|
|
 |
Contamination of Groundwater is more complex than surface water pollution mainly because of difficulty in its timely detection and slow movement , which d requires special expertise to predict the path and rate of Groundwater movement. In addition the complex geo-chemical reactions taking place in the subsurface
between myriad contaminants and earth materials are not always well-understood.
Ideally speaking contamination should be prevented from occurring. After a contaminant
or several contaminants are found in groundwater, a decision must be made on whether
to rehabilitate the aquifer or find alternative groundwater resources.
| Tracer | Lowest Detectable Concentration | Advantages
| Disadvantages |
| Dyes: Uranine (Sodium fluorescein Rhodamine B Sulforhodamine G. Extra |
0.01-0.04 µg/l | Easy to use, safe concentration can be measured in the field
| Some dyes affected by pH, temperature, or are absorbed by clays and organic soils. |
| Strong Electrolytes Sodium chloride Potassium Chloride Lithium Chloride | 1-0.01 mg/l | Can be measured in field or laboratory by electrical conductivity or electrical resistivity, Smaller concentrations measured by atomic absorption spectroscopy |
Must use large amounts of salts if ordinary analytical methods are used. |
|
Radioactive Isotopes Tritiated water Iodine ion Many others
| 0.01 mg/l
or lower
|
Can be used in such small quantities that don’t effect on physical and chemical properties of water occurs, Concentrations easily measured by sophisticated equipment. Cost of tritiated water very low |
May be radiation danger (not in case of tritiated water). Requires expensive detection equipment
|
| Detergents Alkylbensol-sulfonates | 0.05 mg/l | Easy to use, safe | May be confused with sewage-related detergents. Material disperses in soil, thereby changing its permeability. |
| Responsible Factor | Most Probable Response |
| Groundwater pollution originating on land surface | |
| Infiltration of contaminated surface water | Contamination of stream side aquifer due to polluted stream |
|
Land disposal of waste |
Contamination due to direct disposal of waste |
| Stock piles (ore) tailings (over burden dumps |
Release of mineralized leachate |
| Disposal of Sewage/Sludge | Release of Biological mineralized leachate |
|
Salt spreading on road |
Pollution due to winter time road salting |
| Animals feed lots | Biological waste |
| Fertilizer and Pesticides | Run-off resulting from indiscriminate use of such items |
|
Accidental spills |
Spill of in-transit chemicals and contamination due to spray water used during such mishap. |
|
Air borne source |
Acid/alkali rain particulates as fall out from smoke/ flue dust automobile pollutants |
|
Groundwater pollution originating above the groundwater table | |
| Septic tanks Cess pools | Biological contamination of groundwater |
|
Surface impoundment |
Leachate from lagoons for storage/treatment of sewage industrial wastewater oil field brines spent acids etc. |
| Underground storage tanks/pipelines | Corrosion and /or leakage |
| Artificial Recharge | In case of improper operation the recharge may lead to increased concentration of nitrates, metals, bacteria, viruses, detergents etc. |
|
Groundwater pollution originating below the groundwater table
| |
|
Waste disposal in wet excavations |
Contamination through abandoned mines |
| Agriculture drainage wells | Drainage of agricultural residues from marshes /ponds |
|
Well disposal of waste |
Contamination due to direct injection of waste |
| Secondary recovery of petroleum | Migration and ingress of hydrocarbons |
|
Mines |
Percolation of mine water |
| Exploratory wells and test holes | Inter-linking of aquifers leading to dissemination of pollutants |
|
Abandoned wells |
Direct migration of mineralized fluids |
| Water supply wells | Contamination by surface run-off |
| Excessive groundwater development | Salt water ingress |
Aquifer Restoration
Once it is established that contamination of groundwater has occurred , some action must be taken to -
· Find and eliminate the sources ;
· Contain the contaminants
in the area already affected ;
· Restore the water quality of the aquifer.
In majority of the areas, groundwater may be the only fresh water resource, hence restoration of aquifers may be of the highest priority regardless of the cost involved.
Wherever feasible identification and elimination of contaminants must be given the first priority. In case of difficulty, containment of the contaminant source is the other option in aquifer restoration. Recent research has shown that leakage is observed in virtually all land-fills, even if various types of plastic liners or clay liners have been used to retain the leachate. Capping abandoned land-fill sites with bentonite or other low permeability material prevents rain water from entering the site, thus eliminating the formation of leachate.
A combination of 'containment' and 'abatement' is one way to effect aquifer restoration. Containment- usually focuses on some hydraulic means of preventing the spread of the contaminant either through withdrawal of contaminated water or the injection of clean water to create a pressure ridge, Withdrawal of groundwater can reverse the local groundwater gradient, thereby preventing the advance of the contaminant front.
Slurry walls can also be used to isolate areas of contaminated groundwater.
Slurry walls consists of bentonite, water and backfill material placed in deep
trenches as deep as 100 ft. Ordinarily slurry walls last 20 to 40 years however
its life is greatly affected by the groundwater chemistry.
 |
Additional methods of aquifer restoration include
a) Re-oxygenation of
groundwater by means of air compressors and wells to accelerate the growth of
aerobic bacteria that metabolize contaminates.
b) Addition of nutrients to wells to stimulate the growth of bacteria
c) Recharge
of aquifers to facilitate flushing out contaminants adsorbed onto soil particles.
d) Air stripping the contaminated water in an air-stripping tower to remove volatile
organics.
Fluctuation in Groundwater levels
Water level fluctuation can result from a wide variety of hydrologic phenomena, some natural and source induced by man. In many cases, there may be more than one mechanism operating simultaneously. Summary of these mechanisms is discussed ahead wherein, they are classified according to whether they are natural or man-induced , whether they produce fluctuation in confined or unconfined aquifers and whether they are short lived, diurnal, seasonal or long term in time frame.
While monitoring the groundwater level it has been observed that in most of man-induced factors leading to over exploitation of groundwater the effective groundwater resource management stresses on two-pronged strategy, the one relates to 'Static' groundwater level and the other 'Dynamic' groundwater level. While the static water level pertains to the levels not affected by recharge and other seasonal influence, the dynamic water level - as the name implies has greater, influence of seasonal impacts. In an effective groundwater management strategy, the static water level and its restoration should be assigned priority to obviate the possible hazards of groundwater mining(over-exploitation) and associated geo-technical problems.
 |
| 
|