Determination of total volatile organic compound emissions from furniture polishes

In this study, an environmental test chamber with controlled temperature, relative humidity, and airflow rate was developed to evaluate emissions of total volatile organic compounds (TVOCs) from three different kinds of furniture polish. The time dependence of TVOC concentrations produced from the emissions of furniture polish products in the chamber was tested. TVOC emissions from each furniture polish were compared. The main volatile organic compounds emitted from each polish were identified by gas chromatography/mass spectrometry. A double-exponential equation was developed to evaluate the characteristics of emissions of TVOCs from these furniture polish products. With this double-exponential model, the physical processes of TVOC emissions can be explained. A variety of emission parameters can be calculated. These emission parameters could be used to estimate real indoor TVOC concentrations.     

Characterization of total volatile organic compound emissions from paints

Recently, Homeswest in Western Australia and Murdoch University developed a project to construct low allergen houses (LAH) in a newly developed suburb. All potential volatile organic compound (VOC) emission materials used in LAH are required to be measured before the construction of LAH, to ensure they are low VOCs emission materials. To protect people sensitive to exposure to VOCs it is important to evaluate and select low VOCs emitting paints. In this paper, therefore, twelve different paints provided by local manufacturers were selected for analysis to characterize total volatile organic compounds (TVOC) emissions. Emissions of TVOCs from six organic solvent-soluble paints and six water-soluble paints were evaluated using a small test chamber under controlled temperature, relative humidity and air exchange rates. The major volatile organic compounds in these paints were also identified. The time dependence of TVOC emissions from paint products in the chamber was evaluated. TVOC emissions from organic solvent-soluble and water-soluble paints were compared. The influence of air exchange rate on the TVOC concentrations emitted from organic solvent-soluble and water-soluble paints was also investigated. A double-exponential equation was used to evaluate emission characteristics of TVOC from paint products. With this double-exponential model, the physical processes of TVOC emissions can be explained. A variety of emission parameters can be calculated and used to estimate real indoor TVOCs concentrations.     

Volatile organic compound emissions from wastewater treatment plants in Taiwan: legal regulations and costs of control

This study assessed volatile organic compound (VOC) emission characteristics from wastewater treatment plants (WWTPs) in five Taiwanese industrial districts engaged in numerous manufacturing processes, including petrochemical, science-based industry (primarily semiconductors, photo-electronics, electronic products and biological technology), as well as multiple manufacturing processes (primarily pharmaceuticals and paint manufacturing). The most aqueous hydrocarbons dissolved in the wastewater of Taiwanese WWTPs were acetone, acrylonitrile, methylene chloride, and chloroform for the petrochemical districts; acetone, chloroform, and toluene for the science-based districts; and chlorinated and aromatic hydrocarbons for the multiple industrial districts. The aqueous pollutants in the united WWTPs were closely related to the characteristics of the manufacturing plants in the districts. To effectively prevent VOC emissions from the primary treatment section of petrochemical WWTPs, the updated regulations governing VOC emissions were issued by the Taiwanese Environmental Protection Administration in September 2005, legally mandating a seal cover system incorporating venting and air purification equipment. Cost analysis indicates that incinerators with regenerative heat recovery are optimal for treating high VOC concentrations, exceeding 10,000ppm as CH(4), from the oil separation basins. However, the emission concentrations, ranging from 100 to 1000ppm as CH(4) from the other primary treatment facilities and bio-treatment stages, should be collected and then injected into the biological oxidation basins via existing or new blowers. The additional capital and operating costs required to treat the VOC emissions of 1000ppm as CH(4) from primary treatment facilities are less than US$0.1 for perm(3) wastewater treatment capacity.     

A dynamic two-dimensional system for measuring volatile organic compound volatilization and movement in soils

