Looking at the accelerated speed of work (due to lower intensity of sample generation, i.e., 1/80th of standard NGCM programme) and advantages of larger area coverage (16 times more than the area coverage in the same time period by routine NGCM programme), the over bank sampling method has a definite time and money advantage. Not only it can provide the first generation geochemical maps rapidly, but can also act as an area selection criterion for various purposes, including prioritizing the areas for NGCM type of geochemical mapping. The map products of 45K, which have been generated from over bank sampling and selected for this write-up (Fig. 3 to 6) very clearly bring out the importance of a geochemical map. The importance of this type of geochemical map is further enhanced if the same can be made available to the nation in the shortest possible time.

Notwithstanding the usefulness of the programme, Geological Survey of India could achieve very little by the end the 10th plan period in terms of national area coverage. Out of a total of 2 million sq. km identified initially for the National Geochemical Mapping (NGCM) programme, mainly covering the peninsular part and the Indo-Gangetic alluvium of the country, a mere 1 lakh sq. km area has been covered in five years. The problem of low pace of area coverage is due to limitations in the availability of adequate manpower for sampling and for the analytical laboratories in the department. This situation may not improve during the 11th plan period and alternate means of accelerated area coverage may have to be found. One of the view-points is to augment the manpower by involving other Central and State Government agencies in the programme or alternatively, invoking radical changes in the geochemical sampling design. This particular write up makes an emphatic point on the second option by carrying out pilot study in a part of Rajasthan (degree sheet 45K) and display some of the map products for considered evaluation for efficacy of the methodology and results.

During this time, the officers of Project Geochemical Mapping, Operation Rajasthan, viz., Sarvashree S.K. Dash, A. Chishti, B. Behera, S. Banerjee, Pawan Kumar, Dinesh Kumar, Sanjay Singh and Ms Archana Sharma did a commendable work by completing the ‘overbank’ (geochemical) sampling of one degree sheet (45K) covering parts of the districts of Bhilwara, Rajsamand, Chittorgarh and Ajmer. A total of 12,200 sq. km area, (Fig.1. & 2.) was covered in this experimental programme with collection of ‘overbank’ samples from 144 carefully selected sample location points. The entire sample collection work for this programme was done as an additional work and conducted before commencement of the regular NGCM programme.

Whereas the routine. NGCM programme envisages collection of stream sediments in small streams restricted to 1 sq. km area basin. the present method invokes an ‘over bank’ sample generation concept from basin areas as large as 80 to 100 sq. km. The ‘over bank’ samples are usually produced in greater than 3^rd order streams during large scale discharge of water charged with sediments. A section through the ‘overbank’ sediments represents a sedimentation history over a longer period of geological time and the finer fractions in the layered sequence of overbank carry signature from farthest point in the basin. The sample collection in this method is therefore restricted to the finer fraction of various sedimentary cycles in an ‘overbank’ section at a carefully selected sampling point.