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

INTRODUCTION

Arsenic, in very low concentration has its adverse effect on health. In India, 0.05 ppm (or50μg/l) concentration of arsenic has been considered the upper safe limit in drinkingwater, 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 in2003 has been endorsed by Bureau of Indian Standard (BIS).

The purpose of the present work is to document the status of the Arsenic contaminationin ground water in Ambagarh Chowki block, Rajnandgaon district .The scope of workinvolves understanding the extent of Arsenic contamination in space and time and itsvariation with environmental conditions in the area. Attempts have also been madetowards establishing various controls over the occurrence of Arsenic contamination in theArea.

Ambagarh Chowki block of Rajnandgaon district is situated in the west central part ofChhattisgarh adjacent to Maharashtra . (Fig.1) The study area is situated in eastern partof Ambagarh Chowki block covering an area of about 330 sq.km, lying between the northlatitudes 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, mainlydominated by schedule tribes. The SC and ST community together constitutes 51% ofPopulation.

The study area is located in the northern side of the regional water divide of the Godavariand Mahanadi basin. Area is drained by tributaries of the Mahanadi. Seonath is the majorperennial tributary draining the area. The drainage pattern is generally sub-dendritic innature. In areas of volcanic rocks the drainage density is high especially in the southernportion of the study area. The drainage density is poor to moderate in the area of graniteterrain and laterite capping areas. The area experiences sub-tropical climate,characterized by extreme summer and extreme winter. The rainy season extends fromJune to September with well-distributed rainfall during southwest monsoon. Duringsummer 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, timebetween collection and analysis, analytical process all are very significant in determinationof Arsenic in aqueous phase, which mainly occurs in ppb (parts for billion) level. Inabsence of prescribed sampling Protocol in the country, the conventionally followedsampling procedure has been adopted to collect ground water samples from the Arsenicaffected 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 Cellousenitrate (Sagitarious make) using hand operated vacuum pump (Tarson make). The bottlewas filled up to top by sample water, rinsed thoroughly by distilled water first and then bysample water. Two ml of ultra pure HCl (1:1) was added for bringing the pH
The dug well samples were collected at least 20 cm below the water surface by rope andbucket. The water samples from hand pumps were collected after running hand pumps for5 minutes by putting the sample bottles directly below the tap. Samples from borewellsand water supply system bores were collected directly after running the pump for 5minutes. Samples from exploratory wells drilled in the area for delineation of Arsenic freezones was first collected by running the air compressor against the respective zone, andafter completion of drilling by lowering of submersible pump and rejecting the first 20minutes sample from each Piezometer.

40 wells were selected representing all the geological formations of the area for monthlymonitoring after the first random sampling of 72 wells from 36 villages. The samples weretested at the site by Arsenic kit (Merck. 1.17927.0001 Arsenic kit, strip type, semiquantitatively by visual comparison) and subsequently in the CGWB regional laboratory.Arsenic analysis was done by ECIL make Atomic Absorption Spectrometer (AAS) usinghydride generation technique. The detection limit of the method is 0.001 ppm. Furtherdetailed sampling (July 2006) was carried out in five villages where Arsenic contaminationbeyond 0.050 ppm level was reported through monthly monitoring. Sampling stationswere randomly selected within these villages. Drinking water supply scheme if presentwere invariably taken as sampling point. Samples were also collected from 40 identifiedwells in one liter plastic bottle for major ion determination in December 2005. The majorion determination for 40 samples was done following the standard procedure at Regionalchemical laboratory of CGWB, NCCR, Raipur. EC & pH were analysed by EC and pHmeter. Ca, CO3, HCO3 and Cl were analysed using volumetric methods The fieldmeasurement of quality parameters for all the monthly monitoring points was determinedin November 2005 and July 2006. EC and pH were measured by calibrated pocket typeportable instruments (Hanna made). Coordinates of all the sampling points weredetermined through GPS for plotting of sample location on map. Reduced level of all themonthly sampling points were determined through surveying from nearest Survey of India bench mark. Water level data were collected for each month during sampling by graduatedsteel tape.

