attenuation of pollutants in the hyporheic zone: river tame, west midlands michael rivett...
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Attenuation of Pollutants in the Hyporheic Zone: River Tame, West Midlands
Michael Rivett M.O.Rivett@bham.ac.uk
School of Geography, Earth & Environmental Sciences
Groundwater modellers’ Forum: Groundwater – Surface Water Interaction Modelling Workshop Birmingham, UK. 28 Mar 2007
Outline
• Pollutant natural attenuation in the hyporheic zone
• River Tame field studies
• Issues / questions relevant to modelling
• Summary discussion points
Pollutant Natural Attenuation (NA)• ‘Natural Attenuation’ (NA) refers to the
“naturally occurring physical, chemical or biological processes that act within an aquifer to reduce contaminant mass, concentration, flux or toxicity” (Environment Agency, 2000)
m - Xylene
CH3
CH3
Data from: J.F. Barker (Univ. Waterloo, Canada)
Attenuation – Hyporeheic Zone (HZ)
• Infiltration of oxygenated SW
• High organic carbon, nutrients
• High microbiological activity
• Steep redox gradients• Dynamic flows
(USGS Circular 1139, 2001)
River
Hyporheic Zone – Groundwater – surface water mixing
• “Last gasp” remediation of groundwater contaminant plumes
Attenuation – Hyporeheic Zone (HZ)
• “HZ natural attenuation (NA) capacity”: Contaminant flux that can be “treated” by attenuation processes during transport through the HZ
• “Treatment”: Reduction in contaminant concentration and transformation to benign products. ( MNA strategies)
HZ: the reality is complex
Some issues:• Defining an “attenuation capacity” that is…
– Temporally and spatially predictable and sufficient
• Receptor definition: riverbed, benthic life, river …
• Scales of measurement, regulation, modelling
(Conant, 2000)
• Spatial variable
• Temporally dynamic
River Tame field studies
River Tame - Birmingham aquifer
• Rivett, M.O., Ellis, P.A., Greswell, R.B., Ward, R.S., Roche, R.S., Cleverly, M., Walker, C., Conran, D., Fitzgerald, P.J., Willcox, T., Dowle, J., in subm. Cost-effective mini drive-point piezometers and multilevel samplers for monitoring the hyporheic zone. In submittal to Quarterly Journal of Engineering Geology & Hydrogeology.
• Ellis, P.A., Mackay, R., Rivett, M.O., 2007. Quantifying urban river–aquifer fluid exchange processes: A multi-scale problem. Journal of Contaminant Hydrology 91, 51-80.
• Ellis, P.A., Rivett, M.O., 2007. Assessing the impact of VOC-contaminated groundwater on surface-water at the city scale. Journal of Contaminant Hydrology 91, 107-127.
• Rivett, M.O., Greswell, R.B., Mackay, R., Lydon, C., Conran, D.J., Ellis, P.A., 2007. Natural attenuation potential of the urban hyporheic zone: Foundational studies to the River Tame (Birmingham, UK) dipole field experiments. In: Proceedings of the First SWITCH Scientific Meeting, Birmingham, UK, 9-10 Jan 2007.
• Shepherd, K.A., Ellis, P.A., Rivett, M.O. 2006. Integrated understanding of urban land, groundwater, baseflow and surface-water quality – The City of Birmingham, UK. The Science of the Total Environment 360, 180-195.
• Ellis, P.A., 2003. The impact of urban groundwater upon surface water quality: Birmingham – River Tame study, UK. PhD thesis, School of Geography, Earth & Environmental Sciences, University of Birmingham, UK, 360 pp.
• Ellis, P.A., Rivett, M.O., Henstock, J, Dowle., Mackay, R., Ward, R. and Harris, R., 2002. Impacts of contaminated groundwater on urban river quality – Birmingham, UK. In: Groundwater quality: Natural and enhanced restoration of groundwater pollution. IAHS publication No. 275, 71-77.
• Ellis, P.A., Rivett, M.O. and Mackay, R. 2004. Estimation of groundwater-contaminant fluxes to urban rivers. In: Hydrology: Science & Practice for the 21st Century, British Hydrological Society (Publ.) 272-279.
