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Geogenic Arsenic Contamination to Ground Water in Parts of Ambagarh Chowk Block, Rajnandgaon District, Chhattisgarh

Author: 
Arunangshu Mukherjee, Dinesh Tewari, Janak Ram Verma, S Subramanian Ranjan Kumar Ray and Rakesh Devangan
Source: 
Bhujal News Quarterly Journal, April-Sept, 2009

INTRODUCTION


Arsenic, in very low concentration has its adverse effect on health. In India, 0.05 ppm (or 50μg/l) concentration of arsenic has been considered the upper safe limit in drinking water, though in the provisional guidelines (2001) of World Health Organisation (WHO), 0.01 ppm has been taken as the upper safe limit of Arsenic in drinking water which in 2003 has been endorsed by Bureau of Indian Standard (BIS).

The purpose of the present work is to document the status of the Arsenic contamination in ground water in Ambagarh Chowki block, Rajnandgaon district .The scope of work involves understanding the extent of Arsenic contamination in space and time and its variation with environmental conditions in the area. Attempts have also been made towards establishing various controls over the occurrence of Arsenic contamination in the Area.

Ambagarh Chowki block of Rajnandgaon district is situated in the west central part of Chhattisgarh adjacent to Maharashtra . (Fig.1) The study area is situated in eastern part of Ambagarh Chowki block covering an area of about 330 sq.km, lying between the north latitudes 20°39’35” to 20°51’00” and east longitudes 80°38’50” to 80°49’28”, as given in. Fig. 2. The study area covers 92 villages where total population is 74604, mainly dominated by schedule tribes. The SC and ST community together constitutes 51% of Population.

Location Map The study area is located in the northern side of the regional water divide of the Godavari and Mahanadi basin. Area is drained by tributaries of the Mahanadi. Seonath is the major perennial tributary draining the area. The drainage pattern is generally sub-dendritic in nature. In areas of volcanic rocks the drainage density is high especially in the southern portion of the study area. The drainage density is poor to moderate in the area of granite terrain and laterite capping areas. The area experiences sub-tropical climate, characterized by extreme summer and extreme winter. The rainy season extends from June to September with well-distributed rainfall during southwest monsoon. During summer season, day temperatures often go above 46°C .During winter, the night temperatures may sometimes drop below 10°C. Evaporation is maximum in the month of May, which is more than 250 mm. In the district summer season humidity is the lowest, about 35% while it is higher during the monsoon period, about 86%.The long term average rainfall of 28 years (1978-2005) worked out to be 1225 mm for Mohala and 1187mm for the Ambagarh Chowki ( Tewari,2003).

METHODOLOGY


Determination of Arsenic in natural water is a skillful job. The collection of samples, time between collection and analysis, analytical process all are very significant in determination of Arsenic in aqueous phase, which mainly occurs in ppb (parts for billion) level. In absence of prescribed sampling Protocol in the country, the conventionally followed sampling procedure has been adopted to collect ground water samples from the Arsenic affected area. 250 ml samples were collected for Arsenic analysis in polyethylene bottle (Tarson made) after filtering through membrane filter of 0.04 micron made of Cellouse nitrate (Sagitarious make) using hand operated vacuum pump (Tarson make). The bottle was filled up to top by sample water, rinsed thoroughly by distilled water first and then by sample water. Two ml of ultra pure HCl (1:1) was added for bringing the pH <3 and the cap was placed.

The dug well samples were collected at least 20 cm below the water surface by rope and bucket. The water samples from hand pumps were collected after running hand pumps for 5 minutes by putting the sample bottles directly below the tap. Samples from borewells and water supply system bores were collected directly after running the pump for 5 minutes. Samples from exploratory wells drilled in the area for delineation of Arsenic free zones was first collected by running the air compressor against the respective zone, and after completion of drilling by lowering of submersible pump and rejecting the first 20 minutes sample from each Piezometer.

40 wells were selected representing all the geological formations of the area for monthly monitoring after the first random sampling of 72 wells from 36 villages. The samples were tested at the site by Arsenic kit (Merck. 1.17927.0001 Arsenic kit, strip type, semi quantitatively by visual comparison) and subsequently in the CGWB regional laboratory. Arsenic analysis was done by ECIL make Atomic Absorption Spectrometer (AAS) using hydride generation technique. The detection limit of the method is 0.001 ppm. Further detailed sampling (July 2006) was carried out in five villages where Arsenic contamination beyond 0.050 ppm level was reported through monthly monitoring. Sampling stations were randomly selected within these villages. Drinking water supply scheme if present were invariably taken as sampling point. Samples were also collected from 40 identified wells in one liter plastic bottle for major ion determination in December 2005. The major ion determination for 40 samples was done following the standard procedure at Regional chemical laboratory of CGWB, NCCR, Raipur. EC & pH were analysed by EC and pH meter. Ca, CO3, HCO3 and Cl were analysed using volumetric methods The field measurement of quality parameters for all the monthly monitoring points was determined in November 2005 and July 2006. EC and pH were measured by calibrated pocket type portable instruments (Hanna made). Coordinates of all the sampling points were determined through GPS for plotting of sample location on map. Reduced level of all the monthly sampling points were determined through surveying from nearest Survey of India bench mark. Water level data were collected for each month during sampling by graduated steel tape.

