This chapter discusses when and where overburden analysis (OBA) is needed and how OBA can be used as one of the tools in the prediction of mine drainage quality. It also describes procedures for overburden sampling and sample handling. Sampling is addressed from a practical experience-oriented perspective, rather than theoretical.

The results of overburden analyses are generally used in two ways:

1. as a permitting tool and

2. as a management tool.

Permitting Tool

As a permitting tool, OBA can be used to:

1. demonstrate that the proposed mining can be accomplished without causing pollution to surface or ground water;

2. assess the probable cumulative impacts of mining on the hydrologic balance; and,

3. aid in the design of the mining and reclamation plan to prevent or minimize damage to the hydrologic balance within and outside the proposed permit area.

Pennsylvania regulations require an OBA to be submitted for a mine site unless it is determined by the Department that "it has equivalent information…" (25 PA Code Chapter 87, §87.44). OBA is seldom waived for sites that meet any of the following criteria:

1. the proposed mine site lies within a High Quality, Exceptional Value, Wilderness Trout Stream, or other stream sensitive to mining impact;

2. the proposed mine site is in close proximity to a public water supply or mining has the potential to impact a public water supply or numerous private water supplies;

3. postmining water quality data in the watershed on the seam(s) of interest is poor or unavailable;

4. there is a history of acid mine drainage (AMD) in the local area, and the applicant is proposing to mine the same seams that caused AMD in the past;

5. alkaline materials: e.g., limestone or calcareous shale, sandstone, or siltstone, are proposed to be removed from the mine site.

OBA Waivers

When the proposed mine site does not meet any of the above criteria, waiving of the requirement for OBA may be considered. A waiver request must show that there is equivalent information available. The Department of Environmental Protection’s Bituminous Surface Mine application requires that the request include:

1. a discussion of the relationship between the proposed mine site and the five categories discussed above;

2. an explanation of the existing hydrogeologic information that supports the waiver (for example, stratigraphy, water chemistry, and nearby overburden analyses); and

3. previous mining history on the watershed or adjacent areas (including percentage or relative acreages of mined and unmined portions of the watershed) and the postmining water quality associated with the proposed seams (including specific examples).

Other geochemical clues, such as water quality from previously mined areas, can often help support a request for a waiver of overburden analysis. These other prediction tools and how they are used are discussed in Chapters 8, 9, and 10.

Management Tool

As a management tool OBA can be used to:

1. calculate alkaline addition rates;

2. determine the distribution of pyritic zones which may require special handling or avoidance;

3. identify alkaline zones which can be incorporated into a mining plan to prevent acidic drainage; and,

4. determine mining feasibility, including potential environmental impacts, before investing a large amount of money in leasing (advance royalties) and permit application preparation.

The site specific mining data needed to properly plan an OBA includes:

1. Mining limits:

a. boundaries of the proposed area to be affected by coal removal;

b. proposed maximum highwall heights;

c. type of mining - for example, contour/block cut or hill top removal; and

d. accessibility.

2. Geologic considerations such as coal seam identification, depth of weathering, and stratigraphic variation.

3. Information that is available in the permit files of the District Mining Offices, such as water quality data from previous permits or applications covering the same or adjacent areas.

4. OBAs from the same or adjacent areas.

5. Publications of the Pennsylvania Topographic and Geologic Survey, United States Geologic Survey (USGS), the former United States Bureau of Mines (USBM), US Army Corps of Engineers, and Previous Mine Drainage Pollution Abatement Watershed projects conducted under Operation Scarlift funded under Act #443, entitled the "Land and Water Conservation and Reclamation Act" from the late 1960s and early 1970s. These publications can include information such as:

a. coal-bed outcrop maps,

b. generalized stratigraphic sections,

c. coal seam isopach maps,

d. structure contour maps.

Particularly useful publications include "Map 61" the "Atlas of Preliminary Geologic Quadrangle Maps of Pennsylvania," (Thomas M. Berg and Christine M. Dodge), and Mineral Resource Reports M86, M89, M90, M91, M92, M93, M94, M96 and M98 which are about the coal resources of Greene, Allegheny, Butler, Fayette, Clarion, Washington, Westmoreland, Cambria and Blair, and Indiana Counties respectively. These publications are distributed by the Bureau of Topographic and Geologic Survey and are available from the State Book Store. Additional sources of information can be identified from the "Pennsylvania Geological Publications." This catalogue is published by the Bureau of Topographic and Geologic Survey and contains listings of many of the geologic reports written about Pennsylvania.

