Oagile Dikinya

Comparative analysis of heavy metal concentration in secondary treated wastewater irrigated soils cultivated by different crops.

The use of treated urban wastewater for irrigation is a relatively recent innovation in Botswana and knowledge is still limited on its impact on soil heavy metal levels. The aim of this study is to analyze and compare heavy metal concentration in secondary wastewater irrigated soils being cultivated to different crops: olive, maize, spinach and tomato in the Glen Valley near Gaborone City, Botswana. The studied crop plots have been cultivated continuously under treated wastewater irrigation for at least 3 years. Most crop farms have sandy loam, loamy sand soils. Based on food and agriculture organization, heavy metal threshold values for crop production have been studied. Results showed that the wastewater irrigated soils in the Glen Valley have higher cadmium, nickel and copper than desirable levels, while the levels of mercury, lead and zinc are lower than the maximum threshold values recommended for crop production. The control sites show that the soils are naturally high in some of these heavy metals (e.g copper, zinc, nickel) and that crop cultivation under wastewater irrigation has actually lowered the heavy metal content. Comparing between the crops, mercury and cadmium levels are highest in soils under maize and decline linearly from maize to spinach to olive to tomato and control site. By contrast, concentrations of the other metals are at their lowest in maize and then increase from maize to spinach to olive to tomato and to control site.

Dispersion and re-deposition of fine particles and their effects on saturated hydraulic conductivity

Measurements of saturated hydraulic conductivity (K) have been used to study the rates of structural breakdown and pore clogging which occurred during leaching of 2 contrasting soils, Balkuling agricultural soil and a mining residue. Soil columns were leached with solutions of different electrolyte concentrations under saturated conditions using a Marriotte constant-head device. Measurements were made with either abrupt change of concentration from 500 to 1 mmol/L or gradual decreasing concentration from 500 through 100, 50, 10, to 1 mmol/L of NaCl followed by deionised water. The relative hydraulic conductivity (K/Ko) decreased substantially with time and with decrease in electrolyte concentrations for both soils. The decreases were attributed to various extents of internal swelling, dispersion, and decreases of pore radii as a result of detachment followed by re-deposition of the clay fraction during leaching. There was little difference in ultimate reductions in permeability between the abrupt and gradual changes of concentration from 500 to 1 mmol/L for both soils. The mining residue was clearly more prone to structural deterioration than the agricultural soil. Whereas the effluent particles from the Balkuling soil included domains or aggregates of clay crystals, those from the mining residue columns appeared to consist essentially of primary clay crystals indicating the less cohesive nature of this disturbed material. We also estimated the extent of permeability reductions using theoretical investigations based on particle size distribution. Predicted simulations agreed reasonably well with experimental data for both soils following column leaching with 1 mmol/L, with the best agreement observed for the mining residue.

Modeling the Spatio-temporal dynamics and evolution of land use and land cover (1984–2015) using remote sensing and GIS in Raya, Northern Ethiopia

This study models the spatio-temporal dynamics, patterns, and evolution of land use and land cover (LULC) (1984–2015) using remote sensing and GIS supported by in-suit measurements in Raya, Northern Ethiopia. Landsat thematic mapper (TM), and operational land imager (OLI) path 168/169 and row 051/52 were acquired from the United States Geological Survey Landsat archive. All necessary image pre-processing techniques were applied to remove the distortions due to sensors. Eight major LULC types based on a supervised image classification and maximum likelihood decision rule were identified. Post-classification change detection method was also applied to detect the dynamics in LULC. Significant change in forestland, shrub/bush land, built-up area, grassland, cropland, barren land, and floodplain areas were observed over the period 1984–2015. Considerable losses were observed in grasslands (36.9%), water body (8.7%), and floodplain areas (74.4%), while other LULC types increased. This explains why the study area is frequently affected by drought and other related disasters. An overall accuracy of 88.5, 86.5 and 90.5% were observed for the 1984, 1995 and 2015 LULC, respectively. The overall Kappa coefficient of 0.87, 0.85, and 0.90 were also observed for the same periods. Besides, the Pearson pairwise correlation matrix among the 1984–1995, 1995–2015 and 1984–2015 LULC shows positive and strong correlation (r = 0.916, r = 0.908, r = 0.914) at p < 0.005 significance level. Therefore, there is no much difference in identifying LULC types using TM and OLI products. This study is crucial to implement scientific land use policies and strategies in the study area.

Influence of sodium adsorption ratio on sodium and calcium breakthrough curves and hydraulic conductivity in soil columns

The paper examines the effects of electrolyte concentration and sodium adsorption ratio (SAR) on the relative saturated hydraulic conductivity (RHC) and the ionic behaviour of calcium (Ca) and sodium (Na) ions in the Na-Ca exchange complex. Batch binary exchange and saturated column transport experiments were carried out to quantify these effects using an agricultural Balkuling soil and a mining residue. Generally, RHC has been found to decrease with time, with increasing SAR, and with decreasing electrolyte concentration. The more rapid decrease in RHC in the mining residue, particularly at the lowest concentration (1 mmol/L), was consistent at all SAR values. The decreases in RHC were likely to be caused by partial blocking of pores by dispersed clay particles, as evidenced by the appearance of suspended clay particles in the effluent during leaching. Significant differences in RHC were observed in the passage of fronts of decreasing electrolyte concentrations for CaCl2 and SAR 15 solutions through the soil columns. These differences were attributable to structural alterations (slaking) of the media and the nature of the particles released and mobilised within the porous structure at any given point in the column. Measurements at the critical threshold concentration and turbidity concentration at SAR 15 revealed structural breakdown of the pore matrix system as evidenced by decreased RHC. The increase in SAR to 15 is initially accompanied by erratic RHC, presumably due to the break up of soil aggregates under the increased swelling forces. The less coherent mining residue soil was substantially more vulnerable to blockage of pores than the Balkuling soil in which clay particles are likely to be more readily mobilised, and hence available to re-deposit and occlude the matrix pores.

