Xiangyu Tang

A review of rapid transport of pesticides from sloping farmland to surface waters: Processes and mitigation strategies

Pesticides applied to sloping farmland may lead to surface water contamination through rapid transport processes as influenced by the complex topography and high spatial variability of soil properties and land use in hilly or mountainous regions. However, the fate of pesticides applied to sloping farmland has not been sufficiently elucidated. This article reviews the current understanding of pesticide transport from sloping farmland to surface water. It examines overland flow and subsurface lateral flow in areas where surface soil is underlain by impervious subsoil or rocks and tile drains. It stresses the importance of quantifying and modeling the contributions of various pathways to rapid pesticide loss at catchment and regional scales. Such models could be used in scenario studies for evaluating the effectiveness of possible mitigation strategies such as constructing vegetated strips, depressions, wetlands and drainage ditches, and implementing good agricultural practices. Field monitoring studies should also be conducted to calibrate and validate the transport models as well as biophysical-economic models, to optimize mitigation measures in areas dominated by sloping farmland.

A review of colloid transport in fractured rocks

Recent recognition of colloid and colloida-ssociated transport of strongly sorbing contaminants in fractured rocks highlights the importance of exploring the transport behavior of colloids under conditions prevailing in the field. The rapid transport of colloids through fractured rocks-as affected by the hydraulic properties of the flow system, the properties of fracture surface and the geochemical conditions-has not been sufficiently elucidated, and predictions of colloid transport through fractures have encountered difficulties, particularly at the field scale. This article reviews the current understanding of the mechanisms and modeling of colloid transport and retention in fractured rocks. Commonly used experimental techniques and approaches for conducting colloid transport experiments at different scales, ranging from the laboratory to the field scale, are summarized and commented upon. The importance of various interactions (e.g., dissolution, colloid deposition, generation, mobilization and deposition of filling materials within fractures) between the flowing solution and the fracture walls (in many cases, with skin or coating on the host rock at the liquid — solid interface) has been stressed. Colloid transport through fractures of high heterogeneity has not yet been well understood and modeled at the field scale. Here, we summarize the current knowledge and understanding accumulated in the last two decades in regard to colloid and colloid-associated transport through fractures. Future research needs are also discussed.

Assessing Water Quality of Three Gorges Reservoir, China, Over a Five-Year Period From 2006 to 2011

Understanding temporal variability in water quality in the Three Gorges Reservoir (TGR) is crucial for evaluating environmental effects of damming and protecting China’s largest freshwater resource. This study examined water quality changes in the main channel of the Yangtze River after dam completion as well as its relationship with water level fluctuation (WLF), controlled by annual impoundment operations and conditioned by flooding. Finally, the mass balance budget and integrative water quality indexing (WQI) methods were applied to elucidate the status of overall water quality since dam completion. Results showed that TGR outlet water (Yichang) exhibited higher pH and CODMn values and lower concentrations of dissolved oxygen (DO) and ammonia nitrogen (NH3-N) than inlet water (Zhutuo). Temporal variations in water quality parameters displayed similar trends for the outlet and inlet. Water quality parameters all showed negative correlations to water level, revealing the different effects of damming on water quality. It was estimated that reservoir impoundment led to a DO depletion of 1495.5 (±1482.0) × 103 tons/yr and a CODMn increase of 564.0 (±405.0) × 103 tons/yr, likely deriving from various internal pollutant loads from the WLF zone and tributary watersheds. According to WQI, TGR water quality remained at healthy levels. However, WQI linear regression showed that water quality at the outlet significantly decreased over time, indicating that the construction of the Three Gorges Dam generally caused water quality deterioration. Further investigation is required to determine the spatial distribution of point and non-point pollution sources and to identify major factors that influence TGR water quality.