Old and current deep mine maps are available from the Office of Surface Mining, Appalachian Region Coordination Center, at 3 Parkway Center, Pittsburgh, PA; Pennsylvania Bureau of District Mining Operations, Division of Deep Mine Permitting in McMurray; and Pennsylvania Bureau of Deep Mine Safety in Pottsville and Ebensburg. These agencies have map repositories containing prints, originals, and microfilm. Copies of these documents are readily obtained. These repositories include the Works Progress Administration (W.P.A.) deep mine maps prepared in the 1930s. These maps cover nearly all the coal fields. Each map covers an area that is 1/9 of a 15' quadrangle. In addition to showing mining limits, deep mine maps frequently show structure contours. This information can be very helpful in planning OBA drilling.

Other considerations in developing an OBA drilling plan include:

The obvious and most frequently asked questions that operators and consultants have when preparing an OBA proposal are:

1. Should holes be drilled for OBA?

2. How many OBA holes are needed?

3. Where should they be drilled?

Once these details have been worked out, preliminary work can be started.

The first step in the development of an OBA proposal is to plan for the drilling. While there may appear to be savings associated with performing the drilling for the overburden analysis at the same time as the initial exploration drilling, it is generally preferable to exploratory drill the entire site first. This preliminary drilling enables the determination of cover heights and the lateral extent of the various lithologies. This information can then be used to better locate the overburden holes to represent lithologic variation and degree of weathering within the site. If research and exploration are done prior to drilling the OBA holes, it is less likely that there will be a need to drill additional OBA holes later in the permitting process.

Areal Coverage---Number of Holes per Ac (ha)

A rule of thumb developed in Pennsylvania in the 1980s to determine a suggested minimum number of overburden holes was:

Number of ac (ha) to be mined Number of

100 ac (40.47 ha) + 2 = Overburden Holes

If the first part of the equation resulted in a fraction, it was rounded to the closest whole number. For example:

143 ac 57.9 ha

100 ac + 2 = 3 or 40.5 ha + 2 = 3

49.99 ac 20.2 ha

100 ac + 2 = 2 or 40.5 ha + 2 = 2

179 ac 72.4 ha

100 ac + 2 = 4 or 40.5 ha + 2 = 4

The above tables give an idea of the range of sampling intensity used in Pennsylvania. The ranges in the data are due to a multitude of factors. These include stratigraphic complexity of the site, shape of the site, and availability of other prediction tools. The data apply only to permit applications that had overburden analysis data. Approximately 30 to 40% of applications do not require submittal of overburden analysis because of the availability or equivalent prediction data.

Operational Considerations

The overburden analysis drilling program must accurately represent the overburden to be encountered. Therefore, the overburden holes must be located within the limits of the proposed mining area. Some holes must be located at maximum highwall conditions, and the holes must represent all of the strata to be encountered by mining.

Other holes should be located under low and average cover conditions to provide representative sampling of the overburden where zones may be missing or which may have been altered due to surface weathering.

Stratigraphic Variation

It is important to provide enough drill holes to adequately represent the site, including any spatial lithologic variation. One of the first references to the minimum overburden hole spacing was contained in the West Virginia Surface Mine Drainage Task Force’s "Suggested Guidelines for Surface Mining in Potentially Acid-Producing Areas" (1978), which recommended that all surface mining in potentially acid producing areas be within 1 km (approximately 3300 ft) of a sampled overburden analysis hole or highwall.

Donaldson and Renton (1984) and Donaldson and Eble (1991) indicated that although cores spaced up to 2 mi. (3 km) apart in the Pittsburgh coal were adequate to reflect major thickness and sulfur trends, this spacing was not adequate for mine design. They felt that lateral Pittsburgh coal bed sampling at intervals on the order of 1200 to 1400 ft (365 to 427 m) or less than 500 ft (152 m) for the Waynesburg coal along with geostatistics are necessary to determine small-scale sulfur content trends.

