Development of a dynamic model for estimating the food web transfer of chemicals in small aquatic ecosystems

A dynamic combined fate and food web model was developed to estimate the food web transfer of chemicals in small aquatic ecosystems (i.e. ponds). A novel feature of the modeling approach is that aquatic macrophytes (submerged aquatic vegetation) were included in the fate model and were also a food item in the food web model. The paper aims to investigate whether macrophytes are effective at mitigating chemical exposure and to compare the modeling approach developed here with previous modeling approaches recommended in the European Union (EU) guideline for risk assessment of pesticides. The model was used to estimate bioaccumulation of three hypothetical chemicals of varying hydrophobicity in a pond food web comprising 11 species. Three different macrophyte biomass densities were simulated in the model experiments to determine the influence of macrophytes on fate and bioaccumulation. Macrophytes were shown to have a significant effect on the fate and food web transfer of highly hydrophobic compounds with log K_OW > = 5. Modeled peak concentrations in biota were highest for the scenarios with the lowest macrophyte biomass density. The distribution and food web transfer of the hypothetical compound with the lowest hydrophobicity (log K_OW = 3) was not affected by the inclusion of aquatic macrophytes in the pond environment. For the three different hypothetical chemicals and at all macrophyte biomass densities, the maximum predicted concentrations in the top predator in the food web model were at least one order of magnitude lower than the values estimated using methods suggested in EU guidelines. The EU guideline thus provides a highly conservative estimate of risk. In our opinion, and subject to further model evaluation, a realistic assessment of dynamic food web transfer and risk can be obtained using the model presented here.     

Sorption of emerging trace organic compounds onto wastewater sludge solids

This work examined the sorption potential to wastewater primary- and activated-sludge solids for 34 emerging trace organic chemicals at environmentally relevant concentrations. These compounds represent a diverse range of physical and chemical properties, such as hydrophobicity and charge state, and a diverse range of classes, including steroidal hormones, pharmaceutically-active compounds, personal care products, and household chemicals. Solid-water partitioning coefficients (K_d) were measured where 19 chemicals did not have previously reported values. Sludge solids were inactivated by a nonchemical lyophilization and dry-heat technique, which provided similar sorption behavior for recalcitrant compounds as compared to fresh activated-sludge. Sorption behavior was similar between primary- and activated-sludge solids from the same plant and between activated-sludge solids from two nitrified processes from different wastewater treatment systems. Positively-charged pharmaceutically-active compounds, amitriptyline, clozapine, verapamil, risperidone, and hydroxyzine, had the highest sorption potential, log K_d = 2.8-3.8 as compared to the neutral and negatively-charged chemicals. Sorption potentials correlated with a compound’s hydrophobicity, however the higher sorption potentials observed for positively-charged compounds for a given log D_ow indicate additional sorption mechanisms, such as electrostatic interactions, are important for these compounds. Previously published soil-based one-parameter models for predicting sorption from hydrophobicity (log K_ow > 2) can be used to predict sorption for emerging nonionic compounds to wastewater sludge solids.     

Natural versus wastewater derived dissolved organic carbon: implications for the environmental fate of organic micropollutants

The interaction of organic micropollutants with dissolved organic carbon (DOC) can influence their transport, degradation and bioavailability. While this has been well established for natural organic carbon, very little is known regarding the influence of DOC on the fate of micropollutants during wastewater treatment and water recycling. Dissolved organic carbon-water partition coefficients (K_DOC) for wastewater derived and reference DOC were measured for a range of micropollutants using a depletion method with polydimethylsiloxane disks. For micropollutants with an octanol-water partition coefficient (log K_OW) greater than 4 there was a significant difference in K_DOC between reference and wastewater derived DOC, with partitioning to wastewater derived DOC over 1000 times lower for the most hydrophobic micropollutants. The interaction of nonylphenol with wastewater derived DOC from different stages of a wastewater and advanced water treatment train was studied, but little difference in K_DOC was observed. Organic carbon characterisation revealed that reference and wastewater derived DOC had very different properties due to their different origins. Consequently, the reduced sorption capacity of wastewater derived DOC may be related to their microbial origin which led to reduced aromaticity and lower molecular weight. This study suggests that for hydrophobic micropollutants (log K_OW > 4) a higher concentration of freely dissolved and thus bioavailable micropollutants is expected in the presence of wastewater derived DOC than predicted using K_DOC values quantified using reference DOC. The implication is that naturally derived DOC may not be an appropriate surrogate for wastewater derived DOC as a matrix for assessing the fate of micropollutants in engineered systems.     

