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Groundwater flow through till: tortoise, hare, or not in the race?
1Jared Trost, 2William Simpkins, 1,2Anna-Turi Maher, 2,3Alyssa Witt, 1Andrew Berg, 1,4James Stark, 5,6Bob Tipping, 6Justin Blum, 1Daniel Feinstein
1U.S. Geological Survey, 2Iowa State University, 3Golder and Associates, 4Legislative Water Commission, 5MN Geological Survey, 6MN Department of Health
This information is preliminary and is subject to revision. It is being provided to meet the need for timely best science. The information is provided on the condition that neither the U.S. Geological
Survey nor the U.S. Government shall be held liable for any damages resulting from the authorized or unauthorized use of the information.
Cromwell Fm., Cromwell, MN
Well nest, Litchfield, MN
Good Thunder Fm., Olivia, MN
A collaborative effort
• A 5-year collaborative project among:
• U.S. Geological Survey
• Iowa State University
• Cities of Litchfield, Cromwell, and Olivia
• Minnesota Depts. of Health and Natural Resources
• Minnesota Geological Survey
• University of Minnesota
• Major funding:
• MN Environment and Natural Resources Trust Fund
• USGS Cooperative Water Fund Program
• Major in-kind support:
• Bill Simpkins, Iowa State University
A preview of today’s presentations
This talk:
• Study design
• Variability: geology, K, hydraulic relationships, isotope geochemistry, and contamination
• Interpretive modeling
Afternoon talk:
Justin Blum: a presentation about lessons learned from aquifer tests at each site.
Two Posters Hewitt Fm.near Akeley, MN
Outwash
Till
Bedrock
Confined Aquifer
Motivation
• Confined aquifers supply drinking water to thousands of people
• How sustainable are these water supplies?
Outwash
Till
Bedrock
Confined Aquifer
• How we like to conceptualize groundwater flow through till to confined aquifers
• But how realistic is this?
Contaminants
Water
Water WaterWater
Water
Motivation
Outwash
Till
Bedrock
Confined Aquifer
• How we like to conceptualize groundwater flow through till to confined aquifers
• But how realistic is this?
ContaminantsWater
WaterWater
Water
Water
Motivation
Outwash
Till
Bedrock
Confined Aquifer
Leakage? K?
• How does groundwater flow through till?
• How long does it take water to move through till?
• How susceptible are the aquifers to contamination?
Motivation-a focus on till
Project Goal
Better understand the sustainability of water supplies from confined aquifers.
Approach: for different glacial lobes and till units in Minnesota, we quantified:
• Variability of till hydraulic properties
• Leakage of water from till confining units to buried confined aquifers
Rotosonic coring in Olivia, MN, 2017
Study site selection
• Detailed hydrogeologic information available
• Physical setting characteristics
• Small number of high capacity pumping wells
• less than 300 ft below land surface
• High-capacity well construction integrity
• Logistical requirements
• Cooperative water operator
• Accessible with large drill rig Glacial Lobes and Sublobes of Late Wisconsin age (Jennings and Johnson, 2011)
Study sites
Glacial Lobes and Sublobes of Late Wisconsin age (Jennings and Johnson, 2011)
Site Lobe Popula-tion
Pumping (MGY)
UMN hydro-geology field camp (HFC)
Wadena ~30-40 for 3 weeks in July
NA, good for long-term aquifer tests
Cromwell Superior 231 6
Litchfield Des Moines
6,630 340
Olivia Pre-Illinoian
2,350 97
Well nest design and installation
Characterize profile with continuous core to place screens
Till
confinin
gunit
Aquifer
Hig
h-c
apacity
pum
pin
g w
ell
3-ft screen
Aquifer well: 2-in diameter; 5-10 ft screen (rotosonic or mud rotary)
Till wells: 1.25-in diameter, 3-ft screen (rotosonicor hollow-stem)
Continuous core collected to characterize profile and place screens
Well nest design and installation
Core-section retrieved from screen interval and squeezed to get pore-water sample
Till
confinin
gunit
Aquifer
Hig
h-c
apacity
pum
pin
g w
ell
3-ft screen
Aquifer well: 2-in diameter; 5-10 ft screen (rotosonic or mud rotary)
Till wells: 1.25-in diameter, 3-ft screen (rotosonicor hollow-stem)
Characterize profile with continuous core to place screens
Well nest design and installation
Ideally, a well nest would be installed near and far from a pumping well at a site
Data collection
•Hydrology• Water levels and precipitation• Slug tests on all wells• Aquifer tests at 4 sites (led by
Justin Blum)
•Geochemistry of groundwater and pore water• Major ions and nutrients• Stable isotopes (δ18O, δ2H)
• Enriched 3H (GW only)
Confined Aquifer
Till Confining Unit(a.k.a. Till Aquitard)
Till Characterization
Staley et al., 2018
Site Olivia Litchfield Cromwell HFC
Lobe Undetermined Des Moines Superior Wadena
Age Pre-Illinoian Late Wisc. Late Wisc. Late Wisc.
