The chemistry of ground- and surface-water in areas of similar geologic settings are compared by plotting the equivalent concentrations of each component in an XY-diagram. The deviations of each chemical component from the line through origin and the plot of the two ionic sums (line of proportional composition) give information about the differences in composition of the two types of water.
If we assume that the geologic conditions of the ground- and surface-water are similar when the plots of calcium and magnesium will fall close to the ionic sum line, then we find that the alkalinity is relatively higher and the sulphate concentration is relatively lower in groundwater than in surface-water. These observations indicate that in areas influenced by acid precipitation the groundwater is acidified less than the surface water.
The groundwater stations studied so far show a regional tendency to lower pH-values in areas where regional lake surveys show low pH in surface waters, these areas include southernmost Norway and parts of eastern Norway.