RECOVERY of GROUNDWATER LEVELS AFTER LONGWALL MINING Colin J

IMWA Proceedings 1999 | © International Mine Water Association 2012 | www.IMWA.info

MINE, WATER & ENVIRONMENT. 1999 IMWA Congress. Sevilla, Spain

RECOVERY OF GROUNDWATER LEVELS AFTER LONGWALL MINING Colin J. Booth

Northern Illinois University, Department of Geology & Environmental Geosciences DeKalb, IL 60115, USA Phone:+ 1 815 753 7933, Fax:+ 1 815 753 1945 e-mail: [email protected]

ABSTRACT

Longwall underground coal mining lowers water levels in shallow bedrock aquifers overlying the mine, due mainly to increases in fracture porosity caused by subsidence. The water levels commonly recover partially or wholly after mining, but recovery is less consistent and less predictable than the initial decline. This paper examines the recovery after longwall mining reported in US studies and observed at two sites in Illinois, USA. The two sites were similar in low topographic relief and general strata sequence, but had different recovery behavior. Full recovery occurred at one site (Jefferson) where the sandstone aquifer was thicker and more transmissive. At the other site (Saline), there was no recovery, except locally where a sand-and-gravel unit recharged the aquifer. Based on the studies, factors in predicting recovery include topographic setting, hydraulic separation above the mine and caved zone, aquifer transmissivity, and continuity with recharge sources.

INTRODUCTION laterally, and is normally the first response observed in piezo meters ahead of mining. In less transmissive units, the draw This paper examines the recovery of groundwater levels down is very localized and sudden, whereas in more transmis after the completion of longwall underground coal mining. Reco sive units it is more widespread and gradual. very is less predictable than the groundwater declines which The recovery of water levels over a completed mining occur at the time of mining, in wells over and near longwall operation is probably a more important long-term consideration panels. than the initial depression, since it is more costly and difficult to The initial response is well known. During longwall replace a permanent loss than to provide substitute water sup mining, immediate subsidence causes fracturing and bedding plies for a short-term loss. However, the recovery is less well separation in overlying strata, increasing the secondary porosity understood, and it is difficult to predict if (and when and how and permeability of the strata and changing groundwater stora much) a particular well or aquifer zone will recover. In this ge and flow characteristics. There are several deformational paper, we review several studies of recovery conducted in the zones above a longwall mine: a deep caved zone of very fractu US Appalachian coalfield, and summarize recovery observa red, dewatered strata; an intermediate confining zone of cohe tions from our studies at two sites in the Illinois coalfield in the rently subsided strata that hydraulically separates shallow aqui US Midwest region. fers from the mine; the shallow bedrock zone in which aquifers are affected by fracturing but not by drainage to the mine; and in REVIEW OF STUDIES IN THE some cases an unconsolidated cover which is usually not very APPALACHIAN COALFIELD, USA affected by subsidence. The water levels in the shallow bedrock in the actively subsiding zone over the mine face drop sharply The Appalachian Coalfield of the eastern USA contains due to the increase in secondary porosity. From this potentiome mainly bituminous coal resources of Pennsylvanian age. Seve tric low, a potentiometric depression, or drawdown, spreads out ral hydrological studies of longwall mining have been conducted 35

