Example Questions and Answers for Helping Determine Data Representativeness
This table provides some questions to help health assessors examine sampling data and determine if they are representative of site exposures. All sites are unique, and this tables shows some common questions to consider. Health assessors might be able to answer these questions based on a sampling program’s DQOs (assuming that they have been developed) or by attempting to ascertain them by reviewing the documentation and discussing with the entity that conducted the sampling.
What to Do
|What media were sampled?||Find out if the agency or party sampled all affected media, and the media associated with potential exposures.||At a playground with soil contamination, an agency collected subsurface soil samples. You are concerned about children and others who come to the playground and contact soil at the surface. In this case, you might request sampling of surface soils.|
|Were appropriate contaminants sampled and analyzed?||Ensure that the media were sampled and analyzed for the specific contaminants you are investigating.||A health assessor suspected that homes located on a military base had per- and polyfluoroalkyl substances (PFAS) in their drinking water supply. You requested sampling data but were provided only analytical results for volatile organic compounds and pesticides. You might request sampling be conducted for the contaminants of interest.|
|Were the data adequate to evaluate the exposure pathway of concern?||Make sure the data collected can provide information about the specific exposure pathway that you are examining.||A small neighborhood overlies a trichloroethylene (TCE)-contaminated groundwater plume. Residents are concerned about inhaling vapors while showering and bathing. To date, only groundwater samples have been collected and analyzed. In this case, since the available data provide concentrations of TCE in groundwater, you would examine options to evaluate exposures occurring via inhalation of TCE.|
|Were the sampling data collected in the exposure unit?||Examine data for each contaminant in each completed and potential exposure pathway by exposure unit. You will identify exposure units to determine the areas where a person has contact with an environmental medium. You will need sampling data that are representative of the exposure units.||Four homes are located next to a former smelter. The residents living here are concerned about exposure to contaminants in the vegetables they are eating from their gardens. For sampling, you would define four separate exposure units to represent one for each different garden. This decision assumes that residents obtain their vegetables from their own private gardens.|
|Are the sampling methods appropriate?||Examine the sampling methods used to ensure that they are suitable for the contaminants you are evaluating. The selected sampling method usually dictates the contaminants that can be measured and in what concentration ranges.||An agency provided you with the results of a biological sampling data effort. However, upon close examination of the documentation provided, you learn that the agency did not follow the established sampling and analysis methods as outlined in its quality assurance project plan (QAPP). At this point, you need to consider whether the data are appropriate for use, or whether a request for new sampling is warranted.|
|Are the detection limits for the sampling method used low enough to enable an evaluation of health hazards?||Examine whether the detection limits used by the analytical laboratory are low enough to make public health determinations (i.e., they should be below the ATSDR health-based comparison value [CV]). The detection limit is the lowest level of a contaminant that analytical equipment can discern from the “noise” inherent to scientific measurements.||A laboratory reported that a contaminant was not detected in a sample, but the reported detection limit was more than 10 times higher than ATSDR’s contaminant-specific CV. You suspect the contaminant is present in the environment. The non-detect result is not sufficient for determining that contamination was below a CV. Thus, you are able to conclude only that the actual concentration is somewhere between zero and the reported detection limit. Consider requesting additional sampling that uses more sensitive methods to evaluate the concentrations of the contaminant at the range of interest.|
|Were samples collected in areas where exposures to the highest concentrations of site contaminants are expected?
|Evaluate whether samples were collected in locations where potential exposures are expected to be highest.||The nearest ambient air monitoring station to a large municipal landfill is approximately 1 mile downwind. Ask yourself if this proximity is close enough to capture the highest ground-level (or breathing zone) effects of the landfill’s emissions. Knowing that passive releases from landfills tend to have their highest effects closer to the source would help in such evaluations. It is also important to determine where residents live with respect to the monitoring location.|
|Were samples collected over time to understand the temporal extent of contamination?
|Evaluate if the data collected represent past, current, and future levels of pollution.||For an industrial site that has discharged wastewater to a river for 20 years, with surface water monitoring data available only for the last 5 years, ask yourself if the recent data are representative of past levels of pollution. Changes in the facility’s production levels and wastewater treatment practices over the years would be important to consider.|
|Are the number and placement of samples appropriate?
