The diversity of naturally produced organohalogens

More than 3800 organohalogen compounds, mainly containing chlorine or bromine but a few with iodine and fluorine, are produced by living organisms or are formed during natural abiogenic processes, such as volcanoes, forest fires, and other geothermal processes. The oceans are the single largest source of biogenic organohalogens, which are biosynthesized by myriad seaweeds, sponges, corals, tunicates, bacteria, and other marine life. Terrestrial plants, fungi, lichen, bacteria, insects, some higher animals, and even humans also account for a diverse collection of organohalogens.     

Hypersaline cyanobacterial mats as indicators of elevated tropical hurricane activity and associated climate change

The Atlantic hurricanes of 1999 caused widespread environmental damage throughout the Caribbean and US mid-Atlantic coastal regions. However, these storms also proved beneficial to certain microbial habitats; specifically, cyanobacteria-dominated mats. Modern mats represent the oldest known biological communities on earth, stromatolites. Contemporary mats are dominant biological communities in the hypersaline Bahamian lakes along the Atlantic hurricane track. We examined the impacts of varying levels of hypersalinity on 2 processes controlling mat growth, photosynthesis and nitrogen fixation, in Salt Pond, San Salvador Island, Bahamas. Hypersalinity (> 5 times seawater salinity) proved highly inhibitory to these processes. Freshwater input from Hurricane Floyd and other large storms alleviated this salt-inhibition. A predicted 10 to 40 year increase in Atlantic hurricane activity accompanied by more frequent "freshening" events will enhance mat productivity, CO2 sequestration and nutrient cycling. Cyanobacterial mats are sensitive short- and long-term indicators of climatic and ecological changes impacting these and other waterstressed environments.     

Photolysis of atrazine and ametryne herbicides in Barbados sugar cane plantation soils and water

The photodegradation kinetics of atrazine (2-chloro-6-(ethylamino)-4-isopropylamino-1,3,5-triazine) and ametryne (2-methylthio-4-ethylamino-6-isopropylamino-s-triazine), in fresh and coastal salt water from Barbados, were measured under irradiation with artificial solar and UV254-radiation. The first-order rate constants were greater for ametryne than for atrazine, and the rates were reduced in seawater relative to fresh water, and in soil slurries relative to fresh water. However, rates were accelerated in the presence of iron(III) at pH 3 due to photo-Fenton type processes. This rate enhancement was reduced at ambient pH values (pH 7-7.5) representative of surface water in Barbados. These results have important implications for the relative persistence of these contaminants in aquatic environments in tropical areas.     

Aluminium speciation in effluents and receiving waters

The respective speciation of aluminium in sewage effluent and in river water receiving effluent, has been examined. Results showed that concentrations of reactive aluminium changed over a timescale of hours and were controlled predominantly by pH. A minimum concentration of reactive aluminium occurred at a pH of approximately 6.8, coinciding with the prevalence of non-reactive, insoluble Al(OH)3 species. For receiving waters of low pH value, typically < pH 5, a large proportion of the 'naturally present' aluminium can be present in a reactive form at concentrations higher than the proposed Environmental Quality Standard (EQS). Mixing of waters of this type with effluent of a higher pH value leads to the precipitation of aluminium hydroxide. Mixing of effluent of pH value in the range 7.5-8.0 with river water in the same (or slightly higher) pH range appears to result in no appreciable change in the proportion of reactive aluminium; the change in concentration tends to be related simply to dilution. On the basis of a theoretical knowledge of aluminium speciation, results obtained in this work indicate that it is possible to make predictions about the proportion of reactive aluminium present in a receiving water, based on the pH values of the effluent water mixture and the concentration in the effluent. Reasonable comparisons between measured and predicted values were obtained at higher pH values, but the relationship was less certain at pH values less than 6.5 for which levels of reactive metal tended to be higher than the quality standard value.     

