THE EFFECTS OF CHLORINE,ELEVATED TEMPERATURE AND EXPOSURE DURATION OF POWER PLANT EFFLUENTS ON LARVAL WHITE PERCH Morone americana (Gmelin)1

ABSTRACT: Chlorine-temperature interaction studies with various exposure times were conducted on 25–day old larval white perch, Morone Americana, using total residual chlorine (TRC) concentrations of 0.0, 0.15, and 0.30 mg/1 TRC in combination with ΔTs of 2, 6, and 10 C above a base temperature of 18 C. Larval fish were exposed to the chlorine-temperature test conditions for exposure periods of 0.08, 2.0 and 4.0 hours. After each respective exposure period, chlorine concentrations were decayed naturally over a 1.0 to 1.5 hour period to < 0.01 mg/1 TRC; temperatures were decayed over a 4 hour period to 2.0 C above the base temperature. These test conditions were used to simulate chlorine and temperature conditions encountered in power plant discharge canals and near field receiving streams. The interactions of chlorine, ΔT and exposure duration as factors which caused death up to 36 hours after the exposure periods were established by regression model techniques. An initial interaction model showed that ΔT was not a factor which contributed to death. A predictive model for chlorine and exposure duration was constructed which showed that potential impact to larval white perch from chlorine at power facilities with once through cooling systems can be minimized by 1) using short duration exposures (< 1 hour) to chlorine in plants that chlorinate intermittently or 2) by rapid mixing in the receiving stream in plants that chlorinate on a low level (< 0.05 mg/1) continuous basis. Similar considerations should be given to cooling tower blowdown which contain chlorinated water.     

Stable sulfur isotope ratio indicates long-term changes in sulfur deposition in the Broadbalk experiment since 1845

Archived wheat (Triticum aestivum L.) grain and straw, and soil samples from the control plot of the Rothamsted Broadbalk Experiment, located in southeastern England and established in 1843, were used to investigate the effects of dramatically changing SO2 pollution inputs on the concentrations and stable isotope ratios (delta34S) of S in the samples. Representative coal samples from UK major coal fields were also determined for delta34S. Concentrations of S showed no clear trends in either grain or straw over the 155 years from 1845 to 1999. However, grain and straw delta34S decreased rapidly from 6 to 7/1000 in 1845 to -2 to -5/1000 in the early 1970s, and since then have increased to 0.5 to 2/1000 in the late 1990s. This pattern mirrored the trend of UK SO2 emissions over the 155 years. Both grain and straw delta34S correlated strongly and negatively with UK SO2 emissions (R2 > 0.89), but the relationships were different for the pre- and post-1970 data sets. Soil delta34S also decreased considerably, from 8.2/1000 in 1865 to 3.7 to 4.5/1000 during 1965-1999. A negative delta34S value was inferred for the anthropogenic S deposited at the experimental site before 1970, and further confirmed by negative delta34S values (-6 to -10/1000) found in the coal samples from southeastern England and southern Wales. Based on the S isotope ratios, we estimated that anthropogenic S contributed 62 to 78% of the S uptake by wheat at the peak of SO2 emissions, and accounted for 28 to 37% of the topsoil S in 1965.     

