Enhanced Characterization of the Mississippi River Valley Alluvial Aquifer Using Surface Geophysical Methods

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Alluvial Aquifer Using Surface Geophysical Methods Presented by Ryan F.

1 Photo by Shane Stocks, U.S. Geological Survey Enhanced Characterization of the Mississippi River Valley Alluvial Aquifer Using Surface Geophysical Methods Presented by Ryan F. Adams US Geological Survey - Lower Mississippi Gulf WSC Mississippi Alluvial Plain Project U.S. Department of the Interior U.S. Geological Survey 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 may be held liable for any damages resulting from the authorized or unauthorized use of the information. 1

2 Mississippi River Valley Alluvial Aquifer (MRVA) Missouri Kentucky Tennessee Arkansas Mississippi Louisiana Alabama 2

3 Geophysical Properties Gamma-Ray Resistivity Cuttings Geology Sand Clay Jim Hoffman, MDEQ, written communication,

4 Pilot Study Characterize the aquifer system Evaluate potential sources of groundwater recharge Infiltration of precipitation and irrigation water Surface-water/groundwater exchange Use geophysical data to develop a highresolution hydrogeologic framework 4

5 Pilot Study Ground-based Shallow surveys ~30 ft below land surface Deep surveys ~250 ft below land surface Shallow soundings ~80 ft below land surface Waterborne River Surveys ~60 ft below water surface 5

6 Techniques Waterborne resistivity profile Shallow groundbased resistivity profiles Deep ground based resistivity profiles Shallow sounding 6

7 Study Site GPS Resistivity Profile

8 Shallow Surveys Waterborne resistivity profile Shallow groundbased resistivity profiles 8

9 Techniques 9

10 Cooler colors indicate finer sediments and a lower recharge rate to the aquifer Warmer colors indicate coarser sediments and a higher recharge rate to the aquifer Soils made of silty loam at different slopes 10

11 Clay rich alluvial deposit could limit vertical recharge to the aquifer from rainfall or irrigation Shallow groundbased resistivity shows good correlation to deep resistivity profiles 11

12 Deep Surveys Deep groundbased resistivity, Profile 1 12

14 Profile 1 Deep resistivity profiles provide an image of the entire thickness of the alluvial aquifer as well as the base of the aquifer. Elevation, in meters ~ 100 ft thick 0-19 ft ft ft ft ft 14

15 Deep Surveys Waterborne resistivity profile Deep groundbased resistivity profiles 15

16 Aquifer base Clay Aquifer Sand 16

Relatively higher resistivity values near the streambed indicate

17 Low resistivity values in the streambed indicate an area where surfacewater/groundwater exchange could potentially decrease Relatively higher resistivity values near the streambed indicate an area where surfacewater/groundwater exchange could potentially increase 17

18 Shallow Sounding Waterborne resistivity profile Shallow sounding 18

19 Techniques 19

Elevated total water content (red curve) up to nearly 30% at depths of 15-40 m The color image in the background shows partial water content (PWC) as a function of T2* at each depth.

20 Elevated total water content (red curve) up to nearly 30% at depths of m The color image in the background shows partial water content (PWC) as a function of T2* at each depth. T2* indicates the decay time of the NMR signal, with shorter decays related to relatively immobile water in fine-grained pores and large decays related to mobile water in larger pores. 20

21 Summary Study site consists of silty loam soils overlying a sand and gravel aquifer that terminates on a clay confining unit Clay units within the upper 50 feet could limit vertical recharge to the aquifer Comparison of the deep profiles and waterborne profiles indicates a potential pathway for groundwater-surface water exchange Shallow NMR soundings indicate that the aquifer materials have porosities between 15-30% and that the water within the aquifer materials is not bound and is free to move 21

22 Questions Contact: Ryan Adams For more information: Follow us on 22

Soils, Hydrogeology, and Aquifer Properties. Philip B. Bedient 2006 Rice University

Soils, Hydrogeology, and Aquifer Properties Philip B. Bedient 2006 Rice University Charbeneau, 2000. Basin Hydrologic Cycle Global Water Supply Distribution 3% of earth s water is fresh - 97% oceans 1%