Analysis Water Quality Monitoring and Human Health
standard, given the frequent connection between groundwater and source water for drinking water.
• How does the groundwater data compare to criteria established under Section 304 of the Clean Water Act?
Ask what else stands out to a hydrogeologist. Comparing Sampling Data to Flow Data
If you have data from different days, particularly if the results are from before, during and after rain events, compare your monitoring data to flow data. Flow data is available online from the United States Geological Survey (USGS). The USGS maintains gaging stations on many of the nation’s rivers. Unfortunately due to federal budget cuts in the past two decades, some gaging stations have since closed. If you cannot obtain flow data, call the National Weather Service and ask for precipitation data for the days just prior to and during your sampling. Flow (or precipitation) data can provide you with useful clues about the nature of the pollution that you are measuring. Do high levels of contaminants coincide with days of high flow and/or precipitation? If so, the likely source of those pollutants is non-point source run-off from the land. Runoff could be from residential areas, industrial plants, farms, orchards, or other land uses. If high levels of contaminants occur on days with low flow and on days with high flow, chances are, you’re dealing with a point source of pollution, such as a pipe from a factory.
Note the two charts on the next page. The graphs show flow data (gray area graph) plotted alongside bacteria levels (red bar charts). Note that in the first graph bacteria levels seem to rise and fall with flow, indicating that most bacteria are likely coming from non-point source runoff. In the second graph by contrast, bacteria levels appear high on all sampling dates, regardless of flow, indicating that more likely the bacteria is coming from a regularly flowing point source.
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