Methods and Instrumentation used for Investigation of Saltwater Intrusion

Methods and Instrumentation used for Investigation of Saltwater Intrusion

Late in the 1960's, efforts rose toward drilling for chemical analysis of groundwater samples and the determination of flow patterns based on piezometric levels.  Geophysical methods of investigation were introduced later, and were found to provide more information faster than the drilling techniques.  Subsequently, geophysical methods became more important for saltwater intrusion monitoring.

Today, there are numerous methods available including: well logging, chemical analysis of groundwater samples, research into the interaction between aquifer matrix and groundwater, and most common, chloride concentration profiling, and vertical conductivity and temperature profiling.   

Conductivity and Temperature used to Estimate Salinity

An aqueous solution's ability to carry an electrical current by means of ionic motion is measured through conductivity.  Salinity is the measured mass of dissolved salts (ions) in a solution. As such, conductivity readings provide a good indication of salinity.  In general, as salinity increases, the total dissolved solids (TDS) of a solution increases, and so too does conductivity.

As defined by the USGS, salt water has a total dissolved concentration of 35,000 mg/L, of which, 19,000 mg/L is chloride (Barlow, 2003).  Being the major constitute of salt water, chloride concentration profiling is a very common method for saltwater intrusion investigations.  As the concentration of chloride increases in salt water, so does conductivity.   As such, conductivity is a very good indicator of chloride content and salinity.

Conductivity is interdependent with temperature; therefore profiling both of these variables becomes an important factor when determining the behavior of the transition zone and the salt-water interface. 

Through using devices such as the Solinst Model 107 TLC Meter(Temperature, Level, Conductivity), salinity can be estimated through conductivity and temperature readings, both taken at a discrete depth.  The TLC Meter features a 'smart' probe that provides accurate temperature and conductivity measurements, and is attached to high quality flat tape for depth readings.  The probe and tape are mounted on a sturdy reel making operation easy.  Instruments such as this make vertical temperature and conductivity profiling simple.

For example, using standard methods, a conductivity reading of 25,000 µS/cm and a temperature reading of 20˚C yield a salinity estimation of 17ppt (APHA et al, 2005).  Through this method of investigation, borehole profiles of salinity can be used to track the fluctuation of the salt-water interface.  This, in turn increases the potential to control saltwater intrusion problems.

For continuous monitoring of the salt-water interface, an instrument such as the LTC Levelogger Edge allows accurate datalogging of conductivity along with temperature and water levels as often as every 5 seconds.  The LTC Levelogger Edge is ideal for long-term saltwater intrusion monitoring applications due to its compact, low maintenance, waterproof design.

The LTC Levelogger Edge combines a datalogger, 5-year battery, memory for 16,000 sets of readings, pressure transducer, and temperature and conductivity sensors in a small 22 mm x 190 mm housing.  It is simple to deploy, calibrate, and program.

To easily create a network of monitoring wells, the LTC Levelogger Edge can be integrated into a Solinst STS Gold Telemetry System, which allows convenient access to remote, real-time data.  The STS system also sets alarms to trigger when a specific conductivity level is reached, notifying personnel of potential saltwater intrusion conditions.  The LTC Levelogger Edge is also SDI-12 compatible and can be integrated into an SDI-12 or SCADA network.

By using an instrument like the LTC Levelogger Edge, the salt-water interface can be tracked over time, and provide real-time warnings when intrusion conditions occur or worsen.