|Examine if the samples collected explain the spatial extent of potential exposure with the exposure units.||At a site with groundwater contamination, you should ask yourself whether the number, placement, and screen depth of monitoring wells are sufficient for characterizing the spatial extent of contamination to which people are most likely exposed. Also, consider whether an adequate number of residential and municipal water supply wells have been tested.|
|How are the contaminants distributed? Are there “hot spots”?
|A hot spot is an area with elevated contamination. Ask yourself if sampling locations identify such areas of elevated contamination.||When discharged to rivers, hydrophobic contaminants (e.g., polychlorinated biphenyls) tend to accumulate primarily in depositional areas, often resulting in “hot spots.” Consider whether samples were collected at the areas with highest suspected contamination. Review DQOs to determine whether the sampling program was designed to characterize “hot spot” contamination levels in the first place.|
|Are the sampling data from discrete grab sampling or long-term sampling efforts?
|Grab samples, taken at one time and from one place, represent a snapshot in time. In contrast, long-term sampling efforts provide continuous assessment of contaminants over time. Evaluate whether the type of available sampling data represents environmental contamination at the site.||During the wake of a major industrial release, one air grab sample was collected from a plume before people evacuated the area. While one sample may not be representative of air quality over the longer term, the one sampling result might provide insights on the peak exposure levels that occurred in the wake of the release.|
|Is the frequency of sampling adequate to characterize the public health threat?||Assess the timing used for sample collection and determine if samples were collected at an appropriate frequency based on the specific contaminants, media, and possible types of exposures.||Methane is often measured in on-site gas monitoring wells at operating landfills on a weekly basis. However, landfill gas concentrations increase or decrease greatly in just a few hours because of local weather changes. If people live adjacent to the landfill, weekly sampling might not be sufficient to characterize potentially hazardous acute exposures or physical hazards due to an explosion.|
|What are the measured concentrations at the exposure point?||Determine if sampling is being conducted for contaminants at locations where there is potential for contact with the medium of interest.||Sampling from a single municipal water supply well shows elevated levels of chlorinated solvents, but water from this and many other municipal supply wells feed into a complex distribution system before ever reaching homes. For such scenarios, you should check with the water supplier to help inform a decision on how much the water is likely to be diluted before reaching a resident’s tap. If concerned about water quality at the tap, you may recommend sampling at that specific exposure location.|
|In what forms were contaminants sampled and analyzed?||Make sure that the samples are analyzed for the types of contaminants of interest.||When working on a site with extensive electroplating operations, the site owner proposes collecting air samples and analyzing them for chromium. You should ensure that the samples are analyzed for the types of chromium of interest. In this case, analytical methods that can distinguish hexavalent chromium from trivalent chromium should be used.|
|Based on your knowledge of the site, does the pattern of contamination make sense?||Check that the sampling results indicate the breakdown of contaminants and a pattern associated with site activities.||At a site with air releases of tetrachloroethylene (PCE) from a soil excavation project, you would expect the highest concentrations of PCE to occur in the immediate vicinity of the excavation site and to decay with downwind distance. If concentrations increase with downwind distance, however, you should conduct additional research to understand why. In this case, check for sampling data quality issues or the presence of other PCE sources in the area.|
|Are background level data available and were they compared to concentrations for the potential contaminants of concern?||Examine if the statistical analyses of the sampling data demonstrate that the average contamination levels in your exposure unit are lower than, higher than, or consistent with background levels. If concentrations of your potential contaminants of concern are higher than background levels, you can generally conclude that some source — either the site you are evaluating or some other source — has contaminated the media of concern.||You are examining a playground, located within a large park, that has arsenic-contaminated soil. You compare the concentrations detected at the playground to background levels for soil in the local area and learn that the contaminant concentrations are much higher than background levels. Based on this examination, you are confident that the arsenic in the playground is not background contamination. After speaking with city officials who tend to the playground, you learned that arsenic-containing pesticides were historically used in various locations throughout the park area.|