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M/s Birla Corporation Limited has retained M/s Ground Water & Mineral Investigation Consultancy Centre (P) Ltd., Jaipur to carry out Comprehensive Hydrogeological studies with ground water modelling for deep mining dewatering assessment in core zone of 852.50 ha. area and its impact in 10 km radius buffer zone of Sagmania Limestone Mines. Location map of studied area and layout plan of the mine is shown in Fig 1 & 2. The field work was carried out during the month of June & July 2020, by G. Megha Shyam, Sr. Manager (Water Management) and Intekhab Alam, Sr. Manager (Hydrogeology) of GWMICC P. Ltd.
The Problem: –
Topographical Condition: –The studied area is plain with slight undulations. The elevations of core zone are ranging between 323 m amsl & 273 to 382 m amsl in buffer zone. In core zone, there is no significant artificial drainage system in mine lease area. Due to natural slopes, the rain water makes its own path and moves towards mined out pits and in the remining places, water moves towards South and joins natural nallah near Railway track. In Buffer zone
Hydrogeological Condition: –The drainage in area is mainly formed by river Tons and its tributaries. Tons is perennial river with 5-6 orders streams and has dendritic drainage pattern which flows in north and north-east direction. The area received average annual rainfall of 1028.45 mm (1985 to 2019) and the daily maximum rainfall has been 226.60 mm in last 35 years as per data recorded in Rain gauge station installed in BCL premises. The main aquifer in and around Sagmania mines in core zone is fractured limestone and movement of ground water in lime stone is essentially by bedding plains, development and nature of secondary joints and fractures while solution cavities also play an important role for ground water occurrence. Ground water in general is under unconfined to semi-confined conditions (CGWB District Brochure2017). In buffer zone, alluvium followed by limestone, shale, dolomatic limestone are the main aquifers which are mostly unconfined in nature.
Seasonal Variations:- Ground water levels in the core zone range from above 18 m to 30 below ground level in Monsoon/Post-Monsoon (286 to 295 m amsl). The seasonal fluctuation of water level is around average 3 m as per observations made in Piezometer in April 2018 to March 21 near the Sagmania mines in core zone. Whereas, in buffer zone water level range from above 10 to 37 m. below ground level (260 to 320 m amsl). The seasonal fluctuation in water levels are around 3 m as per the field observations. The average TDS of ground water in core zone is around 500 mg/ltr. whereas in buffer zone it goes up to as high as 2000 mg/ltr. & total Hardness in core zone is around 400 mg/ltr. but in buffer zone, it varies from over 500 to 1100 mg/ltr. The elements and compounds analyzed as above in NABL accredited lab indicate that the major ingredients are within permissible limits as per IS: 10500:2015 norms in core zone; whereas, in buffer zone, most of the ingredients are in permissible limits except Total Hardness.
Mine dewatering: – In the sagmaina mine Dewatering is major issue Dewatering a totally or partially flooded mine, or even as a normal procedure in normal mine activity, they carry a number of problems, most of which are environment related. Sagmania mine is open pit mines there is Large problems are developing with abandoned mines that have accumulated acid mine drainage that are growing to become a larger problem as the water sits in the mine and reacts with the exposed rock. There can be mitigation through proper management and with enough funds.
The most relevant concerns with mine dewatering are related to acid mine drainage and the dispersal of contaminated water into other water fonts and the general environment becoming a serious source of pollution.
The Solution
Physiographic Studies: – On the basis of regional levels marked with the help of Shatter Radar Topography Mission (SRTM) based on satellite imaginary and site visit, Digital Elevation Model for core and buffer zone was prepared and general physiographic conditions were studied and physiographic gradient was determined. Besides, Survey of India (SOI) topographic maps were studied and base map for the studies were prepared using villages, roads, railways, streams, canals etc. networks.
Hydrogeological Observations: In hydrogeological studies, observations were made about geology and types of aquifers, water quality, water levels, yield pattern etc. through hydrogeological inventory of existing groundwater structures in core and buffer zones of Sagmania mines.
Aquifer performance test: (Long duration test) A.P.T. is designed to impose hydraulic stress up to three log cycles/equilibrium state of the aquifer such that measurements of the response to the stress will fit in a theoretical model of aquifer signatures. The test set up consists of an exploratory well in which the aquifer is subjected to stress and its effect on the observation well is measured. Observations during APT include measurement of water levels at periodic interval as per log scale & discharge is measured. This test helps to know hydraulic parameters of the area of influence. This long duration test was done in core & buffer zones for detailed studies of all the aquifers encountered.
Geoelectric Survey: Geoelectric survey was done as spot soundings in the core zone to know about the regional behaviour of subsurface disposition of aquifer system to workout hydrogeological characteristics such as saturated thickness, number of aquifers, nature of aquifers viz. confined/unconfined/semi-confined. The geoelectric sections prepared as a result of interpretation of geoelectric data depicting lithology, water level, saturation & expected quality of ground water. The sections give an account of subsurface distribution of aquifers.
