UTILISATION OF SATELLITE DATA IN IDENTIFICATION OF GEOMORPHIC LANDFORM AND ITS ROLE IN ARSENIC RELEASE IN GROUNDWATER .

The present study was conducted to explore the influence of geomorphic features of the area on the mobilization of arsenic in groundwater. In this study, remote sensing and GIS techniques were used to prepare the geomorphic and slope map of the area. Different geomorphic features were identified on the basis of spectral signature on the LISS III and Landsat satellite image and field survey. Groundwater samples were collected from each representative geomorphic feature to inspect the arsenic contamination in the area. The study area is drained by the Brahmaputra river and its tributaries and contain mainly fluvial geomorphic units especially older flood plain, palaeochannels, oxbow lakes, channel islands; and hilly areas at some of the places. In this study it was observed that enrichment of arsenic in groundwater varies along the geomorphic units in following trends Paleochannel> Younger alluvial plain> Active flood plain> channel island > dissected hills. The above trend shows that a higher concentration of arsenic is found in the groundwater samples collected from the fluvial landforms as compared to those collected from structural landforms. Brahmaputra River and its tributaries carry the sediment load from the Himalayan foothills, which get deposited in these features during the lateral shift of the river’s courses. Arsenic bearing minerals may get transported through river and deposit in the geomorphological features along with organic matter. The flat terrain of the area as seen from the slope map provides more residence time to water to infiltrate into the aquifer. The microbial degradation of organic matter generates the reducing environment and facilitates the dissolution of iron hydroxide thus releasing the adsorbed arsenic into the groundwater.


INTRODUCTION
Contamination of drinking water is an environmental challenge which has affected the livelihood of millions of population.( Gaus et al.,2003) Arsenic problem is common to several alluvial plains of South and East Asia as it has been reported in many fluvio-deltaic tracts of the world like Hanoi City and the upper end of Red River Delta (Berg et al., 2001), Tarai belt of Nepal (Gurung et al., 2005), Indus delta of Pakistan (Khan et al., 2010), delta plains of the Mekong river in Cambodia (Polya et al., 2005).In India arsenic enrichment in groundwater has been reported in the Ganga-Brahmaputra river plain (Das et al., 1996) and a number of studies have been conducted in West Bengal and Bangladesh related to the arsenic mobilization in groundwater and its correlation with other affecting parameters (Mukherjee et al., 2005;McArthur et al., 2001;Nickson et al., 2000;Chowdhury et al., 1999;Chakraborti et al., 1996).Continuous exposure to arsenic contaminated groundwater may cause conjunctivitis, hyper pigmentation, painful skin blisters on human body even causing gangrene of the limbs, and the skin cancer.Regional occurrence of arsenic in West Bengal and Bangladesh is mostly attributed to the geogenic source which releases the arsenic in groundwater through natural hydrochemical processes occurring in the study area (Nickson et al., 2000;Bhattacharya et al., 1997).There are several hypotheses regarding the enhanced concentration of arsenic in groundwater but the most accepted hypothesis is reductive dissolution of iron hydroxide (FeOOH), which release the adsorbed arsenic in the aquatic environment (Bhattacharya et al., 2006;McArthur et al., 2004).However identification of source of arsenic is still a matter of study, but some researchers have identified the Himalayan rock as a primary source of arsenic which comes along the river and get deposited in plain areas (Acharyya and Shah, 2007;McArthur et al., 2001).The arsenic enriched groundwater is mainly restricted to the Holocene aquifer at shallow and intermediate depth (Ahmed et al., 2001;Mukherjee and Bhattacharya, 2001).Although arsenic has been extensively studied in West Bengal and Bangladesh but northeastern part of India is also not immune from arsenic contamination.The presence of arsenic has been identified in Assam, Tripura, Arunachal Pradesh, Manipur and Nagaland too (Mukherjee et al., 2006).Assam is an adjoining state of West Bengal with similar hydrogeological pattern.Assam is yet to get attention, though several studies have been done which reported arsenic contamination in the groundwater of 21 districts out of 24 (Singh, 2004;Sailo and Mahanta, 2013).Remote sensing provides quick and useful information about parameters controlling the occurrence and movement of groundwater like soils, geomorphology, geology, lithology; land use/cover, lineaments etc. (Mukherjee et al., 2005;Todd, 1980).Remote sensing and GIS technology has proved to be an important tool in assessment of geomorphic features.Geomorphology of an area has a significant influence over arsenic enrichment in ground water.Ravenscroft et al. (2001) has shown that the occurrence of arsenic has a close relation to geomorphological units.In the present study, geomorphic features were identified in the Bongaigaon and Goalpara districts of lower Western Assam using satellite images.The groundwater samples were collected from each geomorphic unit and were analyzed to understand the groundwater chemistry and to study the influence of geomorphic features on the mobilization of arsenic in the groundwater of the area.

