A.J. Furness

Fate of nine recycled water trace organic contaminants and metal(loid)s during managed aquifer recharge into a anaerobic aquifer: Column studies

Water quality changes associated with the passage of aerobic reverse osmosis (RO) treated recycled water through a deep anaerobic pyritic aquifer system was evaluated in sediment-filled laboratory columns as part of a managed aquifer recharge (MAR) strategy. The fate of nine recycled water trace organic compounds along with potential negative water quality changes such as the release of metal(loid)s were investigated in large-scale columns over a period of 12 months. The anaerobic geochemical conditions provided a suitable environment for denitrification, and rapid (half-life <1-25 days) degradation of the endocrine disrupting compounds (bisphenol A, 17beta-estradiol, 17alpha-ethynylestradiol), and iodipamide. However, pharmaceuticals (carbamazepine and oxazepam), disinfection by-products (N-nitrosodimethylamine, N-nitrosomorpholine) and iohexol did not degrade rapidly (half-life > 100 days). High retardation coefficients (R) determined for many of the trace organics (R 13 to 67) would increase aquifer residence time and be beneficial for many of the slow degrading compounds. However, for the trace organics with low R values (1.1-2.6) and slow degradation rates (half-life > 100 days), such as N-nitrosodimethylamine, N-nitrosomorpholine and iohexol, substantial biodegradation during aquifer passage may not occur and additional investigations are required. Only minor transient increases in some metal(loid) concentrations were observed, as a result of either pyrite oxidation, mineral dissolution or pH induced metal desorption, followed by metal re-sorption downgradient in the oxygen depleted zone.

Behaviour and fate of nine recycled water trace organics during managed aquifer recharge in an aerobic aquifer

The fate of nine trace organic compounds was evaluated during a 12month large-scale laboratory column experiment. The columns were packed with aquifer sediment and evaluated under natural aerobic and artificial anaerobic geochemical conditions, to assess the potential for natural attenuation of these compounds during aquifer passage associated with managed aquifer recharge (MAR). The nine trace organic compounds were bisphenol A (BPA), 17β-estradiol (E2), 17α-ethynylestradiol (EE2), N-nitrosodimethylamine (NDMA), N-nitrosomorpholine (NMOR), carbamazepine, oxazepam, iohexol and iodipamide. In the low organic carbon content Spearwood sediment, all trace organics were non-retarded with retardation coefficients between 1.0 and 1.2, indicating that these compounds would travel at near groundwater velocities within the aquifer. The natural aerobic geochemical conditions provided a suitable environment for the rapid degradation for BPA, E2, iohexol (half life <1day). Lag-times for the start of degradation of these compounds ranged from <15 to 30days. While iodipamide was persistent under aerobic conditions, artificial reductive geochemical conditions promoted via the addition of ethanol, resulted in rapid degradation (half life <1days). Pharmaceuticals (carbamazepine and oxazepam) and disinfection by-products (NDMA and NMOR) did not degrade under either aerobic or anaerobic aquifer geochemical conditions (half life >50days). Field-based validation experiments with carbamazepine and oxazepam also showed no degradation. If persistent trace organics are present in recycled waters at concentrations in excess of their intended use, natural attenuation during aquifer passage alone may not result in extracted water meeting regulatory requirements. Additional pre treatment of the recycled water would therefore be required.

Multiple lines of evidence to demonstrate vinyl chloride aerobic biodegradation in the vadose zone, and factors controlling rates

A field-based investigation was conducted at a contaminated site where the vadose zone was contaminated with a range of chlorinated hydrocarbons. The investigation consisted of groundwater and multilevel soil-gas monitoring of a range of contaminants and gases, along with isotope measurements and microbiology studies. The investigation provided multiple lines of evidence that demonstrated aerobic biodegradation of vinyl chloride (VC) was occurring in the vadose zone (i) above the on-site source zone, and (ii) above the downgradient off-site groundwater plume location. Data from both the on-site and off-site locations were consistent in showing substantially greater (an order of magnitude greater) rates of VC removal from the aerobic vadose zone compared to more recalcitrant contaminants trichloroethene (TCE) and tetrachloroethene (PCE). Soil gas VC isotope analysis showed substantial isotopic enrichment of VC (δ¹³C -5.2 to -10.9‰) compared to groundwater (δ¹³C -39.5‰) at the on-site location. Soil gas CO₂ isotope analysis at both locations showed that CO₂ was highly isotopically depleted (δ¹³C -28.8 to -33.3‰), compared to soil gas CO₂ data originating from natural sediment organic matter (δ¹³C= -14.7 to -21.3‰). The soil gas CO2 δ¹³C values were consistent with near-water table VC groundwater δ¹³C values (-36.8 to -39.5‰), suggesting CO₂ originating from aerobic biodegradation of VC. Bacteria that had functional genes (ethene monooxygenase (etnC) and epoxyalkane transferase (etnE)) involved in ethene metabolism and VC oxidation were more abundant at the source zone where oxygen co-existed with VC. The distribution of VC and oxygen vadose zone vapour plumes, together with long-term changes in soil gas CO₂ concentrations and temperature, provided information to elucidate the factors controlling aerobic biodegradation of VC in the vadose zone. Based on the overlapping VC and oxygen vadose zone vapour plumes, aerobic vapour biodegradation rates were independent of substrate (VC and/or oxygen) concentration. The high correlation (R=0.962 to 0.975) between CO₂ concentrations and temperature suggested that aerobic biodegradation of VC was controlled by bacterial activity that was regulated by the temperature within the vadose zone. When assessing a contaminated site for possible vapour intrusion into buildings, accounting for environmental conditions for aerobic biodegradation of VC in the vadose zone should improve the assessment of environmental risk of VC intrusion into buildings, enabling better identification and prioritisation of contaminated sites to be remediated.

