. Several fragments weighing <2 kg and a single large fragment weighing ∼10 kg were recovered from the strewn field, which extended over several tens of square kilometers. Chemical, petrographic, and oxygen isotopic studies indicate it to be, in most aspects, a typical H5 chondrite, except the unusually low K content of ∼340 ppm. A cosmic ray exposure of 9.7 Ma is inferred from the cosmogenic noble gas records. Activities of eleven cosmogenic radionuclides were measured. 26 Al and 22 Na activities as well as the 22 Na/ 26 Al activity ratio are close to the values expected on the basis of solar modulation of galactic cosmic rays. The low 60 Co activity (<1 dpm/kg) is indicative of a small preatmospheric size of the meteorite. Cosmic ray heavy nuclei track densities in olivine grains range from ∼10 6 cm −2 in samples from the largest fragment to approximately (4-9) × 10 5 cm −2 in one of the smaller fragments. The combined track, radionuclide, and noble gas data suggest a preatmospheric radius of ∼20 cm for the Dergaon meteorite.

A comparative assessment of the hydrological quality of the shallow aquifers and surface water sources in the Jia Bharali River basin and adjoining areas of the Himalayan foothills has been made in this work. Six set of seasonal samples were analysed with respect to the common physicochemical parameters including major anions (carbonate-bicarbonate, chloride, sulphate, fluoride, nitrate and phosphate), and cations (calcium, magnesium, sodium, potassium). 74 % of the aquifers had higher TDS and 88 % had higher hardness in the wet seasons. The major cations were in the ranges of Ca: 7.1 -43.9 mg/L, Mg: 2.1 -25.9 mg/L, Na: 3.8 -40.8 mg/L, and K: 2.1 -22.7 mg/L. The total alkalinity was almost entirely due to the presence of bicarbonate. The aquifers are rich in chloride (5.6 -110.7 mg/L) and sulphate (3.1 -83.1 mg/L) with nitrate (BDL to 0.72 mg/L) and phosphate (BDL -1.27 mg/L), the latter being significantly high for some aquifers. In case of surface water sources, the major ions show wide variability (Ca: 1.56 -45.45 mg/L; Mg: 0.56 -14.56 mg/L; Na: 3.10 -14.30 mg/L; K: 1.0 -11.50 mg/L). The surface water rich in chloride (2.28 -38.45 mg/L) and sulphate (0.38 -29.45 mg/L), is also contains the nutrients, nitrate (BDL -1.45 mg/L) and phosphate (BDL -1.48 mg/L).

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The lower Jia Bharali catchment and adjoining areas in central part of North Brahmaputra Plain (NBP) is characterized by more than 800m thick Older and Younger Alluvium deposited by the west flowing Brahmaputra river and the south flowing trans Himalayan rivers. Unconfined, shallow alluvial aquifers of the area with a general southward flow are largely tapped for domestic use through numerous dug well attached with almost every household. Monitoring of 180 dug wells for two hydrological years show strong seasonal fluctuation of the groundwater table linked to the summer monsoon that brings more than 1500mm precipitation in the area between June-September. This study has presents the first ever systematic database on toxic trace elements viz., As, Cr, Fe, Mn, Ni, Pb and Zn from the shallow aquifers in north Brahmaputra plain based on water samples from 50 monitoring wells collected in both dry and wet seasons. The data was analysed with respect to WHO standards for drinking water and significantly, 2 % of the measurements show As in excess of the WHO limit while 60% of the samples in the wet season and as much as 90 % of the same in dry season have Cr content more than the WHO permissible limit. Pb concentration is above permissible limit of 0.05 mg/L in most of the dry season samples although 88 % of the rainy season samples show Pb concentration exceeding this limit. 34 % of the samples in the wet seasons and 86 % of the samples in the dry seasons have Mn above the permissible limit of 0.1 mg/L while in case of Ni, 56 % of the aquifers in the wet season and 72 % of the aquifers in the dry season show Ni content above the permissible limit of 0.02 mg/L. Zn contents of the aquifers are however very low throughout the year.

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