GEOLOGICAL SET UP


The Chowki area of Rajnandgaon district is part of Indian shield and fall in the Central India Craton. Regionally the area forms part of Dongargarh- Kotri rift zone and is surrounded by Paleo Proterozoic Dongargarh Batholithic granites in west, Meso Proterozoic platform sequence of Chhattisgarh Supergroup in NE and Paleo Proterozoic Supra Crustal sediments of Iron ore series in South. The area is situated within one of the India’s most significant mineralization province where Dalli Rajhra Iron ore deposit, Malajkhand porphyry copper deposit, Chandi-Dondri Fluoride- lead deposit, Kotri Gold prospect and Bodal Uranium prospects are already being exploited where as base metal prospecting in the near by area is under progress.

Arsenic occurrence in the area is controlled by lithology and structure. Therefore the geology, chronostratigraphic sequence, structural history is important to understand the behavior of Arsenic occurrence in the area. The geological relationship of the rocks within the area is complex and poorly understood. The available published geological map on 1:50,000 scale (Krishnamurthy et.al 1998, Ashyiya and Patel 1998, Yogesh Pandey et.al. 2002, Acharyya et.al 2005) of the area by GSI and AMD are having differences. The area needs to be mapped on 1:25000 scale for further detailed information.

Geological map of study area The rocks in the area have regional strike in N-S to NW-SE direction with sub vertical-tovertical dips suggesting antiform or synforms. The rock types represented here by rhyolite, rhyolite porphyries, basalts and thin sheets of mildly ultrabasic (tremolite schists) rocks. The Dongargarh granite batholith emplaced into or formed comagmatically with rhyolite sequence. (Fig 3) .The metamorphism is of low-grade green schist to epidoteamphibolite facies. The sheared rock and metamorphism is considered due to emplacement of epizonal Dongargarh granite batholith, (Krishnamurthy et.al.1988) which during hydrothermal phase deposited the extremely fine veins containing disseminated uranium, fluoride and sulfide minerals.

HYDROGEOLOGICAL SET UP


Detailed hydrogeological study in the Arsenic affected area was carried out to establish hydrogeological relationship between geology and Arsenic concentration. During field investigation 40 key observation wells including both dugwell and bore well were established. Pumping tests were conducted in different formations to know the aquifer characteristics. Ground water level monitoring of all observation wells were carried out on monthly basis and water samples were also collected to assess Arsenic concentration. Bore-wells constructed by CGWB and other agencies were studied.

Water bearing property of aquifer


Hydrological map Ground water occurs in weathered and fractured portion in hard rocks and in porous zones of laterite and alluvium in the area. Ground water occurs under phreatic condition in laterite, alluvium and weathered mantle of crystalline rocks and under semi confined to confined condition in deeper fractured zone. Ground water development in the area is done through dugwells, handpumps and bore wells. Maximum ground water development is observed through irrigation dug well and bore wells in flood plain area of Seonath alluvium. Ground water conditions in different rock formations are discussed below.

In Ferruginous shale and BHQ of the Iron ore Group exposed in southern boundary of the study area, in this formation Ground water occurs in the weathered portion under phreatic condition. In fractured zone (within depth of 100 mbgl) ground water occurs under semi confined to confined condition. Yield of the formation is poor due to their low secondary porosity. Depth to water level during post monsoon observed to vary from 1.5 to 5.00 mbgl and in pre monsoon from 7 to 13 mbgl. The gradient of water table contour in the area is steep. ( Fig 4).

In Basic volcanics, secondary porosity are poorly developed. Weathering has developed permeable zones in weathered mantle of the formation. The thickness of the weathered mantle ranges from 9.9 m at Somatola to 22.57 m at Kumhli. Ground water in weathered mantle occurs in phreatic condition and in fractured aquifer under semi confined to confined condition. Yield potential of the fractured aquifer depends upon inter connectivity and dimension of fractures and the yield range has been recorded from 0.23 lps in Somatola to 12.39 lps in the well at Kumhli. Depth to water level during premonsoon was found to be shallowest at Taramtola (7.02 mbgl) and deepest at Devasur (17.70 mbgl). The post monsoon depth to water level ranges from 3.2 to 10.63 mbgl. Fluctuation of water level varies from 1.00 m at Bhagwantola to 7.72 m at Devasur. Four exploratory wells were constructed in basic rocks at Bhagwantola, Biharikhurd, Gaulitola and Murethitola. Bihrikhurd and Murethitola villages are situated on volcanic tuff where the tuff thickness varying from 13 to 16 m. Basic rock Andesite was encountered below the volcanic tuff and it extended down to maximum drilled depth i.e. 150 m bgl. In basic rock fractured zones are encountered at 32-38, 42-45 mbgl with yield ranges from 2 to 4.5 lps. Transmissivity and storativity of aquifers are moderate (Jogi,2001,Tewari. 2003).

