Integrated geophysical investigation for understanding agriculturally induced landslides in southern Peru

Abstract

This thesis presents results of near-surface geophysical surveys conducted at the Majes I agricultural development (60 km west of Arequipa, Peru) where the nexus of geology and agriculture has increased landslide activity along the Majes-Siguas River Valley. The specific project goals were to refine current understanding of local geology, investigate the local water table, and update subsequent landslide modeling to inform discussions on mitigation strategies for Majes I and prevention at the future Majes II site. To accomplish this, we conducted a combination of DC resistivity, transient electromagnetics (TEM), and seismic surveys. Interpreting geophysical data to image artificial water accumulations in such cases is best performed with either a sequence of time-lapse data with a start time prior to irrigation onset (i.e., time-lapse monitoring), or, as available here, through co-located geophysical surveys over comparable geology where the influence of anthropogenic activities is not observed (i.e., a control site). We performed the geophysical surveys within the agricultural development of Majes I, across the valley at the future site of Majes II which shares similar geology but has yet to be impacted by significant human development, and along the Majes-Siguas River Valley floor. Based on our integrated interpretation of the geophysical data we have refined the stratigraphic interpretation beneath the agricultural development, located the natural water table at Majes I and II, identified an increase in water table and water saturation at Majes I due to irrigation, and incorporated this information in landslide modeling at Majes I. The landslide modeling predicts that the pre-slip topography at Majes I prior to water presence was relatively stable, whereas the addition of excess irrigation water resulted in retrogressive failure that started at the lower slope and is now focused along the scarp near critical infrastructure including the Carretera Panamericana (Pan-American Highway). Majes II is currently stable. However, landslide prevention practices should be implemented to prevent such landslide onset at Majes II. Prevention techniques include switching to more efficient irrigation practices (drip versus flood techniques), using soil ameliorants such as polymers and/or biochar to hold water near the roots, and engineering sub-surface drainage. Combined this work shows the value of integrated hydrological and geophysical research for landslide management and optimized irrigation.