The U.S. Geological Survey says groundwater levels are declining in an aquifer that serves parts of Oklahoma and seven other states.

The USGS has released a report on changes in groundwater levels in the High Plains Aquifer, also known as the Ogallala Aquifer. USGS scientist Virginia McGuire says measurements between 2011 and 2013 represent a large decline in groundwater levels and are likely due to increased groundwater pumping.

In 2011, water in the aquifer totaled about 2.92 billion acre-feet, a decline of about 267 million acre-feet, or 8 percent, since 1950. Change in water levels from 2011 to 2013 was a decline of 36 million acre-feet in just two years.

The aquifer underlies about 175,000 square miles in Oklahoma, Colorado, Kansas, Nebraska, New Mexico, South Dakota, Texas and Wyoming.