Rhyolites predominate in the study area. In this formation groundwater occurs under phreatic and semi confined conditions and is more productive having potentiality higher than the granite. The thickness of the weathered mantle ranges from 5.6 m (Keshrtola) to 36.2 m( Telitola). The dug wells have a depth range from 6.79 mbgl (Biharikala, 64D/9) to 13.90 mbgl. (Devasur 64D/10). Premonsoon water level was shallowest (2.97 mbgl) at Biharikala and deepest at Ambagarh Chowki (12.42 mbgl) .Seasonal water level fluctuation is in the range of 0.97 m to 6.12 m. The groundwater is under semiconfined conditions in the fractures and joints of rhyolite below the weathered zone. The Atargaon bore well-yielded a discharge of 0.47 lps and Iragaon has the maximum discharge of 15.68 lps. Eight exploratory wells were constructed in Rhyolite in the area, the villages are Kaurikasa, Arajkund, Biharikala, Atargaon , Keshritola, Borhanbhedi and Telitola. Telitola village is situated on volcanic tuffs, with the thickness of 36 m, rhyollite is encountered below volcanic tuff which extended down to the drill depth of 152 m bgl. Fractured zones recorded at 40-45, 58, 95-100, 135-140 m bgl. Yield of these borewells ranges from 1.5 lps to 12 lps. The Transmissivity and storativity of these wells are moderate. Salient features of the Borewell drilled in Rhyolite is given in Table 1

Tabel Ground water in granites occurs within weathered zone, joints and fractures, (deep aquifers) under phreatic to semi confined conditions. The thickness of the weathered zone ranges from 8.4 m at Ratanbhat to 26.78 m at Panabaras and the general thickness of the weathered overburden is 10 to 23 m. Four exploratory wells were constructed in the granitic terrain down to the depth range between 100 to 152 mbgl. The deeper fractures was encountered at 91.4 mbgl in Ratanbhat and 141.87 mbgl at Kaneri and Rangakatherea borewells . The maximum yield was recorded at Ratanbhat 11.39 lps. Most of the exploratory wells in granites have yielded meager discharge .

The depth of dug wells ranges from 4.95 mbgl at Hathea (64D/15) to 14.35 at Matiya (64D/13). Depth to water level ranges between 1.16 mbgl to 11.75 mbgl during premonsoon period and from 3. 1.16 mbgl at Dighwari to 8.20 mbgl at Kandadi (64D/11).in postmonsoon.. The seasonal water level fluctuation in observation wells in granite ranges from 0.22 m at Ghotia (64D/15) to 6.37 m at Markatola (64D/14).

Depth to water level


Depth to water level data of 40 observation key wells, for post monsoon ( Nov 2005 ) indicates that in the major part of the area, depth to water level lies between 3 to 5 mbgl. The deepest water level of 19 m bgl was recorded at Devasur where as the shallowest, 1.94 mbgl was recorded at Dadhutola. About 10% of the wells show depth to water level ranges between 0-3 mbgl, 55% of well shows depth to water level in range of 3 to 5 mbgl and 35% of well shows depth to water level in the range of 5 to 10 mbgl .

Depth to water level for the premonsoon period indicates that in the major part of the area depth to water level is between 5 to 10 mbgl. The deepest water level of 18.35 mbgl was recorded at Devasur, whereas the shallowest 2.49 mbgl at Seonath river bank. In area about 2.5% the wells represented depth to water level between 0-5 mbgl, 67.5% shows 5- 10 mbgl and remaining 30% well shows more than 10 mbgl.

Water level fluctuation


Seasonal water level fluctuation was calculated using depth to water of Nov 2005 and May 2006. Fluctuation indicates that more than 50% of the study area, the water level fluctuation ranges between 3 to 5 m. The maximum fluctuation of 7.7 m is recorded at Devasur and the minimum of 1.25 m at Joratarai.

Ground Water Flow


Hydrograph Ground Water Flow To study the ground water flow during premonsoon June 2006, the reduced level of water levels recorded from 40 key wells were plotted and water table contours, with 10 m. interval was drawn. Water table ranges from 310 to 340 m amsl, (Fig 4). It is clear from the map that the ground water flows towards Seonath River from both side of river, The ground water more or less flow as per the surface drainage pattern. The gradient of ground water is steeper in north west of the Seonath River and is gentler in the south east of the study area. However, gradient of ground water varies from 3.0m /km to 0.3 m/km and the Seonath River is effluent type throughout the year.

Analysis of National Hydrograph Stations


There is one National Hydrograph stations in the study area and the NH station is situated at Ambagarh Chowki. Hence is considered to study long term behavior of water levels. This NH stations was monitored four times every year for study DTW, seasonal fluctuation and long-term fluctuation. To assess the long term behavior of the water level ,data of last 30 years of the station was analysed and hydrographs (Fig 5) prepared. The decadal trend was calculated separately.