There is an important need to develop instrumentation that allows better understanding of atmospheric emission of toxic volatile compounds associated with soil management. For this purpose, chemical movement and distribution in the soil profile should be simultaneously monitored with its volatilization. A two-dimensional rectangular soil column was constructed and a dynamic sequential volatilization flux chamber was attached to the top of the column. The flux chamber was connected through a manifold valve to a gas chromatograph (GC) for real-time concentration measurement. Gas distribution in the soil profile was sampled with gas-tight syringes at selected times and analyzed with a GC. A pressure transducer was connected to a scanivalve to automatically measure the pressure distribution in the gas phase of the soil profile. The system application was demonstrated by packing the column with a sandy loam in a symmetrical bed-furrow system. A 5-h furrow irrigation was started 24 h after the injection of a soil fumigant, propargyl bromide (3-bromo-1-propyne; 3BP). The experience showed the importance of measuring lateral volatilization variability, pressure distribution in the gas phase, chemical distribution between the different phases (liquid, gas, and sorbed), and the effect of irrigation on the volatilization. Gas movement, volatilization, water infiltration, and distribution of degradation product (Br-) were symmetric around the bed within 10%. The system saves labor cost and time. This versatile system can be modified and used to compare management practices, estimate concentration-time indexes for pest control, study chemical movement, degradation, and emissions, and test mathematical models.     

Sorption kinetics and equilibria of organic pesticides in carbonatic soils from South Florida

A batch reactor was used to determine sorption kinetic parameters (k2, F, and K*) and the equilibrium sorption coefficient (K). The two-site nonequilibrium (TSNE) batch sorption kinetics model was used to calculate the kinetic parameters. Two probe organic pesticides, atrazine [2-chloro-4-ethylamino-6-isopropylamino-s-triazine] and diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea] were studied using three carbonatic soils from South Florida (Chekika, Perrine, and Krome), one noncarbonatic soil from Iowa (Webster), and one organic soil (Lauderhill) from South Florida. Carbonatic soils contained more than 600 g kg(-1) CaCO3. Sorption is initially very fast up to 3 h and then slowly reaches equilibrium. All soil-chemical combinations reached sorption equilibrium after about 24 h and all sorption isotherms were linear. The sorption kinetics data were well described by the TSNE model for all soil-chemical combinations except for the marl soil data (Perrine-Atrazine), which were better described by the one-site nonequilibrium (OSNE) model. Diuron, with higher K, undergoes slower sorption kinetics than atrazine. The Lauderhill soil containing organic carbon (OC) of 450 g kg(-1) exhibited slowest sorption kinetics for both pesticides. An inverse relationship between k3 and K was observed for atrazine and diuron separately in Chekika, Webster, and Lauderhill soils but not in Perrine and Krome soils. The sorption kinetic parameters were used to distinguish the sorption behavior between atrazine and diuron and to identify differences between soils. Normalizing the sorption coefficient (K) to OC showed that atrazine and diuron had K oc values in carbonatic soils that were a third of reported literature values for noncarbonatic soils. Using existing literature K oc values in solute transport models will most likely underestimate the mobility of atrazine, diuron, and other neutral organic chemicals in carbonatic soils.     

Identification and quantitation of volatile organic compounds emitted from dairy silages and other feedstuffs

High ground-level ozone continues to be an important human, animal, and plant health impediment in the United States and especially in California's San Joaquin Valley (SJV). According to California state and regional air quality agencies, dairies are one of the major sources of volatile organic compounds (VOCs) in the SJV. A number of recently conducted studies reported emissions data from different dairy sources. However, limited data are currently available for silage and otherfeed storages on dairies, which could potentially contribute to ozone formation. Because the impact of different VOCs on ozone formation varies significantly from one molecular species to another, detailed characterization of VOC emissions is essential to include all the important contributors to atmospheric chemistry and especially atmospheric reactivity. The present research study identifies and quantifies the VOCs emitted from various silages and other feedstuffs. Experiments were conducted in an environmental chamber under controlled conditions. Almost 80 VOCs were identified and quantified from corn (Zea mays L.), alfalfa (Medicago sativa L.),and cereal (wheat [Triticum aestivum L.] and oat [Avena sativava L.] grains) silages, total mixed ration (TMR), almond (Amygdalus communis L.) shells and hulls using gas chromatography-mass spectrometry and high performance liquid chromatography. The results revealed high concentrations of emitted alcohols and other oxygenated species. Lower concentrations of highly reactive alkenes and aldehydes were also detected. Additional quantitation and monitoring of these emissions are essential for assessment of and response to the specific needs of the regional air quality in the SJV.     