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in the Northern Appalachian region, which is mostly characteri where the panel width is more than twice the overburden width, zed by moderate topographic relief (typically 100-200 m), long wells do not significantly recover within a few years, unless they wall mining depths of 100 to 400 m, gently dipping strata, and are located near a stream. Rauch noted that many supplies mostly poorly permeable sequences of interbedded shales, which do not recover within a measurable time will nevertheless siltstones, sandstones, minor limestones, underclays and coal. ultimately do so after the mine is abandoned and flooded. Shallow, thin sandstones form local aquifers. The potentiome tric response to mining follows the model described above, but DESCRIPTION OF STUDY SITES IN is affected by the topography. Wells and springs located on hill ILLINOIS tops and hillsides are usually more severely impacted by under mining than those located in valleys (Johnson and Owili-Eger, From 1988 to 1995, Northern Illinois University (NIU) 1987; Tieman et al., 1992; Leavitt and Gibbens, 1992; Werner and the Illinois State Geological Survey (ISGS) conducted a and Hempel, 1992; Johnson, 1992), and recovery is generally comprehensive study into the geotechnical and hydrogeological better in valley settings. effects of longwall mine subsidence at two sites in Jefferson As Werner and Hempel (1992) commented, reports of and Saline Counties, south-central and south-eastern Illinois recoveries after longwall mining are inconsistent. Walker (1988) (Mehnert et al., 1994; Van Roosendaal et al., 1994; Trent et al., observed that, of ten shallow wells over or near four adjacent 1996; Booth et al., 1997). Studies at both sites included pum panels, only one well, over the center of a panel, did not exhibit ping, slug and packer tests to determine changes in hydraulic some recovery. In contrast, Cifelli and Rauch (1986), in a study properties, monitoring of ground subsidence through monu in West Virginia, found that of 19 wells (30-1 00 m deep) directly ments and of subsurface strains via borehole geotechnical met over total extraction mining, only one showed any significant hods, monitoring of water levels in piezometers and wells, and recovery; they also observed a correlation between numbers of geochemical sampling of groundwater. wells impacted and percentage of recharge area undermined. Both mines were into the Herrin Coal. Both sites are In one study of two adjacent panels, Matetic and Trevits located in areas of gently rolling topography with local relief less (1990) found that the largest water-level fluctuations occurred in than 15 m, in a humid continental climate with annual rainfall wells that were directly undermined, and that recovery occurred about 1 m. Both have a cover of glacial material overlying in all affected measurable wells. At other sites, Matetic and Tre Pennsylvanian bedrock. The geological units are poorly perme vits (1991, 1992) and Trevits and Matetic (1991) observed that able but include minor sandstone aquifers and thin, disconti water levels in shallow aquifers declined substantially when the nuous sand and gravel deposits in the drift. Specific site infor sites were undermined, but recovery began before subsidence mation is summarized in Table 1. was completed, starting about the time of maximum compressi ve strain, or approximately when the face had progressed Subsidence and potentiometric behavior at the beyond the well by about 40 % of the overburden thickness. Jefferson County Site Topographic setting and depth of the well relative to the At the Jefferson site, a thin cover of glacial material deep caved zone affect recovery. Johnson and Owili-Eger (mainly till, some sand and gravel) overlies bedrock dominated (1987), in a study of dewatered wells in "perched" aquifers at a by shales with minor sandstone, siltstone, clay, coal, and limes- hilltop location, observed that two wells 182 m deep (into the caving zone) never recovered, whereas two 57-m-deep wells Item recovered within 6 months. They concluded that the recovery occurred after the fractured confining layers between the various aquifers had resealed. Leavitt and Gibbens (1992) exa mined the responses of 17 4 domestic water supplies to long wall mining of the Pittsburgh coal seam and determined that, once over about 90 m, the overburden thickness is not a factor 1.46 m in well response, but topography is dominant, with significantly 1989-1993 more of the valley wells recovering than hilltop wells. Tieman et al. (1992) observed that longwall-impacted spring supplies recovered better near stream channels, and that lost spring Approximate strata thicknesses at the study sites waters were not draining to the mine but were simply dischar ging downgradient. Rauch (1989) concluded that dewatered shallow wells m 6-9 m in valley locations above the mine recovered rapidly (in hours to 174m 65-70 m weeks), whereas wells on steep hillsides took months to reco ver and hillside springs had poor to no recovery. However, Table 1. Summary of study site information. 36

