Abstract
Vertical electrical sounding (VES) survey can help indentify fresh groundwater formation for pumping to minimize secondary salinization problems. Two VES surveys (VES 1and VES 2) were conducted at farmer’s field, Chak No. 405/JB in Tehsil Toba Tek Singh, Punjab, Pakistan. About 100 soil and water samples were collected for water quality analysis and preparing well log profile to compare with VES interpretation of subsurface lithology. Pumping test was carried out to verify aquifer parameters of hydraulic conductivity (k) and transmissivity (T) estimated from VES survey data. The resistivity data were collected using resistivity meter Tarrameter (SAS 4000, Sweden). The resistivity data were analyzed using 1X1D computer software (Interpex, USA). The aquifer was recharged using treated canal water of volume 51, 71, and 99 m3 and was recovered after 7 days. The data analysis revealed that fresh groundwater (EC < 1.5 dS/m) was available from 8 to 15 m depth below ground surface having resistivity values of more than 41 Ω-m. Groundwater was of marginal quality from 15 to 20 m depth with resistivity values of 41 to 21 Ω-m and groundwater quality deteriorated further downwards. The recovery efficiency was found to be 80% for injected volume of 51 m3; 91% for 71 m3 and 98% for 99 m3. These results suggest that VES survey has the potential to assess the groundwater quality and quantity profile to skim freshwater as well as store water in the aquifer for its later recovery and usage.

Problem Statement
Groundwater is considered a reliable source of supplemental irrigation water because of its easy and flexible access to the farmers. This resource, however, is under serious threat due to its over-exploitation. Groundwater levels have started declining because of higher pumping rates. This huge abstraction of groundwater has resulted in lowering the depth of groundwater level in the range of 5 to 7 m in and in some cases even mining the aquifer where discharge rate has exceeded the recharge rates. This practice has also resulted in continuous falling water tables, deteriorating groundwater quality and inducing the secondary salinization problems in the irrigated agriculture of Pakistan, emphasizing the need of groundwater recharge using efficient techniques. The techniques of storing surplus good quality water into the aquifer and pumping the same water during periods of high crop water requirements is called Aquifer Storage and Recovery (ASR). These techniques provide an option to the farmer like water bank deposit especially when the groundwater quality is brackish.

Objectives
1.  To study the physical and chemical characteristics of the aquifer planned for Aquifer Storage and Recovery (ASR) techniques in the saline groundwater zone of Tehsil Toba Tek Singh at farmer’s field using resistivity survey meter.
2.  To investigate the effects of volume, rate and periods of surplus water storage on groundwater quality and lateral and vertical extent of the injected water in the aquifer.
3. To monitor the quality of groundwater injected and pumped and evaluate the recovery efficiency and suitability of the aquifer for implementing the ASR concepts by ensuring the farmer’s participation and the Agri. Extension personnel.

Methodology
The study was conducted at Chak No. 405/JB Tehsil Toba Tek Singh with Latitude, 300 96`.67`` and Longitude, 720 48`.33`` to develop aquifer storage and recovery technology. Two resistivity surveys were conducted using schlumberger array in collaboration with the Agricultural Engineering Department, Field Wing, Govt. of Punjab (Figure below).  The resistivity meter (ABEM Terrameter SAS-4000, Sweden) was used to conduct VES survey. The schlumberger electrode configuration with current electrode distance (AB/2) of 2, 4, 6, 8, 10, 10, 15, 20, 25, 25, 30, 35, 40, 45, 50, 50, 60, 70, 80, 90, 100, 100, 120, 140, 160 and 180 m was followed. Similarly, the potential electrode spacing (MN/2) was kept at 0.5, 2, 5, 10, 20 meter. Using computer software, 1X1D (Interpex, USA), the field resistivity data were interpreted and groundwater depth, layer thickness, groundwater quality, formation factor, longitudinal conductance and transverse resistance were determined. A pumping test was performed to determine the aquifer characteristics. The aquifer was recharged using 51, 71 and 99 m3 treated canal water, which was recovered after seven days.

Results
The results from the analysis of borehole data and VES results closely resembled with each other. The resistivity of the first layer (Figure above) had value of 22.2 Ω -m at VES 1 and 24.2 Ω -m at VES 2 position for a depth of 1.66 m and 1.14 m, respectively, which indicated sandy and clayey layers. These values were compared with the borehole data, which also showed the mixture of sand and clay up to depth of 1.5 m. The second layers had resistivities of 95 Ω-m and 56 Ω-m at both the VES positions indicating the sand and clay with alternate layer of gravel up to depth of 15 m. This also showed that good quality groundwater was available in this layer. Similarly, resistivities of the 3rd layer were compared with borehole data, which indicated medium fine sand with alternate layer of clay having poor quality groundwater up to the depth of 42 m. The fourth layer contained brackish groundwater and fine sand mixed with clay. The decreasing trend in resistivity values showed groundwater quality deteriorating as moving deeper.
Tubewell discharge was measured during pumping test, which was1.52 m3/min using calibrated pitot. The pumping test data analysis average transmissivity, storage coefficient and hydraulic conductivity were determined as 1160 m2/day, 0.476 and 44 m/day, respectively. Average transmissivity from the recovery test data for all these observation wells was 1320 m2/day, which was closer to the transmissivity values determined from the pumping test data. The groundwater quality improved during all three treatments during the aquifer storage and recovery techniques. The increasing trend in the values of EC and SAR was observed with increase in the pumping duration.  The values of injection time, pumping time and recovery times were also calculated separately for each treatment. The value of injection time was 48, 65 and 93 minutes for first, second and third treatment respectively. The measured pumping and recovery times were 28, 43 and 65 minutes and 34, 47 and 66 minutes for first, second and third treatment, respectively.

Conclusions

• The analysis based on VES and borehole data showed that fresh groundwater was available at 15 m depth below the ground surface at the study area, which can be skimmed carefully. 
• The results indicated that fresh groundwater was available for resistivity value of more than 42 Ω-m and marginally fresh groundwater was found within range of 22 to 42 Ω-m, below the bottom layer, groundwater was of poor quality with the resistivity value of less than 22 Ω-m.
• The transmissivity found from the borehole data and pumping test techniques were closer as 1210 m2/day and 1160 m2/day, respectively.
• A well was designed and installed at study area up to depth of 36 m below the ground surface with screen length of 21 m, opening area of 20% and screen diameter of 20 cm, which was efficient design for the design discharge of 1.50 m3/min.
• The results indicated that the recovery efficiency increased with increase in stored volume. The recovery efficiency was found to be 80% for injected volume of 51 m3; 91% for injected volume of 71 m3 and 98% for injected volume of 99 m3.
• The recovered time was found to be 70, 70 and 71 % of injected time for injected volumes of 51, 71 and 99 m3 respectively.
• The recovered volumes till EC value of 3 dS/m were 42.37 m3 for 51 m3; 64.5 m3 for 71 m3 and 97.5 m3 for 99 m3.

The results suggested that Aquifer Storage and Recovery (ASR) technology has the potential to store water for its later usage.

Recommendations

• Before installing the tubewell assembly, appropriate design of the well assembly such as screen length, screen diameter, slot size and opening area needs to be prepared by the design engineer.

• The injected volume of canal water should be more than 100 m3 to recover the same volume of injected water having quality permissible for agriculture use.
• Farmer can pump the tubewell to irrigate the crops for the period of 42 minutes against the injection time of one hour, however, depending on the recharge and discharge rates of the site.

Further information can be obtained from PI of the project.