Sinking land caused by intensive groundwater pumping in the San Joaquin Valley is releasing trapped arsenic — a known carcinogen — into aquifers that supply irrigation and drinking water for a million people, according to a new study published Tuesday in the journal Nature Communications. Arsenic, a naturally occurring chemical in the Earth’s crust, is undetectable by the human senses and has been linked to a host of diseases including heart disease, diabetes, and cancer. Researchers at Stanford University say residents should be concerned about arsenic levels in their water supply.