Coal is the world’s most abundant and important primary source of energy andplays a major role in world energy scenario. It contributes about 26% of the totalglobal primary energy demand and is also a key input for the steel and other industries(http://scclmines.com/downloads/exploration.pdf.). Reserves of coal are spreadworldwide throughout some 100 developed and developing countries, sufficient tomeet global needs for the next 250 years. It is an important fossil fuel for generationof electricity and for other industrial purposes. India has the world’s third largesthard coal reserves, after the United States and China with an output of 328 milliontones in 2001-2002. The contribution of public sector coal companies towards coalproduction in India is 95%, of which Coal India Limited accounts for 80% andSingareni Collieries Company Limited for 10%. Other private companies make upthe balance (http://www.iea.org/ text base/nppdf/free/2000/coalindia 2002.pdf).
Traditionally, coal mining is considered to be the most polluted industry. Mining ofcoal resources either by opencast or by underground process has serious insinuationfor environmental security if proper management strategies are not adopted (Singhand Singh, 1998). Expansion of industrialization for human development needsmassive energy generation, for which huge quantity of coal is extracted throughmining, causing extensive landscape destruction (Singh and Zeng, 2008).
Underground mining of coal began in the early 1800’s and continues to current day.Most mining is accomplished by direct human action utilizing heavy machinery toremove the material (http://ema.ohio.gov/Documents/SOHMPsec214.pdf).However, underground mining is progressively being abandoned due to problems
Abstract
The impact of underground coal mine subsidence in forest soil ecosystems of SingareniCoalfields area was studied with respect to physico-chemical characteristics, inorganicnutrients (plant available nutrients), fine root biomass, net nitrification and net nitrogen(N) -mineralization rates. Soil quality of the studied forest was slightly acidic in nature(6.78-6.80). The forest soil texture with respect to course sand content, was higher in freshsubsided site as compared to old subsided site, while the fine textured soil was maximumin old subsided site. Subsidence caused increased of 34.3%, 9.82% in soil moisture andwater holding capacity (WHC) compared to unsubsided sites. After subsidence, organiccarbon (OC), total N (TN) and phosphorus (TP) was found to increase by 41.7%, 0.45%, and5.6% in subsided site, respectively.
Mean plant available soil nutrients viz. nitrate-N ammonia-N and phosphate-P wereincreased in subsided site by 82.2%, 21.4% and 14.5%, respectively. On an average,subsidence resulted into a 1.84 fold increase in fine root biomass, 1.66-fold in net nitrificationand 1.44-fold increase in net N-mineralization.
of profitability. Presently 60% of the mine materials in the world are extracted byopencast method causing devastation of the ecosystem (Saleque, 2008).
Two basically different methods are used for underground mining coal mining;longwall and room-and-pillar mining. In longwall mining all coal is recovered fromthe mined ponds; hence subsidence occurs at the surface almost immediately and itis planned for. Room-and-pillar mining leaves about half of the coal in the pillars orthe floor strata under them fail, sometimes decades after mining. In room-and-pillar mining, the subsidence is unplanned and, therefore, quite problematic.
Underground coal mining, although not as surface mining, can alter the surface andimpact agriculture ecosystem (Darmody et al, 1989). Surface alteration includessubsidence that creates depressions. Albeit, the underground mining does not disruptthe soil and geologic overburden as radically as surface mining, but it can createsignificant disturbance of surface soils if subsidence occurs. Subsidence hasdeleterious effects on man-made structures. Nevertheless, because the subsidencefrom longwall mining is predictable and short-term, damage to structures can bereduced. Damage is most severe to structures that span the edge of subsided troughs(Boscardin, 1992). Structures toward the center of the subsidence trough aregenerally less prone to damage because they are let down more uniformly after thedynamic subsidence wave passes. Repairs can begin soon after mining becausemost of the subsidence occurs within a few days after undermining, and the surfacetypically within three to six months (Mehnert et al., 1992).
According to Darmody (http://www.mcrcc.osmre.gov/PDF/Forums/Prime.Farmland201998/4d.pdf), the strength of a rock mass changes with time, therefore thesubsidence in a coal mine is also time dependent. It is found that the subsidenceincreases with time, however, the subsidence attains a maximum value which remainsalmost constant with the increase in time.