Tracing nitrate transport and environmental impact from intensive swine farming using delta nitrogen-15

Natural-abundance delta15N showed that nitrate generated from commercial land application of swine (Sus scrofa domesticus) waste within a North Carolina Coastal Plain catchment was being discharged to surface waters by ground water passing beneath the sprayfields and adjacent riparian buffers. This was significant because intensive swine farms in North Carolina are considered non-discharge operations, and riparian buffers with minimum widths of 7.6 m (25 ft) are the primary regulatory control on ground water export of nitrate from these operations. This study shows that such buffers are not always adequate to prevent discharge of concentrated nitrate in ground water from commercial swine farms in the Mid-Atlantic Coastal Plain, and that additional measures are required to ensure non-discharge conditions. The median delta15N-total N of liquids in site swine waste lagoons was +15.4 +/- 0.2% vs. atmospheric nitrogen. The median delta15N-NO3 values of shallow ground water beneath and adjacent to site sprayfields, a stream draining sprayfields, and waters up to 1.5 km downstream were + 15.3 +/- 0.2 to + 15.4 +/- 0.2%. Seasonal and spatial isotopic variations in lagoons and well waters were greatly homogenized during ground water transport and discharge to streams. Neither denitrification nor losses of ammonia during spraying significantly altered the bulk ground water delta15N signal being delivered to streams. The lagoons were sources of chloride and potassium enrichment, and shallow ground water showed strong correlation between nitrate N, potassium, and chloride. The 15N-enriched nitrate in ground water beneath swine waste sprayfields can thus be successfully traced during transport and discharge into nearby surface waters.     

Simulating endosulfan transport in runoff from cotton fields in Australia with the GLEAMS model

Endosulfan (6,7,8,9,10,10-hexachloro-1,5,5a,6,9,9a-hexahydro-6,9methano-2,4,3-benzodioxathiepin 3-oxide), a pesticide that is highly toxic to aquatic organisms, is widely used in the cotton (Gossypium hirsutum L.) industry in Australia and is a risk to the downstream riverine environment. We used the GLEAMS model to evaluate the effectiveness of a range of management scenarios aimed at minimizing endosulfan transport in runoff at the field scale. The field management scenarios simulated were (i) Conventional, bare soil at the beginning of the cotton season and seven irrigations per season; (ii) Improved Irrigation, irrigation amounts reduced and frequency increased to reduce runoff from excess irrigation; (iii) Dryland, no irrigation; (iv) Stubble Retained, increased soil cover created by retaining residue from the previous crop or a specially planted winter cover crop; and (v) Reduced Sprays, a fewer number of sprays. Stubble Retained was the most effective scenario for minimizing endosulfan transport because infiltration was increased and erosion reduced, and the stubble intercepted and neutralized a proportion of the applied endosulfan. Reducing excess irrigation reduced annual export rates by 80 to 90%, but transport in larger storm events was still high. Reducing the number of pesticide applications only reduced transport when three or fewer sprays were applied. We conclude that endosulfan transport from cotton farms can be minimized with a combination of field management practices that reduce excess irrigation and concentration of pesticide on the soil at any point in time; however, discharges, probably with endosulfan concentrations exceeding guideline values, will still occur in storm events.     

Endosulfan transport: I. Integrative assessment of airborne and waterborne pathways

To reduce endosulfan (C9H6O3Cl6S; 6,7,8,9,10,10-hexachloro-1,5, 5a,6,9,9a-hexahydro-6,9-methano-2,4,3-benzodioxathiepin 3-oxide) contamination in rivers and waterways, it is important to know the relative significances of airborne transport pathways (including spray drift, vapor transport, and dust transport) and waterborne transport pathways (including overland and stream runoff). This work uses an integrated modeling approach to assess the absolute and relative contributions of these pathways to riverine endosulfan concentrations. The modeling framework involves two parts: a set of simple models for each transport pathway, and a model for the physical and chemical processes acting on endosulfan in river water. An averaging process is used to calculate the effects of transport pathways at the regional scale. The results show that spray drift, vapor transport, and runoff are all significant pathways. Dust transport is found to be insignificant. Spray drift and vapor transport both contribute low-level but nearly continuous inputs to the riverine endosulfan load during spraying season in a large cotton (Gossypium hirsutum L.)-growing area, whereas runoff provides occasional but higher inputs. These findings are supported by broad agreement between model predictions and observed typical riverine endosulfan concentrations in two rivers.     