Abundances and flux estimates of volatile organic compounds from a dairy cowshed in Germany

Animal husbandry and manure treatment have been specifically documented as significant sources of methane, ammonia, nitrous oxide, and particulate matter. Although volatile organic compounds (VOCs) are also produced, much less information exists concerning their impact. We report on chemical ionization mass spectrometry and photo-acoustic spectroscopy measurements of mixing ratios of VOCs over a 2-wk measurement period in a large cowshed at the Federal Agricultural Research Centre (FAL) in Mariensee, Germany. The high time resolution of these measurements enables insight into the sources of the emissions in a typical livestock management setting. During feeding hours and solid manure removal, large mixing ratio spikes of several VOCs were observed and correlated with simultaneous methane, carbon dioxide, and ammonia level enhancements. The subsequent decay of cowshed concentration due to passive cowshed ventilation was used to model emission rates, which were dominated by ethanol and acetic acid, followed by methanol. Correlations of VOC mixing ratios with methane or ammonia were also used to calculate cowshed emission factors and to estimate potential nationwide VOC emissions from dairy cows. The results ranged from around 0.1 Gg carbon per year (1 Gg = 10(9) g) for nonanal and dimethylsulfide, several Gg carbon per year for volatile fatty acids and methanol, to over 10 Gg carbon per year of emitted ethanol. While some estimates were not consistent between the two extrapolation methods, the results indicate that animal husbandry VOC emissions are dominated by oxygenated compounds and may be a nationally but not globally significant emission to the atmosphere.     

Alcohol, volatile fatty acid, phenol, and methane emissions from dairy cows and fresh manure

There are approximately 2.5 million dairy cows in California. Emission inventories list dairy cows and their manure as the major source of regional air pollutants, but data on their actual emissions remain sparse, particularly for smog-forming volatile organic compounds (VOCs) and greenhouse gases (GHGs). We report measurements of alcohols, volatile fatty acids, phenols, and methane (CH4) emitted from nonlactating (dry) and lactating dairy cows and their manure under controlled conditions. The experiment was conducted in an environmental chamber that simulates commercial concrete-floored freestall cow housing conditions. The fluxes of methanol, ethanol, and CH4 were measured from cows and/or their fresh manure. The average estimated methanol and ethanol emissions were 0.33 and 0.51 g cow(-1) h(-1) from dry cows and manure and 0.7 and 1.27 g cow(-1) h(-1) from lactating cows and manure, respectively. Both alcohols increased over time, coinciding with increasing accumulation of manure on the chamber floor. Volatile fatty acids and phenols were emitted at concentrations close to their detection limit. Average estimated CH4 emissions were predominantly associated with enteric fermentation from cows rather than manure and were 12.35 and 18.23 g cow(-1) h(-1) for dry and lactating cows, respectively. Lactating cows produced considerably more gaseous VOCs and GHGs emissions than dry cows (P < 0.001). Dairy cows and fresh manure have the potential to emit considerable amounts of alcohols and CH4 and research is needed to determine effective mitigation.     

Greenhouse gas emissions from conventional, agri-environmental scheme, and organic Irish suckler-beef units

The problems of overproduction within the European Union countries and the environmental impact of agriculture have lead to the introduction of schemes that aim to reduce both. Beef (Bos taurus) production forms a large component of the Irish agricultural industry and accounts for more than one quarter of agricultural economic output. Recently, the European CAP (Common Agricultural Policy) has been re-evaluated to include supplementary measures that encompass the environmental role of agriculture rather than just the production role. A life cycle assessment (LCA) method was adopted to estimate emissions per kilogram of CO2 equivalent per kilogram of live weight (LW) leaving the farm gate per annum (kg CO2 kg(-1) LW yr(-1)) and per hectare (kg CO2 ha(-1) yr(-1)). Fifteen units engaged in suckler-beef production (five conventional, five in an Irish agri-environmental scheme, and five organic units) were evaluated for emissions per unit product and area. The average emissions from the conventional units were 13.0 kg CO2 kg(-1) LW yr(-1), from the agri-environmental scheme units 12.2 kg CO2 kg(-1) LW yr(-1), and from the organic units 11.1 kg CO2 kg LW yr(-1). The average emissions per unit area from the conventional units was 5346 kg CO2 ha(-1) yr(-1), from the agri-environmental scheme units 4372 kg CO2 ha(-1) yr(-1), and from the organic units 2302 kg CO2 ha(-1) yr(-1). Results indicated that moving toward extensive production could reduce emissions per unit product and area but live weight production per hectare would be reduced.     

