Understanding the uncertainty of estimating herbicide and nutrient mass loads in a flood event with guidance on estimator selection.

The aim of this study was to understand the uncertainty of estimating loads for observed herbicides and nutrients during a flood event and provide guidance on estimator selection. A high-resolution grab sampling campaign (258 samples over 100 h) was conducted during a flood event in a tropical waterway in Queensland, Australia. Ten herbicides and three nutrient compounds were detected at elevated concentrations. Each had a unique chemograph with differences in transport processes (e.g. dependence on flow, dilution processes and timing of concentration pulses). Resampling from the data set was used to assess uncertainty. Bias existed at lower sampling efforts but depended on estimator properties as sampling effort increased: the interpolation, ratio and regression estimators became unbiased. Large differences were observed in precision and the importance of sampling effort and estimator selection depended on the relationship between the chemograph and hydrograph. The variety of transport processes observed and the resultant variability in uncertainty suggest that useful load estimates can only be obtained with sufficient samples and appropriate estimator selection. We provide a rationale to show the latter can be guided across sampling periods by selecting an estimator where the sampling regime or the relationship between the chemograph and hydrograph meet its assumptions: interpolation becomes more correct as sampling effort increases and the ratio becomes more correct as the r2 correlation between flux and flow increases (e.g. > 0.9); a stratified composite sampling approach, even with random samples, is a promising alternative.

Monitoring Herbicide Concentrations and Loads during a Flood Event: A Comparison of Grab Sampling with Passive Sampling.

The suitability of passive samplers (Chemcatcher) as an alternative to grab sampling in estimating time-weighted average (TWA) concentrations and total loads of herbicides was assessed. Grab sampling complemented deployments of passive samplers in a tropical waterway in Queensland, Australia, before, during and after a flood event. Good agreement was observed between the two sampling modes in estimating TWA concentrations that was independent of herbicide concentrations ranging over 2 orders of magnitude. In a flood-specific deployment, passive sampler TWA concentrations underestimated mean grab sampler (n = 258) derived concentrations of atrazine, diuron, ametryn, and metolachlor by an average factor of 1.29. No clear trends were evident in the ratios of load estimates from passive samplers relative to grab samples that ranged between 0.3 and 1.8 for these analytes because of the limitations of using TWA concentrations to derive flow-weighted loads. Stratification of deployments by flow however generally resulted in noticeable improvements in passive sampler load estimates. By considering the magnitude of the uncertainty (interquartile range and the root-mean-squared error) of load estimates a modeling exercise showed that passive samplers were a viable alternative to grab sampling since between 3 and 17 grab samples were needed before grab sampling results had less uncertainty.

Passive sampling of perfluorinated chemicals in water: in-situ calibration.

Perfluorinated chemicals (PFCs) have been recognised as environmental pollutants that require monitoring. A modified polar organic chemical integrative sampler (POCIS) is able to quantify aqueous PFCs. However, with varying external water velocity, PFC sampling rates (Rs) may change, affecting accuracy of derived water concentrations. To facilitate field deployment of this sampler, two methods of in-situ calibration were investigated: performance reference compounds (PRCs) and passive flow monitors (PFMs). Increased Rs's (by factors of 1.2-1.9) with PFM loss rate (g d(-1)) were observed for some PFCs. Results indicate PFMs can be used to correct PFC specific Rs's for more reliable estimates of environmental concentrations with a precision of about 0.01 L d(-1). Empirical models presented provide an improved means for aquatic monitoring of PFCs. The PRC approach was unsuccessful, confirming concern as to its applicability with such samplers.

Method for the in situ calibration of a passive phosphate sampler in estuarine and marine waters.

Passive samplers for phosphate were calibrated in the laboratory over a range of flow velocities (0-27 cm s(-1)) and ionic strengths (0-0.62 mol kg(-1)). The observed sampling rates were between 0.006 and 0.20 L d(-1). An empirical model allowed the estimation of these sampling rates with a precision of 8.5%. Passive flow monitors (PFMs), based on gypsum dissolution rates, were calibrated for the same range of flow velocities and ionic strength. Mass loss rates of the PFMs increased with increasing ionic strength. We demonstrate that this increase is quantitatively accounted for by the increased gypsum solubility at higher ionic strengths. We provide a calculation scheme for these solubilities for an environmentally relevant range of temperatures and salinities. The results imply that co-deployed PFMs can be used for estimating the flow effect on the in situ sampling rates of the phosphate samplers, and we expect that the same may hold for other passive samplers.

A new method to quantify the application thickness of sunscreen on skin.

Proper application of sunscreen is essential as an effective public health strategy for skin cancer prevention. Insufficient application is common among sunbathers, results in decreased sun protection and may therefore lead to increased UV damage of the skin. However, no objective measure of sunscreen application thickness (SAT) is currently available for field-based use. We present a method to detect SAT on human skin for determining the amount of sunscreen applied and thus enabling comparisons to manufacturer recommendations. Using a skin swabbing method and subsequent spectrophotometric analysis, we were able to determine SAT on human skin. A swabbing method was used to derive SAT on skin (in mg sunscreen per cm(2) of skin area) through the concentration-absorption relationship of sunscreen determined in laboratory experiments. Analysis differentiated SATs between 0.25 and 4 mg cm(-2) and showed a small but significant decrease in concentration over time postapplication. A field study was performed, in which the heterogeneity of sunscreen application could be investigated. The proposed method is a low cost, noninvasive method for the determination of SAT on skin and it can be used as a valid tool in field- and population-based studies.