Influence of plant age and size on simazine toxicity and uptake

Improper pesticide management can lead to environmental problems such as water quality degradation and ecological stress. Recent research in our laboratory has focused on development of constructed wetlands to assimilate pesticide-contaminated water. For improved aesthetics, these wetlands have been established with ornamental plant species. The effectiveness of a plant species for phytoremediation depends in part on its tolerance for the contaminant. Plant tolerance for pesticides may vary depending on plant age and size. This study examined the influence of plant age and size on the uptake, distribution, and toxicity of the herbicide simazine [2-chloro-4,6-bis(ethylamino)-1,3,5-triazine] in two ornamental wetland plants: parrot feather [Myriophyllum aquaticum (Vell.) Verdc.] and canna (Canna x hybrida L. 'Yellow King Humbert'). Plants of different ages and sizes were exposed to simazine in 10% Hoagland's nutrient solution. Toxicity was characterized using plant growth, water uptake, and photosynthetic yield during exposure and postexposure periods. In addition, other plants were exposed to [14C] simazine in nutrient medium to characterize pesticide uptake and translocation. Four-week-old parrot feather and canna were more tolerant of simazine than two-week-old plants. The two-week-old plant tissues of both species had higher tissue burdens of simazine than four-week-old plants. Simazine was primarily accumulated in the leaves of both parrot feather and canna. These results suggest that plants in a constructed wetland designed for simazine assimilation would be more vulnerable to simazine toxicity shortly after emergence.     

Carbon isotope ratios in logged and unlogged boreal forests: Examination of the potential for determining wildlife habitat use

Due to assimilation of recycled CO₂ from litter decomposition and photosynthetic changes in carbon fractionation at low light levels, the foliage at the base of a forest is often more depleted in¹³C compared to that exposed to the atmosphere in either the canopy or in open clearings. This is referred to as the canopy effect. African research has indicated that these habitat differences in foliar ¹³C can be substantial enough to affect the carbon isotope ratios of resident fauna. Previous work documenting a 30-year chronology on moose teeth from Isle Royale National Park indicated a progressive depletion in¹³C and suggested that this could be due to forest regrowth following extensive burning. The present study examined the assumption implicit in this hypothesis that foliar ¹³C varies between open and closed boreal forest sites. I found a marginal canopy effect of 2 ¹³C difference between upper canopy and ground flora for a forest in northwestern Ontario and an average difference of 1.2 in under- and mid-story vegetation between closed forests and open clear-cuts. Because of these small differences, the utility of carbon isotope analysis in quantifying temporally integrated exploitation of deforested habitats will be low for northern boreal locations. In denser forests, such as those in the tropics or western North American where the canopy effect can be expected to be much greater, ¹³C analysis may still offer some promise for determining selection by wildlife of disturbed habitats.     

Tracing sources of sulfur in the Florida Everglades

We examined concentrations and sulfur isotopic ratios (34S/32S, expressed as delta34S in parts per thousand [/1000] units) of sulfate in surface water, ground water, and rain water from sites throughout the northern Everglades to establish the sources of sulfur to the ecosystem. The geochemistry of sulfur is of particular interest in the Everglades because of its link, through processes mediated by sulfate-reducing bacteria, to the production of toxic methylmercury in this wetland ecosystem. Methylmercury, a neurotoxin that is bioaccumulated, has been found in high concentrations in freshwater fish from the Everglades, and poses a potential threat to fish-eating wildlife and to human health through fish consumption. Results show that surface water in large portions of the Everglades is heavily contaminated with sulfate, with the highest concentrations observed in canals and marsh areas receiving canal discharge. Spatial patterns in the range of concentrations and delta34S values of sulfate in surface water indicate that the major source of sulfate in sulfur-contaminated marshes is water from canals draining the Everglades Agricultural Area. Shallow ground water underlying the Everglades and rain water samples had much lower sulfate concentrations and delta34S values distinct from those found in surface water. The delta34S results implicate agricultural fertilizer as a major contributor to the sulfate contaminating the Everglades, but ground water under the Everglades Agricultural Area (EAA) may also be a contributing source. The contamination of the northern Everglades with sulfate from canal discharge may be a key factor in controlling the distribution and extent of methylmercury production in the Everglades.     