The underground mines generally have less visible impacts on the environmentthan opencast mines. There is less disturbance of the ground surface but it canaffect the water by contaminating with acids and metals and by intercepting aquifers.Underground mines not only impact groundwater hydrology, they are prone tosubsidence. Subsidence occurs when the ground above the mine sinks because theroof of the mine either shifts or collapses.
Mine subsidence can be defined as movement of the ground surfaces as a result ofreadjustments of the overburden due to collapse or failure of underground mineworkings. Surface subsidence features usually take the form of either sinkholes ortroughs. Subsidence can alter ground slopes to such an extent that roads, water andgas lines and buildings are damaged (Op. cit. Office of Technology Assessment,1979). Subsidence can cause loss of productive land (Guither, 1986), damage to
underground pipelines (Hucka et al., 1986) and above-ground structures (Kaneshige,1971), decreased stability of slopes and escarpments (Shea-Albin, 1992; Slaughteret al., 1995), contamination of groundwater by acid drainage (Emrich and Merritt,1969), and dewatering of streams (Cifelli and Rauch 1986; Dixon and Rauch, 1990)and groundwater supplies (Stoner, 1983; Matetic and Trevits, 1992).
Mining causes drastic disturbances in soil properties. Typical activities during theconstruction and mining phase include ground clearing (removal of vegetative coverand topsoil), drilling, blasting, trenching, excavation, and vehicular and pedestriantraffic. Mining activities usually cause catastrophic and extensive environmentalchanges, and eventually cause major damage to the whole ecosystem. During surfacemining, 2-11 times more land is damaged than with underground mining (Li, 2006).The direct effects of mining activities can be an unsightly landscape, loss of cultivatedland, forest and pasture land, and the overall loss of production. The indirect effectscan be multiple, such as soil erosion, water and air pollution, toxicity, geo-environmental disasters, loss of biodiversity, and ultimately loss of economic wealth(Wong, 2003; Xia and Cai, 2002).
2. IMPACT OF SUBSIDENCE MOVEMENTS
Damage to different surface features and structures due to underground coal mininghas been a serious problem and becoming more widespread as the demand for coalincreases. It is, also increasing due to population explosion in and around the miningleasehold area. Damage from subsidence movements can be caused by change insurface slope, differential vertical displacement and horizontal strains. Further,mining may restore the original slope or close tensile fractures thereby renderingsome remedial measures unnecessary or even harmful to the environment.Compressive strains with concave curvature of the ground surface results in crushing,over-thrusting and horizontal openings in brick-wall. Tensile strain accompanyingconvex curvature of the ground resulting in fractures tapering from the groundupwards. Similarly, bridges, roads etc may experience movement towards or awayfrom each other depending upon the nature of ground strains. Differential verticalground movement can adversely affect surface drainage, tall structures, factoryplant and machinery. Sewerage pipes may be broken or cracked due to verticalmovement which results in malfunctioning of the system. Similarly, water mainsbreakages commonly occur at intersection points where service pipes tap into themains. Tilting of factory machines and plants results in inoperative equipment(CIMFR, 2007). Subsidence trough can give rise to potential formation of smallpond and flooding on the surface of the land resulting in increase in surface water.The crack at the edge of the panel causes damage to the plant root system andtilting of the trees and sometime fall of the tree (Tripathi et al., 2009)
Different surface features and structures can sustain certain magnitude of groundmovement which are called safe limit of ground movements. The safe limit ofmaximum permissible subsidence movements to different surface features andstructures for Indian geo-mining conditions (Anon., 1991) are given below:
Railway line of jointed construction : Strain=3mm/m: Limiting operating gradient=1 in 100
Railway line of welded construction : No movement permittedBuildings : Total elongationor compression=60 mmWater bodies : Tensile strain=4.5 mm/mSurface topography : Strain=3 mm/mForest cover (Slight impact) : Strain=20 mm/m
As per Ministry of Environment and Forest, Government of India guidelines, themaximum permissible tensile stain and width of surface cracks in forest land are 20mm/m and 300 mm, respectively.
