The cost-effective removal ...

The cost-effective removal with minimum disturbance of both arsenic oxyanion and heavy metal cations from natural water bodies is a challenge. Concentration depends on the pH of the water.  The solubility of the negatively charged arsenic complex increases when the pH is raised to decrease the solubilities of the positively charged heavy metal ions, and vice versa. Besides, the optimum for arsenic removal more often is below pH 5, while the optimum for manganese is above pH 9. If you do not want to change the pH of the water body to reach such extreme values, you may like to use a sparely soluble compound that only creates the desired conditions in a thin surface layer between the solid and surrounding water. Ideally, a compound consisting of natural minerals with a high adsorption capacity for arsenic, and of alkalinity producing minerals allowing for the adsorption and precipitation of the heavy metals. Depending on your conditions, various available compounds can be advised or else might be readily prepared for testing.  They must be very fine-grained to have enough reactivity and therefore they also must be very dense so that they readily sink to the bottom of the waterbody in which they are introduced. You can mix the compound with a bit of water to produce a slurry that you then spray evenly over the surface of the water body you like to treat. The compound removes the contaminants, while sinking to the bottom, where it binds them long term once concentrated in the sediment. In the laboratory, you first test the effect of the addition of the compound using meter long cylinders with natural water. Then you perform a pilot test on a small screened-off part of the waterbody. You may investigate effect on nekton on the short term, but long-term studies might be required to investigate the effect on benthos.