GEOLOGICAL SET UP

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

Arsenic occurrence in the area is controlled by lithology and structure. Therefore thegeology, chronostratigraphic sequence, structural history is important to understand thebehavior of Arsenic occurrence in the area. The geological relationship of the rocks withinthe area is complex and poorly understood. The available published geological map on1:50,000 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 areaneeds to be mapped on 1:25000 scale for further detailed information.

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 disseminateduranium, fluoride and sulfide minerals.

HYDROGEOLOGICAL SET UP

Detailed hydrogeological study in the Arsenic affected area was carried out to establishhydrogeological relationship between geology and Arsenic concentration. During fieldinvestigation 40 key observation wells including both dugwell and bore well wereestablished. Pumping tests were conducted in different formations to know the aquifercharacteristics. 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

Ground water occurs in weathered and fractured portion in hard rocks and in porouszones of laterite and alluvium in the area. Ground water occurs under phreatic conditionin laterite, alluvium and weathered mantle of crystalline rocks and under semi confined toconfined 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 Seonathalluvium. 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 phreaticcondition. 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 developedpermeable zones in weathered mantle of the formation. The thickness of the weatheredmantle ranges from 9.9 m at Somatola to 22.57 m at Kumhli. Ground water in weatheredmantle occurs in phreatic condition and in fractured aquifer under semi confined toconfined condition. Yield potential of the fractured aquifer depends upon inter connectivityand dimension of fractures and the yield range has been recorded from 0.23 lps inSomatola to 12.39 lps in the well at Kumhli. Depth to water level during premonsoon wasfound to be shallowest at Taramtola (7.02 mbgl) and deepest at Devasur (17.70 mbgl). Thepost monsoon depth to water level ranges from 3.2 to 10.63 mbgl. Fluctuation of waterlevel varies from 1.00 m at Bhagwantola to 7.72 m at Devasur. Four exploratory wellswere constructed in basic rocks at Bhagwantola, Biharikhurd, Gaulitola and Murethitola.Bihrikhurd and Murethitola villages are situated on volcanic tuff where the tuff thicknessvarying from 13 to 16 m. Basic rock Andesite was encountered below the volcanic tuff andit extended down to maximum drilled depth i.e. 150 m bgl. In basic rock fractured zonesare encountered at 32-38, 42-45 mbgl with yield ranges from 2 to 4.5 lps. Transmissivityand storativity of aquifers are moderate (Jogi,2001,Tewari. 2003).

Rhyolites predominate in the study area. In this formation groundwater occurs underphreatic and semi confined conditions and is more productive having potentiality higherthan the granite. The thickness of the weathered mantle ranges from 5.6 m (Keshrtola) to36.2 m( Telitola). The dug wells have a depth range from 6.79 mbgl (Biharikala, 64D/9) to13.90 mbgl. (Devasur 64D/10). Premonsoon water level was shallowest (2.97 mbgl) at Biharikala and deepest at Ambagarh Chowki (12.42 mbgl) .Seasonal water levelfluctuation is in the range of 0.97 m to 6.12 m. The groundwater is under semiconfinedconditions in the fractures and joints of rhyolite below the weathered zone. The Atargaonbore well-yielded a discharge of 0.47 lps and Iragaon has the maximum discharge of 15.68lps. Eight exploratory wells were constructed in Rhyolite in the area, the villages areKaurikasa, Arajkund, Biharikala, Atargaon , Keshritola, Borhanbhedi and Telitola.Telitola village is situated on volcanic tuffs, with the thickness of 36 m, rhyollite isencountered 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 borewellsranges from 1.5 lps to 12 lps. The Transmissivity and storativity of these wells aremoderate. Salient features of the Borewell drilled in Rhyolite is given in Table 1

Ground water in granites occurs within weathered zone, joints and fractures, (deepaquifers) under phreatic to semi confined conditions. The thickness of the weathered zoneranges from 8.4 m at Ratanbhat to 26.78 m at Panabaras and the general thickness ofthe weathered overburden is 10 to 23 m. Four exploratory wells were constructed in thegranitic terrain down to the depth range between 100 to 152 mbgl. The deeper fractureswas encountered at 91.4 mbgl in Ratanbhat and 141.87 mbgl at Kaneri andRangakatherea 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 duringpremonsoon 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 ingranite 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 levelranges between 0-3 mbgl, 55% of well shows depth to water level in range of 3 to 5 mbgland 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 areadepth to water level is between 5 to 10 mbgl. The deepest water level of 18.35 mbgl wasrecorded at Devasur, whereas the shallowest 2.49 mbgl at Seonath river bank. In areaabout 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 May2006. 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 atDevasur and the minimum of 1.25 m at Joratarai.

