evaluation of plume stability · 2018. 6. 5. · 2d presentation of data across plume (area under...
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Evaluation of Plume Stability
David L. Pate, RGMidwest Environmental ConsultantsMidwest Environmental Consultants
Typical Groundwater Plume
Typical Sequence of Events
Characterize, delineate impacts
Identify receptors, pathways
Risk assessment
Source removal
Groundwater Monitoring –
Today’s Issue: Ways to Evaluate plume stability
Groundwater Plume Map View
Plume Stability Assessment
Objectives:–
Demonstrate plume is not migrating
No future risk to downgradient receptors
–
Demonstrate that COCs are attenuating
Reduction in risk to receptors over time
–
COC fluctuations do not create unacceptable risk to
receptors
–
Closure of R‐file and issuance of NFRA
Assessment Methodologies
Variety of statistical & graphical methods–
Individual Wells Methods
Linear regression analysis–
Evaluates COC concentrations vs. time
–
How closely does trend line match data?
–
Correlation coefficient (R2) ‐
“goodness of fit”
Mann Kendall Analysis–
Measure of stability of COCs vs. time
–
Stable, unstable, decreasing, increasing
Assessment Methodologies
Whole Plume Evaluation Methods–
Ricker Method
Graphical 3D Analysis using “Surfer”
software
–
Average COC concentration linear regression
Average COC concentrations for all wells vs. time
(proportional to plume mass vs. time)
–
Cross Sectional Analysis
2D presentation of data across plume (area under curve
proportional to plume mass)
Assessment Methodologies
Combination of methods –
Can make a more convincing case for plume stability
Individual well + whole plume techniques –
Puts individual well fluctuations at heart of plume
area in a big picture context
–
Make more convincing case for plume stability
Example Site Using Multiple Methods
Site located in western Missouri–
Active convenience store
–
USTs replaced 1991
–
Residual impacts in tank pit continue to feed
groundwater plume
–
Groundwater data from 2004 to 2010
Assessment of Individual Wells
Good News:–
Individual well ‐
concentration vs. time plots
Generally show attenuating trend
COC’s
decreasing
Bad News:–
Individual wells with seasonal COC fluctuations
Linear regression R2
values <0.5
Benzene Concentration Vs. Time
Monitoring Well MW-7
R2 = 0.1802
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
08/01/0408/01/05
08/01/0608/01/07
08/01/0808/01/09
Sample Date
Ben
zene
Con
cent
ratio
n (m
g/l)
BenzeneConcentrationTrendline
TPH-GRO Concentration Vs. Time
Monitoring Well MW-7
R2 = 0.4580
5
10
15
20
25
30
35
Aug-04Aug-05
Aug-06Aug-07
Aug-08Aug-09
Sample Date
TPH
-GR
O C
once
ntra
tion
(mg/
l)
TPH-GROConcentrationTrendline
Assessment of Individual Wells
Are the fluctuations in individual source area wells really indicative of unstable plume?
Stability Assessment Alternatives
My subjective assessment of the data:
Despite source area fluctuations:–
Wells away from residual source consistently < DTLs
–
Perimeter wells ‐
no evidence of expanding plume
Need alternate way to quantify my subjective assessment
Stability Assessment Alternatives
Option 1: Average COC concentrations vs. time–
Average for individual COCs vs. time
–
Average is proportional to total plume mass
–
Linear Regression of average vs. time
more indicative of behavior of total plume
generally yields higher correlation coefficients
Average Benzene Concentration Vs. Time
R2 = 0.9715
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Date
Benz
ene
(mg/
L)
Average TPH-GRO Concentration Vs. Time
R2 = 0.9508
0
1
2
3
4
5
6
7
Date
TPH
-GR
O (m
g/L
Stability Assessment Alternatives
Option 2: Plume centerline cross sections –
Published USEPA method (Dupont, et al, 1998)
Use cross sections through plume center
Plot data from individual monitoring events (separate line
for each sampling event)
Area under curve proportional to total plume mass
For attenuating plume, area under the curve should
decrease with time
Benzene Cross Section A - A1 vs. Time
0
0.5
1
1.5
2
2.5
3
3.5
MW-14 MW-3 MW-1 MW-5 MW-6
M onitoring Well
4/ 18/ 2007
8/ 14/ 2007
6/ 18/ 2008
3/ 25/ 2010
Benzene Cross Section B - B1 vs Time
0
0.5
1
1.5
2
2.5
3
3.5
MW-10 MW-8 MW-7 MW-1 MW-2 MW-4
M onitoring Well
4/ 18/ 2007
8/ 14/ 2007
6/ 18/ 2008
3/ 25/ 2010
Summary
Statistical evaluation of Individual wells can sometimes be misleading
Plume centerline cross sections & average COC vs. time plots can provide better assessment of plume attenuation
Looking at the forest rather than the trees
Summary
Additional options to evaluate plume stability–
Additional tools in your tool box
Can combine with individual well or 3D analysis to make a more convincing case for plume
stability
Plume Stability Evaluation
Thank You
Questions?