Tillage, intercrop, and controlled drainage-subirrigation influence atrazine, metribuzin, and metolachlor loss

Atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] have been found with increasing occurrence in rivers and streams. Their continued use will require changes in agricultural practices. We compared water quality from four crop-tillage treatments: (i) conventional moldboard plow (MB), (ii) MB with ryegrass (Lolium multiflorum Lam.) intercrop (IC), (iii) soil saver (SS), and (iv) SS + IC; and two drainage control treatments, drained (D) and controlled drainage-subirrigation (CDS). Atrazine (1.1 kg a.i. ha-1), metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazine-5(4H)-one] (0.5 kg a.i. ha-1), and metolachlor (1.68 kg a.i. ha-1) were applied preemergence in a band over seeded corn (Zea mays L.) rows. Herbicide concentration and losses were monitored from 1992 to spring 1995. Annual herbicide losses ranged from < 0.3 to 2.7% of application. Crop-tillage treatment influenced herbicide loss in 1992 but not in 1993 or 1994, whereas CDS affected partitioning of losses in most years. In 1992, SS + IC reduced herbicide loss in tile drains and surface runoff by 46 to 49% compared with MB. The intercrop reduced surface runoff, which reduced herbicide transport. Controlled drainage-subirrigation increased herbicide loss in surface runoff but decreased loss through tile drainage so that total herbicide loss did not differ between drainage treatments. Desethyl atrazine [6-chloro-N-(1-methylethyl)-1,3,5-triazine-2,4-diamine] comprised 7 to 39% of the total triazine loss.     

The relationship between microbial carbon and the resource quality of soil carbon

The biological health of soil is an important aspect of soil quality because of the many critical functions performed by organisms in soil. Various indicators of soil quality have been proposed, but measurements of microbial biomass are most commonly used. During decomposition of plant residues in soil the relative intensities of the O-alkyl-C signal decreases and the alkyl-C signal increases in nuclear magnetic resonance (NMR) spectra. This leads to the suggestion that the alkyl-C to O-alkyl-C ratio of a soil may indicate the degree of decomposition. Consequently, the overall resource quality of soil C as a substrate for heterotrophic microorganisms may be inversely related to the alkyl-C to O-alkyl-C ratio. Our hypothesis is that a relationship exists between the size of the soil microbial community (microbial biomass) and the quality of soil carbon as a resource for microorganisms. New data have been combined with previously published data to show that there was a significant, negative correlation between the biomass C to total C (Cmic, to Corg) ratio and the alkyl-C to O-alkyl-C ratio (p < 0.01), which supports our hypothesis.     

Hypoxia in the Gulf of Mexico

Seasonally severe and persistent hypoxia, or low dissolved oxygen concentration, occurs on the inner- to mid-Louisiana continental shelf to the west of the Mississippi River and Atchafalaya River deltas. The estimated areal extent of bottom dissolved oxygen concentration less than 2 mg L-1 during mid-summer surveys of 1993-2000 reached as high as 16,000 to 20,000 km2. The distribution for a similar mapping grid for 1985 to 1992 averaged 8000 to 9000 km2. Hypoxia occurs below the pycnocline from as early as late February through early October, but is most widespread, persistent, and severe in June, July, and August. Spatial and temporal variability in the distribution of hypoxia exists and is, at least partially, related to the amplitude and phasing of the Mississippi and Atchafalaya discharges and their nutrient flux. Mississippi River nutrient concentrations and loadings to the adjacent continental shelf have changed dramatically this century, with an acceleration of these changes since the 1950s to 1960s. An analysis of diatoms, foraminiferans, and carbon accumulation in the sedimentary record provides evidence of increased eutrophication and hypoxia in the Mississippi River delta bight coincident with changes in nitrogen loading.