Triassic Sandstone aquifer effluent to 7km reach of the
River Tame
• Qmin~180 Ml/d• Baseflow ~7% Qtotal
• Quality Class E/F
• K ~ 2 m/d• n ~ 0.27• Sy ~ 0.1• Chlorinated solvents,
metals
Methods:
• Drivepoint piezometers
• Drivepoint multilevel samplers
(c) (d)
(e) (f)
(a) (b)
Methods:
MLS Transect #1
(g) (h)
(i) (j)
Issue 1: assessment scale(s)
• Field investigation– City reach 7 km– 400 m reach– 50 m reach– Single multilevel profile
• Attenuation process investigation• Regulatory compliance• Up-scaling issues• ? Models developed at appropriate
scale(s)
0.1
1
10
100
5 11 12 4 2 1 13 14 6 19 15 10 8 9 16 7 17 18
Profile number
Con
cent
ratio
n (u
g/l)
PCE
TCE
cDCE
tDCE
1,1-DCE
1,1,1-TCA
1,1-DCA
TCM
0.1
1
10
100
5 11 12 4 2 1 13 14 6 19 15 10 8 9 16 7 17 18
Profile number
Con
cent
ratio
n (u
g/l)
PCE
TCE
cDCE
tDCE
1,1-DCE
1,1,1-TCA
1,1-DCA
TCM
0.5 m below riverbed
0.1
1
10
100
5 11 12 4 2 1 13 14 6 19 15 10 8 9 16 7 17 18
Profile number
Con
cent
ratio
n (u
g/l)
PCE
TCE
cDCE
tDCE
1,1-DCE
1,1,1-TCA
1,1-DCA
TCM
0.1
1
10
100
5 11 12 4 2 1 13 14 6 19 15 10 8 9 16 7 17 18
Profile number
Con
cent
ratio
n (u
g/l)
PCE
TCE
cDCE
tDCE
1,1-DCE
1,1,1-TCA
1,1-DCA
TCM
0.5 m below riverbed
Profile Number
Bank 1
Bank 2
VOCs in groundwater baseflow (7 km reach)
7 km Reach
400 m reach scale
• TCA is parent solvent– DCE is abiotic degradation product– DCA is biotic degradation product
0.1
1
10
100
1000
375350325300275250225200175150125100755025
Longitudinal reach distance (m)
Con
cent
ratio
n (u
g/l)
1,1,1-Trichloroethane
1,1-Dichloroethene
1,1-Dichloroethane
TCADCEDCA
Cross river transect:
• Evidence of negligible (if any) NA in HZ
Modelling at various scales• Based of River Tame studies:
Ellis, P.A., Mackay, R., Rivett, M.O., 2007. Quantifying urban river–aquifer fluid exchange processes: A multi-scale problem. Journal of Contaminant Hydrology 91, 51-80.
Modelling• Examine GW-SW interations / mixing
zone• 4 models at various scales (+ aquifer
model)
Sandstone
Gravel
Made Ground
River
Cross Sectional Model
300 m wide
100 m deep
Riverbed
11 12 13 # # # # # # 96 97 98 99 100
Piezom eter P 10 Piezom eter P 11
River
Sandstone
W eathered Sandstone
River bed Sedim ents
Sands and Gravels
Made Ground
Column Number
RIVER
11 12 13 # # # # # # 96 97 98 99 100
Piezom eter P 10 Piezom eter P 11
River
Sandstone
W eathered Sandstone
River bed Sedim ents
Sands and Gravels
Made Ground
Column Number11 12 13 # # # # # # 96 97 98 99 100
Piezom eter P 10 Piezom eter P 11
River
Sandstone
W eathered Sandstone
River bed Sedim ents
Sands and Gravels
Made Ground
Column Number11 12 13 # # # # # # 96 97 98 99 100
Piezom eter P 10 Piezom eter P 11
River
Sandstone
W eathered Sandstone
River bed Sedim ents
Sands and Gravels
Made Ground
Column Number
RIVER
Sandstone
Gravel
Made Ground
River
Cross Sectional Model
300 m wide
100 m deep
Riverbed
11 12 13 # # # # # # 96 97 98 99 100
Piezom eter P 10 Piezom eter P 11
River
Sandstone
W eathered Sandstone
River bed Sedim ents
Sands and Gravels
Made Ground
Column Number
RIVER
11 12 13 # # # # # # 96 97 98 99 100
Piezom eter P 10 Piezom eter P 11
River
Sandstone
W eathered Sandstone
River bed Sedim ents
Sands and Gravels
Made Ground
Column Number11 12 13 # # # # # # 96 97 98 99 100
Piezom eter P 10 Piezom eter P 11
River
Sandstone
W eathered Sandstone
River bed Sedim ents
Sands and Gravels
Made Ground
Column Number11 12 13 # # # # # # 96 97 98 99 100
Piezom eter P 10 Piezom eter P 11
River
Sandstone
W eathered Sandstone
River bed Sedim ents
Sands and Gravels
Made Ground
Column Number
RIVER
Model 2: 300 m transect
Issue #2 How to improve our understanding of flow nuances
• Dominance of lateral flows into the river through the river sides, e.g. central 50% of the river bed contributes only 25 % of total baseflow
• River water entering the river bed during the passage of the flood wave occurs only for very short times (<10 mins) and in negligible quantities - aquifer head changes derive mostly from damming of water entering the river rather than flow reversal
• After passage of flood wave a persistent (rather than large short-term) release of groundwater occurs as the stored water progressively drains to the river
• Vertical pressure gradients through the bed of the river are higher at the river channel edge than near the centre
• Local scale geological heterogeneity (particularly clay lenses) substantially influences the flow geometry beneath the river.
