A. Considerations
- Field water quality measurements should always be collected at the sample collection point, including pH, temperature, dissolved oxygen and conductivity. Ion-specific probes are often available for some water quality instruments to screen for nitrate and chloride. These may be useful tools in evaluating the best sampling location(s).
- If stream flow gaging is part of the assessment, sample(s) should be collected along the same profile.
- Sample(s) should be collected in straight-segment stream reaches that have uniform flow and a uniform bottom contour. Strong stream current can create eddies that represent non-uniform flow and should be avoided when present.
- All sampling locations should be clearly documented, along with GPS coordinates.
- Bridges and roads should be noted when up-stream from a sampling location. The goal of the baseline assessment should be clearly defined (i.e. to document the possible influence of such infrastructure or to document stream water quality conditions up-stream).
- Stream intersections (confluences) can result in poorly-mixed flow, if sampled at the immediate confluence. If the aggregate water quality of two streams is desired, the sample should be collected far enough down-stream from the confluence to ensure well-mixed and unidirectional flow.
- Larger streams will likely require field water quality profiling, both perpendicular to the stream and with depth to assess any water quality variations. Observance of significant variations should prompt the collection of multiple samples or compositing for appropriate sample analytes.
- Stream water quality should be expected to vary between predominantly base-flow conditions and when surface run-off is the main contributing water source. Baselines should consider and document under what conditions samples are collected.
- Seasonal variations should be expected and considered. For example, ice/snow pack melt-water provides a significant contribution to stream flow in the spring. At the end of summer/fall with few precipitation events, stream flow may be entirely derived from base-flow.
- The USGS's National Field Manual for the Collection of Water-Quality Data should be consulted when preparing for the sampling of flowing-water sites.
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