SESSION I-2. NAPL DISTRIBUTION
Reading Assignment: Continue now by reading pages 41 – 66 in Chapter 2. This material provides in-depth coverage of NAPL dissolution leading to the formation of dissolved contaminant plumes. The coverage actually begins with the 4-scenarios for contaminant release shown in Figure 2.1, pg 29. These include aqueous release to saturated zone, NAPL release in vadose zone only, LNAPL release to water table, and DNAPL release to saturated zone. On page 41 a process description for scenario 1 is presented. A process description for scenario 2 begins on page 46 and scenarios 3 and 4 are covered in section 2.6 beginning on page 60.
Format. As in the case for session I-1, the suggested format here is to complete the written assignment questions as you proceed through pages 41 - 66. The written assignments, along with additional commentary, will emphasize concepts important to understanding how NAPLs distribute and dissolve and form contaminant plumes.
Written Assignment: Questions I-9 through I-11.
Now please read section 2.5, pg 46 entitled “Process Description for Source Scenario 2: Contaminant Release to Vadose Zone Only”. Here the concept is that the dissolved contaminant or NAPL infiltrates into the vadose (partially saturated) zone, mixes with pore water and infiltrates vertically downward toward the water table. If the contaminant is in NAPL phase, individual contaminant compounds will partition out of the NAPL and become dissolved in theaqueous phase. The dissolved contaminants, or “leachate” ultimately mix with the ground water at the water table. On pg 47 the concept of equilibrium partitioning is introduced and discussed briefly. Please note that this topic will be covered in detail later in Module I in the session entitled “Equilibrium Partitioning”. For now please answer question I-12 below realizing that more in-depth coverage will follow.
Written Assignment: Question I-12.
Equilibrium partitioning between the NAPL and aqueous phase is discussed on pg 49. The important concept here is that when NAPL and water come into direct contact, individual contaminant compounds will partition out of the NAPL into the aqueous phase until chemical equilibrium is reached. At this point the maximum aqueous concentration of a particular compound is attained. This is referred to as the “effective solubility” of the compound. The effective solubility relationship is given by Equation 2.11, which is an approximation similar to Raoult’s Law for gases.
Written Assignment: Question I-13.
Pages 51-60 discuss methods and equations for modeling infiltration and leachate mixing with groundwater along with several example problems for illustration. A brief discussion of available software packages for complex vadose zone modeling (SESOIL, VADSAT, Jury’s model) appears on pages 58-60. There are no written questions for this material.
Section 2.6 beginning on page 60 give a process description for source scenarios 3 and 4: LNAPL and DNAPL dissolution in the saturated zone. This topic discusses the very important concept of scales of observation (site-wide or “field” scale, core, pore and sub-pore scales) which also appears the introduction section to Module I. Answer problem I-14 below to help develop an appreciation for NAPL behavior at each scale of observation.
Written Assignment: Question I-14.
Figures 2.14, 2.15, and 2.16, along with the relevant discussion in the text, reveal some interesting concepts on NAPL behavior in the field over short and long time periods. Similarly Figures 2.17, 2.18, and 2.19 illustrate how NAPL ganglia (blobs) persist or dissolve depending on either NAPL composition and/or pore geometry. The very important concept of residual saturation of NAPL (pg 71) is illustrated by written assignment I-15 below.
Written Assignment: Question I-15.
The material from pages 73-81 provides anecdotal and laboratory information observed NAPL saturation, ganglia size, and observed NAPL distributions at field sites. We now proceed in the following session with an in-depth discussion of Equilibrium Partitioning which will supplement the foregoing material from the textbook.