Persistence and partitioning of eight selected pharmaceuticals in the aquatic environment: Laboratory photolysis, biodegradation, and sorption experiments

We selected eight pharmaceuticals with relatively high potential ecological risk and high consumption—namely, acetaminophen, atenolol, carbamazepine, ibuprofen, ifenprodil, indomethacin, mefenamic acid, and propranolol—and conducted laboratory experiments to examine the persistence and partitioning of these compounds in the aquatic environment. In the results of batch sunlight photolysis experiments, three out of eight pharmaceuticals—propranolol, indomethacin, and ifenprodil—were relatively easily photodegraded (i.e., half-life < 24 h), whereas the other five pharmaceuticals were relatively stable against sunlight. The results of batch biodegradation experiments using river water suggested relatively slow biodegradation (i.e., half-life > 24 h) for all eight pharmaceuticals, but the rate constant was dependent on sampling site and time. Batch sorption experiments were also conducted to determine the sorption coefficients to river sediments and a model soil sample. The determined coefficients (K_d values) were much higher for three amines (atenolol, ifenprodil, and propranolol) than for neutral compounds or carboxylic acids; the K_d values of the amines were comparable to those of a four-ring polycyclic aromatic hydrocarbon (PAH) pyrene. The coefficients were also higher for sediment/soil with higher organic content, and the organic carbon-based sorption coefficient (log K_oc) showed a poor linear correlation with the octanol-water distribution coefficient (log D_ow) at neutral pH. These results suggest other sorption mechanisms—such as electrochemical affinity, in addition to hydrophobic interaction—play an important role in sorption to sediment/soil at neutral pH.     

Probabilistic ecological risk assessment of DDTs in the Bohai Bay based on a food web bioaccumulation model

The fugacity-based food web model was developed to simulate the bioaccumulation of dichloro-diphenyl-trichloroethanes (DDTs) in the aquatic ecosystem in the Bohai Bay. The internal exposure levels (IELs) of DDTs in various organism categories were calculated. Monte Carlo-based uncertainty analysis was performed to get the of IEL distributions of DDTs in organisms. Probabilistic ecological risk assessment (ERA) was performed based on IEL distributions and internal species sensitivity distributions (SSDs). The results show that fugacities and bioaccumulation factors (BAFs) generally increased with increasing trophic level in the food web. Octanol-water partition coefficient (K_ow), DDT levels in water and the lipid contents had the greatest influences on IELs in the organism bodies. The ecological risks of DDTs were relatively high. The risk order was p,p′-DDT > p,p′-DDE > p,p′-DDD. At an internal hazard quotient (HQ_int) criterion of 1/5, the risk probabilities were 0.10 (0.055-0.17), 0.079 (0.045-0.13) and 0.053 (0.028-0.092) for p,p′-DDT, p,p′-DDE and p,p′-DDD, respectively. The results from ERA based on the internal exposure approximated those based on external exposure. The food web model is a feasible method to predict the extent of bioaccumulation and IELs of hydrophobic organic pollutants in organisms as a step to evaluate their risk posed on aquatic ecosystems.     