STUDY AREA
The study area (Bongaigaon and Goalpara districts) lies in the lower western part of the Assam (Fig. 1), and is occupied by both hills and plains.Both districts are situated in the Foreland depression between the comparatively younger mountain chains of the Himalayas in the north and the Block Mountains of Shillong plateau in the south.This contribution has been peer-reviewed.The double-blind peer-review was conducted on the basis of the full paper.doi:10.5194/isprsannals-II-8-29-2014are formed in the regions which frequently experiences flooding and comes under the active flood plain of the area.The Brahamputra basin have the deposition of sediments from the river and their tributaries, with the visible presence of numerous palaeochannels, oxbow lakes, and flood plains.

Groundwater Chemistry
The results of the groundwater quality have been tabulated in the form of maximum, minimum and mean (Table 1).pH of the groundwater in the study area is slightly acidic to alkaline in nature and the value ranges between 6.8 to 8.4.The electrical conductivity is defined as the ability of a substance to conduct electric current which depends upon dissolution of minerals in the sampled area.The mean value of electrical conductivity in the groundwater samples is 269.7 ± 64.1 μmhos/cm, and TDS value is 188.8 ± 44.9 mg/L which indicates that the water samples are mineralized.The results of ground water samples indicate the dominance of cations in the order of Na + >Ca 2+ >Mg 2+ >K + while in the anions bicarbonate dominates in the samples followed by chloride, and nitrate HCO 3

Mobilization of arsenic in groundwater
Out of 45 groundwater samples collected from each geomorphic unit, 8 samples have concentration higher than WHO permissible limit of 10 ppb (Fig. 5).The arsenic enrichment is found in the groundwater samples collected from the areas lying along or near the flowing and dry rivers.The geography of study area shows that the area is drained by several rivers from both sides of the Brahmaputra, and arsenic concentration is found to be higher mainly along the river (Manas, Aie) coming from northern Himalayan foothills.(Fig. 4) The flood plain sediments drained by the rivers originating from the Himalayan Mountains and Tibetan Plateau are expected to contain arsenic as these are reported to be the geological origin of arsenic in South Asia (Das et al., 2008).The sediments and mineralogy of the basin depend on the type of sediments eroded and transported by the Brahmaputra river and its tributaries.Local geology, geomorphology, hydrogeology, geochemistry of sediments influences the occurrence, origin and mobilization of arsenic in sedimentary aquifers (Bhattacharya et al., 1997;Kinniburgh and Smedley, 2001).

Variations of Arsenic concentration in different Geomorphic features.
Enrichment of arsenic in groundwater varies along the geomorphic units in following trends Paleochannel> Younger alluvial plain> Active flood plain> channel island > dissected hills (Table 2).The above trend shows that a higher concentration of arsenic is found in the groundwater samples collected from the fluvial landforms as compared to those collected from structural landforms (Fig. 5).The occurrence of arsenic in alluvial plain is usual (Welch et al., 1988) and its enrichment is associated with Holocene sediment brought by rivers originated from Himalayas (Ahamed et al., 2006;Chakraborti et al., 2002;McArthur et al., 2004).In the present study, it was found that the groundwater samples collected from paleochannel geomorphic features have higher arsenic concentration.Palaeochannels are the remnant geomorphic unit formed through the lateral shift of the rivers.Brahmaputra plain is covered by younger alluvial sediments at most of the places deposited from the sediment load carried by the river and its tributaries (Sarma and Phukan, 2004).The Brahmaputra is a braided stream showing the rapid lateral migration of as much as 800 m/year (Allison, 1998).
The Brahmaputra river originates from the eastern part of the Mansarovar Lake in the Tibetan Plateau between the Kailash range and the Himalayan range and flows through China, India   The groundwater collected from geomorphic units were slightly acidic to alkaline in nature with most of the parameters well within the permissible limit of WHO except for fluoride, iron and arsenic.Arsenic concentration was found to be higher in the samples collected from the fluvial geomorphic features especially palaeochannels.Geomorphological map prepared from the satellite images shows that most part of the area is covered with geomorphic features of fluvial origin formed by the rivers.River Brahmaputra and their tributaries often change their course periodically thus creating palaeochannels, oxbow lakes, channel islands as major geomorphic units.During the lateral shift of the river, lot of minerals, sediments and organic matter get deposited into these geomorphic units.Over a period of time, microbial degradation of these organic matters create reducing environment which is evident from negative redox potential in the study area.A low Eh brought on by the degradation of organic matter may be responsible for the dissolution of the iron hydroxide present in the sediments leading to the desorption of any adsorbed arsenic.Slope map shows the flat terrain of the area with a very less slope which facilitates the infiltration of the stored water deep into the aquifer.In the present study, authors found that reductive dissolution of iron hydroxide is the main source of arsenic release into the groundwater.The arsenic may have been present in adsorbed from with the minerals deposited in sediments carried from the Himalayas.Poor correlation between arsenic and iron may be due to the precipitation of iron as siderite.

Figure. 3 .
Figure.3.Geomorphology map of the study area