Ground Water Flow

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 tablecontours, 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-97methodology is 4021.04 ham and the ground water draft for all uses is 969.12 ham. Thestage of ground water development is only 24.10% and the area is categorized as safe fromground water development point of view.

ARSENIC IN GROUND WATER: SOURCE, EXTENT AND VARIATIONS

Sources of Arsenic in Study Area
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.al2002) 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 volcanicand 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 ChhattisgarhState ground water samples during May 02 were collected and analysed from 313permanent 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 groundwater all along the Kotri- Dongargarh rift zone. The state PHED through NEERI Nagpurhas made village wise inventory of ground water of entire Chowki block to assess the valueof Arsenic in ground water .

The results of 313 samples of Ground water of entire state (excluding Chowki block) byCentral Ground Water Board shows that 68% of samples the Arsenic values are foundbelow detection limit (i.e.

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

Geogenic Arsenic contamination in ground water at different degree of severity occurs ineastern part of Ambagarh Chowki block, Rajnandgaon district, Chhattisgarh and isconfined 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 insmall isolated area and in a cluster of few villages within widely place area of 330 sq.kmwhich is 21.5 km long and 18.5 km wide (broadly 10 km radius area). This area lyingbetween N latitudes 20^°39 to 20^°51 and E longitudes 80^°40’ to 80^°49’ and falling under1:50000 toposheet 64D./ 9,10, 13 & 14. (Fig.6) .The village boundary in the map is as perCensus report (1998).

Distribution of high Arsenic Ground water

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

While working in the Ambagarh Chowki block and all those villages of the block in whichother 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 watersamples and 24 surface water samples were analyzed in laboratory ( Table-2) and nearly100 samples insitu at field by Arsenic testing kit and have conformed presence of highArsenic 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 ofArsenic values in the analysed samples varies from 0.0001 to 0.720 ppm.The surfacewater 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 conductedin selected 40 wells located in 29 villages to evaluate the arsenic concentration in groundwater 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 canbroadly be divided into three major groups.
(a) Arsenic was never detected in any of the months of entire study period of 14months in 9 wells.
(b) Arsenic is generally non detected but suddenly occurs for very few months (1 to 3or 4 months) mostly during rainy season ( June to August ) and concentration ismuch below permissible limit of 0.050 ppm. i .e safe for drinking purpose as perBIS 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 Jorataraioccurs in Northern part of study area while Kaurikasa, Mulethetola andBhagwantola is located in the southern part and the Sonsayatola is situated inwestern part ( Fig 6). It was observed that in rest 5 villages only 8 wells (5 borewells and 3 dug wells) content arsenic all through and out of 8 wells only in 5 wellsarsenic occurs above permissible limit ( >0.050 ppm). The maximum concentrationof 0.720 ppm has been noted from a dug well located in house of Sh. Daulat Ramin Kaurikasa village and nearby bore well in market complex of same villagecontent maximum of 0. 575 ppm arsenic.

