A comparison of numerical and field modeling techniques in tracking contaminant plumes :

A comparison of numerical and field modeling techniques in tracking contaminant plumes:

1. Analysis of subsurface contaminant migration and remediation using high performance computing

Tompson, Andrew F., Falgout, Robert D., Smith, Steven G., Bosl, William J. Ashby, Steven F.

2. A controlled field evaluation of continuous vs. pulsed pump-and-treat remediation of VOC-contaminated aquifer: site

characterization, experimental setup, and overview of resultsMackay, D.M., Wilson, R.D., Brown, M.J., Ball, W.P., Xia, G., Durfee, D.P.

Introduction:I. Computer modeling of contaminant migration and

remediation

II. Field modeling of two different methods of remediation; constant vs. pulse pump-and-treat remediation of groundwater contamination

• both addressed the issue of Volatile Organic Compounds and the formation of contaminant plumes

Helpful Terminology

• Volatile Organic Compound (VOCs): highly reactive, charged molecules including trichloroethylene (TCE), perchloroethylene (PCE), trichloroethane (TCA)

• Hydrostratigraphic Unit: an assemblage of sediments, rocks, etc. that share similar hydraulic properties, but are not bound by lithology

I. Computer Modeling of Contaminant Plumes

• Volatile organic compounds introduced into the groundwater form complex geometries and move along hydraulic gradients as contaminant plumes

• These plumes follow hydtrostratigraphic units, while heterogeneities in the geology and hydrogeology of the aquifer system greatly influences the migration of the contaminant plumes

• The current model generates 6 realizations to model formation properties: 5 distinct random realizations of the properties, and a sixth in which individual layer properties were held constant

• Understanding the geometry of contaminant plumes and their behavior given certain hydraulic factors is critical in isolating and removing the contaminants as a potential threat to drinking water.

Conceptual model of the upper aquifer beneath Lawrence Livermore National Labs and its Hydraulic Barriers

Conceptual Model of Hydrostratigraphic Units

Potentiometric surface of water table across study site:

Head Profiles for the first 3 Realizations

Realization 1 Realization 2 Realization 3

Ambient

Remedial

Head Profiles for heterogeneous realization (1) vs uniform realization (6)

Realization 1 Realization 6

Ambient

Remedial

Migration of a Contaminant Plume and Subsequent Extraction

Time =41 years

Time = 82 years

Contaminant Migration

ContaminantExtraction

Incomplete Extraction of Contaminants

Horizontal Conductivity vs. Geometric mean Conductivity

(a) indicates displacement as a function of time of 5 realizations

(b) indicates the average displacement for the first 5 (s(t)) realizations and 6th realization (s6(t))

• s(t) indicates horizontal conductivity, while s6(t) indicates mean conductivity

(a)

(b)

Longitudinal Spreading of Plume with Displacement

(a) plot of longitudinal spreading of first 5 realizations with displacement

(b) average longitudinal spreading as a function of displacement

(a)

(b)

Transverse spreading of contaminant plume

(a) transverse spreading of 5 realizations with ambient-only displacement

(b) average of the spreading behavior for all 5 realizations

(a)

(b)

Summary:• Based on mathematical relations not included in

this discussion, plume morphology can be used to indicate the point source

• model indicates heterogeneities responsible for incomplete extraction of contaminants

II. Continuous vs. Pulse pump-and-treat remediation

• Pump-and-treat method of remediation most commonly applied technique

• The technique is problematic because concentrations of contaminants decrease with time

• Study sought to determine whether periodic cessation of pumping (pulse pumping) would disrupt the equilibrium conditions and decreasing concentrations

Primary Cause for Waning Concentrations

Slow mass transfer of contaminants into the flowing aqueous phase because:

(1) slow dissolution of non-aqueous phase liquids (NAPLs)

(2) slow diffusion from less permeable strata

(3) slow desorption of sorbed contaminants from aquifer solids

Location of study site

Experimental design: one cell set for continuous pumping and the other for pulse pumping

Cell Design:

• Pulse Pumping Cell on left side; cell closed at downstream end

• Continuous Pumping Cell on right; cell closed on right as well

• arrow indicates direction of groundwater flow

Cross section of study site indicating depth of aquitard, water table, and plume location.

Aquifer systems represented in area:

General Hydrostratigraphy:• Unconfined aquifer (0 - 6.1 m bgs)• confining layer (6.1 - 9.1 m bgs)• Confined Aquifer (Columbia Aquifer, 9.1- 14.5 m)• Basal Confining layer (14.5 m bgs)

Experimental Set-up for stripping VOC’s from water:

Core Sample Locations:

• Cores of the aquifers were taken before and after pumping to determine the effectiveness of the two methods.

Conductivity profile for the study area:

Graphical form for pumping, coring, and aqueous ‘snapshot’ sampling

Cumulative volume of extracted water v. elapsed time for the CPC and PPC:

Change in dissolved oxygen (DO) within the cells pre and post pumping:

• Profiles conducted for lower portion of the aquifer

• flushing process has oxygenated the aquifer

Extraction of Perchloroethane (PCE) from aquifer:

• Data obtained exclusively from core data

• both methods appear to increase the concentrations of PCE within the aquitard

Extraction of Trichloroethylene (TCE) from aquifer:

• Data sampled exclusively from core data

• preferential loading of the aquitard with VOCs?

Extraction of Perchloroethane (PCE) from aquifer

• Results measured from ‘snapshot’ of aqueous fluid flushed through the aquifer

• indicates effective treatment and extraction

Extraction of Trichloroethylene (TCE) immediately above aquifer/aquitard horizon:

• The incomplete extraction of TCE for both methods suggests retarded diffusion of VOCs from aquitard

Extraction of cis-DCE from aquifer:

• Cis-DCE is an aqueous phase VOC

• concentrations taken from aqueous ‘snapshot’

Extraction of Trichloroethane (TCA) from aquifer:

• Incomplete extraction resulting in residual TCA along aquifer/aquitard boundary

• residual suggests retarded dissolution

Elution curves for PPC vs. CPC sampling PCE and TCE from the combined effluent:

• Both patterns indicate a typical, rapid local extraction and then decrease in concentration of contaminant

• (b) broken segments indicate when the pump was off

Elution curves for VOC’s sampled at aquifer/aquitard interface:

• The rapid response in the PPC (b) indicates a more rapid diffusion of PCE and TCE from the aquitard.

• CPC also shows a rapid response and diffusion

Elution Curves for extraction of PCE and TCE from the middle of the aquifer:

• Despite the vertical location, PPC still exhibits evidence of contaminant recharge

• depth in aquifer correlates to upper confining layer for confined aquifer

Total Contaminant Mass removed from aquifer:

• Suggests CPC method more efficient than PPC at removing VOCs

Percent of Initial contaminant Mass removed during experiment from aquifer:

• Normalizing graph yields PPC as more efficient method

• values normalized due to contaminants in aquitard yielding apparent loading of system with VOCs

Percent total extracted mass from the aquifer system:

Summary• The partial pressure method of extraction is more efficient

because it allows for diffusion of the contaminant from the less permeable spaces. The stagnant interval also reduces the onset of equilibrium.

• The diffusion of contaminants from an aquitard can contaminate the entire overlying aquifer.

• A given aquifer can suffer multiple contaminant events complicating its source and remediation

Conclusions:In order to stop the progression or flow of a

contaminant plume, it s necessary to hydraulically isolate the plume with a series of perimeter wells and

a single or multiple extraction wells. In order to remediate the area containing the plume, a solid understanding of the regional hydrology must be known, and the aquitards must be the focus of the

extraction.