Journal Article

Gravel-Corrected Kd Values

Kaplan, Daniel I., Serne, R. Jeff, Parker, Kent E., Kutnyakov, Igor V., Gamerdinger, Amy P.

2000

Overview

Standard measurements of solute sorption to sediments are typically made on the <2 mm sediment fraction. This fraction is used by researchers to standardize the method and to ease experimental protocol so that large labware is not required to accommodate the gravel fraction (>2 mm particles). Since sorption is a phenomenon directly related to surface area, sorption measurements based on the <2 mm fraction would be expected to overestimate actual whole‐sediment values for sediments containing gravel. This inaccuracy is a problem for ground water contaminant transport modelers who use laboratory‐derived sorption values, typically expressed as a distribution coefficients (Kd), to calculate the retardation factor (Rf), a parameter that accounts for solute‐sediment chemical interactions. The objectives of this laboratory study were to quantify the effect of gravel on strontium Kd and Rf values and to develop an empirical method to calculate gravel‐corrected Kdgc values for the study site (Hanford Site in Richland, Washington). Three gravel corrections, Kd values, were evaluated: a correction based on the assumption that the gravel simply diluted the Kd<2mm and had no sorption capacity (Kdgc,g=0), a correction based on the assumption that the Kd of the intact sediment (Kdtot was a composite of the Kd<2mm and the Kd>2mm (Kdgc,g = x), and a correction based on surface area (Kdgc,surf). On average, Kd<2mm tended to overestimate Kdtot by 28% to 47%; Kdgc,g = x overestimated Kdtot by only 3% to 5%; and Kdgc,g = 0 and Kdgc,surf underestimated Kdtot by 10% to 39%. Kdgc,g = x provided the best estimate of actual values (Kdtot); however, Kdgc,g = 0 was appreciably easier to acquire. Although other contaminants will likely have different gravel‐correction values, these results have important implications regarding the traditional approach to modeling contaminant transport which uses Kd<2mm values. Such calculations may overestimate the tendency of gravel‐containing sediments to retard contaminant migration.

• Publisher: Blackwell Publishing Ltd,National Ground Water Association,Ground Water Publishing Company
• Subject: aquifers / equations / Gravel / Groundwater / Groundwater flow / mathematical models / Measurement / Sand & gravel / Sediments / Soil chemistry / solutes / sorption / strontium / USA, Hanford Site / USA, Washington, Richland / Water, Underground