-1.6
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
Sp
ecif
ic d
isch
arge
to
the
rive
r m
3d
-1
(neg
ativ
e in
dic
ates
dis
char
ge
to t
he r
iver
)
0
10
20
30
40
50
60
70
80
90
100
Dry Weather Flow Conditions, R24
Seepage face
Cumulative discharge (2nd y axis)
River boundary cells in order starting at the top of the Eastern bank down across the river bed and up to the top of the Western bank, breaks between vertical and horizontal faces.
River Bank (East) River Bank (West)River Bed
Cell DimensionsHorizontal 0.3 mVertical 0.1 m
River boundary cells
North Bank
South BankRiverbed
Surface water – Groundwater mixing• River Tame chloride: a convenient
tracer.0
0.2
0.4
0.6
50 100 150 200
De
pth
(m
)
Chloride (mg/l)
23-Jul-06 (MLS-1)
02-Aug-06 (MLS-1)
08-Aug-06 (MLS-1)
14-Aug-06 (MLS-2)
0
0.2
0.4
0.6
50 100 150 200 250 300
De
pth
(m
)
Chloride (mg/l)
23-Jul-06 (MLS-3)
08-Aug-06 (MLS-3)
14-Aug-06 (MLS-3)
Proposed
dipole site
Transect 8
Transect 11
T8-ML1T8-ML3
T11-ML2T11-ML4
Riverbed monitoring network
Proposed
dipole site
Transect 8
Transect 11
T8-ML1T8-ML1T8-ML3T8-ML3
T11-ML2T11-ML2T11-ML4T11-ML4
Riverbed monitoring network
• Test hypothesis that local-scale mixing can be explained by bed material heterogeneity and undulations in bed form (Conant, 2004)
• 10-m reach model populated with slug test hydraulic conductivity data. The modelling indicated that:– a laterally persistent shallow HZ of less than 10 cm only should exist; – where flows are concentrated, e.g., near river edges, HZ existence is
very unlikely– HZ is physically created if the bed material is protected from significant
vertical flow.
• These results are not entirely consistent with field-observed deeper mixing zones, may be due to:– low permeability structures deeper in the profile are having a significant
influence in generating the apparently high hydraulic gradients; – other processes are operative, e.g. in the summer aquatic plants are
more common and may induce locally steeper river gradients; – upper layer material structure is radically different to that modelled, e.g.
due to bioturbation or more likely river reworking during the commonly observed armouring process leading to the winnowing of fine material in the near riverbed surface.
Surface water – Groundwater mixing
• Impacts on the local mixing in the river bed may occur due to – flow variations arising from rapid pressure perturbations at the base of the river
caused by turbulent eddies and surface wave forms (with time periods on the order of a second).
– Penetration of pressure transients to reasonable depth (up to 0.5 m) in the river bed on these timescales may be hypothesized and that enhanced chemical diffusion may operate in this zone as a consequence of fluctuating velocity induced mixing.
• Whether such pressure transients can operate at levels that may cause enhanced diffusion and thereby enhance the depth of penetration of the stream aquifer mixing zone was evaluated my momentum exchange modelling.
