hydrologic properties (e.g., [5]), epikarst hydrogeochemical properties (e.g., [6–8]),
the influence of drip rates on isotopic composition of speleothems used for
paleoclimate interpretation (e.g., [9–11]), and the impact of seasonal climatic variations on cave dripwater (e.g., [12, 13]), among other topics.
Approaches to measure cave drip rates include manual measurement of the timing
between water droplets (e.g., [6, 14]); automated counting of water droplets by some
triggering mechanism, such as a drum (e.g., [15, 16]), or passage by an electroluminescent diode (e.g., [12]); manual timed volumetric measurement by collection
into a vessel of known volume; and automated volumetric measurement using a
tipping bucket style gauge, generally a modified rain gauge ([5], among many others).
Drop-counting methods suffer from the assumption of known and consistent volume
per drop. This has been shown experimentally to vary, but there is a general mathematical relation between drop mass and the radius of curvature of the drip point of
detachment [17]. The tipping bucket gauge approach is preferred for moderate to
high-flow dripwater, but may suffer from inaccuracies at very high discharges.
Decisions to use one method over another should consider these limitations.
One major advantage of using automated methods for hydrologic research in
karst systems is the ability to collect high-resolution datasets over long time
periods, from years to decades. Automated methods have greatly expanded over
the past several decades as sensor, data storage, and battery technologies have
improved. Data loggers can hold millions of data points, allowing for deployment
of equipment for months at a time, depending on temporal measurement frequency.
Thus, automated equipment has allowed for both higher resolution and longer term
monitoring. Researchers can address more detailed research questions and return to
repeatedly “mine” the dataset as additional questions regarding data of varying
temporal resolutions emerge.
In addition to cave drip rates, sensors that measure specific conductance can be
deployed in caves and programmed to collect data as frequently as needed,
depending on the research need and available data storage. For example, Baldini
et al. [10] and Shade and Veni [18] collected data on the rate and specific conductance of cave dripwater to examine connections between discharge and geochemical
composition. Sensors for temperature, relative humidity, and barometric pressure
are also available, easily deployable, and relatively robust under cave conditions.
Other sensors, such as those for dissolved oxygen and pH, are currently less robust
for use in caves and, depending on sensor type, can require more frequent calibration
and may be subject to fouling. Sensors for turbidity, a parameter of interest at many
sites affected by surface water, are available and robust, but are generally an order of
magnitude more expensive than the aforementioned sensors.
Many studies on epikarst and cave dripwater have included geochemical sampling of the dripwater for a variety of constituents, including major ions, alkalinity,
trace elements, organic carbon, and isotopes of water, strontium, and dissolved
inorganic and organic carbon (e.g., see [6, 8, 13, 16, 19, 20]). Methods for sample
collection of cave dripwater for geochemical analysis in these studies vary
depending on whether individual or groups of stalactites are being targeted for
sampling, the time period of collection, and the analytes of interest, many of which
Instrumenting Caves to Collect Hydrologic and Geochemical Data: Case Study. . .
207
the influence of drip rates on isotopic composition of speleothems used for
paleoclimate interpretation (e.g., [9–11]), and the impact of seasonal climatic variations on cave dripwater (e.g., [12, 13]), among other topics.
Approaches to measure cave drip rates include manual measurement of the timing
between water droplets (e.g., [6, 14]); automated counting of water droplets by some
triggering mechanism, such as a drum (e.g., [15, 16]), or passage by an electroluminescent diode (e.g., [12]); manual timed volumetric measurement by collection
into a vessel of known volume; and automated volumetric measurement using a
tipping bucket style gauge, generally a modified rain gauge ([5], among many others).
Drop-counting methods suffer from the assumption of known and consistent volume
per drop. This has been shown experimentally to vary, but there is a general mathematical relation between drop mass and the radius of curvature of the drip point of
detachment [17]. The tipping bucket gauge approach is preferred for moderate to
high-flow dripwater, but may suffer from inaccuracies at very high discharges.
Decisions to use one method over another should consider these limitations.
One major advantage of using automated methods for hydrologic research in
karst systems is the ability to collect high-resolution datasets over long time
periods, from years to decades. Automated methods have greatly expanded over
the past several decades as sensor, data storage, and battery technologies have
improved. Data loggers can hold millions of data points, allowing for deployment
of equipment for months at a time, depending on temporal measurement frequency.
Thus, automated equipment has allowed for both higher resolution and longer term
monitoring. Researchers can address more detailed research questions and return to
repeatedly “mine” the dataset as additional questions regarding data of varying
temporal resolutions emerge.
In addition to cave drip rates, sensors that measure specific conductance can be
deployed in caves and programmed to collect data as frequently as needed,
depending on the research need and available data storage. For example, Baldini
et al. [10] and Shade and Veni [18] collected data on the rate and specific conductance of cave dripwater to examine connections between discharge and geochemical
composition. Sensors for temperature, relative humidity, and barometric pressure
are also available, easily deployable, and relatively robust under cave conditions.
Other sensors, such as those for dissolved oxygen and pH, are currently less robust
for use in caves and, depending on sensor type, can require more frequent calibration
and may be subject to fouling. Sensors for turbidity, a parameter of interest at many
sites affected by surface water, are available and robust, but are generally an order of
magnitude more expensive than the aforementioned sensors.
Many studies on epikarst and cave dripwater have included geochemical sampling of the dripwater for a variety of constituents, including major ions, alkalinity,
trace elements, organic carbon, and isotopes of water, strontium, and dissolved
inorganic and organic carbon (e.g., see [6, 8, 13, 16, 19, 20]). Methods for sample
collection of cave dripwater for geochemical analysis in these studies vary
depending on whether individual or groups of stalactites are being targeted for
sampling, the time period of collection, and the analytes of interest, many of which
Instrumenting Caves to Collect Hydrologic and Geochemical Data: Case Study. . .
207