Sorption of ionized and neutral emerging trace organic compounds onto activated sludge from different wastewater treatment configurations

The objective of this study was to examine sorption of a suite of 19 trace organic contaminants (TOrCs) to activated sludge. Compounds examined in this study included neutral, nonionized TOrCs as well as acidic TOrCs which may carry a negative charge and basic TOrCs which may carry a positive charge at the pH of wastewater. These TOrCs were evaluated to examine how sorptive behavior might differ for TOrCs in different states of charge. Additionally, multiple sludges from geographically and operationally different wastewater treatment plants were studied to elicit how solid-phase characteristics influence TOrC sorption. Characterization of sludge solids from 6 full scale treatment facilities and 3 bench-scale reactors showed no significant difference in fraction organic carbon (f_oc) and cation exchange capacity (CEC). Sorption experiments demonstrated that sorption of TOrCs also exhibits little variation between these different sludges. Organic carbon normalized partition coefficients (log K_oc) were determined as a measure of sorption, and were found to correlate well with octanol-water partition coefficients (log K_ow) for nonionized TOrCs, and log D_ow for anionic TOrCs where log D_ow is greater than 2. These data were used to construct a linear free energy relationship (LFER), which was comparable to existing LFERs for sorption onto sludge. No trend in sorption was apparent for the remaining anionic TOrCs or for the cationic TOrCs. These data suggest that predicting sorption to activated sludge based on K_ow values is a reasonable approach for neutral TOrCs using existing LFERs, but electrostatic (and likely other) interactions may govern the sorptive behavior of the charged organic chemicals to sludge.     

Passive sampling combined with ecotoxicological and chemical analysis of pharmaceuticals and biocides - evaluation of three Chemcatcher™ configurations

Passive sampling is a tool to monitor the presence and concentrations of micropollutants in the aquatic environment. We investigated the duration of integrative sampling and the effects of flow rate on the performance of three configurations of the Chemcatcher - a sampler for polar organic compounds. Chemcatchers were fitted with Empore™ styrenedivinylbenzene (SDB) XC disks (XC), SDB-RPS disks (RPS) or SDB-RPS disks covered with a polyethersulfone membrane (RPS-PES). Samplers were either exposed to treated sewage effluent for 5 days at various flow rates, or at a single flow rate with overlapping exposures of 3-24 days. Chemical analysis focused on a set of pharmaceuticals and biocides and ecotoxicological analysis measured inhibition of photosystem II in algae. For compounds with log K_OW > 2, both XC and RPS disks respond dynamically to higher flow rates; uptake increased up to five-fold when flow increased from 0.03 to 0.37 m s⁻¹. At a flow rate of 0.13 m s⁻¹ the integrative window of SDB disks approached 6 days for more hydrophobic compounds (log K_OW > 3.5). The RPS-PES configuration was less affected by flow and also showed an extended integrative window (up to 24 days). The membrane causes a lag phase of up to 2.3 days which thwarts a sound interpretation of data from sampling periods of less than 10 days.     

Evaluating dissolved organic carbon-water partitioning using polyparameter linear free energy relationships: Implications for the fate of disinfection by-products

The partitioning of micropollutants to dissolved organic carbon (DOC) can influence their toxicity, degradation, and transport in aquatic systems. In this study carbon-normalized DOC-water partition coefficients (K_DOC-w) were measured for a range of non-polar and polar compounds with Suwannee River fulvic acid (FA) using headspace and solid-phase microextraction (SPME) methods. The studied chemicals were selected to represent a range of properties including van der Waal forces, cavity formation and hydrogen bonding interactions. The K_DOC-w values were used to calibrate a polyparameter linear free energy relationship (pp-LFER). The difference between experimental and pp-LFER calculated K_DOC-w values was generally less than 0.3 log units, indicating that the calibrated pp-LFER could provide a good indication of micropollutant interaction with FA, though statistical analysis suggested that more data would improve the predictive capacity of the model. A pp-LFER was also calibrated for Aldrich humic acid (HA) using K_DOC-w values collected from the literature. Both experimental and pp-LFER calculated K_DOC-w values for Aldrich HA were around one order of magnitude greater than Suwannee River FA. This difference can be explained by the higher cavity formation energy in Suwannee River FA. Experimental and pp-LFER calculated K_DOC-w values were compared for halogenated alkanes and alkenes, including trihalomethane disinfection by-products, with good agreement between the two approaches. Experimental and calculated values show that DOC-water partitioning is generally low; indicating that sorption to DOC is not an important fate process for these chemicals in the environment.     