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

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

Through no definite pattern in arsenic concentration, variation with respect to time hasbeen noted. In certain cases, the concentration has been found to reduce gradually duringrainy seasons with minimum concentration mostly during October/November followed byincreased gradually in November-December and in rest of the months till May-June thisincreased values show some oscillating pattern instead of continuous increase. Thisvariation in concentration had also been tried to correlate with monthly rainfall and depthto 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 VariationTo establish the cause of temporal variation in the Arsenic concentration level in groundwater, monthly water level data at the time of sampling were taken and monthly rainfalldata from the nearby stations Chowki and Mohla were collected and plotted in graph. Theanalytical values clearly indicate a dilution of Arsenic concentration during monsoon (Julyto September when > 90% of annual rainfall occurs) and resultant ground water levelremains shallowest during July to October (Fig 7)

Reasoning of occasional positive test of few well samples

The study reveals that dynamic ground water levels influence Arsenic concentration inground water. The dynamic levels are results of recharge (mainly during monsoon) anddischarge (draft and base flow) phenomenon due to hydro meteorological and climaticparameters and anthropogenic influence. Since the study area is mono-cropped and anyground water irrigational draft is mainly takes places during monsoon and just after monsoonand 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 steadybase flow from the phreatic aquifer coupled with limited ground water draft. The highArsenic ground water source show gradual enrichment in overall Arsenic concentrationafter monsoon and is found highest in pre monsoon. (Table 3). Leaching effects fromArsenic 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 theavailability/ expose of Arsenic source.

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

The bore hole provides direct conduit for atmospheric oxygen to interact with water andsulfide minerals when due to pumping water level (draw down) goes below the presence ofsulfide mineral in the aquifer material. The hydrogeological studies in the present areareveal drawdown of maximum 38 m in potential wells. However in poor yielding wells itwill go further down. Further, deepest maximum water level in the area is noted around18 mbgl in summer thus the exposed level of aquifer material during pumping can bearound 60 mbgl or even more. All the hand pumps in this area are working in this depthrange hence can exhibit high Arsenic if situation other wise is favorable/ source mineral ispresent.

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 timeoccurred between well construction and initiation of sulphide oxidation. The currentpositive arsenic test in Joratari (0.210 ppm) and Sonsaytola (0.240 ppm) water supplybore 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 inground water. Therefore it can be stated that entire stretch of 80 km Kotri- Dongargarhrift zone where sulphide minerilization is in abandoned can be declared as “ArsenicAdvisory area”

Arsenic concentrations in dug well and bore wells
Study in the area have established that though many dug wells are highly contaminatedin 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 phreaticaquifer. Pandey P et al (1999, 2002) established negative correlation between diameter ofdug well and Arsenic concentration. This relationship is obvious as per the chemicalbehavior of Arsenic. Due to better atmospheric interaction in dug wells arsenic getoxidized and removed from the aquous phase. Due to the same reason and dilution effectthe base flow to surface water body have not effected the level of Arsenic in rivers andponds. The contamination of nala (0.010 ppm) or some ponds (0.020 ppm) in Kaurikasavillage (Acharyya et al. 2005) is probably due to direct pumping of ground water to pondsunder Indira Gaon Ganga Yojna.

Relation of individual fracture zone and arsenic concentration
Detailed investigation to establish Arsenic free deeper aquifer in Chowki area has beentaken up in the Rajnandgaon district. The exploratory drilling have proved occasionalpresence of potential deeper fracture which can yield water up to 7 lps. Deeper fractureshave been encountered between 60 m and 150 m in number of wells in meta volcanic andgranitic rocks in the district. (Tewari, 2003). Based on the findings of regional explorationdetailed 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 constructedtapping individual fractures available in between 0-150 mbgl in a particular siteeliminating others by cement sealing. The results of the exploration is summarized in thetable 5 and depth wise ground water sample collected while drilling by compressor andafter drilling by deploying submersible pumps in the individual wells are analysed at fieldby Arsenic testing kit as well as in laboratory to find out depth wise Arsenicconcentration.( table 5).

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

Where as at Bihrikala the Arsenic concentration in well water collected by compressed airlifted sample, just after drilling shown constant increase in Arsenic level from BDL to0.027ppm. However sample collected by lowering of pump have not found any arsenic in thisdeeper zone i.e. 135 mbgl. Importantly the exploration have provided two high dischargewell 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 ofdrinking water.