4. IMPACTS ON SOIL
The coal extraction process drastically alters the physical and biological nature ofthe mined area. The principal surface impact of underground coal mining issubsidence (Booth, 1990). In general, soil physical properties are sensitive to miningsubsidence and they become worse from the top to the centre of the subsidencetrough. Nevertheless, the soil chemical properties except for electrical conductivityare not so sensitive to mining subsidence and might be changed after subsidenceprocess. The studies show that the bottom of prone land accumulates nutrients andsalt. Thus, the most important impact of mining subsidence on soil chemicalproperties can be seen in soil electrical conductivity reflecting high salt content,which might occur after the subsidence process. The soil biomass C in newlysubsided land has also shown a significant tendency in the old subsided land andsubsiding land.
Mining subsidence causes decreased stability of slopes and escarpments,contamination of ground water by acid drainage, increased sedimentation, bankinstability and loss, creation or alteration of riffle and pool sequences, changes toflood behavior, increased rates of erosion with associated turbidity impacts anddeterioration of water quality due to a reduction in dissolved oxygen and to increased
6 Underground Coal Mine Subsidence
salinity, iron oxides, manganese, and electrical conductivity (Booth and Bertsch,1999; Sidle et al., 2000; DLWC, 2001; Gill 2000; Stout, 2003) resulting into deathof fringing vegetation (NSW, 2007). The occurrence of iron precipitate and iron-oxidizing bacteria is particularly evident in rivers where surface cracking hasoccurred.
Much of the impact of subsidence on soils and landspcape is related to thepre-mining surface topography. Landscapes with erosive soils on long slopes maybe subject to increased erosion potential because of slope increase or displacementof erosion control structures. The effect of longwall mine subsidence on soil hasbeen studied for two years over two mines at Queensland in Australia (http:/www.acarp.com.au) above two physical properties of soil. Large cracks that developat the soil surface after subsidence can pose a hazard and may alter soil watermovement. Most subsidence cracks are small and are quickly obscured by normalcultivation. Larger cracks are generally backfilled or graded to prevent them fromposing a hazard to foot or wheel traffic. Along the panel edge, cracks remain openafter the dynamic subsidence wave passes. This may allow surface water to infiltratemore easily and may increase the hydraulic conductivity of some soil horizons.These changes are in a very small portion of the mined area and may revert to theoriginal conditions with time (Seils et al., 1992).
Subsidence effects on agriculture land have been documented in Illinois (Darmodyet al., 1989; Guither, 1986; Guither et al., 1985; Guither and Neff, 1983), the UnitedKingdom (Selman, 1986), India (Kundu and Ghose, 1994), China (Hu and Gu,1995), South Africa (van der Merwe, 1992), and Australia (Holla and Bailey, 1990).These effects include soil erosion, disruption of surface and subsurface drainageand reduction of crop yields. Hu et al. (1997) observed that the physical propertiesof soil sensitive to mining subsidence were bulk density, moisture content andhydraulic conductivity, and they showed worsening from the top to the centre ofthe subsidence trough. The soil biomass C in newly subsided area showed adecreasing trend from the top to the centre of the subsidence trough, but no obvioustrend was observed in the old subsidence areas. Based on the soil analysis of thesubsided land, soil erosion was identified as a serious problem, most severe in themiddle of the prone land. In low areas with high water tables, ponding is a particularproblem. In some situations ponding might be viewed in a positive way because itcreates wetlands beneficial to wildlife, but negatively when it reduces net returnsto a food or fiber producer. In Southern Illinois landscape, subsidence fromunderground longwall coal mining creates wet or ponded areas that delay and disruptsfarming practices, causes low seed germination, and reduces crop growth and grain
yields. Darmody et al (1989) found a 4.7% average reduction in overall corn yieldson subsidence affected land in southern affected land in southern Illinois.