Formation Good Thunder New UlmVillard Mbr.
Cromwell Hewitt
Grain Size [Sand:Silt:Clay]
37:40:23 50:32:17 57:31:13 67:22:11
Texture Clay loam to loam
Loam to sandy loam
Sandy loam Sandy loam Loamy sand
Cromwell Fm.Outwash
Till -Cromwell
Fm.
Aquifer
Slate?
Aquifer
0
Aquifer
Till -New
Ulm Fm.VillardMbr.
Till -New
Ulm Fm.VillardMbr.
Site Stratigraphy
Deltaic and LacustrineSedimentNew Ulm Fm.
Olivia Litchfield 1 Litchfield 2 Cromwell
Univ. of MN Hydrogeology Field Camp (HFC)
Till -Good Thunder Fm.
Till -Hewitt Fm.
Clay loam till Sandy loam till
Staley et al., 2018
Box plots of Log K for all study sites
Data from slug tests
Units in feet/day
Olivia L1 L2 Cromwell HFC
N=5 N=2
N=5
N=5 N=4
Preliminary Information-Subject to Revision. Not for Citation or Distribution.
Olivia L1 L2 Cromwell HFC
Box plots of Log K for all study sites
Data from slug tests
Units in meters/sec
N=5 N=2
N=5
N=5 N=4
Preliminary Information-Subject to Revision. Not for Citation or Distribution.
L1 L2 Dows Fm.
Box plots of Log K for Litchfield and the Dows Fm. in Iowa
Data from slug tests
Units in meters/sec
N=2
N=5
N=48
Preliminary Information-Subject to Revision. Not for Citation or Distribution.
Mean hydraulic head measured in piezometers vs. depth at all sites
Preliminary Information-Subject to Revision. Not for Citation or Distribution.
Hart et al., 2008. Ground Water 46(4): 518-520
Till i = 0.12
Blue (neg.) downward flowRed (pos.) upward flow
Preliminary Information-Subject to Revision. Not for Citation or Distribution.
Till i=0.24
Preliminary Information-Subject to Revision. Not for Citation or Distribution.
Till i =0.37
Preliminary Information-Subject to Revision. Not for Citation or Distribution.
Till Sites from the Des Moines Lobe in Central Iowa
Beth Johnson and Jim Eidem, personal communication
Till i=0.025 up
Potentiometric high in the slate aquifer
Preliminary Information-Subject to Revision. Not for Citation or Distribution.
Till i = 0.05
Preliminary Information-Subject to Revision. Not for Citation or Distribution.
SiteALV from Kh (ft/d)
Bottom of Till (yrs)
ALV from Kv (ft/d)
Bottom of Till (yrs)
LF-1 7E-02 2 2E-03 91
LF-2 3E-04 1,026 2E-04 2,129
Olivia 3E-03 125 3E-04 1,588
HFC 9E-03 30 2E-03 147
Groundwater Velocity and Travel Time
𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑙𝑖𝑛𝑒𝑎𝑟 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 (𝐴𝐿𝑉) =𝐾𝑖
𝑛n = 0.25
Preliminary Information-Subject to Revision. Not for Citation or Distribution.