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RECOVERY OF GROUNDWATER LEVELS AFTER LONGWALL MINING

tone. The sequence includes a moderately permeable aquifer, ground, then began a rise that by 1995 reached 10 m below the Mt Carmel Sandstone, overlain by a confining shale. This ground, well above the initial levels. A similar recovery was section of the mine consisted of four longwall panels, each observed in the two new piezometers, indicating that it was a about 183 m wide and over 1.5 km long. The study began after general phenomenon across the panel. the mining of panels 1 and 2, so that the initial water levels were Durng recovery, the following phenomena were obser already affected by mining. ved (Booth et al., 1998): ISGS studies (Mehnert et al., 1994) during the mining of • Subsidence had increased the permeabilities of the panel 3 in 1988 showed a progressive decline in water levels in aquifer (from pre-mining values in the range 10-7 m/s) the sandstone aquifer directly over the panel as the panel face by about an order of magnitude over the interior of the approached, to a minimum during the maximum tensional subsi longwall panel, and by about two orders of magnitude in dence phase. The water levels then started to recover immedia the tensional zone along the edges. tely and, despite being affected by adjacent panel 4, eventually • Pumping tests conducted after mining indicated leakage recovered a total of about 10 m in five years. of groundwater from the overlying confining shale, Our studies (Booth et al., 1997, 1998) over panel 4 sho which had been fractured during subsidence, to the wed that water levels in the sandstone declined due to the pas upper bench of the sandstone aquifer. sage of adjacent panel 3, then recovered slightly until they star • Geochemical results showed a marked increase in total ted a gradual decline as panel 4 approached. They dropped dissolved solids, especially sulfate levels, which could rapidly at undermining to reach a total head loss of about 14 m be attributed to both influx of poor quality water from the compared to initially measured levels. Subsidence produced a overlying shale and lateral flow of aggressive recharge heavily fractured zone extending to about 60 m above the mine, water through the aquifer itself. and substantial fracturing and bedding separations in the shallo wer bedrock, and also damage to most piezometers. Monitoring Subsidence and potentiometric behavior at the continued in a 15-cm-diameter well (P350) on the panel centerli Saline County site ne, and in two new piezometers constructed into the sandstone At the Saline site, the strata dip gently northwards and over the inner panel and tension zone the cover of glacial materials (tills, lacustrine deposits, sand and Panel 4, and all mining in this section, was completed in gravel) varies considerably in thickness over an irregular April 1989. Figure 1 shows the water levels in well P350 from bedrock surface. This section of the mine consisted of six long initial declines to final recovery. The recovery occurred in two wall panels, variously 186-287 m wide and 2.2 - 3.1 km long, distinct phases. First, there was a rapid partial recovery when mined successively southwards. Studies were conducted over the site entered the compressional phase, within a month of panels 1 and 5, mined in 1989-1990 and 1992-1993 (Booth et undermining. This is presumably related to the reclosure of frac al., 1994, 1997). tures and bedding plane openings, and corresponds to the early Panel1: At the panel 1 study site, the mine was overlain recovery noted by Trevits and Matetic (1991) and Matetic and by about 90 m of bedrock and 30 m of glacial drift. The bedrock Trevits (1992) (see above). The water level in P350 then remai sequence was very poorly permeable, and even the shallow Tri ned approximately stable for a year at about 25-30 m below voli Sandstone (less than 10 m thick, at depths around 55 m) had low permeabilities in the range 1o- 8 to 1o- 7 m/s. The deep Jefferson Panel4 ~Sandstone caved zone extended only about 6 m above the mine, and the overburden subsided largely as a coherent mass with little inter nal vertical displacement (Van Roosendaal et al., 1994). Water levels in the Trivoli Sandstone declined very rapidly some 37 to 43 m in the period from just before undermi ning until the subsidence tensile phase (Booth et al., 1994). Then, except for a 6 m rise in some piezometers during the compressional phase, no further recovery was observed in the sandstones during the subsequent three years of monitoring. Also, a 42-m-deep well into a shallow sandstone, located 300 m north of the panel, declined rapidly by about 22 m during mining Panel 4 started mining 12/88 Site subsided 2/89 in 1989 and recovered only about 1.5 m in winter 1994-1995. Mining completed 4/89 Panel 5: Panel 5, approximately 1 km south of panel 1, was mined at a depth of about 91 m at the study site, which 08/14192 08115193 08/16194 08117195 was undermined at New Year 1993. The glacial cover was 12 - DATE 20m thick and the Trivoli Sandstone was either sub-cropping at Figure 1. Water-level decline and recovery in Mt Carmel Sanstone, Jefferson Panel 4. the bedrock surface or overlain by up to 2 m of shale beneath

