Optimal Control of Lake pH for Mercury Bioaccumulation Control

Mercury is recognized internationally as an important pollutant, since mercury and its compounds are persistent, bioaccumulative and toxic, and pose human and ecosystem risks. Although mercury can cycle in the environment in all media, an important aspect of this cycling is the bioaccumulation of mercury along the aquatic food chain. Mercury, in the form of methylmercury, bioaccumulates up the aquatic food chains so that organisms in higher trophic levels have higher mercury concentrations. This leads to humans getting exposed to mercury through contaminated fish consumption. Since methyl mercury is the primary bioaccumulative form of mercury, controlling the conversion of mercury in water bodies to methyl mercury is a possible option to control bioaccumulation. Since lake acidity (pH) is correlated with mercury methylation, this work proposes to reduce bioaccumulation by controlling water pH. This is to be achieved through time dependent liming strategy, derived using optimal control theory. The work incorporates uncertainty for a robust and realistic analysis. This calls for effective uncertainty modeling using stochastic processes from real options theory and efficient solution techniques using stochastic optimal control. The work also presents results for a multi-objective problem highlighting the tradeoff between good control and liming cost. The analysis is expected to make liming operation more reliable, thereby presenting one more tool to manage the harmful effects of mercury pollution.