Nitrate in groundwater: an isotopic multi-tracer approach

In spite of increasing efforts to reduce nitrogen inputs into groundwater from intensive agriculture, nitrate (NO3) remains one of the major pollutants of drinking-water resources worldwide. Determining the source(s) of NO3 contamination in groundwater is an important first step for improving groundwater quality by emission control, and it is with this aim that we investigated the viability of an isotopic multi-tracer approach (delta15N, delta11B, 87Sr/86Sr), in addition to conventional hydrogeologic analysis, in two small catchments of the Arguenon watershed (Brittany, France). The main anthropogenic sources (fertilizer, sewage effluent, and hog, cattle and poultry manure) were first characterized by their specific B, N and Sr isotope signatures, and compared to those observed in the ground- and surface waters. Chemical and isotopic evidence shows that both denitrification and mixing within the watershed have the effect of buffering NO3 contamination in the groundwater. Coupled delta11B, delta15N and 87Sr/86Sr results indicate that a large part of the NO3 contamination in the Arguenon watershed originates from the spreading of animal manure, with hog manure being a major contributor. Point sources, such as sewage effluents, contribute to the NO3 budget of the two watersheds.     

Hydrochemical and isotopic effects associated with petroleum fuel biodegradation pathways in a chalk aquifer

Hydrochemical data, compound specific carbon isotope analysis and isotopic enrichment trends in dissolved hydrocarbons and residual electron acceptors have been used to deduce BTEX and MTBE degradation pathways in a fractured chalk aquifer. BTEX compounds are mineralised sequentially within specific redox environments, with changes in electron acceptor utilisation being defined by the exhaustion of specific BTEX components. A zone of oxygen and nitrate exhaustion extends approximately 100 m downstream from the plume source, with residual sulphate, toluene, ethylbenzene and xylene. Within this zone complete removal of the TEX components occurs by bacterial sulphate reduction, with sulphur and oxygen isotopic enrichment of residual sulphate (epsilon(s) = -14.4 per thousand to -16.0 per thousand). Towards the plume margins and at greater distance along the plume flow path nitrate concentrations increase with delta15N values of up to +40 per thousand indicating extensive denitrification. Benzene and MTBE persist into the denitrification zone, with carbon isotope enrichment of benzene indicating biodegradation along the flow path. A Rayleigh kinetic isotope enrichment model for 13C-enrichment of residual benzene gives an apparent epsilon value of -0.66 per thousand. MTBE shows no significant isotopic enrichment (delta13C = -29.3 per thousand to -30.7 per thousand) and is isotopically similar to a refinery sample (delta13C = -30.1 per thousand). No significant isotopic variation in dissolved MTBE implies that either the magnitude of any biodegradation-induced isotopic fractionation is small, or that relatively little degradation has taken place in the presence of BTEX hydrocarbons. It is possible, however, that MTBE degradation occurs under aerobic conditions in the absence of BTEX since no groundwater samples were taken with co-existing MTBE and oxygen. Low benzene delta13C values are correlated with high sulphate delta34S, indicating that little benzene degradation has occurred in the sulphate reduction zone. Benzene degradation may be associated with denitrification since increased benzene delta13C is associated with increased delta15N in residual nitrate. Re-supply of electron acceptors by diffusion from the matrix into fractures and dispersive mixing is an important constraint on degradation rates and natural attenuation capacity in this dual-porosity aquifer.     

Occurrence of sulfonamide antimicrobials in private water wells in Washington County, Idaho, USA

Samples from six private wells formerly used as sources for drinking water by the residents of Washington County (Weiser, Idaho) were collected to assess the impact of a nearby confined animal feeding operation (CAFO) on the quality of the local groundwater. All six samples were found contaminated by two veterinary antimicrobials, sulfamethazine (at concentrations from 0.076 to 0.22 μg/l) and sulfadimethoxine (at concentrations from 0.046 to 0.068 μg/l). These groundwater samples also contained elevated concentrations of nitrate and ammonium. Three of the sampled wells have nitrate levels that exceeded the maximum contaminant level set by the US Environmental Protection Agency for drinking water, with nitrate concentration as high as 39.1 mg/l. All but one well showed nitrate, which instead contained ammonium at 1.22 mg/l. Analysis of the nitrate and ammonium in these samples by isotopic ratio mass spectrometry indicated δ¹⁵N characteristic of an animal or human waste source. Results from this study underscore the role of CAFO as an important source of antibiotic contamination of groundwater.     