Pilot study of temporal variations in lead bioaccessibility and chemical fractionation in some Chinese soils

The effect of ageing, following the addition of approximately 400mgkg(-1) lead (Pb) as Pb(NO(3))(2), on Pb bioaccessibility was examined in five typical Chinese soils using a physiologically based extraction test. Sequential extraction was employed to identify the source fraction(s) of bioaccessible Pb in the soils. Pb bioaccessibility decreased exponentially to nearly steady levels in mildly acidic or alkali (pH 6.09-7.43) soils, for both gastric (69.91-71.75%) and small intestinal (7.53-9.63%) phases within the first 2-4 weeks and 1-2 months of incubation, respectively; however, it took only 1-2 weeks for strongly acidic ( approximately pH 4.5) soils to reach nearly steady levels of Pb bioaccessibility (73.01-74.46% and 10.30-10.98% in the gastric and small intestinal phases, respectively). In addition to the water-soluble and exchangeable fractions, the carbonate fraction of mildly acidic or alkali soils appeared to be a third main source of bioaccessible Pb in the small intestinal phase; however, bioaccessible Pb was likely to derive principally from Pb in the water-soluble and exchangeable fractions of strongly acidic soils. Bioaccessible Pb in the gastric phase appeared to derive from all the fractions in all five studied soils, even the residual fraction.

Colloid-facilitated transport of lead in natural discrete fractures.

Colloid-facilitated transport of lead (Pb) was explored in a natural chalk fracture with an average equivalent hydraulic aperture of 139 microm. Tracer solution was prepared by adding montmorillonite (100 mg L(-1)) and/or humic acid (HA) (10 mg L(-1)) to modified artificial rainwater containing dissolved Pb (21.4 mg Pb L(-1)), naturally precipitated PbCO(3) particles (16.4 mg Pb L(-1)) and LiBr (39.0 mg L(-1)). We found that Pb is only mobile when associated with colloids. PbCO(3) particles were not mobile in the fracture. The addition of HA to the montmorillonite suspension increased the suspensions mobility and therefore promoted the colloid-facilitated transport of Pb. The increases in pH and sodium absorption ratio induced by the chalk-tracer solution interactions appeared to increase the dispersion and mobilization of colloids entering the fracture. The dominant colloid-facilitated transport of Pb reported in this study has significant implications for risk assessment of Pb mobility in fractured rocks.

Formation of aldehydes and carboxylic acids in ozonated surface water and wastewater: A clear relationship with fluorescence changes

This study examined the formation of aldehydes and carboxylic acids in ozonated surface water and municipal wastewater secondary effluent and addressed correlations between the generation of these compounds and concurrent changes of the fluorescence of natural/effluent organic matter (NOM/EfOM) substrates. Ozonation was effective in removing fluorophores in all excitation/emission matrix (EEM) regions, with those operationally assigned to humic- and protein-like species showing relatively higher reactivity than fulvic-like species. Examination of HO exposures and attendant changes of fluorescence-based parameters allows establishing strong linear relationships between formation of the aldehydes and carboxylic acids and the relative changes of integrated fluorescence (ΔIF/IF0). This demonstrates the feasibility of surrogate monitoring of the formation of biodegradable ozonation by-products via online measurements of water/wastewater EEM fluorescence.

137Cs depth distribution in haplic-udic ferrosols of southern China and its implication for soil erosion

The depth distribution of 137Cs (cesium-137) and its relationship with organic carbon and different particle size fractions in Haplic-Udic Ferrosols in Yujiang, Southern China, are described in this paper. Compared with other sites, the lower 137Cs inventories associated with uncultivated soils on or close to the crests indicate the relatively low 137Cs in situ retention ability of the soil. Therefore, the use of 137Cs techniques in erosion studies on uncultivated Haplic-Udic Ferrosols may create some significant difficulties and limitations, including identifying the suitable 137Cs reference site and its validity on and near the crests. The impact of the soil erosion process to a depth of more than 4 to 6 cm on the sloping dry farmland and dry terrace farmland indicates that the widely accepted assumption of uniform 137Cs depth distribution within the plough layer may not apply in these soils, although it does basically hold true for the paddy terrace field, in the area dominated by Haplic-Udic Ferrosols. For uncultivated Haplic-Udic Ferrosols, the organic matter may not control the downward migration of 137Cs because no significant relationship was found between the depth distribution of 137Cs and that of organic matter. It was also demonstrated that the 137Cs fallout is associated mainly with the clay fraction, whereas the organic matter is associated mainly with the silt fraction. For Haplic-Udic Ferrosols, the waterinduced erosion removes preferentially the clay associated with the relatively high concentration of 137Cs and low content of organic matter.