5. IMPACT OF SUBSIDENCE ON GROUND WATERSubsidence depressions caused due to U/G coal mining is accompanied by rockfracturing, dilations of joints and separation along bedding planes. Rock movementsoccur above the mine workings and at an angle projected away from the mined-outarea. Mine induced fracturing within the angle can result in hydrological impacts,i.e. disruption of surface and underground water bodies (aquifer), contamination ofaquifer, beyond the margins of the mine workings.
Walker (1988) correlated the subsidence movements with fluctuation in piezometriclevels in 10 shallow observation well lying above and around 4 longwall panels inthe northern Appalachian Coalfields. He reported that water level decrease in thewell is greater when the ground surrounding the well is in tension. The rate ofrecovery is greatest when the ground was subjected to maximum compressive strain.The fluctuation of water levels appears to be a function of both the position of thewell relative to the layout of the panel and the proximity of the mining. A well isunaffected by mining of a preceding panel unless it was positioned within the angleof draw for the panel. Well located over the centre of the panel exhibit the greatestfluctuation and head loss. Further, wells located in stream valley, exhibited alesser response to mining. Nine of the ten wells investigated recovered to their pre-mining level after mining was completed. Booth et al, (1998) also made similarobservation after conducting seven years of study over longwall mining in Illinois.Moebs and Barton (1985) reported complete loss of water after mining on fourshallow wells directly above the panel. While Liu (1981) stated that beyond aninfluence angle of 20-260 from the panel edges the effect on well water were minoror none at all. Cifelli and Rauch (1986) also reported that water wells located withinan influence angle of 200 from the panel edge of the opening were affected byunderground mining. Singh and Singh (1998) reported 6.5m water depletion levelin the unconfined aquifer due to longwall mining in Kamptee coalfield, India.
In this paper an attempt has been made to quantify the impact of underground coalmine subsidence on changes in soil physico-chemical characteristics, nitrogentransformation rate and hydrological status of SCCL coal mines of India.
6. MATERIALS AND METHODS
Location
The study mine sites spread over an area of 2,85 km2 are located in SingareniCoalfields of Singareni Collieries Company Ltd. (SCCL), Kothagudem (Andhra
Pradesh, India) at 17030’N latitude and 80040’E longitude. The topography is almostundulating plain terrain to gently sloping towards the river Godavari in the southeastwith the average elevation varying from 119 to 157 m above mean sea level. Thereis no effective drainage developed in this are due to sandy soil cover and number offaults and fractures.
Climate
The climate is seasonally tropical and divisible into 3 distinct seasons, namely, rainy(mid June to October, winter (November to February) and hot summer (March tomid June). According to the mean monthly rainfall data (1996-2006) of Kothagudem,the annual rainfall of the area is 1084 mm, being maximum in July (300.31) andminimum in Janurary (7.82mm). The study site receives about 86.8% of rainfallduring S-W monsoon and 7.4% during N-E monsoon season. The mean airtemperature varies from as low as 11.420C in December to as high as 46.60C duringMay. The predominant wind direction is southeast to west. The relative humidity ofthe area fluctuates from 49% during February to 73.21% during April (CIMFR,2007).
Land use pattern
Within 10 km radius from the edge of mine site 23,004 ha area is covered byRamvaram Reserve Forest. An area of 2781 ha is barren and uncultivated land,while 2377 ha area is fallow land and 1757 ha of land is put to non-agricultural uses.The net area sown is 14,005 ha and 535 ha during Kharif (July to October) and Rabi(November to February) seasons, respectively.
Forest
Vegetation of this region has been classified as Southern Tropical Dry DeciduousForest (Champion and Seth, 1968). The total forest area is spread over an area of748,882 ha, constituting nearly 46.72% of the total geographical area. The densityof forest ranges from 45-65stems 100m2. The forest is dominated by Tectonagrandis. The other co-dominant species are Holarrhena antidysentrica, Hardwikisbinnata, Choroxylon swietenia, Anogiessus latifolia, Morind tomentos,Diospyros melanoxylon, Terminalis tomentosa, Strychnos-nux-vomica, Acaciacatechu, Boswellia serrata, emblica offcinalis, Xylia xylocarpa etc. The groundflora is dominated with Andrographis paniculata, Gymnema sylvestris, abrusprecatorius, Aristolochia indica, Desmodium triflorum, Glorisa superb,Hemidesmus indicus, etc. (ICFRE, 2004).
