No abstract

Over the past decade, San Francisco Bay surface waters have remained enriched with dissolved (<0.45 microm) silver and lead concentrations (decadal means of 5.7 ng kg(-1) Ag[filtered] and 31 ng kg(-1) Pb[filtered]) compared with those (0.26 ng kg(-1) Ag[filtered] and 2.7 ng kg(-1) Pb[filtered]) of adjacent oceanic surface waters of the northeast Pacific, despite efforts to reduce pollutant loadings to the Bay during that period. While time series models show that there has been a 40% decline in total lead concentrations in the southern reach of the estuarine system between 1989 and 1999, the filtered lead fraction has not changed significantly during that time. That persistence is attributed to (i) the ongoing input from previous atmospheric depositions and industrial lead to its drainage basin, which are slowly being advected into the estuary and (ii) the internal recycling of lead between the surface sediments and the water column within the Bay. In contrast, both filtered and total silver concentrations in the southern reach have declined by 70% and 40%, respectively, within the past decade. These temporal declines are attributed to a 2-fold decrease in silver loadings from publicly owned treatment works and a comparable decline in the silver concentration of surficial sediments within that region during the past decade. In the northern reach, silver and lead concentrations have remained essentially constant between 1989 and 1999, reflecting invariable anthropogenic input of these elements to this embayment over that decade.

Inter-organisational sampling trials were performed at a reclaimed landfill site in Bedfont, West London, UK, to assess the uncertainty in measured concentrations of landfill gas. Eight participants independently measured concentrations of methane, carbon dioxide and oxygen, each on one occasion, over a two week period in February 1999. The organisers monitored the natural variability of the gas concentration over this sampling period using a control borehole in order to separate the variance that arose as a result of the sampling procedure from that caused by natural geochemical variation. A collaborative trial in sampling showed the extent of agreement between participants' results when they measured gases using the same, nominal protocol and equipment. The protocol was identified as fit for the purpose of estimating the mean concentration of methane, carbon dioxide and oxygen in a single borehole to within 21%, 15% and 321% of their assigned values, respectively (at 95% confidence). A realistic estimate of uncertainty in measurements of concentration from different protocols was estimated by the use of a sampling proficiency test, in which participants collected measurements using a protocol selected on the basis of their own professional judgement. Each participant's performance was compared with the consensus mean for each of the three gases using the z-score assessment proposed by the International Harmonised Protocol. This test highlighted the potentially large uncertainty in measured gas concentrations that can arise (91% relative uncertainty for CH4 in one borehole), and identified that an improved sampling protocol is required if misclassification of the explosive potential of methane is to be avoided.

No abstract

The concept of constructing a synthetic reference sampling target (RST), for the purpose of estimating bias and uncertainty caused by the process of sampling, has been shown to be feasible. An example of an RST has been prepared for measuring the concentration of contaminants, and delineating their spatial extent in 'hot spots', in contaminated land investigations. A circular hot spot was prepared by spiking a defined volume of top soil with barium sulfate in a previously 'clean' site in Ascot, UK. The assigned values for the concentration and spatial distribution of the contamination were based largely on the mass and position of the spike added to the site. These values were corroborated and refined by thorough sampling of the whole RST and of the hot spot in detail. The temporal stability and heterogeneity of the RST were tested and found to be acceptable for the intended use of the RST. The use of the site for inter-organisational sampling trials to estimate measurement uncertainty will be reported in subsequent papers.

The performance of samplers in their ability to delineate spatially a region of contamination has been assessed by the use of a sampling proficiency test (SPT) on a synthetic reference sampling target (RST). The RST was used to compare the participant's estimate of the extent of contamination with that of the assigned dimensions. These dimensions were defined based on the analyte spiked into the site and verified by the collection of field samples. Nine participants collected soil samples independently using protocols selected on the basis of their own professional judgement. Test portions from these samples were then analysed by the SPT coordinator under repeatability conditions. The resulting concentration measurements were sent to each participant for them to make their own interpretation of the spatial distribution of the contamination above a defined threshold concentration. For each participant the reported spatial extent of contamination was compared with the assigned dimensions of the hot spot to provide a score based on a novel adaptation of the International Harmonised Protocol. The consensus of the participants' spatial delineations that were deemed fit-for-purpose using a cost-effectiveness scoring system were used to estimate the area within which the assigned hot spot was positioned with 95% confidence. The score for this 'consensus' hot spot area was 0.64 when compared with the assigned dimensions, where a score of 0 indicates perfect spatial delineation and a score of @3 indicates acceptable performance for this particular investigation.

No abstract