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the glacial deposits. Further south, the sandstone becomes mis Discussion sing by erosion over panel 6, which was completed in July 1994. The Jefferson and Saline County sites were similar in The subsidence, strata deformation, and generally low climate, topography, mined seam, and overall geological permeabilities at panel 5 were similar to those at panel 1. The sequence. The initial potentiometric decline due to mine subsi water levels in the sandstone piezometers (P51 B-P53B) over dence followed the same general pattern at both sites, though the panel declined rapidly some 16-18 m during the period from was more abrupt at the less transmissive Saline site. However, just before undermining to the tensional phase of subsidence the potentiometric recovery in the shallow sandstone aquifers at (Booth et al., 1997). The water level over the southern barrier the two sites was quite different. At Jefferson, recovery began pillar P54B) declined about 12 m. Figure 2 shows the sandstone soon after undermining and continued for over five years to an piezometer hydrographs. There was no appreciable recovery of over-recovered level. No recovery occurred at either of the Sali sandstone water levels over the panel in the three years after ne panels during the post-mining monitoring periods, except mining, but the water level in the piezometer over the barrier locally over the southern barrier at panel 5 where the sandstone recovered rapidly by about 4 m a few weeks after mining. was recharged from overlying sand and gravel. Recovery should be the normal response after a poten Saline 5 Sandstone Water Levels tiometric level has been lowered by temporary stress. However, it can be prevented by any of three factors:

Continued loss of water from the aquifer In areas of significant topographic relief such as the Appalachians, drainage can continue from upland aquifers through fractured aquitards to lower aquifers. However, this is not a factor in our low-relief setting. Drainage can continue to the mine and deep caved zone. Generally, hydraulic separation is maintained between shallow aquifers and the mine and caved zone, provided that a sufficiently thick confining sequence is present. The Pennsylva nian coal-measure rocks in Illinois and Appalachia are domina ted by shales which tend to retain their confining nature despite

07/15/92 01/13/93 07/15/93 01/13/94 07/15/94 01/13/95 07/15/95 subsidence. Situations in which the confining separation is inadequa

~ P51B ~ P52B ~ P53B ~ P54B te could include highly permeable settings (such as the karst mining areas of China), localized buried bedrock valleys which Figure 2. Hydrographs of Trivoli Sandstone piezometers over PanelS, Saline Sit. bring sand and gravel aquifers close to the mine roof (as Cartw Figure 3 is a cross-section over panel 5 showing the pie right and Hunt, 1976, reported for some wet mines in Illinois}, zometers, geology, panel position, pre-mining (November 1992) water levels in the sandstone (b-b) and deep drift (d-d), and post

mining (November 1994) water levels (b'-b' and d'-d'). The initial Ps1 PSW GTJ GT4 sandstone water level (b-b) was flat; the stable post-mining level (b'-b') is depressed over the panel, the aquifer being almost dewa IAnM"""'""" tered, but rises steeply to the partially recovered level at the I barrier pillar. The potentiometric response in the lower drift (d-d, I

d'-d') varied across the panel. Over the center, the lower drift was 1 poorly permeable clay till and its water levels fluctuated greatly ~ during mining, then declined gradually by about 2-3 m. Over the 1- southern barrier, the lower drift was sand and gravel, in which the water level fell rapidly during subsidence, concurrent with the head drop in the underlying sandstone. It then remained low at a

level approximately equal to the head in the sandstone. The water 0 50 100tn I.~'---' --'·~HOriiOOlru s~i~_ __.___,_ __ ..J,_ __ _, level in the sandy till in the tension zone near the edge of the panel also declined substantially. These permeable drift units apparently lost water to the underlying sandstone, with which they Panef6 were in good hydraulic continuity, and thus contributed to the local Figure 3. Cross-section over southern half of Saline panel 5 and barrier pillar, showing shallow recovery of the sandstone in the barrier pillar area. strata, piezometer locations, and pre- and post-mining ground surface and water levels. 38