Assessment of groundwater contamination from fertilizers in the Delhi area based on 0, N03 and K composition

Increasing application of nitrogen fertilizers in the irrigated lands of the studied area is likely to create a blanket non-point source of nitrate. Groundwater contamination from fertilizers, in this context, has been reported as derived from N0₃⁻, K⁺ and ¹⁸0 composition of groundwater. The data suggest both point and non-point sources of groundwater pollution. Thirty-three percent of the groundwater samples showed nitrate contents exceeding the general acceptable limit of 20 p.p.m. and 15% of the samples crossed the maximum permissible limit of 45 p.p.m. High nitrate levels are associated with high δ¹⁸O values, clearly indicating that significant quantities of evaporated (isotopically enriched) irrigation water infiltrate along with fertilizer nitrate to the groundwater system. Different δ¹⁸O---N0₃⁻ trends suggest isotopically distinct, non-point source origins which vary spatially and temporally, due to different degrees of evaporation/recharge and amounts of fertilizer applied. A scatter diagram of N0₃⁻ vs K⁺ suggests a common source of these ions when the concentration is less than 40 p.p.m. The investigation indicates that a combination of isotope (¹⁸0) and hydrochemical data can clearly characterize the impact of fertilizer on groundwater. Application of high nitrate, high potassium groundwater irrigation can minimize the requirement for inorganic fertilizers and bring down the cost of cultivation considerably, through appropriate management of fertilizer and water and modifications in agronomic practices and strategies on crops grown. Such practices will help protect groundwater from further degradation.     

Comparison of biotic and abiotic treatment approaches for co-mingled perchlorate, nitrate, and nitramine explosives in groundwater

Biological and abiotic approaches for treating co-mingled perchlorate, nitrate, and nitramine explosives in groundwater were compared in microcosm and column studies. In microcosms, microscale zero-valent iron (mZVI), nanoscale zero-valent iron (nZVI), and nickel catalyzed the reduction of RDX and HMX from initial concentrations of 9 and 1 mg/L, respectively, to below detection (0.02 mg/L), within 2 h. The mZVI and nZVI also degraded nitrate (3 mg/L) to below 0.4 mg/L, but none of the metal catalysts were observed to appreciably reduce perchlorate ( approximately 5 mg/L) in microcosms. Perchlorate losses were observed after approximately 2 months in columns of aquifer solids treated with mZVI, but this decline appears to be the result of biodegradation rather than abiotic reduction. An emulsified vegetable oil substrate was observed to effectively promote the biological reduction of nitrate, RDX and perchlorate in microcosms, and all four target contaminants in the flow-through columns. Nitrate and perchlorate were biodegraded most rapidly, followed by RDX and then HMX, although the rates of biological reduction for the nitramine explosives were appreciably slower than observed for mZVI or nickel. A model was developed to compare contaminant degradation mechanisms and rates between the biotic and abiotic treatments.     

Trends in nitrate concentrations and determination of its origin using stable isotopes (18O and 15N) in groundwater of the Western Central Valley, Costa Rica

A study was conducted to evaluate long-term trends in nitrate concentrations and to try to identify the origin of nitrate using stable isotopes (15N(NO3-) and 18O(NO3-)) in the aquifers of the western Central Valley, Costa Rica, where more than 1 million people depend on groundwater to satisfy their daily needs. Data from 20 sites periodically sampled for 4 to 17 years indicate an increasing trend in nitrate concentrations at five sites, which in a period ranging from 10 to 40 years, will exceed recommended maximum concentrations. Results of isotopic analysis indicate a correspondence between land use patterns and the isotopic signature of nitrate in groundwater and suggest that urbanization processes without adequate waste disposal systems, followed by coffee fertilization practices, are threatening water quality in the region. We conclude that groundwater management in this area is not sustainable, and that land use substitution processes from agricultural activity to residential occupation that do not have proper sewage disposal systems may cause a significant increment in the nitrate contaminant load.     

