Geology Reference
In-Depth Information
GROUNDWATER RECHARGE STUDIES
Groundwater recharge has been measured at many locations in India by the
tritium tagging or tritium injection method. The method is based on the
assumption that the soil water in the unsaturated zone moves vertically
downward as discrete layers. Water added on the surface either as precipitation
or irrigation will move downwards by pushing the older water beneath and
this in turn will push the still older water further below, thereby the water
from the unsaturated zone is added to the groundwater reservoir. This flow
mechanism is known as the piston flow. Therefore, the vertical movement
of the injected tritium can be monitored in the soil column. The position of
the tracer is indicated by a peak or a maximum in the tritium activity versus
depth plot. However, molecular diffusion, dispersion and aquifer
heterogeneties may cause broadening of the peak. The methodology provides
spot measurements of natural recharge.
This method has been used by a number of workers in different
hydrogeological environments in India. Measurements have been carried out
for the last more than 25 years in 35 watersheds, water basins and
administrative blocks by various research workers (Table 3).
Natural recharge rates obtained from the tritium injection method were
compared with other methods e.g. water level fluctuation and groundwater
modelling. The recharge rates calculated from tritium injection method range
from 24 to 198 mm/yr. or 4.1 to 19.7% of the local average seasonal rainfall
depending upon the hydrogeological and climatic conditions (Table 4). The
replenishable groundwater potential of India, for normal monsoon years
based on tritium injection method, is calculated as 476 × 10
9
m
3
per year.
Table 4:
Rainfall recharge measurements in India
using tritium injection method
SI.. Basin/watershed/
Main rock
Rainfall
Natural recharge derived
(mm yr
-1
)
No.
Blocks
types
Median Mean Rainfall
(mm yr
-1
) (mm yr
-1
)
(%)
1.
Punjab
Alluvium
460
35
56
12.2
2.
Haryana
Alluvium
470
43
70
14.9
3.
Western Uttar Pradesh
Alluvium
174
195
19.7
4.
Churu district, Rajasthan
Alluvium
491
67
62
12.6
5.
Godavari-Puma basin,
Maharashtra
Basalt
652
50
56
8.6
6.
Lower Maner basin,
Warangal and Karimnagar
Sandstone
dists. Andhra Pradesh
and shale
1250
103
117
9.4
7.
Neyveli basin, Tamilnadu
Sandstone
1398
150
181
12.9
8.
Neyveli basin, Tamilnadu
Alluvium
1004
50
161
16.0
9.
Noyil basin, Tamilnadu
Granite,
Gneiss
715
35
69
9.6
(after Rangarajan and Athavale, 2000)