Geology and soil
The coal of Godavari valley Coalfield belongs to Lower and Upper Gondwana.
9Impacts Forest Ecosystem
Lower Gondawana consists of the Talchir, Barakar, and Kamthi series and theUpper Gondwana are classified into Maleri, Kota and Chikiala formations. Theaverage thickness of Barakar formation is about 100-200 m thick and lower non-coalliferous one of a less thickness. The coal mining area is blanketed by soil on thesurface. Coal seams, less thickness. The coal mining area is blanketed by soil onthe surface, coal seams, shales, clays and felspathic, medium to coarse grained andbrown gray sandstones are the main litho-units. The average thickness of soil in thearea is about 1.5m. Alluvium soil layer is of recent origin, underlain by Kamthis,Barakars and Talchirs boulder bed (CIMFR, 2007). The texture of the soil is mostlysandy loam. The texture of the soil is mostly sandy loam. The pH of the soil extractvaries from 6.0 to 7.9. In terms of soil pH, the soil characteristics vary from ‘slightlyacidic’ to moderately alkaline’ in nature.
Subsidence investigation
Geo-mining details of the subsided study areas (panel) are given in Table 1. A panelis an area demarcated for the excavation of coal. The surface subsidence investiga-tion was conducted over the selected panels MK-4 A-19, 5B N-18 and MK4 Y-12.The topography of all the panels before and after subsidence was undulating. Thevertical ground movement was monitored by self-aligning level with precision lev-eling staves. A total of 6 sets of observation were taken during 2004-2005 withrespect to vertical and horizontal movement of the ground surface by theodoliteequipment. The least count of the instrument was 0.05 mm. The recorded subsid-ence ranged from 315 to 992.4 with compressive strain of 4.5-16.41 mm/m andtensile strain of 3.5-14.06 mm/m (Table 1). The safer limit of tensile strain of sur-face topography is 3 mm/m (Anon, 1991). The extraction period of coal was fromAugust 2001 to June 2002. There was a trough subsidence after depillaring, so theground surface indicated saucer shaped depression sites with undulations (Fig.1).Maximum slopes in these panels after subsidence were 12.48-57.76 mm/m. Forma-tion of cracks occurred in tensile strain zone at the edge of panel. The angle ofbreak varied from about 5-100 from the vertical (Fig.1). All the trees lying at theedge of the subsided sites were lying down with broken roots.
Field sampling
One plot, 100 x 100 m in size, in undamaged forest (undisturbed forest site) andone subsided site each in 5B incline (N-18 panel) of Kothagudem and MK-4 (A-19panel) incline of Mandamari areas of SCCL mining area were selected for the studyto understand the impact of mine subsidence on the surface land. Ten soil sampleswere collected at random locations from upper 0-10cm from each of the undis-
10 Underground Coal Mine Subsidence
turbed and adjacent subsided seasonally during 2004-2005. The seasonal samplesof the respective site were composited to get one representative sample from eachsite. The mean values of each site are used for comparison in the study.
Physico-chemical analysis
Each soil sample was divided into two parts. One part in the field-moist conditionwas used for the measurement of available nutrients (NO3-N, NH4-N and PO4-P).The other part was used for the determination of dry weight, total organic C, total Nand total P, pH, water holding capacity (WHC) and bulk density (BD).
Soil pH was measured by Orion Ion Analyser, using glass electrode (1:10 soil:water).WHC was measured following Piper (1994) and BD was determined by inserting ametal tube of known internal volume in the soil and oven drying the encased soil,BD was estimated as the weight of oven dried soil per unit volume. For moisturecontent determination, the pre-weight was obtained. Organic C of soil was deter-mined by Walkley Black’s method and total N by modified Kjeldahl method (Jack-son, 1958). Total P was determined following perchloric acid digestion method(Mehta et al. 1954). Organic P was calculated by subtracting extractable phosphatefrom total P, particle size distribution was analyzed by using soil sieves of differentmesh sizes.