The Problem of Obtaining Representative Samples

The maximum thickness of each lithologic unit to be represented by one vertical sample interval will be discussed later under "Compositing and Laboratory Preparation." It is also discussed on pages 29 to 30 of Part 1 "Collection and Preparation of Sample" in the "Overburden Sampling and Testing Manual" by Noll et al. (1988).

Noll et al. (1988) do not however discuss the subtle complexity of ensuring accurate, non-biased, representative samples. It does stress that it is critical that 100% of the sample volume be included for a sample interval for compositing purposes, because of possible geochemical variations within the three foot (0.9 m) interval. The ultimate sample size used in Acid Base Accounting (ABA) is 1 g for total percent sulfur and 2 g for the neutralization potential (NP) test. Therefore, assuming no loss or contamination of the zone being sampled, only 1 g to 2 g are tested out of a 25,550 g sample (based upon a 4.5 in. (11.4 cm) diameter drill bit and using an average rock density of 170 lbs/ft³ (2,723 kg/m³)).

In reality, this 1 g to 2 g represents a lithologic unit in a drill hole which itself may extend laterally to 25 ac (10 ha) and up to a half a billion grams of material. To better comprehend this degree of representativeness, consider the following analogy: It is equivalent to evaluating the quality of an apple crop by examining one apple in a 7,000 acre (2,833 ha) orchard - for example, an average of 238,000 lbs of apples per ac and approximately 3 apples per lb. (U.S. Dept of Agriculture personal communication, 1996). There has been extensive literature, and, in fact, a complete science integrating geology and statistics to spatial sampling and the determination of optimal sampling patterns for estimating the mean value of spatially distributed geologic variables. For information on geostatistics the authors suggest Journel and Huijbregts (1981), Webster and Burgress (1984), and J.C. Davis (1986).

Fortunately, the geologic systems responsible for the deposition and alteration of the sediments and their chemical quality do not operate in a completely random fashion at the cubic centimeter level and, thus, do not produce overburden samples which are statistically independent. Although there are exceptions, most of the geologic systems, especially those which produce calcareous material, operate over large areas with some degree of order, and deposit laterally pervasive units (Caruccio et al., 1980). Lateral continuity has also been observed in high sulfur strata. Facies changes can provide variations in lithology and the degree of surface weathering can cause changes to the percent total sulfur and NP over short distances. Therefore, it is imperative to know the areal extent of any alkaline or acidic material, high energy paleodepositional environments (for example, channel sandstones), and the degree or depth of weathering. Adequate exploratory drilling is essential to the development of a representative overburden sampling plan.

A recent study suggested that the sulfur is not uniformly distributed in a homogeneous fashion, but is controlled by the nugget effect; i.e., distributed in clusters of hot spots similar to large chips or chunks of chocolate in a cake. If this is the case, accurately determining the mean percent total sulfur of a particular stratum may be difficult, which could in turn lead to under-predicting the potential to produce AMD (Rymer and Stiller, 1989). It could also result in under predicting the site’s alkaline potential.

Concern over the nugget effect may be over-emphasized. The concentration of total sulfur at a mine site may not be the critical factor of whether or not AMD will be produced. The acidity produced in laboratory experiments appears only to be strongly related to percent total sulfur for sulfur values above 1.0 parts per thousand (ppt) with acidity production being negligible for sulfur of lesser value unless there is a paucity of NP (Rose et al., 1983). Department experience has shown that, in the field, sulfur as low as 0.5% (and possibly somewhat less) can be a problem. As discussed in Chapter 11, the presence of significant NP appears to be a more critical factor, than percent sulfur.

The tendency for the mean percent total sulfur at a site to be skewed to the right is probably just a natural distribution of data involving the plotting of a quantity where the left boundary or minimum abscissa is zero, and there is essentially no right boundary. For example, most sulfur values in coal overburden are less than 0.5%; a rare few are as high as 10 to 20%. Pure pyrite has a percent sulfur of 53.4% (the maximum right-hand value). Thus, a few high values will skew data to the right. This is a commonly observed distribution in geologic data (Koch et al., 1980).

