Water Chemistry Field Sampling
46
data (e.g., North Carolina data are found at http://nc.water.usgs.gov/info/h2o.
html) can be examined prior to going to the field to evaluate ambient stream
flow from stream gauges in the general area of the sample site. This precaution is more important when sampling stream, as opposed to lake, chemistry
and when water characterization will be based on a single sample rather than
multiple samples collected at different times. It is also important to consider
the potential influence of climatological wet/drought cycles on the chemistry
of surface waters.
2.2.3.1 Lake or Stream Characterization
The water chemistry of lakes or streams is often characterized on the basis
of a single sample, collected during the index season (spring or summer for
streams; summer or fall for lakes). In general, however, it is preferable, but not
necessary, to base surface water characterization, assessment, or modeling on
multiple samples (either collected throughout the annual cycle or restricted to
the index season) collected over several years.
2.2.3.2 Synoptic Survey
The time at which the water sample is collected during a synoptic survey can
influence the resulting chemistry and the ways in which the data can be used
and interpreted. Stream water chemistry, and to a lesser extent lake water chemistry, can vary diurnally (Burns 1996), seasonally (Lawrence et al. 2004), and
annually (Murdoch and Shanley 2006). Stream water chemistry can also vary
on an hourly basis in response to changes in flow (Lawrence 2002). Therefore, if
the objective is to conduct a synoptic survey to compare measurements among
different streams or lakes within the same region (a spatial assessment), all
sites would ideally be sampled at the same time, which is seldom possible. Even
this approach might not ensure the same sampling conditions if a localized
storm was affecting only part of the study region. Nevertheless, approaches can
be chosen to minimize the effects of temporal variability when conducting synoptic sampling.
The major causes of temporal variability in water quality are generally associated with flow and climate. Diel changes in pH and metal concentrations caused
by patterns of photosynthesis and respiration can also be important. Climate is
particularly important in regions where snow accumulation occurs in winter,
and water chemistry is affected by snowmelt (Campbell et al. 1995, Lawrence
et al. 2004). Seasonal effects can be easily addressed by restricting the sampling
to a period that falls within a single season. However, the choice of season can
also affect the frequency and magnitude of flow variations. For example, in
some regions, summer is the season that typically has the lowest flows and the
fewest rain- or snowmelt-driven hydrological events. Scheduling a synoptic
46
data (e.g., North Carolina data are found at http://nc.water.usgs.gov/info/h2o.
html) can be examined prior to going to the field to evaluate ambient stream
flow from stream gauges in the general area of the sample site. This precaution is more important when sampling stream, as opposed to lake, chemistry
and when water characterization will be based on a single sample rather than
multiple samples collected at different times. It is also important to consider
the potential influence of climatological wet/drought cycles on the chemistry
of surface waters.
2.2.3.1 Lake or Stream Characterization
The water chemistry of lakes or streams is often characterized on the basis
of a single sample, collected during the index season (spring or summer for
streams; summer or fall for lakes). In general, however, it is preferable, but not
necessary, to base surface water characterization, assessment, or modeling on
multiple samples (either collected throughout the annual cycle or restricted to
the index season) collected over several years.
2.2.3.2 Synoptic Survey
The time at which the water sample is collected during a synoptic survey can
influence the resulting chemistry and the ways in which the data can be used
and interpreted. Stream water chemistry, and to a lesser extent lake water chemistry, can vary diurnally (Burns 1996), seasonally (Lawrence et al. 2004), and
annually (Murdoch and Shanley 2006). Stream water chemistry can also vary
on an hourly basis in response to changes in flow (Lawrence 2002). Therefore, if
the objective is to conduct a synoptic survey to compare measurements among
different streams or lakes within the same region (a spatial assessment), all
sites would ideally be sampled at the same time, which is seldom possible. Even
this approach might not ensure the same sampling conditions if a localized
storm was affecting only part of the study region. Nevertheless, approaches can
be chosen to minimize the effects of temporal variability when conducting synoptic sampling.
The major causes of temporal variability in water quality are generally associated with flow and climate. Diel changes in pH and metal concentrations caused
by patterns of photosynthesis and respiration can also be important. Climate is
particularly important in regions where snow accumulation occurs in winter,
and water chemistry is affected by snowmelt (Campbell et al. 1995, Lawrence
et al. 2004). Seasonal effects can be easily addressed by restricting the sampling
to a period that falls within a single season. However, the choice of season can
also affect the frequency and magnitude of flow variations. For example, in
some regions, summer is the season that typically has the lowest flows and the
fewest rain- or snowmelt-driven hydrological events. Scheduling a synoptic