A polymer-ceramic composite membrane for recovering volatile organic compounds from wastewaters by pervaporation

A composite membrane was constructed on a porous ceramic support from a block copolymer of styrene and butadiene (SBS). It was tested in a laboratory pervaporation apparatus for recovering volatile organic compounds (VOCs) such as 1,1,1-trichloroethane (TCA) and trichloroethylene (TCE) from dilute aqueous solutions. This polymer-ceramic composite membrane yielded significantly higher VOC selectivity than an SBS membrane without the ceramic support, for comparable fluxes. At VOC concentrations near 100 ppm, fluxes and selectivities of VOCs were essentially independent of the number of VOCs in solution. The liquid-side boundary layer resistance dominated mass transport. The experimental data fitted the resistance-in-series model and yielded reliable membrane permeability values. This method using this high-performing membrane offers potentially cleaner and cost-effective means of recovering VOCs from contaminated streams.     

Maximum permissible and negligible concentrations for some organic substances and pesticides

The aim of the paper is to provide interested parties the methods that were used for generic hazard assessment in The Netherlands, and the resulting so-called maximum permissible concentrations (MPCs) and the negligible concentrations (NCs) for approximately 150 organic substances and pesticides. The MPCs and NCs were derived for water, sediment, and soil. The concentration in the environment above which the risk of adverse effects was considered unacceptable to ecosystems is called the MPC. The MPCs take into account that the substances are distributed among the different environmental compartments, and are harmonized accordingly. The MPCs served as a basis for the Dutch government to set generic environmental quality standards (EQS) in The Netherlands (IWINS,[15]). EQS in turn are used by the Dutch Government to assess the environmental quality and for other environmental policy purposes. Concentrations in the environment below which the occurrence of adverse effects is considered to be negligible are called NCs. Hazards must be reduced when the environmental concentration of a substance exceeds its MPC. In-between this limits reduction of hazards is preferable. The MPC is a scientifically derived hazard limit. The NC is simply defined as 1% of the MPC. In general, there is a great demand for ecotoxicological data that currently limits a more reliable estimate of many MPCs. For water, approximately half of the MPCs are derived on the basis of four or more NOECs (no observed effect concentrations). For the other half, MPCs are based on only a few chronic or acute tests. For soil and sediment, however, almost no ecotoxicological data are available, and MPCs for those compartments have, in many cases, been derived from MPCs in water applying the equilibrium partitioning method (EqP-method), resulting in MPCs with greater uncertainty. Some of the methods and underlying assumptions that have been used may need improvement. For example, the factor between MPC and NC, the statistical extrapolation method, the method that is used for secondary poisoning, the role of the background concentrations of ‘naturally’ occurring substances, and the bioavailability and the EqP-method. There is a great need for hazard limits, and the present compilation tries to provide those as well as identifying research gaps.     

医化行业挥发性有机废气(VOCs)排放特征及防治对策

为了防治医化行业挥发性有机废气(VOCs)环境污染,推动医化行业的可持续发展,本文通过全面调查VOCs的排放源头、排放方式、防治措施,对比分析了各种末端处理技术的优劣,确定了医化行业有机废气(VOCs)防治的有效方式。     

Attenuation of methane and volatile organic compounds in landfill soil covers

The potential for natural attenuation of volatile organic compounds (VOCs) in landfill covers was investigated in soil microcosms incubated with methane and air, simulating the gas composition in landfill soil covers. Soil was sampled at Skellingsted Landfill at a location emitting methane. In total, 26 VOCs were investigated, including chlorinated methanes, ethanes, ethenes, fluorinated hydrocarbons, and aromatic hydrocarbons. The soil showed a high capacity for methane oxidation resulting in very high oxidation rates of between 24 and 112 microg CH4 g(-1) h(-1). All lower chlorinated compounds were shown degradable, and the degradation occurred in parallel with the oxidation of methane. In general, the degradation rates of the chlorinated aliphatics were inversely related to the chlorine to carbon ratios. For example, in batch experiments with chlorinated ethylenes, the highest rates were observed for vinyl chloride (VC) and lowest rates for trichloroethylene (TCE), while tetrachloroethylene (PCE) was not degraded. Maximal oxidation rates for the halogenated aliphatic compounds varied between 0.03 and 1.7 microg g(-1) h(-1). Fully halogenated hydrocarbons (PCE, tetrachloromethane [TeCM], chlorofluorocarbon [CFC]-11, CFC-12, and CFC-113) were not degraded in the presence of methane and oxygen. Aromatic hydrocarbons were rapidly degraded giving high maximal oxidation rates (0.17-1.4 microg g(-1) h(-1)). The capacity for methane oxidation was related to the depth of oxygen penetration. The methane oxidizers were very active in oxidizing methane and the selected trace components down to a depth of 50 cm below the surface. Maximal oxidation activity occurred in a zone between 15 and 20 cm below the surface, as this depth allowed sufficient supply of both methane and oxygen. Mass balance calculations using the maximal oxidation rates obtained demonstrated that landfill soil covers have a significant potential for not only methane oxidation but also cometabolic degradation of selected volatile organics, thereby reducing emissions to the atmosphere.     