Ground Water Resource


The Net annual Ground water availability in the study area as per the GEC-97 methodology is 4021.04 ham and the ground water draft for all uses is 969.12 ham. The stage of ground water development is only 24.10% and the area is categorized as safe from ground water development point of view.

ARSENIC IN GROUND WATER: SOURCE, EXTENT AND VARIATIONS


Sources of Arsenic in Study Area
Distribution of arsenic The sources of Arsenic for the high Arsenic ground water in Chowki area is being established geogenic (Acharyya et.al 2001 and 2005, Y.Pandey et.al 2002, P.Pandey et.al 2002) and is related to Kotri- Dongargarh rift zone. All over the rift zone, insitu soil, weathered rocks and fresh rocks are found locally enriched with Arsenic. The volcanic and shear zone rocks (quartz ribs and vains) contain hydrothermal sulphide mineralization. The most common metal sulphide is pyrite, which many a time is Arsenic bearing. Acharyya et.al (2005) reported presence of 150 ppm Arsenic within the fresh pyrite, occurring in rhyolite from Joratari and 1.6% Arsenic from Dadhutola.

Baseline Concentration Of Arsenic In Ground Water


To know the baseline value of Arsenic in the ground water occurring in Chhattisgarh State ground water samples during May 02 were collected and analysed from 313 permanent monitoring National Hydrograph stations spread over on all 16 district of state ( Tewari, 2003). Apart from this GSI Raipur has carried out intensive sampling of ground water all along the Kotri- Dongargarh rift zone. The state PHED through NEERI Nagpur has made village wise inventory of ground water of entire Chowki block to assess the value of Arsenic in ground water .

The results of 313 samples of Ground water of entire state (excluding Chowki block) by Central Ground Water Board shows that 68% of samples the Arsenic values are found below detection limit (i.e. <0.001 ppm). In other 32% sample the Arsenic values varies from 0.001 to 0.011 ppm. The maximum value observed in one well of Dongargarh (0.011 ppm). However higher Arsenic concentration is being reported by GSI in ground water from the Auriferous Kotri- Dongargarh rift zone extending over 80 km length from Kanker to Rajnandgaon district represented by bimodal volcanics, granite and Iron Ore Group rocks. Apart from the present study area, higher concentration (more than 0.05 ppm) of Arsenic is being reported from few other villages along the rift area are Gurwandi (0.250 to 0.680 ppm) Kaneri (0.060 ppm) Umarpal (0.058 ppm) Jabkasar (0.060 ppm). In other remaining samples the Arsenic concentration was found in the range of < 0.001 to 0.049 ppm. NEERI(2000) in their study have shown the ground water samples are having higher Arsenic concentration in the western part of Chowki block. However in other part of Chowki block ground water samples containing Arsenic concentration between 0.001 and 0.010 ppm.

Spatial And Temporal Extent Of High Arsenic Ground Water In Study Area.


Geogenic Arsenic contamination in ground water at different degree of severity occurs in eastern part of Ambagarh Chowki block, Rajnandgaon district, Chhattisgarh and is confined to the early Proterozoic rhyolitic rocks and granite along the N-S trending Kotri- Dongargarh rift zone. The high Arsenic ground water in the area is restricted to wells in small isolated area and in a cluster of few villages within widely place area of 330 sq.km which is 21.5 km long and 18.5 km wide (broadly 10 km radius area). This area lying between N latitudes 20°39 to 20°51 and E longitudes 80°40’ to 80°49’ and falling under 1:50000 toposheet 64D./ 9,10, 13 & 14. (Fig.6) .The village boundary in the map is as per Census report (1998).

Distribution of high Arsenic Ground water


The geographical distribution of high arsenic ground water is sporadic in the area. Five villages are found severely effected with high Arsenic ground water. These are Kaudikasa > Joratarai > Sonsaytola > Muletitola > Jadutola respectively in order of abundance. Even in the worst affected village -Kaudikasa not all ground water abstraction structures are found contaminated with high values of arsenic (i.e. >0.05 ppm). Only 10 to 70% of the ground water abstraction structures are found with high Arsenic value (>0.05 ppm) in the Area.

Tabel-2 While working in the Ambagarh Chowki block and all those villages of the block in which other organization have reported higher concentration of Arsenic have taken in to account. During investigations in the area from the year 1999-2006 total 793 ground water samples and 24 surface water samples were analyzed in laboratory ( Table-2) and nearly 100 samples insitu at field by Arsenic testing kit and have conformed presence of high Arsenic ground water ( >0.050 ppm ) in 8 villages during one or the other time namely Kaurikasa , Joratarai, Sonsaytola, Jadutola, Muletitola, Telitola, Bharsena, Nichakhore . Out of which first five are severely effected with high Arsenic ground water. The range of Arsenic values in the analysed samples varies from 0.0001 to 0.720 ppm.The surface water samples tested during this period are not found contaminated with Arsenic. (Table.2)

Analytical results: A study for continuous 14 months ( June 05 to July 06) was conducted in selected 40 wells located in 29 villages to evaluate the arsenic concentration in ground water in different months, The ground water analysis results are presented in Table 3. The analysis results reveal the following facts.

