Introduction

The age of ground water is defined as the time of travel of a parcel of water from the water table to a downgradient measurement point such as a well. Knowledge of ground-water age is essential to constraining rates of ground-water movement and of contaminants transported with ground water. To better characterize apparent (piston-flow) ground-water age in selected recharge areas in the Portland basin, the City of Portland Bureau of Environmental Services (BES) and the U.S. Geological Survey (USGS) collected water samples for tritium/helium-3 analysis from 12 shallow monitoring wells (fig. 1). Apparent ground-water age was determined by the tritium/helium-3 method. Samples were collected during October 1997, and again during April 1998. Ten of these twelve wells were originally installed as part of the National Water-Quality Assessment (NAWQA) Program in 1995 (Hinkle, 1997). Two wells were subsequently installed by BES, following NAWQA protocols (Lapham and others, 1995).

This report documents tritium/helium-3 data collected in 1997–98. Unprocessed data and interpreted apparent ground-water ages are briefly presented. Additional information about the NAWQA monitoring wells and the quality of water sampled from those wells is provided in Hinkle (1997). The environmental setting of these wells is described in Uhrich and Wentz (1999), and the ground-water flow system is described in McFarland and Morgan (1996).