Ethel R. Groundwater, 75, a registered nurse who organized a nurse's office and first aid center at The Washington Post and ran it from 1963 until her retirement in 1973, died of cancer Wednesday at her home in Arlington.

Mrs. Groundwater's duties at The Washington Post included first aid, helping relieve employes' minor ailments and carrying out other treatment under the supervision of a physician. She is widely remembered among employes here for the good humor and sympathy she brought to her job.

A native of Hazleton, Pa., she graduated from the Thomas Jefferson University Hospital School of Nursing in Philadelphia and was licensed as a registered nurse. She also took a degree from its School of Anaesthesia. After working in Philadelphia and Atlantic City for a few years, she moved to Honolulu and joined the staff of the Kapiolani Maternity and Gynecological Hospital.

While there, she met her husband, Benjamin Groundwater, a Navy communicator. They were married and began the peripatetic life typical of service families. They were stationed in Maine in the late 1930s, in Shanghai. China, where Mrs. Groundwater was active in helping an orphanage, in the Washington area, where they established their permanent home during World War II, and later in Japan, Germany and Britain. In 1956, following Lt. Cmdr. Groundwater's retirement from the Navy, they returned here permanently.

Mrs. Groundwater was the company nurse for Frank R. Jelleff Inc., for five years before joining The Post. After her retirement from the newspaper, she remained on call for part-time duty until 1976.

Mrs. Groundwater was chairman of the Red Cross Visitor Program in Arlington and received a commendation in 1978 from the county for her work with the elderly. That year she also received a certificate recognizing 50 years of volunteer service with the American Red Cross.

Her husband died in 1978.

Survivors include a son, Robert B. of Memphis, Tenn.; a daughter, Carol G. Preston of Annandale; one sister, Alice M. Alderson of Silver Spring; one brother, Donald Roth of Florida, and three grandchildren.

The family suggests that expressions of sympathy be in the form of contributions to the Hospice of Northern Virginia Inc., 4715 N. 15th St., Arlington, or to the American Cancer Society.