) or specify an annual maintenance cost 

amount. ALDs have one of the lowest annual maintenance requirements of any passive treatment 

component when applied to the appropriate water chemistry and implemented properly.

3.3.4 Net Present Value: The Net Present Value (NPV) section determines the cost to operate a 

treatment system component over a specified time frame. The NPV calculates the present-day financial 

investment required to generate the income to pay for future operation and equipment/materials 

replacement costs. Both Financial Variables and Cost Categories are required to calculate the NPV.

3.3.4.1 Financial Variables - The Term of Analysis, Inflation Rate, and Rate of Return are three 

variables used in the NPV calculations. The default values for these terms are shown under the 

Net Present Value section of each module. Users must access the Net Present Value menu at the 

top of the main user interface to change the default values as they would apply to all modules 

used for an entire treatment system. While NPV is determined for each AMDTreat module 

activated by the user, the goal is to determine a total NPV for an entire mine drainage treatment 

system project (a collection of cost estimates for individual modules creates a treatment system 

project in AMDTreat). Therefore, a single value for Term of Analysis, Rate of Return, and 

Inflation Rate is applied to all modules and cannot vary between modules.

• Term of Analysis: The time period used for the NPV calculation is to determine the 

financial investment required to pay for all future costs of the treatment system

components.

• Inflation Rate: Represents the average price increase of goods and services over time. 

AMDTreat uses the inflation rate to calculate the future cost of the annual operation and 

maintenance (O&M) and recapitalization items (e.g., limestone). 

• Rate of Return: Describes the expected profit on an investment. 

3.3.4.2 Cost Categories - For each treatment module, AMDTreat provides a list of recommended 

equipment and materials that require recapitalization. In addition, AMDTreat provides 

recommendations (default values) for life cycle and replacement percentage. Users can click on 

the default values for Life Cycle or Replacement Percentage and use the +/- buttons to change the 

default values. In addition, users can select Custom Cost and enter a new cost to represent the 
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current cost of the equipment. Users can add new recapitalization items or deactivate/delete 

existing items for calculating the NPV.

An example of how the recapitalization variables are used to determine NPV is to consider the 

following hypothetical scenario. Assume a vertical turbine pump has a life cycle of 50 years but 

requires the motor component of the pump to be rebuilt every 20 years. Let’s assume the presentday cost to purchase the pump is $500,000 and the cost to remove, rebuild, and reinstall the pump 

motor is only $20,000. Now assume we want to determine the amount of investment required 

today (NPV) to generate the income to pay for the future cost of rebuilding the pump motor over 

a 50-year Term of Analysis, which is also equal to the life cycle of the pump. In addition, let’s 

assume an inflation rate of 5.0% and Rate of Return of 8.1%. The goal is to place the money in a 

relatively secure investment vehicle to generate 8.1% annually. The NPV will tell us the size of 

investment required to generate the income to pay for the future costs over 50 years.

There are several ways to model the replacement cost. One way is to replace 4% of the present 

day cost of the pump (4% of $500,000 = $20,000) with a life cycle of every 20 years. If the Term 

of Analysis is 50 years, then the entire pump would not require recapitalization since the life 

cycle of the pump is 50 years. However, the motor would require two replacements (50 years / 20 

years = 2.5 rounded down to 2). 

To determine the NPV to recapitalize rebuilding of the motor, AMDTreat calculates the future 

cost to rebuild the motor at each life cycle, 20 and 40 years. The program uses the Inflation Rate 

to inflate the present day default cost to rebuild the motor in 20 and 40 years from now. While 

the present day cost to rebuild the pump motor is $20,000, the future cost to rebuild the motor in 

20 years at a 5.0% Inflation Rate is $53,065 and $140,799 in 40 years (Equation 8). Assuming an 

8.1% Rate of Return, the 50-year NPV for the pump is $17,422. In other words, an initial 

investment of $17,422 is needed at an annual Rate of Return of 8.1% to generate the investment 

income required for the two motor rebuilds over the 50-year life cycle of the pump.

