Groundwaters were sampled from 730 wells being used for drinking purposes during 2007-2009. These samples were analysed using a gas-flow type GPC (Gas Proportional Counter) according to the USEPA method (900.0). We obtained a gross-alpha counting TDS (total dissolved solid) efficiency curve (Y = 0.0017X2 - 0.3122X + 19.165, X = TDS, Y = efficiency, $R^2$ = 0.9734) using natural uranium standard to get gross ${\alpha}$ value of the samples. The gross alpha values ranged from MDA (minimum detectable activity) to 14.88 pCi/L and 429 samples showed values higher than MDA (< 0.9 pCi/L). Correlations of the uranium values with the total alpha values and the gross-alpha values indicate that uranium values have high impacts on gross-alpha values. Groundwater samples of study areas were classified into four regions according to the rock types; plutonic (granite) rock region (427 areas), metamorphic rock region (181 aeras), sedimentary rock region (70 areas), volcanic rock region (52 areas). Groundwater of Cretaceous granite presented the highest gross-alpha value. Gross alpha in groundwaters showed no relationship with uranium in terms of the geological ages, rocks and minerals.

Keywords: Gross alpha;GPC;TDS efficiency curve;Groundwater;Geological age;