Chemistry to the Rescue
Groundwater Sampling
Core for Pore-Water Samples
Age Estimates from δ18OSource δ18O (‰)
VSMOW
Glacier Ice (Paterson and Hammer, 1987) -40 to -55
Lake Agassiz, southern Manitoba and North Dakota (Remenda et al., 1994 )
-19 to -25
Beneath Lake Agassiz sediments in ClayCounty-Geologic Atlas B (Berg, 2018)
-19 to -23
Oak Creek Fm. till, Southeast Wisconsin (Simpkins and Bradbury, 1992)
-18
This study: pore water and groundwater are not glacial age
-8.33 to -11.61
Preliminary Information-Subject to Revision. Not for Citation or Distribution.
Radioactive Isotopes
Nuclear weapon test Bravo on Bikini Atoll, Feb. 28, 1954. USDOE, public domain.
3H
Litchfield 2 Site
Pre-bomb 1953
1953
Recent
Aquifer
Till –New Ulm Fm.
Till -Good Thunder Fm.
Higher K, Moderate head gradient Lowest K, Steepest head gradient
TU=Tritium units
Preliminary Information-Subject to Revision. Not for Citation or Distribution.
1966?
Litchfield 1 Site
1967
Till –New Ulm Fm.
Till -Hewitt Fm.
Aquifer
Higher K, Steep gradient Highest K, Shallow gradient
TU=Tritium units
Preliminary Information-Subject to Revision. Not for Citation or Distribution.
Anthropogenic Tracers
Photo credit: WKOW 27 News, Madison, WI
Photo credit: University of WI, Madison, extension
Aquifer
Sewer Pipe
Ground Surface
Well
0 50 100 150 200 250 300
18
33
57
82
111
DEP
TH (F
T)
Cl/Br Mass Ratio
GW Cl/Br PW Cl/Br
Depth
GroundwaterPore water
Indic
atio
n o
f anth
ropogenic
sourc
ein
MN
gro
undw
ate
r (Berg
, 2018)
Example
Preliminary Information-Subject to Revision. Not for Citation or Distribution.
0 20 40 60 80 100
13
20
35
60
105
145
175
DE
PT
H (F
T)
Cl (mg/L)
GW-Cl PW-Cl
Olivia3H data suggest GW below 60 ft is older than 1953
Depth
Preliminary Information-Subject to Revision. Not for Citation or Distribution.
*Estimate Br <.01
0 200 400 600 800 1000 1200 1400
13
20
35
60
105
145
175
DEP
TH (F
T)
Cl/Br Mass Ratios
GW Cl/Br PW Cl/Br
Olivia3H data suggest GW below 60 ft is older than 1953
Olivia has some of the lowest Br concentrations
Depth
Preliminary Information-Subject to Revision. Not for Citation or Distribution.
0 10 20 30 40 50
18
33
57
82
111
DEP
TH (F
T)Cl (mg/L)
GW-Cl PW-Cl
Litchfield 23H data suggest GW below 57 ft is older than 1953
Depth
Preliminary Information-Subject to Revision. Not for Citation or Distribution.
0 50 100 150 200 250 300
18
33
57
82
111
DEP
TH (F
T)Cl/Br Mass Ratio
GW Cl/Br PW Cl/Br
Litchfield 2
3H data suggest GW below 57 ft is older than 1953
Depth
Preliminary Information-Subject to Revision. Not for Citation or Distribution.
0 10 20 30 40 50 60
115
137
172
196
DEP
TH (F
T)Cl (mg/L)
GW-Cl PW-Cl
HFC3H data suggest GW from 115 and 196 ft is between 1953 and ~1967.
Depth
Preliminary Information-Subject to Revision. Not for Citation or Distribution.
0 20 40 60 80 100 120 140
115
137
172
196
DEP
TH (F
T)Cl/Br Mass Ratio
GW Cl/Br PW Cl/Br
HFC
*Estimate Br <.01
*Estimate Br <.01
250
3H data suggest GW from 115 and 196 ft is between 1953 and ~1967.