1. The 40 wells (dug & bore wells) of 29 villages based on analytical results can broadly be divided into three major groups.
(a) Arsenic was never detected in any of the months of entire study period of 14 months in 9 wells.
(b) Arsenic is generally non detected but suddenly occurs for very few months (1 to 3 or 4 months) mostly during rainy season ( June to August ) and concentration is much below permissible limit of 0.050 ppm. i .e safe for drinking purpose as per BIS norm in 1-20 wells.
(c) Arsenic was detected in all 14 months in 6 villages viz- Jadutola , Joratarai , Sonsaytola , Kaurikasa , Mulethetola , Bhagwantola . The Jadutola and Joratarai occurs in Northern part of study area while Kaurikasa, Mulethetola and Bhagwantola is located in the southern part and the Sonsayatola is situated in western part ( Fig 6). It was observed that in rest 5 villages only 8 wells (5 bore wells and 3 dug wells) content arsenic all through and out of 8 wells only in 5 wells arsenic occurs above permissible limit ( >0.050 ppm). The maximum concentration of 0.720 ppm has been noted from a dug well located in house of Sh. Daulat Ram in Kaurikasa village and nearby bore well in market complex of same village content maximum of 0. 575 ppm arsenic.

2. The ground water structure of above 6 village which containt arsenic (more or less all the 14 months) are located in different rock types such as rhyolite (Kaurikasa and Joratarai), rhyolite tuff (Jadutola and Murethetola) and Granite (Sonsaytola) .

3. The concentration of arsenic varies widely in different months. The arsenic concentration of 0.190 ppm in July 2005 was found to reduced to 0.040 ppm in October 2005 and was found0.097 ppm in July 2006 in the same bore well in Joratarai .Similarly0. 575 ppm arsenic in June2005 remained much below permissible limit in all other months except in June 06 (0.169ppm) in bore well of Kaurikasa village.

Through no definite pattern in arsenic concentration, variation with respect to time has been noted. In certain cases, the concentration has been found to reduce gradually during rainy seasons with minimum concentration mostly during October/November followed by increased gradually in November-December and in rest of the months till May-June this increased values show some oscillating pattern instead of continuous increase. This variation in concentration had also been tried to correlate with monthly rainfall and depth to water level data but any definite conclusion could not be obtained with 14 months data (Fig 7). Perhaps long term data of 5 to 6 years may speak something definite

Causes of Temporal Variation To establish the cause of temporal variation in the Arsenic concentration level in ground water, monthly water level data at the time of sampling were taken and monthly rainfall data from the nearby stations Chowki and Mohla were collected and plotted in graph. The analytical values clearly indicate a dilution of Arsenic concentration during monsoon (July to September when > 90% of annual rainfall occurs) and resultant ground water level remains shallowest during July to October (Fig 7)

Tabel-3

Reasoning of occasional positive test of few well samples


Fig-7 The study reveals that dynamic ground water levels influence Arsenic concentration in ground water. The dynamic levels are results of recharge (mainly during monsoon) and discharge (draft and base flow) phenomenon due to hydro meteorological and climatic parameters and anthropogenic influence. Since the study area is mono-cropped and any ground water irrigational draft is mainly takes places during monsoon and just after monsoon and confined to Kharif crops only, the water level decline from post monsoon to pre monsoon is mainly due to evapo-transpiration (more than 15% area is forested) and steady base flow from the phreatic aquifer coupled with limited ground water draft. The high Arsenic ground water source show gradual enrichment in overall Arsenic concentration after monsoon and is found highest in pre monsoon. (Table 3). Leaching effects from Arsenic enriched soil and weathered rocks can be observed until first few showers. Therefore the wide change in values of Arsenic concentration in some wells in different year (Table 4) may be the result of variation in precipitation amount and pattern along with the availability/ expose of Arsenic source.

The study in the area suggested presence of elevated arsenic concentration in wells is many a time caused by localized borehole interaction of air, water and sulfides. Although Arsenic contamination is caused by oxidation of naturally occurring sulfides, it is influenced by water level fluctuations caused by pumping wells or climatic change, which can shift geographic areas in which contamination occurs (Schrieiber et al, 2000, Smedley and Kinniburgh 2002).

The bore hole provides direct conduit for atmospheric oxygen to interact with water and sulfide minerals when due to pumping water level (draw down) goes below the presence of sulfide mineral in the aquifer material. The hydrogeological studies in the present area reveal drawdown of maximum 38 m in potential wells. However in poor yielding wells it will go further down. Further, deepest maximum water level in the area is noted around 18 mbgl in summer thus the exposed level of aquifer material during pumping can be around 60 mbgl or even more. All the hand pumps in this area are working in this depth range hence can exhibit high Arsenic if situation other wise is favorable/ source mineral is present.