Effect of organic material on field-scale emissions of 1,3-dichloropropene

Soil fumigation is important for growing many fruits and vegetable crops, but fumigant emissions may contaminate the atmosphere. A large-scale field experiment was initiated to test the hypothesis that adding composted municipal green waste to the soil surface in an agricultural field would reduce atmospheric emissions of the 1,3-dichloropropene (1,3-D) after shank injection at a 133 kg ha(-1) application rate. Three micrometeorological methods were used to obtain fumigant flux density and cumulative emission values. The volatilization rate was measured continuously for 16 d, and the daily peak volatilization rates for the three methods ranged from 12 to 24 μg m(-2) s(-1). The total 1,3-D mass that volatilized to the atmosphere was approximately 14 to 68 kg, or 3 to 8% of the applied active ingredient. This represents an approximately 75 to 90% reduction in the total emissions compared with other recent field, field-plot, and laboratory studies. Significant reductions in the volatilization of 1,3-D may be possible when composted municipal green waste is applied to an agricultural field. This methodology also provides a beneficial use and disposal mechanism for composted vegetative material.     

Simultaneous chemical and sensory characterization of volatile organic compounds and semi-volatile organic compounds emitted from swine manure using solid phase microextraction and multidimensional gas chromatography-mass spectrometry-olfactometry

Swine manure is associated with emissions of odor, volatile organic compounds (VOCs) and other gases that can affect air quality on local and regional scales. In this research, a solid phase microextraction (SPME) and novel multidimensional gas chromatography-mass spectrometry-olfactometry (MDGC-MS-O) system were used to simultaneously identify VOCs and related odors emitted from swine manure. Gas samples were extracted from manure headspace using Carboxen/polydimethylsiloxane (PDMS) 85-microm SPME fibers. The MDGC-MS-O system was equipped with two columns in series with a system of valves allowing transfer of samples between columns (heartcutting). The heartcuts were used to maximize the isolation, separation, and identification of compounds. The odor impact of separated compounds was evaluated by a trained panelist for character and intensity. A total of 295 compounds with molecular weights ranging from 34 to 260 were identified. Seventy one compounds had a distinct odor. Nearly 68% of the compounds for which reaction rates with OH* radicals are known had an estimated atmospheric lifetime <24 h.     

土壤中挥发酚的测定及质量控制

采用分光光度法测定土壤中的挥发酚，详细介绍了样品预处理方法，分析过程中的质量控制及土壤中有机物、矿物油对测定干扰的消除方法。分析结果表明：平行双样的相对偏差都低于其最大允许值，加标回收率在78.6％～81．7％范围内，测定结果具有较好的精密度和准确度。     

Correlation of human olfactory responses to airborne concentrations of malodorous volatile organic compounds emitted from swine effluent

Direct multicomponent analysis of malodorous volatile organic compounds (VOCs) present in ambient air samples from 29 swine (Sus scrofa) production facilities was used to develop a 19-component artificial swine odor solution that simulated olfactory properties of swine effluent. Analyses employing either a human panel consisting of 14 subjects or gas chromatography were performed on the air stream from an emission chamber to assess human olfactory responses or odorant concentration, respectively. Analysis of the olfactory responses using Fisher's LSD statistics showed that the subjects were sensitive to changes in air concentration of the VOC standard across dilutions differing by approximately 16%. The effect of chemical synergisms and antagonisms on human olfactory response magnitudes was assessed by altering the individual concentration of nine compounds in artificial swine odor over a twofold concentration range while maintaining the other 18 components at a constant concentration. A synergistic olfactory response was observed when the air concentration of acetic acid was increased relative to the concentration of other VOC odorants in the standard. An antagonistic olfactory response was observed when the air concentration of 4-ethyl phenol was increased relative to the other VOC odorants in the standard. The collective odorant responses for nine major VOCs associated with swine odor were used to develop an olfactory prediction model to estimate human odor response magnitudes to swine manure odorants through measured air concentrations of indicator VOCs. The results of this study show that direct multicomponent analysis of VOCs emitted from swine effluent can be applied toward estimating perceived odor intensity.     

