Kenneth K. Tanji

Soil salinization in the Nile Delta and related policy issues in Egypt

Abstract The soil salinization problem in Egypts Nile Delta and related policy issues are addressed in regards to the need to create water supplies for new irrigation projects by, for instance, re-use of irrigation drainage waters and limits on rice plantings, and at the same time improving the agricultural productivity of the Nile Delta through, for instance, subsurface drainage in water-logged lands, land leveling and use of gypsum amendments. A brief background on the climate, agriculture and crop rotation patterns, and water and land resources and their salinities is provided. The paper describes countermeasures taken by the government on a national and regional scale and farmers on a local or field scale. The farmers participatory role in adapting to changes in cropping, water and soil salinity, and national needs are crucial. Emphasis is laid on the advantages and benefits of rice cultivation in drainage- and salt-impacted lands of the northern Delta. But use of rice paddies to control salinity is faced with a number of constraints such as periodic shortfalls in supply of irrigation water, salinity of supply waters consisting of a blend of fresh and drainage waters and diversified cropping, including rice, in a given subsurface drainage system. We strongly support the recommendations for rice cultivation only in saline soils of the Delta but perceive that enforcement of such a policy may be difficult to achieve and long-term changes in salinity of delta waters are not clearly known with increased drainwater re-use.

Production of dissolved organic carbon (DOC) and trihalomethane (THM) precursor from peat soils.

Water passing through the Sacramento-San Joaquin Delta contains elevated concentrations of dissolved organic carbon (DOC) and trihalomethane (THM) precursor relative to upstream waters from the Sacramento River and the San Joaquin River. Drainage from agricultural peat soils has been identified as one of the major sources of DOC and THM precursor. A series of controlled laboratory experiments were conducted to evaluate abiotic and biotic effects on the quantity and the nature of DOC and THM precursors produced from oxidized surface and reduced subsurface soils in the Delta. For abiotic effects, DOC was extracted from both soils with synthetic solutions containing a range of salinity (0-4 dS/m) and sodicity (0 to infinity ). The results showed that an increase in salinity significantly decreased the concentration of DOC in the soil-water from both soils but increased its aromaticity, as indicated by specific ultraviolet absorbance at 254 nm (SUVA). For biotic effects, peat soils were incubated over a range of temperatures (10 degrees C, 20 degrees C and 30 degrees C) and soil moisture contents (0.3-10 g water/g soil). After 8 weeks of incubation, only extracted DOC from flooded conditions and flooded and non-flooded cycles showed an increase in DOC. These findings indicate that neither salinity nor sodicity is the major factor for DOC production, but both can affect the solubility and mobility of DOC in the Delta soils. We believe wetting processes in oxidized peat soils produce significant amounts of DOC found in agricultural drainage discharged into the Delta waters.

PHYSICAL AND MATHEMATICAL MODELLING OF ANAEROBIC DIGESTION OF ORGANIC WASTES

Anaerobic digestion of the organic food fraction of municipal solid waste (OFMSW), on its own or co-digested with primary sewage sludge (PSS), produces high quality biogas, suitable as renewable energy. We report the results from one such bench scale laboratory experiment, on the co-digestion of OFMSW and PSS. The experiment used a continuously stirred tank reactor and operated at 36°C for 115 days. Prior to the experiments, activity tests verified that the inoculum sludges were suitable for the biodegradation of the volatile fatty acid substrate and so producing biogas. The experimental data were used to develop and validate a two-stage mathematical model of acidogenesis and methanogenesis. In simulating the behavior of the anaerobic digestion process, including ammonia inhibition, the mathematical model successfully predicts the performance of methane production. Simulations of the pH and ammonia in the MSW anaerobic reactor were also satisfactory. Sensitivity analysis on the 18 model parameters indicated that eight of these parameters were in the most sensitive and highly sensitive range, while the remainder were in the moderate to least sensitive range.

