M. Jahiruddin

Variation in Arsenic Concentration Relative to Ammonium Nitrogen and Oxidation Reduction Potential in Surface and Groundwater

Abstract Arsenic (As), ammonium‐nitrogen (N), nitrate‐N concentrations, and oxidation–reduction potential (ORP) in the water samples from the river, pond, dug well, and shallow and deep tube wells (TW) were investigated in a farming village of southwestern Bangladesh. Concentrations of As and ammonium‐N were the highest, whereas ORP was the lowest in the shallow TW water among the water sources. The ammonium‐N concentration correlated positively with the As concentration and negatively with ORP for all samples, irrespective of the water sources. A rise in the ammonium‐N concentration was hypothesized to enhance microbial activity, which in turn would lower ORP, and then As was released from sediments to the surrounding water in a reducing condition. The source of ammonium‐N in the shallow TW water was identified as N fertilizer, based on the δ15N analysis. Thus, the influence of N fertilizer application on As contamination in groundwater was suggested.

Mechanisms and Drivers of Soil Salinity in Coastal Bangladesh

Determining soil salinity within the delta is crucial as it is the dominant factor determining crop productivity. There are numerous interacting drivers that influence soil salinity, including climate variability, saline river water inundation, storm surge inundation, depth to groundwater table, groundwater salinity, and shrimp farming (Bagda). For the study area, tidal river salinity appears to influence the soil salinity most, particularly in the south-west of the delta. In northern areas, high groundwater salinity levels, combined with a high groundwater table, are a major contributor to soil salinity. In addition, an increase in salinity of dry season irrigation water is expected to increase salt accumulation in soils, with a possibility of irrigation water salinity exceeding five parts per thousand.

Prospects for Agriculture Under Climate Change and Soil Salinisation

Agriculture is the largest and most important provisioning ecosystem in the Ganges-Brahmaputra-Meghna delta and is significantly affected by levels of soil and water salinity. Model-based assessment using both soil moisture and salt balance models indicate that whilst monsoon rains supply adequate water to grow a main season rice crop, agricultural diversity is currently constrained by the limited availability of good quality irrigation water in the dry season. There is a tipping point of water salinity around four parts per thousand beyond which soil salinity accumulates. Although the development of soil salinity is an environmental process, soil salinisation is closely linked to farmers’ behaviour and land use practices. It is also closely associated with the decline in other ecosystem services associated with water regulation.