Arsenic has four valence states, pentad, trivalence, null valence and trivalent negative. Arsenic in water commonly exists in the form of compounds. In shallow water where the dissolved oxygen and pH measures are higher, arsenic exists in the form of pentavalent arsenate. Contrarily in deep water where the dissolved oxygen and pH measures are lower, arsenic exists in the form of trivalent arsenate or arsenic sulfide. Arsenide is extremely toxic. Its long-term harm to the human body include blackfoot disease, cancers (skin cancer, lung cancer, bladder cancer), heart disease, diabetes and hypertension.