More than 600 Bucks and Montgomery Counties residents have agreed to participate in a state study to determine the health impact after drinking water that was possibly tainted for years by chemicals leaching off former military bases.

The study, testing the blood of people who live in neighborhoods surrounding the old Naval Air Warfare Center Warminster and Naval Air Station Willow Grove, had been delayed last month because of a lack of participation. But after receiving more responses — 625 in all — the Pennsylvania Department of Health said last week it won't need to send out any more invitations.

As of Tuesday, 509 adults and 116 children — all from households randomly selected for the study — had said they would participate. But they must follow through by returning a form and getting tested — and only 22 had made appointments and 11 had given samples as of Tuesday.

Also, 140 have returned the form necessary to schedule a blood test. Officials have not imposed a deadline for responses, though that could change based on how long it takes blood tests to be completed. They hope to test about 500 people.

"We are waiting to hear from as many people as we can," said spokesperson Nate Wardle.

The tests are part of a pilot program to study the effects of drinking water that was contaminated by chemicals that for years leached off the now-closed bases into water supplies in Horsham, Warminster, and Warrington Townships. The same water contamination had been found in more than 550 public or private drinking water systems in communities near military bases nationwide as of August, according to a recent military report.

The pilot study, supported by the Centers for Disease Control and Prevention and the Agency for Toxic Substances and Disease Registry, is a unique opportunity for some residents to find out what levels of the chemicals — known as per- or polyfluoroalkyl substances, or PFAS, and used on the bases mainly in firefighting foam — are in their blood. The agencies hope to use the data gathered in Pennsylvania to create a federal blood-testing program for PFAS.

The state could not accept volunteers for the testing and was required for scientific integrity to randomly select participants. Volunteers can skew study results because they often come from similar groups. A random sample of all types of residents in the area was required to represent the entire population, health officials said. Clinics in Bucks and Montgomery Counties are conducting blood tests for the study.

By the end of May, before the agency sent out more invitations to participate in the study, only 355 people had indicated interest and 50 had turned in the necessary form. In total, 550 households received letters asking them whether they'd be interested in taking part.

Are you getting your blood tested? Email jmcdaniel@philly.com if you are interested in talking to a reporter about your participation.