RECOVERY OF GROUNDWATER LEVELS AFTER LONGWALL MINING

and very shallow longwall mining. Rauch (1989) suggested that tantly, laterally through the aquifer from adjacent areas unaffec a critical overburden thickness is about half the width of the ted by mining. At the Saline site, there was no apparent vertical panel. However, Matetic and Trevits (1991) reported complete recharge from the thick, poorly permeable confining beds over recovery over shallow (61-91 m) longwall mining in Ohio, and the sandstone at panel 1 or from the till over the interior of Van Roosendaal et al. (1995) reported that confining separation panel 5, but there was local recharge from the sand and gravel was maintained and shallow water levels not depressed by a over the barrier at the southern edge of panel 5. Laterally, the 106-m-deep longwall mine in Illinois. sandstone was missing south of panel 5, but again the steep In our case studies, inflow was not a general problem at hydraulic gradient near the edge of the panel suggests that the either mine, and the confining layer separations were probably poor transmissivity is a more important factor. sufficient to prevent drainage from the aquifer to the mine or deep caved zone. At Jefferson, the sandstone aquifer was sepa CONCLUSIONS rated from the mine by about 168 m of bedrock, of which the lower 60 m were heavily fractured. At the Saline site, the shallo Numerous studies show that a bedrock well directly wer mining depth was offset by the limited extent (about 6 m) of over or close to a longwall mining panel typically experiences a the deep fractured zone above the mine. In both cases, geo drop in water level due to mine subsidence. The controls of this technical and packer tests showed that the confining zone response include topographic setting and hydraulic properties above this contained isolated fractures and bedding separations of the aquifer. The depth of mining does not have a significant that only locally increased permeability. At the Saline site the effect on shallow aquifers provided that a sufficiently thick confi entire sequence, including the sandstone, remained tight, with ning zone maintains hydraulic separation between the aquifers isolated fractures into which injected water flowed only briefly. and the deep caved zone and mine. Thus, the differences in mining depth and possibility of conti To predict whether a particular well or aquifer zone will nued drainage of water from the aquifer were probably not fac exhibit significant potentiometric recovery from the depression tors in the recovery process at the Illinois study sites. after longwall mining, useful guidelines are: • In areas of significant topographic relief, if the aquifer is The prevention of recharge water from flowing back through or on an upland and is separated from a deeper aquifer of into the aquifer lower head only by a thin, easily fractured confining The transmissivity of the aquifer controls the rate at unit, continued vertical drainage will tend to prevent which recharge water can flow back into the affected areas of recovery. the aquifer. Considering the differences in thickness, initial per • If the well or aquifer is not adequately separated from meability, and degree by which permeability was enhanced by the deep caved zone above the mine by a sufficiently subsidence, the Mt Carmel Sandstone at Jefferson was several thick confining unit, continued drainage to the deep hundred times more transmissive than the Trivoli Sandstone at zone will prevent significant recovery in a reasonable the Saline site. This is probably the major cause of the differen time. ces in recovery between the sites. It is also reflected in the initial • If the aquifer is thin and very poorly permeable, it will drawdown response to mining, which was more rapid and inten probably not recover in a reasonable time period; an se at the Saline site, more diffuse and gradual at the Jefferson indication of poor transmissivity is the suddenness and site. intensity of the initial potentiometric decline due to Additionally, the continued mining up-dip at Saline (com mining. pared to the end of mining at Jefferson) may have disrupted • The aquifer is less likely to recover if potential sources groundwater flow through the aquifer for part of the time. On the of recharge (e.g. overlying aquifers, lateral extent of the other hand, the active subsidence zone is a relatively small aquifer into unmined areas, location in a valley in an area, generally distant from the study sites, and for most of the area of significant relief) are not available or are separa time there was much more aquifer available for recharge, in all ted from the affected site by boundaries, continued directions, than was being impacted by active mining. Further mining, etc. more, at panel 5, the continuing steep hydraulic gradient within the aquifer at the southern edge of the panel (Figure 3) sug ACKNOWLEDGMENTS gests that the lack of recovery was a localized feature due to low transmissivity rather than to any influence of subsequent The Illinois research in Illinois was supported by the Illi mining. nois Department of Energy and Natural Resources through the Illinois Mine Subsidence Research Program, and by the US Lack of a source of recharge Department of the Interior, Office of Surface Mining Reclama At the Jefferson site, the sandstone was recharged both tion and Enforcement under agreement GR196171. This sup vertically from the overlying fractured shale and, more impor- port does not constitute an endorsement by either DENR or