Depth
Preliminary Information-Subject to Revision. Not for Citation or Distribution.
Summary
• Hydraulic head relationships suggest that groundwater in till “leaks” to the underlying confined aquifer (except Cromwell), but the “leak” magnitude reflects site-specific conditions− Travel times shorter than expected for “impermeable” till
• δ18O data suggest post-glacial groundwater age at all sites• 3H data suggest ~1960s age at Litchfield 1 and HFC sites
• Cl and Cl/Br ratio data suggest possible penetration of anthropogenic tracers (deicing salt?) to depth, particularly at urban sites
• Pore-water vs. groundwater geochemistry?−Possible explanations: sampling scale, crushing of grains with Cl,
sample contamination, and fracture/matrix interaction
Outwash
Till
Bedrock
Confined Aquifer
K
Quick Review
• Field studies demonstrated:
• Groundwater does flow through till (groundwater is “in the race”)
• Till hydraulic properties are highly variable
KKK
K K K
Water Water
Water
K
KLow
High
Hydraulic conductivity
Outwash
Till
Bedrock
Confined Aquifer
K
Quick Review
• Field studies demonstrated:
• Groundwater does flow through till (groundwater is “in the race”)
• Till hydraulic properties are highly variable
KKK
K K KWater
WaterWater
K
KLow
High
Hydraulic conductivity
Outwash
Till
Bedrock
Confined Aquifer
K
Modeling challenges
• Till hydraulic properties are variable and often unknown
• The extents of and connections between buried aquifers is largely unknown
KKK
K K K?
K
KLow
High
Hydraulic conductivity
Outwash
Till
Bedrock
Confined Aquifer
K
Questions for Modeling
• How does pumping affect groundwater flow through till?
• Where does the water enter the aquifer?
• Which model parameters are most important?
KKK
K K K?
K
KLow
High
Hydraulic conductivity
Model construction
Explanation
• MODFLOW 2000• 500 x 500 ft cells• N-to-S hydraulic gradient
of 0.001• Vertical downward
gradient=0.15• Recharge = 4 in/yr• Local area used to
calculate fluxes20
mile
s
Rivers (RIV pkg)
Lakes (DRN pkg)
Pumping wells (MNW2 pkg)
Local areaBuried
aquifer
N
5 mi
Specified head boundary
Specified head boundary
No
-flo
w b
ou
nd
ary
No
-flo
w b
ou
nd
ary
Model construction
50 ft
10,000 ft
Surficial unit
Upper till (3 layers)
Middle unit and buried aquifer (2 layers)
Pumping wellsSouth
North
Buried aquifer
Lower till (Kvand Kh fixed at 0.05 ft/day
Local area
General description of model scenarios
Surficial unit
Upper till
Lower till
Middle unit and buried aquifer
Vary vertical hydraulic conductivity (Kv) of upper till
Vary horizontal hydraulic conductivity (Kh) of the middle unit
Leakage (flux) into aquifer
The percent of the total volume of water entering the aquifer from:
Response variable
Above (downward flux)Laterally (lateral flux)
Below (upward flux)
Surficial unit
Upper till
Lower till
Middle unit and buried aquifer
Outwash
Till
Bedrock
Confined Aquifer
Leakage (flux) into aquifer
• Vary Kv of till 0.001 – 2.0 ft/d
• Vary Kh of middle unit: 0.05 – 30 ft/d
• Wide range of relative fluxes into the aquifer
10 – 84 %
15 – 90 %
0.1 - 5%
Preliminary Information-Subject to Revision. Not for Citation or Distribution.
