Contaminations at the derma of Gaia: a case study in environmental geology

Crucial problems of natural resource conservation in Gaia are related to industrial, agricultural and urban pollutions of its atmosphere, water bodies and upper crust; today, one of the main human concerns is how to survive and to manage the environmental changes caused by widespread disasters related to improper disposal of large industrial and urban waste. During the last decade, we have developed hydro-bio-geological methods to quantitatively describe the soil and water contamination within a 4.0 km² area, including two landfill deposits and a tannery industry of Alagoinhas city, Bahia state, Brazil. Geological reconnaissance works, and a detailed geophysical survey consisting of 59 Schlumberger resistivity and time-domain induced polarization soundings, were conducted around this urban–industrial disposal area to detect and delineate the 3-D extent of an underground contamination plume. These indirect results pointed out the presence of a strong electrical conductive anomaly within the aquifer, resulting from invasive fluids from the landfills, and from the surface disposal lagoons from the tannery. Complimentarily, permanent multi-electrode logging arrays were installed on three monitoring wells, constructed with thin rings copper electrodes of 1.0 cm width, externally mounted at 0.5-m distance interval, on the entire length of the rigid PVC well casings and filter sections of such wells. In each well, the ring electrodes are accessed in different electrode combinations at the ground surface, through electrical insulated multi-cables. Repeated apparent resistivity and chargeability logs were obtained for these wells using normal electrode arrays. Such measurements were also used to characterize the plume extent and to estimate changes in water chemistry below the water table and throughout the upper vadose section of the Marizal-São Sebastião aquifer system. Chemical analyses of water samples, collected at available wells and along the neighboring Sauípe River, have shown drastic changes in the total dissolved solids, major cations and anions contents, total chromium and other heavy metals, inorganic macro-components, biochemical oxygen demand, chemical oxygen demand, nutrients and bacterial content. This work has conclusively shown that the subsurface is severely contaminated by liquids generated from these three surface sources. In addition, the monitoring technique using jointly electrical soundings and multi-electrode wells is an efficient and low-cost complement for an environmental evaluation of extensive aquifers.