Comparison of the Instantaneous Profile Method and inverse modelling for the prediction of effective soil hydraulic properties

Soil hydraulic conductivity K(θ) and soil water retention θ(h) have been determined from a drainage experiment. Two lysimeters, one filled with a sandy soil and the other with a loamy soil, were set up for a 1-dimensional transient flow experiment. The data were collected after flooding the lysimeters with water. Soil water contents were measured by time domain reflectrometry (TDR) and pressure heads were measured by tensiometers with mercury manometers. The experimental data determined by the instantaneous profile method (IPM) were compared with the results obtained by inverse modelling. The inverse modelling proved to be superior to the IPM methodology in effective prediction of hydraulic properties. The measurable properties water content and pressure head were optimised for the following datasets: water content (WC), pressure head (P-h), and a combination of WC and P-h. For both soils the optimisation of the dataset with both WC and P-h resulted in parameters that corresponded closely to the soil hydraulic data generated by the IPM method. The correspondence for the water retention data was better than for the hydraulic conductivity data. The datasets with WC only or P-h only did not contain enough information to accurately estimate the soil hydraulic properties. In most cases the results indicated that the sandy soil gave better agreement than the loamy soil. This was attributed to the faster drainage of the sandy than the loamy soil.

Cellular automata and Markov Chain (CA_Markov) model-based predictions of future land use and land cover scenarios (2015–2033) in Raya, northern Ethiopia

Little is known about the future land use and land cover (LULC) type in some parts of Ethiopia, but not in the study area. This study aims to predict and analyze the future scenarios of LULC (2015–2033) using cellular automata and Markov Chain model (CA_Markov) by taking into consideration the physical and socio-economic drivers of LULC dynamics. The historical LULC change data of 1984–1995, 1995–2015, and 1984–2015 were used as a baseline. Both transition rules and transition area matrix for the period 1984–1995, 1995–2015, and 1984–2015 were produced quantitatively using the Markov chain model. After that, the physical and socio-economic factors were standardized using fuzzy and then Multi-Criteria Evaluation (MCE) was used to produce the transition suitability image. The CA_Markov model was then applied as a standard contiguity filter of 5 × 5 to predict the 2033 LULC condition using the TerrSet Geospatial Modeling and Monitoring System software. The result indicated that forestland are predicted to increase by 108 sq km (44.5%), shrub/bush lands 710 sq km (20%), built-up area 286.2 sq km (48.3%), and grasslands 31 sq km (15%), respectively. However, significant reductions (losses) in a water body (Wb) 5.2 sq km (11.2%), croplands (Cl) 78.9 sq km (1.3%), barren lands (Bl) 800 sq km (27.4%), and floodplain area (Fp) 251.68 sq km (33.7%), respectively. Furthermore, the Pearson correlation result between the historical and predicted LULC type indicated that there are positive, strongly correlated, and are statistically significant relationships (r = 0.981, p = 0.000). The increase in forest land and reduction in barren and flood plain may benefit the study area. However, the decrease in the water body may contribute to the severity of drought in the area. This study may help to use as useful information to foster better decisions and improve policies in land use within the framework of sustainable land use planning system.

Land Suitability Assessment of the Proposed Uranium Mining Area in North-East Botswana

Land suitability assessment of the proposed uranium mining site was made to mitigate the possible environmental hazards arising from its potential use. The FAO framework of land evaluation and guidelines for land use planning were employed to assess the land suitability for arable, pasture/grazing and forestry uses. Typical soil textures are sandy loam to sandy clay loam, generally characterized by high base saturation and good fertility. The soils are deep enough for plant growth. Water-holding capacities are favourable for most of the agronomic crops and pastures. The mining area is assessed as marginally to moderately suitable for arable farming and grazing. Poor drainage, soil salinity and rainfall are the limiting factors for arable crops, whilst poor quality of vegetation species and rainfall are the major constraints to pasture and grazing. The area is marginally suitable to forestry due to rainfall being the main constraint. Overall, the area is suitable for grazing/pastures and arable farming, with few pockets suitable for forestry use.

Variations on soil carbon dioxide flux with land-use type and selected soil properties in the hardveld of Botswana

The dynamics of carbon dioxide (CO2) flux in the hardveld regions of Botswana are poorly understood, with previous studies having concentrated on sandveld and pan environments. In particular, soil organic carbon (SOC) has a positive influence on CO2 emissions to the atmosphere. To examine the effects on soil CO2 flux of land use, namely conventional tillage, horticultural and grazing land uses, measurements were carried out in sites having similar climatic conditions. To enhance the understanding of soil carbon dynamics in these typical land uses, an investigation on the influence of soil pH, moisture content, organic carbon and temperature on CO2 flux rates was carried out. The results showed a relatively stronger dependence of CO2 flux rates on soil temperature for conventional tillage (R 2 = 0.71) and grazing (R 2 = 0.56) compared with horticultural land use (R 2 = 0.05). High SOC content in the grazing and horticultural land uses was due to increased oxidation of organic matter following soil disturbance. High organic matter turnover was probably from animal wastes and plant decomposition. Similarly, a decrease in CO2 flux with pH in the conventional tillage and grazing land uses suggests there is clear potential for these land uses to become carbon sources depending on the acidity of the soil.