The Peconic River: Concerns associated with different risk evaluations for fish consumption

Risk evaluation and assessment have been used as tools to regulate and manage the risks to consumers of eating self-caught fish that have high levels of contaminants. Armed with these risk assessments, health agencies issue consumption advisories, and in some cases, close some waters to fishing. Recently, regulatory agencies have used contaminant levels in fish as a benchmark for remedial action on contaminated sites, using human health risk assessment as the justification. The US Environmental Protection Agency's new surface water criterion for mercury is based on mercury levels in fish tissue. When multiple regulatory agencies have jurisdiction over the same waters or remediation site there is the potential for differing risk evaluations. Using the Peconic River on Long Island, New York as a case study, the paper examines how and why county, state, and federal health risk evaluations for fish contaminated with mercury differed. While the same risk methodology was applied by all agencies, the assessments were conducted for different purposes, applied different consumption and fish biomass assumptions, and arrived at different conclusions. The risk evaluations invoked to design fish consumption advisories use mercury levels currently in fish, and are designed to prevent current exposure. However, the risk assessments that provide a basis for remediation consider many different pathways of exposure (not just ingestion), and deal with long-term exposure. The risk evaluations, and recommendations promulgated by those agencies, differ because they have different goals, use different assumptions, and often fail to communicate among agencies. It is suggested that it is valuable to have these different levels of risk evaluations to adequately address health issues. However, there are policy implications, which include making the distinctions between the types of risk assessments, their methods and assumptions, and the rationale for these assumptions. Further, assessors and managers should involve all interested stakeholders (including regulators and state health officials) in discussions about the use of risk, the assumptions of risk assessment, and the goals of those evaluations. The difficulties in the case of the Peconic were not due to differences in the original data, but rather in the goals and type of risk assessments performed. If all deliberations had been transparent during all phases of the decision-making and management process, the conflicts within the minds of the public, regulators and other agencies might have been avoided. This case study suggests that more reliability, circumspection and transparency should be built into the process where multiple agencies and multiple objectives are involved.     

Role of Wild Game in the Diet of Recreationists in South Carolina

Risk assessors have devoted considerable attention to the consumption of fish in the diet of recreational and subsistence anglers, but little attention has been directed toward the percentage that wild game contributes to total protein intake for people who engage in hunting and fishing. While recall studies have limitations, the relative errors should be similar for different types of fish and game. We interviewed 454 people attending the Palmetto Sportsmen's Classic in South Carolina to determine their consumption patterns of domesticated animals, fish (both wild-caught and commercial) and wild game. The percentage of people who consumed each type of meat was: chicken (98%); beef (95%); wild-caught fish (79%); deer (79%); restaurant fish (73%); pork (71%); dove (47%); commercial fish (41%); wild turkey (40%), duck, squirrel and self-caught quail (about 25% each); restaurant quail (10%); and raccoon (11%). Although a similar proportion of white respondents and black respondents consumed wild game overall (90%), there were ethnic differences in the number of meals of wild-caught fish and game. Black respondents ate more wild-caught fish, rabbit, raccoon and squirrel, and less deer, than did white respondents. Wild-caught fish and game made up 50% of the meat and fish diet of black sportsmen, but only 32% for whites. Wild-caught fish and game were being eaten disproportionately more by low-income black respondents, while more deer was consumed by higher-income black respondents. The data suggest that managers and planners should take into account age, ethnicity and income when (1) conducting exposure assessments, (2) considering consumption patterns for wild-caught fish and game and (3) managing risk from wild-caught fish and game. The data will be especially useful to policy makers and risk managers who are designing consumption advisories, for risk communicators in identifying the target audience and for managers designing long-term stewardship for sites with contamination.     

Managing Water to Protect Fish: A Review of California’s Environmental Water Account, 2001–2005