Occurrence and removal of priority pollutants by lamella clarification and biofiltration

This study investigates the occurrence of all priority substances (n = 41) listed in the Water Framework Directive and additional substances (n = 47) in raw sewage, as well as the removal performance of lamella clarification and biofiltration techniques. Once the efficiency of both types of techniques has been assessed for typical wastewater parameters, the differences in each technique's ability to remove pollutants becomes obvious; nevertheless, pollutant removal in quantitative terms still depends on the physico-chemical properties of the compounds used and operating conditions within the selected facility. For lamella clarification, the removal of organic chemicals was found to be primarily correlated with their sorption potential and, hence, strongly dependent upon log K_ow of the compound under study. Compounds with a strong hydrophobic character (log K_ow > 4.5) are removed to a significant extent (approx. 85%), while hydrophilic compounds (log K_ow < 3.5) are poorly removed (<20%). For biofiltration, the removal of chemicals appears to be compound-dependent, although this outcome involves several mechanisms, namely: i) physical filtration of total suspended solids, ii) volatilisation, iii) sorption, and iv) biotransformation of substances. Even if the complex processes within a biofilter system do not yield an accurate prediction of pollutant removal, two groups of chemicals can still be clearly identified: i) hydrophobic or volatile compounds, for which moderate to high removal rates are observed (from 50% to over 80%); and ii) hydrophilic, non-volatile and refractory compounds for which a low removal rate would be expected (<20%).     

Role of cation exchange processes on the sorption influenced transport of cationic β-blockers in aquifer sediments

The influence of cation exchange processes on the transport behavior of the cationic β-blockers atenolol and metoprolol was investigated by applying saturated laboratory column experiments. Breakthrough curves using natural sediments under different competitive conditions were generated and resulting sorption coefficients were compared. For the cationic species of atenolol (at pH = 8), the existence and dominating role of cation exchange processes were demonstrated by varying calcium concentrations. No effect of atenolol concentration on its retardation was observed within a wide concentration range. The breakthrough curve comparison of atenolol and the more hydrophobic metoprolol under constant conditions showed a significantly stronger retardation for metoprolol than for atenolol. However, additional non-polar interactions cannot explain the observed differences as they are determined to be negligible for both compounds. Due to the dominating role of cation exchange processes for the cationic species on overall sorption, a simple prediction of β-blocker transport in the subsurface by using K_OC values derived from log K_OW-log K_OC correlations is not feasible.     

Dynamic multi-phase partitioning of decamethylcyclopentasiloxane (D5) in river water

The behaviour of decamethylcyclopentasiloxane (D5) in river water was evaluated by measuring concentration changes in open beakers. Effective values for the partition coefficient between organic carbon and water (K_OC) were derived by least-squares optimisation of a dynamic model which accounted for partitioning between the sorbed and dissolved phases of D5, and for losses via volatilisation and hydrolysis. Partial mass transfer coefficients for volatilisation were derived from model fits to controls containing deionised water. Effective values of log (K_OC) were between 5.8 and 6.33 (mean 6.16). These figures are higher than some other experimentally-derived values but much lower than those estimated from the octanol: water partition coefficient using single-parameter linear free energy relationships (LFERs). A poly-parameter LFER gave a predicted log (K_OC) of 5.5. Differences in partitioning are believed to be due to the nature of the organic matter present. The new value for effective K_OC was employed in a simple model of D5 behaviour in rivers to ascertain the extent to which a higher affinity for organic carbon would depress volatility. The results suggest that despite the revised K_OC value, volatilisation of D5 remains a significant removal mechanism in surface waters.     