Tabel-4 The borehole construction play important role in sulfide mineral oxidation. Schrieiber et al (2000) during their study in USA found in several heavily impacted wells a lag time occurred between well construction and initiation of sulphide oxidation. The current positive arsenic test in Joratari (0.210 ppm) and Sonsaytola (0.240 ppm) water supply bore well water and constant increase of Arsenic values in Kaudikasa water supply well (Table 4) may be an indication of lag time of sulfide oxidation and release of Arsenic in ground water. Therefore it can be stated that entire stretch of 80 km Kotri- Dongargarh rift zone where sulphide minerilization is in abandoned can be declared as “Arsenic Advisory area”

Arsenic concentrations in dug well and bore wells
Study in the area have established that though many dug wells are highly contaminated in the most affected five villages, still the Arsenic in dug wells are less than the bore wells. The dug wells in the area are maximum 15 m in depth and invariably tap the phreatic aquifer. Pandey P et al (1999, 2002) established negative correlation between diameter of dug well and Arsenic concentration. This relationship is obvious as per the chemical behavior of Arsenic. Due to better atmospheric interaction in dug wells arsenic get oxidized and removed from the aquous phase. Due to the same reason and dilution effect the base flow to surface water body have not effected the level of Arsenic in rivers and ponds. The contamination of nala (0.010 ppm) or some ponds (0.020 ppm) in Kaurikasa village (Acharyya et al. 2005) is probably due to direct pumping of ground water to ponds under Indira Gaon Ganga Yojna.

Relation of individual fracture zone and arsenic concentration
Detailed investigation to establish Arsenic free deeper aquifer in Chowki area has been taken up in the Rajnandgaon district. The exploratory drilling have proved occasional presence of potential deeper fracture which can yield water up to 7 lps. Deeper fractures have been encountered between 60 m and 150 m in number of wells in meta volcanic and granitic rocks in the district. (Tewari, 2003). Based on the findings of regional exploration detailed exploratory drilling programme has been taken up to construct specially design/ well nest in four villages in the area. Deploying DTH rig total 12 wells were constructed tapping individual fractures available in between 0-150 mbgl in a particular site eliminating others by cement sealing. The results of the exploration is summarized in the table 5 and depth wise ground water sample collected while drilling by compressor and after drilling by deploying submersible pumps in the individual wells are analysed at field by Arsenic testing kit as well as in laboratory to find out depth wise Arsenic concentration.( table 5).

The depth wise exploratory wells constructed by tapping only a single water bearing fracture zone in 4 villages have not lead to any definite conclusion. In Mandirpara middle fracture (between 47-54 mbgl) is contaminated and shallow and deeper are free of Arsenic Table 5

Where as at Bihrikala the Arsenic concentration in well water collected by compressed air lifted sample, just after drilling shown constant increase in Arsenic level from BDL to0.027 ppm. However sample collected by lowering of pump have not found any arsenic in this deeper zone i.e. 135 mbgl. Importantly the exploration have provided two high discharge well of 4.5 and 6.3 lps at Bihiri khurd (Arsenic free) and Bihri kala (Arsenic0.027ppm) respectively. This relatively arsenic free source can be utilized for community supply of drinking water.

Tabel-5

Lithological Control Over Arsenic Contamination


The high Arsenic ground water (>0.05 ppm) in the study area is preferably associated with acid meta volcanic and granitic rocks and is situated close to shear zone rocks. Ground water in the relatively younger metabasic, basic and pyroclastic rocks are having conspicuously low Arsenic level (below 0.050 ppm). These are very important observation for the area as basic rocks are occupied nearly 28% of the area and are closely associated with rhyolite. Some of the villages are situated over both the rock type. For example Muletitola, Deversur, Kalkasa, Sangli, Chowki, Arajkund etc, many of them are found one or other time with high arsenic contamination (i.e. > 0.05 ppm). Careful site selection on basic rocks in these villages can eliminate the high Arsenic source. There exists an urgent need of large scale geological mapping 1:25000 or even 1:10000 scale of the area. The occurrence of high Arsenic ground water preferably in rhyolite and granite is reported by Acharyya et al (2005) but they have not discussed the cause behind. Our search for arsenic free aquifer is mainly targeted to basic rocks of the area however large many numbers of villages situated on rhyolite are totally free of arsenic contamination in the area. The hydrothermal phase followed by acid magma intrusion has enriched the rhyolite-granite rocks with Arsenic. The basic rocks intruded after hydrothermal phase, this is the main cause of less abundance of Arsenic in these rocks. The limited occasional occurrence of Arsenic in basic rocks is due to assimilation of rhyolite rocks and enriched through remobilization of sulfide mineral. Because of the difference in the grain size, the glassy rhyolites probably have produced more fine ruptures than granite to host mineralization through moving hydrothermal solution.