Fate of N-nitrosodimethylamine in recycled water after recharge into anaerobic aquifer.

Laboratory and field experiments were undertaken to assess the fate of N-nitrosodimethylamine (NDMA) in aerobic recycled water that was recharged into a deep anaerobic pyritic aquifer, as part of a managed aquifer recharge (MAR) strategy. Laboratory studies demonstrated a high mobility of NDMA in the Leederville aquifer system with a retardation coefficient of 1.1. Anaerobic degradation column and (14)C-NDMA microcosm studies showed that anaerobic conditions of the aquifer provided a suitable environment for the biodegradation of NDMA with first-order kinetics. At microgram per litre concentrations, inhibition of biodegradation was observed with degradation half-lives (260±20 days) up to an order of magnitude greater than at nanogram per litre concentrations (25-150 days), which are more typical of environmental concentrations. No threshold effects were observed at the lower ng L(-1) concentrations with NDMA concentrations reduced from 560 ng L(-1) to <6 ng L(-1) over a 42 day 14C-NDMA aerobic microcosm experiment. Aerobic (14)C-NDMA microcosm studies were also undertaken to assess potential aerobic degradation, likely to occur close to the recharge bore. These microcosm experiments showed a faster degradation rate than anaerobic microcosms, with a degradation half-life of 8±2 days, after a lag period of approximately 10 days. Results from a MAR field trial recharging the Leederville aquifer with aerobic recycled water showed that NDMA concentrations reduced from 2.5±1.0 ng L(-1) to 1.3±0.4 ng L(-1) between the recharge bore and a monitoring location 20 m down gradient (an estimated aquifer residence time of 10 days), consistent with data from the aerobic microcosm experiment. Further down gradient, in the anaerobic zone of the aquifer, NDMA degradation could not be assessed, as NDMA concentrations were too close to their analytical detection limit (<1 ng L(-1)).

The use of mass depletion-mass flux reduction relationships during pumping to determine source zone mass of a reactive brominated-solvent DNAPL

Mass depletion-mass flux relationships usually applied to a groundwater plume were established at field scale for groundwater pumped from within the source zone of a dense non-aqueous phase liquid (DNAPL). These were used as part of multiple lines of evidence in establishing the DNAPL source mass and architecture. Simplified source mass-dissolved concentration models including those described by exponential, power, and error functions as well as a rational mass equation based on the equilibrium stream tube approach were fitted to data from 285 days of source zone pumping (SZP) from a single well which removed 152 kg of dissolved organics from a multi-component, reactive brominated solvent DNAPL. The total molar concentration of the source compound, tetrabromoethane and its daughter products was used as a single measure of contaminant concentration to relate to source mass. A partitioning inter-well tracer test (PITT) conducted prior to the SZP provided estimates of groundwater travel times, enabling parameterisation of the models. After accounting for capture of the down-gradient dissolved plume, all models provided a good fit to the observed data. It was shown that differentiation between models would only emerge after appreciably more pumping from the source zone. The model fits were not particularly sensitive to the exponent parameters and variance of groundwater travel time. In addition, the multi-component nature of the DNAPL did not seem to affect the utility of the models for the period examined. Estimates of the DNAPL mass prior to the start of SZP from the models were greatest where the log of the variance of travel time was used explicitly in the source depletion models (mean 295kg) compared to where the associated power exponent and variance was fitted freely (mean 258 kg). The estimates of source mass were close to that of 220kg determined from the PITT. In addition to the PITT, multi-level groundwater sampling from within the source zone provided important supporting information for developing the conceptual model of the source zone. It is concluded that SZP may be an effective and relatively simple means for characterising DNAPL source zones.

Fate of N-nitrosomorpholine in an anaerobic aquifer used for managed aquifer recharge: A column study

The fate of N-nitrosomorpholine (NMOR) was evaluated at microgram and nanogram per litre concentrations. Experiments were undertaken to simulate the passage of groundwater contaminants through a deep anaerobic pyritic aquifer system, as part of a managed aquifer recharge (MAR) strategy. Sorption studies demonstrated the high mobility of NMOR in the Leederville aquifer system, with retardation coefficients between 1.2 and 1.6. Degradation studies from a 351 day column experiment and a 506 day stop-flow column experiment showed an anaerobic biologically induced reductive degradation process which followed first order kinetics. A biological lag-time of less than 3 months and a transient accumulation of morpholine (MOR) were also noted during the degradation. Comparable half-life degradation rates of 40-45 days were observed over three orders of magnitude in concentration (200 ng L(-1) to 650 μg L(-1)). An inhibitory effect on microorganism responsible to the biodegradation of NMOR at 650 μg L(-1) or a threshold effect at 200 ng L(-1) was not observed during these experiments.

