Heavy metal (Cr, Cu, Zn, As, Cd, and Pb) contamination in irrigation water, soil, and vegetables was investigated in farmland adjacent to a multi-industry zone in Bangladesh in dry and wet seasons. In the zone, many factories release wastewater into nearby irrigation canals, and vegetables cultivated with this water could be a major food chain route for human exposure. In the irrigation water and vegetables, heavy metal concentrations exceeded permissible levels in the two seasons, but this was not the case in soil. Zn had the highest concentration, and Cd had the lowest concentration in irrigation water, soil, and vegetables. All heavy metal concentrations were found to be lower in the wet than in the dry season, which is due to the dilution of water by rainfall, lower absorption of heavy metals from the diluted irrigation water, and heavy metal absorption from low concentrated irrigation water and/or soil. The cluster analysis data of irrigation water, soil, and vegetables revealed that the heavy metals in vegetables were considered to be absorbed from irrigation water in the wet season and from soil in the dry season. In the dry season, the high heavy metal concentrations in vegetables might be caused by a high bioconcentration factor (mostly > 20%).

Absorption of heavy metals and contaminants through a rice-based diet may have serious consequences for human health. The present study determined the levels of arsenic (As), cadmium (Cd), lead (Pb) and chromium (Cr) in 71 irrigated and rain-fed rice and assessed dietary (rice) exposure to the heavy metals. The concentration in rice grains were generally higher in the irrigated season which may be related to the use of contaminated irrigation water: As 0.153 ± 0.112 and 0.140 ± 0.080 mg kg -1 , Cd 0.073 ± 0.069 and 0.038 ± 0.032 mg kg -1 , Pb 0.264 ± 0.125 and 0.147 ± 0.077 mg kg -1 and Cr 1.208 ± 0.913 and 0.986 ± 0.796 mg kg -1 in irrigated and rain-fed rice, respectively. Elevated concentration of grain As was recorded for rice samples that were collected from the locations with As-contaminated groundwater used for irrigation. For Cd, Pb and Cr, relatively higher concentration was noted for the areas which could be linked to industrial effluent contamination. Daily intake of As and heavy metals from rice is estimated as 18.6-214 µg for As, 2.6-119 µg for Cd, 25.0-241 µg for Pb and 59.0-1846 µg for Cr, based on 400g daily rice consumption for 60 kg Bangladeshi adult people. The rice component of the diet alone may contribute up to 46%, 57%, 50% and 60% of the Maximum Tolerable Daily Intake (MTDI) for As, Cd, Pb and Cr, respectively, making it a more important factor in the dietary intake for these elements than other food stuffs and drinking water. Hence, heavy metals accumulation in rice grains is a big concern in south Asia where people's daily meal largely contains rice or rice based products.

Heavy metal contamination of soil and sediment in estuaries warrants study because a healthy estuarine environment, including healthy soil, is important in order to achieve ecological balance and good aquaculture production. The Ba Lat estuary of the Red River is the largest estuary in northern Vietnam and is employed in various land uses. However, the heavy metal contamination of its soil has not yet been reported. The following research was conducted to clarify contamination levels, supply sources, and the effect of land use on heavy metal concentrations in the estuary. Soil samples were collected from the top soil layer of the estuary, and their arsenic (As), chromium (Cr), cadmium (Cd), copper (Cu), lead (Pb), and zinc (Zn) concentrations were analyzed, as were other soil properties. Most soils in the estuary were loam, silt loam, or sandy loam. The pH was neutral, and the cation exchange capacity ranged from 3.8 to 20 cmol·kg−1. Manganese and iron concentrations averaged 811 µg·g−1 and 1.79%, respectively. The magnitude of the soil heavy metal concentrations decreased in the order of Zn > Pb > Cr > Cu > As > Cd. The concentrations were higher in the riverbed and mangrove forest than in other land-use areas. Except for As, the mean heavy metal concentrations were lower than the permissible levels for agricultural soils in Vietnam. The principal component analyses suggested that soil As, Pb, Zn, Cd, and Cu were of anthropogenic origin, whereas Cr was of non-anthropogenic origin. The spatial distribution of concentration with land use indicated that mangrove forests play an important role in preventing the spread of heavy metals to other land uses and in maintaining the estuarine environment.

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It has recently been proposed that water purification could be performed using aquatic plants, since they absorb nutrient salts. The behavior of a substance in a closed water area is affected by turbulent flows from wind-induced flow, which is a mechanical disturbance, and convective flow, which is a thermal disturbance. In a closed density stratified water area, wind-induced flow gives rise to the entrainment phenomenon at the density interface. This phenomenon, which is based on mixing between the upper and lower water layers, lowers the density interface and so affects the water quality. We experimentally investigated the effect of aquatic plants on the turbulent flow from a mechanical disturbance in the closed water area. Results indicated that the presence of floating and submerged plants had a significant effect on the scale of the turbulent entrainment, and that the entrainment velocity depended on the overall Richardson number to the power of 3/2.

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