<add name="PureWater.Properties.Settings.purewaterConnectionS"
        connectionString="Data Source=.;Initial Catalog=purewater;Integrated Security=True"
        providerName="System.Data.SqlClient" />

pureWaterCxtDataContext cxt = new pureWaterCxtDataContext(ConfigurationManager.ConnectionStrings["PureWater.Properties.Settings.purewaterConnectionS"].ConnectionString);