The Pennsylvania Department of Environmental Protection (DEP) is investigating a hydraulic fracturing (fracking) fluid spill at a natural gas well pad operated by ExxonMobil Corp. subsidiary XTO Energy Corp.

The spill, in Penn Township in Lycoming County, was discovered last week by a DEP inspector.

“This spill was initially estimated at more than 13,000 gallons by the company and has polluted an unnamed tributary to Sugar Run and a spring,” said DEP Northcentral Regional Director Nels Taber. “There are also two private drinking water wells in the vicinity that will be sampled for possible impacts.”

A DEP official was at the well site to inspect the well pad when the spill was discovered. The bottom valve on a 21,000-gallon fracking fluid tank was found to be open and discharging fluid off the well pad. No one else was at the pad, which has one producing Marcellus Shale well.

The DEP inspector was able to close the valve and XTO officials were immediately contacted about the spill. XTO had not provided any explanation for the open valve, DEP said late Monday.

According to officials, XTO hired cleanup contractor Minuteman Response, which was vacuuming spilled fluids and mobilizing equipment to excavate soil. A fence also was installed to prevent cattle from grazing in the impacted area of the pasture.

Samples of nearby soil samples and surface water have been collected by DEP. Initial field meter readings showed “elevated levels of conductivity and salinity” in the spring and unnamed tributary.

Conductivity measures water’s ability to carry an electric current, while salinity measures the dissolved salt content in water, DEP noted. “Elevated levels of both are indications that spilled fracking fluid is present.”

A notice of violation letter was to be sent to XTO and the company will be required to remediate the site, said state officials.