Data used in the graphs came only from the "high intensity" sampling period, which was a period of 3 - 4 years during which each Study Unit collected most of their samples. Outside of this "high intensity" sampling period, some other samples were collected, less frequently, for long-term monitoring purposes; these "low intensity" samples are EXCLUDED.
In general, for all data types, only regular samples were included in the graphs; samples taken for quality assurance and quality control purposes were NOT included in the graphs.
Other sample selection criteria used include:
Most NAWQA ground water sites were sampled only once. Therefore, from each well (even those sampled MORE than once), only one sample was used in the graphs. For all data groups except volatile organic compounds (VOCs), the sample identified by the Study Unit as the "primary sample" for that well, was selected for inclusion. In cases where more than one sample was identified as the "primary sample", only one sample was used (the first in time).
However, some VOC samples were inadequately collected originally, requiring some wells to be re-sampled for VOCs. Therefore, for wells that had re-samples taken, the VOC data from the re-samples were used.
For surface water nutrients and pesticides, only regular samples taken in fixed frequency, extreme high flow, or extreme low flow sampling modes, were included in the graphs. Thus, these graphs do NOT include nutrient and pesticide samples taken in storm, diurnal, or synoptic sampling modes.
All sediment and tissue samples taken as part of NAWQA's "occurrence and spatial distribution surveys" were included in these graphs, if they were from sites identified as Agricultural, Urban, or Mixed landuse. This excludes only a few samples taken in special efforts to sample certain locations for specific reasons, which are excluded so as not to bias the national dataset.
For tissue, only samples of fish species were used in the graphs; insect, clam, and other invertebrate samples were excluded to maintain comparablity in this national context. Only whole fish samples for Organochlorines were used, and only fish liver samples for Trace Elements were used in the graphs.