Occurrence and behavior of arsenic in groundwater-aquifer system of irrigated areas

Groundwater arsenic pollution has received much attention worldwide for decades as a serious threat to public health, but the mechanisms responsible for arsenic mobilization are not fully understood. Groundwater and bore drilling sediment samples from Qiji county, a small geographical agricultural area with endemic arsenicosis, are collected for demonstrating the occurrence and speciation of arsenic in groundwater and sediments, and arsenic release between solid-liquid phase influenced by human activities. Results show that arsenic concentrations in groundwater vary from 5 μg/L to 19.6 μg/L, with 80% exceeding the maximum permissible limits required by WHO (10 μg/L) for drinking water and therefore constituting a health risk for humans. In a weak oxidizing environment (oxidation-reduction potential (ORP): 12.9 mV–151 mV), inorganic As(V) accounts for 85% of total dissolved As, which to some extent alleviates the harm of As pollution on humans. Total As content in the sediments is in the range of 6.98 mg/kg and 14.34 mg/kg (median of 10.71 mg/kg), three times higher than the average value of many countries. Sequential chemical leaching indicates that 11% of arsenic in sediments is labile bound and may be closely related to the arsenic in groundwater. Additionally, irrigation intensity contributes to arsenic release with diverse As3+/As5+ by dissolving weakly bound arsenic rapidly. Subsequently part of As(III) is oxidized to As(V). Competitive and/or alkaline desorption of As(V), which had been adsorbed by FeMn (hydrous)-oxides and carbonates in the unsaturated zone and the aquifer, exerts a significant role in releasing arsenic into the groundwater. Our study indicates that systematic management and regulation of irrigation intensity are required to prevent further deterioration of groundwater resources. [Display omitted] •Over 80% of the groundwater has elevated arsenic levels, with As (V) being the major species.•Labile bound arsenic in sediments may be closely related to arsenic in groundwater.•High irrigation intensity in the central facilitates arsenic release with high As3+/As5+ into groundwater.•Oxidation of As(III) to As(V) and adsorption-desorption of Fe/Mn oxides and carbonates play major roles.