Sorption of dichlorodiphenyltrichloroethane (DDT) and its metabolites by activated carbon in clean water and sediment slurries

Polyethylene-water partitioning coefficients (K_PE) and mass transfer coefficients (k_PE) for the ortho and para isomers of the organochlorine pesticide dichlorodiphenyltrichloroethane (DDT) and its metabolites dichlorodiphenyldichloroethane (DDD), dichlorodiphenyldichloroethylene (DDE) and, dichlorodiphenylmonochloroethylene (DDMU) were measured. These data were used to derive activated carbon (AC) sorption isotherms in clean water in the sub-nanogram per litre free aqueous concentration range for a virgin and a regenerated AC. The sorption strength of AC for DDT and its metabolites was very high and logarithmic values of the AC-water partitioning coefficients, log K_AC, ranged from 8.47 to 9.26. A numerical mass transfer model was calibrated with this data to interpret previously reported reductions in DDT uptake by semipermeable membrane devices after AC amendment of sediment from Lauritzen Channel, California, USA. The activated carbon-water partitioning coefficient values (K_AC) measured in clean water systems appear to overestimate the AC sorption capacity in sediment up to a factor 32 for DDT and its metabolites at long contact time with fine-sized AC. Modelling results show decreased attenuation of the AC sorption capacity with increased sediment-AC contact time. We infer that increased resistance in mass transfer of DDTs to sorption sites in the microporous region likely caused by deposits of dissolved organic matter in the macro- and mesopores of AC appears to be the most relevant fouling mechanism. These results suggest that DDTs may diffuse through possible deposits of dissolved organic matter over time, implying that the effects of sediment on the sorption of DDTs by AC may be more kinetic than competitive.     

Sorption of antibiotics to biofilm

Using a continuous-flow rotating annular bioreactor, sorption of three selected antibiotics (sulfamethoxazole (SMX), ciprofloxacin (CIP), and erythromycin (ERY)) to bacterial biofilm was investigated. CIP had the greatest biofilm partition coefficient (K_oc = 92,000 ± 10,000 L/kg) followed by ERY (K_oc = 6000 ± 1000 L/kg) and then SMX (K_oc = 4000 ± 1000 L/kg). Antibiotic sorption to biofilm did not correlate with experimentally-determined K_ow values (CIP: −0.4; ERY: 0.98; SMX: <-0.59 at pH 7), suggesting that hydrophobic interactions do not drive the sorption of these relatively hydrophilic compounds to the biofilm. It appears that speciation (i.e. charge) and molecular size of the antibiotics are important in explaining their sorption to typically negatively charged biofilm. SMX is neutral to negatively charged at circumneutral pH while CIP and ERY are both positively charged. The decreased extent of sorption of ERY relative to CIP is likely due to the larger molecular size of ERY that results in a decreased rate of mass transfer (i.e. diffusion) to and through the biofilm. In conclusion, the results of this research suggest that hydrophobic interactions (predicted by K_ow) do not control sorption of relatively hydrophilic antibiotics to biofilm and that antibiotic speciation and molecular size are important factors affecting the interactions between antibiotics and biofilm.     

Effects of Mg and H on the toxicity of Ni to the unicellular green alga Pseudokirchneriella subcapitata: Model development and validation with surface waters