The Sacramento-San Joaquin Delta, the landward reach of the San Francisco Estuary, provides habitat for threatened delta smelt, endangered winter-run Chinook salmon, and other species of concern. It is also the location of huge freshwater diversion facilities that entrain large numbers of fish. Reducing the entrainment of listed fishes into these facilities has required curtailment of pumping, reducing the reliability of water deliveries. We reviewed the first 5 years (2001–2005) of the Environmental Water Account (EWA), a program instituted to resolve conflicts between protecting listed fishes and providing a reliable water supply. The EWA provided fishery agencies with control over 0.2–0.4 km³ of water to be used for fish protection at no cost to users of exported water, and fish agencies guaranteed no disruption of water supply for fish protection. The EWA was successful in reducing uncertainty in water supply; however, its contribution to the recovery of listed fishes was unclear. We estimated the effectiveness of the EWA to be modest, increasing the survival of winter-run Chinook salmon by 0–6% (dependent on prescreen mortality), adult delta smelt by 0–1%, and juvenile delta smelt by 2–4%. Allocating EWA water for a single life stage of one species could provide larger gains in survival. An optimally allocated EWA of equal size to the median of the first 5 years could increase abundance of juvenile delta smelt up to 7% in the springs of dry years. If the EWA is to become a long-term program, estimates of efficacy should be refined. If the program is to be held accountable for quantitative increases in fish populations, it will be necessary to integrate scientific, possibly experimental, approaches.     

Interacting effects of pyrethroid and hormone on catalase and protein for toxicity amelioration in the freshwater fish, <Emphasis Type="Italic">Clarias batrachus</Emphasis>

Ameliorating effects of cortisol and T₃ on alphamethrin-dependent changes in the catalase (CAT) and protein profile of the freshwater catfish, Clarias batrachus, were studied. CAT activity and specific activity decreased in the liver, heart, kidney, and intestine after exposure to alphamethrin for 14 days. The administration of cortisol to alphamethrin-exposed fish brought the activity and specific activity of CAT up to their control values. The protein content decreased in different tissues of the fish after exposure to alphamethrin. The administration of T₃ to alphamethrin-exposed fish reduced the protein content to the control value. Therefore, it can be suggested that cortisol and T₃ may be used to ameliorate the toxicological effects of alphamethrin in fish.     

Regional differences in mercury levels in aquatic ecosystems: A discussion of possible causal factors with implications for the Tennessee river system and the Northern Hemisphere

Concern about mercury pollution from atmospheric deposition has risen markedly in the last decade because of high levels of mercury in freshwater fish from relatively pristine waters. Whereas high concentrations have been found principally in Canada, the northern United States, and Scandinavia, they have also recently been observed throughout much of Florida. Recent surveys of the Tennessee River system, however, have found no locations where fish levels exceed EPA guidelines for fish consumption. This paper evaluates a number of factors that may cause certain regions in the northern hemisphere to experience unacceptable fish mercury levels while other regions do not. Relevant regional differences include: (1) Waters of the Tennessee River system are generally nonacidic (pH>6) and well buffered, whereas 16%, 22%, and 40% of the lakes in upper Midwest, Northeast, and Florida, respectively, have acid-neutralizing capacities below 50 μeq/liter. Acidity correlates highly with fish mercury levels in a number of lake surveys, and experimental manipulations of acidity have significantly raised fish mercury levels. (2) The ratio of land area to water surface area in the Tennessee Valley averages about 30, whereas it is 15 in the upper Midwest and 6 in Florida. Low ratios allow mercury in precipitation to be directly deposited to aquatic bodies, without an opportunity for the mercury to be sequestered by terrestrial ecosystems. (3) Stream organic matter concentrations in Florida, the upper Midwest, and Sweden are 2–10 times those in the Tennessee Valley. Mercury binds strongly to organic matter, and organic matter transport in runoff is a major pathway by which mercury enters aquatic ecosystems.     

