Monitoring Stormwater Redistribution Into Low Resistivity Soils Using Noninvasive Geophysical Techniques

Water held in the unsaturated zone is important for agriculture and construction and is replenished by infiltrating rainwater. Monitoring the soil water content of clay soils using ground-penetrating radar (GPR) has not been researched, as clay soils cause attenuation of GPR signal. In this study, GPR common-midpoint soundings (CMPs) are used in the clayey soils of the Miller Run floodplain to monitor changes in the soil water content (SWC) before and after rainfall events. GPR accomplishes this task because increases in water content will increase the dielectric constant of the subsurface material, and decrease the velocity of the GPR wave. Using an empirical relationship between dielectric constant and SWC, the Topp relation, we are able to calculate a SWC from these velocity measurements. Non-invasive electromagnetics, resistivity, and seismic were performed, and from these surveys, the layering at the field site was delineated. EM characterized the horizontal variation of the soil, allowing us to target the most clay rich area. At the CMP location, resistivity indicates the vertical structure of the subsurface consists of a 40 cm thick layer with a resistivity of 100 ohm*m. Between 40 cm and 1.5 m is a layer with a resistivity of 40 ohm*m. The thickness estimates were confirmed with invasive auger and trenching methods away from the CMP location. GPR CMPs were collected relative to a July 2013 and September 2013 storm. The velocity observations from the CMPs had a precision of +/- 0.001 m/ns as assessed by repeat analysis. In the case of both storms, the GPR data showed the expected relationship between the rainstorms and calculated SWC, with the SWC increasing sharply after the rainstorm and decreasing as time passed. We compared these data to auger core samples collected at the same time as the CMPs were taken, and the volumetric analysis of the cores confirmed the trend seen in the GPR, with SWC values between 3 and 5 percent lower than the GPR estimates. Our data shows that we can, with good precision, monitor changes in the SWC of conductive soils in response to rainfall events, despite the attenuation induced by the clay.

Microscopic and Spectroscopic Methods for Probing the Clay Water Interface

Clay minerals have a fundamental importance in many processes in soils and sediments such as the bioavailability of nutrients, water retention, the adsorption of common pollutants, and the formation of an impermeable barrier upon swelling. Many of the properties of clay minerals are due to the unique environment present at the clay mineral/water interface. Traditional techniques such as X-ray diffraction (XRD) and absorption isotherms have provided a wealth of information about this interface but have suffered from limitations. The methods and results presented herein are designed to yield new experimental information about the clay mineral/water interface.A new method of studying the swelling dynamics of clay minerals was developed using in situ atomic force microscopy (AFM). The preliminary results presented here demonstrate that this technique allows one to study individual clay mineral unit layers, explore the natural heterogeneities of samples, and monitor swelling dynamics of clay minerals in real time. Cation exchange experiments were conducted monitoring the swelling change of individual nontronite quasi-crystals as the chemical composition of the surrounding environment was manipulated several times. A proof of concept study has shown that the changes in swelling are from the exchange of interlayer cations and not from the mechanical force of replacing the solution in the fluid cell. A series of attenuated total internal reflection Fourier transform infrared spectroscopy (ATR-FTIR) experiments were performed to gain a better understanding of the organization of water within the interlayer region of two Fe-bearing clay minerals. These experiments made use of the Subtractive Kramers-Kronig (SKK) Transform and the calculation of difference spectra to obtain information about interfacial water hidden within the absorption bands of bulk water. The results indicate that the reduction of structural iron disrupts the organization of water around a strongly hydrated cation such as sodium as the cation transitions from an outer-sphere complex with the mineral surface to an inner-sphere complex. In the case of a less strongly hydrated cation such as potassium, reduction of structural iron actually increases the ordering of water molecules at the mineral surface. These effects were only noticed with the reduction of iron in the tetrahedral sheet close to the basal surface where the increased charge density is localized closer to the cations in the interlayer.