OSM of any views expressed in this article, which are solely Matetic R.J. and M.A. Trev its, 1992. Longwall Mining and its Effect those of the author. Thanks are due to Robert Bauer and the on Ground Water Quantity and Quality at a Mine site in the staff of the Earth Hazards section of the Illinois State Geological Northern Appalachian Coal Field. Ground Water Manage Survey, and to former NIU graduate students Erik Spande, Tom ment, Book 13: Proc. of the Focus Conference on Eastern Pattee, Alan Curtiss, Joe Miller, and Larry Bertsch for their con Regional Ground Water Issues, AGWSE, October 13-15, tributions to this research. 1992, Newtown, Mass. p. 573-587. Mehnert B.B., D.J. Van Roosendaal, R.A. Bauer, P.J. DeMaris, REFERENCES and N. Kawamura, 1994. Final Report of Subsidence Investigations at the Rend Lake Site, Jefferson County, Booth C.J., A.M. Curtiss and J.D. Miller, 1994. Groundwater Illinois. Illinois State Geological Survey, US Department Response to Longwall Mining, Saline County, Illinois. of the Interior Coop Agreement CO 267001. Proc. Fifth International Mine Water Congress, Notting Rauch H.W., 1989. A Summary of the Ground Water Impacts ham, UK, September, p. 71-81. from Underground Mine Subsidence in the North Cen Booth C.J., P.J. Carpenter and R.A. Bauer, 1997. Aquifer Res tral Appalachians: Chapter Two in Coal Mine Subsiden ponse to Longwall Mining, Illinois. US Department of ce Special Institute, December, Pittsburgh PA, Eastern the Interior, Office of Surface Mining, Library Report No. Mineral Law Foundation. 637, Grant/Coop Agreement GR 196171, 400+ pp. Tieman G.E., H.W. Rauch and L.S. Carver, 1992. Study of Booth C.J., E.D. Spande, C.T. Pattee, J.D. Miller and L.P. Dewatering Effects at a Longwall Mine in Northern Bertsch, 1998. Positive and Negative Impacts of Long West Virginia. Third Subsidence Workshop due to wall Mining on a Sandstone Aquifer. Environmental Underground Mining, Morgantown, WV, June, p. 214- Geology, v. 34, no. 213, p. 223-233. 221. Cartwright K. and C.S. Hunt, 1983. Hydrogeologic Aspects of Trent B.A., R.A. Bauer, P.J. DeMaris and N. Kawamura, 1996. Coal Mining in Illinois: An Overview. Illinois State Geo Findings and Practical Applications from the Illinois Mine logical Survey, Environmental Geology Notes EGN 90. Subsidence Research Program, 1985-1993: Final Report Cifelli R.C. and H.W. Rauch, 1986. Dewatering Effects from to Illinois Clean Coal Institute. IMSRP XII., 146 pp. Selected Underground Coal Mines in North-Central Trevits M.A. and R.J. Matetic, 1991. A Study of the Relationship West Virginia. Proc. Second Workshop on Subsidence Between Saturated Zone Response and Longwall due to Underground Mining. Morgantown, WV, June 9- Mining-Induced Ground Strain. Proc. NWWA Fifth 11, 1980. National Outdoor Action Conference on Aquifer Resto Johnson K.L. and A.S.C. Owili-Eger, 1987. Hydrogeological ration, Ground Water Monitoring, and Geophysical Met Environment of Full Extraction Mining. Longwall USA hods. May. Las Vegas, NV., p. 1101-1109. Conference, Pittsburgh, June, pp. 147-158. Van Roosendaal D.J., F.S. Kendorski and J.T. Padgett, 1995. Leavitt B.R. and J.F. Gibbens, 1992. Effects of Longwall Coal Application of Mechanical and Groundwater-Flow Mining on Rural Water Supplies and Stress Relief Models to Predict the Hydrogeologic Effects of Longwall Fracture Flow Systems. Proc. Third Subsidence Subsidence - A Case Study. Proc. Fourteenth Confe Workshop due to Underground Mining, Morgantown, rence on Ground Control in Mining, Morgantown, West WV, June, p. 228-236. Virginia, p. 252-260. Matetic R.J. and M.A. Trevits, 1990. Case Study of Longwall Walker J.S., 1988. Case Study of the Effects of Longwall Mining Effects on Water Wells. SME Annual Meeting, Mining Induced Subsidence on Shallow Ground Water Salt Lake City, UT, February. Preprint 90-141, 7 p. Sources in the Northern Appalachian Coalfield. US Matetic R.J. and M.A. Trevits, 1991. Does Longwall Mining Bureau of Mines, Report of lnvs 9198, 17 p. have a Detrimental Effect on Shallow Groundwater Werner E. and J.C. Hempel, 1992. Effects of Coal Mine Subsi Sources? Ground Water Management, Book 5: Proc dence on Shallow Ridge-Top Aquifers in Northern Fifth National Outdoor Action Conference on Aquifer West Virginia. Proc. Third Subsidence Workshop due Restoration, Groundwater Monitoring, and Geophysical to Underground Mining, Morgantown, WV, June. Methods, AGWSE. p. 237-243.

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