Project Title: 
Application of Isotope Techniques for the Groundwater Investigations in the Kolar Semi-arid Regions.

Thematic Area:
Rampatna watershed in the Kolar Semi-arid Regions of Karnataka State

Location:
* Located in the semi-arid region of Kolar district in the Karnataka State. 
* Latitude: 3035’ -- 13040’ 
* Longitude: 77040’ -- 77043’. 
* The watershed area is spread out in two talukes, namely Chickaballapur and Gudibanda (Figure1).

Objective:
Groundwater investigations with the application of isotope techniques in the semi-arid regions of Kolar.

Abstract:
The increasing water demand has led to heavy exploitation of groundwater resources in many parts of the country, particularly in the arid and semi-arid regions. The water resources available for exploitation in the arid and semi-arid regions are often restricted to groundwater sources. Exploitation of this vital source leads to lowering of the groundwater table and water quality deterioration. Groundwater is recharged primarily by rainfall and surface water bodies such as rivers, lakes, tanks etc. Better understanding by the groundwater system and its recharge characteristics are very much important to know the safe yield and to evolve plans for optimal utilization of the groundwater resources. Groundwater sources are heavily exploited in the semi-arid region for various purposes. Isotopes and isotope-based techniques are increasingly used for the groundwater investigations especially in the semi-arid regions. The groundwater scenario of the Rampatna watershed of Kolar in the semi-arid regions was investigated with the application of isotope techniques. 

Data used: 
Estimation of groundwater recharge
The tritium tagging technique is one of the isotope methods applied to investigate the groundwater recharge through precipitation and irrigation. This method is based on piston flow model. The piston flow model assumes that the moisture movement in unsaturated soil media is vertically downwards in discrete layers. Water added near the surface due to precipitation or irrigation would percolate downwards by pushing an equal amount of water beneath it further down, and so on, till the moisture of the last layer in the unsaturated zone is added to the saturated zone. Further, it is assumed that the soil moisture in the unsaturated soil media remains constant during the infiltration process due to very slow movement and in the piston type flow, the effective soil moisture content remains constant, which is the field capacity. Seven locations have been identified in the study area for the tracer studies (Figure2). Tritiated water with tritium activity of 100m Ci for each site was prepared. At each site, five holes, equidistant along the perimeter of a circle of diameter 10-cm were driven to depths of 30 to 60 cm by using an iron rod of 10mm diameter. The tritium activity is divided into 5 equal parts of 20mCi/ml. The tracer was injected using a syringe through copper tubes, the end of which almost touched the bottom of the hole and the opening was closed by filling up with the soil to prevent the loss of the tracer due to evaporation. Tracer injection in the study area was carried out during the end of May 1999, before the onset of the southwest monsoon. Proper identification marks were established at the sites of tracer injection so as to facilitate easy identification of the tracer sites for collection of soil samples afterwards. The soil samples at intervals of 10-cm depth from each site were collected during January 2000, after the completion of the northeast monsoon. Soil moisture was extracted by vacuum distillation method and counted for tritium using Liquid Scintillation Counting System. Soil moisture content was also determined. The graphical representation of depth of soil sampling versus tritium concentration will indicate the movement of peak concentration of tritium and the depth at which the tritium peak is found. Vertical recharge due to rainfall is computed by knowing the displacement of tritium peak, the initial and final moisture content of the soil using the equation:

         Dθv
R = ---------- 100
           P
Where, R is the groundwater recharge in percentage, θv the average volumetric moisture content in the tritium peak shift region, D the shift of tritium peak (cm), P the precipitation (cm). 
Interaction of shallow and deep well aquifer systems, surface water and groundwater systems 
Investigation on the influence of surface water bodies in recharging groundwater aquifers is one of the critical aspects in groundwater investigations. Environmental stable isotope investigations are extensively used to study the interaction of two water bodies. Surface water system has a stable isotope composition significantly different from that of the local groundwater system. The stable isotope content of the surface and groundwater could be used to determine the interaction of the surface water and groundwater. Evaporation from the surface water body causes enrichment in stable isotope components, Deuterium and Oxygen-18, while the stable isotope composition of groundwater has not undergone evaporation. Interaction of the surface water body with the groundwater system indicates no significant difference in the stable isotope composition of their water samples. Insignificant difference in the stable isotope compositions of two aquifer systems indicates their interaction with each other.