• The results confirmed that: – The momentum exchange increases the depth of penetration of reverse flows
into the river bed although the magnitudes are lower. – The flow reversals extend to a depth of 50 cm for the same system with
momentum suggesting that enhanced diffusive mixing into the river bed profile could occur against a general upflow.
– This does lend support to the concept that small scale pressure transients could be influential in increasing the mixing at depth which when combined with flow mixing created through the heterogeneity patterns in the river bed may explain HZ (mixing zone) thicknesses that are observed to be much greater than ~10 cm.
Surface water – Groundwater mixing
Issue #3: Complexity, heterogeneity & predictability of attenuation capacity (reactions)
Attenuation of chlorinated VOCs• Anaerobic or aerobic pathways (5 sub-
types)• DOC, DO, Eh, H2, pH, bacteria type, electron
acceptor(s)
0
50
100
150
200
-105 -75 -45 -15 15 30 45
Conc
entr
ation
(μg/
l)
Distance downstream from Transect #1 (m)
TCE
cDCE
VC
150 m Reach
0
200
400
600
800
0 1.67 3.33 7.5 10 15 20 25 30 35 40 45 50
Conc
entr
ation
(μg/
l)Distance downstream (m)
TCE
0
200
400
600
800
0 1.67 3.33 7.5 10 15 20 25 30 35 40 45 50
Conc
entr
ation
(μg/
l)
Distance downstream (m)
cDCE
0100200300400500600700800
0 1.67 3.33 7.5 10 15 20 25 30 35 40 45 50
Conc
entr
ation
(μg/
l)
Distance downstream from Transect #1 (m)
VC
50 m Sub-reach
0.1 m
0.5 m Depth
profile0.3 m
Geologic controls ?
• Grain-size distribution of surface sediments
cis-DCE/TCE ratios with depth
0
10
20
30
40
50
0 5 10 15 20
cis-DCE/TCE concentration (ug/L)
De
pth
be
low
riv
erb
ed
(c
m)
ML 1
ML 3
ML 4
ML 5
ML 7
ML 12
ML 14
ML 2
TCE conc with depth
0
10
20
30
40
50
60
0 500 1000 1500
TCE conc (ug/L)
De
pth
be
low
riv
erb
ed
(c
m)
ML 1
ML 4
ML 5
ML 8
ML 2
• ~ 120 sample points in riverbed
Cross river transect
TCE cDCE VC
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0 1 2 3 4 5 6 7 8 9 10
Distance from river bank (m)
De
pth
be
low
riv
erb
ed
(m
)
477 73 1
757 102 8
275 241 54
35 323 191
2 71 132
0.9 nd nd
523 56 nd
99 11 nd
33 2 nd
27 3 nd
19 2 nd
nd nd nd
4 1 nd
1 3 nd
5 2 nd
4 2 nd
4 2 nd
nd nd nd
nd 1 nd
nd 1 nd
nd 2 nd
nd 1 nd
nd 1 nd
nd nd nd
nd nd nd
nd nd nd
nd nd nd
1 nd nd
1 1 nd
nd nd nd
Concentration (μg/l): TCE cDCE VC
Issue #3: Complexity, heterogeneity & predictability of attenuation capacity (reactions)
• Sophisticated kinetic, multi-process, NA models exist as research tools (e.g. Gerhard, Univ. Edinburgh)
• In practice, modeling of HZ natural attenuation is fairly limited (simple half-life approach?)
• How sophisticated do our (site) NA models need to be ?
Issue #4: By passing high attenuation zones
• Contaminated groundwater may by-pass “high attenuation capacity” zones (silty/organic carbon-rich)
• Majority of flow may be via more permeable “low-attenuation capacity”, zones (sands and gravels) of low residence time
Modelling must have a role here ?
Issue #5: Prediction of NAPL migration
• Little multi-phase flow modelling has been applied to the groundwater – surface-water interface Temporal behaviour NAPL property (density, viscosity...)
controls Preferential pathways
Issue #6: Communication of modellers with the “rest of hydro-world”
You want what in your model ?
Attenuation of Pollutants in the Hyporheic Zone
Summary of modelling issues
#1 - Appropriate scale(s) for the problem(s)#2 - Flow nuances: complexity required#3 - Attenuation reaction: complexity required#4 - By passing of high attenuation zones #5 - NAPLs: controls on migration#6 - Communication of model needs/outputs
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