Click here to download the NDM DNAPL Case Study presentation.

Reference: Carey, G.R., 2015. Using the NAPL Depletion Model (NDM) for Forensic Analysis of NAPL Architecture at a Field Site, presented at the 30th Biennial Groundwater Conference, Sacremento, California, Oct. 6-7, 2015.

ABSTRACT

NAPL architecture in a source zone will influence the rate of source strength reduction and remediation timeframe for natural and enhanced dissolution alternatives.  For example, the presence of a thick NAPL pool will result in a substantially longer remediation timeframe compared to a zone with multiple thin pools or layers of residual NAPL.  It is difficult to distinguish between these types of NAPL architectures when reviewing field data.

One approach for refining the characterization of NAPL architecture at a site is to use the screening-level NAPL Depletion Model, or NDM (Carey et al., 2014a and b) to simulate historical observations of source strength reduction.  The NAPL Depletion Model is a semi-analytical, process-oriented screening model for simulating the depletion of horizontal layers of residual or free phase NAPL.  NDM simulates surface discharge, through-NAPL discharge, as well as forward and back-diffusion from thin or thick layers of low-permeability lenses in contact with NAPL at some point in time.

An overview is presented to demonstrate how the NAPL Depletion Model may be used to simulate source strength decline and remediation timeframe for natural and enhanced dissolution alternatives such as EISB, ISCO, and strategic pump-and-treat (SP&T). A case study application is then presented, where NDM was used as a forensic tool for evaluating NAPL architecture at a Connecticut.  NDM was used to simulate historical source strength trends for three types of architectures that included:  (i) one large thick TCE pool; (ii) multiple, vertically-distinct pools of free phase TCE with varying thicknesses; and (iii) multiple layers of residual and free-phase TCE pools.  The influence of reduced groundwater flow through each NAPL layer is considered in these simulations. 

Simulated source strength trends for each type of architecture are compared to those predicted by Chapman and Parker (2005), to identify the type of NAPL architecture which was most likely present at the site in the late 1990s when an extensive investigation was conducted.  This application demonstrates how a simple screening tool like NDM may be used as a forensic tool to refine the characterization of NAPL architecture as part of a weighted lines of evidence approach.