White spruce response to co-composted hydrocarbon-contaminated drilling waste: effects of compost age and nitrogen fertilization

There are growing interests to use co-composted drilling wastes contaminated with hydrocarbons as growth media for planting in land reclamation. However, such use of the compost may have potential problems such as inherent toxicity of residual hydrocarbon and microbial N immobilization due to high compost C to N ratios. We investigated the growth, biomass production, N uptake, and foliar delta13C of white spruce (Picea glauca [Moench] Voss) seedlings in a pot experiment using 1-, 2-, 3-, and 4-yr-old composts (with different hydrocarbon concentrations and C to N ratios) and a local noncontaminated soil with (200 kg N ha(-1)) or without N fertilization. Growth and N content of seedlings (particularly N content in roots) were lower when grown in the compost media as compared with those grown in the soil. Within the compost treatments seedling growth was affected by compost age, but the magnitude of growth reduction was not linearly proportional to hydrocarbon concentrations. Plant N uptake increased with compost age, which corresponds with an increase in indigenous mineral N concentration. Effects of N fertilization on N uptake were curtailed by the presence of indigenous mineral N (e.g., in the 4-yr-old compost) and by fertilization-induced stimulation of microbial activities (e.g., in the 1-yr-old compost). The differences in foliar delta13C values between seedlings grown in compost and soil (P < 0.05) suggest that limitations on water uptake caused by the residual hydrocarbon might have been the predominant factor limiting seedling growth in the compost media. This study suggests that water stress caused by residual hydrocarbons may be a critical factor for the successful use of co-composted drilling wastes as a growth medium.     

Science, Policy, Stakeholders, and Fish Consumption Advisories: Developing a Fish Fact Sheet for the Savannah River

In recent years there has been a startling rise in the issuance of fish consumption advisories. Unfortunately, compliance by the public is often low. Low compliance can be due to a number of factors, including confusion over the meaning of advisories, conflicting advisories issued by different agencies, controversies involving health benefits versus the risks from consuming fish, and an unwillingness to act on the advisories because of personal beliefs. In some places, such as along the Savannah River, one state (South Carolina) had issued a consumption advisory while the other (Georgia) had not, although at present, both states now issue consumption advisories for the Savannah River. Herein we report on the development of a fish fact sheet to address the confusing and conflicting information available to the public about consuming fish from the Savannah River. The process involved interviewing fishers to ascertain fishing and consumption patterns, evaluating contaminant levels and exposure pathways, discussing common grounds for the provision of information, and consensus-building among different regulatory agencies (US Environmental Protection Agency, South Carolina Department of Health and Environmental Control, Georgia Department of Natural Resources) and the Department of Energy. Consensus, a key ingredient in solving many different types of “commons” problems, was aided by an outside organization, the Consortium for Risk Evaluation with Stakeholder Participation (CRESP). The initial role for CRESP was to offer scientific data as a basis for groups with different assumptions about risks to reach agreement on a regulatory response action. The process was an example of how credible science can be used to implement management and policies and provide a basis for consensus-building on difficult risk communication issues. The paper provides several lessons for improving the risk process from stakeholder conflicts, through risk assessment, to risk management. It also suggests that consensus-building and risk communication are continuing processes that involve assimilation of new information on contaminants and food-chain processes, state and federal law, public policy, and public response.     

Chemical and isotopic compositions of the Minjiang River, a headwater tributary of the Yangtze River

We describe the anthropogenic impacts on the major dissolved elements (Cl(-), NO(3)(-)-N, SO(4)(2-), and Na(+)) in the water from the Minjiang River (a headwater tributary of the Yangtze River) and upper Yangtze River in relation to increasing human activity. The major element chemistry and hydrogen, oxygen, and sulfur isotopic compositions were investigated. When the Minjiang River flows through the populated Sichuan Basin, the concentrations of Cl(-), NO(3)(-)-N, SO(4)(2-), and Na(+) gradually increase. The increasing SO(4)(2-) in the highly polluted Minjiang River had high delta(34)S values (+6.3 to approximately +13.6 per thousand), implicating the anthropogenic sources of sulfur from air pollutants, domestic wastewater, industrial effluents, and agricultural fertilizers. The water quality of the upper Yangtze River does not worsen after receiving the Minjiang River because the water from the lightly polluted Jinshajiang River contributes most of the total flux in the Yangtze River. However, these rivers deserve attention and further research because the Yangtze River is the most important river in China in terms of water quality.     