Characterizing redox status of paddy soils with incorporated rice straw

Redox status is one of the more difficult soil quality criteria to characterize especially in paddy soils. The soil system chosen in this study was an ongoing paddy field trial on alternative practices of incorporating rice straw instead of traditional burning of straw. Upon flooding, rapid changes in redox potential (Eh) occur in paddy soil due to the decomposition of soil organic matter (SOM), including rice straw. This investigation evaluated three methods of characterizing redox status: (i) Eh, a conventional method using Pt black electrode; (ii) terminal electron-accepting processes (TEAPs), a method of diagnosing microbially mediated electron acceptor (oxidized species) consumption, intermediate product concentration of H2 and accumulation of final products (reduced species); and (iii) oxidative capacity (OXC), a comprehensive analysis of the algebraic sum of oxidized and reduced species into a single descriptive parameter. There was a need to develop a sampling technique for anaerobic soil pore water in rice paddy (without exposure to the atmosphere) to measure all the redox parameters required by these methods. The setup comprised of a capped piezometer (sampling well) back-filled with sand and sealed with bentonite, and vacuum extraction of pore water containing dissolved constituents. The pore water containing dissolved gases was collected into previously evacuated 400-ml two-port PVC bags from which gaseous phase sub-samples were withdrawn with a syringe into 10-ml vacutainers previously flushed with N2 gas for H2 and CH4 determinations in the laboratory. Unstable water quality parameters such as DO, Eh, pH and S2− were measured on site using fresh samples, while other parameters such as NO3−, SO42−, EC, Fe(II) and Fe(III), and Mn(II) were analyzed in the laboratory. The above three methods of evaluating redox status were applied to monitoring the paddy pore water over 3 years of field observations (1997–1999). Eh is a simple measure, but it gives at best only qualitative assessment of redox status because this electrode may not respond to many of the important redox couples. The adaptation of TEAPs to assess non-equilibrium redox conditions in flow pathways in large-scale ground water systems to small-scale rice rootzone was not entirely successful because of differences in the two systems and the significant overlapping among electron acceptors in paddy soils. OXC, a non-equilibrium capacity-type redox parameter, clearly identified geochemical redox classes of oxic, post-oxic, sulfidic and methanic conditions in the paddy pore waters during the course of the rice-growing season. Based on data observed for 3 years, it is concluded that straw incorporation does enhance more reducing conditions development compared to that without straw incorporation. We conclude that OXC provides a better characterization of redox status in paddy soils.

Groundwater contamination by trace elements

Abstract Trace element contamination of groundwaters by agricultural activities is addressed. Accumulation o of trace elements in groundwaters from natural sources is differentiated from agriculture-induced contamination. An analysis of reactivity and mobility of trace elements indicates that cationic trace elements (mainly heavy metals) tend to be strongly retained by earth materials owing to ion exchange, sorption and mineral solubility, and do not typically accumulate in groundwaters to very high concentration levels, the exception being shallow soil profiles with coarse-textured soils and large water fluxes. In contrast, anionic trace elements (mainly oxyanions), although retained by earth materials (clays and sesquioxides) to some extent, are subject to greater mobility and may accumulate to high concentrations in groundwaters. A case study on application of sewage sludges to cropped soils indicates that heavy metals (Cd, Cr, Cu, Ni, Pb, Zr) accumulated almost entirely on the surface 15 cm soil depth owing to sorption by clays and hydrous oxides of Fe and Mn, complexation by soil organic matter, and precipitation as inorganic minerals. A second case study is presented on trace element accumulation in shallow groundwaters under irrigated lands in the San Joaquin Valley, California, where Se toxicosis to fish and waterfowl was recently discovered resulting from the disposal of saline agricultural drainage waters containing naturally occurring toxic elements. The shallow-groundwater problem is described, with extensive data on trace element contamination but with a principal focus on Se, As, Mo and B. A study of the processes affecting the distribution and mobility of Se indicates that it is highly correlated to salinity levels in the groundwater, and its accumulation of up to 4200 μg l−1 is the result of leaching of soil Se derived from the Coast Range mountains. These mountains are composed of marine sedimentary rocks and subsequent evaporative salinization of shallow groundwaters, as is documented by delta 18O2 data. The practice of irrigation in this arid climate is shown to have escalated the toxic element problem by mobilizing soil Se, and the particular design of artificial subsurface drainage used in this area appears to collect high concentrations of Se from deeper portions of the shallow groundwater.

PESTICIDE OCCURRENCE IN GROUNDWATER IN TULARE COUNTY, CALIFORNIA

Geographic Information Systems (GIS) and statistical methodswere used to identify the major factors affecting pesticideleaching in groundwater from agricultural fields in TulareCounty, California. Residues of bromacil, diuron, and simazineincreased in groundwater during the 1980s. Bromacil, diuron,and simazine contamination were positively correlated to cropdiversity and water demand. Diuron and simazine were positively correlated to groundwater depth and negatively correlatedto soil water-holding capacity. DBCP concentration in groundwater was related to the crop coverage. The Goss model wasused to examine soil-pesticide interactions and a PesticideContamination Index (PCI) was developed. Areas having highleaching potentials were mainly associated with citrus andorchards and coarse-textured sandy soils along the SierraNevada foothills, while areas having low leaching potentialswere associated with field crops and clay soils of the southwest region. The PCI was largest for DBCP during the 1980s,suggesting that it was the most significant contaminant before1977 when it was widely used; however, wells were not testedfor this pesticide during that period. Twelve years after DBCPwas banned, it was still the most significant health riskcontaminant. Spatial maps showing the distribution of leachingpotentials and soil interactions for these pesticides canprovide useful information to regulatory and planning agenciesfor land use planning and pesticide management.