Estimation of fine root biomass
Fine root biomass (<5 mm diameter) was determined seasonally from five soilmonoliths of 20x20x15cm size at all the sites randomly. The monoliths were col-lected separately on a 0.5mm mesh screen after washing the roots with a fine jet ofwater. The roots were oven dried at 800C to constant weight. Root production wascalculated by subtracting the maximum biomass of rainy season to minimum biom-ass of summer season.
Microbial biomass N
Field-moist soil samples from undisturbed and subsided microsites were analyzedfor soil MBN by the chloroform extraction method using purifying CHCl3 treat-ment followed by 0.5M extraction of fumigated and non-fumigated soil (Singh andSingh, 1993). Soil MBN was calculated by subtracting the total N content of K2SO4extract of non-fumigated soil from that of fumigated soil and dividing the valueobtained by a kN (fraction of biomass N extracted after fumigation) factor of 0.54(Brookes et al, 1985). The Kjeldahl digestion method was used to measure the totalN contents (inorganic+organic) of K2SO4 extracts of fumigated and non-fumigatedsoils.
Inorganic N (NO3-N and NH4-N) was analyzed in the field moist condition of soil.For NO3-N in soil, CaSO4 was used as soil extractant and estimation was done byphenol-disulfonic acid method (Jacson, 1958). NH4-N was measured by phenatemethod (Wetzel and Likens, 1979), using 2M KCl as soil extractant.
N-mineralization was measured following the in situ buried bag technique (Eno,1960) for which about 150-200g of freshly collected soil material was sealed inpolyethylene bags and buried in field at 0-10 cm depth for 1 month incubation.Course roots and large fragments of organic debris were removed in order to avoidany marked immobilization during incubation. An increase in nitrate-N and ammo-nia-N, during the course of incubation indicated nitrification and ammonification,respectively. The increase in the concentrations of ammonium-N and nitrate-N overthe course of field incubation is defined as net N-mineralization and the increase innitrate-N only as net nitrification. All results are expressed on an oven-dry soil(1050C, 24h) basis.
Hydrological study
Hydrological study was done to know the impact of subsidence movement causeddue to caving on ground water. This study includes water level monitoring in thepiezometric well lying over and around the panel. The constructed depth and diameterof these piezometers are 30-36m and 4 inch respectively. Water levels was monitoredand analyzed on seasonal basis of six piezometric wells lying over and around N-31 and Y-12 panel during 2006-2007 by electronic water level indicator.
7. RESULTS AND DISCUSSION
Soil samples of all the sites were acidic in nature (6.78-6.80). The sand content washigher in fresh subsided site as compared to old subsided site. Coarse textured soilpercentage was maximum in fresh subsided site while the fine textured soil wasmaximum in old subsided site. The higher percentage of fine textured soil contentin old subsided site may be attributed to the accumulation of finer particles andorganic matter deposition in depression zone, which tends to accumulate moreorganic carbon and total nitrogen in soil. Prescott et al (2000) suggested that oncoarse textured soil, accumulation of organic matter, C and N is slower. The valuesof bulk density across the sites ranged from 1.06-1.18 g m-3. There was a decline inthe values of bulk density in both the subsided sites with 11.3% decline in old and10.9% decline in fresh subsided sites. As compared to control sites, there was anincrease of 34.3% and 35.5% in soil moisture of old and freshly subsided sites,respectively. Water holding capacity was increased by 9.82% in old subsided siteand 12.59% in freshly subsided site. Organic carbon content ranged from 1.32-1.86%, with higher value in subsided site and lower in undisturbed site. Aftersubsidence, organic carbon was found to increase by 41.6% in old and 41.9% in
freshly subsided sites. Total nitrogen and phosphorus contents in control and subsidedsites ranged from 1325-1331 μg g-1 and 49.5-52.3 μg g-1, respectively with highervalue in subsided site. In old and freshly subsided sites, the total N content wasincreased by 0.45% and 4.53% and total P by 5.6% and 5.8%, respectively (Table 2and 3).