Stable carbon isotope ratio and composition of microbial fatty acids in tropical soils

The soil microbial community plays a critical part in tropical ecosystem functioning through its role in the soil organic matter (SOM) cycle. This study evaluates the relative effects of soil type and land use on (i) soil microbial community structure and (ii) the contribution of SOM derived from the original forest vegetation to the functioning of pasture and sugarcane (Saccharum spp.) ecosystems. We used principal components analysis (PCA) of soil phospholipid fatty acid (PLFA) profiles to evaluate microbial community structure and PLFA stable carbon isotope ratios (delta13C) as indicators of the delta13C of microbial substrates. Soil type mainly determined the relative proportions of gram positive versus gram negative bacteria whereas land use primarily determined the relative proportion of fungi, protozoa, and actinomycetes versus other types of microorganisms. Comparison of a simple model to our PLFA delta13C data from land use chronosequences indicates that forest-derived SOM is actively cycled for appreciably longer times in sugarcane ecosystems developed on Andisols (mean turnover time = 50 yr) than in sugarcane ecosystems developed on an Oxisol (mean turnover time = 13 yr). Our analyses indicate that soil chronosequence PLFA delta13C measurements can be useful indicators of the contribution that SOM derived from the original vegetation makes to continued ecosystem function under the new land use.     

Fathead Minnow and Bluegill Sunfish Life‐Stage Responses to 17β‐Estradiol Exposure in Outdoor Mesocosms

Developmental and reproductive effects of 17β‐estradiol (E2) exposure on two generations of fathead minnows and one generation of bluegill sunfish were assessed. Fish were exposed to E2 for six continuous weeks in outdoor mesocosms simulating natural lake environments. First generation fish were exposed while sexually mature. Second generation fathead minnows were exposed either during early development, sexual maturity, or both stages. Multiple endpoints were measured to assess effects of E2 exposure on fecundity and fish health and development. Plasma vitellogenin concentrations were highly variable in all fish. Differences in egg production timing for both species indicate differences in fecundity between females exposed to E2 and controls. First generation fathead minnows exposed to E2 had lower body condition factors and reduced secondary sexual characteristic expression by males. Only a difference in relative liver weight was observed in second generation fathead minnows. First generation bluegill males exposed to E2 had significantly smaller testes compared to controls. Although fish response was highly variable, results indicate that exposure to E2 at environmentally relevant concentrations affect fathead minnow and bluegill sunfish health and development, which may have implications for the health and sustainability of fish populations. Furthermore, exposure timing and environmental factors affect fish response to E2 exposure.     

EFFECTS OF SALINITY ON PREFERRED AND LETHAL TEMPERATURES OF THE BLACKCHIN TILAPIA,SARATHERODON MELANOTHERON1

Acclimation to varying salinities did not affect the preferred temperature of Saratherodon melanotheron, which had a final preferred temperature of 33.5 C. However, fish acclimated to 25 C and 0, 15, and 30 o/oo salinities had lower lethal temperatures of 15, 13, and 15 C, respectively, and an upper lethal temperature of 37 C. The thermal zone of tolerance for fish acclimated in freshwater was 17-35 C, and was not dependent upon acclimation temperatures. Data suggest this species could expand its range in North America.     

Dissolved organic carbon and disinfection by-product precursor release from managed peat soils