Lithological Control Over Arsenic Contamination

The high Arsenic ground water (>0.05 ppm) in the study area is preferably associated withacid meta volcanic and granitic rocks and is situated close to shear zone rocks. Groundwater in the relatively younger metabasic, basic and pyroclastic rocks are havingconspicuously low Arsenic level (below 0.050 ppm). These are very important observationfor the area as basic rocks are occupied nearly 28% of the area and are closely associatedwith rhyolite. Some of the villages are situated over both the rock type. For exampleMuletitola, Deversur, Kalkasa, Sangli, Chowki, Arajkund etc, many of them are found oneor other time with high arsenic contamination (i.e. > 0.05 ppm). Careful site selection onbasic rocks in these villages can eliminate the high Arsenic source. There exists an urgentneed of large scale geological mapping 1:25000 or even 1:10000 scale of the area. Theoccurrence of high Arsenic ground water preferably in rhyolite and granite is reported byAcharyya et al (2005) but they have not discussed the cause behind. Our search forarsenic free aquifer is mainly targeted to basic rocks of the area however large manynumbers of villages situated on rhyolite are totally free of arsenic contamination in thearea. The hydrothermal phase followed by acid magma intrusion has enriched therhyolite-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 occasionaloccurrence of Arsenic in basic rocks is due to assimilation of rhyolite rocks and enrichedthrough remobilization of sulfide mineral. Because of the difference in the grain size, theglassy rhyolites probably have produced more fine ruptures than granite to hostmineralization through moving hydrothermal solution.

Results Of Field Kit Test
Merck arsenic test kit (1.17927.001) is used for insitu field determination of Arsenic inground water. In this when zinc powder, a solid acid, and- for the elimination ofinterfering 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-brownmixed arsenic- mercury halogenides. The concentration of arsenic (III) and arsenic (V) aremeasured semi- quantitavely by visual comparison of the reaction zone of the analyticaltest strip with the fields of a colour scale. The field kit provides unreliable resultsregarding 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, assummarized for few samples Table 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 organicand inorganic forms in water. In Inorganic arsenic systems the -3,0, +3 and the +5oxidation states are common in aqueous systems. The +3 form is more toxic and hasgreater mobility compared to the other forms. Arsenic is known to readily participate inoxidation, reduction, methylation and acid base reaction. Aqueous arsenic in the form ofarsenite, arsenate and organic arsenicale may result from mineral dissolution, industrialdischarge or the application of herbicides. The toxicity of arsenic depends on its chemicalForm.

In previous para attempt has been made to discuss temporal and spatial variations ofArsenic content in ground water also discussed its geological control in various formationsand different zones. Huge data had been collected to study arsenic in ground water ofChowki block. Apart form specific arsenic analysis, 39 ground water samples wereanalysed to determine chemical composition of ground water in the study area tounderstand the general characteristic of ground water in study area and the results aretabulated in Table 7

Discussion On Analytical Results

The results of chemical analysis of 39 samples from study area show that groundwater in the area is generally fresh as EC values were found in the range of 201μs/cm to 1388 μs/cm. EC values in excess of 1000 μs/cm were found in only 6wells water. Water in the area is alkaline in nature as pH varies in the range of 7.6to 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 eightwells whereas more than 100 ppm was found in a well of Nichagoda Village whereits 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 interestingfeatures were noted from the results. As usual EC has positive correlation with all themajor ions. Bicarbonates ions have better correlation of +0.61 with magnesium ion incomposition 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 thebicarbonate, lower is the chloride ions. Remaining correlations are normal and usual.

Relation Of Arsenic With Other Ions

An attempt has been made to establish a correlation between major and minor ions of thewater samples together with arsenic. It has been observed that high arsenic is associatedwith 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 isthe 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 nosodium 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 blockRajnandgaon district detailed study have been made to (i) establish geological andhydrogeological relationship to high arsenic concentration in ground water, itsvariations and controls (ii) For source of arsenic free aquifer within the area byexploratory drillings. (iii) Inventory the present status of arsenic poisoning and possibleremedial measures and alternative arrangements. The investigation carried out so far isable to understand many aspects of high Arsenic ground water in the area, these aresummarized below.