Applications of Anthropogenic Lead ArchaeoStratigraphy (ALAS Model) to Hydrocarbon Remediation

A model, which employs the use of high precision stable lead isotopic analyses, has been developed to estimate the age of hydrocarbon releases. The ALAS Model (Anthropogenic Lead ArchaeoStratigraphy) is based on calibrated, systematic increases in lead isotope ratios of gasolines caused by shifts in sources of lead ores used by the U.S. lead industry, including manufacturers of alkylleads, to more radiogenic Mississippi Valley Type (MVT) deposits. Acquisition of high quality samples (free product, gasoline-impacted soil and groundwater) of known age and subsequent analyses of the hydrocarbon component by high precision lead isotopic analyses by thermal ionization mass spectrometry (TIMS) have produced the ALAS Model calibration curve. Age uncertainties range from - 1 to 2 years for gasoline releases which occurred between 1965 and 1990, the major era of leaded gasoline usage. Analytical methods required to measure lead isotope ratios on ~5 nanograms of lead with precisions and accuracy of < - 0.1% (2 SEM ) are discussed in detail. Published lead isotopic measurements of gasoline-derived anthropogenic lead of samples throughout the United States are used to demonstrate the wide geographic range over which the ALAS Model may be applied. Two representative case studies involving an early 1970s free product release in California and the discrimination of a 1970s from modern unleaded gasoline release in Florida demonstrate the use of the model on single and multiple hydrocarbon releases, respectively, in different geographic regions of the United States. A third investigation focuses on the use of lead isotopes to correlate dissolved phase hydrocarbons with their source, in this case, unleaded (aka low lead) gasoline releases in New Jersey. Dissolved phase hydrocarbons (BTEX/MTBE) are shown to carry the lead isotopic signature of the unleaded gasoline into groundwater, allowing the specific source of the release to be identified. Investigations of lead isotopes as tracers of MTBE in groundwater are ongoing. However, both laboratory and field data indicate MTBE carries the lead isotopic signature of its unleaded gasoline source into groundwater, demonstrating the potential of the lead isotopic system as a discriminant of MTBE sources. Although developed to estimate the age of leaded gasoline releases, the ALAS Model has been successfully applied in studies requiring age dating of jet-A, diesel, kerosene, motor oil, and heating oil. These petroleum distillates are suspected of accidentally acquiring small, yet significant quantities of alkylleads during refining, allowing accurate ALAS Model ages to be determined. When lead levels in these petroleum distillates are within their normal range, typically tens to hundreds of ppb lead, it is possible to use lead isotopic ratios to correlate environmental releases of these products to their source or other releases.     

Applications of Anthropogenic Lead ArchaeoStratigraphy (ALAS Model) to Hydrocarbon Remediation

A model, which employs the use of high precision stable lead isotopic analyses, has been developed to estimate the age of hydrocarbon releases. The ALAS Model (Anthropogenic Lead ArchaeoStratigraphy) is based on calibrated, systematic increases in lead isotope ratios of gasolines caused by shifts in sources of lead ores used by the U.S. lead industry, including manufacturers of alkylleads, to more radiogenic Mississippi Valley Type (MVT) deposits. Acquisition of high quality samples (free product, gasoline-impacted soil and groundwater) of known age and subsequent analyses of the hydrocarbon component by high precision lead isotopic analyses by thermal ionization mass spectrometry (TIMS) have produced the ALAS Model calibration curve. Age uncertainties range from  ± 1 to 2 years for gasoline releases which occurred between 1965 and 1990, the major era of leaded gasoline usage. Analytical methods required to measure lead isotope ratios on ∼5 nanograms of lead with precisions and accuracy of < ± 0.1% (2_SEM) are discussed in detail. Published lead isotopic measurements of gasoline-derived anthropogenic lead of samples throughout the United States are used to demonstrate the wide geographic range over which the ALAS Model may be applied. Two representative case studies involving an early 1970s free product release in California and the discrimination of a 1970s from modern unleaded gasoline release in Florida demonstrate the use of the model on single and multiple hydrocarbon releases, respectively, in different geographic regions of the United States. A third investigation focuses on the use of lead isotopes to correlate dissolved phase hydrocarbons with their source, in this case, unleaded (aka low lead) gasoline releases in New Jersey. Dissolved phase hydrocarbons (BTEX/MTBE) are shown to carry the lead isotopic signature of the unleaded gasoline into groundwater, allowing the specific source of the release to be identified. Investigations of lead isotopes as tracers of MTBE in groundwater are ongoing. However, both laboratory and field data indicate MTBE carries the lead isotopic signature of its unleaded gasoline source into groundwater, demonstrating the potential of the lead isotopic system as a discriminant of MTBE sources. Although developed to estimate the age of leaded gasoline releases, the ALAS Model has been successfully applied in studies requiring age dating of jet-A, diesel, kerosene, motor oil, and heating oil. These petroleum distillates are suspected of accidentally acquiring small, yet significant quantities of alkylleads during refining, allowing accurate ALAS Model ages to be determined. When lead levels in these petroleum distillates are within their normal range, typically tens to hundreds of ppb lead, it is possible to use lead isotopic ratios to correlate environmental releases of these products to their source or other releases.     

