Image processing-based study of soil porosity and its effect on water movement through Andosol intact columns

The soil pore network and marcoporosity are important factors affecting water and solute transport. The transfer of contaminants to water resources is of particular importance in the Valle de Bravo watershed as it provides 10% of the drinking water for the 20 million inhabitants of Mexico City. This watershed is composed mainly of Andosols with unique mineralogical and physical characteristics. Soil porosity is usually examined on thin sections, using various image analysis techniques. We propose a novel methodology combining image analysis and a displacement experiment to study relationships between soil structure and water tracer transport parameters. H₂¹⁸O displacement experiments were conducted through intact soil columns sampled at three depths from a representative cultivated Andosol profile. The soil structure and pore characteristics were obtained by image analysis on thin sections obtained from each column at the end of the displacement experiment. The total 2D porosity (for pores larger than 50 μm) varied from 80% of the total section area in the topsoil to around 60% in the subsoil. Tubular pores were the most abundant in the soil profile, but ploughing of the topsoil had destroyed sections of these pores and replaced them with packing pores. Water transport in the intact subsoil columns was always in physical non-equilibrium, showing the existence of preferential flow pathways. In the topsoil, one column out of three showed no preferential flow, demonstrating that soil ploughing also homogenised pore connections. Pore connectivity was larger in the ploughed topsoil than in their deeper soil horizon counterparts. Our methodology offers a 2D quantitative characterisation of the macroporous network at 50 μm resolution and the determination of water transport parameters on the same intact soil samples. 3D characterisation of soil porosity using X-ray computed tomography (CT) gives a better picture of pore connection but usually has lower spatial resolution and a larger cost.     

How Uncontrolled Urban Expansion Increases the Contamination of the Titicaca Lake Basin (El Alto,La Paz,Bolivia)

Cities in developing countries encounter rapid waves of social transformation and economic development where the environment is mostly a neglected aspect. The Katari watershed encompasses mining areas, El Alto city (one of the fastest growing urban areas in South America and the biggest in the Altiplano) as well as agricultural areas. Its outlet is Cohana Bay, one of the most polluted areas of Lake Titicaca. Here we propose an integrative approach (hydrological, physicochemical, chemical and bacterial data) to understand the pollution problem of this developing area, in which a variety of anthropogenic activities takes place. Both mining and urban areas appear to be sources of metal pollution. Nutrient and bacterial contaminations are mainly related to urban and industrial discharges. These situations have impacts in the basin from the mining area down to Cohana Bay of Lake Titicaca. Pollutant concentration patterns are highly influenced by seasonal hydrology variations. The poor quality of surface waters in the basin represents a risk for human and animal populations, as well as for the quality of aquifers located underneath El Alto city.     

(Methyl)Mercury,Arsenic, and Lead Contamination of the World’s Largest Wastewater Irrigation System: the Mezquital Valley (Hidalgo State—Mexico)

We investigate the potential of near-infrared (NIR) spectroscopy to predict some heavy metals content (Zn, Cu, Pb, Cr and Ni) in several soil types in Stara Zagora Region, South Bulgaria, as affected by the size of calibration set using partial least squares (PLS) regression models. A total of 124 soil samples from the 0–20 and 20–40 cm layers were collected from fields with different cropping systems. Total Zn, Cu, Pb, Cr and Ni concentrations were determined by Atomic Absorption Spectrometry. Spectra of air dried soil samples were obtained using an FT-NIR Spectrometer (spectral range 700–2,500 nm). PLS calibration models were developed with full-cross-validation using calibration sets of 90 %, 80 %, 70 % and 60 % of the 124 samples. These models were validated with the same prediction set of 12 samples. The validation of the NIR models showed Cu to be best predicted with NIR spectroscopy. Less accurate prediction was observed for Zn, Pb and Ni, which was classified as possible to distinguish between high and low concentrations and as approximate quantitative. The worst model performance in cross-validation and prediction was for Cr. Results also showed that values of root mean square error in cross-validation (RMSE_cv) increased with decreasing number of samples in calibration sets, which was particularly clear for Cu, Pb, Ni and Cr content. A similar tendency was observed in the prediction sets, where RMSEP values increased with a decrease in the number of samples, particularly for Pb, Ni and Cr content. This tendency was not clear for Zn, while even an increase in RMSEP for Cu with the sample size was observed. It can be concluded that NIR spectroscopy can be used to measure heavy metals in a sample set with different soil type, when sufficient number of soil samples (depending on variability) is used in the calibration set.     

Estimation of nitrate retention in a Ferralsol by a transient-flow method

Anion retention is important in highly weathered soils that contain large amounts of iron and aluminium oxides with surfaces of variable charge. Sorption mechanisms retard anionic solute transfer through these soils. We determined the retardation factor for nitrate in highly weathered Ferralsols from New Caledonia from dynamic experiments using a transient‐flow method, and we evaluated the effect of soil solution concentration and organic matter content. A simple method with sectionable tubes was used to determine the nitrate isotherm during non‐steady‐state water flow under unsaturated conditions. The topsoil retarded the movement of nitrate, and the sorption followed a linear isotherm. In subsoils, retardation factors were larger and increased from 1.15 to 2.05 at soil pH as the NO₃^–‐N concentration of the input solution decreased from 71.43 to 0.35 mm , indicative of a non‐linear isotherm. Positive surface charge sites were considered to be of two types: one with strong affinity for nitrate at small concentrations and one with weak affinity for adsorption of nitrate at larger concentrations. This type of isotherm with high‐ and low‐energy sites is similar to those found for oxyanions and heavy metals. The related anion exchange capacity was larger than that usually observed in soils of variable charge. Not all exchange sites were detected with our method, and some sites were obviously not available for nitrate retention.