The high arsenic ground water occurrence in eastern part of Chowki block, Rajnandgaondistrict, Chhattisgarh is confined to the early Proterozoic meta volcanic- granite rocksalong the N-S trending Kotri-Dongargarh rift zone. The geographical extent of this higharsenic ground water occurrence is found in isolated clusters distributed over an area of330 sq.km, nearly in a 10 km radius zone. The most severely affected villages arepreferably situated on rhyolite and granite rocks close to shear zone. The relativelyyounger metabasic, basic and pyroclastic aquifers are less contaminated. This is animportant since 28 % of the area is covered with basic rock closely associated asanasthomosing network with rhyolite, the dominant rock type of the area. The lithologicalcontrol thus observed may contributes toward providing safe and alternative drinking andirrigation water. The intrusion of rhyolitic- granitic magma is followed by the hydrothermalphase which is responsible for arsenic enriched sulfide mineralization and Arsenicenrichment 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 andremobilization reaction only.

In this total 330 sq.km area 92 villages are situated where the present population (census2001) is around 75000, mainly dominated by aboriginals (SC & ST). Ground water is theprincipal source for drinking water for the area including the five most arsenic affectedvillages. State PHED has nearly 780 HP and 20 power pumps in these villages forproviding drinking water for people. Since 1997, State PHED has sealed nearly 25 handpumps in 11 villages which were identified as high Arsenic ground water source. Thealternative safe drinking water supply in the most Arsenic affected villages are stillthrough ground water abstraction. During the present course of study, in two villagesSonsaytola & Joratarai it is found that the public water supply source (through groundwater) is contaminated with high arsenic 0.240 and 0.210 ppm respectively .Regarding theirrigation water, the area is predominantly mono-cropped area. Kharif is the croppingseason and mainly rain fed, supported by surface canal irrigation and occasional groundwater irrigation. Second crop area is very scanty and is invariably based on ground waterirrigation. The area is having limited ground water potential, which is restricted to theupper 150 m weathered and fractured zone . Present annual ground water draft forirrigation is 860 ham only and other industrial requirement is negligible. Overall groundwater development of the area is 24% . Apart from the most arsenic affected five villagesKaurikasa> Joratari> Sonsaytola> Jadutola> Muletitola, the occurrence of high arsenicground water in the remaining area is sporadic and occasional. Even in the most affectedvillages not all the ground water source are affected with high arsenic contamination. Borewells and hand pumps are more affected than dug wells in general, and this is moreparticularly applicable to the area where arsenic contamination is less. However largenumbers of dug wells are severely arsenic contaminated in most effected Kaurikasa,Sonsaytola, Joratarai village. During the ground water exploration in the area somearsenic free or limited arsenic contaminated (
Persons with arsenic toxicity manifestations are identified only in Kaurikasa andSonsaytola village. Report says 400 persons (nearly 30% population) of village Kaurikasaare affected with high arsenic ground water poisoning and 130 people are criticallyaffected. The clinical symptoms like palmoplanter keratosis can be frequently observed inthe area. The critically affected patients are early adulthood or middle age group. Microwatershed management work under Rajiv Gandhi National Watershed ManagementProgramme is going on in the area. Since the soil and weathered rocks are enriched witharsenic hence method of artificial recharge which can bypass the soil and weathered zonemust be applied for the area. The Vadose zone available in the area for recharge to dilutethe Arsenic contamination is calculated 41 MCM. Long term analysis of hydrograph ofAmbagarh Chowki have shown significant decline in pre and post monsoon water level inlast 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 isexposing more and more aquifer material to oxidizing environment. The deepest staticwater level is at present around 18 m and maximum drawdown is recorded 38 m hencepresent zone of expose is around 60 m bgl which is the zone of hand pump operation inthe area. Ground water in the Amabagarh Chowki block is otherwise geochemically freshwith low to medium salinity; therefore Arsenic free ground water is good both for drinkingand 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 andpublish 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. ShriA.K.Sinha, the then Regional Director, Sh Ashish Chokraborty, Regional Director, CGWB,NCCR Raipur , Shri T.M. Hunse, the then Superintending Hydrogeologist and Sh AbhijeetRoy, RD, CGWB. Sincere thanks to Dr. S. Shekhar, Assistant Editor for inviting us tocontribute 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.