A wetland restoration demonstration project examined the effects of a permanently flooded wetland on subsidence of peat soils. The project, started in 1997, was done on Twitchell Island, in the Sacramento-San Joaquin Delta of California. Conversion of agricultural land to a wetland has changed many of the biogeochemical processes controlling dissolved organic carbon (DOC) release from the peat soils, relative to the previous land use. Dissolved organic C in delta waters is a concern because it reacts with chlorine, added as a disinfectant in municipal drinking waters, to form carcinogenic disinfection byproducts (DBPs), including trihalomethanes (THMs) and haloacetic acids (HAAs). This study explores the effects of peat soil biogeochemistry on DOC and DBP release under agricultural and wetland management. Results indicate that organic matter source, extent of soil organic matter decomposition, and decomposition pathways all are factors in THM formation. The results show that historical management practices dominate the release of DOC and THM precursors. However, within-site differences indicate that recent management decisions can contribute to changes in DOC quality and THM precursor formation. Not all aromatic forms of carbon are highly reactive and certain environmental conditions produce the specific carbon structures that form THMs. Both HAA and THM precursors are elevated in the DOC released under wetland conditions. The findings of this study emphasize the need to further investigate the roles of organic matter sources, microbial decomposition pathways, and decomposition status of soil organic matter in the release of DOC and DBP precursors from delta soils under varying land-use practices.     

Modeling of power plant impacts on fish populations

A simulation model was developed for assessing the effects of power plant impacts on fish populations. The model was based on biomass changes in a fish population that divided into 4 life stages: eggs, larvae, juveniles, and adults. Metabolism of fish was included in the model as assimilation and respiration with the effects of temperature push-pull regulation. A cohort technique was used for life stage transfers in the young stages. Compensation was included in terms of density-dependent mortality of the young fish. Power plant impacts on a fish population included entrainment (eggs and larvae) and impingement (juveniles and adults).The model was applied to the gizzard shad (Dorosoma cepedianum) population in Pool 14 of the Mississippi River in Illinois and lowa. The nearby nuclear power plant, the Quad-Cities Station, is located on the Illinois side. The simulation of the model with the 1976 field data estimates of the power plant entrainment and impingement predicted a 10 percent potential reduction of the population over 30 years. The simulated reduction of the population with the effects of different river flows showed that the result with the 1976 river flow data gave 1.5 times higher reduction than the results with data of other plant operation years, 1972 through 1975. Because the 1976 data recorded low river flow, the 10 percent reduction quoted above may be high.     

反演同化技术在激光雷达中的应用研究

为增强模式预报准确性,提高激光雷达数据在化学模式中的应用水平,建立激光雷达同化系统,将激光雷达数据引入模式,利用WRF-Chem模拟2016年12月6日～10日一次污染过程,将激光雷达监测的气溶胶光学深度(AOD)反演成近地面PM2.5浓度,通过三维变分同化技术订正模式原始场,调整模式预报结果.实验对比发现,反演同化后...     

Persistent elevated nitrate in a riparian zone aquifer

Streamside vegetated buffer strips (riparian zones) are often assumed to be zones of ground water nitrate (NO3(-)) attenuation. At a site in southwestern Ontario (Zorra site), detailed monitoring revealed that elevated NO3(-) -N (4-93 mg L(-1)) persisted throughout a 100-m-wide riparian floodplain. Typical of riparian zones, the site has a soil zone of recent river alluvium that is organic carbon (OC) rich (36 +/- 16 g kg(-1)). This material is underlain by an older glacial outwash aquifer with a much lower OC content (2.3 +/- 2.5 g kg(-1). Examination of NO3(-), Cl(-), SO4(2-), and dissolved organic carbon (DOC) concentrations; N/Cl ratios; and NO3(-) isotopic composition (delta15N and delta18O) provides evidence of four distinct NO3(-) source zones within the riparian environment. Denitrification occurs but is incomplete and is restricted to a narrow interval located within ~0.5 m of the alluvium-aquifer contact and to one zone (poultry manure compost zone) where elevated DOC persists from the source. In older ground water close to the river discharge point, denitrification remains insufficient to substantially deplete NO3(-). Overall, denitrification related specifically to the riparian environment is limited at this site. The persistence of NO3(-) in the aquifer at this site is a consequence of its Pleistocene age and resulting low OC content, in contrast to recent fluvial sediments in modern agricultural terrain, which, even if permeable, usually have zones enriched in labile OC. Thus, sediment age and origin are additional factors that should be considered when assessing the potential for riparian zone denitrification.