Microbial in situ degradation of aromatic hydrocarbons in a contaminated aquifer monitored by carbon isotope fractionation

We present an approach for characterizing in situ microbial degradation using the 13C/12C isotope fractionation of contaminants as an indicator of biodegradation. The 13C/12C isotope fractionation of aromatic hydrocarbons was studied in anoxic laboratory soil percolation columns with toluene or o-xylene as the sole carbon and electron source, and sulfate as electron acceptor. After approximately 2 months' of incubation, the soil microbial community degraded 32 mg toluene l(-1) and 44 mg o-xylene l(-1) to less than 0.05 mg l(-1), generating a stable concentration gradient in the column. The 13C/12C isotope ratio in the residual non-degraded fraction of toluene and o-xylene increased significantly, corresponding to isotope fractionation factors (alphaC) of 1.0015 and 1.0011, respectively. When the extent of biodegradation in the soil column was calculated based on the measured isotope ratios (R(t)) and an isotope fractionation factor (alphaC=1.0017) obtained from a sulfate-reducing batch culture the theoretical residual substrate concentrations (C(t)) matched the measured toluene concentrations in the column. This indicated that a calculation of biodegradation based on isotope fractionation could work in systems like soil columns. In a field study, a polluted, anoxic aquifer was analyzed for BTEX and PAH contaminants. These compounds were found to exhibit a significant concentration gradient along an 800-m groundwater flow path downstream of the source of contamination. A distinct increase in the carbon isotope ratio (delta13C) was observed for the residual non-degraded toluene (7.2 per thousand ), o-xylene (8.1 per thousand ) and naphthalene fractions (1.2 per thousand ). Based on the isotope values and the laboratory-derived isotope fractionation factors for toluene and o-xylene, the extent to which the residual substrate fraction in the monitoring wells had been degraded by microorganisms was calculated. The results revealed significant biodegradation along the groundwater flow path. In the wells at the end of the plume, the bioavailable toluene and o-xylene fractions had been almost completely reduced by in situ microbial degradation. Although indane and indene showed decreasing concentrations downstream of the groundwater flow path, suggesting microbial degradation, their carbon isotope ratios remained constant. As the physical properties of these compounds are similar to those of BTEX compounds, the constant isotope values of indane and indene indicated that microbial degradation did not lead to isotope fractionation of all aromatic hydrocarbons. In addition, physical interaction with the aquifer material during the groundwater passage did not significantly alter the carbon isotope composition of aromatic hydrocarbons.     

Back-trajectory analysis and source-receptor relationships: particulate matter and nitrogen isotopic composition in rainwater

The southeastern portion of North Carolina features a dense crop and animal agricultural region; previous research suggests that this agricultural presence emits a significant portion of the state's nitrogen (i.e., oxides of nitrogen and ammonia) emissions. These findings indicate that transporting air over this region can affect nitrogen concentrations in precipitation at sites as far as 50 mi away. The study combined nitrate nitrogen isotope data with back-trajectory analysis to examine the relationship between regional nitrogen emission estimates independent of pollutant concentration information. In 2004, the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model was used to determine potential sources of nitrogen in rainwater collected at an urban receptor site in Raleigh, NC. The delta 15N isotope ratio signatures of each sample were used to further differentiate between sources of the rainwater nitrate. This study examined the importance of pollution sources, including animal agricultural activity, and meteorology on rainfall chemistry as well as the implications in fine particulate matter (PM2.5) formation. Samples that transited the dense crop and animal (swine) agricultural region of east-southeastern North Carolina (i.e., the source region) had lower delta 15N isotope ratios in the nitrate ion (average = -2.1 +/- 1.7 per thousand) than those from a counterpart nonagricultural region (average = 0.1 +/- 3 per thousand.) An increase in PM2.5 concentrations in the urban receptor site (yearly average = 15.1 +/- 5.8 microg/m3) was also found to correspond to air transport over the dense agricultural region relative to air that was not subjected to such transport (yearly average = 11.7 +/- 5.8 microg/m3).     