Sample Collection

Overburden sampling is accomplished by drilling or direct collection of the sample from an open source such as a highwall. Four types of drilling methods are generally used to obtain overburden sample:

Air rotary rig: normal circulation - This type of drill is the most commonly used drill for the collection of overburden samples in Pennsylvania. Drilling in this manner uses air to blow the rock chips to the surface for collection. The most common pitfall with normal circulation air rotary drilling is that the individual samples of stratum can be contaminated by an overlying sample zone as the rock chips are blown up the annular space of the drill hole.

The rock chips traveling in this space can dislodge loose particles from an overlying source. Care should be taken to stop the downward progression of the drill stem after each interval has been sampled and allow any upper loose particles to blow out prior to continuing downward. Contamination of the sample can also occur at the surface due to the pile of ejected material that forms near the drill hole. These piled materials, if not removed during drilling, can sluff back into the open hole and the chip stream. This can be avoided by shoveling the materials away from the hole during the period when drilling is stopped to blow out the hole. Another option is to add a short length of casing to the top of the hole after the upper few feet have been collected. Samples are collected by placing a shovel under the chip stream. Care should be taken to clean the shovel of any accumulated materials from previous usage or sampling. This is particularly important in sampling of wet test holes where the ejecta consist primarily of mud. Before drilling the overburden hole, the driller should be instructed to clean the dust collector hood to remove any accumulated materials which may dislodge and contaminate the samples being collected.

Reverse Circulation Rotary Rig - This type of drill rig is less commonly used as a drilling platform for the collection of overburden samples, primarily because of availability. A reverse circulation rig uses a double-walled drill stem through which water or air is forced down the outer section of the drill stem and the cuttings/chips are forced up the inner section of the drill stem. The cuttings and water or air are brought into a separator and dropped near the rig where the samples can be collected. The samples are isolated from contact with overlying strata, so this type of drilling offers a much cleaner and quicker means of obtaining overburden samples. Further, the drilling does not need to be stopped to blow out the hole. If water is employed in the drilling process, the materials are also washed free of the fine dust coating which accumulates on the chips during drilling with air. This allows for much easier rock type identification and logging.

Diamond Cores - Diamond core barrels can be used on both types of rotary drilling platforms. Coring provides a continuous record of the lithology present and can provide the geologist with more information than can be obtained through the collection of rock chips (cuttings). Cores can provide a better overall view of the lithology underlying a proposed site by providing the geologist with the ability to judge rock color, gross mineralogy, grain size/texture, fossil content and relative hardness. This type of information is not always readily available from rock chips. Although a core provides an uncontaminated and better source of reliable lithologic data, coring is very time consuming and costly, especially if the entire overburden section is to be sampled by this means. Diamond cores can be used as a secondary means of data collection to isolate previously identified problem zones, or as a primary sampling tool in the area of the coal, i.e. the interval 5 ft above and below the coal horizon. The entire core section must be collected and processed for analysis to ensure representative sampling. A problem that can occur with coring is "core loss."

The problems of core loss during core drilling with a dedicated coring rig can be overcome or minimized by the driller and his helper by regulating the drilling speed, (i.e., rotational speed of the bit, and down pressure), diameter and type of core bit, amount of water, by minimizing the overbearing weight in the core barrel by emptying it prior to drilling the coal interval, and by keeping the equipment in good condition. Knowing what drilling adjustments to make can prevent blocking of the core barrel or causing the rock being cored to crumble and wash.

Successful coring is mostly dependent upon the experience of the on-site geologist, project engineer, or driller. Factors that are important include total years of core drilling experience, experience with the drill being used, and previous drilling experience in the same area, including exposure to the same rock formations and weathering characteristics (Personal Communication Clifford Dodge, William Marks, and Richard Beam). Having as much geologic data as possible (approximate depth to the coal, and extent of weathering) prior to drilling is also particularly useful. It is especially useful to have air rotary pilot holes to evaluate the site prior to the core drilling. These pilot holes allow particularly troublesome formations to be identified and avoided. Particularly troublesome conditions include highly fractured rocks or drilling into a joint or intersection of joints or fractures. Other problem areas include slickensided zones and areas indicative of paleoslump features. In some cases this may require re-drilling nearby, off of the joint or fracture zone (Personal Communication Clifford Dodge).

