PNNL

Abstract

Accurate representation of artificial recharge is requisite to calibration of a ground water model of an unconfined aquifer. For semi-arid or arid sites with a thick vadose zone, attenuation of liquid transport by the vadose zone is an important consideration. Artificial recharge occurs in response to liquid disposal to the vadose zone in areas that are small relative to the ground water model domain. In contrast, natural recharge is spatially variable and occurs over the entire upper boundary of a typical unconfined ground water model. An improved technique for partitioning artificial recharge from simulated total recharge is presented. The improved technique is applied using data from the semi-arid Hanford Site in southeast Washington State. During the operational period from 1944 until the late 1980s, when Hanford’s mission was the production of nuclear materials, the quantities of liquid discharged from production facilities to the ground vastly exceeded natural recharge. Nearly all hydraulic head data available for use in calibrating a ground water model at this site were collected during this period or later, when the aquifer was under the subsiding influence of the massive water disposals. The vadose zone is typically 80 to 90 meters thick at the Central Plateau where most production facilities were located at this semi-arid site, and its attenuation of liquid transmission to the aquifer can be significant. The new technique is shown to improve the representation of artificial recharge and thereby contribute to improvement in the calibration of a site-wide ground water model.