Hydraulic constraints on the performance of a groundwater denitrification wall for nitrate removal from shallow groundwater

Denitrification walls are a practical approach for decreasing non-point source pollution of surface waters. They are constructed by digging a trench perpendicular to groundwater flow and mixing the aquifer material with organic matter, such as sawdust, which acts as a carbon source to stimulate denitrification. For efficient functioning, walls need to be permeable to groundwater flow. We examined the functioning of a denitrification wall constructed in an aquifer consisting of coarse sands. Wells were monitored for changes in nitrate concentration as groundwater passed through the wall and soil samples were taken to measure microbial parameters inside the wall. Nitrate concentrations upstream of the wall ranged from 21 to 39 g N m(-3), in the wall from 0 to 2 g N m(-3) and downstream from 19 to 44 g N m(-3). An initial groundwater flow investigation using a salt tracer dilution technique showed that the flow through the wall was less than 4% of the flow occurring in the aquifer. Natural gradient tracer tests using bromide and Rhodamine-WT confirmed groundwater bypass under the wall. Hydraulic conductivity of 0.48 m day(-1) was measured inside the wall, whereas the surrounding aquifer had a hydraulic conductivity of 65.4 m day(-1). This indicated that during construction of the wall, hydraulic conductivity of the aquifer had been greatly reduced, so that most of the groundwater flowed under rather than through the wall. Denitrification rates measured in the center of the wall ranged from 0.020 to 0.13 g N m(-3) day(-1), which did not account for the rates of nitrate removal (0.16-0.29 g N m(-3) day(-1)) calculated from monitoring of groundwater nitrate concentrations. This suggested that the rate of denitrification was greater at the upstream face of the wall than in its center where it was limited by low nitrate concentrations. While denitrification walls can be an inexpensive tool for removing nitrate from groundwater, they may not be suitable in aquifers with coarse textured subsoils where simple inexpensive construction techniques result in major decreases in hydraulic conductivity.     

Impacts of swine manure pits on groundwater quality

Manure deep-pits are commonly used to store manure at confined animal feeding operations. However, previous to this study little information had been collected on the impacts of deep-pits on groundwater quality to provide science-based guidance in formulating regulations and waste management strategies that address risks to human health and the environment. Groundwater quality has been monitored since January 1999 at two hog finishing facilities in Illinois that use deep-pit systems for manure storage. Groundwater samples were collected on a monthly basis and analyzed for inorganic and bacteriological constituent concentrations. The two sites are located in areas with geologic environments representing different vulnerabilities for local groundwater contamination. One site is underlain by more than 6 m of clayey silt, and 7-36 m of shale. Concentrations of chloride, ammonium, phosphate, and potassium indicated that local groundwater quality had not been significantly impacted by pit leakage from this facility. Nitrate concentrations were elevated near the pit, often exceeding the 10 mg N/l drinking water standard. Isotopic nitrate signatures suggested that the nitrate was likely derived from soil organic matter and fertilizer applied to adjacent crop fields. At the other site, sandstone is located 4.6-6.1 m below land surface. Chloride concentrations and delta15N and delta15O values of dissolved nitrate indicated that this facility may have limited and localized impacts on groundwater. Other constituents, including ammonia, potassium, phosphate, and sodium were generally at or less than background concentrations. Trace- and heavy-metal concentrations in groundwater samples collected from both facilities were at concentrations less than drinking water standards. The concentration of inorganic constituents in the groundwater would not likely impact human health. Fecal streptococcus bacteria were detected at least once in groundwater from all monitoring wells at both sites. Fecal streptococcus was more common and at greater concentrations than fecal coliform. The microbiological data suggest that filtration of bacteria by soils may not be as effective as commonly assumed. The presence of fecal bacteria in the shallow groundwater may pose a significant threat to human health if the ground water is used for drinking. Both facilities are less than 4 years old and the short-term impacts of these manure storage facilities on groundwater quality have been limited. Continued monitoring of these facilities will determine if they have a long-term impact on groundwater resources.     

