2.3 Field Methods
63
measured by a pressure transducer that is installed in a deep portion of the
stream so that it will remain below water during low flows. A large number
of pressure transducers that vary in design and price are commercially available. The water level measured by the pressure transducer is typically recorded
by the data logger at 15-min intervals. The data logger transfers this information to a data storage module for retrieval during site visits. The data logger
can be programmed to trigger the autosampler to collect a sample based on
the rate and direction of change of the water level, so that samples can be collected on ascending and descending limbs of the hydrograph as well as at the
peak. Programming the data logger for this type of sampling will require some
knowledge of flow variability of the stream being monitored. A weatherproof
box is required to protect the data logger, storage module, and battery needed
to run the data logger and autosampler. Autosamplers are usually kept in a
shelter to limit the chance of vandalism.
For automatic collection of samples, the autosampler should be placed on
the bank where there is no risk of it being washed into the stream during high
flows. This also allows the sample tube to drain freely after sample collection.
Autosampling requires that the sampling tube extend into the stream and that
the suction head at the end of the tube is anchored in a deep, well-mixed portion of the channel where it will not be easily dislodged by high flows. The suction head should be positioned so that it will not draw in sediment from the
bottom. The entire section of the sampling tube underwater also needs to be
well anchored. The pressure transducer and its line to the data logger should be
similarly anchored on the stream bottom.
Flow-activated autosamplers should be visited promptly after hydrological
events to retrieve the samples and reset the autosampler. Disposable, powderless latex or nitrile gloves should be worn while handling sampling equipment
to reduce the chance of contamination. Care must be taken to avoid touching
potential contaminating surfaces while wearing the gloves.
On arrival at the site, bottles in the autosampler that have collected samples
should be capped and labeled with the date, site, and their position number
in the autosampler. These bottles are then removed and replaced with clean,
empty bottles with caps removed. The autosampler is then prepared for sampling by resetting the counter and sampler spout to sample position number 1.
Data are then downloaded from the data logger. These data provide the date,
time, and water level associated with each sample that was collected. This
information can then be used to select samples for chemical analysis.
2.3.2.2 Collection of Lake Water
Lake sampling should normally be done by boat, raft, or float tube over the
deepest part of the lake. Such a sample is intended to represent the average
lake chemistry. If boat sampling is not possible, an alternative protocol is to
63
measured by a pressure transducer that is installed in a deep portion of the
stream so that it will remain below water during low flows. A large number
of pressure transducers that vary in design and price are commercially available. The water level measured by the pressure transducer is typically recorded
by the data logger at 15-min intervals. The data logger transfers this information to a data storage module for retrieval during site visits. The data logger
can be programmed to trigger the autosampler to collect a sample based on
the rate and direction of change of the water level, so that samples can be collected on ascending and descending limbs of the hydrograph as well as at the
peak. Programming the data logger for this type of sampling will require some
knowledge of flow variability of the stream being monitored. A weatherproof
box is required to protect the data logger, storage module, and battery needed
to run the data logger and autosampler. Autosamplers are usually kept in a
shelter to limit the chance of vandalism.
For automatic collection of samples, the autosampler should be placed on
the bank where there is no risk of it being washed into the stream during high
flows. This also allows the sample tube to drain freely after sample collection.
Autosampling requires that the sampling tube extend into the stream and that
the suction head at the end of the tube is anchored in a deep, well-mixed portion of the channel where it will not be easily dislodged by high flows. The suction head should be positioned so that it will not draw in sediment from the
bottom. The entire section of the sampling tube underwater also needs to be
well anchored. The pressure transducer and its line to the data logger should be
similarly anchored on the stream bottom.
Flow-activated autosamplers should be visited promptly after hydrological
events to retrieve the samples and reset the autosampler. Disposable, powderless latex or nitrile gloves should be worn while handling sampling equipment
to reduce the chance of contamination. Care must be taken to avoid touching
potential contaminating surfaces while wearing the gloves.
On arrival at the site, bottles in the autosampler that have collected samples
should be capped and labeled with the date, site, and their position number
in the autosampler. These bottles are then removed and replaced with clean,
empty bottles with caps removed. The autosampler is then prepared for sampling by resetting the counter and sampler spout to sample position number 1.
Data are then downloaded from the data logger. These data provide the date,
time, and water level associated with each sample that was collected. This
information can then be used to select samples for chemical analysis.
2.3.2.2 Collection of Lake Water
Lake sampling should normally be done by boat, raft, or float tube over the
deepest part of the lake. Such a sample is intended to represent the average
lake chemistry. If boat sampling is not possible, an alternative protocol is to