Leakage (flux) into till from surficial unit
The percent of total surficial unit groundwater flux that flows into upper till
Response variable
Vdown
Vlat Vrech+ +Vind
Percent downward flux
= X 100 Vdown+
Surficial unit
Upper till
Lower till
Middle unit and buried aquifer
25 mi2 “local area” of model
2%
16%
35%31%
34%
46%
0%
10%
20%
30%
40%
50%
Aquifer size = 4.5 sq. miOverlying till Kv = 0.001 ft/dMiddle unit Kh = 0.05 ft/d
Pumping = 900 gpm
Aquifer size = 4.5 sq. miOverlying till Kv = 0.05 ft/dMiddle unit Kh = 0.05 ft/d
Puming = 900 gpm
Aquifer size = 4.5 sq. miOverlying till Kv = 2.0 ft/dMiddle unit Kh = 0.05 ft/d
Pumping = 900 gpm
Perc
en
t o
f Su
rfic
ial U
nit
G
rou
nd
wat
er F
lux
No Pumping
Pumping 900 gpm
Pumping-induced changes to groundwater flow into till from surficial aquifer
• Aquifer areal extent: 4.5 mi2
• Middle unit Kh: 0.05 ft/d
Scenario 32.0
Kv of Upper Till Unit (ft/d)
Scenario 10.001
Litchfield-like till Cromwell-like till
Varying till vertical hydraulic conductivity (Kv)
Surficial unit
Upper till
Middle unit and
buried aquifer
Scenario 20.05
Preliminary Information-Subject to Revision. Not for Citation or Distribution.
2% 3%5%
31%
22%
14%
0%
5%
10%
15%
20%
25%
30%
35%
Aquifer size = 4.5 sq. miOverlying till Kv = 0.001 ft/dMiddle unit Kh = 0.05 ft/d
Pumping = 900 gpm
Aquifer size = 4.5 sq. miOverlying till Kv = 0.001 ft/d
Middle unit Kh = 5 ft/dPuming = 900 gpm
Aquifer size = 4.5 sq. miOverlying till Kv = 0.001 ft/d
Middle unit Kh = 30 ft/dPumping = 900 gpm
Perc
en
t o
f Su
rfic
ial U
nit
G
rou
nd
wat
er F
lux
No Pumping
Pumping 900 gpm
Pumping-induced changes to groundwater flow into till from surficial aquiferSurficial
unitUpper
till
• Aquifer areal extent: 4.5 mi2
• Upper till unit Kv: 0.001 ft/d
Scenario 330
KH of Middle Unit (ft/d)
Scenario 10.05
Aquitard-like Aquifer-like
Varying horizontal connectivity of aquifer
Middle unit and
buried aquifer
Scenario 25.0
Preliminary Information-Subject to Revision. Not for Citation or Distribution.
Sensitivity Analysis
• Calculated the relative sensitivity of downward flux for all model runs compared to the base model
𝑃𝑒𝑟𝑐𝑒𝑛𝑡 𝑐ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑑𝑜𝑤𝑛𝑤𝑎𝑟𝑑 𝑓𝑙𝑢𝑥
|𝑃𝑒𝑟𝑐𝑒𝑛𝑡 𝑐ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑚𝑜𝑑𝑒𝑙 𝑝𝑎𝑟𝑎𝑚𝑒𝑡𝑒𝑟 𝑣𝑎𝑙𝑢𝑒|× 100
Surficial unit
Upper till
Lower till
Middle unit and buried
aquifer
Sensitivity Analysis
• Most sensitive parameters:• Kv of upper till unit• Areal extent of conductive materials:
• Buried aquifer• Lateral connectivity (middle unit Kh)
• Less sensitive parameters:• Aquifer Kh
• Pumping rate• Screen length (partial penetration)• Upper till thickness
Surficial unit
Upper till
Lower till
Middle unit and buried
aquifer
Outwash
Till
Bedrock
Confined Aquifer
Modeling conclusions
• Groundwater flows through till to a confined aquifer. Preliminary Information-Subject to Revision. Not for Citation or Distribution.
Outwash
Till
Bedrock
Confined Aquifer
Modeling conclusions
• Groundwater flows through till to a confined aquifer.
• Pumping increases the leakage of water into till
Preliminary Information-Subject to Revision. Not for Citation or Distribution.
Outwash
Till
Bedrock
Confined Aquifer
Modeling conclusions
• Groundwater flows through till to a confined aquifer.