• Annual Operation and Maintenance Cost: By default, AMDTreat transcribes the annual 

O&M cost from the Annual Cost section to the Net Present Value section. The program 

assumes the module is being used to first estimate the annual cost for a treatment system 

component, so it automatically transcribes the annual cost to the NPV section. If this is 

not the case or the user wants to use some other annual cost, the “Use Custom” box can 

be selected to allow the user input of a different annual cost to utilize in the NPV 

calculation. 

• Recapitalization Cost: Certain treatment system components, especially mechanical and 

water conveyance equipment, require periodic replacement. The recapitalization cost of 

an item is an estimate of the amount of money required to pay for future replacement 

costs for the item. In addition to the Financial Variables described above, three additional 

values are required to calculate the NPV of recapitalization costs, the Present-Day

Equipment Cost, the Life Cycle, and the Replacement Percentage.
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• Default Cost: This represents the current cost to purchase the equipment or material.

• Life Cycle: The time span between equipment or material replacement is termed as its

Life Cycle. Some equipment manufacturers provide recommended life cycles for their 

equipment to provide consumers with an estimate of how long the equipment is expected 

to be operational. Some life cycles, such as those used for treatment media (limestone),

are based on best professional judgement. Some operators prefer to periodically purchase 

and replace equipment before failure to preserve the continuity of operations, while 

others wait until failure to replace an item.

• Replacement Percentage: The Replacement Percentage is an adjustment factor to the 

Default Cost to accommodate situations where the entire piece of equipment or all of the 

material does not require recapitalization. For example, a passive treatment component 

may be designed to contain enough limestone to neutralize the acidity load for 20 years, 

however, the accumulation of metal hydroxide precipitates within the void space of the 

limestone layer may require that 25% of the limestone be replaced every 7 years to 

prevent hydraulic failure such as plugging or short-circuiting. For this scenario, the 

initial cost of the limestone making up the limestone layer is discounted by 75% and 

assigned a life cycle of 7 years to determine the amount of money required to cover the 

cost of replacing 25% of the limestone layer every 7 years over the Term of Analysis. 

3.3.4.3 Rationale for Recapitalization Recommendations: 

Recapitalization recommendations are based on professional experience of the 

AMDTreat Team and may not apply to all situations. Users are encouraged to customize 

the recapitalization assumptions to their treatment scenario. AMDTeat Team members 

are located in Pennsylvania and West Virginia and have collective experience in design, 

funding, and/or operation/maintenance for over 100 passive treatment systems. Most 

passive treatment systems constructed in Pennsylvania and West Virginia are less than 15 

years old, thus the industry is still gaining experience in lifecycles for various 

components. However, the AMDTreat Team held discussions on personal experience to 

develop a list of recapitalization recommendations. Users may have different experience 

and opinions. 

By default, AMDTreat includes a list of five recapitalization items for Limestone Bed. 

The items are listed even if they are not selected for the treatment system. The purpose of 

this is to inform the user of items they should think about recapitalizing or including in 

their treatment design. For example, Liner is listed as a recapitalization item even if no 

liner is selected in the cost estimation. In this case the recapitalization cost is set to $0.00 

but the item is still shown as a reminder to the user that a Liner is a common item that 

should be considered. If a liner is included in the cost estimation, the estimated liner cost 

is used to calculate a recapitalization cost for liner. Users can delete or modify any of the 

default Recapitalization items by either deselecting the item or by setting the 

Replacement % to zero. If the item is deselected the Total Cost for the item will still be 

shown but the cost will be subtracted from the Net Present Value Cost, shown in the Net 

Present Value Heading.
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Limestone: The limestone can require maintenance and periodic partial replacement, 

mostly due to clogging from metal precipitate. Therefore, the default recommendation in 

AMDTreat is to replace 50% of the limestone every 10 years. 

