Groundwater dating and residence time measurements
When atmospheric measurements are not available (early end of CFC dating ranges), estimates of production and release must be made using information from the Chemical Manufacturers Association (CMA) and the Alternate Fluorocarbon Environmental Acceptability Study (AFEAS).The quality of production data is deteriorating due to unreported data and under-reporting.When sampling in urban or industrial areas, local atmospheric measurements may be more useful for CFC reconstructions than global atmospheric CFC measurements that do not account for the local anomalies.Arsenic contamination of groundwater continues to threaten the health of millions of people in southeast Asia. Seasonal variations also occur, with the highest concentrations in the spring.CFC concentrations are higher over cities and urban areas.Calculations indicate significant under-reporting after the Montreal Protocol.The release of CFCs into the atmosphere must account for the following: 95% of the CFCs are released in the northern hemisphere with interhemispheric exchange occurring within 1.1-1.4 years.
The sedimentary pools of both iron and arsenic also declined with the burial age of the sediments. The oxidation of organic carbon, coupled to the reductive dissolution of arsenic-bearing iron oxides, is thought to control the release of sediment-bound arsenic into groundwater. However, the cause of the high spatial variability in groundwater arsenic concentrations—which can range from 5 to 500 within distances of a few kilometres—has been uncertain. We suggest that the age of aquifer sediments is a key determinant of groundwater arsenic concentrations.
Here, we combine measurements of sediment age, organic-matter reactivity and water chemistry at four locations along a cross-section of the arsenic-contaminated Red River floodplain in Vietnam to determine the origin of variations in groundwater arsenic concentrations. The burial age of the aquifer sediments, determined using optical stimulated luminescence, ranged from 460 years near the course of the present-day river to 5,900 years at the margin of the floodplain.