Case Study: Sensors in China

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SensorKit was chosen to study mercury contamination in the soil in a rice farming province of China. Understanding mercury cycling in a rice paddy has broad implications for human health, as ingestion of rice may be an important mercury exposure pathway in regions where rice is a staple.

Geography

SensorKit was launched near Wanshan, located in Guizhou Province in southwestern China. Wanshan is a mountainous area with limited arable land. To compensate, farmers cultivate the mountainside using terraces. Land is plowed with the help of oxen.

Mercury was mined for over 600 years in the Wanshan province, and official operations were ceased in 2001. Although the mercury mine is closed, the tailings are still being mined by local farmers. The tailings contain mercury cinnabar (HgS), which are smelted using a crude oven. The mercury residue is collected and sold, but this process is inefficient, and residual mercury is released to the atmosphere. Nearby rice paddies are contaminated by the wet and dry deposition of atmospheric mercury.

Mercury methylation is the process by which less toxic inorganic mercury is converted to methyl-mercury, a known neurotoxin. This process is primarily mediated by microbes, which thrive in flooded environments (e.g. wetlands). Like wetlands, rice paddies are potential hot spots for the conversion of inorganic mercury to methyl-mercury. Atmospheric deposition of mercury to rice paddies may lead to higher methylmercury yields and uptake of methylmercury by rice plants.

According to recent research, "It is demonstrated that rice growing in [mercury]-contaminated soil can accumulate methylmercury to a level to pose health threat to local inhabitants whose staple food is rice."1 Even though most human exposure to methylmercury occurs through the consumption of fish, methymercury exposure may also occur through the ingestion of rice. This exposure pathway may be important in regions where rice is a staple.

Deployment Configuration

Research

This research includes a 2-year investigation of mercury methylation in rice paddies in Guizhou Province, China. In addition to analyzing methylmercury in the pore water, sediments, surface water and rice plants, sensors are used to collect data for pore water pH and oxidizing reduction potential (ORP), as well as data for photosynthetic available radiation (PAR) and ultra-violet radiation (UV). Both pH and ORP will help elucidate biogeochemical controls on mercury methylation, while PAR and UV may be used to assess whether mercury cycling is correlated with plant growth. The sensor deployment will extend through the end of the rice growing season (early September), and data for all matrices will be collected during the rice growing season, and after rice plants are harvested.

1Feng, Xinbin et al., “Mercury Pollution in Guizhou, Southwestern China--An Overview.” Science of the Total Environment: 9 July 2008.