A distinct seasonal variation was observed in available nitrogen (NO3-N andNH4-N) and phosphorus concentrations in soil, with the maximum values attainedin summer and minimum in rainy seasons. The mean seasonal concentrations ofnitrate-N and ammonia-N across the sites ranged from 3.2 to 5.83 μg g-1 and from4.3 to 5.22 μg g-1, respectively. The mean seasonal value of Phosphate-P rangedfrom 6.2 to 7.1 μg g-1. An increase in inorganic nutrients in the summer season maybe attributed to the reduced uptake by the plants, increased microbial biomass andalso the reduced activity of the plants. Nitrate-N, ammonia-N and phosphate-P wereincreased in old and fresh subsided sites by 82.2% and 15.53%; 21.4% and 15.38%;and 16.12% and 32.84%, respectively.
Increase in the inorganic nutrient concentration of the soil in subsided depressionmicrosite may be due to the higher microbial activity because of the high moistureand organic carbon contents and also increased fine root biomass in this zone.Singh (1993) also reported increase in N and P in disturbed site. There is a run-offof nutrients from the undisturbed to the subsided site that increases the concentrationof inorganic nutrients in subsided site. There was a marked seasonal variation inconcentration of fine root biomass. It was maximum during rainy season andminimum during summer season. Mean seasonal fine root biomass across the sitesranged from 74 to 136 g m-2. On an average, subsidence resulted into a 1.84 foldincrease in fine root biomass over non-subsided site. The reason for higher fineroot biomass in subsided site may be the greater proliferation of roots due to highmoisture and greater accumulation of organic C contents due to run-off from theadjacent undisturbed sites. Besides, the fine roots are comparatively more damagedin cracked zone of the subsided site than the non-subsided site. In subsided site, theproliferating roots and also the other invasive species growing in this zone give riseto more organic carbon. Growing roots of plants release considerable amount oforganic carbon into the rhizosphere (Noble and Randall, 1998) and consequentlybiomass increases.
Net nitrification and net N-mineralization rates were highest during rainy seasonfollowed by winter and summer seasons. The mean values of net nitrification andnet N-mineralization ranged from 3.2-5.3 μg g-1 mo-1 and 4.8-6.94 μg g-1 mo-1,respectively. Subsidence, on an average, gave rise to a 1.66-fold and 1.44-foldincrease in net nitrification and net N-mineralization, respectively. The rates of netnitrification and net N-mineralization were higher in rainy season because of high
soil moisture, which is the limiting factor in tropical dry deciduous forest (Tripathiand Singh, 2007). Singh and Kashyap (2007) proposed that when adequate soilmoisture is available, a large number of nitrifier bacteria become viable or free-living during the rainy season (wet period) and result in high nitrification rate.During summer season (dry period), the soil moisture decreases and results in adecline in nitrifier population size. Further, they also reported that this decrease inpopulation size would also reflect a low rate of nitrification in the soil. The reasonfor higher rates of both the processes in both the subsided sites may also be the highorganic carbon. Moreover, the total nitrogen contents in adjacent undisturbed sitemay suppress the nitrification and N-mineralization rates. Although Hu et al. (1999)reported that most of the chemical properties showed a worsening change from topto the centre of the trough. Singh and Kashyap (2007) postulated that the contactbetween the plant residues and microbes in erosional soil is reduced and may resultin lower decomposition rate, which render the reduced inputs and increased outputsof organic matter in the erosional soils. Higher rates of net nitrification and net N-mineralization in subsided site may also be envisaged due to the higher fine rootbiomass. Roots make a continuous contribution to soil organic matter through decayand their annual contribution to the organic pool can be as high as that from above-ground litter (Nadelhoffer et al., 1985, Tripathi et al., 2009). According to Jacksonet al. (2008), roots influence the complex set of nitrogen (N) transformations thatregulate the production, flow and loss of N in ecosystems. The linkages among rootphysiology, activity of soil biota, and N availability occur at various scales, affectingplant productivity, N use efficiency. These processes thereby contribute to theprovision of ecosystem services, i.e., ecosystem functions and processes of theenvironment (Daily, 1997). The reason for significant increase (P<0.05) in fineroot biomass in subsided site may be due to the greater proliferation of roots due tohigher soil moisture (Tripathi et al., 2009) and greater accumulation of organicmatter runoff toward the centre of subsidence trough. Conversely the fine roots arecomparatively more damaged in the slope due to high tensile strain Structure towardsthe centre of the subsidence trough are generally less prone to damage because theylet down more uniformly after the dynamic subsidence wave passes.
