Acidification is the most serious threat to the groundwater quality in the province, causing both corrosion on water pipe systems and inoreased leaching of metals from the ground. The regional groundwater monitoring programme focuses on parameters as pH, alkalinity, sulphate and total aluminium. Sampling is done regularly in 1) natural groundwater springs in the countryside, 2) public water supplies, 3) observation wells in the countryside and 4) observation wells in urban areas.
For most of the sites, analysing has been carried out during a time period that is shorter than five years. These sites give valuabM information about the general relationship between groundwater acidity and the hydrogeological conditions in the recharge area. Results from regular sampling of public water supplies have been used for studies of time series. The trends indicate that the hydrogeological conditions are of great importance for how the groundwater responds to acid input. In general, surficial groundwater in Quaternary deposits is more sensitive to acidification than groundwater in crystalline bedrock.
One example of progressing groundwater acidification since the early 1970 "s origins from the water supply at Backamo, 20 km south of Uddevalla. The well is situated at the slope of a delta deposit and water is taken from a depth of 2,5 meters below ground level. During the time period 1970-1994 pH has decreased about 0,2 units and alkalinity about 9 nag HCO 3 L -1. The ratio total hardness/alkalinity increased with time.
During 1995 the regional groundwater monitoring programme is being reviewed.