2021 journal article
Evaluating the impact of back diffusion on groundwater cleanup time
JOURNAL OF CONTAMINANT HYDROLOGY, 243.
author keywords: Back diffusion; Cleanup time; Sensitivity; Parameters; Damkohler
MeSH headings : Diffusion; Groundwater; Models, Theoretical; Water Pollutants, Chemical / analysis; Water Wells
TL;DR:
This work identifies the aquifer and contaminant characteristics that have the greatest influence on the time (TOoM) after complete source removal for contaminant concentrations to decline by 1, 2 and 3 Orders ofMagnitude (T1, T2 and T3).
(via Semantic Scholar)
UN Sustainable Development Goal Categories
6. Clean Water and Sanitation
(Web of Science; OpenAlex)
Source: Web Of Science
Added: October 12, 2021
Back diffusion of groundwater contaminants from low permeability (K) zones can be a major factor controlling the time to reach cleanup goals in downgradient monitor wells. We identify the aquifer and contaminant characteristics that have the greatest influence on the time (T OoM ) after complete source removal for contaminant concentrations to decline by 1, 2 and 3 Orders-of-Magnitude (T 1 , T 2 and T 3 ). Two aquifer configurations are evaluated: (a) layered geometry (LG) with finite thickness low K layers; and (b) boundary geometry (BG) with thick semi-infinite low K boundaries. A semi-analytical modeling approach (Muskus and Falta, 2018) is used to simulate the concentration decline following source removal for a range of conditions and generate ≈21,000 independent values of T 1 , T 2 and T 3 . Linear regression is applied to interpret this large dataset and develop simple relationships to estimate T OoM from three characteristic parameters - the mass residence time (T M ), diffusion time (T D ), and ratio of low K to high K mass storage (γ). T M is most important predictor of T 1 , T 2 and T 3 for both geometries and is equal to the combined high and low K contaminant mass divided by the mass flux, at the end of the loading period (T L ). For LG, T 3 is strongly influenced by T D = R L L D 2 /(4D*), where R L is the low K retardation factor, L D is the half-thickness of the embedded low K layers, and D* is the effective diffusion coefficient. For BG, T 3 is strongly influenced by γ. Contaminant decay in low K zones can significantly reduce cleanup times when λ L T D > 0.01, where λ L is the effective first order decay rate in the low K zone. The 1st Damköhler (Da), equal to T M /T D , provides a useful indicator of the relative importance of back diffusion on T OoM . Back diffusion impacts are greatest on T 3 when 0.01 > Da > 0.1, then decrease with increasing Da. Back diffusion has less impacts on T 2 , with limited influence on T 1 . The results are summarized in a simple conceptual model to aid in evaluating the impact of back diffusion on the time for concentrations to decline by 1-3 OoM.