Dissolved and colloidal transport of cesium in natural discrete fractures.

Transport of cesium (Cs) was investigated in a saturated natural chalk fracture with an average equivalent hydraulic aperture of 129 microm. The results show that Cs (inflow concentration of 0.22 mmol L(-1)) can be transported in its dissolved form and in association with montmorillonite. Humic acid (HA) did not sorb Cs but enhanced colloid-associated Cs transport by 12.5% in terms of breakthrough curve (BTC) recovery. The BTCs clearly showed desorption of Cs from the fracture walls during the artificial rainwater (ARW)-injection period. Cesium transport associated with montmorillonite colloids was significant, with a maximum colloid-associated Cs C/C(0) (outflow-to-inflow concentration ratio) value of 16.6 +/- 1.1% during the tracer (colloids and LiBr)-injection period. However, the relative contribution of colloid-associated Cs transport to total Cs transport was relatively low, amounting to 10.3 +/- 0.7% and 14.5 +/- 0.7% with montmorillonite (500 mg L(-1)) and the montmorillonite-HA (10 mg L(-1)) mixture, respectively. Readsorption of Cs onto the colloids occurred immediately on switching from the tracer suspension to the background solution of ARW. The significant colloid-associated Cs transport, the stripping effect of Cs from colloids, and the slow desorption of Cs from fracture walls reported in this study have important implications for risk assessments of Cs mobility in fractured carbonatic rocks.

Dispersive solid phase extraction coupled with HPLC-UV for simultaneous determination of chlorpyrifos and 3,5,6-trichloro-2-pyridinol in soil samples

A simple method was proposed for simultaneous determination of chlorpyrifos and its hydrolysis and biodegradation product 3,5,6-trichloro-2-pyridinol (TCP) in soil samples. The analytes were extracted with ethyl acetate (EtOAc) under optimized pretreatment conditions such as the type, volume and pH of extraction solvent. Clean-up of the extract from soil was conducted by dispersive solid phase extraction (DSPE) using activated carbon as the adsorbent. High performance liquid chromatography with ultraviolet detection (HPLC-UV) was used for simultaneous determination of the two analytes. Under the optimized HPLC conditions, the limits of detection (LOD), obtained on signal-to-noise ratio (S/N) of 3, for chlorpyrifos and TCP were 0.0080 mg kg−1 and 0.0120 mg kg−1, respectively. Linear calibration curves were obtained in the range of 0.05–2.00 mg kg−1 and 0.03–2.00 mg kg−1 for chlorpyrifos and TCP, respectively. The spike recoveries from soil ranged from 89.4% to 114%, and the relative standard deviation (RSD) was in the range of 0.80%–14%.

DISTRIBUTION OF 137CS AND ORGANIC CARBON IN PARTICLE SIZE FRACTIONS IN AN ALUMI-HAPLIC ACRISOL OF SOUTHERN CHINA

The distributions of 137Cs and organic carbon and their relationships in various particle size fractions of an Alumi-Haplic Acrisol in Southern China were studied. 137Cs was enriched primarily in the clay fraction (<2 μm), indicating that 137Cs was preferentially adsorbed by finer soil particles. Organic carbon content in the clay fraction was no higher than in the very fine silt fraction (2–5 μm) for most soil samples. For different particle size fractions of the Alumi-Haplic Acrisol, the relationship between 137Cs and organic carbon could be described by a power function for the clay fraction (<2 μm) and by a linear function for the other fractions. The significant correlation (r = 0.85, P = 0.05) between 137Cs and organic carbon in the clay fraction (<2 μm) demonstrated that organic carbon contributed to 137Cs sorption in the clay of the soil samples. A particle size correction factor was determined based on the distribution of 137Cs in different particle size fractions. The value of 1.3–1.5 for the particle size correction factor indicated that selective removal of soil particles during soil erosion had an important effect on the accuracy of the estimate of the soil erosion rate for the Alumi-Haplic Acrisol of Southern China Therefore, it is necessary to consider the particle size correction factor in the quantitative models of soil erosion in the Alumi-Haplic Acrisol of Southern China characterized by a high clay content.