Determination of Lateral and Vertical Penetration of Canal Water in Rechna Doab Using Environmental Isotopes

Abstract

The agricultural economy of the Indus Plains has been deteriorating for the last many years due to progressive waterlogging and salinization of the soil. Major cause of this soil disease is the leakage from present day irrigation system in the Rechna Doab which are unlined canals. The poor surface and subsurface drainage in the project area further enhance the problem. As a result, large areas of irrigated land have been lost either completely or partly to cultivation and yields of practically all crops have been seriously reduced by the rise of water table and soil & water salinity. It was planned to use the environmental isotopes to identify recharge sources, estimation of the relative contribution of different recharge sources, determination of temporal variations of isotopes in the groundwater regime and the assessment of lateral and vertical penetration of canal water into the groundwater. Two sites were selected for this study, one at Sukheki and its surrounding area located in district Hafizabad and the other at Pansera and its surrounding area located in district Faisalabad. Ninety 96 sampling points (forty from Sukheki area and fifty-six from Pansera area) were selected for regular water sampling. These sampling points included already existing hand-pumps, tube-wells and canals. Five samplings in Sukheki area and four in Pansera area were carried out periodically on quarterly basis. Physicochemical parameters like electrolytic conductivity (EC) and temperature were measured in the field. Water samples were analysed for isotopes of hydrogen and oxygen in the laboratory. Electrical resistivity survey was performed at eight locations in Sukheki area and at nine locations in the area of Pansera, to estimate the vertical penetration of canal water. The range of δ18O for the groundwater in Sukheki area is -9.46 % to -5.36 %. The wide spread of isotopic data along δD - δ18O line suggests that the Final Report iv groundwater is a mixture of varying proportions of more than one sources, each having its own characteristic isotopic composition. From frequency histogram of δ18O, it can be seen that three distinct groups exist. The enriched isotopic values show rain recharge, depleted isotopic values are from canal system. The intermediate isotopic values show mixing of both sources of water. Spatial variations have been observed in this area. The canal water has lateral penetration in whole of the project area. The maximum depth of penetration of canal water came to be 88 meters in Sukheki area. Frequency histogram shows that major contribution towards groundwater comes from rain water. The δ18O values of the groundwater in the Pansera area range from -9.77 to -7.52 %. In Pansera area, the lateral penetration of canal water is more pronounced. In eastern part of the area, the isotopic data show mixing of rain water and canal water. But major contribution towards groundwater recharge comes from canal system. The maximum depth of penetration of canal water came to be 200 meters near the canal and it reduces to 11 meters away from the canal. The EC is generally low near the canal and it increases as the distance from the canal increases. The flow velocity of groundwater in Sukheki area, comes to be 25.0 meters/annum and the seepage rate from Jhang Branch canal in this area comes to be 1460.0 m3 per annum per meter length of canal.