Groundwater Management | Geography

Groundwater management involves the planning and operation of the groundwater resources to provide safe and reliable water supplies. In groundwater management the factors that need to be considered include aquifer yield, recharge, water quality, socioeconomic and legal factors.

Groundwater Basins and Safe Yield:

The occurrence and movement of groundwater is confined to independent hydrologic units known as groundwater basins. Groundwater basins can be demarcated using hydrogeological boundaries like hill ranges, rivers, deep drainage channels or any other groundwater divides.

For a groundwater basin, the water balance equation can be written as –

Inflow – outflow = change in storage.

The inflow to the groundwater basin consists of subsurface inflows, recharge from rainfall, seepage from unlined channels and irrigated areas and any other imported water. The outflows are subsurface outflows, withdrawals by pumping and water use by vegetation (in case of shallow water tables).

Safe yield of a groundwater basin is defined as the amount of water which can be withdrawn annually from the basin without producing any undesirable results. Withdrawal in excess of safe yield is known as overdraft. Excess withdrawal from the groundwater basin will result in several undesirable results.

The amount of water available for pumping will be reduced gradually. The water levels in the aquifers decline leading to higher pumping costs. It is therefore, desirable to maintain a water balance in the groundwater basins.

The concept of safe yield is based on the principle that groundwater is a renewable resource. The safe yield of a basin could change from time to time depending upon the natural and economic considerations. There are several methods for determining the safe yield of groundwater basins.

Two methods given by Todd (1980) are the Hills method and the Harding method. In the Hills method (Fig. 9.2a) the annual changes in elevations of groundwater levels are plotted against annual draft. Safe yield is considered as the draft corresponding to no change in groundwater level.

In the Harding Method (Fig. 9.2b) the retained inflow i.e., the difference between the total inflow and outflow from the basin is plotted against the annual average change in elevation of the water levels in the basin. Again, the safe yield is considered as the quantity at which there is no change in groundwater elevation.

When a groundwater basin is subjected to overdraft, to reduce the consequent undesirable effects, withdrawals could be controlled. In addition, more water could be added to the aquifers by means of artificial recharge methods.

Hydrological Investigations for Groundwater Management:

Management of groundwater resources is usually planned on the basis of groundwater basins. However management of individual tubewells is equally important.

The following hydrological investigations are generally useful in planning for groundwater management:

1. Study and analysis of meteorological factors like precipitation, evaporation, etc., in a given area.

2. Rainfall—runoff relations and rainfall—infiltration studies to estimate availability of water for recharge and also contribution of rainfall for recharge.

3. Study of groundwater levels during and before monsoon.

4. Inventory of existing wells and their discharge.

5. Aquifer tests to determine their properties.

6. Collection and analysis of water samples for their quality.

7. Estimation of seepage and recharge contributions from canals, ponds and agricultural areas.

8. Study of flows in streams and drainage channels in the area and their relationship with groundwater.

Groundwater Recharge:

Recharge of the aquifers occurs from natural resources like rainfall, seepage from canals, ponds etc. In areas where groundwater is extracted beyond the natural recharge, artificial recharge may become necessary.

Artificial recharge is the process of replenishing the aquifers by adding water to the storage by establishing some facilities for recharge. Artificial recharge helps in increasing the groundwater supply and conservation of water resources.

In addition to these benefits, artificial recharge is used sometimes for preventing land subsidence due to excessive groundwater withdrawal and for controlling sea water intrusion into coastal aquifers.

Artificial recharge projects require both water for recharging the aquifers and aquifers which are capable of absorbing the water and making it available for pumping later.

Methods of Recharge:

The various methods for artificial recharge may be classified as follow:

1. Water spreading.

2. Percolation ponds.

3. Pits and shafts.

4. Injection wells.

5. Induced recharge.

Water spreading for recharge can be achieved by using simple flooding, basins, ditches, furrows, natural channels or irrigated areas. These methods are practiced when large areas are available and aquifers to be recharged are unconfined with a permeable layer above it.

Percolation ponds require less area than the spreading methods, but involve construction costs. Farm ponds could also serve the purpose of groundwater recharge. In percolation ponds, the recharge rate is to be maintained by clearing the bed frequently but in farm ponds seepage is usually controlled.

Pits and shafts are used in areas where an impervious layer is encountered at shallow depths. Pits and shafts could break the impervious layer and maintain a high recharge rate. Pits and shafts may be constructed for the purpose of recharge or sometimes existing ones made for other purposes could be used with minor modifications.

Injection wells or recharge well are used to recharge deep confined aquifers. Flow in such wells is the reverse of pumping wells. Injection wells are relatively costly to install and operate and have to be used under special conditions like demand on groundwater and controlling intrusion of salt water using recharge wells.

Induced recharge refers to the withdrawal of water near a surface source like river so that seepage rate increases into the aquifers due to lowered water table. This method is effective when the source is connected to the adjoining aquifer.

Requirements for Artificial Recharge:

For undertaking recharge projects, the following requirements need to be satisfied:

1. The groundwater basin should have aquifers with enough storage capacity and transmissibility properties.

2. Adequate water should be available for recharge.

3. The recharge method selected should be able to maintain the rate of recharge.

4. Quality of water recovered after recharges should be satisfactory.

5. The pumping lifts for pumping the recharged water should not be excessive.

Groundwater Contamination:

Groundwater contamination refers to the degradation of water quality in the aquifers and can occur either due to natural or artificial sources. Natural contamination is a result of the chemical and physical processes that transfer impurities to the water through the earth’s crust. Sea water intrusion is another source of natural contamination. Artificial contamination or pollution results from human activities.

Groundwater contamination can occur from several sources. These include industrial wastes, solid waste disposal sites, waste water treatment ponds, agricultural area, mine spills, septic tank, tile fields, etc. The type of contaminants added will depend on each particular situation that is considered. In general, substances which are of main concern in a drinking water supply can be considered as contaminants to a groundwater reservoir.

Domestic Wastes:

Domestic wastes and the methods of their disposal are of primary concern in an urban situation. The quantity and quality of domestic wastes from an area could vary, but the four prime quality concerns are pathogenic organisms, oxygen demand, nutrients and solids. Nutrients, particularly nitrogen and phosphorous are of major importance.

A part of the domestic wastes are taken to the solid waste disposal sites. They are partly burned and partly incorporated in the soil. These could form potential groundwater contamination sources and the leachates from such areas enter the aquifers below. If the same aquifers are exploited for drinking water supplies, care should be taken regarding the movement of contaminants in these aquifers.

Runoff from Urban Areas:

The runoff from rainfall in the urban area is another potential source as it may contain relatively large concentrations of oil, grease, nutrients pesticides and heavy metals. The runoff, where there are no proper outlets could accumulate into small ponds and depending upon the subsoil strata may join the aquifers below.

The rainfall in the city could also pick up substantial contamination from dust and air pollution before it reaches the ground. Its pollution capacity is substantial and cannot be neglected.

Agricultural Wastes:

The wastes from agriculture could be significant in certain situations. Agricultural wastes may be categorized as animal wastes, processing wastes and agricultural chemicals. A major problem in agricultural areas is the runoff from farm ladus using large quantities of fertilizers. The contribution of agricultural wastes to groundwater contamination appears to be marginal, but specific situations needs to be investigated.

Industrial Wastes:

Industrial development has become a part in several urban areas. Most of the industries, as a result of manufacturing, produce some kind of wastes. Industrial wastes could mainly consist of inorganic chemicals and heavy metals.

Industries also draw heavy supplies from groundwater and also produce contaminants on a large scale. Wherever industrial complexes form a part of the urban environment, the situation needs a thorough analysis from a groundwater contamination point of view.

Contamination of groundwater is somewhat different from contamination of surface water. In case of surface water, contamination may be transient lasting a few days or weeks, whereas contamination of groundwater could persist over long time. As groundwater is used for drinking purposes directly at many locations, it is desirable to protect groundwater from contamination.

Protecting Groundwater Quality:

The following general steps are suggested for this purpose:

1. The contaminant sources should be surveyed in a given groundwater basin.

2. Location of a disposal site (either for industrial or municipal wastes) should be done considering the groundwater hydrology of the area.

3. In case of likely presence of harmful constituents particularly in industrial effluents, steps should be taken for some predisposal treatment by the industry itself.

4. Regular monitoring or groundwater quality in suspected areas of contamination should be carried out.

5. Location of wells for drinking water supplies should be done with utmost caution. Surrounding contaminant sources and flow directions should be taken into account. It is also not advisable to tap the uppermost aquifer in case of drinking water wells. For drinking water wells, disinfection using standard procedures should be carried out before the well supplies are used for human consumption.

Groundwater Models:

Groundwater models aim in describing the behaviour of a groundwater system. These models are useful for purposes like predicting aquifer response for external changes like extraction and recharge, movement of contaminants within the aquifer system etc. These models are also useful as management tools as several management alternatives like number and spacing of wells and their effect on water table, effect of location and quantity of recharge, etc. can be analysed.

Groundwater models can be grouped into three general categories:

i. Physical models,

ii. Analogue models and

iii. Mathematical models.

In the earlier stages of development of this area, physical and analogue models were used. Mathematical models which are more versatile than the other approaches are now widely used in groundwater management.