In this study, increasing Mg concentrations and decreasing pH were observed to decrease Ni toxicity to the green alga Pseudokirchneriella subcapitata. To investigate to what extent the original biotic ligand model (BLM) concept could explain Ni toxicity as a function of water chemistry, the protective effects of Mg²⁺ and H⁺ were modeled as BLM-type single-site competition effects. The model parameters representing these effects were log K_MgBL = 3.3 and log K_HBL = 6.5. The BLM was capable of predicting Ni toxicity by an error of less than a factor of 2 in most synthetic and natural waters used in this study. However, since the relationship between 72-h E_rC50_Ni²⁺ (i.e. the 72-h E_rC50 expressed as Ni²⁺ activity) and H⁺ activity was not linear over the entire tested pH range, only the ‘linear part’ between pH 6.45 and 7.92 was used for derivation of log K_HBL. This nonlinearity indicates that the effect of pH can probably not be attributed to H⁺ competition with Ni²⁺ for a single site alone. When modeling the effect of pH as a linear relation between 72-h E_rC50_pNi²⁺? (= − log (72-h E_rC50_Ni²⁺ corrected for the presence of Mg)) and pH, the applicability of the model was successfully extended to pH levels as low as 6.01. This type of empirical model has also been used in our previous studies on the development of a chronic Ni bioavailability model for Daphnia magna and a long-term Ni bioavailability model for rainbow trout. Finally, we could not detect a statistically significant interactive effect of pH and Mg on the toxicity of Ni²⁺ to P. subcapitata and this is in line with the formulation of our empirical model.     

Sorption of perfluorooctane sulfonate and perfluorooctanoate on activated carbons and resin: Kinetic and isotherm study

Perfluorooctane sulfonate (PFOS) and perfluorooctanoate (PFOA) have increasingly attracted global concerns in recent years due to their global distribution, persistence, strong bioaccumulation and potential toxicity. The feasibility of using powder activated carbon (PAC), granular activated carbon (GAC) and anion-exchange resin (AI400) to remove PFOS and PFOA from water was investigated with regard to their sorption kinetics and isotherms. Sorption kinetic results show that the adsorbent size influenced greatly the sorption velocity, and both the GAC and AI400 required over 168 h to achieve the equilibrium, much longer than 4 h for the PAC. Two kinetic models were adopted to describe the experimental data, and the pseudo-second-order model well described the sorption of PFOS and PFOA on the three adsorbents. The sorption isotherms show that the GAC had the lowest sorption capacity both for PFOS and PFOA among the three adsorbents, while the PAC and AI400 possessed the highest sorption capacity of 1.04 mmol g⁻¹ for PFOS and 2.92 mmol g⁻¹ for PFOA according to the Langmuir fitting. Based on the sorption behaviors and the characteristics of the adsorbents and adsorbates, ion exchange and electrostatic interaction as well as hydrophobic interaction were deduced to be involved in the sorption, and some hemi-micelles and micelles possibly formed in the intraparticle pores.     

A comparison of PCB bioaccumulation factors between an arctic and a temperate marine food web

To test how environmental conditions in the Arctic and the resulting ecological adaptations affect accumulation of persistent organic pollutants (POPs) in the marine food web, bioaccumulation of four polychlorinated biphenyls (PCBs) in an arctic (Barents Sea 77 °N-82 °N) and a temperate marine (Baltic Sea 54 °N-62 °N) food web were compared. Three different trophic levels were studied (zooplankton, fish, and seal), representing the span from first-level consumer to top predator. Previously published high-quality data on PCB water concentrations in the two areas were used for calculation of bioaccumulation factors (BAF). BAF was calculated as the ratio of the PCB concentration in the organism ([PCB]_org; pg/kg lipid) to the dissolved water concentration (C_w; pg/L). The BAF_Arctic:BAF_Temperate ratios were above 1 for all four PCB congeners in zooplankton (6.4-13.8) and planktivorous fish (2.9-5.0)), whereas the ratios were below 1 in seal. The mean ratio between arctic and temperate BAFs for all trophic levels and congeners (BAF_Arcti:BAF_Temperate) was 4.8. When the data were corrected for the seawater temperature difference between the two ecosystems, the ratio was 2.0. We conclude that bioaccumulation differences caused by ecological or physiological adaptations of organisms between the two ecosystems were well within a water concentration variability of 50%. Further, our data support the hypothesis that lower seawater temperature lead to a thermodynamically favoured passive partitioning to organic matrices and thus elevated ambient BAFs in the Arctic compared to the Baltic Sea. This would imply that bioaccumulation in the Arctic may be described in the same way as bioaccumulation in temperate regions, e.g. by the use of mechanistic models parameterised for the Arctic.     

