2.3 Field Methods
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2.3.3.2.2 Lake Level
For interpreting lake chemistry data, the lake level can be especially useful.
Of particular importance is the likelihood that lake chemistry will vary with
precipitation cycles. During drought periods, a higher proportion of inflowing
water may follow relatively deep flow paths, allowing for greater acid neutralization and BC mobilization. During wet periods, drainage water may preferentially follow shallow flow paths, allowing less contact with soils and geologic
materials and therefore limited acid neutralization. Seepage lakes may receive
proportionately greater inflow of groundwater (which, depending on geological and soil conditions, may be rich in BC) during drought periods. It is also
possible that some seepage lakes might lose their connection with the groundwater during drought, causing the opposite effect.
The extent of such influences on hydrology and consequent acid neutralization is expected to be region and watershed specific. Measurement of lake level
at the time of sampling can provide critical data to help sort such effects. The
simplest way to collect such data is to install a fixed staff gauge in the lake and
record the lake level at each sampling time. In wilderness settings, or other
locations where installation of a staff gauge is not allowed or is impractical,
relative lake level can be documented by measuring the vertical height of the
lake surface below a fixed landmark (e.g., a large shoreline rock or tree root).
2.3.3.3 Ancillary Data (Chemical and Physical Information
That Is Not Necessary But May Be Useful)
2.3.3.3.1 Chemical Data
Physicochemical data can be collected on site with field equipment for measurements such as pH, specific conductance, DO, and turbidity. This information can
be useful for reconnaissance work and sample site selection or for other investigations in which real-time information is needed to direct field activities. In
general, however, we recommend that assessments or monitoring of acid-base
chemistry or nutrient status should be made using chemical analyses conducted
in the laboratory rather than the field. Commercially available equipment can
produce data of quality similar to that of laboratory equipment for some variables. However, reproducing the clean, controlled environment of a laboratory
in the field is difficult. Therefore, if real-time data are not required to satisfy the
objectives of a particular study or other requirements, * conducting the chemical analysis in the laboratory is recommended to ensure high data quality. If
chemical analysis will be required in the field, premobilization and field calibration checks should be conducted. Other types of chemical measurements can
* For example, in situ measurement of pH is sometimes required for data used by state 303(d)
water quality assessment programs to determine waters to be classified as water quality limited according to the Clean Water Act.
69
2.3.3.2.2 Lake Level
For interpreting lake chemistry data, the lake level can be especially useful.
Of particular importance is the likelihood that lake chemistry will vary with
precipitation cycles. During drought periods, a higher proportion of inflowing
water may follow relatively deep flow paths, allowing for greater acid neutralization and BC mobilization. During wet periods, drainage water may preferentially follow shallow flow paths, allowing less contact with soils and geologic
materials and therefore limited acid neutralization. Seepage lakes may receive
proportionately greater inflow of groundwater (which, depending on geological and soil conditions, may be rich in BC) during drought periods. It is also
possible that some seepage lakes might lose their connection with the groundwater during drought, causing the opposite effect.
The extent of such influences on hydrology and consequent acid neutralization is expected to be region and watershed specific. Measurement of lake level
at the time of sampling can provide critical data to help sort such effects. The
simplest way to collect such data is to install a fixed staff gauge in the lake and
record the lake level at each sampling time. In wilderness settings, or other
locations where installation of a staff gauge is not allowed or is impractical,
relative lake level can be documented by measuring the vertical height of the
lake surface below a fixed landmark (e.g., a large shoreline rock or tree root).
2.3.3.3 Ancillary Data (Chemical and Physical Information
That Is Not Necessary But May Be Useful)
2.3.3.3.1 Chemical Data
Physicochemical data can be collected on site with field equipment for measurements such as pH, specific conductance, DO, and turbidity. This information can
be useful for reconnaissance work and sample site selection or for other investigations in which real-time information is needed to direct field activities. In
general, however, we recommend that assessments or monitoring of acid-base
chemistry or nutrient status should be made using chemical analyses conducted
in the laboratory rather than the field. Commercially available equipment can
produce data of quality similar to that of laboratory equipment for some variables. However, reproducing the clean, controlled environment of a laboratory
in the field is difficult. Therefore, if real-time data are not required to satisfy the
objectives of a particular study or other requirements, * conducting the chemical analysis in the laboratory is recommended to ensure high data quality. If
chemical analysis will be required in the field, premobilization and field calibration checks should be conducted. Other types of chemical measurements can
* For example, in situ measurement of pH is sometimes required for data used by state 303(d)
water quality assessment programs to determine waters to be classified as water quality limited according to the Clean Water Act.