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

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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 results

Mackay, D.M., Wilson, R.D., Brown, M.J., Ball, W.P., Xia, G., Durfee, D.P.


Introduction
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
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
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.


A comparison of numerical and field modeling techniques in tracking contaminant plumes
Conceptual model of the upper aquifer beneath Lawrence Livermore National Labs and its Hydraulic Barriers


Conceptual model of hydrostratigraphic units
Conceptual Model of Hydrostratigraphic Units Livermore National Labs and its Hydraulic Barriers


Potentiometric surface of water table across study site
Potentiometric surface of water table across study site: Livermore National Labs and its Hydraulic Barriers


Head profiles for the first 3 realizations
Head Profiles for the first 3 Realizations Livermore National Labs and its Hydraulic Barriers

Realization 1

Realization 2

Realization 3

Ambient

Remedial


Head profiles for heterogeneous realization 1 vs uniform realization 6
Head Profiles for heterogeneous realization (1) vs Livermore National Labs and its Hydraulic Barriersuniform realization (6)

Realization 1

Realization 6

Ambient

Remedial


Migration of a contaminant plume and subsequent extraction
Migration of a Contaminant Plume and Subsequent Extraction Livermore National Labs and its Hydraulic Barriers

Contaminant

Migration

Time =41 years

Contaminant

Extraction

Time = 82 years


Incomplete extraction of contaminants
Incomplete Livermore National Labs and its Hydraulic BarriersExtraction of Contaminants


Horizontal conductivity vs geometric mean conductivity
Horizontal Conductivity vs. Geometric mean Conductivity Livermore National Labs and its Hydraulic Barriers

(a)

(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

(b)


Longitudinal spreading of plume with displacement
Longitudinal Spreading of Plume with Displacement Livermore National Labs and its Hydraulic Barriers

(a)

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

(b) average longitudinal spreading as a function of displacement

(b)


Transverse spreading of contaminant plume
Transverse spreading of contaminant plume Livermore National Labs and its Hydraulic Barriers

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

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

(a)

(b)


Summary
Summary Livermore National Labs and its Hydraulic Barriers:

  • 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
II. Continuous vs. Pulse pump-and-treat remediation Livermore National Labs and its Hydraulic Barriers

  • 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
Primary Cause for Waning Concentrations Livermore National Labs and its Hydraulic Barriers

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
Location of study site Livermore National Labs and its Hydraulic Barriers



Cell design
Cell Design: the other for pulse pumping

  • 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



Aquifer systems represented in area
Aquifer systems represented in area: water table, and plume location.

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)



Core sample locations
Core Sample Locations: water table, and plume location.

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


Conductivity profile for the study area
Conductivity profile for the study area: water table, and plume location.


Graphical form for pumping coring and aqueous snapshot sampling
Graphical form for pumping, coring, and water table, and plume location.aqueous ‘snapshot’ sampling



Change in dissolved oxygen do within the cells pre and post pumping
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
Extraction of Perchloroethane (PCE) from aquifer: post pumping:

  • Data obtained exclusively from core data

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


Extraction of trichloroethylene tce from aquifer
Extraction of Trichloroethylene (TCE) from aquifer: post pumping:

  • Data sampled exclusively from core data

  • preferential loading of the aquitard with VOCs?


Extraction of perchloroethane pce from aquifer1
Extraction of Perchloroethane (PCE) from aquifer post pumping:

  • 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
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
Extraction of aquifer/aquitard horizon:cis-DCE from aquifer:

  • Cis-DCE is an aqueous phase VOC

  • concentrations taken from aqueous ‘snapshot’


Extraction of trichloroethane tca from aquifer
Extraction of Trichloroethane (TCA) from aquifer: aquifer/aquitard horizon:

  • 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
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
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
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
Total Contaminant Mass removed from aquifer: of the aquifer:

  • Suggests CPC method more efficient than PPC at removing VOCs


Percent of initial contaminant mass removed during experiment from aquifer
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



Summary1
Summary experiment from aquifer:

  • 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


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

Conclusions: experiment from aquifer: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.