Delineation of Pb contamination pathways in two Pectinidae: The variegated scallop Chlamys varia and the king scallop Pecten maximus

Bioaccumulation of Pb was determined in Chlamys varia and Pecten maximus exposed to ²¹⁰Pb via seawater, food and sediment. Both scallops readily concentrated dissolved Pb with whole-body 7-d concentration factors of 250 ± 40 and 170 ± 70, respectively. In both species, more than 70% of Pb taken up from seawater was strongly retained within tissues (biological half-life > 1.5 month) whereas Pb ingested with phytoplankton was poorly assimilated (< 20%). As P. maximus lives buried in the sediment, this exposure pathway was assessed and showed low bioaccumulation efficiency for sediment-bound Pb (transfer factor < 0.015). Despite the poor transfer efficiency of Pb from food and sediment, the use of a global bioaccumulation model indicated that the particulate pathway (food and/or sediment) constituted the major bioaccumulation route of Pb in both scallops. Whatever the exposure pathway, the digestive gland and kidneys always played a major role in Pb accumulation. In scallop tissues, Pb was predominantly associated with the insoluble subcellular fraction, suggesting a low bioavailability of Pb for scallop consumers.     

Runoff of pharmaceuticals and personal care products following application of dewatered municipal biosolids to an agricultural field

Municipal biosolids are a useful source of nutrients for crop production, and commonly used in agriculture. In this field study, we applied dewatered municipal biosolids at a commercial rate using broadcast application followed by incorporation. Precipitation was simulated at 1, 3, 7, 21 and 34 days following the application on 2 m² microplots to evaluate surface runoff of various pharmaceuticals and personal care products (PPCPs), namely atenolol, carbamazepine, cotinine, caffeine, gemfibrozil, naproxen, ibuprofen, acetaminophen, sulfamethoxazole, triclosan and triclocarban. There was little temporal coherence in the detection of PPCPs in runoff, various compounds being detected maximally on days 1, 3, 7 or 36. Maximum concentrations in runoff ranged from below detection limit (gemfibrozil) to 109.7 ng L^− 1 (triclosan). Expressing the total mass exported as a percentage of that applied, some analytes revealed little transport potential (< 1% exported; triclocarban, triclosan, sulfamethoxazole, ibuprofen, naproxen and gemfibrozil) whereas others were readily exported (> 1% exported; acetaminophen, carbamazepine, caffeine, cotinine, atenolol). Those compounds with little transport potential had log K_ow values of 3.18 or greater, whereas those that were readily mobilized had K_ow values of 2.45 or less. Maximal concentrations of all analytes were below toxic concentrations using a variety of endpoints available in the literature. In summary, this study has quantified the transport potential in surface runoff of PPCPs from land receiving biosolids, identified that log K_ow may be a determinant of runoff transport potential of these analytes, and found maximal concentrations of all chemicals tested to be below toxic concentrations using a variety of endpoints.     

Sorption of organics on clay and synthetic humic-clay complexes simulating aquifer processes

Sorption/partition of several organic solute (contaminants) of a wide range of hydrophobicities was studied on clay and on clay-humic complexes representing aquifer-soil systems. The role of the mineral and of the organic (humic) fractions was elucidated and a model considering both fractions in the sorption process was proposed. The adsorption constants on humic (organic fraction), K_∝, were 8–20 times higher than on “pure” clay, K_m. But with soils with low to medium organic fractions (ƒ_∝ < 0.05) the contribution of the clay mineral to adsorption was quite significant, in spite of the fact that half of the sorption sites on the mineral surface were blocked by the humic. In the range of very low organic content in aquifer soil a non-linear pattern going through a minimum is observed between the overall partition coefficient and soil-organic fraction, transforming to the familiar linear relationship at higher ƒ_∝s. Both the K_∝ and K_m followed the linear-free energy relationship to the octanol-water partition coefficient K_ow.