Monitoring changes in soil moisture during artificial infiltration with geophysical methods

Derek Lichtner 1 , Jonathan Nyquist 1 , Laura Toran 1 , Li Guo 2 , and Henry Lin 2 (1) Earth and Environmental Science, Temple University, Philadelphia, PA 19122 (2) Crop and Soil Sciences, Penn State University, University Park, PA 16802 From Lin, 2010.

Funding was provided by NSF EAR-0725019 for the Susquehanna/Shale Hills Critical Zone Observatory and the Temple CST URP

Flow

in the Vadose Zone

Important for: 1) 2) 3) 4) Making agricultural decisions Understanding contaminant propagation Describing groundwater recharge Understanding soil formation

The Critical Zone Vadose zone Unconfined Aquifer

Susquehanna Shale Hills CZO

• • • 7.9-ha forested research site in Huntingdon County, PA Ephemeral stream runs roughly east to west Weikert series soil: a well-drained, shallow soil

Geophysical Methods

• Surface reflection GPr • Ground wave GPR • Electrical resistivity tomography

Artificial Infiltration Experiments

Artificial Infiltration Experiments

• • • 53 L or 26.5 L of water at constant head Horizontal flow Geophysical data were collected at 15 minute intervals

Ground-Penetrating Radar (GPR)

Tx Rx GPR signal trace GPR images: • Reflective interfaces with contrasting dielectric permittivities, e.g. • soil layers, moisture Scattering objects, e.g. rocks, tree roots 800 MHz, 1 GHz, and 2.3 GHz antennas were used

GPR Data Processing

• • • • Software: MatLab scripting MatGPR opensource add-on Reflex2DQuick Surfer 11 Gridding Strong reflectors (weathered shale) Heterogeneous soil/root fabric Example radargram of Weikert soil site, pre-infiltration

New Approach: Surface Reflection GPR air Tx Rx dry soil Tx Rx Elevated GPR unit • • Stronger surface reflections wet soil • • GPR unit is elevated Increased reflection amplitudes where the soil is moist Water content is proportional to the surface reflection coefficient Travel time proportional to microtopography

Microtopography from off-ground GPR

Subsurface flow followed this topography

Position, W to E (m) • Relative elevations determined from off-ground GPR travel times • 5 cm contour interval

Time-lapse GPR Surface Amplitude

(%) (%)

1. At end of 26.5 L (7 gal) injection

(%)

3. 45 minutes after injection ended

(%)

2. 15 minutes after injection ended 4. At end of additional 26.5 L (7 gal) injection

• Perspective is overhead map view • Blue = percent increases in soil moisture, orange = background • Water appears rapidly and subsequently fades

Another Approach: Ground Wave GPR air layer ε 1 Tx air wave ground wave layer ε 2 refracted wave Propagation paths of GPR waves in soils. After Huisman et al., 2003.

Rx • • • • Air wave arrives first Ground wave arrives second Water content is proportional to the difference in arrival times Lower velocity ground waves indicate higher moisture content

Ground Wave GPR

Ground Wave Delay due to Moisture

Unwetted Trace: Wetted Trace:

Normalized Amplitude Normalized Amplitude After wetting: Ground wave delay • Left: Unwetted GPR signal trace is very reproducible • Right: Ground wave in wetted soil shows delay and amplitude increase Before After Before After

Ground Wave Arrival Time Picking

Air wave echo Air wave Distance (m) Slow ground wave = moist soil • Air wave (top red line) and ground wave arrivals (bottom red line) • Ground wave velocity is dependent on moisture

Time-lapse Soil Water Content

Large increases at grid’s center fade with time 2 nd infiltration more to E with microtopography Position, W to E (m) Time-lapse water contents calculated with ground wave GPR Map view Trench Ground wave line Soil grid

Electrical Resistivity

• Super Sting R8 Resistivity meter with 28 electrodes • Changes in resistivity are proportional to changes in saturation battery voltmeter V subsurface with apparent resistivity ρ equipotential lines electric field lines

Time-lapse Resistivity

Trench Position, W to E (m) Water promotes near surface current, creating positive inversion artifact Conductive anomaly from water At end of 53 L injection Anomaly fades slightly with time, spreads 30 minutes after injection ended Greater extent after 2 nd infiltration At end of additional 53 L injection • Negative percent-changes in resistivity (blue) correspond to increases in soil moisture • Increased water contents appear quickly and subsequently fade, indicating rapid infiltration

• •

Conclusions

Infiltration followed the site’s microtopography to the S and SE Infiltration was rapid, with geophysical signatures strongest at the conclusion of injection and fading with each subsequent 15 minute measurement

Off-Ground GPR Ground Wave GPR Resistivity

At end of 53 L injection 30 minutes after injection ended • A second infiltration pulse utilized already activated flowpaths

Questions?

At end of additional 53 L injection