Modeling China's semiconductor industry fluorinated compound emissions and drafting a roadmap for climate protection

Fluorinated compounds (FC) are high-global warming potential (GWP) greenhouse gases used and emitted during the manufacture of silicon semiconductor devices. Following the U.S. EPA's PFC Emissions Vintage Model (PEVM), uncontrolled FC emissions are modeled as proportional to total manufactured layer area (TMLA) of silicon. FC emissions of World Semiconductor Council (WSC) charter member countries (Europe, Japan, Korea, Taiwan and the United States), which voluntarily committed in 1999 to lower FC emissions by 2010 to 10% of baseline year emissions, are modeled for the period 1995–2020. For this same period, emissions from Chinese manufacturers under alternative emission reduction scenarios are modeled. If Chinese manufacturers were to adopt a baseline year of 2005 and a reduction target of 10% below baseline year emissions to be achieved by 2020, emissions would be 3.4 MMTCO₂eq, comparable to the similarly projected controlled emissions of an average WSC charter member country (=16.3/5 MMTCO₂eq) in 2020. The relative stringency of the alternative reduction scenarios considered for China vary between 50 and 95% reduction compared to business as usual (BAU). This is comparable to the stringency of the WSC charter members’ goals for which FC emission reduction technologies are currently available.     

Designing reverse-flow packed bed reactors for stable treatment of volatile organic compounds

Reverse-flow packed bed reactors can be used to treat gaseous pollutants from chemical plants. This article describes the design and operation of a modified reverse-flow reactor (MRFR) which has a recuperator on each end of the reactor and a reaction zone in the middle. The recuperators have low thermal dispersion and the reaction zone has a high thermal dispersion, obtained by placing metal inserts into the bed, parallel with the gas flow. Performance of the MRFR during extended lean and rich conditions is determined with analytical analysis and compares well with numerical simulations of CO oxidation; however, the theory is expected to be useful for any reaction kinetics. A major advantage of this MRFR design is an extended time for the reactor to extinguish during lean conditions. This work also describes MRFR performance with internal reactor cooling, which can be used as a control mechanism to maintain reactor temperature for proper removal of volatile organic compounds.     

Nitrite formation and nitrous oxide emissions as affected by reclaimed effluent application

The effect of irrigation with reclaimed effluent (RE) (after secondary treatment) on the mechanisms and rates of nitrite formation, N2O emissions, and N mineralization is not well known. Grumosol (Chromoxerert) soil was incubated for 10 to 14 d with fresh water (FW) and RE treated with 15NO3- and 15NH4+ to provide a better insight on N transformations in RE-irrigated soil. Nitrite levels in RE-irrigated soil were one order of magnitude higher than in FW- irrigated soil and ranged between 15 to 30 mg N kg(-1) soil. Higher levels of NO2- were observed at a moisture content of 60% than at 70% and 40% w/w. Nitrite levels were also higher when RE was applied to a relatively dry Grumosol (20% w/w) than at subsequent applications of RE to soil at 40% w/w. Isotopic labeling indicated that the majority of NO2 was formed via nitrification. The amount of N2O emitted from RE-treated Grumosol was double the amount emitted from FW treatments at 60% w/w. Nitrification was responsible for about 42% of the emissions. The N20 emission from the RE-treated bulk soil (passing a 9.5-mm sieve) was more than double the amount formed in large aggregates (4.76-9.5 mm in diameter). No dinitrogen was detected under the experimental conditions. Results indicate that irrigation with secondary RE stimulates nitrification, which may enhance NO3 leaching losses. This could possibly be a consequence of long-term exposure of the nitrifier population to RE irrigation. Average gross nitrification rate estimates were 11.3 and 15.8 mg N kg(-1) soil d(-1) for FW- and RE-irrigated bulk soils, respectively. Average gross mineralization rate estimates were about 3 mg N kg(-1) soil d(-1) for the two water types.