Impact of mining on ground water
Lateral and vertical fluctuation of water level in five piezometric and one observationwell were analyzed over and around N-31 panel. Piezometric well 1 was damaged.Piezometric well 2 lying on the rise side had depletion in water level by 2.8-5.3mduring extraction period and up to eight months after completion of extraction inthe panel. Piezometric wells 3 and 4 lying over barrier between N-31 and N-32panels having 5mm/m tensile strain and centre of N-31 panel having compressionof upto 8mm/m, respectively became dry during extraction of N-31 panel. Thesethree wells are likely to recover after a few years of mining following the complete
settlement of the overburden. Shale layers present below these three wells in theoverburden are likely to help in recovery of water level. Piezometric well 5 lyingover western boundary of N-31 panel in the faulted zone was dry all along the studyand is not likely to recover due to interconnection of faulted zone with theunderground workings. One observation well located 395 m away from the panelwas not affected at all due to mining.
In Y-12 panel piezometric well 1 lying outside the angle of draw on the dip side wasnot affected at all due to mining. Dried wells 2 and 3 lying over barrier pillar betweenY-12 and Y-13 panels having 3mm/m tensile strain and over centre of the panelhaving compressive strain of 5mm/m respectively had water only before thecommencement of extraction in the panel. Piezometric wells 4, 5 and 6 lying on therise side of the panel in fouled zone had water only before commencement ofextraction in Y-12 panel. Wells 2 and 3 are likely to recover after complete settlementof the ground in a few years whereas wells 4, 5, and 6 are not expected to recoverdue to interconnection of underground workings with faulted zone. However, thesewells are likely to recover after a few years of mining (Booth et al.,1998) exceptthose present in faulted zone. Test well located other side of down throw fault wasnot affected at all due to mining. Aquifer parameters are assessed in pumping testbefore and after extraction to know the impact of mining.
The values of transmissivity and hydraulic conductivity of aquifer were almostunaffected due to mining at 5B incline whereas they become almost doubled atMK4 incline due to multi-seam extraction in four panels at a time. This may be dueto development of subsidence-induced fractures in the overburden and due tocombined effect of all the four multi-seam depillared caved panels.
8. CONCLUSION
Although several reports show the detrimental impact of underground minesubsidence on soil fertility status, microbial populations and also plant biomass.While the present study on underground coal mine subsidence has positive impactwith respect to soil texture, soil moisture, WHC, total and plant available soilnutrients. Subsidence increases the soil microbial biomass and soil nitrogentransformation rate which leads to greater release of nutrients which ultimatelycauses proliferation of root biomass and enhancement of plant productivity. Thesubsidence has negative impact with respect to bulk density, development of surfacecracks, undulation of land and tilting of trees at the edge of the subsided sites. Thetransmissivity and hydraulic conductivity of aquifer were significantly affecteddue to multi-seam extraction in four panels at a time. However, there is no impacton the phytosociological and population structure of the forest area having up to1m wide cracks and 23.74 mm/m tensile strain. The findings of this study have
Table 2: Characterization of soil samples of forest ecosystem (control) and oldsubsided site (5 B incline) (±1SE)
16 Underground Coal Mine Subsidence
Table 3: Characterization of soil samples of forest ecosystem (control) and freshsubsided site (MK-4 incline) (±1SE)
Fig. 1: A typical view of subsidence in coal mining area
17Impacts Forest Ecosystem
implications for the management of nutrient budget and hydrology in subsidedunderground coal mine sites.
9. ACKNOWLEDGEMENTS
Ministry of Coal and DST, WOS-B, Govt. of India are gratefully acknowledged forfinancial support to the authors. Thanks are also due to CSIR, New Delhi and CIMFR,Dhanbad for permission.
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