• Pumping increases the leakage of water into till
• Leakage into till induced by pumping is sensitive to hydraulic properties of till
K KKK
K K
Preliminary Information-Subject to Revision. Not for Citation or Distribution.
Outwash
Till
Bedrock
Confined Aquifer
Modeling conclusions
• Groundwater flows through till to a confined aquifer.
• Pumping increases the leakage of water into till
• Leakage into till induced by pumping is sensitive to hydraulic properties of till
• Leakage into aquifer dependent on hydraulic properties of till
K KKK
K K10 – 84 %
15 – 90 %
0.1 - 5%
Preliminary Information-Subject to Revision. Not for Citation or Distribution.
Project Goal
Better understand the sustainability of water supplies from confined aquifers.
Approach: for different glacial lobes and till units in Minnesota, we quantified:
• Variability of till hydraulic properties
• Leakage of water from till confining units to buried confined aquifers
Rotosonic coring in Olivia, MN
Overall conclusions
• Groundwater in till is “in the race”; multiple lines of evidence for groundwater flow through till: • Hydraulic head relationships demonstrated primarily downward
flow with some faster than anticipated rates• Post-glacial groundwater found throughout till profiles• Anthropogenic tracers at depth at urban sites• Modeling showed that pumping can increase leakage rates
• Tortoise or hare? Both!• Evidence for wide-ranging leakage rates, even at a single site.
Preliminary Information-Subject to Revision. Not for Citation or Distribution.
Implications
• Implications for sustainability:• Modeling suggests that hydraulic information about till is just
as important as aquifer mapping for understanding sustainable withdrawals.
Preliminary Information-Subject to Revision. Not for Citation or Distribution.
Implications
• Implications for conceptualization of till:• Expect surprises! • Simple conceptual models are likely not applicable. • Till is not impermeable and velocities may be higher than
previously assumed.
Preliminary Information-Subject to Revision. Not for Citation or Distribution.
Questions?
Jared TrostU.S. Geological Surveyjtrost<at sign>usgs.gov
Iowa State Field Crew
Olivia, MN field site
Bill SimpkinsIowa State Universitybsimp<at sign> iastate.edu
References Cited
• Berg, J.A., 2018, Geologic Atlas of Clay County, Minnesota County Atlas Series C-29 Part B, Hydrogeology. Report to accompany Plates 6,7,8. Accessed 4/1/2019 at https://files.dnr.state.mn.us/waters/groundwater_section/mapping/cga/c29_clay/clay_report.pdf.
• Jennings, C. E., and M.D. Johnson. 2011. The Quaternary of Minnesota. Developments in Quaternary Science.15: 499-511.
• Paterson, W.S.B and Hammer C.U., 1987. Ice core and other glaciological data. In: W.F. Ruddimanand H.E. Wright (Eds.), North America and Adjacent Oceans During the Last Glaciation. Geol. Soc. Am., Decade of North American Geology. Boulder, CO, K-3: 91-109.
• Remenda, V. H., J. A. Cherry, and T. W. D. Edwards. "Isotopic composition of old ground water from Lake Agassiz: implications for late Pleistocene climate." Science266, no. 5193 (1994): 1975-1978.
• Simpkins, W. W., and K. R. Bradbury. Groundwater flow, velocity, and age in a thick, fine-grained till unit in southeastern Wisconsin. Journal of Hydrology 132, no. 1-4 (1992): 283-319.
• Staley, A., Wagner, K., Nguyen, M., and Tipping, R. 2018. Core descriptions, borehole geophysics, and unit interpretations in support of phase I and II USGS hydrologic properties of till investigation. Minnesota Geological Survey Open File Report 18-03.
Image references
https://scubasanmateo.com/explore/turtle-clipart-hare/
http://clipart-library.com/tortoise-clipart.html
http://clipart-library.com/snow-plowing-cliparts.html
https://publicdomainvectors.org/en/free-clipart/Vector-graphics-of-younger-oak-tree/24737.html
http://clipart-library.com/clipart/344789.htm
http://clipart-library.com/clipart/8cA6xxXXi.htm