The objective of this study was to report on the arsenic species present in tube-well water samples collected from West Bengal, India, especially dimethylarsinic acid (DMA) and monomethylarsonic acid (MMA), whose existence has not been reported in the literature. The water samples were collected from Jalangi Gram Panchayet (Murshidabad district, West Bengal, India). The samples were speciated for arsenic 11 days after collection. The samples were collected in duplicate. One part was acidified with nitric acid (final concentration 0.1%), whereas the other part was left unacidified. A quick and highly sensitive high-performance liquid chromatography±inductively coupled plasma mass spectrometry (HPLC±ICPMS) technique was employed for the separation and detection of the arsenic species. Four arsenic species, namely arsenite [arsenic(III)], DMA, MMA and arsenate [arsenic(V)] were separated and analysed in less than 5 min. Total arsenic concentration was determined by flow injection (FI)-ICPMS. Most of the samples were found to contain low concentrations of DMA and MMA (<2.1 ppb) and high concentrations of inorganic arsenic (>300 ppb). The existence of DMA and MMA in both acidified and unacidified water samples and in similar concentrations suggests that their presence is natural and not due to acidification. The detection limit of the four arsenic species was 0.06±0.10 ppb. The method was validated by spike recovery and analysis of two water standard reference materials (SRMs). The percentage recoveries of added spikes of all four species were 97±112%. The total arsenic concentration obtained by FI-ICPMS and the sum of the four arsenic species obtained by HPLC±ICPMS for the two water SRMs agreed with the certified values. Moreover, the difference between the total arsenic and the sum of the four arsenic species for most of the water samples was less than 10%.

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The highest net return was found in continuous application (two times) especially in the treatment biochar @ 10 t + 100 % NPK + FYM with Rs.67,928 ha-1 followed by biochar @ 10 t + 75 % NPK + FYM treatment with Rs.67,172 ha-1. The next higher net return was registered in biochar @ 10 t + 75 % NPK + FYM treatment under one time application with Rs. 62,970/-.Comparing the studies, the two times application (biochar @ 10 t ha-1 time-1) recorded an addition of Rs. 5,565/- only over one time application. Therefore one time application (biochar @ 10 t ha-1) considered to be an economically viable management technology than two times application in the cotton – maize – cowpea cropping sequence. One time conjoint application of biochar @ 10 t ha-1 along with 75 % recommended dose of NPK fertilizers and 12.5 t ha-1 of FYM i.e., (biochar @ 10 t + 75 % NPK + FYM) to cotton and following the general recommended practices for maize and cowpea to be the best combination suited for enhancing higher benefit cost ratio of 1.45 under cotton – maize – cowpea cropping system in Inceptisol (Vertic Ustropept) of Periyanaickenpalayam series of Coimbatore District.

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