Natural and enhanced biodegradation of propylene glycol in airport soil

Aircraft de-icing fluids (ADF) are a source of water and soil pollution in airport sites. Propylene glycol (PG) is a main component in several commercial formulations of ADFs. Even though PG is biodegradable in soil, seasonal overloads may result in occasional groundwater contamination. Feasibility studies for the biostimulation of PG degradation in soil have been carried out in soil slurries, soil microcosms and enrichment cultures with and without the addition of nutrients (N and P sources, oligoelements), alternative electron acceptors (nitrate, oxygen releasing compounds) and adsorbents (activated carbon). Soil samples have been taken from the contaminated area of Gardermoen Airport Oslo. Under aerobic conditions and in the absence of added nutrients, no or scarce biomass growth is observed and PG degradation occurs by maintenance metabolism at constant removal rate by the original population of PG degraders. With the addition of nutrient, biomass exponential growth enhances aerobic PG degradation also at low temperatures (4 ° C) that occur at the high season of snowmelt. Anaerobic PG degradation without added nutrients still proceeds at constant rate (i.e. no biomass growth) and gives rise to reduced fermentation product (propionic acid, reduced Fe and Mn, methane). The addition of nitrate does not promote biomass growth but allows full PG mineralization without reduced by-products. Further exploitation on the field is necessary to fully evaluate the effect of oxygen releasing compounds and adsorbents.     

Carbon and nitrogen composition and stable isotope as potential indicators of source and fate of organic matter in the salt marsh of the Changjiang Estuary, China

Elemental (TOC, TN, C/N) and stable carbon and nitrogen isotopic (delta(13)C, delta(15)N) compositions were measured for surface sediments, three sediment vibrocores, plants, and suspended particulate matter (SPM) collected from salt marsh of the Changjiang Estuary. The purpose of this study is to characterize the sources of organic matter in sediments and to further elucidate the factors influencing the isotope signature in the salt marsh. Our results indicate that organic matter preserved in the sediments is predominantly controlled by the particulate organic matter in the Changjiang Estuary. The in situ contribution of marsh plants carbon to sediment organic matter is clearest in the high marsh, where the low delta(13)C of the plants (-28.1 per thousand) is reflected by a sediment delta(13)C (-24.7 per thousand) lower than values found for the low marsh and bare flat sediments (-23.4 per thousand and -23.0 per thousand, respectively). The effect of grain size on the spatial difference of isotope composition in the marsh sediments is insignificant, based on the observation that similar isotope values are found in different size particles, both for delta(13)C and delta(15)N. Nutrient utilization by plant assimilation, however, shows great impact on the surface sediment delta(15)N composition, due to the isotope fractionation. With extensive plant coverage and the consequent low surface water nitrate concentration, delta(15)N values of the high marsh surface sediments show (15)N enrichment.     

Modelling the geochemical fate and transport of wastewater-derived phosphorus in contrasting groundwater systems

A 1D reactive transport model (RTM) is used to obtain a mechanistic understanding of the fate of phosphorus (P) in the saturated zone of two contrasting aquifer systems. We use the field data from two oxic, electron donor-poor, wastewater-impacted, sandy Canadian aquifers, (Cambridge and Muskoka sites) as an example of a calcareous and non-calcareous groundwater system, respectively, to validate our reaction network. After approximately 10 years of wastewater infiltration, P is effectively attenuated within the first 10 m down-gradient of the source mainly through fast sorption onto calcite and Fe oxides. Slow, kinetic sorption contributes further to P removal, while precipitation of phosphate minerals (strengite, hydroxyapatite) is quantitatively unimportant in the saturated zone. Nitrogen (N) dynamics are also considered, but nitrate behaves essentially as a conservative tracer in both systems. The model-predicted advancement of the P plume upon continued wastewater discharge at the calcareous site is in line with field observations. Model results suggest that, upon removal of the wastewater source, the P plume at both sites will persist for at least 20 years, owing to desorption of P from aquifer solids and the slow rate of P mineral precipitation. Sensitivity analyses for the non-calcareous scenario (Muskoka) illustrate the importance of the sorption capacity of the aquifer solids for P in modulating groundwater N:P ratios in oxic groundwater. The model simulations predict the breakthrough of groundwater with high P concentrations and low N:P ratios after 17 years at 20 m from the source for an aquifer with low sorption capacity (<0.02% w/w Fe(OH)(3)). In this type of system, denitrification plays a minor role in lowering the N:P ratios because it is limited by the availability of labile dissolved organic matter.     