Results Of Field Kit Test
Merck arsenic test kit (1.17927.001) is used for insitu field determination of Arsenic in ground water. In this when zinc powder, a solid acid, and- for the elimination of interfering sulfide irons- an oxidizing agent are added to compounds of arsenic (III) and arsenic (V), Arsenic hydride is liberated, which is turn reacts with mercury (II) bromide contained in the reaction zone of the analytical test strip to form yellow-brown mixed arsenic- mercury halogenides. The concentration of arsenic (III) and arsenic (V) are measured semi- quantitavely by visual comparison of the reaction zone of the analytical test strip with the fields of a colour scale. The field kit provides unreliable results regarding for arsenic concentration between 0.010-0.100 (ppm) (Md.Jakuriya et al 2000). During the present study results of kit test many time differ with lab test results, as summarized for few samples Table 6.

Tabel-6

CHEMISTRY OF ARSENIC IN GROUND WATER OF CHOWKI BLOCK


Arsenic is the element of group V-A of the periodic table. Arsenic occurs in both organic and inorganic forms in water. In Inorganic arsenic systems the -3,0, +3 and the +5 oxidation states are common in aqueous systems. The +3 form is more toxic and has greater mobility compared to the other forms. Arsenic is known to readily participate in oxidation, reduction, methylation and acid base reaction. Aqueous arsenic in the form of arsenite, arsenate and organic arsenicale may result from mineral dissolution, industrial discharge or the application of herbicides. The toxicity of arsenic depends on its chemical Form.

In previous para attempt has been made to discuss temporal and spatial variations of Arsenic content in ground water also discussed its geological control in various formations and different zones. Huge data had been collected to study arsenic in ground water of Chowki block. Apart form specific arsenic analysis, 39 ground water samples were analysed to determine chemical composition of ground water in the study area to understand the general characteristic of ground water in study area and the results are tabulated in Table 7

Discussion On Analytical Results


Piper tri The results of chemical analysis of 39 samples from study area show that ground water in the area is generally fresh as EC values were found in the range of 201 μs/cm to 1388 μs/cm. EC values in excess of 1000 μs/cm were found in only 6 wells water. Water in the area is alkaline in nature as pH varies in the range of 7.6 to 8.2. All the major ions (cations and anions) including fluoride as minor ion were found in safe limit for drinking water use. Nitrate ion in excess of 45 ppm was found only in eight wells whereas more than 100 ppm was found in a well of Nichagoda Village where its concentration was 116 ppm. Plot of water quality data on tri-linear diagram (Fig.8) shows that calcium and magnesium ion together dominant over sodium and potassium, as such water is calcium and magnesium type in the area with regards to anions. Most of samples are carbonate and bicarbonate type. In some of the cases chloride and sulphate ions dominates.

An attempt has been made to find inter correlation been two ions. Some interesting features were noted from the results. As usual EC has positive correlation with all the major ions. Bicarbonates ions have better correlation of +0.61 with magnesium ion in composition to calcium ions (+ 0.27). This shows the dominance of MgHCO3 in the area. Chloride ions have poor correlation of +.08 with bicarbonate. This shows higher the bicarbonate, lower is the chloride ions. Remaining correlations are normal and usual.

Tabel-7

Relation Of Arsenic With Other Ions


An attempt has been made to establish a correlation between major and minor ions of the water samples together with arsenic. It has been observed that high arsenic is associated with low EC, low bicarbonates and comparatively high sulphate ions.

Quality of Ground Water For Drinking And Irrigation Use


As discussed above, it is inferred that arsenic free water if available; then ground water is the study area is safe for drinking water. In one cases nitrate concentration was found more than 100 mg/l this need to be monitored in future. Water quality data was plotted on US Salinity diagram (Fig-9) to assess irrigation quality of ground water. Ground water samples were found to fall in CS, C2S, C3S, class of irrigation quality classification. Since, there is no sodium hazard in the ground water and EC is below 1500 μs/cm, therefore, ground water in the area is safe for irrigation.

CONCLUSIONS


During the present investigation in high arsenic ground water area of Chowki block Rajnandgaon district detailed study have been made to (i) establish geological and hydrogeological relationship to high arsenic concentration in ground water, its variations and controls (ii) For source of arsenic free aquifer within the area by exploratory drillings. (iii) Inventory the present status of arsenic poisoning and possible remedial measures and alternative arrangements. The investigation carried out so far is able to understand many aspects of high Arsenic ground water in the area, these are summarized below.