Environmental stable isotope investigations have been attempted to investigate the interaction of the shallow and deep well aquifers and the interaction of the percolation tank (Bairasagara kere) located 2 km downstream of the watershed with the groundwater system in the study area. The groundwater samples collected from the shallow (dug well) and deep wells (bore well) located in different parts of the study area and the water samples collected from the percolation tank (Bairasagara kere) were analyzed for stable isotope composition (Oxygen-18 and Deuterium). 

Highlights/ Findings:
Investigation using Tritium tagging techniques shows that the groundwater recharge through rainfall in the study area is about 5 % of the annual rainfall. Environmental stable isotope investigations reveal that there exists interaction between the shallow and deep well aquifers in the study area. It also reveals that the percolation tank (Bairasagara kere) located about 2 km downstream of the watershed, has no significant influence in recharging the groundwater system of the Rampatna watershed. Of the 252 ha-meter of the net utilizable groundwater, the present utilization is about 80 percent and within a short span of time, the area will fall under the dark category from the point of view of the groundwater utilization. 

Output:
Estimation of groundwater recharge
The results of recharge estimation using tracer technique are presented in Table (Table1). The tracer studies indicated that there was very limited vertical movement of tritium, the soil moisture even after the onset of both the monsoons. The groundwater recharge, due to the rainfall in the study area is estimated to be very low. The study area is predominant in latertic soil and the recharge through the latertic soil media is very limited. In this experimental set up, estimation of soil moisture in the soil samples is very important and a small variation in the measurement can affect the estimation. Soil moisture depletion due to evaporation is also an important factor. Also, it is assumed that the soil moisture moment is vertically downwards. But in the soil media, along with the vertical soil moisture movement, there is also a horizontal movement.
Interaction of shallow and deep well aquifer systems, surface water and groundwater systems 
The stable isotope composition of the water samples from shallow wells (dug well) and deep wells (bore well) are presented in Figure (Figure3). It shows that there is no significant difference in the stable isotopic composition of the two systems (falls on the local meteoric line). It was therefore concluded that the water from the shallow well aquifers has mixed with that of the deep well aquifers i.e., the deep well aquifers are recharged by the shallow aquifers. The stable isotope composition of water samples collected from the percolation tank and groundwater in the study area is presented in Figure (Figure4). It shows that the stable isotope composition of the percolation tank is markedly different from that of the groundwater samples, which fall on the local meteoric line. It is therefore concluded that the percolation tank, which is located 2 km downstream, has no significant influence in recharging the groundwater system of the Rampatna watershed, as is generally expected. 

Groundwater evaluation
Based on a survey, well use statistics in the study area was investigated, the extent and pattern of groundwater and the scope for future development were determined. Drilling of bore wells for domestic and irrigation purposes has increased drastically due to the fact that the open wells are drying up during the summer periods. The private irrigation bore wells range in depth from 40 to 150 m and their yields range from 0.5 to 200 m3 per day. The watershed area in general can be categorized under low to moderately yielding from1000 to 8000 liter per hour in the case of bore wells. The total groundwater recharge from all the sources, such as due to rainfall and irrigation return flow, has been computed. The area under different lithological units was demarcated and the annual replenishable recharge was computed based on the specific yield and water level fluctuations. The present annual groundwater draft is estimated using the unit annual draft and the existing groundwater abstraction structures in the study area. Of the 252 ha-meter of the net utilizable groundwater, the present utilization is about 80 % and the projected groundwater utilization by 2005 AD is estimated as 96%. It is shown that within a short span of time the study area will come under the Dark category from the point of view of groundwater utilization. 

Date of project Completion:
15th November 2001

Project Investigator:
V P Dinesan, Scientist – C
Centre for Water Resources Development and Management (CWRDM),
Kunnamangalam PO, 
Kerala- 673 571
Phone: 0495 - 2355864
Email: dineshcwrdm[at]yahoo[dotcom