Stable isotope dynamics of nitrogen sewage effluent uptake in a semi-arid wetland

Our objectives were to determine (1) how much N is transferred into the food web via plants from a wetland receiving not only inputs of treated sewage effluent, but also containing contaminants such as polychlorinated biphenyls (PCBs), (2) how birds, as consumers, utilize exogenous N and uptake PCBs in relation to the food web of the wetlands, (3) the feasibility of using isotopic analysis in estimating trophic levels in a semi-arid system. Our results demonstrate that there is very high spatial variability in the N isotopic composition of primary producers. Birds had lower variability in delta15N, despite feeding at multiple trophic levels. In very high spatial variability in delta15N of primary producers, it is difficult to use N isotope techniques to define trophic levels relevant to the bioaccumulation of organic pollutants, but it is possible to track the flow of exogenous N through the food web.     

Anaerobic biotransformation of explosives in aquifer slurries amended with ethanol and propylene glycol

Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), and 2,4,6-trinitrotoluene (TNT) are explosives that are frequently found as environmental contaminants on military installations. Hydrogen has been shown to support the anaerobic transformation of these explosives. We investigated ethanol and propylene glycol as electron donors for providing syntrophically produced H2 for stimulating the anaerobic biodegradation of explosives in contaminated soil. The study was conducted using anoxic microcosms constructed with slurries of the contaminated soil and groundwater. The addition of 5mM ethanol and propylene glycol enhanced the biodegradation of RDX and HMX relative to the control bottles. Ethanol was depleted within about 20 days, resulting in the transient formation of hydrogen, acetate, and methane. The hydrogen headspace concentration increased from 8 ppm to 1838 ppm before decreasing to background concentrations. Propylene glycol was completely degraded after 15 days, forming hydrogen, propionate, and acetate as end-products. The hydrogen headspace concentrations increased from 56 ppm to 628 ppm before decreasing to background concentrations. No methane formation was observed during the incubation period of 48 days. Our findings indicate the addition of ethanol and propylene to the aquifer slurries increased the hydrogen concentrations and enhanced the biotransformation of RDX and HMX in the explosive-contaminated soil.     

Stable chlorine intramolecular kinetic isotope effects from the abiotic dehydrochlorination of DDT

INTENTION, GOAL, SCOPE, BACKGROUND: Identifying different sources and following reaction pathways of chlorinated organic contaminants in the environment can be challenging, especially when only their concentrations are available. Compound-specific stable chlorine measurements of some contaminants have recently been shown to provide additional information and an increased understanding of their biogeochemistry. These studies, however, have been generally limited to volatile molecules. OBJECTIVE: Here, the stable chlorine isotope ratios of the semi-volatile pesticide, 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT) were investigated. Specifically, the intramolecular stable chlorine isotopic compositions of DDT and the kinetic isotope effect (KIE) for the abiotic dehydrochlorination of DDT to 2,2-bis(p-chlorophenyl)-1,1-dichloroethene (DDE) were determined. METHODS: Selective chemical oxidation of DDT to dichlorobenzophenone (DCBP) and analysis of each compound was used to calculate the stable chlorine isotope ratios of the alkyl and aromatic chlorines in DDT. To determine the KIE for dehydrochlorination, DDT was reacted in a basic solution to yield DDE at 52 degrees C, 60 degrees C, and 72 degrees C for 3, 5, and 5 days, respectively. RESULTS AND DISCUSSION: Significant intramolecular stable chlorine isotopic differences were observed in one sample of DDT where the alkyl and aromatic delta 37Cl values were -5.76 +/- 0.45 and -2.21 +/- 0.24%@1000, respectively. Dehydrochlorination of DDT to DDE in basic solutions at 52, 60, and 70 degrees C resulted in a substantial intramolecular KIE where the alkyl chlorines of DDE shifted by approximately 3%@1000 relative to the alkyl chlorines in DDT. However, no temperature dependence was observed. The KIE, calculated by an iterative program, was 1.009. CONCLUSIONS: Intramolecular differences in the stable chlorine isotope ratios were observed in DDT and this is the first such finding. Dehydrochlorination of DDT yields a measurable and distinct intramolecular stable chlorine KIE. RECOMMENDATION AND OUTLOOK: The results of this study demonstrate the existence of significant intramolecular differences in chlorinated organic compounds. Many other chlorinated semi-volatile and volatile organic contaminants are synthesized from multiple sources of chlorine, and we recommend that similar studies be performed on many such molecules in order to attain a clear understanding of their intramolecular chlorine isotopic differences. The existence of a measurable KIE for the dehydrochlorination of DDT to DDE shows the potential strength of using isotopic measurements to investigate the biogeochemistry of these important compounds. For example, the isotopically depleted aqueous chloride produced by dehydrochlorination of DDT to DDE may be a useful tracer of these reactions in freshwater environments.     