Rain Water Harvesting: Existing rain water harvesting measures were studied and recharge to ground water was estimated and the impact of these recharge measures were evaluated. Through seepage studies, constant rate of infiltration of water is determined after complete saturation of the soil in the prevailing condition of soil in the field. This study helps to know about the seepage of water through the soil in core and buffer zones. Recharge test was also carried out by injecting known volume of water in existing tube well & constant rate of recharge was determined for the aquifers present in core & buffer zones, to know the recharge capacity of the aquifers for designing most feasible RWH structures.
Water Resources Evaluation: Based on Hydraulic parameters & water level fluctuation of the area, all recharge components were determined, ground water draft due to all factors were considered and Water Resources have been evaluated in the area to assess extent of subsurface water availability in the study year.
Groundwater Modelling: The broad objectives of this modelling exercise is to replicate the field parameters numerically into the ground water model, development of ground water flow model and simulation of the existing field conditions, calibration and testing of the proposed groundwater scenario through budgeting and water level trend predictions. The numerical approach used for solving ground water flow equations used are Finite Difference Method (FDM) in the model MODFLOW-NWT. This model is a simple analytical and numerical model which has applicability and usefulness in hydrogeological and remedial investigations.
Dewatering of Mines and Water Utilization Plan: According to mine plan, hydrogeologic characteristics of the aquifers to be encountered during mining, water inflow conditions, dewatering estimates are done. Besides, quantities of dewatering were worked out for 1 st 5 years of mining as per mining plan. The water extraction and its utilization plan for gainful purpose in and around mine lease area was also worked out.
Impact Assessment of Mining on Ground Water: Based on above comprehensive studies, impact of mining activities on water environment of the area is evaluated and its management is suggested such that on ground water regime of the area is not affected adversely. Based on mining plan, water level, ultimate mine limits and quantifications of both surface and ground water, impact on water quality and dewatering is also assessed
Results :-
The hydrogeological, hydro-chemical, dewatering & rainwater harvesting studies reveal that due to mining activities there will not be any adverse impact on the groundwater regime in terms of water level, quantity and quality including socio economic aspects of buffer zone in immediate future. The quantities of mine seepage water has been calculated as per CGWA norms. The proposed recharge potential through applied irrigation by dewatered water and recharge potential through existing mine pits. Artificial recharge measures to be taken up by BCL in first five years of mining would be more than 50 % of water extraction/dewatering. Therefore, recharge measures to be adopted would satisfy recharge criteria for Semi-Critical areas as per CGWA guidelines, 2015. Average mixed quality of rainwater and groundwater would be lower than normal groundwater quality It is clear that in Monsoon/Post-Monsoon season water inception in the mine would not cause adverse impact on ground water quality of core and buffer zone as major chemical ingredients are within permissible limits of IS10500 norms. There have been positive impact on ground water regime of the area due to canal irrigation system and development of ponds/check dams etc. Therefore in immediate future, no adverse impact on ground water regime of the area is envisaged on the people of the area due to groundwater abstraction/dewatering of ground water by BCL. There are no wells or agriculture fields or any other business activities within the area of influence determined by APT. Hence, negative socio-economic effects are not expected in next five years
Impact:-
The stage of ground water development at 96 % rendering area under Semi Critical category as per CGWA. Therefore, there will be no negative effect on ground water regime of the area due to mining. However, groundwater shows rising trend from pre-monsoon of 2021 to pre-monsoon of 2025 by 0.8 m. Hence, water levels will rise nearly @ 0.16 m per annum in next five years as predicted by ground water modelling. The surface water component of intercepted water in mining pit would cause direct & positive impact on ground water quality of the core area as it has relatively low TDS around 200 mg/ltr. TDS of ground water is around 506 mg/ltr. Average mixed quality of rainwater and groundwater would be lower than normal groundwater quality It is clear that in Monsoon/Post-Monsoon season water inception in the mine would not cause adverse impact on ground water quality of core and buffer zone as major chemical ingredients are within permissible limits of IS10500 norms. There have been positive impact on ground water regime of the area due to canal irrigation system and development of ponds/check dams etc. Therefore in immediate future, no adverse impact on ground water regime of the area is envisaged on the people of the area due to groundwater abstraction/dewatering of ground water by BCL. There are no wells or agriculture fields or any other business activities within the area of influence determined by APT. Hence, negative socio-economic effects are not expected in next five years. The hydrogeological, hydro-chemical & rainwater harvesting studies reveal that due to mining activities there will not be any adverse impact on the groundwater regime in terms of waterlevel, quantity and quality including socio economic aspects of bufferzone in immediate future.