Many areas of the world use groundwater as their main source of fresh water supply. With the worlds population increasing at alarming rates, the fresh water supply is being continually depleted, increasing the importance of groundwater monitoring. One of the major concerns most commonly found in coastal aquifers, is the induced flow of salt water into fresh water aquifers caused by groundwater development, known as salt water intrusion. In places where groundwater is being pumped from aquifers that are in hydraulic connection with the sea, the induced gradients may cause the migration of salt-water from the sea toward the well. The key to controlling this problem is to maintain the proper balance between water being pumped from the aquifer and the amount of water recharging it. Constant monitoring of the salt-water interface is necessary in determining proper control measures.
The Ghyben-Herzberg Relation
Under hydrostatic conditions, the weight of a unit column of fresh water extending from the water table to the interface is balanced by a unit column of salt water extending from sea level to the same depth as the point on the interface.
This analysis assumes hydrostatic conditions in a homogeneous, unconfined coastal aquifer. According to this relation, if the water table in an unconfined coastal aquifer is lowered by 1 m, the saltwater interface will rise 40 m.
Since the early 1960's, the coastal aquifers of China have been studied for salt-water intrusion. With a transition zone of 1.5 to 6.0 km, and an aquifer area of more than 580 km2, the increasing extension of the salt-water intrusion is a major concern in this area. Throughout the transition zone, mixing of local fresh water, sea water, and cation exchange can be recognized. Through the depletion of Na+ (Sodium) and enriched Ca2+ (Calcium), the cation (positively charged ion) exchange plays an ever increasingly important role.
A study conducted in the City of Laizhou in 1971, and in the City of Longkou in 1979 illustrated that salt-water intrusion had been caused by excessive pumping of the groundwater in these areas. In the beginning the observations were taken from some specific, isolated spots (0.5 km2). Eventually the intrusion area spread as increases persisted in agriculture and industry. In 1979, the salt-water intrusion area covered 16 km2, 39 km2 in 1982, 71 km2 in 1984, and 196 km2 in 1987. By 1989, the salt-water intrusion area became a continuous zone covering an area of 238 km2 in Laizhou. In the 1970's, the salt-water intrusion area in the southwestern part of the study area increased by 4 km2 each year. In the early 1980's, this number increased to 11.1 km2, and after the mid-1980's to 30 km2. This rapid increase reinforces the need for proper monitoring and controlling methods for salt-water intrusion.
Methods and Instrumentation used for Investigation
Late in the 1960's, efforts rose toward drillings for chemical analysis of groundwater samples and the determination of flow patterns based on piezometric levels. Subsequently, geophysical methods of investigation became more important, as more information could be obtained, faster than by using drilling techniques.
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. Conductivity is interdependent with temperature, therefore profiling of both these variables becomes an important factor when determining the behaviour of the transition zone and the salt-water interface. Through using a device such as the Solinst Model 101 Water Level Meter with a P4 probe, C4 Conductivity Sleeve, and T4 Temperature Sleeve, salinity can be estimated through conductivity and temperature readings taken at the same depth. For example, a conductivity reading of 25,000 µ,S/cm, and a temperature reading of 20C yields and salinity estimation of 17 ppt. Through this method of investigation,borehole profiles of salinity can be used to track the fluctuation of the salt-water/fresh-water interface. This, in turn increases the potential to control the salt-water intrusion problem.
Control of Salt-Water Intrusion
The increased use of groundwater has caused the salt-water interface to move inland and closer to the ground surface in Long Island, New York, southern California, and Florida. In the past, many communities coming across salt-water intrusion problems simply set up new production wells further inland. This only complicated the problem, but efforts to maintain groundwater levels by ponding surface runoff or river water to recharge the groundwater table have been implemented.
Other methods to control salt-water intrusion have been successful by creating a high potentiometric surface, which still allows for the pumping of groundwater below sea level landward of a groundwater ridge created by deep recharge wells. Potentiometric surface mapping of an aquifer can provide important information determining the direction of groundwater flow within a confined aquifer. This can be determined by plotting water-level elevations on a map and contouring the results. This contoured surface is known as the potentiometric surface, which is actually a map of the hydraulic head in the aquifer. In some instances, barrier wells have been set up near the shore to pump out salt-water, and recharge a fresh-water gradient toward the sea.
Therefore, recharge wells, recharge basins, and barrier wells have proven to be very useful in maintaining the proper equilibrium between groundwater recharge and pumping. Proper groundwater monitoring techniques, groundwater management, combined with groundwater conservation are needed to keep salt-water intrusion under control.