Both the drought and high nitrate levels in shallow groundwater have necessitated deeper drilling of new wells in the San Joaquin Valley, only to expose water with heightened arsenic levels. Arsenic usually exists in water as arsenate or arsenite, the latter of which is more frequent in deep lake sediments or groundwater with little oxygen and is both more harmful and difficult to remove.

Arsenic has long been on California’s list of known cancer-causing agents (carcinogens) and can cause gastrointestinal and skin distress. Although the presence of arsenic is largely natural, its mobilization in water is exacerbated by groundwater pumping, mining and the presence of petroleum products. 

Removing arsenic from groundwater is done by precipitation, adsorption (the process by which liquid particles bind to a solid or liquid surface), ion-exchange and membrane filtration.

Adsorption uses charcoal filters with pores big enough for arsenic to become trapped to its surface.

Precipitation forces dissolved arsenic to solidify into particles in the water and be filtered out. 

Membrane filtration uses reverse osmosis and electro-dialysis. Reverse osmosis pressurizes arsenic-containing water and moves it through a filter to remove toxins. Electro-dialysis exerts an electric field and uses a membrane that attracts the arsenic based on its charge (much like two opposite poles of a magnet attract).