Other than encountering mine voids or solution channels, the first 10 ft (3 m) or so of unconsolidated soil and rocks as well as the transition through weathered rock into competent rock, is the zone most subject to core loss. Core recovery on the order of only 50 to 60% or less is not unusual. When drilling is done in the unweathered zone (sometimes indicated by an absence of iron staining) core recovery approaching 100% is the norm rather than the exception.

When coring the coal it is advisable to use a core barrel long enough to core the entire thickness of the coal and one that is not more than 20% full when first encountering the coal. It is preferable to have a nearly empty core barrel containing only 6 in. to 12 in. (15 to 30 cm) of overburden, before starting into the coal. The small amount of overburden aids in determining if the entire coal section has been sampled; i.e., knowing the starting and ending points of the coal. It also helps protect the coal from being crushed by the "ram" when extracting the coal from the core barrel (Personal Communication Clifford Dodge, Richard Beam, William Marks).

Besides actual core loss encountered while drilling, drilling data can be lost due to the improper handling of the cores. Possible problems include placing cores in the core boxes in the wrong order or upside down, or damage caused to the core during handling and shipping (Personal Communication Cliff Dodge).

Augering - Auger drilling is not recommended for general overburden sampling. It is typically used for unconsolidated or highly weathered materials. The auger lifts the materials on the auger screw. They are in constant contact with the overlying stratum, thus providing for intermixing. However, augering can be successfully used in homogeneous materials such as glacial till and/or old mine spoil.

Highwall Sampling - Direct collection of samples from an open source, such as a highwall within or near a proposed permit area, can be used for overburden analysis, provided several caveats are understood. First, samples may be weathered to such a degree that they do not represent the strata to be mined. Second, highwall sampling is limited by the availability and accessibility of highwalls. Therefore, care should be taken to collect only unweathered samples from the highwalls in close proximity to and representative of the proposed mining. It is recommended that open source (outcrop, highwall, etc.) samples be used primarily as a supplement to drilled samples.

Sample Description (Log)

Field Preparation

The following procedures should be followed to ensure proper sample preparation in the field:

1. Use the drilling techniques previously discussed.

2. Samples should be immediately placed in airtight containers such as bottles or bags (plastic snap closing). This limits the exposure to air (oxygen), which can change the sulfur from the pyritic form to any of many secondary sulfate minerals.

3. The sample container should be immediately labeled with a permanent marker as to the interval which was sampled.

4. Field logs of the overburden holes should be kept to later help composite and log the represented lithologies at the site.

5. Each hole’s individual sample containers should be immediately placed in a sealable mass container and labeled with hole number, mine name, and operator, if applicable, to avoid possible intermixing of samples from one hole to another from that site or from a different site.

Compositing and Laboratory Preparation

dilution of the zone and result in a falsely low percent sulfur, or make a thicker zone (e.g., 3 ft (0.9 m)) resemble a high sulfur zone, when in fact only the lowest third of the interval is high sulfur. The coal seam may also require greater sample resolution than the suggested 3 ft (0.9 m) if a portion of the coal will be left in the pit as pit cleanings or unmarketable coal. The coal that remains behind should be sampled separately.

As can be seen from Table 5.4, if too many 1 ft (0.3 m) intervals are composited or too large a vertical sampling interval is chosen, a high total sulfur (2.34%), potentially acid producing zone can be masked by dilution with adjacent low sulfur strata. The net effect is an underprediction of the potential for a site to produce acid mine drainage. Compositing one foot of high sulfur black shale with the overlying four feet of low sulfur sandstone results in a 0.48% total sulfur for the composited five foot zone, thus under-predicting the acid producing potential of the black shale.

Sobek et al (1978) suggested that for cores, a 5 inch (12.7cm) section out of the middle of a 1 ft interval can be assumed to be representative of that one-foot interval. The best way to ensure representativeness is to sample the entire interval. In order to avoid bias, one of the following two methods is recommended:

Purpose of Sample Preparation

The procedures for sample preparation are explained by Noll et al. (1988). The two main reasons for splitting and crushing are to provide: