With the increasing use of ground water for agricultural, municipal and industrial needs, the annual extractions of ground water are far in excess of net average recharge from natural sources. Consequently, ground water is being withdrawn from storage and water levels are declining, resulting in crop failures, adverse salt balance, and sea water intrusion in coastal aquifers and land subsidence in areas where drafts result in compaction of sediments.
In many instances, the overdraft is due to the diminishing opportunity for natural recharge of ground water basins due to such factors as:
(i) Lining of stream channels and concentration of surface runoff by flood control projects.
(ii) Discharge of sewage and industrial wastes through closed sewage disposal systems.
(iii) Sealing of natural recharge areas with impervious side walls, streets, air ports, parking lots and buildings.
(iv) Diversion and export of waters which might otherwise percolate naturally in the stream channels.
Artificial recharge (or replenishment) is one method of modifying the hydrological cycle and thereby providing ground water in excess of that available by natural processes. It is accomplished by augmenting the natural infiltration of precipitation or surface water into underground formations by some method of construction, by ponding or spreading of water, or by artificially changing the natural conditions.
The following are the favourable conditions for natural or artificial recharge:
(i) Formation of sand, gravel, or highly fractured rocks either underground or exposed over a large area or in stream channels.
(ii) The presence of caverns fractured or faulted zones or numerous small cavities in rock formations (limestone areas) either underground or exposed on the land surface or stream channels.
(iii) Karsts or sinkhole topography.
(iv) The absence of barriers for horizontal or vertical movement of ground water.
(v) Feasible locations for installation of recharge wells, dams, diversions or other recharge structures.
(vi) Wide braided streams, broad alluvial fans and glaciofluvial deposits may present excellent opportunities for water spreading.
Methods of Artificial Recharge:
The methods of artificial recharge are by direct flooding, water spreading in basins, ditches and furrows, Fig. 15.1, irrigation, modified stream bed or natural channel, percolation dams, pits and shafts, injection wells, and induced recharge by lowering the water table by pumping water from wells, collectors, galleries located near surface water sources like lakes or streams, Fig. 15.2 (a), (b).
For confined aquifers or shallow beds where ponding is not practicable, recharging is effected by pumping water down the wells at rates rather less than the corresponding withdrawal rates. Mostly recharge water is excess surface water, but industrial waste water, sewage and uncontaminated cooling water from industrial and air-conditioning plants are used in some countries.
In areas where the pervious formations are at a shallow depth, recharging is done by digging pits or shafts. Abandoned gravel pits have been utilised occasionally. If storm waters are to be recharged through shafts, consideration should be given to removal of silt. Injection rates of 20 m/day was noticed in the first 1/17 ha of Peoria pit at Illinois, USA. Fig. 15.3.
Injection wells may be drilled downstream of a dam and the water released from the spillway is conveyed into the wells. In areas of cavernous limestones and gypsum, recharge wells may be placed upstream of a flood water retarding basin or other structure, Fig. 15.4 (a), (b). The intake, provided with trash rack, should be well below the crest of the spillway but several metres above the bottom to aid in de-silting. A pumping well may also be used as a recharge well.
The quality of the injected water is very important. Suspended solids, biological and chemical impurities, dissolved air and gases, turbulence and temperature of both the aquifer and injected water will have an effect on the life and efficiency of a well by clogging or corrosion of the well screen. Large amounts of dissolved air in the recharge water tend to reduce the permeability of the aquifer by ‘air binding’. The injected water should have temperature only slightly higher than the temperature of the aquifer.
The use of fresh water barriers by ground water recharge to prevent sea water intrusion is practised extensively on the sea coast in southern California, Fig. 15.5. The fresh water barrier should be far enough inland to force all the wedge back seaward. Otherwise, the fresh water will separate the wedge and force the landward edge still farther inland, creating a saline wave. A series of spreading grounds or injection wells or a combination of both could be utilised as dictated by the geologic conditions encountered.
Injection rates are maintained in wells along the coast of Manhattan Beach in Southern California by using chlorinated water, free from suspended solids. The high cost of chlorinated water is justified in this case, since the system of injection wells protects an inland ground water basin from sea water intrusion.
In Netherlands, the fine-sand beds are recharged with treated water from the Rhine, to provide water storage for supply as well to act as a barrier against the inward seepage of sea water from the North Sea.
Spreading in natural stream channels, Fig. 15.6 that are not subject to year round flow is an effective method. No additional land is required and the stream beds tend to be self-cleaning. Inexpensive small levee systems can be constructed between storm periods to maximise coverage, or permanent drop type structures can be incorporated.
Water meandering the canals over a part of 32 ha Rohrer Island, Dayton, Ohio has recharged ground water at the rate of 13.2 ha-m/day, (Fig. 15.6).
A typical plan of a basin type recharge project is shown in Fig. 15.7. In projects designed principally for the purpose of spreading storm waters, multiple basins are advantageous since the first of a series of basins can be utilised for settlement of silt. The de-silting or detention basin should be large enough to reduce the velocity of flow substantially, and its inlet and outlet facilities should be so located that short circuiting is prevented.
In Fig. 15.8, at the spreading ground, the infiltration rate, f = K (hs + ls/ls)
In impeding layer, percolation rate, p = K1 × (h’s + ls1)/ls1.
When mound meets saturated soil column, infiltration is controlled by area through which lateral flow moves × K × (h’s/l) + area through which water moves into impeding layer × K1 × (h’s + ls1)/ls1.
The mechanics of recharge by spreading is illustrated in Fig. 15.8. The infiltration rates were determined with infiltrometers—open ended cylindrical or square units driven into the soil and constant heads maintained, prior to the construction of ELRIO spreading ground, Fig. 15.9. In the figure, if hs = 30 cm, ls = 7.5 cm, K= 30 cm/day, the infiltration rate is-
Beneath the saturated soil column the soil moisture content trends to be between field capacity and field saturation, Fig. 15.10. Water reaching an impeding layer creates a water table or mound which moves upward and outward or laterally. The amount of water moving through the impeding layer largely depends on its permeability. If for clay soil K1 = 0.06 cm/ day, average height of the water table on the impeding layer is 3 m and the length of the saturated column beneath the top of the impeding layer is 7.5 cm. The percolation gradient is very high ip = (3 + 0.005)/0.075 = 41
But the percolation rate is only V = K1 ip = 0.06 × 41 = 2.46 cm/day compared to f ;(V = p).
If for a mound height of 6 m, the water has moved out 30 m, the lateral flow gradient = 6/30 = 0.2. If the lateral flow is not great enough, the mound will build up high enough to contact the saturated column at the surface, i.e. there is hydraulic continuity between the impeding layer and soil surface. When this occurs the infiltration rate declines and is controlled largely by the lateral flow. The same situation occurs when percolating water builds up on a water table. This is an important consideration in the design as the type, size and shape of the basin affect the rate of lateral flow.
A multipurpose spreading operation which emphasises the disposal of suitably treated agricultural waste waters and spreading of local storm of flood waters and imported water for additional ground water replenishment, provides a promising method. Rotational spreading basins may be used to allow significant drying periods for recovery in infiltration rate.
The infiltration rates can be increased by certain soil treatments like vegetation, organic residues (bacteria), chemicals (ferric sulphate), grits and sand materials, and certain physical approaches like spreading when the infiltration rate is high (rates decline with time due to microbial sealing) and using relatively high depths of water to increase the gradient.
Cotton trash consisting of boll hulls, leaves, stems, a few seeds and a small amount of lint, when mixed with soil and given a moist incubation period, is effective in increasing infiltration rates. Various grasses, particularly the Bermuda, improve the intake rate of the underlying soil. Alternate wetting and drying periods of 7-14 days with cultivation during the dry cycle gives maximum spreading rate. Drying kills microbial growths and this, along with scarification of soil, reopens soil pores.
Strips or portions of an area may be used where subsurface layers limit the flow. Water will accumulate and spread laterally on a subsurface layer. Schiff (1954) suggested spacing strips on the infiltration—percolation rate ratio. If the ratio is 10, about the same amount of recharge could be obtained by using 1/10 of the land as by using the entire area. Strips treated to increase the infiltration rates, or strips in the form of channels or shafts in the bottom of channels may further reduce the areas required.
Multiple rectangular basins have more lateral flow opportunity for recharge. This is illustrated in the following example:
Assume a 400 ha basin with uniform medium textured and (f = 0.6 m/day) up to 60 m and little or no permeability below 60 m. Recharge at f= 0.6 m/day for 3 months = 21,600 ha- m. Assuming a specific yield of 24%, water stored between depths 3 to 60 m = 57 × 400 (0.24) = 5472 ha-m. Lateral flow to adjacent area = 21,600 – 5472 = 16,128 ha-m.
The greater the perimeter for a given area the greater the lateral flow opportunity:
(а) A 400 ha square basin of size 2,000 × 2,000 m has a perimeter of 8000 m.
(b) 10 square basis of 40 ha each of size 630 × 632 m has a total perimeter of 25,280 m.
(c) 10 rectangular basins (L = 4B) of 40 ha each of size 1264 × 316 m have a total perimeter of 31,600 m.
The ratio of perimeters or lateral flow opportunity-
a:b = 1:3.2
a:c = 1:3.9
b:e = 1:1.25
Hence, ten rectangular basins of 40 ha each may be adopted for spreading. Thus, more lateral flow will occur if a number of spreading areas are used.
Water spreading ranges from 0.3 to 3 m/day in USA. Estimates of the rates of recharge in full scale basins conducted in USA indicate an average discharge rate of 1,125 lpd/m2, recharge through wells of 180 to 3900 lpm, recharge through wells of storm drainage 360 to 5000 lpm or more and sewage and waste water 0.06 to 0.36 m/day. Treated sewage can also be recharged through wells. Continuous operation is possible with regular chlorine injections and redevelopment by pumping about 4% of the recharged water. Pit recharge rates of 0.3, 0.4 and 3 m/day are used for the underlying aquifers of sandstone, fine sand, and sand and gravel, respectively.
In Maharashtra, a number of percolation dams are built and the artificial lake created will improve the ground water conditions in the areas in the vicinity, where a number of wells may be sunk.
Deep, confined aquifers can be recharged by a recharge well. Its flow is the reverse of a pumping well but its construction may or may not be the same.
If water is passed into a recharge well, a cone of recharge will be formed which is reverse of a cone of depression for a pumping well, Fig. 15.11.
The approximate steady-state equations for recharge rate Q into a completely penetrating well are:
Though the above equations are similar to discharge equations from a pumping well, the recharge rates are seldom equal to pumping rates.
Well recharge rates in USA vary from 500 to 5000 m3/day. Initially the intake rates are high and gradually decrease or become constant. High intake rates are found in porous formations like limestones and lavas.