Liner: Often liners are destroyed, especially synthetic, while conducting maintenance on 

passive treatment systems, if the equipment operator is inexperienced with passive 

systems. Thus, AMDTreat recommends planning to replace a portion of the liner while 

the Limestone is being replaced. The default assumption is to replace 50% of the 

Limestone every 10 years, so the same assumptions are recommended for Liner 

recapitalization.

Pipe: Piping is often exposed at the surface and can become damaged by vandalism, 

falling trees, and other factors. AMDTreat assumes 50% of the piping will require 

replacement after 25 years of service.

Other Items: The Other Items section in AMDTreat includes Flow Distribution 

Structures, Water Level Control Structures, and Outlet Protection Structures. These 

structures are often durable, and many treatment systems have never experienced 

replacement of these items, however, AMDTreat Team members have had at least one 

experience of each of these items requiring replacement, thus the Team wanted to include 

these items for replacement consideration. 

3.4 Assumptions of Design Sizing and Costs: AMDTreat is a cost estimation model that 

uses assumptions to provide treatment sizing and both capital and annual cost estimates. While there are 

many assumptions in the program, the ones provided below are important to consider for the LB module. 

1. The Limestone bed is assumed to be constructed on a flat surface, where the water and limestone 

layers are excavated at or below the existing or original ground. A portion of the excavated

material is then used to construct the freeboard embankments above the existing or original 

ground (see Figure 2). AMDTreat assumes an on-site balance of cut and fill material. However, if 

this is not the case, the user can modify excavation cost default values to reflect the anticipated 

conditions based on the site characteristics.

2. The geometric shape of a ALD is an inverted trapezoidal prism (Figure 1). The volumetric 

equation for a trapezoidal prism is used to calculate the ALD layer volumes.

3. None of the sizing methods provide a “design life” that properly accounts for both: A) the amount 

of limestone required to neutralize the acidity loading for a defined time frame, and B) hydraulic 

and treatment issues caused by the accumulation of metal hydroxide precipitates within the void 

spaces of the treatment media. Some sizing methods, such as the Bureau of Mines, account for 

reagent requirements.

4. In general, the goal of the ALD sizing methods is to provide an estimate of the limestone mass 

required to generate net alkaline conditions. None of the sizing methods are based on achieving 

effluent water quality criteria or requirements. With that said, a properly sized ALD treatment 

system should generate pH > 6.0 and net alkaline water, which in theory will decrease aluminum 

and ferric iron solubility to less than 1.0 mg/L.
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4.0 References

Hedin, R.S. and Watzlaf, G.R., 1994. The effects of anoxic limestone drains on mine water chemistry. 

Proceedings of the 3rd International Conference of the Abatement of Acidic Drainage, Volume 1, 

Pittsburgh, PA. U.S. Bureau of Mines SP 06B-94, p. 185–194.

Rose, A. W., 2004. Vertical flow systems – Effects of time and acidity relations. Proceedings, American 

Society of Mining and Reclamation, Morgantown, WV, April 18-24, 2004, p. 1595-1615. 

https://www.asrs.us/Portals/0/Documents/Conference-Proceedings/2004/1595-Rose.pdf
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5.0 Figures

Figure 1: Typical Inverted Trapezoidal Prism

Figure 2: Typical Cross Section View of an Anoxic Limestone Drain (ALD)

with a water layer. 
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Figure 3: Plan View of the ALD to Illustrate Common Piping Layout and 

Components (Used in AMDTreat Piping Calculator) and the Layer Surface 

Areas
22

Figure 4: Excavator unrolling Non-woven Geotextile to act as a protective 

barrier between soil and synthetic liner as part of constructing an ALD. 
23

Figure 5: Excavator unrolling synthetic liner overtop of a layer of non-woven 

Geotextile. 
24

Figure 6: Triaxle truck dumping limestone on top of synthetic liner during 

ALD construction.
25

Figure 7: Long narrow ALD. Workers wrapping the limestone with synthetic 

liner before burial. Vertical pipes are used to measure head loss caused by 

the gradual accumulation of precipitate within the porosity of the limestone.
26

Figure 7: Completed and operating ALD.