Photocatalytic Treatment of RDX Wastewater with Nano-Sized Titanium Dioxide (5 pp)

Background, Aim and Scope The polynitramines, hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), are important military explosives and regulated toxic hazardous compounds. Production, testing and use of the compounds has resulted in numerous acres of contaminated soils and groundwater near many munitions facilities. Economical and efficient methods for treatment of wastewater and cleanup of soils or groundwater containing RDX and HMX are needed. This study focuses on the photocatalytic treatment of RDX wastewater with nano-sized titanium dioxide (nano-TiO2) under simulated sunlight, whose intensity and wavelength are similar to that of the real sunlight in Xi'an at noon. The objective is to determine the potential for RDX destruction with nano-TiO2 in aqueous solution. Materials and Methods: An activated carbon fiber (ACF) cloth-loaded with nano-TiO2 was put into the RDX containing solution, and the concentration of RDX was measured (by HPLC–UV) at regular time intervals under simulated sunlight. Results: The RDX degradation percentage of the photocatalytic process is higher than that of Fenton oxidation before 80 min, equivalent after 80 min, and it reaches 95% or above after 120 min. The nano-TiO2 catalyst can be used repeatedly. Discussion: The photocatalytic degradation kinetics of RDX under simulated sunlight can be described by a first-order reaction kinetics equation. The possible degradation mechanism of RDX was presented and the degradation performance was compared with that of biological method. Conclusions: It was demonstrated that the degradation of RDX wastewater is very effective with nano-TiO2 as the photocatalytic catalyst under simulated sunlight. The efficiency of the nano-TiO2 catalyst for RDX degradation under simulated sunlight is nearly identical to that of Fenton oxidation. Recommendations and Perspectives: To date, a number of catalysts show poor absorption and utilization of sunlight, and still need ultraviolet light irradiation during wastewater degradation. The nano-TiO2 used in the described experiments features very good degradation of RDX under simulated sunlight, and the manufacturing costs are rather low (around 10 Euro/m2). Moreover, the degradation efficiency is higher compared to that of the biological method. This method exhibits great potential for practical applications owing to its easiness and low cost. If it can be applied extensively, the efficiency of wastewater treatment will be enhanced greatly.     

Monthly variations in nitrogen isotopes of ammonium and nitrate in wet deposition at Guangzhou,south China

Monthly nitrogen isotopes of ammonium and nitrate in wet deposition in the city of Guangzhou, and the causes of their variability, are reported in this paper. Nitrate δ¹⁵N showed nearly constant values around zero in the dry season (October to April), but oscillating values from negative to positive in the rainy season (May to September). By contrast, ammonium δ¹⁵N displayed lower values during the rainy season than in the dry season. The rural area north of the city was considered as the prominent source of ammonium and nitrate in spring and early summer (May and June), as suggested by their concurrent negative isotopic trends and higher NH₄⁺/NO₃^? ratios. From July to September, different dominating sources from the city, i.e., fossil fuel combustion for nitrate, and sewage and waste emission for ammonium, caused disparate δ¹⁵N trends of the two species, showing positive nitrate δ¹⁵N, but still negative ammonium δ¹⁵N. During the cool dry season, the high values of ammonium δ¹⁵N and concurrently low NH₄⁺/NO₃^? ratios suggested the decrease in NH₃ volatilization and relatively important thermogenic origin of ammonium, but the intermediate nitrate δ¹⁵N values around zero may be a result of a balanced emission of NO_x from the city and the rural areas. The isotopic effects of chemical conversion of NO_x to nitrate and washout of nitrate were ruled out as significant causes of nitrate δ¹⁵N variability, but ammonium washout, during which ¹⁵N is assumed to be preferentially removed, may partly contribute to the ammonium δ¹⁵N variability.