Stalker: Shadow of Chernobyl → Wild Territory
You'll have to make your way to Amber Lake through the Wildlands (directly from the northern checkpoint). You'll have the opportunity to rescue one of the scientists by escorting them to the lake. This isn't as difficult as it seems—the scientist tries to stay out of trouble.
Beyond the bridge with fire anomalies, you'll encounter the first zombies. They wander slowly, but shoot accurately. Along the way, you'll reach the vicinity of the dried-up Yantar Lake.
One crazy shooter sits on the tower with sniper rifle.
See also:
Assault rifles: from the Viper to the FT-200M