Selenium Removal from Irrigation Drainage Water Flowing Through Constructed Wetland Cells with Special Attention to Accumulation in Sediments

A flow-through experimental wetland system has been under investigation since 1996 to remove selenium (Se) fromagricultural drainage water in the Tulare Lake Drainage Districtat Corcoran, California, U.S.A. The system consists of ten cellswhich have dimensions of 15 × 76 m continuously flooded andvarious substrates planted. The objectives of this article are topresent the overall performance in Se removal after establishingthe wetland for three years, and to examine factors affecting Seremoval with special attention to accumulation in the sediments.In 1999, The wetland cells reduced Se from inflow water by 32 to65% in concentration and 43 to 89% in mass. Vegetationplays an important role in Se removal as non-vegetated cellshowed the least removal of Se. The inflow drainage water wasdominated by selenate (Se(VI), 91%) with smaller percentages ofselenite (Se(IV), 7%) and organic Se (org-Se(II-), 2%). Theoutflow water from the cells contained an average of 47% Se(VI),32% Se(IV) and 21% org-Se indicating reduction processesoccurring in the wetland cells. The surface sediment appears as alarge sink of Se removal. The highest Se concentration was foundin fallen litter, followed by the fine organic detrital layer onthe sediment surface. The sediment Se concentration dramaticallydecreased with increasing sediment depth. The mass distribution of Se, however, was sediment (0-20 cm) > fine detrital matter >fallen litter. Fractionation of surface sediment (0-5 cm) reveals that elemental Se was the largest fraction (ave. 47%) followedby organic matter-associated Se (34%). Soluble, adsorbed, and carbonate-associated Se accounted for 1.2, 3.1 and 2.5% ofthe total sediment Se, respectively. The major Se sink mechanism in the cells is the reduction of selenate to elemental Se andimmobilization into the organic phase of the sediments.

QSAR treatment of electronic substituent effects using frontier orbital theory and topological parameters

A methodology for the estimation of Hammett substituent constants from computational-based descriptors utilizing quantitative structure activity/property relationships (QSAR/QSPR) formalism is presented. Electronic descriptors derived from quantum chemical calculations and molecular topology were used to generate computational-based analogues of empirical Hammett substituent constants from statistical analysis. Global quantum chemical reaction indices were drawn from frontier orbital theory and density functional theory and formulated from AM1-based calculations. A localized index based on the electrotopological state index was used to encode information on individual group properties. From a training set consisting of 150 meta and para-substituted benzoic acids, statistical analysis of computational-based descriptors as a function of empirical substituent constants yielded a five-parameter QSAR/QSPR model which generates computational-based constants exhibiting a strong correlation with empirical values (r2 = 0.958). Both internal (PRESS) and external (independent testing set of benzoic acids) validation procedures suggest that the electronic effects QSAR/QSPR model derived in this work from computational-based parameters is a statistically viable paradigm. Both predicted and empirical constants were used in Hammett-type validation analyses as functions of chemical, biological, and spectroscopic data for thirty structurally diverse meta and parasubstituted aromatic testing sets. Statistical measures of ensuing correlations were examined and compared, and the empirical and predicted results were of similar quality. Validation results reveal that a large number of computational-based substituent constants can be accurately estimated from semiempirical AM1 frontier orbital energies and electronic structure information obtained directly from substituted benzoic acids without the aid of empirical parametrization.

COMPUTER SIMULATION MODELING FOR NITROGEN IN IRRIGATED CROPLANDS

ABSTRACT This paper reviews the state of the art of nitrogen simulation modeling of the soil-plant-water system in irrigated croplands, and identifies research needs. It focuses on only those models that have been formulated at the systems level for field application and not on the single-process mathematical models. The modeling approaches taken to simulate the physical, biological, and chemical mechanisms are empirical, mechanistic, or combinations of both. The programming languages used are either FORTRAN or CSMP. The system being modeled is an exceedingly complex one which is difficult to simulate let alone verify the simulated results. These models contain parameters and coefficients, some of which are not commonly measured in the field and have to be approximated. But, these modeling efforts are providing an increased insight into the system and better conceptual understanding. It appears that we do not have a single field experiment in the literature by which a nitrogen simulation model can be adequately documented. A concerted joint effort is of high priority to design, monitor, and evaluate data taken under field conditions to determine the credibility of these models.

Processes and conditions affecting elevated arsenic concentrations in groundwaters of Tulare Basin, California, USA

Abstract High concentrations of arsenic (As) in the shallow and deep groundwaters of the Tulare Basin in California, USA, pose a hazardous risk to drinking water supplies. Both natural processes and anthropogenic activities affect As concentrations and movement in groundwater in this region. Agricultural activities, particularly irrigation and drainage, enhance As mobilization; and high evapotranspiration rates concentrate As in surface and shallow groundwaters. The complexities of interactions among multi aquifers and water flow present challenges in identifying the source of As and its movement. Important biogeochemical processes affecting As solubility and mobility include adsorption/desorption and redox reactions. The later processes, which transform inorganic arsenate [As(V)] to arsenite [As(III)], remain poorly understood as to what extent they affect As concentration and its mobilization into groundwater. The presence of impermeable clay layers, e.g., E-clay, and their roles in potentially retarding As movement into deeper aquifers are also not well understood because significant increase of vertical leakage was predicted by computer modeling due to drilling of many irrigation wells penetrating impermeable clay layers. Future research on the reactivity of As with sediments through biogeochemical processes under natural physical setting conditions, as well as reactive transport modeling are necessary to better our understanding of As mobility and contamination in the groundwaters of the Tulare Basin.