Granites

Knowledge of the granites of Sumatra has been gathered mainly as the result of systematic mapping programmes conducted with the aim of identifying mineral resources and providing a geological data base for more detailed studies. Mapping programmes were conducted principally by Dutch and Indonesian geologists prior to the second world war, mainly in southern Sumatra and the Tin Islands. In the 1970s a combined Indonesian Directorate of Mineral Resources (DMR)/British Geological Survey (BGS) project was set up to map the geology of Sumatra to the north of the Equator. On completion of this project in the mid-1980s geological and geochemical maps for the region were published at the scale of 1:250 000, together with descriptive sheet bulletins. Another useful compilation which may be refered to is the 1:2.5 million scale geological map for the whole of the Indonesian Archipelago which includes Sumatra (Clarke 1990). Subsequently BGS undertook a similar but smaller project in southern Sumatra in order to upgrade geological mapping and mineral exploration programmes which were being conducted by the Indonesian Geological Research and Development Centre (GRDC) and DMR. As part of this programme a specific effort was made to investigate the granites of this region. A combined granite workshop/regional mapping programme resulted in the identification of many granite units within batholiths such as Lassi, Bungo and Garba, as well as numerous isolated plutons. Full geochemical and isotopic analyses were provided for these granites (McCourt & Cobbing 1993; McCourt et al. 1996). Gasparon & Varne (1995) have provided furthe

Chalk recharge beneath thick till deposits in East Anglia

This report describes the results of a project to investigate the Chalk-till groundwater system in East Anglia and to estimate rates of recharge to the Chalk aquifer through thick Lowestoft Till (chalky boulder clay). The project has involved drilling two cored boreholes, monitoring groundwater levels, sampling Chalk and till fracture waters and porewaters, numerical modelling of groundwater levels and the development of a conceptual model of the Chalk-till groundwater system. The main findings of the report are that: • the till has a significant impact on recharge quantity and distribution to the underlying Chalk aquifer. Beneath the interfluves recharge appears to be lower than previous estimates of 20 – 40 mm/a (Klink et al., 1996; Soley and Heathcote, 1998), maybe as low as 5 mm/a; • the Chalk groundwater beneath the interfluves is old (probably a minimum of several hundreds of years) and has negligible nitrate concentrations. This groundwater makes only a relatively small contribution to the active circulation system in the valleys; • recharge rates to the Chalk aquifer at the edge of the till are greater than the effective rainfall (rainfall minus actual evapotranspiration) because of the contribution of large volumes of runoff from the till sheet. This water characterises the modern (post- 1960s), high-nitrate, groundwaters of the main Chalk valleys with potentially short travel times from recharge to discharge. The arable land on the till sheet has had field drains installed and these contribute to the bulk of the runoff; as a consequence nitrate concentrations in the runoff are high; • the Chalk-till groundwater system and the spatial distribution of recharge to the Chalk aquifer determine the shape and dimensions of the catchment areas of abstraction boreholes. This in turn controls the proportion of modern water pumped by abstraction boreholes, which has implications for the concentration of nitrate in pumped water. One consequence of the redistribution of recharge by the till is that boreholes close to the edge of the till sheet are likely to pump a greater proportion of modern recharge than previously believed and these are likely to produce water with higher nitrate concentrations; • the Chalk groundwaters at the edge of the till sheet are vulnerable to pollution because of the potentially high recharge rates (due to runoff recharge) and the relatively shallow depth to the water table. As a consequence, travel times through the unsaturated zone may be short