On-line groundwater velocity probe: Laboratory testing and field evaluation

An automated on-line instrument has been developed to rapidly measure groundwater velocity within a screened well. The instrument consists of a carbon dioxide gas tracer that is periodically delivered to a permeable chamber located within a screened well. The rate of diffusion of the tracer through the wall of the permeable chamber was rapid and the effective diffusion into the groundwater was controlled by the mass transfer limitations at the groundwater/chamber interface with gas entrainment proportional to the groundwater velocity past the chamber. By periodically delivering the gas tracer and monitoring the reduction in concentration of the tracer from the permeable chamber, the groundwater velocity was determined multiple times daily. Laboratory experiments undertaken within a calibrated flow chamber have demonstrated that the instrument can be used to accurately and reliably determine groundwater flow velocities at 3h intervals for flow rates between 25 and 300 m y(-1). Field testing of the velocity probe at multiple well locations in a sandy aquifer gave velocities consistent with another monitoring technique (passive flux meter) and site modelling.

Fate of benzotriazole and 5-methylbenzotriazole in recycled water recharged into an anaerobic aquifer: Column studies

The fate of benzotriazole (BTri) and 5-methylbenzotriazole (5-MeBT) was investigated under anaerobic conditions at nano gram per litre concentrations in large-scale laboratory columns to mimic a managed aquifer recharge replenishment strategy in Western Australia. Investigations of BTri and 5-MeBT sorption behaviour demonstrated mobility of the compounds with retardation coefficients of 2.0 and 2.2, respectively. Degradation processes over a period of 220 days indicated first order biodegradation of the BTri and 5-MeBT under anaerobic aquifer conditions after a biological lag-time of approximately 30-60 days. Biodegradation half-lives of 29 ± 2 and 26 ± 1 days for BTri and 5-MeBT were respectively observed, with no threshold effect to biodegradation observed at the 200 ng L(-1). The detection of degradation products provided further evidence of BTri and 5-MeBT biodegradation. These results suggested that if BTri and 5-MeBT were present in recycled water recharged to the Leederville aquifer, biodegradation during aquifer passage is likely given sufficient aquifer residence times or travel distances between recycled water injection and groundwater extraction.

Concentration effects on biotic and abiotic processes in the removal of 1,1,2-trichloroethane and vinyl chloride using carbon-amended ZVI

A permeable reactive barrier, consisting of both zero valent iron (ZVI) and a biodegradable organic carbon, was evaluated for the remediation of 1,1,2-trichloroethane (1,1,2-TCA) contaminated groundwater. During an 888 day laboratory column study, degradation rates initially stabilized with a degradation half-life of 4.4±0.4 days. Based on the accumulation of vinyl chloride (VC) and limited production of 1,1-dichloroethene (1,1-DCE) and 1,2-dichloroethane (1,2-DCA), the dominant degradation pathway was likely abiotic dichloroelimination to form VC. Degradation of VC was not observed based on the accumulation of VC and limited ethene production. After a step reduction in the influent concentration of 1,1,2-TCA from 170±20 mg L(-1) to 39±11 mg L(-1), the degradation half-life decreased 5-fold to 0.83±0.17 days. The isotopic enrichment factor of 1,1,2-TCA also changed after the step reduction from -14.6±0.7‰ to -0.72±0.12‰, suggesting a possible change in the degradation mechanism from abiotic reductive degradation to biodegradation. Microbiological data suggested a co-culture of Desulfitobacterium and Dehalococcoides was responsible for the biodegradation of 1,1,2-TCA to ethene.

Soil gas carbon dioxide probe: Laboratory testing and field evaluation

An automated semi-continuous on-line instrument has been developed to measure CO2 gas concentrations in the vadose zone. The instrument uses semi-permeable polymer tubing (CO2 probe) for diffusion based sampling, coupled to an infra red sensor. The system operated automatically by intermittently purging the CO2 probe, which was installed in the vadose zone, with a non-CO2 gas at a low flow rate. The gas exiting the CO2 probe was monitored at the ground surface using a miniature infra red sensor and the response related to the vadose zone soil gas CO2 concentration. The in situ CO2 probes provided a reliable monitoring technique under long-term (18 months) aggressive and dynamic field conditions, with no interference observed from non-CO2 gases and volatile organic compounds. The probes provided data that were comparable to conventional grab sampling techniques without the labour-intensive sample collection and processing associated with these conventional techniques. Also, disturbance to vadose zone CO2 profiles from repeated grab samples during long-term semi-continuous monitoring could potential be reduced by using the diffusion based sampling technique.