Water Journal : Water Journal September 2012-1
potable reuse refereed paper technical features 54 SEPTEMBER 2012 water Materials and Methods Initially, a stepwise framework was developed that could be applied to determine the prioritisation category of all target parameters. The framework goes through simple dichotomous questions about each parameter and takes into account method detection limits, chemical characteristics and expected occurrences above guideline values before producing PDFs where required (Figure 2). Based on this stepwise analysis, chemicals were sorted into three prioritisation categories: Category 1 (high priority), Category 2 (medium priority) and Category 3 (low priority). The scientific basis for the guideline value of chemicals binned in Category 1 was examined to determine the significance of any potential short-term exceedances. Where the monitoring dataset does not provide sufficient data points to generate a PDF, the margin of safety (MoS) between the highest concentration and the guideline value was used. We cautiously chose 0.1% as our trigger for likelihood of exceedance from the PDF, as this would translate into 1 in every 1,000 samples being above the guideline value. The second trigger value of 0.001% was 100× lower than the first trigger to incorporate a large safety factor. This means that the parameter would be detected above guideline value in 1 out of 100,000 samples. We used "10× below the guideline value" when dealing with the MoS as a commonly used safety factor to buffer for uncertainty in the data. Our criteria requiring at least 8 data points to apply the PDF approach is based on a minimum sample size needed to produce a reliable distribution (Khan, 2010). Chemicals in Category 1 were also assessed separately in terms of both a chemical fate model (Hawker et al., 2011) and hydrodynamic model (Gibbes et al., 2010) to determine the predicted extent of natural attenuation and the expected range of concentrations at the dam wall (location of the closest downstream water treatment plant off-take), and again compared with the relevant guideline. Results and Discussion The following paragraphs describe a chemical identified at each of the steps of the framework, starting with chemicals singled out because of their detection limit, chemicals that trigger PBT alerts, chemicals that were detected only occasionally, and chemicals detected routinely for which is it possible to produce a PDF. Method detection limit trigger The first step of the framework is to determine if the chemical has ever been detected during the monitoring period, and if not, to ensure that the method detection limit used is appropriate. Of the 468 parameters monitored in Lowood PRW between 13/5/2009 and 23/1/2012, 402 (86%) were never detected above the method detection limit (MDL). For those parameters, it is essential to compare the MDL with the guideline value to ensure that these have not been eliminated due to insufficiently sensitive analytical methods. Of those 402 parameters, 16 had an MDL higher than or equal to the guideline value (GV /MDL≤1)andafurther21hadanMDL lower but less than 10× lower than the guideline value (GV / MDL < 10). Those 16 parameters with an MDL higher than or equal to the current PHR guideline (GV / MDL ≤ 1) fall in Category 1n (High Priority) (Table 1). In this case, it becomes important to understand the basis for the guideline (e.g., chronic animal data, acute animal data) to determine the significance and consequence of any potential exceedances. Where a guideline was based on the threshold of toxicological Figure 2. Stepwise decision tree to assign parameters to their priority categories. MDL = method detection limit; PBT = persistent, bioaccumulative and toxic; GV = guideline value. Prioritisation sub-grouping letters are: n = "not detected"; R = "radiological"; P = "PBT triggered"; d = "detected only once"; h = "highest concentration -- not based on PDF".
Water Journal November 2012-1
Water Journal August 2012