The high arsenic ground water occurrence in eastern part of Chowki block, Rajnandgaon district, Chhattisgarh is confined to the early Proterozoic meta volcanic- granite rocks along the N-S trending Kotri-Dongargarh rift zone. The geographical extent of this high arsenic ground water occurrence is found in isolated clusters distributed over an area of 330 sq.km, nearly in a 10 km radius zone. The most severely affected villages are preferably situated on rhyolite and granite rocks close to shear zone. The relatively younger metabasic, basic and pyroclastic aquifers are less contaminated. This is an important since 28 % of the area is covered with basic rock closely associated as anasthomosing network with rhyolite, the dominant rock type of the area. The lithological control thus observed may contributes toward providing safe and alternative drinking and irrigation water. The intrusion of rhyolitic- granitic magma is followed by the hydrothermal phase which is responsible for arsenic enriched sulfide mineralization and Arsenic enrichment in bedrock. The emplacement of basic rocks took place after the hydrothermal phase, the limited occurrence of Arsenic in basic rocks are due to assimilation and remobilization reaction only.

In this total 330 sq.km area 92 villages are situated where the present population (census 2001) is around 75000, mainly dominated by aboriginals (SC & ST). Ground water is the principal source for drinking water for the area including the five most arsenic affected villages. State PHED has nearly 780 HP and 20 power pumps in these villages for providing drinking water for people. Since 1997, State PHED has sealed nearly 25 hand pumps in 11 villages which were identified as high Arsenic ground water source. The alternative safe drinking water supply in the most Arsenic affected villages are still through ground water abstraction. During the present course of study, in two villages Sonsaytola & Joratarai it is found that the public water supply source (through ground water) is contaminated with high arsenic 0.240 and 0.210 ppm respectively .Regarding the irrigation water, the area is predominantly mono-cropped area. Kharif is the cropping season and mainly rain fed, supported by surface canal irrigation and occasional ground water irrigation. Second crop area is very scanty and is invariably based on ground water irrigation. The area is having limited ground water potential, which is restricted to the upper 150 m weathered and fractured zone . Present annual ground water draft for irrigation is 860 ham only and other industrial requirement is negligible. Overall ground water development of the area is 24% . Apart from the most arsenic affected five villages Kaurikasa> Joratari> Sonsaytola> Jadutola> Muletitola, the occurrence of high arsenic ground water in the remaining area is sporadic and occasional. Even in the most affected villages not all the ground water source are affected with high arsenic contamination. Bore wells and hand pumps are more affected than dug wells in general, and this is more particularly applicable to the area where arsenic contamination is less. However large numbers of dug wells are severely arsenic contaminated in most effected Kaurikasa, Sonsaytola, Joratarai village. During the ground water exploration in the area some arsenic free or limited arsenic contaminated (< 0.040 ppm) fractures were tapped through specially design wells by CGWB which can be utilized as alternative safe drinking water source. Geochemical control on occurrence of high arsenic ground water has been established. The ground water with more than 800 μs/cm EC are found invariably containing lower arsenic value, below 0.050 ppm.

Persons with arsenic toxicity manifestations are identified only in Kaurikasa and Sonsaytola village. Report says 400 persons (nearly 30% population) of village Kaurikasa are affected with high arsenic ground water poisoning and 130 people are critically affected. The clinical symptoms like palmoplanter keratosis can be frequently observed in the area. The critically affected patients are early adulthood or middle age group. Micro watershed management work under Rajiv Gandhi National Watershed Management Programme is going on in the area. Since the soil and weathered rocks are enriched with arsenic hence method of artificial recharge which can bypass the soil and weathered zone must be applied for the area. The Vadose zone available in the area for recharge to dilute the Arsenic contamination is calculated 41 MCM. Long term analysis of hydrograph of Ambagarh Chowki have shown significant decline in pre and post monsoon water level in last decade, where as in previous decade the level were either stable or rising in trend. This clearly indicates enhancement of draft during recent past, which gradually is exposing more and more aquifer material to oxidizing environment. The deepest static water level is at present around 18 m and maximum drawdown is recorded 38 m hence present zone of expose is around 60 m bgl which is the zone of hand pump operation in the area. Ground water in the Amabagarh Chowki block is otherwise geochemically fresh with low to medium salinity; therefore Arsenic free ground water is good both for drinking and irrigation use.

ACKNOWLEDGMENT:


The authors are highly thankfully to Shri B M Jha, Chairman and Sh S Kunar, Member (T&TT), Central Ground Water Board, Faridabad for kind permission to take up and publish the work. The authors thankfully acknowledge the suggestions and review done to the early manuscript by Shri P K Das ,the then HOO, CGWB, NCCR Raipur. Shri A.K.Sinha, the then Regional Director, Sh Ashish Chokraborty, Regional Director, CGWB, NCCR Raipur , Shri T.M. Hunse, the then Superintending Hydrogeologist and Sh Abhijeet Roy, RD, CGWB. Sincere thanks to Dr. S. Shekhar, Assistant Editor for inviting us to contribute in this prestigious volume. Data collected from various agencies such as IMD, District Statistical and Economic Department, PHED, State Irrigation department, GSI, Raipur are thankfully acknowledged.

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Arunangshu Mukherjee, Dinesh Tewari, Janak Ram Verma, S Subramanian Ranjan Kumar Ray and Rakesh Devangan - Central Ground water Board, North Central Chhattisgarh region, Raipur.

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